Electric motors - Educypedia

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Cahier technique no. 207

Electric motors... and how to improve their control andprotection

E. Gaucheron

Building a New Electric World

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Graduate electronics engineer by training. After a short period withThomson he joined the VsD (Variable speed drives and starters)division of Telemecanique in 1970. He then completed his training atENSAM (the national college for arts and vocational training) in Parisduring his position with Telemecanique's after sales service.As a specialist in motor control, he was involved in the developmentof variable speed control for AC motors. He has gained a wide rangeof experience in various positions: systems platform engineer,product manager for machine tool drive products, product managerfor drives for asynchronous motors (Altivar products) and manager ofthe VSD project marketing team.He is currently a motor control "applications" specialist within theadvance development team for the Schneider Electric PCP (PowerControl and Protection) division.

no. 207Electric motors... and how to improve their control andprotection

ECT 207 first issue, October 2004

Etienne Gaucheron

Cahier Technique Schneider Electric no. 207 / p.2

Cahier Technique Schneider Electric no. 207 / p.3

Electric motors... and how to improve their control and protection

Contents

1 Three-phase asynchronous motors 1.1 Operating principle p. 4

1.2 Construction p. 6

1.3 The various types of rotor p. 6

2 Other types of electric motor 2.1 Single phase asynchronous motors p. 10

2.2 Synchronous motors p. 10

2.3 DC motors p. 14

3 Operation of asynchronous motors 3.1 Squirrel cage motors p. 17

3.2 Slip-ring motors p. 19

3.3 Other speed variation systems p. 20

4 Conclusion p. 22

Nowadays, apart from lighting devices, electric motors represent thelargest loads in industry and commercial installations. Their function, toconvert electrical energy into mechanical energy, means they areparticularly significant in economic terms, and hence, they cannot beignored by installation or machinery designers, installers or users.

There are many types of motor in existence, but 3-phase asynchronousmotors, and in particular squirrel cage motors, are the most commonlyused in industry and in commercial buildings applications above a certainpower level. Moreover, although they are ideal for many applications whencontrolled by contactor devices, the increasing use of electronic equipmentis widening their field of application. This is the case for start/stop controlwith soft start/soft stop units, and when precise speed adjustment is alsonecessary with variable speed drives/regulators.

However, slip-ring asynchronous motors are used for certain high powerapplications in industry, and single phase asynchronous motors remainsuitable for limited power applications, mainly for buildings applications.

The use of synchronous motors, known as brushless or permanent magnetmotors, combined with converters is becoming increasingly common inapplications requiring high performance levels, in particular in terms ofdynamic torque (on starting or on a change of duty), precision and speedrange.

After presenting the various types of electric motor and their operatingprinciples, this "Cahier Technique" describes the technical and operatingfeatures of asynchronous motors, covering in particular the main startingdevices, speed control and braking methods. It provides a solid groundingfor the reader to gain a good understanding of all the problems involvedwith motor control and protection.

This "Cahier Technique" touches briefly on variable speed control ofelectric motors. However, this subject is covered more specifically in"Cahier Technique" CT 208 "Electronic starters and variable speed drives".Motor protection is the subject of another "Cahier Technique" currently inpreparation.

Cahier Technique Schneider Electric no 207 / p.4

1 Three-phase asynchronous motors

This section covers 3-phase asynchronousmotors, the most commonly used motors fordriving machinery. The use of this type of motorhas become the norm in a large number ofapplications because of its numerousadvantages: it is standardized, rugged, easy tomaintain and use, and inexpensive.

Other types of motor are presented in section 2.Section 3 describes and compares the mainstarting devices, and speed control and brakingmethods associated with the motors.

1.1 Operating principle

The operating principle of an asynchronousmotor is based on the creation of an inducedcurrent in a conductor when the conductor cutsthe lines of force of a magnetic field, hence thename "induction motor". The combined action ofthis induced current and the magnetic fieldcreates a motive force on the motor rotor.Let us take the example of a turn with shortcircuit ABCD, located in magnetic field B, androtating around an axis xy (see Fig. 1 ).

