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U.S. Patent 381968: Mode and plan of operating electric motors by progressive shifting; Field
Magnet; Armature; Electrical conversion; Economical; Transmission of energy; Simple
construction; Easier construction; Rotating magnetic field principles.
10 July 1856
Smiljan, Austrian Empirea
(Croatian Military Frontier)
Died 7 January 1943 (aged 86)
New York City, New York, USA
United States of America
Citizenship Austro-Hungariana (1856–1891)
Fields Mechanical and electrical
Institutions Edison Machine Works
Tesla Electric Light &
Westinghouse Electric &
Edison Medal (1916)
Elliott Cresson Medal (1893)
John Scott Medal (1934)
Notes a Austrian Empire (1804–1867) reorganized and
renamed into the Austro-Hungarian Monarchy
(1867–1918) in 1867
Nikola Tesla's AC dynamo used to generate AC which is used to transport electricity across great
distances. It is contained in U.S. Patent 390,721.
An AC motor is an electric motor that is driven by an alternating current. It consists of two basic parts, an
outside stationary stator (there are examples such as some of the Papst motors that have the stator on the
inside and the rotor on the outside to increase the inertia and cooling, they were common on high quality tape and film machines where speed stability is important) having coils supplied with alternating current to produce a rotating magnetic field, and an inside rotor attached to the output shaft that is given a torque by the rotating field.
There are two types of AC motors, depending on the type of rotor used (not including eddy current motors also AC/DC mechanically commutated machines which are speed dependant on voltage and winding connection). The first is the synchronous motor, which rotates exactly at the supply frequency or a submultiple of the supply frequency. The magnetic field on the rotor is either generated by current delivered through slip rings or by a permanent magnet.
The second type is the induction motor, which runs slightly slower than the supply frequency. The magnetic field on the rotor of this motor is created by an induced current.
In 1882 Nikola Tesla identified the rotating magnetic induction field principle used in alternators and pioneered the use of this rotating and inducting electromagnetic field force to generate torque in rotating machines. He exploited this principle in the design of a poly-phase induction motor in 1883. In 1885, Galileo
Ferraris independently researched the concept. In 1888, Ferraris published his research in a paper to the Royal Academy of Sciences in Turin.
Introduction of Tesla's motor in 1888 initiated what is sometimes referred to as the Second Industrial Revolution, making possible both the efficient generation and long distance distribution of electrical energy
using the alternating current transmission system, also of Tesla's invention (1888). Before widespread use of Tesla's principle of poly-phase induction for rotating machines, all motors operated by continually passing a conductor through a stationary magnetic field (as in homopolar motor).
Initially Tesla suggested that the commutators from a machine could be removed and the device could
operate on a rotary field of electromagnetic force. Professor Poeschel, his teacher, stated that would be akin to building a perpetual motion machine. This was because Tesla's teacher had only understood one half of Tesla's ideas. Professor Poeschel had realized that the induced rotating magnetic field would start the rotor of the motor spinning, but he did not see that the counter electromotive force generated would gradually bring the machine to a stop. Tesla would later obtain U.S. Patent 0,416,194, Electric Motor (December 1889), which resembles the motor seen in many of Tesla's photos. This classic alternating current electro-magnetic motor was an induction motor.
Michail Osipovich Dolivo-Dobrovolsky later invented a three-phase "cage-rotor" in 1890. This type of motor is now used for the vast majority of commercial applications.
Most common AC motors use the squirrel cage rotor, which will be found in virtually all domestic and light industrial alternating current motors. The squirrel cage refers to the rotating exercise cage for pet animals. The motor takes its name from the shape of its rotor "windings"- a ring at either end of the rotor, with bars
connecting the rings use cast copper to reduce the resistance in the rotor.running the length of the rotor. It is typically cast aluminum or copper poured between the iron laminates of the rotor, and usually only the end rings will be visible. The vast majority of the rotor currents will flow through the bars rather than the higher-resistance and usually varnished laminates. Very low voltages at very high currents are typical in the bars and end rings; high efficiency motors will often
In operation, the squirrel cage motor may be viewed as a transformer with a rotating secondary. When the rotor is not rotating in sync with the magnetic field, large rotor currents are induced; the large rotor currents magnetize the rotor and interact with the stator's magnetic fields to bring the rotor almost into synchronization with the stator's field. An unloaded squirrel cage motor at rated no-load speed will consume electrical power only to maintain rotor speed against friction and resistance losses. As the mechanical load increases, so will the electrical load - the electrical load is inherently related to the mechanical load. This is
similar to a transformer, where the primary's electrical load is related to the secondary's electrical load.
This is why a squirrel cage blower motor may cause household lights to dim upon starting, but doesn't dim the lights on startup when its fan belt (and therefore mechanical load) is removed. Furthermore, a stalled squirrel cage motor (overloaded or with a jammed shaft) will consume current limited only by circuit resistance as it attempts to start. Unless something else limits the current (or cuts it off completely) overheating and destruction of the winding insulation is the likely outcome.
To prevent the currents induced in the squirrel cage from superimposing itself back onto the supply, the squirrel cage is generally constructed with a prime number of bars, or at least a small multiple of a prime number (rarely more than 2). There is an optimum number of bars in any design, and increasing the number of bars beyond that point merely serves to increase the losses of the motor particularly when starting.
Virtually every washing machine, dishwasher, standalone fan, record player, etc. uses some variant of a squirrel cage motor.
Squirrel cage rotor
A squirrel cage rotor is the rotating part used in the most common form of AC induction motor. An electric motor with a squirrel cage rotor is termed a squirrel cage motor.
Diagram of the squirrel-cage (showing only three laminations)
In overall shape, it is a cylinder mounted on a shaft. Internally it contains longitudinal conductive bars (usually made of aluminum or copper) set into grooves and connected together at both ends by shorting
rings forming a cage-like shape. The name is derived from the similarity between this rings-and-bars winding and a squirrel cage (or, as it is commonly known, a hamster wheel).
The core of the rotor is built with stacks of electrical steel laminations. Figure 3 shows three of many laminations used.
Stator and rotor laminations
The field windings in the stator of an induction motor set up a rotating magnetic field around the rotor. The relative motion between this field and the rotation of the rotor induces electric current in the conductive bars. In turn these currents lengthwise in the conductors react with the magnetic field of the motor to produce force acting at a tangent orthogonal to the rotor, resulting in torque to turn the shaft. In effect the rotor is carried around with the magnetic field but at a slightly slower rate of rotation. The difference in speed is called slip and increases with load.
The conductors are often skewed slightly along the length of the rotor to reduce noise and smooth out torque fluctuations that might result at some speeds due to interactions with the pole pieces of the stator. The number of bars on the squirrel cage determines to what extent the induced currents are fed back to the stator coils and hence the current through them. The constructions that offer the least feedback employ prime numbers of bars.
The iron core serves to carry the magnetic field across the motor. In structure and material it is designed to minimize losses. The thin laminations, separated by varnish insulation, reduce stray circulating currents that would result in eddy current loss. The material is a low carbon but high silicon iron with several times the resistivity of pure iron, further reducing eddy-current loss. The low carbon content makes it a magnetically soft material with low hysteresis loss.
The same basic design is used for both single-phase and three-phase motors over a wide range of sizes. Rotors for three-phase will have variations in the depth and shape of bars to suit the design classification.
To demonstrate how the cage rotor works, the stator of a single-phase motor and a copper pipe (as rotor) may be used. If adequate ac power is applied to the stator, an alternating magnetic field revolves around the stator. If the copper pipe is inserted inside the stator, there will be an induced current in the pipe, and this current produces another magnetic field. The interaction of the stator revolving field and rotor induced field produce a torque and thus rotation.
Use in synchronous motors
Synchronous motors must use other types of rotors although they may employ a squirrel cage winding to allow them to reach near-synchronous speed while starting. Once operating at synchronous speed, the
magnetic field is rotating at the same speed as the rotor, so no current will be induced into the squirrel cage windings and they will have no further effect on the operation of the synchronous motor.
Three phase squirrel cage induction motors can also be used as generators. For this to work the motor must either be connected to a grid supply or an arrangement of capacitors. If the motor is run as a self exciting induction generator (SEIG) the capacitors can either be connected in a delta or c2c arrangement. The c2c method is for producing a single phase output and the delta method is for a three phase output. For the motor to work as a generator instead of a motor the rotor must be spun just faster than its nameplate speed, this will cause the motor to generate power after building up its residual magnetism.
If the rotor of a squirrel runs at the true synchronous speed, the flux in the rotor at any given place on the
rotor would not change, and no current would be created in the squirrel cage. For this reason, ordinary squirrel-cage motors run at some tens of rpm slower than synchronous speed, even at no load. Because the rotating field (or equivalent pulsating field) actually or effectively rotates faster than the rotor, it could be said to slip past the surface of the rotor. The difference between synchronous speed and actual speed is called slip, and loading the motor increases the amount of slip as the motor slows down slightly.
Two-phase AC servo motors
A typical two-phase AC servo-motor has a squirrel cage rotor and a field consisting of two windings:
1. a constant-voltage (AC) main winding. 2. a control-voltage (AC) winding in quadrature (i.e., 90 degrees phase shifted) with the main
winding so as to produce a rotating magnetic field. Reversing phase makes the motor reverse.
An AC servo amplifier, a linear power amplifier, feeds the control winding. The electrical resistance of the rotor is made high intentionally so that the speed/torque curve is fairly linear. Two-phase servo motors are inherently high-speed, low-torque devices, heavily geared down to drive the load.
Single-phase AC induction motors
Three-phase motors produce a rotating magnetic field. However, when only single-phase power is available, the rotating magnetic field must be produced using other means. Several methods are commonly used:
A common single-phase motor is the shaded-pole motor and is used in devices requiring low starting torque,
such as electric fans or the drain pump of washing machines and dishwashers or in other small household appliances. In this motor, small single-turn copper "shading coils" create the moving magnetic field. Part of each pole is encircled by a copper coil or strap; the induced current in the strap opposes the change of flux through the coil. This causes a time lag in the flux passing through the shading coil, so that the maximum field intensity moves across the pole face on each cycle. This produces a low level rotating magnetic field which is large enough to turn both the rotor and its attached load. As the rotor picks up speed the torque builds up to its full level as the principal magnetic field is rotating relative to the rotating rotor.
A reversible shaded-pole motor was made by Barber-Colman several decades ago. It had a single field coil, and two principal poles, each split halfway to create two pairs of poles. Each of these four "half-poles" carried a coil, and the coils of diagonally-opposite half-poles were connected to a pair of terminals. One terminal of each pair was common, so only three terminals were needed in all.
The motor would not start with the terminals open; connecting the common to one other made the motor run one way, and connecting common to the other made it run the other way. These motors were used in industrial and scientific devices.
An unusual, adjustable-speed, low-torque shaded-pole motor could be found in traffic-light and advertising-lighting controllers. The pole faces were parallel and relatively close to each other, with the disc centred between them, something like the disc in a watthour meter. Each pole face was split, and had a shading coil on one part; the shading coils were on the parts that faced each other. Both shading coils were probably closer to the main coil; they could have both been farther away, without affecting the operating principle, just the direction of rotation.
Applying AC to the coil created a field that progressed in the gap between the poles. The plane of the stator core was approximately tangential to an imaginary circle on the disc, so the travelling magnetic field dragged the disc and made it rotate.
The stator was mounted on a pivot so it could be positioned for the desired speed and then clamped in position. Keeping in mind that the effective speed of the travelling magnetic field in the gap was constant, placing the poles nearer to the centre of the disc made it run relatively faster, and toward the edge, slower.
It is possible that these motors are still in use in some older installations.
Split-phase induction motor
Another common single-phase AC motor is the split-phase induction motor, commonly used in major
appliances such as washing machines and clothes dryers. Compared to the shaded pole motor, these motors can generally provide much greater starting torque by using a special startup winding in conjunction with a centrifugal switch.
In the split-phase motor, the startup winding is designed with a higher resistance than the running winding. This creates an LR circuit which slightly shifts the phase of the current in the startup winding. When the motor is starting, the startup winding is connected to the power source via a set of spring-loaded contacts pressed upon by the stationary centrifugal switch. The starting winding is wound with fewer turns of smaller wire than the main winding, so it has a lower inductance (L) and higher resistance (R). The lower L/R ratio creates a small phase shift, not more than about 30 degrees, between the flux due to the main winding and the flux of the starting winding. The starting direction of rotation may be reversed simply by exchanging the connections of the startup winding relative to the running winding.
The phase of the magnetic field in this startup winding is shifted from the phase of the mains power, allowing the creation of a moving magnetic field which starts the motor. Once the motor reaches near design operating speed, the centrifugal switch activates, opening the contacts and disconnecting the startup
winding from the power source. The motor then operates solely on the running winding. The starting winding must be disconnected since it would increase the losses in the motor.
Capacitor start motor
Schematic of a capacitor start motor.
A capacitor start motor is a split-phase induction motor with a starting capacitor inserted in series with the startup winding, creating an LC circuit which is capable of a much greater phase shift (and so, a much greater starting torque). The capacitor naturally adds expense to such motors.
Resistance start motor
A resistance start motor is a split-phase induction motor with a starter inserted in series with the startup
winding, creating capacitance. This added starter provides assistance in the starting and initial direction of rotation.
Permanent-split capacitor motor
Another variation is the permanent-split capacitor (PSC) motor (also known as a capacitor start and run motor). This motor operates similarly to the capacitor-start motor described above, but there is no centrifugal starting switch, and what correspond to the start windings (second windings) are permanently
connected to the power source (through a capacitor), along with the run windings. PSC motors are frequently used in air handlers, blowers, and fans (including ceiling fans) and other cases where a variable speed is desired.
A capacitor ranging from 3 to 25 microfarads is connected in series with the "start" windings and remains in
the circuit during the run cycle. The "start" windings and run windings are identical in this motor, and reverse motion can be achieved by reversing the wiring of the 2 windings, with the capacitor connected to the other windings as "start" windings. By changing taps on the running winding but keeping the load constant, the motor can be made to run at different speeds. Also, provided all 6 winding connections are available separately, a 3 phase motor can be converted to a capacitor start and run motor by commoning two of the windings and connecting the third via a capacitor to act as a start winding.
An alternate design, called the wound rotor, is used when variable speed is required. In this case, the rotor has the same number of poles as the stator and the windings are made of wire, connected to slip rings on the shaft. Carbon brushes connect the slip rings to an external controller such as a variable resistor that
allows changing the motor's slip rate. In certain high-power variable speed wound-rotor drives, the slip-frequency energy is captured, rectified and returned to the power supply through an inverter. With bidirectionally controlled power, the wound-rotor becomes an active participant in the energy conversion process with the wound-rotor doubly-fed configuration showing twice the power density.
Compared to squirrel cage rotors and without considering brushless wound-rotor doubly-fed technology, wound rotor motors are expensive and require maintenance of the slip rings and brushes, but they were the standard form for variable speed control before the advent of compact power electronic devices. Transistorized inverters with variable-frequency drive can now be used for speed control, and wound rotor
motors are becoming less common.
Several methods of starting a polyphase motor are used. Where the large inrush current and high starting torque can be permitted, the motor can be started across the line, by applying full line voltage to the terminals (direct-on-line, DOL). Where it is necessary to limit the starting inrush current (where the motor is large compared with the short-circuit capacity of the supply), reduced voltage starting using either series inductors, an autotransformer, thyristors, or other devices are used. A technique sometimes used is (star-delta, YΔ) starting, where the motor coils are initially connected in star for acceleration of the load, then switched to delta when the load is up to speed. This technique is more common in Europe than in North America. Transistorized drives can directly vary the applied voltage as required by the starting characteristics of the motor and load.
This type of motor is becoming more common in traction applications such as locomotives, where it is known as the asynchronous traction motor.
The speed of the AC motor is determined primarily by the frequency of the AC supply and the number of poles in the stator winding, according to the relation:
Ns = 120F / p
Ns = Synchronous speed, in revolutions per minute
F = AC power frequency
p = Number of poles per phase winding
Actual RPM for an induction motor will be less than this calculated synchronous speed by an amount known as slip, that increases with the torque produced. With no load, the speed will be very close to synchronous. When loaded, standard motors have between 2-3% slip, special motors may have up to 7% slip, and a class of motors known as torque motors are rated to operate at 100% slip (0 RPM/full stall).
The slip of the AC motor is calculated by:
S = (Ns − Nr) / Ns
Nr = Rotational speed, in revolutions per minute.
S = Normalised Slip, 0 to 1.
As an example, a typical four-pole motor running on 60 Hz might have a nameplate rating of 1725 RPM at full load, while its calculated speed is 1800 RPM.
The speed in this type of motor has traditionally been altered by having additional sets of coils or poles in the motor that can be switched on and off to change the speed of magnetic field rotation. However, developments in power electronics mean that the frequency of the power supply can also now be varied to provide a smoother control of the motor speed.
Three-phase AC synchronous motors
If connections to the rotor coils of a three-phase motor are taken out on slip-rings and fed a separate field
current to create a continuous magnetic field (or if the rotor consists of a permanent magnet), the result is called a synchronous motor because the rotor will rotate synchronously with the rotating magnetic field produced by the polyphase electrical supply.
The synchronous motor can also be used as an alternator.
Nowadays, synchronous motors are frequently driven by transistorized variable-frequency drives. This greatly eases the problem of starting the massive rotor of a large synchronous motor. They may also be started as induction motors using a squirrel-cage winding that shares the common rotor: once the motor reaches synchronous speed, no current is induced in the squirrel-cage winding so it has little effect on the synchronous operation of the motor, aside from stabilizing the motor speed on load changes.
Synchronous motors are occasionally used as traction motors; the TGV may be the best-known example of such use.
One use for this type of motor is its use in a power factor correction scheme. They are referred to as synchronous condensers. This exploits a feature of the machine where it consumes power at a leading power factor when its rotor is over excited. It thus appears to the supply to be a capacitor, and could thus be used to correct the lagging power factor that is usually presented to the electric supply by inductive loads. The excitation is adjusted until a near unity power factor is obtained (often automatically). Machines used for
this purpose are easily identified as they have no shaft extensions. Synchronous motors are valued in any case because their power factor is much better than that of induction motors, making them preferred for very high power applications.
Some of the largest AC motors are pumped-storage hydroelectricity generators that are operated as
synchronous motors to pump water to a reservoir at a higher elevation for later use to generate electricity using the same machinery. Six 350-megawatt generators are installed in the Bath County Pumped Storage Station in Virginia, USA. When pumping, each unit can produce 563,400 horsepower (420 megawatts).
Universal motors and series wound motors
AC motors can also have brushes. The universal motor is widely used in small home appliances and power tools.
Repulsion motors are wound-rotor single-phase AC motors that are similar to universal motors. In a
repulsion motor, the armature brushes are shorted together rather than connected in series with the field. By transformer action ,the stator induces currents in the rotor, which create torque by repulsion instead of attraction as in other motors. Several types of repulsion motors have been manufactured, but the repulsion-start induction-run (RS-IR) motor has been used most frequently. The RS-IR motor has a centrifugal switch that shorts all segments of the commutator so that the motor operates as an induction motor once it has been accelerated to full speed. Some of these motors also lift the brushes out of contact with the commutator once the commutator is shorted. RS-IR motors have been used to provide high starting torque per ampere under conditions of cold operating temperatures and poor source voltage regulation. Few repulsion motors of any type are sold as of 2005.
Other types of motors
Single-phase AC synchronous motors
Small single-phase AC motors can also be designed with magnetized rotors (or several variations on that
idea; see "Hysteresis synchronous motors" below).
If a conventional squirrel-cage rotor has flats ground on it to create salient poles and increase reluctance, it will start conventionally, but will run synchronously, although it can provide only a modest torque at synchronous speed. This is known as a reluctance motor.
Because inertia makes it difficult to instantly accelerate the rotor from stopped to synchronous speed, these
motors normally require some sort of special feature to get started. Some include a squirrel-cage structure to bring the rotor close to synchronous speed. Various other designs use a small induction motor (which may share the same field coils and rotor as the synchronous motor) or a very light rotor with a one-way mechanism (to ensure that the rotor starts in the "forward" direction). In the latter instance, applying AC power creates chaotic (or seemingly chaotic) jumping movement back and forth; such a motor will always start, but lacking the anti-reversal mechanism, the direction it runs is unpredictable. The Hammond organ tone generator used a non-self-starting synchronous motor (until comparatively recently), and had an auxiliary conventional shaded-pole starting motor. A spring-loaded auxiliary manual starting switch connected power to this second motor for a few seconds.
Hysteresis synchronous motors
These motors are relatively costly, and are used where exact speed (assuming an exact-frequency AC source) as well as rotation with a very small amount of fast variations in speed (called 'flutter" in audio
recordings) is essential. Applications included tape recorder capstan drives (the motor shaft could be the capstan). Their distinguishing feature is their rotor, which is a smooth cylinder of a magnetic alloy that stays magnetized, but can be demagnetized fairly easily as well as re-magnetized with poles in a new location. Hysteresis refers to how the magnetic flux in the metal lags behind the external magnetizing force; for instance, to demagnetize such a material, one could apply a magnetizing field of opposite polarity to that which originally magnetized the material.
These motors have a stator like those of capacitor-run squirrel-cage induction motors. On startup, when slip decreases sufficiently, the rotor becomes magnetized by the stator's field, and the poles stay in place. The motor then runs at synchronous speed as if the rotor were a permanent magnet. When stopped and re-started, the poles are likely to form at different locations.
For a given design, torque at synchronous speed is only relatively modest, and the motor can run at below synchronous speed.
Electronically commutated motors
Such motors have an external rotor with a cup-shaped housing and a radially magnetized permanent magnet connected in the cup-shaped housing. An interior stator is positioned in the cup-shaped housing. The interior stator has a laminated core having grooves. Windings are provided within the grooves. The
windings have first end turns proximal to a bottom of the cup-shaped housing and second end turns positioned distal to the bottom. The first and second end turns electrically connect the windings to one another. The permanent magnet has an end face rom the bottom of the cup-shaped housing. At least one galvano-magnetic rotor position sensor is arranged opposite the end face of the permanent magnet so as to be located within a magnetic leakage of the permanent magnet and within a magnetic leakage of the interior stator. The at least one rotor position sensor is designed to control current within at least a portion of the windings. A magnetic leakage flux concentrator is arranged at the interior stator at the second end turns at a side of the second end turns facing away from the laminated core and positioned at least within an angular area of the interior stator in which the at (?-someone got distracted)
ECM motors are increasingly being found in forced-air furnaces and HVAC systems to save on electricity costs as modern HVAC systems are running their fans for longer periods of time (duty cycle). The cost effectiveness of using ECM motors in HVAC systems is questionable, given that the repair (replacement) costs are likely to equal or exceed the savings realized by using such a motor.
These are essentially two-phase induction motors with permanent magnets that retard rotor speed, so their
speed is quite accurately proportional to wattage of the power passing through the meter. The rotor is an aluminium-alloy disc, and currents induced into it react with the field from the stator.
The stator is composed of three coils that are arranged facing the disc surface, with the magnetic circuit completed by a C-shaped core of permeable iron. One phase of the motor is produced by a coil with many
turns located above the disc surface. This upper coil has a relatively high inductance, and is connected in parallel with the load. The magnetic field produced in this coil lags the applied (line/mains) voltage by almost 90 degrees. The other phase of the motor is produced by a pair of coils with very few turns of heavy-gauge wire, and hence quite-low inductance. These coils, located on the underside of the disc surface, are wired in series with the load, and produce magnetic fields in-phase with the load current.
Because the two lower coils are wound anti-parallel, and are each located equidistant from the upper coil, an azimuthally traveling magnetic flux is created across the disc surface. This traveling flux exerts an average torque on the disc proportional to the product of the power factor; RMS current, and voltage. It follows that the rotation of the magnetically-braked disc is in effect an analogue integration the real RMS power delivered to the load. The mechanical dial on the meter then simply reads off a numerical value proportional to the total number of revolutions of the disc, and thus the total energy delivered to the load.
Slow-speed synchronous timing motors
Representative are low-torque synchronous motors with a multi-pole hollow cylindrical magnet (internal
poles) surrounding the stator structure. An aluminum cup supports the magnet. The stator has one coil, coaxial with the shaft. At each end of the coil are a pair of circular plates with rectangular teeth on their edges, formed so they are parallel with the shaft. They are the stator poles. One of the pair of discs distributes the coil's flux directly, while the other receives flux that has passed through a common shading coil. The poles are rather narrow, and between the poles leading from one end of the coil are an identical set leading from the other end. In all, this creates a repeating sequence of four poles, unshaded alternating with shaded, that creates a circumferential traveling field to which the rotor's magnetic poles rapidly synchronize. Some stepping motors have a similar structure.
When considering starting criteria, we must determine how often the motor will be started, once a week,
once a day, once an hour. The reason we are concerned with the frequency of starting is the high starting current.
Common induction motors have locked rotor starting
currents of up to six times their nameplate current. At locked rotor conditions, during starting, the effect of heat on stator windings equals 6 squared, or 36 times, the heating effect under rated load. A motor can sustain locked rotor currents only briefly, for about 10 to 15 seconds. Thereafter, damage results to the motor's windings
Slow acceleration or jogging increases motor temperature in the same manner. For applications involving slow acceleration or jogging, excessive heat input must be offset with idle periods for motor cooling. If the motor is subjected to slow or repeated starts without resting, high starting current is present in the windings until the motor exceeds breakdown torque, and stalls.
Starting a motor involves special torque characteristic
considerations, including locked rotor or starting
torque, pull-up torque, and breakdown torque. The
motor must have sufficient starting and pull-up torque
to bring the driven machine to operating speed,
otherwise the motor will stall and this can eventually
damage the motor.
Some applications require a motor to accelerate the driven load to normal operating speed within in a certain time frame. The longer a motor takes to accelerate, the higher the temperature rise in the motor. The larger the load, the longer is the acceleration time. A motor's ability to accelerate a driven load influences both the safety and performance factors of an operation.
Use this chart as a guide to maximum allowable acceleration times:
Electrical Supply System
Motors require the correct phase, voltage, and frequency from an electrical supply system. The system must also have sufficient capacity, or current carrying capability, to start and operate the motor. Motor nameplates include all this information.
Motors operate satisfactorily on voltage that is within 10 percent, or frequency within 5 percent, of the optimal value listed on the motor nameplate. There is a risk of motor damage if the combined total voltage and frequency variation of a power supply exceeds 10 percent.
Stable Frequency and Voltage
NEMA rated motors can be satisfactorily operated up to a combination voltage and frequency variation of 10 percent, assuming that the frequency variation itself does not exceed 5 percent.
Motors operate on either single or three phase electrical
power. Single phase motors may operate on one phase of a three phase power supply with a compatible voltage rating. Many power suppliers do not allow large single phase motors to be connected to their lines. This is because a single phase motor larger than 10 horsepower started on a three phase source often causes voltage flicker problems.
One advantage of three phase motors is that they cause little or no voltage flicker on starting. Three phase motors cost less and last longer than comparably sized single phase motors.
One disadvantage of three phase power is that it often entails line extension charges upon installation. Operation of a three phase motor on a single phase power source results in permanent damage to the motor.
This is a diagram that may be helpful in understanding how single and three phase power commonly appear.
Physical Environmental Considerations
Motors must be rated to withstand the physical and environmental conditions their operation subjects them to. The selection of the proper enclosure is vital to the successful safe operation of a motor. Motor performance and life can be significantly impacted by using a motor enclosure inappropriate for the application. An examination of the operating conditions of the motor environment will determine the most appropriate enclosure along with attention to applicable codes and standards.
The NEMA frame size of the motor you select depends upon the horsepower and speed you require. After choosing the motor, check the dimensions of that frame size to see if it fits in the space available.
If the standard motor does not fit, determine whether it is most economical to change the driven machine, use a different standard motor, or adapt the standard motor by using a mechanical modification or accessory.
NEMA rates most motors for exposure to ambient temperatures ranging from 32 to 104 degrees F (0 to 40 degrees C). If the motor has water cooling capabilities, NEMA ratings range from 50 to 104 degrees F (10 to 40 degrees C). Standard NEMA maximum ambient temperature is 104 degrees F.
NEMA bases temperature rise ratings on operation altitudes of 3,300 feet (1,000 meters) or less. NEMA considers any altitude up to 3,300 feet equivalent to sea level. NEMA also assumes a maximum ambient temperature of 104 degrees F (40 degrees C) within these parameters.
When a motor operates at high altitude, de-rating is necessary since lower air density does not cool motors as well as high density air. Motors with Class B or F insulation systems and temperature rise in accordance with NEMA operate satisfactorily at altitudes above 3,300 feet; as long as ambient temperature remains lower than 104 degrees F (40 degrees C).
Rigid Mounting Surfaces
A motor's mounting surface must be rigid, and motor drives must accord with NEMA specifications. Furthermore, supplemental enclosures and structural accessories cannot noticeably interfere with motor ventilation. If a motor does not meet these provisions, the acquisition of a specialized motor, more suited to unconventional installation, is necessary.
Duty cycle: Continuous steady-running loads over long periods are demonstrated by fans and blowers. On the other hand, electric motors installed in machines with flywheels may have wide variations in running loads. Often,
electric motors use flywheels to supply the energy to do the work, and the electric motor does nothing but restore lost energy to the flywheel. Therefore, choosing the proper electric motor also depends on whether the load is steady, varies, follows a repetitive cycle of variation, or has pulsating torque or shocks.
For example, electric motors that run continuously in fans and blowers for hours or days may be selected on
the basis of continuous load. But electric motors located in devices like automatically controlled compressors and pumps start a number of times per hour. And electric motors in some machine tools start and stop
many times per minute.
Duty cycle is a fixed repetitive load pattern over a given period of time which is expressed as the ratio of on-time to cycle period. When operating cycle is such that electric motors operate at idle or a reduced load for more than 25% of the time, duty cycle becomes a factor in sizing electric motors. Also, energy required to start electric motors (that is, accelerating the inertia of the electric motor as well as the driven load) is much higher than for steady-state operation, so frequent starting could overheat the electric motor.
For most electric motors (except squirrel-cage electric motors during acceleration and plugging) current is almost directly proportional to developed torque. At constant speed, torque is proportional to horsepower. For accelerating loads and overloads on electric motors that have considerable droop, equivalent horsepower is used as the load factor. The next step in sizing the electric motor is to examine the electric motor's performance curves to see if the electric motor has enough starting torque to overcome machine static friction, to accelerate the load to full running speed, and to handle maximum overload.
Electric Motors - Service factors: A change in NEMA standards for electric motor service factors and temperature rise has been brought about because of better insulation used on electric motors. For instance, a 1.15 service factor -- once standard for all open electric motors -- is no longer standard for electric motors above 200 hp.
Increases in electric motor temperature are measured by the resistance method in the temperature rise
table. Electric motors feature a nameplate temperature rise, which is always expressed for the maximum allowable load. That is, if the electric motor has a service factor greater than unity, the nameplate temperature rise is expressed for the overload. Two Class-B insulated electric motors having 1.15 and 1.25 service factors will, therefore, each be rated for a 90°C rise. But the second electric motor will have to be larger than the first in order to dissipate the additional heat it generates at 125% load.
Electric motors feature a service factor, which indicates how much over the nameplate rating any given electric motor can be driven without overheating. NEMA Standard MGI-143 defines service factor of an ac motor as "...a multiplier which, when applied to the rated horsepower, indicates a permissible horsepower
loading which may be carried under the conditions specified for the service factor..." In other words, multiplying the electric motor's nameplate horsepower by the service factor tells how much electric motors can be overloaded without overheating. Generally, electric motor service factors:
Handle a known overload, which is occasional.
Provide a factor of safety where the environment or service condition is not well defined, especially
for general-purpose electric motors.
Obtain cooler-than-normal electric motor operation at rated load, thus lengthening insulation life.
Electric Motors - Efficiency: Small universal electric motors have an efficiency of about 30%, while 95% efficiencies are common for three-phase machines. In less-efficient electric motors, the amount of power wasted can be reduced by
more careful application and improved electric motor design.