If, for example, we rotate the magnetic fieldclockwise, the turn is subject to a variable fluxand becomes the source of an inducedelectromotive force which causes an inducedcurrent I (Faraday’s law).According to Lenz’s law, the direction of thecurrent is such that it opposes the cause thatproduced it by its electromagnetic action. Both of

the conductors are therefore subject to a Lorentzforce F (also known as Laplace force), in theopposite direction to its relative displacement inrelation to the field coil field.The left hand rule (action of the field on acurrent, see Figure 2 ) helps demonstrate thedirection of the force F applied to eachconductor.The thumb points in the direction of themovement field. The index finger indicates thedirection of the field. The middle finger points inthe direction of the induced current. The turn istherefore subject to a torque that causes it torotate in the same direction as the coil field,called the rotating field. The turn therefore startsto rotate and the electromotive torque producedbalances the resistive torque.

Creation of the rotating field

Three windings, geometrically offset by 120°, areeach supplied by one of the phases of a 3-phase

Fig. 1: Creation of an induced current in a shorted turn

Fig. 2: Fleming’s left hand rule to find the direction ofthe force

North

I

D

F

C

y

x

A

B

I

B

South

F

movements of the wire

direction of magnetic field

direction of current (positive to negative)


AC supply (see Fig. 3 ). The windings have ACcurrents flowing through them that have thesame electrical offset and that each produce asinusoidal AC magnetic field. This field, whichalways follows the same axis, is at its maximumwhen the current in the winding is at itsmaximum.The field generated by each winding is theresultant of two fields rotating in oppositedirections, each field having a constant value of

half that of the maximum field. At any instant t1in the period (see Fig. 4 ), the fields produced byeach winding can be represented as follows:v Field H1 decreases. Its two component fieldstend to move away from the axis OH1.v Field H2 increases. Its two component fieldstend to move towards the axis OH2.

v Field H3 increases. Its two component fieldstend to move towards the axis OH3.

Fig. 3: Principle of a 3-phase asynchronous motor

Fig. 4: Fields generated by the three phases

The flux corresponding to phase 3 is negative.The field is thus directed in the opposite directionto the coil.

If the three diagrams are superimposed, it canbe seen that:

v The three fields rotating anticlockwise areoffset by 120° and cancel one another out.

v The three fields rotating clockwise aresuperimposed on one another. These fields areadded together to form the rotating field withconstant amplitude 3Hmax/2. It is a field with onepair of poles.

This field performs one rotation during oneperiod of the supply current. Its speed isdependent on the line supply frequency (f), andthe number of pairs of poles (p). It is called the"synchronous speed".

Slip

Motor torque can only exist if there is an inducedcurrent flowing in the turn. This torque isdetermined by the current flowing in the turn andcan only exist if there is flux variation in this turn.There must therefore be a difference in speedbetween the turn and the rotating field. This iswhy an electric motor operating according to theprinciple we have just described is called an"asynchronous motor". The difference betweenthe synchronous speed (Ns) and that of the turn(N) is called the "slip" and is expressed as apercentage of the synchronous speed.

slip = [(Ns - N) / Ns] x 100

During operation, the rotor current frequency isobtained by multiplying the supply frequency bythe slip. The rotor current frequency is thereforeat its maximum on starting.

The steady state slip varies according to themotor load and the level of the supply voltageapplied to it: the lower the motor load, the lowerthe slip; and if the motor is under-supplied theslip increases.