Electric motor's feature an efficiency level, which also depends on actual electric motor load versus rated load, being greatest near rated load and falling off rapidly for under and overload conditions.
Mechanical requirements of a Driven Load:
A motor must produce enough torque to accelerate from a standstill to operating speed, and supply enough power for all possible demands without exceeding its design limits. Selecting the right motor for the application is not always a simple task.
In this section we examine the characteristics of driving the load, and see how important each part is in properly specifying a motor.
An example of PWM in an AC motor drive: the phase-to-phase voltage (blue) is modulated as a series of
pulses that results in a sine-like flux density waveform (red) in the magnetic circuit of the motor. The
smoothness of the resultant waveform can be controlled by the width and number of modulated
impulses (per given cycle)
Pulse-width modulation (PWM) is a commonly used technique for controlling power to an electrical device,
made practical by modern electronic power switches. The average value of voltage (and current) fed to the load is controlled by turning the switch between supply and load on and off at a fast pace. The longer the switch is on compared to the off periods, the higher the power supplied to the load is.
The PWM switching frequency has to be much faster than what would affect the load, which is to say the device that uses the power. Typically switchings have to be done several times a minute in an electric stove,
120 Hz in a lamp dimmer, from few kilohertz (kHz) to tens of kHz for a motor drive and well into the tens or hundreds of kHz in audio amplifiers and computer power supplies.
The term duty cycle describes the proportion of on time to the regular interval or period of time; a low duty cycle corresponds to low power, because the power is off for most of the time. Duty cycle is expressed in percent, 100% being fully on.
The main advantage of PWM is that power loss in the switching devices is very low. When a switch is off there is practically no current, and when it is on, there is almost no voltage drop across the switch. Power loss, being the product of voltage and current, is thus in both cases close to zero. PWM works also well with digital controls, which, because of their on/off nature, can easily set the needed duty cycle.
PWM has also been used in certain communication systems where its duty cycle has been used to convey information over a communications channel.
In the past, when only partial power was needed (such as for a sewing machine motor), a rheostat (located in the sewing machine's foot pedal) connected in series with the motor adjusted the amount of current flowing through the motor, but also wasted power as heat in the resistor element. It was an inefficient scheme, but tolerable because the total power was low. This was one of several methods of controlling
power. There were others—some still in use—such as variable autotransformers, including the trademarked Autrastat for theatrical lighting; and the Variac, for general AC power adjustment. These were quite efficient, but also relatively costly.
For about a century, some variable-speed electric motors have had decent efficiency, but they were
somewhat more complex than constant-speed motors, and sometimes required bulky external electrical apparatus, such as a bank of variable power resistors or rotating converter such as Ward Leonard drive .
However, in addition to motor drives for fans, pumps and robotic servos, there was a great need for compact and low cost means for applying adjustable power for many devices, such as electric stoves and lamp dimmers.
One of early applications of PWM was in the Sinclair X10, a 10 W audio amplifier available in kit form in the 1960s. At around the same time PWM started to be used in AC motor control 
Fig. 1: a pulse wave, showing the definitions of ymin, ymax and D.
Pulse-width modulation uses a rectangular pulse wave whose pulse width is modulated resulting in the
variation of the average value of the waveform. If we consider a pulse waveform f(t) with a low value
ymin, a high value ymax and a duty cycle D (see figure 1), the average value of the waveform is given
As f(t) is a pulse wave, its value is ymax for and ymin for .
The above expression then becomes:
This latter expression can be fairly simplified in many cases where ymin = 0 as . From
this, it is obvious that the average value of the signal ( ) is directly dependent on the duty cycle D.
Fig. 2: A simple method to generate the PWM pulse train corresponding to a given signal is the
intersective PWM: the signal (here the green sinewave) is compared with a sawtooth waveform (blue).
When the latter is less than the former, the PWM signal (magenta) is in high state (1). Otherwise it is in
the low state (0).
The simplest way to generate a PWM signal is the intersective method, which requires only a sawtooth or a
triangle waveform (easily generated using a simple oscillator) and a comparator. When the value of the reference signal (the green sine wave in figure 2) is more than the modulation waveform (blue), the PWM signal (magenta) is in the high state, otherwise it is in the low state.
In the use of delta modulation for PWM control, the output signal is integrated, and the result is compared
with limits, which correspond to a reference signal offset by a constant. Every time the integral of the output signal reaches one of the limits, the PWM signal changes state.
Fig. 3 : Principle of the delta PWM. The output signal (blue) is compared with the limits (green). These
limits correspond to the reference signal (red), offset by a given value. Every time the output signal
reaches one of the limits, the PWM signal changes state.
In delta-sigma modulation as a PWM control method, the output signal is subtracted from a reference signal
to form an error signal. This error is integrated, and when the integral of the error exceeds the limits, the output changes state.
Fig. 4 : Principle of the sigma-delta PWM. The top green waveform is the reference signal, on which the
output signal (PWM, in the middle plot) is subtracted to form the error signal (blue, in top plot). This
error is integrated (bottom plot), and when the integral of the error exceeds the limits (red lines), the
output changes state.
Space vector modulation
Space vector modulation is a PWM control algorithm for multi-phase AC generation, in which the reference
signal is sampled regularly; after each sample, non-zero active switching vectors adjacent to the reference vector and one or more of the zero switching vectors are selected for the appropriate fraction of the sampling period in order to synthesize the reference signal as the average of the used vectors.
Direct torque control (DTC)
Direct torque control is a method used to control AC motors. It is closely related with the delta modulation (see above). Motor torque and magnetic flux are estimated and these are controlled to stay within their hysteresis bands by turning on new combination of the device's semiconductor switches each time either of the signal tries to deviate out of the band.
Many digital circuits can generate PWM signals (e.g. many microcontrollers have PWM outputs). They
normally use a counter that increments periodically (it is connected directly or indirectly to the clock of the circuit) and is reset at the end of every period of the PWM. When the counter value is more than the reference value, the PWM output changes state from high to low (or low to high). This technique is referred to as time proportioning, particularly as time-proportioning control – which proportion of a fixed cycle time is spent in the high state.
The incremented and periodically reset counter is the discrete version of the intersecting method's sawtooth. The analog comparator of the intersecting method becomes a simple integer comparison between the current counter value and the digital (possibly digitized) reference value. The duty cycle can only be varied in discrete steps, as a function of the counter resolution. However, a high-resolution counter can provide quite satisfactory performance.
Fig. 5 : Three types of PWM signals (blue): leading edge modulation (top), trailing edge modulation
(middle) and centered pulses (both edges are modulated, bottom). The green lines are the sawtooth
waveform (first and second cases) and a triangle waveform (third case) used to generate the PWM
waveforms using the intersective method.
Four types of pulse-width modulation (PWM) are possible:[dubious – discuss]
1. The pulse center may be fixed in the center of the time window and both edges of the pulse moved to compress or expand the width.
2. The lead edge can be held at the lead edge of the window and the tail edge modulated. 3. The tail edge can be fixed and the lead edge modulated. 4. The pulse repetition frequency can be varied by the signal, and the pulse width can be constant.
However, this method has a more-restricted range of average output than the other three.
The resulting spectra (of the three cases) are similar, and each contains a dc component, a base sideband
containing the modulating signal and phase modulated carriers at each harmonic of the frequency of the
pulse. The amplitudes of the harmonic groups are restricted by a sinx / x envelope (sinc function) and
extend to infinity.
On the contrary, the delta modulation is a random process that produces continuous spectrum without distinct harmonics.
In telecommunications, the widths of the pulses correspond to specific data values encoded at one end and decoded at the other.
Pulses of various lengths (the information itself) will be sent at regular intervals (the carrier frequency of the modulation).
_ _ _ _ _ _ _ _
| | | | | | | | | | | | | | | |
Clock | | | | | | | | | | | | | | | |
__| |____| |____| |____| |____| |____| |____| |____| |____
_ __ ____ ____ _
PWM Signal | | | | | | | | | |
| | | | | | | | | |
_________| |____| |___| |________| |_| |___________
Data 0 1 2 4 0 4 1 0
The inclusion of a clock signal is not necessary, as the leading edge of the data signal can be used as the clock if a small offset is added to the data value in order to avoid a data value with a zero length pulse.
_ __ ___ _____ _ _____ __ _
| | | | | | | | | | | | | | | |
PWM Signal | | | | | | | | | | | | | | | |
__| |____| |___| |__| |_| |____| |_| |___| |_____
Data 0 1 2 4 0 4 1 0
PWM can be used to adjust the total amount of power delivered to a load without losses normally incurred
when a power transfer is limited by resistive means. The drawback are the pulsations defined by the duty cycle, switching frequency and properties of the load. With a sufficiently high switching frequency and, when necessary, using additional passive electronic filters the pulse train can be smoothed and average analog waveform recovered.
High frequency PWM power control systems are easily realisable with semiconductor switches. As has been already stated above almost no power is dissipated by the switch in either on or off state. However, during the transitions between on and off states both voltage and current are non-zero and thus considerable power is dissipated in the switches. Luckily, the change of state between fully on and fully off is quite rapid (typically less than 100 nanoseconds) relative to typical on or off times, and so the average power dissipation is quite low compared to the power being delivered even when high switching frequencies are used.
Modern semiconductor switches such as MOSFETs or Insulated-gate bipolar transistors (IGBTs) are quite ideal components. Thus high efficiency controllers can be built. Typically frequency converters used to control AC motors have efficiency that is better than 98 %. Switching power supplies have lower efficiency due to low output voltage levels (often even less than 2 V for microprocessors are needed) but still more than 70-80 % efficiency can be achieved.
Variable-speed fan controllers for computers usually use PWM, as it is far more efficient when compared to a potentiometer or rheostat. (Neither of the latter is practical to operate electronically; they would require a small drive motor.)
Light dimmers for home use employ a specific type of PWM control. Home use light dimmers typically include electronic circuitry which suppresses current flow during defined portions of each cycle of the AC line voltage. Adjusting the brightness of light emitted by a light source is then merely a matter of setting at what voltage (or phase) in the AC halfcycle the dimmer begins to provide electrical current to the light source (e.g. by using an electronic switch such as a triac). In this case the PWM duty cycle is the ratio of the
conduction time to the duration of the half AC cycle defined by the frequency of the AC line voltage (50 Hz or 60 Hz depending on the country).
These rather simple types of dimmers can be effectively used with inert (or relatively slow reacting) light sources such as incandescent lamps, for example, for which the additional modulation in supplied electrical
energy which is caused by the dimmer causes only negligible additional fluctuations in the emitted light. Some other types of light sources such as light-emitting diodes (LEDs), however, turn on and off extremely rapidly and would perceivably flicker if supplied with low frequency drive voltages. Perceivable flicker effects from such rapid response light sources can be reduced by increasing the PWM frequency. If the light fluctuations are sufficiently rapid, the human visual system can no longer resolve them and the eye perceives the time average intensity without flicker (see flicker fusion threshold).
In electric cookers, continuously-variable power is applied to the heating elements such as the hob or the grill using a device known as a Simmerstat. This consists of a thermal oscillator running at approximately two cycles per minute and the mechanism varies the duty cycle according to the knob setting. The thermal time constant of the heating elements is several minutes, so that the temperature fluctuations are too small to matter in practice.
PWM is also used in efficient voltage regulators. By switching voltage to the load with the appropriate duty
cycle, the output will approximate a voltage at the desired level. The switching noise is usually filtered with an inductor and a capacitor.
One method measures the output voltage. When it is lower than the desired voltage, it turns on the switch. When the output voltage is above the desired voltage, it turns off the switch.
Audio effects and amplification
PWM is sometimes used in sound (music) synthesis, in particular subtractive synthesis, as it gives a sound
effect similar to chorus or slightly detuned oscillators played together. (In fact, PWM is equivalent to the difference of two sawtooth waves. ) The ratio between the high and low level is typically modulated with a low frequency oscillator, or LFO. In addition, varying the duty cycle of a pulse waveform in a subtractive-synthesis instrument creates useful timbral variations. Some synthesizers have a duty-cycle trimmer for their square-wave outputs, and that trimmer can be set by ear; the 50% point was distinctive, because even-numbered harmonics essentially disappear at 50%.
A new class of audio amplifiers based on the PWM principle is becoming popular. Called "Class-D amplifiers", these amplifiers produce a PWM equivalent of the analog input signal which is fed to the loudspeaker via a suitable filter network to block the carrier and recover the original audio. These amplifiers are characterized by very good efficiency figures (≥ 90%) and compact size/light weight for large power outputs. For a few decades, industrial and military PWM amplifiers have been in common use, often for driving servo motors. They offer very good efficiency, commonly well above 90%. Field-gradient coils in MRI machines are driven by relatively-high-power PWM amplifiers.
Historically, a crude form of PWM has been used to play back PCM digital sound on the PC speaker, which is driven by only two voltage levels, typically 0 V and 5 V. By carefully timing the duration of the pulses, and by relying on the speaker's physical filtering properties (limited frequency response, self-inductance, etc.) it
was possible to obtain an approximate playback of mono PCM samples, although at a very low quality, and with greatly varying results between implementations.
In more recent times, the Direct Stream Digital sound encoding method was introduced, which uses a generalized form of pulse-width modulation called pulse density modulation, at a high enough sampling rate
(typically in the order of MHz) to cover the whole acoustic frequencies range with sufficient fidelity. This method is used in the SACD format, and reproduction of the encoded audio signal is essentially similar to the method used in class-D amplifiers.
1. Schönung, A.; Stemmler, H. (August 1964). "Geregelter Drehstrom-Umkehrantrieb mit gesteuertem Umrichter nach dem Unterschwingungsverfahren". BBC Mitteilungen (Brown Boveri et Cie) 51 (8/9): 555–577.
2. www.netrino.com – Introduction to Pulse Width Modulation (PWM) 3. Fundamentals of HVAC Control Systems, by Robert McDowall, p. 21
Three-phase induction motors
An induction motor or asynchronous motor is a type of alternating current motor where power is supplied to the rotor by means of electromagnetic induction.
An electric motor turns because of magnetic force exerted between a stationary electromagnet called the stator and a rotating electromagnet called the rotor. Different types of electric motors are distinguished by how electric current is supplied to the moving rotor. In a DC motor and a slip-ring AC motor, current is provided to the rotor directly through sliding electrical contacts called commutators and slip rings. In an induction motor, by contrast, the current is induced in the rotor without contacts by the magnetic field of the
stator, through electromagnetic induction. An induction motor is sometimes called a rotating transformer because the stator (stationary part) is essentially the primary side of the transformer and the rotor (rotating part) is the secondary side. Unlike the normal transformer which changes the current by using time varying flux, induction motors use rotating magnetic fields to transform the voltage. The current in the primary side creates an electromagnetic field which interacts with the electromagnetic field of the secondary side to produce a resultant torque, thereby transforming the electrical energy into mechanical energy. Induction motors are widely used, especially polyphase induction motors, which are frequently used in industrial drives.
Induction motors are now the preferred choice for industrial motors due to their rugged construction, absence of brushes (which are required in most DC motors) and—thanks to modern power electronics—the ability to control the speed of the motor.
The first induction motors were realized by Galileo Ferraris in 1885 in Italy. In 1888, Ferraris published his research in a paper to the Royal Academy of Sciences in Turin (later, in the same year, Nikola Tesla gained
U.S. Patent 381,968) where he exposed the theoretical foundations for understanding the way the motor operates. The induction motor with a cage was invented by Mikhail Dolivo-Dobrovolsky about a year later.
Principle of operation and comparison to synchronous
A 3-phase power supply provides a rotating magnetic field in an induction motor.
The basic difference between an induction motor and a synchronous AC motor is that in the latter a current is supplied into the rotor (usually DC) which in turn creates a (circular uniform) magnetic field around the rotor. The rotating magnetic field of the stator will impose an electromagnetic torque on the still magnetic
field of the rotor causing it to move (about a shaft) and rotation of the rotor is produced. It is called synchronous because at steady state the speed of the rotor is the same as the speed of the rotating magnetic field in the stator.