120˚

B3 B2

H2H3

Ph2Ph3 Ph1

H1 B1

Ph1

H2 max2

Ph3

Ph2

H2O

H2 max2

H3 max2

H3 max2

H3

O

Ph2

Ph3

Ph1

Ph2

H1 max2

H1 max2

Ph1

Ph3

O

H1


Synchronous speedThe synchronous speed of 3-phaseasynchronous motors is directly proportional tothe supply current frequency and inverselyproportional to the number of pairs of polescomprising the stator.For example:Ns = 60 f/pWhere:v Ns: synchronous speed in rpmv f: frequency in Hzv p: number of pairs of poles

The rotation speeds of the rotating field, orsynchronous speeds, according to the number ofpoles, are given in the table in Figure 5 forindustrial frequencies of 50 Hz and 60 Hz andone other frequency (100 Hz).

In practice it is not always possible to increasethe speed of an asynchronous motor bysupplying it with a higher frequency than that forwhich it is designed, even if the voltage issuitable. It is therefore necessary to checkwhether its mechanical and electrical designallows this.

Fig. 5: Synchronous speeds according to the numberof poles and the current frequency

It should be noted that in view of the slip, the on-load rotation speeds of asynchronous motors areslightly lower than the synchronous speedsindicated in the table.

1.2. Construction

A 3-phase squirrel cage asynchronous motorconsists of two main parts: a field coil or statorand an armature or rotor.

Stator

This is the fixed part of the motor. A cast iron orlight alloy frame surrounds a ring of thinlaminations (around 0.5 mm thick) made ofsilicon steel. The laminations are insulated fromone another by oxidation or an insulatingvarnish. The "lamination" of the magnetic circuitreduces losses via hysteresis and eddy currents.The laminations have slots in them for holdingthe stator windings that produce the rotating field(three windings for a 3-phase motor). Each

winding is made up of a number of coils. Theway these coils are joined to one another definesthe number of pairs of poles of the motor, andthus the speed of rotation.

Rotor

This is the moving part of the motor. Like themagnetic circuit of the stator, it is made up of astack of thin laminations insulated from oneanother, forming a keyed cylinder on the motorshaft. Two different technologies can be used forthis part, which separate asynchronous motorsinto two distinct families: those with a “squirrelcage” rotor and those with a wound rotor whichare referred to as “slip-ring”.

1.3. The various types of rotor

Squirrel cage rotorThere are several types of squirrel cage rotor.They are all designed as shown in the examplein Figure 6 .These motors are (from the least common to themost widely used):c Resistive squirrel cage rotorThe resistive rotor mainly exists in the singlecage version (see later for the definition of the

single cage motor). The cage is closed by tworesistive rings (special alloy, small cross-section,stainless steel rings, etc). These motors havehigh slip at nominal torque. Their starting torqueis high and the starting current is low(see Fig. 7 ). The efficiency of these motors islow due to the losses in the rotor.This type of motor is mainly used forapplications in which it is useful to have some

Number Speed of rotation in rpmof poles

50 Hz 60 Hz 100 Hz

2 3000 3600 6000

4 1500 1800 3000

6 1000 1200 2000

8 750 900 1500

10 600 720 1200

12 500 600 1000

16 375 540 750


Fig. 6: Exploded view of a squirrel cage rotor motor

Fig. 7: Torque/speed curves for the various types ofcage rotor (at Un)

1. This force-cooled asynchronous motor with high slip isused in speed control. Its stalling current is close to itsnominal current, and its torque/speed characteristic fallsvery steeply. With a variable power supply it is possible toadapt this characteristic and adjust the motor torqueaccording to the required traction.

slip in order to adapt the speed to the torque,for example:v In the case of several motors connectedmechanically, across which the load must bedistributed, such as rolling mill roller tables, orthe drive system of a lifting cranev For winder/unwinder functions using Alquist(1)

motors designed for this purposev When a high starting torque with limited inrushcurrent is needed (lifting hoists or conveyors).

They are used to control the speed by modifyingthe voltage alone. But this application is on thedecline, being increasingly replaced byfrequency inverters. Although, in general, motorsare self-cooled, some resistive squirrel cagemotors are force-cooled (separate motorizationof their fans).