By way of contrast, the induction motor does not have any direct supply onto the rotor; instead, a
secondary current is induced in the rotor. To achieve this, stator windings are arranged around the rotor so that when energised with a polyphase supply they create a rotating magnetic field pattern which sweeps past the rotor. This changing magnetic field pattern induces current in the rotor conductors. These currents interact with the rotating magnetic field created by the stator and in effect causes a rotational motion on the rotor.
However, for these currents to be induced, the speed of the physical rotor must be less than the speed of the rotating magnetic field in the stator or else the magnetic field will not be moving relative to the rotor conductors and no currents will be induced. If by some chance this happens, the rotor typically slows slightly until a current is re-induced and then the rotor continues as before. This difference between the speed of the rotor and speed of the rotating magnetic field in the stator is called slip. It is unitless and is the ratio between the relative speed of the magnetic field as seen by the rotor (the slip speed) to the speed of the rotating stator field. Due to this, an induction motor is sometimes referred to as an asynchronous machine.
AC Induction Motor
n = Revolutions per minute (rpm)
f = AC power frequency (hertz)
p = Number of poles per phase (an even number)
Slip is calculated using:
where s is the slip.
The rotor speed is:
A synchronous motor always runs at synchronous speed with 0% slip. The speed of a synchronous motor is determined by the following formula:
where v is the speed of the rotor (in rpm), f is the frequency of the AC supply (in Hz) and p is the number of magnetic poles.
For example, a 6 pole motor operating on 60 Hz power would have a speed of:
Note on the use of p - some texts refer to number of pole pairs per phase instead of number of poles per phase. For example a 6 pole motor, operating on 60 Hz power, would have 3 pole pairs. The equation of synchronous speed then becomes:
with P being the number of pole pairs. For P = 3 and :
The stator consists of wound 'poles' that carry the supply current to induce a magnetic field that penetrates the rotor. In a very simple motor, there would be a single projecting piece of the stator (a salient pole) for each pole, with windings around it; in fact, to optimize the distribution of the magnetic field, the windings
are distributed in many slots located around the stator, but the magnetic field still has the same number of north-south alternations. The number of 'poles' can vary between motor types but the poles are always in pairs (i.e. 2, 4, 6, etc.).
Induction motors are most commonly built to run on single-phase or three-phase power, but two-phase
motors also exist. In theory, two-phase and more than three phase induction motors are possible; many single-phase motors having two windings and requiring a capacitor can actually be viewed as two-phase motors, since the capacitor generates a second power phase 90 degrees from the single-phase supply and feeds it to a separate motor winding. Single-phase power is more widely available in residential buildings, but cannot produce a rotating field in the motor ( the field merely oscillates back and forth), so single-phase induction motors must incorporate some kind of starting mechanism to produce a rotating field. They would, using the simplified analogy of salient poles, have one salient pole per pole number; a four-pole motor would have four salient poles. Three-phase motors have three salient poles per pole number, so a four-pole motor would have twelve salient poles. This allows the motor to produce a rotating field, allowing the motor to start with no extra equipment and run more efficiently than a similar single-phase motor.
There are three types of rotor:
The most common rotor is a squirrel-cage rotor. It is made up of bars of either solid copper (most common) or aluminum that span the length of the rotor, and those solid copper or aluminium strips can be shorted or connected by a ring or some times not, i.e. the rotor can be closed or semiclosed type. The rotor bars in squirrel-cage induction motors are not straight, but have some skew to reduce noise and harmonics.
Slip ring rotor
A slip ring rotor replaces the bars of the squirrel-cage rotor with windings that are connected to slip rings. When these slip rings are shorted, the rotor behaves similarly to a squirrel-cage rotor; they can also be connected to resistors to produce a high-resistance rotor circuit, which can be beneficial in starting
Solid core rotor
A rotor can be made from a solid mild steel. The induced current causes the rotation.
The synchronous rotational speed of the rotor (i.e. the theoretical unloaded speed with no slip) is controlled by the number of pole pairs (number of windings in the stator) and by the frequency of the supply voltage. Under load, the speed of the induction motor varies according to size of the load. As the load is increased, the speed of the motor decreases, increasing the slip, which increases the field strength of the rotor to bear the extra load. Before the development of economical semiconductor power electronics, it was difficult to vary the frequency to the motor and induction motors were mainly used in fixed speed applications. As an induction motor has no brushes and is easy to control, many older DC motors are now being replaced with THR induction motors and accompanying inverters in industrial applications.
Starting of induction motors
In a single phase induction motor, it is necessary to provide a starting circuit to start rotation of the rotor. If this is not done, rotation may be commenced by manually giving a slight turn to the rotor. The single phase
induction motor may rotate in either direction and it is only the starting circuit which determines rotational direction.
For small motors of a few watts, the start rotation is done by means of one or two single turn(s) of heavy copper wire around one corner of the pole. The current induced in the single turn is out of phase with the
supply current and so causes an out-of-phase component in the magnetic field, which imparts to the field sufficient rotational character to start the motor. Starting torque is very low and efficiency is also reduced. Such shaded-pole motors are typically used in low-power applications with low or zero starting torque requirements, such as desk fans and record players.
Larger motors are provided with a second stator winding which is fed with an out-of-phase current to create a rotating magnetic field. The out-of-phase current may be derived by feeding the winding through a capacitor or it may derive from the winding having different values of inductance and resistance from the main winding.
In some designs, the second winding is disconnected once the motor is up to speed, usually either by means of a switch operated by centrifugal force acting on weights on the motor shaft or by a positive temperature coefficient thermistor which, after a few seconds of operation, heats up and increases its resistance to a high
value thereby reducing the current through the second winding to an insignificant level. Other designs keep the second winding continuously energised when running, which improves torque.
Henri Boy de la Tour (1906). The induction motor: its theory and design, set forth by a practical method of calculation. Translated Cyprien Odilon Mailloux. McGraw Pub. Co.. http://books.google.com/books?id=hbM_AAAAYAAJ&printsec=frontcover&dq=induction+motor&source=bl&ots=_JgDsnjN2s&sig=LHXibhTQ9XXIOvzsWATRSHA-xkA&hl=en&ei=X1O3TOekFpCisAPomqGeCQ&sa=X&oi=book_result&ct=result&resnum=14&sqi=2&ved=0CFoQ6AEwDQ#v=onepage&q&f=false.
Benjamin Franklin Bailey (1911). The induction motor. McGraw-Hill. http://books.google.com/books?id=r_dOAAAAMAAJ&printsec=frontcover&dq=induction+motor&source=bl&ots=g7Th09trR-&sig=onxjvgyC920oARs_LUDqnzV2kHg&hl=en&ei=1VS3TNTyNoKKlwfWwJ3MDA&sa=X&oi=book_result&ct=result&resnum=4&ved=0CDcQ6AEwAzgK#v=onepage&q&f=false.
Bernhard Arthur Behrend (1901). The induction motor: A short treatise on its theory and design, with numerous experimental data and diagrams. Electrical world and engineer. http://books.google.com/books?id=ffpOAAAAMAAJ&printsec=frontcover&dq=induction+motor&source=bl&ots=AWJzYuRVCl&sig=Bm0VKBdRKgCfTPpeR5_YU3BCrso&hl=en&ei=1VS3TNTyNoKKlwfWwJ3MDA&sa=X&oi=book_result&ct=result&resnum=7&ved=0CEUQ6AEwBjgK#v=onepage&q&f=false.
1. http://www.wisegeek.com/what-is-an-induction-motor.htm 2. http://www.fi.edu/learn/case-files/tesla/motor.html 3. http://www.electricmotors.machinedesign.com/guiEdits/Content/bdeee11/bdeee11_7.aspx
A synchronous electric motor is an AC motor distinguished by a rotor spinning with coils passing magnets
at the same rate as the alternating current and resulting rotating magnetic field which drives it. Another way of saying this is that it has zero slip under usual operating conditions. Contrast this with an induction motor, which must slip in order to produce torque. They operate synchronously with line frequency. As with squirrel-cage induction motors, speed is determined by the number of pairs of poles and the line frequency.
Synchronous motors are available in sub-fractional self-excited sizes to high-horsepower direct-current excited industrial sizes. In the fractional horsepower range, most synchronous motors are used where precise constant speed is required. In high-horsepower industrial sizes, the synchronous motor provides two important functions. First, it is a highly efficient means of converting ac energy to work. Second, it can operate at leading or unity power factor and thereby provide power-factor correction.
There are two major types of synchronous motors: non-excited and direct-current excited.
Non-excited motors are manufactured in reluctance and hysteresis designs, these motors employ a self-starting circuit and require no external excitation supply.
Reluctance designs have ratings that range from sub-fractional to about 30 hp. Sub-fractional horsepower motors have low torque, and are generally used for instrumentation applications. Moderate torque, integral
horsepower motors use squirrel- cage construction with toothed rotors. When used with an adjustable frequency power supply, all motors in the drive system can be controlled at exactly the same speed. The power supply frequency determines motor operating speed.
Hysteresis motors are manufactured in sub-fractional horsepower ratings, primarily as servomotors and timing motors. More expensive than the reluctance type, hysteresis motors are used where precise constant speed is required.
DC-excited motors — Made in sizes larger than 1 hp, these motors require direct current supplied through slip rings for excitation. The direct current can be supplied from a separate source or from a dc generator directly connected to the motor shaft
Slip rings and brushes are used to conduct current to the rotor. The rotor poles connect to each other and move at the same speed - hence the name synchronous motor.
Synchronous motors fall under the category of synchronous machines which also includes the alternator (synchronous generator). These machines are commonly used in analog electric clocks, timers and other devices where correct time is required.
The speed of a synchronous motor is determined by the following formula:
where v is the speed of the rotor (in rpm), f is the frequency of the AC supply (in Hz) and n is the number
of magnetic poles.
A synchronous motor is composed of the following parts:
The stator is the outer shell of the motor, which carries the armature winding. This winding is spatially distributed for poly-phase AC current. This armature creates a rotating magnetic field inside the motor.
The rotor is the rotating portion of the motor. it carries field winding, which may be supplied by a DC source. On excitation, this field winding behaves as a permanent magnet.
The slip rings in the rotor, to supply the DC to the field winding, in the case of DC excited types
The operation of a synchronous motor is simple to imagine. The armature winding, when excited by a poly-phase (usually 3-phase)Supply, creates a rotating magnetic field inside the motor. The field winding, which acts as a permanent magnet, simply locks in with the rotating magnetic field and rotates along with it. During operation, as the field locks in with the rotating magnetic field, the motor is said to be in synchronization.
Once the motor is in operation, the speed of the motor is dependent only on the supply frequency. When the motor load is increased beyond the break down load, the motor falls out of synchronization i.e., the applied load is large enough to pull out the field winding from following the rotating magnetic field. The motor immediately stalls after it falls out of synchronization.
Synchronous motors are not self-starting motors. This property is due to the inertia of the rotor. When the power supply is switched on, the armature winding and field windings are excited. Instantaneously, the armature winding creates a rotating magnetic field, which revolves at the designated motor speed. The rotor, due to inertia, will not follow the revolving magnetic field. In practice, the rotor should be rotated by some other means near to the motor's synchronous speed to overcome the inertia. Once the rotor nears the synchronous speed, the field winding is excited, and the motor pulls into synchronization.
The following techniques are employed to start a synchronous motor:
A separate motor (called pony motor) is used to drive the rotor before it locks in into synchronization.
The field winding is shunted or induction motor like arrangements are made so that the synchronous motor starts as an induction motor and locks in to synchronization once it reaches speeds near its synchronous speed.
Reducing the input electrical frequency to get the motor starting slowly, Variable-frequency drives can be used here which have Rectifier-Inverter circuits or Cycloconverter circuits.
Synchronous motors show some interesting properties, which finds applications in power factor correction. The synchronous motor can be run at lagging, unity or leading power factor. The control is with the field excitation, as described below:
When the field excitation voltage is decreased, the motor runs in lagging power factor. The power factor by which the motor lags varies directly with the drop in excitation voltage. This condition is called under-excitation.
When the field excitation voltage is made equal to the rated voltage, the motor runs at unity power factor.
When the field excitation voltage is increased above the rated voltage, the motor runs at leading power factor. And the power factor by which the motor leads varies directly with the increase in field excitation voltage. This condition is called over-excitation.
The most basic property of synchro motor is that it can be use both as a capacitor or inductor. Hence in turn it improves the power factor of system.
The leading power factor operation of synchronous motor finds application in power factor correction. Normally, all the loads connected to the power supply grid run in lagging power factor, which increases reactive power consumption in the grid, thus contributing to additional losses. In such cases, a synchronous motor with no load is connected to the grid and is run over-excited, so that the leading power factor created by synchronous motor compensates the existing lagging power factor in the grid and the overall power factor is brought close to 1 (unity power factor). If unity power factor is maintained in a grid, reactive power losses diminish to zero, increasing the efficiency of the grid. This operation of synchronous motor in over-excited mode to correct the power factor is sometimes called as Synchronous condenser.
Synchronous motors find applications in all industrial applications where constant speed is necessary.
Improving the power factor as Synchronous condensers.
Electrical power plants almost always use synchronous generators because it is important to keep the frequency constant at which the generator is connected.
Low power applications include positioning machines, where high precision is required, and robot actuators.
Mains synchronous motors are used for electric clocks.
Record player turntables
Synchronous motors have the following advantages over non-synchronous motors:
Speed is independent of the load, provided an adequate field current is applied. Accurate control in speed and position using open loop controls, eg. stepper motors. They will hold their position when a DC current is applied to both the stator and the rotor
windings. Their power factor can be adjusted to unity by using a proper field current relative to the load.
Also, a "capacitive" power factor, (current phase leads voltage phase), can be obtained by increasing this current slightly, which can help achieve a better power factor correction for the whole installation.
Their construction allows for increased electrical efficiency when a low speed is required (as in ball mills and similar apparatus).
They run either at the synchronous speed or they do not run at all.
brushless DC electric motor. stepper motor. Three-phase AC synchronous motors. Switched reluctance motor.
Rotor and stator of an electric motor
Rotor from Hoover Dam generator
The rotor is the non-stationary part of a rotary electric motor, electric generator or alternator, which rotates
because the wires and magnetic field of the motor are arranged so that a torque is developed about the rotor's axis. In some designs, the rotor can act to serve as the motor's armature, across which the input voltage is supplied. The stationary part of an electric motor is the stator. A common problem is called cogging torque.
A DC armature.
In electrical engineering, an armature generally refers to one of the two principal electrical components of
an electromechanical machine– a motor or generator, but may also mean the pole piece of a permanent magnet or electromagnet, or the moving iron part of a solenoid or relay. The other component is the field winding or field magnet. The role of the "field" component is simply to create a magnetic field (magnetic flux) for the armature to interact with, so this component can comprise either permanent magnets, or electromagnets formed by a conducting coil. The armature, in contrast, must carry current so it is always a conductor or a conductive coil, oriented normal to both the field and to the direction of motion, torque (rotating machine), or force (linear machine). The armature's role is two-fold. The first is to carry current crossing the field, thus creating shaft torque in a rotating machine or force in a linear machine. The second role is to generate an electromotive force (EMF).
In the armature, an electromotive force is created by the relative motion of the armature and the field. When the machine is acting as a motor, this EMF opposes the armature current, and the armature converts electrical power to mechanical torque, and power, unless the machine is stalled, and transfers it to the load via the shaft. When the machine is acting as a generator, the armature EMF drives the armature current, and shaft mechanical power is converted to electrical power and transferred to the load. In an induction
generator, these distinctions are blurred, since the generated power is drawn from the stator, which would normally be considered the field.
A growler is used to check the armature for shorts, opens and grounds.
The parts of an alternator or related equipment can be expressed in either mechanical terms or electrical terms. Although distinctly separate, these two sets of terminology are frequently used interchangeably or in combinations that include one mechanical term and one electrical term. This may cause confusion when working with compound machines such as brushless alternators, or in conversation among people who are accustomed to work with differently configured machinery.
In alternating current machines, the armature is usually stationary, and is known as the stator winding. In DC rotating machines other than brushless DC machines, it is usually rotating, and is known as the rotor. The pole piece of a permanent magnet or electromagnet and the moving, iron part of a solenoid, especially if the latter acts as a switch or relay, may also be referred to as armatures.
Mechanical Rotor: The rotating part of an alternator, generator, dynamo or motor. Stator: The stationary part of an alternator, generator, dynamo or motor
Electrical Armature: The power-producing component of an alternator, generator, dynamo or motor. The armature can be on either the rotor or the stator. Field: The magnetic field component of an alternator, generator, dynamo or motor. The field can be on either the rotor or the stator and can be either an electromagnet or a permanent magnet.
Armature reaction in a DC machine
In a DC machine, the main field is produced by field coils. In both the generating and motoring modes, the armature carries current and a magnetic field is established, which is called the armature flux. The effect of armature flux on the main field is called the armature reaction.
The armature reaction:
1. demagnetizes the main field, and 2. cross magnetizes the main field.
The demagnetizing effect can be overcome by adding extra ampere-turns on the main field. The cross magnetizing effect can be reduced by having common poles.
Armature reaction drop
Armature reaction drop is the effect of a magnetic field on the distribution of the flux under main poles of a generator
Since an armature is wound with coils of wire, a magnetic field is set up in the armature whenever a current flows in the coils. This field is at right angles to the generator field, and is called cross magnetization of the armature. The effect of the armature field is to distort the generator field and shift the neutral plane. The neutral plane is the position where the armature windings are moving parallel to the magnetic flux lines. This effect is known as armature reaction and is proportional to the current flowing in the armature coils.
The brushes of a generator must be set in the neutral plane; that is, they must contact segments of the commutator that are connected to armature coils having no induced emf. If the brushes were contacting commutator segments outside the neutral plane, they would short-circuit "live" coils and cause arcing and loss of power.
Armature reaction causes the neutral plane to shift in the direction of rotation, and if the brushes are in the neutral plane at no load, that is, when no armature current is flowing, they will not be in the neutral plane when armature current is flowing. For this reason it is desirable to incorporate a corrective system into the generator design.
These are two principal methods by which the effect of armature reaction is overcome. The first method is to shift the position of the brushes so that they are in the neutral plane when the generator is producing its normal load current. in the other method, special field poles, called interpoles, are installed in the generator to counteract the effect of armature reaction.
The brush-setting method is satisfactory in installations in which the generator operates under a fairly constant load. If the load varies to a marked degree, the neutral plane will shift proportionately, and the brushes will not be in the correct position at all times. The brush-setting method is the most common means of correcting for armature reaction in small generators (those producing approximately 1000 W or less). Larger generators require the use of interpoles.
A balancing machine is a measuring tool used for balancing rotating machine parts such as rotors for
electric motors, fans, turbines, disc brakes, disc drives, propellers and pumps. The machine usually consists of two rigid pedestals, with suspension and bearings on top supporting a mounting platform. The unit under test is bolted to the platform and is rotated either with a belt-, air-, or end-drive. As the part is rotated, the vibration in the suspension is detected with sensors and that information is used to determine the amount of unbalance in the part. Along with phase information, the machine can determine how much and where to add weights to balance the part.
Hard-bearing vs. soft-bearing
There are two main types of balancing machines, hard-bearing and soft-bearing. The difference between them, however, is in the suspension and not the bearings.
In a hard-bearing machine, balancing is done at a frequency lower than the resonance frequency of the suspension. In a soft-bearing machine, balancing is done at a frequency higher than the resonance frequency of the suspension. Both types of machines have various advantages and disadvantages. A hard-
bearing machine is generally more versatile and can handle pieces with greatly varying weights, because hard-bearing machines are measuring centrifugal forces and require only a one-time calibration. Only five geometric dimensions need to be fed into the measuring unit and the machine is ready for use. Therefore, it works very well for low- and middle-size volume production and in repair workshops.
A soft-bearing machine is not so versatile with respect to amount of rotor weight to be balanced. The preparation of a soft-bearing machine for individual rotor types is more time consuming, because it needs to be calibrated for different part types. It is very suitable for high-production volume and high-precision balancing tasks.
Hard- and soft-bearing machines can be automated to remove weight automatically, such as by drilling or milling, but hard-bearing machines are more robust and reliable. Both machine principles can be integrated into a production line and loaded by a robot arm or gantry, requiring very little human control.
How it works
With the rotating part resting on the bearings, a vibration sensor is attached to the suspension. In most soft-bearing machines, a velocity sensor is used. This sensor works by moving a magnet in relation to a fixed coil that generates voltage proportional to the velocity of the vibration. Accelerometers, which measure acceleration of the vibration, can also be used.
A photocell (sometimes called a phaser), proximity sensor, or encoder is used to determine the rotational speed, as well as the relative phase of the rotating part. This phase information is then used to filter the vibration information to determine the amount of movement, or force, in one rotation of the part. Also, the time difference between the phase and the vibration peak gives the angle at which the unbalance exists. Amount of unbalance and angle of unbalance give an unbalance vector.
Calibration is performed by adding a known weight at a known angle. In a soft-bearing machine, trial weights must be added in correction planes for each part. This is because the location of the correction planes along the rotational axis is unknown, and therefore it is unknown how much a given amount of weight will affect the balance. By using trial weights, you are adding a known weight at a known angle and getting the unbalance vector caused by it. This vector is then compared to the original unbalance vector to
find the resultant vector, which gives the weight and angles needed to bring the part into balance. In a hard-bearing machine, the location of the correction plane must be given in advance so that the machine always knows how much a given amount of weight will affect the balance.
Other types of balancing machines
Static balancing machines differ from hard- and soft-bearing machines in that the part is not rotated to take a measurement. Rather than resting on its bearings, the part rests vertically on its geometric center. Once at rest, any movement by the part away from its geometric center is detected by two perpendicular sensors beneath the table and returned as unbalance. Static balancers are often used to balance parts with a diameter much larger than their length, such as fans. The advantages of using a static balancer are speed and price. However a static balancer can only correct in one plane, so its accuracy is limited.
A blade balancing machine attempts to balance a part in assembly, so minimal correction is required later on. Blade balancers are used on parts such as fans, propellers, and turbines. On a blade balancer, each blade to be assembled is weighed and its weight entered into a balancing software package. The software then sorts the blades and attempts to find the blade arrangement with the least amount of unbalance.
Portable balancing machines are used to balance parts that cannot be taken apart and put on a balancing machine, usually parts that are currently in operation such as turbines, pumps, and motors. Portable balancers come with displacement sensors, such as accelerometers, and a photocell, which are then mounted to the pedestals or enclosure of the running part. Based on the vibrations detected, they calculate
the parts unbalance. Many times these devices contain a spectrum analyzer so the part condition can be monitored without the use of a photocell and non-rotational vibration can be analyzed.
AC contactor for pump application.
A contactor is an electrically controlled switch used for switching a power circuit, similar to relay except with higher amperage ratings. A contactor is controlled by a circuit which has a much lower power level than the switched circuit. Contactors come in many forms with varying capacities and features. Unlike a circuit breaker, a contactor is not intended to interrupt a short circuit current.
Contactors range from those having a breaking current of several amps and 24 V DC to thousands of amps and many kilovolts. The physical size of contactors ranges from a device small enough to pick up with one hand, to large devices approximately a meter (yard) on a side.
Contactors are used to control electric motors, lighting, heating, capacitor banks, and other electrical loads.
Albright SPST DC contactor,
sometimes used in EV conversions
A contactor is composed of three different items. The contacts are the current carrying part of the contactor.
This includes power contacts, auxiliary contacts, and contact springs. The electromagnet provides the driving force to close the contacts. The enclosure is a frame housing the contact and the electromagnet. Enclosures are made of insulating materials like Bakelite, Nylon 6, and thermosetting plastics to protect and insulate the contacts and to provide some measure of protection against personnel touching the contacts. Open-frame contactors may have a further enclosure to protect against dust, oil, explosion hazards and weather.
High voltage contactors (greater than 1000 volts) may use vacuum or an inert gas around the contacts.
Magnetic blowouts use blowout coils to lengthen and move the electric arc. These are especially useful in DC power circuits. AC arcs have periods of low current, during which the arc can be extinguished with relative ease, but DC arcs have continuous high current, so blowing them out requires the arc to be stretched further than an AC arc of the same current. The magnetic blowouts in the pictured Albright contactor (which is designed for DC currents) more than double the current it can break, increasing it from 600 A to 1,500 A.
Sometimes an economizer circuit is also installed to reduce the power required to keep a contactor closed; an auxiliary contact reduces coil current after the contactor closes. A somewhat greater amount of power is required to initially close a contactor than is required to keep it closed. Such a circuit can save a substantial amount of power and allow the energized coil to stay cooler. Economizer circuits are nearly always applied on direct-current contactor coils and on large alternating current contactor coils.
A basic contactor will have a coil input (which may be driven by either an AC or DC supply depending on the contactor design). The coil may be energized at the same voltage as the motor, or may be separately controlled with a lower coil voltage better suited to control by programmable controllers and lower-voltage pilot devices. Certain contactors have series coils connected in the motor circuit; these are used, for
example, for automatic acceleration control, where the next stage of resistance is not cut out until the motor current has dropped.
Unlike general-purpose relays, contactors are designed to be directly connected to high-current load
devices. Relays tend to be of lower capacity and are usually designed for both normally closed and normally open applications. Devices switching more than 15 amperes or in circuits rated more than a few kilowatts are usually called contactors. Apart from optional auxiliary low current contacts, contactors are almost exclusively fitted with normally open contacts. Unlike relays, contactors are designed with features to control and suppress the arc produced when interrupting heavy motor currents.
When current passes through the electromagnet, a magnetic field is produced, which attracts the moving core of the contactor. The electromagnet coil draws more current initially, until its inductance increases when the metal core enters the coil. The moving contact is propelled by the moving core; the force developed by the electromagnet holds the moving and fixed contacts together. When the contactor coil is de-energized, gravity or a spring returns the electromagnet core to its initial position and opens the contacts.
For contactors energized with alternating current, a small part of the core is surrounded with a shading coil, which slightly delays the magnetic flux in the core. The effect is to average out the alternating pull of the magnetic field and so prevent the core from buzzing at twice line frequency.
Most motor control contactors at low voltages (600 volts and less) are air break contactors; i.e., ordinary air surrounds the contacts and extinguishes the arc when interrupting the circuit. Modern medium-voltage motor controllers use vacuum contactors.
Motor control contactors can be fitted with short-circuit protection (fuses or circuit breakers), disconnecting means, overload relays and an enclosure to make a combination starter.
Contactors are rated by designed load current per contact (pole), maximum fault withstand current, duty cycle, voltage, and coil voltage. A general purpose motor control contactor may be suitable for heavy starting duty on large motors; so-called "definite purpose" contactors are carefully adapted to such applications as air-conditioning compressor motor starting. North American and European ratings for contactors follow different philosophies, with North American general purpose machine tool contactors generally emphasizing simplicity of application while definite purpose and European rating philosophy emphasizes design for the intended life cycle of the application.
Contactors are rated by designed load current per contact pole, maximum fault withstand current, duty
cycle, voltage, and coil voltage. A general purpose motor control contactor may be suitable for heavy starting duty on large motors; so-called "definite purpose" contactors are carefully adapted to such applications as air-conditioning compressor motor starting. North American and European ratings for contactors follow different philosophies, with North American general purpose machine tool contactors generally emphasizing simplicity of application while definite purpose and European rating philosophy emphasizes design for the intended life cycle of the application.
Current rating of the contactor depends on utilization category. For example IEC Categories are described as:
AC1 - Non-inductive or slightly inductive rows
AC2 - Starting of slip-ring motors
AC3 - Starting of squirrel-cage motors and switching off only after the motor is up to speed. Make Locked Rotor Amps LRA, Break Full Load Amps FLA
AC4 - Starting of squirrel-cage motors with inching and plugging duty. Rapid Start/Stop.
Make and Break LRA
AC11 - Auxiliary control circuits
Contactors are often used to provide central control of large lighting installations, such as an office building
or retail building. To reduce power consumption in the contactor coils, latching contactors are used, which have two operating coils. One coil, momentarily energized, closes the power circuit contacts, which are then mechanically held closed; the second coil opens the contacts.
A magnetic starter is a contactor designed to provide power to electric motors. The magnetic starter has
an overload relay, which will open the control voltage to the starter coil if it detects an overload on a motor. Overload relays may rely on heat produced by the motor current to operate a bimetal contact or release a contact held closed by a low-melting-point alloy. The overload relay opens a set of contacts that are wired in series with the supply to the contactor feeding the motor. The characteristics of the heaters can be matched to the motor so that the motor is protected against overload. Recently, microprocessor-controlled motor digital protective relays relays offer more comprehensive protection of motors.
Every electric motor has to have some sort of controller. The motor controller will have differing features and complexity depending on the task that the motor will be performing.
The simplest case is a switch to connect a motor to a power source, such as in small appliances or power tools. The switch may be manually operated or may be a relay or contactor connected to some form of sensor to automatically start and stop the motor. The switch may have several positions to select different connections of the motor. This may allow reduced-voltage starting of the motor, reversing control or
selection of multiple speeds. Overload and overcurrent protection may be omitted in very small motor controllers, which rely on the supplying circuit to have overcurrent protection. Small motors may have built-in overload devices to automatically open the circuit on overload. Larger motors have a protective overload relay or temperature sensing relay included in the controller and fuses or circuit breakers for overcurrent protection. An automatic motor controller may also include limit switches or other devices to protect the driven machinery.
More complex motor controllers may be used to accurately control the speed and torque of the connected motor (or motors) and may be part of closed loop control systems for precise positioning of a driven machine. For example, a numerically controlled lathe will accurately position the cutting tool according to a preprogrammed profile and compensate for varying load conditions and perturbing forces to maintain tool position.
Types of motor controllers
Motor controllers can be manually, remotely or automatically operated. They may include only the means for starting and stopping the motor or they may include other functions
An electric motor controller can be classified by the type of motor it is to drive such as permanent magnet, servo, series, separately excited, and alternating current.
A motor controller is connected to a power source such as a battery pack or power supply, and control circuitry in the form of analog or digital input signals.
A small motor can be started by simply plugging it into an electrical receptacle or by using a switch or circuit
breaker. A larger motor requires a specialized switching unit called a motor starter or motor contactor. When energized, a direct on line (DOL) starter immediately connects the motor terminals directly to the power supply. A motor soft starter connects the motor to the power supply through a voltage reduction device and increases the applied voltage gradually or in steps
An adjustable-speed drive (ASD) or variable-speed drive (VSD) is an interconnected combination of
equipment that provides a means of driving and adjusting the operating speed of a mechanical load. An electrical adjustable-speed drive consists of an electric motor and a speed controller or power converter plus auxiliary devices and equipment. In common usage, the term ―drive‖ is often applied to just the controller
Motor control centers
A small, early 1960's-vintage motor control center for 480 volt motors.
A motor control center (MCC) is an assembly of one or more enclosed sections having a common power bus and principally containing motor control units. Motor control centers are in modern practice a factory assembly of several motor starters. A motor control center can include variable frequency drives, programmable controllers, and metering and may also be the electrical service entrance for the building. Motor control centers are usually used for low voltage three-phase alternating current motors from 230 volts to 600 volts. Medium-voltage motor control centers are made for large motors running at 2300 V to around
15000 V, using vacuum contactors for switching and with separate compartments for power switching and control.
Motor control centers have been used since 1950 by the automobile manufacturing industry which used large numbers of electric motors. Today they are used in many industrial and commercial applications.
Where very dusty or corrosive processes are used, the motor control center may be installed in a separate air-conditioned room, but often an MCC will be on the factory floor adjacent to the machinery controlled.
A motor control center consists of one or more vertical metal cabinet sections with power bus and provision for plug-in mounting of individual motor controllers. Very large controllers may be bolted in place but
smaller controllers can be unplugged from the cabinet for testing or maintenance. Each motor controller contains a contactor or a solid-state motor controller, overload relays to protect the motor, fuses or a circuit breaker to provide short-circuit protection, and a disconnecting switch to isolate the motor circuit. Three-phase power enters each controller through separable connectors. The motor is wired to terminals in the controller. Motor control centers provide wire ways for field control and power cables.