Connecting box

Stator winding

Bearing

Shield end flange on shaft end side

Bearing

Cage rotor

Stator

FanFan cover

Fan end shield

N0

C

Single cage rotor

Double cage rotor

Resistive cage rotor


c Single squirrel cage rotorConductors are placed in the holes or slotsaround the edges of the rotor (on the outside ofthe cylinder created by the stacking of thelaminations) and connected at either end by ametal ring. The motor torque generated by therotating field is applied to these conductors. Tomake the torque regular, the conductors are at aslight angle in relation to the motor shaft. Thewhole assembly resembles a squirrel cage,hence the name of this type of rotor.The squirrel cage is generally fully molded, (onlyvery large motors are made with conductorsinserted in slots). Aluminum is pressure injectedand the cooling fins, which are cast in the sameoperation, are used for short-circuiting the statorconductors.These motors have a relatively low startingtorque and the current drawn on power-up ismuch higher than the nominal current (seeFigure 7).On the other hand they have low slip at nominalcurrent.These motors are mainly used at high power toimprove the efficiency of installations on pumpsand fans. They are also used with frequencyinverters at variable speed. The torque andcurrent problems on starting are thus fullyresolved.

c Double squirrel cage rotorThis consists of two concentric cages, one outer,with a small cross-section, and highly resistive,and the other inner, with a large cross-sectionand lower resistance.v At the beginning of the starting phase, whenthe rotor current frequency is high, the resultingskin effect causes the whole of the rotor currentto flow around the outer surface of the rotor andthus in a smaller surface area of the conductors.Thus, at the beginning of the starting phase,when the rotor current frequency is high, thecurrent only flows in the outer cage. The torqueproduced by the resistive outer cage is high andthe current inrush low (see Fig. 7 ).v At the end of the starting phase, the frequencydecreases in the rotor and it is easier for the fluxto flow through the inner cage. The motor then

behaves very much as if it had been built with asingle low resistance cage.In steady state the speed is only slightly lowerthan that of a single cage motor.

c Rotor with deep slotsThis is the standard version.The rotor conductors are molded into the rotorslots which are trapezoidal shape, with the smallside of the trapeze located on the outer surfaceof the rotor.Operation is similar to that of a double cagemotor: the intensity of the rotor current variesinversely to its frequency. Thus:v At the beginning of the starting phase, torqueis high and the current inrush is low.v In steady state the speed is more or less thesame as that of a single cage motor.

Wound rotor (slip-ring rotor)

Identical windings to those of the stator areinserted in the slots around the outer edge of therotor (see Fig. 8 ). The rotor is generally3-phase.One end of each of the windings is connected toa common point (star connection). The free endscan be connected to a centrifugal switch or tothree insulated solid copper rings that form partof the rotor. The graphite-based brushesconnected to the starting device rub againstthese rings.Depending on the values of the resistors insertedin the rotor circuit, this type of motor can developa starting torque of up to 2.5 times the nominaltorque.The current on starting is more or lessproportional to the torque developed on themotor shaft.This solution is now being phased out in favor ofelectronic solutions combined with a standardsquirrel cage motor. In fact, the latter solutionsresolve maintenance issues (replacement ofworn rotor power supply brushes, servicing ofadjustment resistors), reduce the energydissipated in these resistors and alsosignificantly improve the efficiency of theinstallation.


Fig. 8: Exploded view of a slip-ring rotor moto

Connecting box

Brush

Shield end flange on shaft end side

Bearing

Wound rotor with open slots

Stator

Fan

Fan cover

Slip-ring end shield

Slip rings

Brush inspection door

Bearing


2 Other types of electric motor

2.1 Single phase asynchronous motors

Although the single phase asynchronous motoris less widely used in industry than its 3-phasecounterpart, it nevertheless represents asignificant proportion of low power and buildingsapplications that use a 230 V single phase linesupply.