Each motor controller in an MCC can be specified with a range of options such as separate control transformers, pilot lamps, control switches, extra control terminal blocks, various types of bi-metal and
solid-state overload protection relays, or various classes of power fuses or types of circuit breakers. A motor control center can either be supplied ready for the customer to connect all field wiring, or can be an engineered assembly with internal control and interlocking wiring to a central control terminal panel board or programmable controller.
Motor control centers (MCC) usually sit on floors, which are often required to have a fire-resistance rating. Firestops may be required for cables that penetrate fire-rated floors and walls.
Speed controls for AC induction motors
Recent developments in drive electronics have allowed efficient and convenient speed control of these
motors, where this has not traditionally been the case. The newest advancements allow for torque generation down to zero speed. This allows the polyphase AC induction motor to compete in areas where DC motors have long dominated, and presents an advantage in robustness of design, cost, and reduced maintenance.
Variable frequency drives
Phase vector drives
Phase vector drives (or simply vector drives) are an improvement over variable frequency drives (VFDs) in that they separate the calculations of magnetizing current and torque generating current. These quantities are represented by phase vectors, and are combined to produce the driving phase vector which in
turn is decomposed into the driving components of the output stage. These calculations need a fast microprocessor, typically a DSP device.
Unlike a VFD, a vector drive is a closed loop system. It takes feedback on rotor position and phase currents. Rotor position can be obtained through an encoder, but is often sensed by the reverse EMF generated on the motor leads.
In some configurations, a vector drive may be able to generate full rated motor torque at zero speed.
Direct torque control drives
Direct torque control has better torque control dynamics than the PI-current controller based vector control.
Thus it suits better to servo control applications. However, it has some advantage over other control methods in other applications as well because due to the faster control it has better capabilities to damp mechanical resonances and thus extend the life of the mechanical system.
Brushed DC motor speed or torque controls
An industrial grade first quadrant PWM DC-motor controller
These controls are applicable to brushed DC motors with either a wound or permanent magnet stator. A valuable characteristic of these motors is that they are easily controlled in torque, the torque being fairly directly proportional to the driving current. Speed control is derived by simply modulating the motor torque.
SCR or thyristor drive
SCR controls for DC motors convert AC power to direct current, with adjustable voltage. Small DC drives are
common in industry, running from line voltages, with motors rated at 90V for 120V line, and 180V for a 240V line. Larger drives, up to thousands of horsepower, are powered by three phase supplies and are used in such applications as rolling mills, paper machines, excavators, and ship propulsion. DC drivers are available in reversing and non-reversing models. The waveform of the current through the motor by a single-phase drive will have strong ripple components due to the switching at line frequency. This can be
reduced by use of a poly phase supply or smoothing inductors in the motor circuit; otherwise the ripple currents produce motor heating, excess noise, and loss of motor torque.
PWM or chopper drives
PWM controls use pulse width modulation to regulate the current sent to the motor. Unlike SCR controls
which switch at line frequency, PWM controls produce smoother current at higher switching frequencies, typically between 1 and 20 kHz. At 20 kHz, the switching frequency is inaudible to humans, thereby eliminating the hum which switching at lower frequency produces. However, some motor controllers for radio controlled models make use of the motor to produce audible sound, most commonly simple beeps.
A PWM controller typically contains a large reservoir capacitor and an H-bridge arrangement of switching elements (thyristors, Mosfets, solid state relays, or transistors).
Servo controllers is a wide category of motor control. Common features are:
precise closed loop position control fast acceleration rates precise speed control
Servo motors may be made from several motor types, the most common being
brushed DC motor brushless DC motors AC servo motors
Servo controllers use position feedback to close the control loop. This is commonly implemented with encoders, resolvers, and Hall effect sensors to directly measure the rotor's position.
A servo may be controlled using pulse-width modulation (PWM). How long the pulse remains high (typically between 1 and 2 milliseconds) determines where the motor will try to position itself. Another control method is pulse and direction.
Other position feedback methods measure the back EMF in the undriven coils to infer the rotor position, or detect the Kick-Back voltage transient (spike) that is generated whenever the power to a coil is instantaneously switched off. These are therefore often called "sensorless" control methods.
Sensorless control methods
Ripple counting works on the 'law of induction', or more specifically Lenz's law, which says that the magnetic
field of any induced current opposes the charge that induces it. This so-called back EMF (sometimes called the counter electromotive force) can be detected by measuring the current flowing through each coil as the motor rotates.
In a fully encapsulated motor and particularly a multi pole motor this is difficult to measure. Therefore ripple counting usually relies on measuring the voltage variations over a small resistor inserted in one of the power supply wires to the motor. The result is a voltage curve representing the accumulated currents running through the coils of the motor assembly as the motor rotates.
The current ripple waveform characteristics are highly dependent of a number of factors such as the supply voltage and the actual load, speed, direction and temperature of the motor. Other factors such as the in-rush current, aging of motor parts and electromagnetic interference can also influence the ripple waveform. The amplitude and the shape of the waveform can vary significantly due to these factors. In other applications noise transients superposed onto the ripple current waveform can generate false counting pulses.
It has proven difficult to design a detection system based on ripple counting that can be used to precisely and reliably count the number of commutations of a rotating motor from start to stop. Numerous attempts to seek to improve the reliability of ripple counting have been described in the literature, examples in  .
Transient counting works on the basic principle of Ohm's law and the behavior of a collapsing magnetic field
in which a Back-Fire or Kick-Back transient (spike) is generated whenever the power to a coil is instantaneously switched off
At each commutation point, when the brush breaks contact with a commutation segment, the energy stored in the motor winding as a magnetic field causes an arc or voltage spike between the brush and the commutator segment. This occurs not only during normal commutation but also in situations where the brushes bounce on the rotating commutator.
A dedicated transient detector circuit (in effect a high pass filter) detects the Kick-Back spike from the collapsing magnetic field in the coil when the power to the coil is turned off. The Kick-Back transients trigger the modulation of an electronic encoder signal for each of the motor commutations. Thus an N pole motor will encode N signals per rotation. The Kick-Back spikes can be measured anywhere on the power supply wires to the motor. The encoded signal can be used as position feedback in the servo controller.
The performance of transient counting is by and large unaffected by the parameters which are influencing ripple counting. Whether the motor is powered or is coasting in generator mode driven by the inertia of a load has no influence on the counting reliability. The amplitude of the Kick-Back transients is mainly influenced by the conductivity of the surrounding air. This is because the intensity of the arc generated between a brush and a commutator depends on the air conductivity.
Stepper motor controllers
A stepper, or stepping, motor is a synchronous, brushless, high pole count, polyphase motor. Control is usually, but not exclusively, done open loop, i.e. the rotor position is assumed to follow a controlled rotating field. Because of this, precise positioning with steppers is simpler and cheaper than closed loop controls.
Modern stepper controllers drive the motor with much higher voltages than the motor nameplate rated voltage, and limit current through chopping. The usual setup is to have a positioning controller, known as an indexer, sending step and direction pulses to a separate higher voltage drive circuit which is responsible for commutation and current limiting.
Relevant circuits to motor control
DC motors are typically controlled by using a transistor configuration called an "H-bridge". This consists of a minimum of four mechanical or solid-state switches, such as two NPN and two PNP transistors. One NPN and one PNP transistor are activated at a time. Both NPN or PNP transistors can be activated to cause a short across the motor terminals, which can be useful for slowing down the motor from the back EMF it creates.
Small variable frequency drive
A variable-frequency drive (VFD) is a system for controlling the rotational speed of an alternating current
(AC) electric motor by controlling the frequency of the electrical power supplied to the motor. A variable frequency drive is a specific type of adjustable-speed drive. Variable-frequency drives are also known as adjustable-frequency drives (AFD), variable-speed drives (VSD), AC drives, microdrives or inverter drives. Since the voltage is varied along with frequency, these are sometimes also called VVVF (variable voltage variable frequency) drives.
Variable-frequency drives are widely used. In ventilation systems for large buildings, variable-frequency motors on fans save energy by allowing the volume of air moved to match the system demand. They are also used on pumps, elevator, conveyor and machine tool drives.
All VFDs use their output devices (IGBTs, transistors, thyristors) only as switches, turning them only on or off. Using a linear device such as a transistor in its linear mode is impractical for a VFD drive, since the power dissipated in the drive devices would be about as much as the power delivered to the load.
Drives can be classified as:
Constant voltage Constant current Cycloconverter
In a constant voltage converter, the intermediate DC link voltage remains approximately constant during each output cycle. In constant current drives, a large inductor is placed between the input rectifier and the output bridge, so the current delivered is nearly constant. A cycloconverter has no input rectifier or DC link and instead connects each output terminal to the appropriate input phase.
The most common type of packaged VF drive is the constant-voltage type, using pulse width modulation to control both the frequency and effective voltage applied to the motor load.
VFD system description
A variable frequency drive system generally consists of an AC motor, a controller and an operator interface.
The motor used in a VFD system is usually a three-phase induction motor. Some types of single-phase motors can be used, but three-phase motors are usually preferred. Various types of synchronous motors
offer advantages in some situations, but induction motors are suitable for most purposes and are generally the most economical choice. Motors that are designed for fixed-speed operation are often used. Certain enhancements to the standard motor designs offer higher reliability and better VFD performance, such as MG-31 rated motors.
Variable frequency drive controllers are solid state electronic power conversion devices. The usual design first converts AC input power to DC intermediate power using a rectifier or converter bridge. The rectifier is usually a three-phase, full-wave-diode bridge. The DC intermediate power is then converted to quasi-
sinusoidal AC power using an inverter switching circuit. The inverter circuit is probably the most important section of the VFD, changing DC energy into three channels of AC energy that can be used by an AC motor. These units provide improved power factor, less harmonic distortion, and low sensitivity to the incoming
phase sequencing than older phase controlled converter VFD's. Since incoming power is converted to DC, many units will accept single-phase as well as three-phase input power (acting as a phase converter as well as a speed controller); however the unit must be derated when using single phase input as only part of the rectifier bridge is carrying the connected load.
As new types of semiconductor switches have been introduced, these have promptly been applied to inverter circuits at all voltage and current ratings for which suitable devices are available. Introduced in the 1980s, the insulated-gate bipolar transistor (IGBT) became the device used in most VFD inverter circuits in the first decade of the 21st century.
AC motor characteristics require the applied voltage to be proportionally adjusted whenever the frequency is changed in order to deliver the rated torque. For example, if a motor is designed to operate at 460 volts at 60 Hz, the applied voltage must be reduced to 230 volts when the frequency is reduced to 30 Hz. Thus the ratio of volts per hertz must be regulated to a constant value (460/60 = 7.67 V/Hz in this case). For optimum performance, some further voltage adjustment may be necessary especially at low speeds, but constant volts per hertz is the general rule. This ratio can be changed in order to change the torque delivered by the motor.
In addition to this simple volts per hertz control more advanced control methods such as vector control and direct torque control (DTC) exist. These methods adjust the motor voltage in such a way that the magnetic flux and mechanical torque of the motor can be precisely controlled.
The usual method used to achieve variable motor voltage is pulse-width modulation (PWM). With PWM voltage control, the inverter switches are used to construct a quasi-sinusoidal output waveform by a series of narrow voltage pulses with pseudosinusoidal varying pulse durations.
Operation of the motors above rated name plate speed (base speed) is possible, but is limited to conditions that do not require more power than nameplate rating of the motor. This is sometimes called "field weakening" and, for AC motors, means operating at less than rated volts/hertz and above rated name plate speed. Permanent magnet synchronous motors have quite limited field weakening speed range due to the constant magnet flux linkage. Wound rotor synchronous motors and induction motors have much wider
speed range. For example, a 100 hp, 460 V, 60 Hz, 1775 RPM (4 pole) induction motor supplied with 460 V, 75 Hz (6.134 V/Hz), would be limited to 60/75 = 80% torque at 125% speed (2218.75 RPM) = 100% power. At higher speeds the induction motor torque has to be limited further due to the lowering of the breakaway torque of the motor. Thus rated power can be typically produced only up to 130...150 % of the rated name plate speed. Wound rotor synchronous motors can be run even higher speeds. In rolling mill drives often 200...300 % of the base speed is used. Naturally the mechanical strength of the rotor and lifetime of the bearings is also limiting the maximum speed of the motor. It is recommended to consult the motor manufacturer if more than 150 % speed is required by the application.
PWM VFD Output Voltage Waveform
An embedded microprocessor governs the overall operation of the VFD controller. The main microprocessor
programming is in firmware that is inaccessible to the VFD user. However, some degree of configuration programming and parameter adjustment is usually provided so that the user can customize the VFD controller to suit specific motor and driven equipment requirements.
VFD operator interface
The operator interface provides a means for an operator to start and stop the motor and adjust the
operating speed. Additional operator control functions might include reversing and switching between manual speed adjustment and automatic control from an external process control signal. The operator interface often includes an alphanumeric display and/or indication lights and meters to provide information about the operation of the drive. An operator interface keypad and display unit is often provided on the front of the VFD controller as shown in the photograph above. The keypad display can often be cable-connected and mounted a short distance from the VFD controller. Most are also provided with input and output (I/O) terminals for connecting pushbuttons, switches and other operator interface devices or control signals. A serial communications port is also often available to allow the VFD to be configured, adjusted, monitored and controlled using a computer.
When an induction motor is connected to a full voltage supply, it draws several times (up to about 6 times) its rated current. As the load accelerates, the available torque usually drops a little and then rises to a peak while the current remains very high until the motor approaches full speed.
By contrast, when a VFD starts a motor, it initially applies a low frequency and voltage to the motor. The starting frequency is typically 2 Hz or less. Thus starting at such a low frequency avoids the high inrush current that occurs when a motor is started by simply applying the utility (mains) voltage by turning on a switch. After the start of the VFD, the applied frequency and voltage are increased at a controlled rate or ramped up to accelerate the load without drawing excessive current. This starting method typically allows a motor to develop 150% of its rated torque while the VFD is drawing less than 50% of its rated current from the mains in the low speed range. A VFD can be adjusted to produce a steady 150% starting torque from standstill right up to full speed. Note, however, that cooling of the motor is usually not good in the low speed range. Thus running at low speeds even with rated torque for long periods is not possible due to
overheating of the motor. If continuous operation with high torque is required in low speeds an external fan is usually needed. The manufacturer of the motor and/or the VFD should specify the cooling requirements for this mode of operation.
In principle, the current on the motor side is in direct proportion of the torque that is generated and the
voltage on the motor is in direct proportion of the actual speed, while on the network side, the voltage is constant, thus the current on line side is in direct proportion of the power drawn by the motor, that is U.I or C.N where C is torque and N the speed of the motor (we shall consider losses as well, neglected in this explanation).
(1) n stands for network (grid) and m for motor (2) C stands for torque [Nm], U for voltage [V], I for current [A], and N for speed [rad/s] We neglect losses for the moment : Un.In = Um.Im (same power drawn from network and from motor) Um.Im = Cm.Nm (motor mechanical power = motor electrical power) Given Un is a constant (network voltage) we conclude : In = Cm.Nm/Un That is "line current (network) is in direct proportion of motor power".
With a VFD, the stopping sequence is just the opposite as the starting sequence. The frequency and voltage applied to the motor are ramped down at a controlled rate. When the frequency approaches zero, the motor is shut off. A small amount of braking torque is available to help decelerate the load a little faster than it would stop if the motor were simply switched off and allowed to coast. Additional braking torque can be obtained by adding a braking circuit (resistor controlled by a transistor) to dissipate the braking energy. With 4-quadrants recifiers (active-front-end), the VFD is able to brake the load by applying a reverse torque and reverting the energy back to the network.
Power line harmonics
While PWM allows for nearly sinusoidal currents to be applied to a motor load, the diode rectifier of the VFD takes roughly square-wave current pulses out of the AC grid, creating harmonic distortion in the power line voltage. When the VFD load size is small and the available utility power is large, the effects of VFD systems slicing small chunks out of AC grid generally go unnoticed. Further, in low voltage networks the harmonics caused by single phase equipment such as computers and TVs are such that they are partially cancelled by three-phase diode bridge harmonics.
However, when either a large number of low-amperage VFDs, or just a few very large-load VFDs are used, they can have a cumulative negative impact on the AC voltages available to other utility customers in the same grid.