It is more bulky than a 3-phase motor of thesame power rating.Moreover, its efficiency and its power factor aremuch lower than with the 3-phase motor andthey vary considerably depending on the powerand the manufacturer.Single phase motors up to ten or so kW arewidely used in the United States.

Construction

Like the 3-phase motor, the single phase motorconsists of two parts: the stator and the rotor.

c StatorThis consists of an even number of poles and itscoils are connected to the line supply.

c RotorMore often than not this is a squirrel cage rotor.

Operating principleLet us consider a stator consisting of twowindings, L1 and N, connected to the line supply(see Fig. 9 ).

The single phase AC current creates a single ACfield H in the rotor, which is the superimpositionof two rotating fields H1 and H2 with the samevalue rotating in opposite directions.On stopping, because the stator is energized,these fields have the same slip in relation to the

rotor and consequently produce two equal andopposite torques. The motor cannot start.A mechanical pulse on the rotor causes the slipsto become unequal. One of the torquesdecreases while the other increases. Theresulting torque causes the motor to start in thedirection in which it has been set going.In order to solve this torque problem during thestarting phase, a second coil, offset by 90°, isinserted in the stator. This auxiliary phase ispowered by a phase angle device (capacitor orinductance). Once starting is complete theauxiliary phase can be disabled.Note: A 3-phase motor can also be used insingle phase operation. The starting capacitor isthen connected in series or in parallel with theunused winding.

Fig. 9: Operating principle of a single phaseasynchronous motor

2.2 Synchronous motors

Construction

Like the asynchronous motor, the synchronousmotor consists of a stator and a rotor separatedby the air gap. It differs from the asynchronousmotor in that the flux in the air gap is not due to acomponent of the stator current: it is created bymagnets or by the field coil current provided byan external DC source energizing a windingplaced in the rotor.

c Stator The stator consists of a housing and a magneticcircuit generally comprising silicon steel

laminations and a 3-phase coil similar to that ofan asynchronous motor supplied with 3-phaseAC to produce a rotating field.

c RotorThe rotor carries field magnets or coils throughwhich a direct current flows and which createinterposed North and South poles. Unlikeasynchronous machines, the rotor rotates withno slip at the speed of the rotating field.There are therefore two different types ofsynchronous motor: magnet motors and woundrotor motors.

L1 N

H1 H2

HStator winding

Stator winding

Rotor


v With magnet motors, the motor rotor is fittedwith permanent magnets (see Fig. 10 )(generally rare earth magnets), in order toachieve increased field strength in a smallvolume. The stator has three-phase windings.

These motors can tolerate significant overloadcurrents in order to achieve high-speedacceleration. They are always used with avariable speed drive, and these motor-driveassemblies are intended for specific marketssuch as robots or machine tools, for whichsmaller motors, acceleration and passband areessential.

Fig. 10: Cross-section of a permanent magnet motor

v The second type of synchronous machine hasa wound coil, and is a reversible machine thatcan operate as either a generator (alternator) ora motor. For many years these machines havebeen mainly used as alternators. Their use asmotors was virtually confined to applicationswhere it was necessary to drive loads at fixedspeed despite relatively wide variations in theirresistive torque.

The development of direct (cycloconverters) orindirect frequency inverters operating withnatural switching due to the ability ofsynchronous machines to provide reactivepower, has enabled the creation of highperformance, reliable variable speed electricdrives that are particularly competitive in relationto competitors' solutions for power ratings overone megawatt.

Although it is possible to find synchronousmotors used industrially in power ratings rangingfrom 150 kW to 5 MW, it is above 5 MW thatelectric drives using synchronous motorsbecome virtually essential, for the most partcombined with variable speed drives.

Operating characteristics

The motor torque of the synchronous machine isproportional to the voltage at its terminals,whereas that of the asynchronous machine isproportional to the square of that voltage.