When the utility voltage becomes misshapen and distorted the losses in other loads such as normal AC motors are increased. This may in the worst case lead to overheating and shorter operation life. Also substation transformers and compensation capacitors are affected, the latter especially if resonances are aroused by the harmonics.
In order to limit the voltage distortion the owner of the VFDs may be required to install filtering equipment to smooth out the irregular waveform. Alternately, the utility may choose to install filtering equipment of its own at substations affected by the large amount of VFD equipment being used. In high power installations decrease of the harmonics can be obtained by supplying the VSDs from transformers that have different phase shift.
Further, it is possible to use instead of the diode rectifier a similar transistor circuit that is used to control the motor. This kind of rectifier is called active infeed converter in IEC standards. However, manufacturers call it by several names such as active rectifier, ISU (IGBT Supply Unit), AFE (Active Front End) or four quadrant rectifier. With PWM control of the transistors and filter inductors in the supply lines the AC current
can be made nearly sinusoidal. Even better attenuation of the harmonics can be obtained by using an LCL (inductor-capacitor-inductor) filter instead of single three-phase filter inductor.
Additional advantage of the active infeed converter over the diode bridge is its ability to feed back the energy from the DC side to the AC grid. Thus no braking resistor is needed and the efficiency of the drive is improved if the drive is frequently required to brake the motor.
The output voltage of a PWM VFD consists of a train of pulses switched at the carrier frequency. Because of the rapid rise time of these pulses, transmission line effects of the cable between the drive and motor must be considered. Since the transmission-line impedance of the cable and motor are different, pulses tend to reflect back from the motor terminals into the cable. The resulting voltages can produce up to twice the rated line voltage for long cable runs, putting high stress on the cable and motor winding and eventual insulation failure. Increasing the cable or motor size/type for long runs and 480v or 600v motors will help offset the stresses imposed upon the equipment due to the VFD (modern 230v single phase motors not
effected). At 460 V, the maximum recommended cable distances between VFDs and motors can vary by a factor of 2.5:1. The longer cables distances are allowed at the lower Carrier Switching Frequencies (CSF) of 2.5 kHz. The lower CSF can produce audible noise at the motors. For applications requiring long motor cables VSD manufacturers usually offer du/dt filters that decrease the steepness of the pulses. For very long cables or old motors with insufficient winding insulation more efficient sinus filter is recommended. Expect the older motor's life to shorten. Purchase VFD rated motors for the application.
Further, the rapid rise time of the pulses may cause trouble with the motor bearings. The stray capacitance of the windings provide paths for high frequency currents that close through the bearings. If the voltage between the shaft and the shield of the motor exceeds few volts the stored charge is discharged as a small spark. Repeated sparking causes erosion in the bearing surface that can be seen as fluting pattern. In order to prevent sparking the motor cable should provide a low impedance return path from the motor frame back to the inverter. Thus it is essential to use a cable designed to be used with VSDs.
In big motors a slip ring with brush can be used to provide a bypass path for the bearing currents. Alternatively isolated bearings can be used.
The 2.5 kHz and 5 kHz CSFs cause fewer motor bearing problems than the 20 kHz CSFs. Shorter cables are recommended at the higher CSF of 20 kHz. The minimum CSF for synchronize tracking of multiple conveyors is 8 kHz.
The high frequency current ripple in the motor cables may also cause interference with other cabling in the building. This is another reason to use a motor cable designed for VSDs that has a symmetrical three-phase structure and good shielding. Further, it is highly recommended to route the motor cables as far away from signal cables as possible.
Available VFD power ratings
Variable frequency drives are available with voltage and current ratings to match the majority of 3-phase motors that are manufactured for operation from utility (mains) power. VFD controllers designed to operate at 111 V to 690 V are often classified as low voltage units. Low voltage units are typically designed for use with motors rated to deliver 0.2 kW or 1/4 horsepower (hp) up to several megawatts. For example, the largest ABB ACS800 single drives are rated for 5.6 MW . Medium voltage VFD controllers are designed to operate at 2,400/4,162 V (60 Hz), 3,000 V (50 Hz) or up to 10 kV. In some applications a step up transformer is placed between a low voltage drive and a medium voltage load. Medium voltage units are typically designed for use with motors rated to deliver 375 kW or 500 hp and above. Medium voltage drives rated above 7 kV and 5,000 or 10,000 hp should probably be considered to be one-of-a-kind (one-off) designs.
Medium voltage drives are generally rated amongst the following voltages : 2.3 kV - 3,3 kV - 4 kV - 6 kV - 11 kV The in-between voltages are generally possible as well. The power of MV drives is generally in the range of 0,3 to 100 MW however involving a range a several different type of drives with different technologies.
Line regenerative variable frequency drives, showing capacitors (top cylinders)and inductors attached
which filter the regenerated power.
Using the motor as a generator to absorb energy from the system is called dynamic braking. Dynamic braking stops the system more quickly than coasting. Since dynamic braking requires relative motion of the
motor's parts, it becomes less effective at low speed and cannot be used to hold a load at a stopped position. During braking of an electric motor the electrical energy produced by the motors is dissipated as heat, inside the rotor of a motor (which is potentially hazardous to life of a electric motor ) so this energy is usually transferred outside to a bank of onboard resistors. Cooling fans may be used to protect the resistors from damage. Modern systems have thermal monitoring, so if the temperature of the bank becomes excessive, it will be switched off.
Regenerative variable-frequency drives
Regenerative AC drives have the capacity to recover the braking energy of an overhauling load and return it to the power system.
sample of how a regenerative drive would look
Cycloconverters and current-source inverters inherently allow return of energy from the load to the line; voltage-source inverters require an additional converter to return energy to the supply.
Regeneration is only useful in varible-frequency drives where the value of the recovered energy is large compared to the extra cost of a regenerative system, and if the system requires frequent braking and starting. An example would be use in conveyor belt during manufacturing where it should stop for every few minutes, so that the parts can be assembled correctly and moves on. Another example is a crane, where the hoist motor stops and reverses frequently, and braking is required to slow the load during lowering. Regenerative variable-frequency drives are widely used where speed control of overhauling loads is required.
Brushless DC motor drives
Much of the same logic contained in large, powerful VFDs is also embedded in small brushless DC motors such as those commonly used in computer fans. In this case, the chopper usually converts a low DC voltage (such as 12 volts) to the three-phase current used to drive the electromagnets that turn the permanent magnet rotor.
Vector control (motor)
Vector control (also called Field Oriented Control, FOC) is one method used in variable frequency drives to
control the torque (and thus finally the speed) of three-phase AC electric motors by controlling the current fed to the machine.
The stator phase currents are measured and converted into a corresponding complex (space) vector. This
current vector is then transformed to a coordinate system rotating with the rotor of the machine. For this the rotor position has to be known. Thus at least speed measurement is required, the position can then be obtained by integrating the speed.
Then the rotor flux linkage vector is estimated by multiplying the stator current vector with magnetizing inductance Lm and low-pass filtering the result with the rotor no-load time constant Lr/Rr, that is the ratio of the rotor inductance to rotor resistance.
Using this rotor flux linkage vector the stator current vector is further transformed into a coordinate system where the real x-axis is aligned with the rotor flux linkage vector.
Now the real x-axis component of the stator current vector in this rotor flux oriented coordinate system can be used to control the rotor flux linkage and the imaginary y-axis component can be used to control the motor torque.
Typically PI-controllers are used to control these currents to their reference values. However, bang-bang type current control, that gives better dynamics, is also possible.
With PI-controllers the outputs of the controllers are the x-y components of the voltage reference vector for the stator. Usually due to the cross coupling between the x- and y-axes a decoupling term is further added to the controller output to improve control performance when big and rapid changes in speed, current and flux linkage occur. Usually the PI-controller also needs low-pass filtering of either the input or output of the controller to prevent the current ripple due to transistor switching from being amplified excessively and
unstabilizing the control. Unfortunately, the filtering also limits the dynamics of the control system. Thus quite high switching frequency (typically more than 10 kHz) is required to allow only minimum filtering for high performance drives such as servo drives.
Next the voltage references are first transformed to the stationary coordinate system (usually through rotor
d-q coordinates) and then fed into a modulator that using one of the many Pulse Width Modulation (PWM) algorithms defines the required pulse widths of the stator phase voltages and controls the transistors (usually IGBTs) of the inverter according to these.
This control method implies the following properties of the control:
Speed or position measurement or some sort of estimation is needed Torque and flux can be changed reasonably fast, in less than 5-10 milliseconds, by changing the
references The step response has some overshoot if PI control is used The switching frequency of the transistors is usually constant and set by the modulator The accuracy of the torque depends on the accuracy of the motor parameters used in the
control. Thus large errors due to for example rotor temperature changes often are encountered. Reasonable processor performance is required, typically the control algorithm has to be
calculated at least every millisecond.
Although the vector control algorithm is more complicated than the Direct Torque Control (DTC), the algorithm is not needed to be calculated as frequently as the DTC algorithm. Also the current sensors need not be the best in the market. Thus the cost of the processor and other control hardware is lower making it suitable for applications where the ultimate performance of DTC is not required.
Vector control was patented by Felix Blaschke in U.S. Patent 3,824,437 filed originally on August 14, 1969 in Germany while he worked for Siemens.
Another important contemporary publication about the same topic was
Karl Hasse: Zur Dynamik drehzahlgeregelter Antriebe mit stromrichtergespeisten Asynchron-Kurzschlußläufermotoren. Dissertation, TH Darmstadt, 1969.
In the Blaschke's patent the rotor flux linkage was calculated from the measured air-gap magnetic field. Thus this method is called direct rotor oriented vector control. However, to use standard induction machines, the method to estimate the rotor flux linkage from the measured stator currents, as proposed by Hasse, is more practical. Versions based on flux estimation instead of measuring are called indirect rotor oriented vector controls. An early review of the possible alternatives was published in the paper:
Blaschke, F., Böhm, K.: Verfahren der Felderfassung bei der Regelung stromrichtergespeister Asynchronmaschinen. IFAC Symposium: Control in Power Electronics and Electrical Drives, Düsseldorf, October 7 – 9, 1974, Proceedings Vol I, pp. 635...649.
Vector control has later been dealt with in numerous publications. Several methods have been developed to make possible the operation without speed or position sensor. Also methods to estimate the rotor time constant and other parameters have been presented. One good book dealing with these issues is:
Peter Vas: Sensorless Vector and Direct Torque Control, Oxford University Press, 1998, ISBN 0-19-856465-1
In addition to induction machines, the vector control has also been applied to synchronous machines and doubly fed machines.
After the major Siemens' patents expired in the end of 80's and beginning of 90's, many other manufacturers begin to use this method in their products making this the de facto standard in demanding motor control applications the only alternative being the Direct Torque Control (DTC) developed by ABB.
Direct torque control
Direct torque control (DTC) is one method used in variable frequency drives to control the torque (and
thus finally the speed) of three-phase AC electric motors. This involves calculating an estimate of the motor's magnetic flux and torque based on the measured voltage and current of the motor.
Stator flux linkage is estimated by integrating the stator voltages. Torque is estimated as a cross product of estimated stator flux linkage vector and measured motor current vector. The estimated flux magnitude and torque are then compared with their reference values. If either the estimated flux or torque deviates from the reference more than allowed tolerance, the transistors of the variable frequency drive are turned off and on in such a way that the flux and torque will return in their tolerance bands as fast as possible. Thus direct torque control is one form of the hysteresis or bang-bang control.
This control method implies the following properties of the control:
Torque and flux can be changed very fast by changing the references High efficiency & low losses - switching losses are minimized because the transistors are
switched only when it is needed to keep torque and flux within their hysteresis bands The step response has no overshoot No coordinate transforms are needed, all calculations are done in stationary coordinate system
No separate modulator is needed, the hysteresis control defines the switch control signals directly
There are no PI current controllers. Thus no tuning of the control is required The switching frequency of the transistors is not constant. However, by controlling the width of
the tolerance bands the average switching frequency can be kept roughly at its reference value. This also keeps the current and torque ripple small. Thus the torque and current ripple are of the same magnitude than with vector controlled drives with the same switching frequency.
Due to the hysteresis control the switching process is random by nature. Thus there are no peaks in the current spectrum. This further means that the audible noise of the machine is low
The intermediate DC circuit's voltage variation is automatically taken into account in the algorithm (in voltage integration). Thus no problems exist due to dc voltage ripple (aliasing) or dc voltage transients
Synchronization to rotating machine is straightforward due to the fast control; Just make the torque reference zero and start the inverter. The flux will be identified by the first current pulse
Digital control equipment has to be very fast in order to be able to prevent the flux and torque from deviating far from the tolerance bands. Typically the control algorithm has to be performed with 10 - 30 microseconds or shorter intervals. However, the amount of calculations required is small due to the simplicity of the algorithm
The current and voltage measuring devices have to be high quality ones without noise and low-pass filtering, because noise and slow response ruins the hysteresis control
In higher speeds the method is not sensitive to any motor parameters. However, at low speeds the error in stator resistance used in stator flux estimation becomes critical
The direct torque method performs very well even without speed sensors. However, the flux estimation is
usually based on the integration of the motor phase voltages. Due to the inevitable errors in the voltage measurement and stator resistance estimate the integrals tend to become erroneous at low speed. Thus it is not possible to control the motor if the output frequency of the variable frequency drive is zero. However, by careful design of the control system it is possible to have the minimum frequency in the range 0.5 Hz to 1 Hz that is enough to make possible to start an induction motor with full torque from a standstill situation. A reversal of the rotation direction is possible too if the speed is passing through the zero range rapidly enough to prevent excessive flux estimate deviation.
If continuous operation at low speeds including zero frequency operation is required, a speed or position sensor can be added to the DTC system. With the sensor, high accuracy of the torque and speed control can be maintained in the whole speed range.
Direct torque control was patented by Manfred Depenbrock in U.S. Patent 4,678,248 filed originally on October 20, 1984 in Germany. He called it "Direct Self-Control" (DSC). However, Isao Takahashi and Toshihiko Noguchi presented a similar idea only few months later in a Japanese journal. Thus direct torque control is usually credited to all three gentlemen.
The only difference between DTC and DSC is the shape of the path along which the flux vector is controlled to follow. In DTC the path is a circle and in DSC it was a hexagon. Today DTC uses hexagon flux path only when full voltage is required at high speeds.
Since Depenbrock, Takahashi and Noguchi had proposed direct torque control (DTC) for induction machines in the mid 1980s, this new torque control scheme has gained much momentum. From its introduction, the Direct Torque control or Direct Self Control (DSC) principle has been used for Induction Motor (IM) drives with fast dynamics. Despite its simplicity, DTC is able to produce very fast torque and flux control, if the torque and flux are correctly estimated.
Among the others, DTC/DSC was further studied in Ruhr-University in Bochum, Germany at the end of 80's. A very good treatment of the subject can be found from the doctoral thesis:
Uve Baader: Die Direkte-Selbstregelung (DSR), Ein Verfahren zur hochdynamischen Regelung von Drehfeldmaschinen. Fortschr.-Ber. VDI Reihe 21, Nr. 35. VDI-Verlag 1988. ISBN 3-18-143521-X
The first commercial application was in traction. At the end of 80's DSC was tried in German diesel-electric locomotives DE502  and DE1003  by ABB, please see paper:
Jänecke, M., Kremer, R., Steuerwald, G.: Direct Self-Control (DSC), A Novel Method Of Controlling Asynchronous Machines In Traction Applications. Proceedings of EPE 1989, October 9–12, 1989 Aachen, Germany, Vol. 1, pp. 75–81.
The first major commercial application was, however, the ACS600 variable speed drive by ABB that saw the daylight in 1995. ACS600 has later been replaced with ACS800 . A good presentation of ACS600 and a full theoretical treatment of direct torque control can be found in the book:
Peter Vas: Sensorless Vector and Direct Torque Control, Oxford University Press, 1998, ISBN 0-19-856465-1
Also dealing with DTC and ACS600 are:
Tiitinen, P., Pohjalainen, P., Lalu, J.: The Next Generation Motor Control Method: Direct Torque Control (DTC). EPE Journal, Vol. 5., no 1, March 1995, pp. 14–18 .