Unlike the asynchronous motor, it can work witha power factor equal to one or very close to it.

The synchronous motor therefore has a numberof advantages over the asynchronous motor withregard to its ability to be powered via theconstant voltage/frequency line supply:

v The speed of the motor is constant, regardlessof the load.

v It can supply reactive power and increase thepower factor of an installation.

v It can withstand relatively large voltage drops(around 50% due to its over-excitationproperties) without stalling.

However, the synchronous motor supplieddirectly by the constant voltage/frequency linesupply has two disadvantages:

v It has starting difficulties. If the motor is notcombined with a variable speed drive, startingmust be performed at no-load, either by DOLstarting for small motors, or using a startingmotor that drives it at a speed close tosynchronous speed before direct connection tothe line supply.

v It may stall if the resistive torque exceeds itsmaximum electromagnetic torque. In this case,the entire start process must be repeated.

Other types of synchronous motor

To conclude this overview of industrial motors,we must also mention linear motors,synchronized asynchronous motors and steppermotors.

c Linear motorsTheir structure is identical to that of synchronousrotary motors: they consist of a stator (plate) anda rotor (forcer) which are in line. In general theplate moves along the forcer on a guide.

This type of motor does away with allintermediate kinematics for converting themovement, which means there is no play ormechanical wear on this drive.

c Synchronized asynchronous motorsThese are induction motors. During the startingphase, the motor operates in asynchronousmode and when it has reached a speed close tosynchronous speed, it switches to synchronousmode.

If it has a high mechanical load, it can no longeroperate in synchronous mode and returns toasynchronous mode. This feature is obtained byspecial construction of the rotor and is generallyfor low power motors.

11

10

12 12

3

4

5

678

9

S S

N

N

Stator winding

Permanent magnet rotor (4 poles)


c Stepper motorsThe stepper motor is a motor that operatesaccording to the electrical pulses supplying itscoils. Depending on its electrical power supply, itmay be:v Unipolar if its coils are always supplied in thesame direction by a single voltage, hence thename unipolar.v Bipolar when its coils are supplied sometimesin one direction and sometimes in the other.They sometimes create a North pole, andsometimes a South pole, hence the namebipolar.Stepper motors can be of variable reluctance ormagnet type or a combination of the two(see Fig. 11 ).

The minimum angle of rotation between twomodifications of the electrical pulses is called astep. A motor is characterized by the number ofsteps per revolution (that is, for 360°). The mostcommon values are 48, 100 or 200 steps perrevolution.

The motor therefore rotates discontinuously. Toimprove the resolution, the number of steps maybe increased in a purely electronic way (micro-step operation). By varying the current in thecoils in steps (see Fig. 12 ), a resulting field iscreated that slides from one step to another, thuseffectively reducing the step.

The circuits for micro-steps multiply the numberof motor steps by 500, thus changing, forexample, from 200 to 100,000 steps.The electronics can be used to control thechronology of these pulses and count thenumber of pulses. Stepper motors and theircontrol circuits thus enable a shaft to rotate witha high degree of precision in terms of both speedand amplitude.

Their operation is thus similar to that of asynchronous motor when the shaft is rotatingcontinuously, which corresponds to specifiedfrequency, torque and driven load inertia limits(see Fig. 13 ). If these limits are exceeded, themotor stalls, the effect of which is to stop themotor.

Fig. 11: The three types of stepper motor

45˚ 15˚ 30˚

Type Permanent Variable Hybridmagnet reluctance bipolarbipolar unipolar

Characteristics 2 phases, 4 wires 4 phases, 8 wires 2 phases, 4 wires

No. of steps/rev. 8 24 12

Operating stages

Step 1

Intermediate state

Step 2


Fig. 12: Current steps applied to the coils of a stepper motor to reduce its step

Accurate angular positioning is possible withouta measurement loop. The small models of thesemotors, generally with power ratings of less thanone kW, have a low voltage power supply. Inindustry, these motors are used for positioncontrol applications such as setting stops forcutting to length, controlling valves, optical ormeasurement devices, loading and unloadingpresses or machine tools, etc.