Nash, J.: Direct Torque Control, Induction Motor Vector Control Without an Encoder. IEEE Tr. on Industry Applications, Vol. 33, No. 2, March/April 1997 .
DTC has also been applied to three-phase grid side converter control (U.S. Patent 5,940,286). Grid side
converter is identical in structure to the transistor inverter controlling the machine. Thus it can in addition to rectifying AC to DC also feed back energy from the DC to the AC grid. Further, the waveform of the phase currents is very sinusoidal and power factor can be adjusted as desired. In the grid side converter DTC version the grid is considered to be a big electric machine (which, actually, there are many in the grid!). A paper dealing with grid side converter DTC was presented already in 1995:
Manninen, V.: Application of Direct Torque Control Modulation to a Line Converter. Proc. of EPE 1995, Sept. 19-21, 1995, Sevilla, Spain, Proceedings pp. 1.292-1.296.
In the late 1990s DTC techniques for the Interior Permanent Magnet Synchronous Machine (IPMSM) appeared.
Further, in the beginning of 2000's DTC was applied to doubly fed machine control (U.S. Patent 6,448,735). Doubly fed generators are today commonly used in wind turbine applications.
Thinking of the outstanding torque control dynamics of the DTC it was somewhat surprising that the first servo drive using DTC, ABB's ACSM1, was introduced quite late, in 2007.
During 2000's several papers have been published about DTC. Also several modifications such as space vector modulated DTC that has constant switching frequency, has been presented.
Due to the expiring of the DTC patent by Depenbrock in 2004 it is to be expected that other companies than ABB will also start to sell drives using DTC.
Torque and speed of a DC motor
The torque of an electric motor is not necessarily dependent on its speed. It is rather a function of flux and armature current.
Increase in flux decreases the speed but increases the torque. If torque is decreased by decreasing the field current, the following sequences are found:
1. Back EMF drops instantly, the speed remaining constant because of the inertia of heavy armature.
2. Due to decrease of EMF armature current I is increased because of I = (V − E)/R. 3. A small decrease of flux is more than counterbalanced by a large increase of I which means net
increase of torque. 4. If torque increases the speed also increases.
If applied voltage is kept constant, motor speed has inverse relation with flux.
N = revolutions per minute (RPM) ,i.e. motor speed K = proportional constant R = resistance of armature (ohms) V = electromotive force (volts) I = current (amperes) Φ = flux (webers)
Characteristics of DC motors
DC motors respond to load changes in different ways, depending on the arrangement of the windings.
Shunt wound motor
A shunt wound motor has a high-resistance field winding connected in parallel with the armature. It responds to increased load by trying to maintain its speed and this leads to an increase in armature current. This makes it unsuitable for widely-varying loads, which may lead to overheating.
Series wound motor
A series wound motor has a low-resistance field winding connected in series with the armature. It responds
to increased load by slowing down and this reduces the armature current and minimises the risk of overheating. Series wound motors were widely used as traction motors in rail transport of every kind, but are being phased out in favour of AC induction motors supplied through solid state inverters. The counter-EMF aids the armature resistance to limit the current through the armature. When power is first applied to a
motor, the armature does not rotate. At that instant, the counter-EMF is zero and the only factor limiting the armature current is the armature resistance. Usually the armature resistance of a motor is less than 1 Ω; therefore the current through the armature would be very large when the power is applied. Therefore the need arises for an additional resistance in series with the armature to limit the current until the motor rotation can build up the counter-EMF. As the motor rotation builds up, the resistance is gradually cut out.
Permanent magnet motor
A permanent magnet DC motor is characterized by its locked rotor (stall) torque and its no-load angular velocity (speed).
A thyristor drive is a motor drive circuit where AC supply current is regulated by a thyristor phase control to provide variable voltage to a DC motor.
Thyristor drives are very simple and were first introduced in the 1960s. They remained the predominant type of industrial motor controller until the end of the 1980s when the availability of low cost electronics led to their replacement by chopper drives for high performance systems and inverters for high reliability with AC motors.
They are still employed in very high power applications, such as locomotives, where the high power capability of the thyristors and the simplicity of the design can make them a more attractive proposition than transistor based controllers.
A derivative of the thyristor drive is the simple AC phase controller. This uses a single phase controlled triac
to provide a variable voltage AC output for regulating a universal motor. This is the type of motor speed control most commonly used in domestic appliances, such as food mixers, and small AC powered tools, such as electric drills.
A fire-resistance rating typically means the duration for which a passive fire protection system can
withstand a standard fire resistance test. This can be quantified simply as a measure of time, or it may entail a host of other criteria, involving other evidence of functionality or fitness for purpose. Common rating systems
The following depict the most commonly used international time/temperature curves:
Time/Temperature Curves used for testing the fire-resistance rating of passive
fire protection systems such as firestops, fire doors, wall and floor assemblies, etc.,
Curves used for testing the fire-resistance rating of passive fire protection systems in tunnels in Germany, The Netherlands and France.
used for testing the fire-resistance rating of passive fire protection system|systems in tunnels in The Netherlands.
used for testing the fire-resistance rating of passive fire protection systems in Europe.
which are used in compartmentalisation in buildings and the petrochemical industry in Europe and North America.
used for testing the fire-resistance rating of passive fire protection systems in tunnels in France.
Time/Temperature Curve used for testing the fire-
resistance rating of passive fire protection systems in tunnels in Germany.
Furnace pressure is also subject to standardised tolerances for testing to
obtain fire-resistance ratings. This image shows European tolerances, subject to NEN-EN 1363-1.
Furnace Temperatures for fire testing to obtain fire-resistance ratings are subject to certain tolerances. This graph
shows the tolerance applicable to the European building elements / cellulosic curve.
International fire-resistance ratings
For instance, in Germany, a fire door may have a "T90" designation, which means that the door has documented evidence of its ability to withstand that country's fire door test regime for a duration of 90 minutes.
There are many international variations for nearly countless types of products and systems, some with multiple test requirements.
Canada's Institute for Research in Construction (a part of the National Research Council and publisher of Canada's model building code - NBC) requires a special test regime for firestops for plastic pipe penetrants. Fire endurance tests for this application must be run under 50Pa positive furnace pressure in order to adequately simulate the effect of potential temperature differences between indoor and outdoor temperatures in Canada's winters. Special hoods are applied here to provide suction on the top side of a test assembly in order to reach the 50Pa pressure differential. Afterwards, a 30PSI hose-stream test may be applied.
Some fire doors in the North American Free Trade Agreement (NAFTA) area may even open up somewhat during a fire endurance test, although not too far, while a wall or floor assembly proper may not even be allowed an average temperature rise above 140°C or single point increases over 180°C.
Outdoor spray fireproofing methods that must be qualified to the hydrocarbon curve may be required to pass a host of environmental tests before any burn takes place, to minimise the likelihood that ordinary operational environments cannot render a vital system component useless before it ever encounters a fire.
If critical environmental conditions are not satisfied, an assembly may not be eligible for a fire-resistance rating.
Regardless of the complexity of any given test regime that may lead to a rating, the premise is generally product certification and, most importantly listing and approval use and compliance. Testing without certification and installations that cannot be matched with an appropriate certification listing, are not usually recognised by any Authority Having Jurisdiction (AHJ) unless it is in a realm where product certification is optional.
Tests for fire resistance of record protection
The following classifications may be attained when testing in accordance with UL 72.
Class 125 Rating
This rating is the requirement in data safes and vault structures for protecting digital information on
magnetic media or hard drives. Temperatures inside the protected chamber must be held below 125°F (51.7°C) for the time period specified, such as Class 125-2 Hour, with temperatures up to 2,000°F (1,093.3°C) outside the vault. The temperature reading is taken on the inside surfaces of the protective structure. Maintaining the temperature below 125°F. is critical because data is lost above that temperature threshold, even if the media or hard drives appear to be intact.
Class 150 Rating
This is the rating required to protect microfilm, microfiche, and other film-based information storage media.
Above 150°F (65.5°C) film is distorted by the heat and information is lost. A Class 150-2 Hour vault must keep the temperature below 150°F. for at least two hours, with temperatures up to 2,000°F. (1,093.3°C) outside the vault.
Class 350 Rating
This rating is the requirement for protecting paper documents. Above 350°F (176.7°C) paper is distorted by
the heat and information is lost. A Class 350-4 Hour vault must keep the temperature below 350°F. for at least four hours, with temperatures up to 2,000°F. (1,093.3°C) outside the vault.
Different time/temperature curves
Typically, most countries use the building elements curve, which is nearly identical in most countries as that is what results by burning wood. The building elements curve is characterized jointly by, including, but not limited to, DIN4102, BS476, ASTM E119, ULC-S101, etc. For exterior systems used in the petrochemical industry, the hydrocarbon curve is used. The only exposure beyond this, apart from the more recent tunnel curves shown above, would be the British "jetfire" exposure, which is not commonly used.
Big differences between different countries in terms of the use of the curves include the use of pipes, which shield the furnace thermocouples inside of NAFTA testing laboratories. This slows down the response time and results in a somewhat more conservative test regime in North America. On the other hand, the ISO based European curves run somewhat hotter for most of the test. North America also selectively uses a
hose-stream test between 30 and 45PSI, to simulate real-world impacts and damages that may not be simulated in a laboratory. The US Navy even insists on a 90PSI hose-stream test for some of its assemblies, which may simulate the pressure available to firefighters in fighting a fire, but which has little to do with countermeasures against damaging effects of manual fire suppression. The hose-stream is simply intended to add a level of toughness to matters because without this, some fairly flimsy systems can pass a test, thus receive a rating and thus be permissible by a building code but be so weak that ordinary building use may damage a thus qualified system before it encounters a fire. See firestop.org treatise on the hose stream test.
Germany's DIN4102 also includes a significant impact test for a potential firewall, which is, however, applied from the wrong side: the cold side. Applying the impact from the cold side is more practical to do in a lab setting, however, potential impacts should come from the exposed side, not the unexposed side. Still, for the person designing, building and paying for the test, the fire resistance itself may be rather uneventful unless major problems appear. The burn itself is the long duration, up to 4 hours, but the hose stream test only lasts a few minutes, with large damage potential due to the sudden thermal and kinetic impacts, as the
fire was upwards of 1,100°C (see curves above), whereas the sudden hose-stream test is as cold as the domestic water fed to the fire hose used in the test, which might be 10-20°C. This combined impact explains the debris that can be seen coming from test specimens during the hose stream test, as seen herein.
Because of the large differences in test regimes all over the world, even for identical products and systems, organizations that intend to market their products internationally are often required to run many tests in many countries. Even where test regimes are identical, countries are often reluctant to accept the test results and particularly the certification methods of other countries.
During a fire in a tunnel, as well as in the petrochemical industry, temperatures exceed those of ordinary building (cellulosic) fires. This is because the fuel for the fire is hydrocarbons, which burn hotter (compare hydrocarbon curve above to ASTM E119 curve), faster and typically run out of fuel faster as well, compared against timber. The added complication with tunnels is that the environment inside a "tube" is best described as a "microclimate". The heat cannot escape as well as it can in a burning refinery, which is in the open. Instead, the fire is confined to a narrow tube, where pressure and heat build up and spread rapidly, with little room for escape and little chance of compartmentalization. This scenario was amply tested and
quantified, particularly during the "Eureka Project", run by Technische Universität Braunschweig's iBMB, Dr. Ekkehard Richter, which has profoundly affected tunnel regulations in the Nations that took part in the project. The Netherlands, through Rijkswaterstaat in particular, mandated an extremely tough standard, the curve of which is shown in the gallery above.
Example of a test leading to a fire-resistance rating
The following is a series of pictures depicting a typical fire test, in this case for a firestop, which led to an active fire-resistance rating, backed up by active product certification. A copy of the resulting certification listing can be seen under the certification listing article.
Construction of a test sample, consisting of a mock-up concrete floor frame, complete with penetrants. The
concrete frame measures approximately 5’ x 9’ x 4― (ca. 1.5m x 2.3m x 10 cm). It has a large hole in the centre with many mechanical and electrical services traversing. The penetrants extend 1’ (30 cm) into the furnace and 3’ (91 cm) on the unexposed side. A firestop mortar is being applied here. Notice the intumescent wrap strip surrounding the fibreglass pipe insulation. When the fire starts, this embedded intumescent will swell to take up the place of the melting insulation. The test was conducted in accordance with the Canadian firestop test method ULC S-115 in Scarborough, Ontario.
The completed test sample is being lifted by crane to the test furnace for the fire resistance test. By
contrast, European furnaces can typically allow up to a 1m penetrant depth to reach into the furnaces. North American panel furnaces are not deep enough to accommodate this more realistic exposure.
After the completed test sample has been seated on a ceramic fibre gasket on the top of the furnace, gas is
let in through perforated pipes at the bottom of the furnace. The ULC technician is now igniting the gas on each pipe to start the test. Thermocouples are located inside the furnace to make sure the fire resistance test is run in accordance with the prescribed time/temperature curve. Further thermocouples are located on the firestop, 1‖ or 25mm away from each penetrant and on each penetrant, 1‖ or 25mm up from the surface of the firestop. The length of time the test is run and/or however long it takes for fire to penetrate the firestop determines the F-Rating. The length of time required for a penetrant and/or the sample on average
to exceed an average heat rise above ambient at the start of the test to exceed 140°C or 180°C at any single location – this determines the duration for the FT Rating (Fire and Temperature). If the hose-stream test is passed afterwards, the rating can then be expressed as an FTH Rating (Fire, Temperature and Hose-stream). The lowest of the three determines the overall rating, though it is possible to have a wide variety of T results, which can vary depending upn how well each such penetrant conducts heat.
At the conclusion of the fire resistance test, the test sample is lifted off the furnace and readied for a hose-
stream test, which is NOT intended to simulate the effects of firefighting. Instead, it is to add a measure of reality of possible impacts, thermal shock and generally the brutal environment of a real fire, which is hard to simulate in pristine laboratory conditions. See this. With combustible penetrants like cables or combustible firestops like silicone foam, it is not entirely unlikely even after two hours of fire to see residual flaming on the exposed side. For an example, see this picture. This was proven during a highly publicised fire test at ULC that encased the silicone foam with noncombustible sheathing in an attempt to justify in-situ installations in US and Canadian nuclear reactor facilities, as per submissions provided to Select Committee on Ontario Hydro Nuclear Affairs by Pickering, Ontario Regional Councillor Maurice Brenner
The duration and pressure of the hose-stream test are a direct function of the length of the test and the size
of the test sample. The most typical test pressure is 30PSI, though 45PSI may also be used for fire tests of 4 hours duration or longer.
The test can be considered passed if no fire, water and no excessive heat traversed the sample or
penetrants. All results are tabulated and form part of the rating designations, which can be quite complex in the case of busy firestop tests such as this. For instance, an uninsulated copper pipe may have only a 5 minute T Rating, whereas that same pipe insulated with 2‖ or 50mm of rockwool may achieve a 2 hour T-Rating. Product certification listings resulting from such successful testing can be used to obtain the approval and acceptance of installed configurations on the part of the Authority Having Jurisdiction on construction sites. Listings are considered public knowledge, whereas the test report itself would be a proprietary item.
] Picture 7
Observations from both sides of the test assembly continue on the next day, once the sample has cooled down sufficiently. It is also customary to destroy the test sample, for two reasons: to learn from the effects of the test on the inside of the sample Research and development, as well as for a test laboratory which would issue a certification listing on the basis of the test to ensure that the listing reflects accurately what was installed inside the test assembly, in case and changes occurred that were not previously documented.
Firestop after fire exposure during fire test in Tulsa, Oklahoma. This was an R & D test leading to a fire-
resistance rating of three hours.
A firestop is a passive fire protection system of various components used to seal openings and joints in fire-resistance rated wall and/or floor assemblies, based on fire testing and certification listings.
Unprotected openings in fire separations void the fire-resistance ratings of the fire separations that contain them, allowing spread of fire past the limits of the fire safety plan of the entire building. Firestops are designed to restore the fire-resistance ratings of rated wall and/or floor assemblies by impeding the spread of fire through the opening by filling the openings with fire resistant materials.