Fig. 13: Maximum torque according to step frequency

The simplicity of this solution makes itparticularly economical (no feedback loop).Magnet stepper motors also have the advantageof a standstill torque when there is no powersupply.On the other hand, the initial position of themoving part has to be known and taken intoaccount by the electronics in order to provideeffective control.

0.86

0.5

B1 I1B1

tB2

I2B2

t

Starting frequency limit

Acceleration range

Working torque limit

Maximum step frequency

Step frequency (Hz)

Torque

Standstill torque

Starting range


2.3 DC motors

Separate field excitation DC motors are stillsometimes used for driving machines at variablespeed.

These motors are very easy to miniaturize, andessential for very low powers and low voltages.They are also particularly suitable, up to highpower levels (several megawatts), for speedvariation with simple, uncomplicated electronictechnologies for high performance levels(variation range commonly used from1 to 100).

Their characteristics also enable accurate torqueregulation, when operating as a motor or as agenerator. Their nominal rotation speed, which isindependent of the line supply frequency, is easyto adapt by design to suit all applications.They are however less rugged thanasynchronous motors and much moreexpensive, in terms of both hardware andmaintenance costs, as they require regularservicing of the commutator and the brushes.

Construction

A DC motor is composed of the following parts:

c Field coil or statorThis is a non-moving part of the magnetic circuiton which a winding is wound in order to producea magnetic field. The electro-magnet that iscreated has a cylindrical cavity between itspoles.

c Armature or rotorThis is a cylinder of magnetic laminations thatare insulated from one another andperpendicular to the axis of the cylinder. Thearmature is a moving part that rotates round itsaxis, and is separated from the field coil by an airgap. Conductors are evenly distributed aroundits outer surface.

c Commutator and brushesThe commutator is integral with the armature.The brushes are fixed. They rub against thecommutator and thus supply power to thearmature conductors.

Operating principle

When the field coil is energized, it creates amagnetic field (excitation flux) in the air gap, inthe direction of the radii of the armature. Thismagnetic field "enters" the armature from theNorth pole side of the field coil and "exits" thearmature from the South pole side of the fieldcoil.

When the armature is energized, currents passthrough the conductors located under one fieldcoil pole (on the same side of the brushes) in the

same direction and are thus, according toLaplace's law, subject to a force. The conductorslocated under the other pole are subject to aforce of the same intensity in the oppositedirection. The two forces create a torque whichcauses the motor armature to rotate(see Fig. 14 ).When the motor armature is powered by a DC orrectified voltage supply U, it produces backemf E whose value is E = U – RIRI represents the ohmic voltage drop in thearmature.The back emf E is linked to the speed and theexcitation by the equation E = k ω Φ in which:v k is a constant specific to the motorv ω is the angular speedv Φ is the flux

This equation shows that at constant excitationthe back emf E (proportional to ω) is an image ofthe speed.

The torque is linked to the field coil flux and thecurrent in the armature by the equation:T = k Φ IIf the flux is reduced, the torque decreases.

There are two methods for increasing the speed.

c Either increase the back emf E, and thus thesupply voltage at constant excitation: this isknown as "constant torque" operation.

c Or decrease the excitation flux, and thus theexcitation current, while keeping the supplyvoltage constant: this is known as "reduced flux"or "constant power" operation. This operationrequires the torque to decrease as the speedincreases (see Fig. 15 ). However, for highreduced flux ratios this operation requires

Fig. 14: Production of torque in a DC motor

F

F

i f i f

I

I

NS

Field coil pole Field coil pole

Brush

Brush


specially adapted motors (mechanically andelectrically) to overcome switching problems.

The operation of this type of device (DC motor)is reversible:

v If the load opposes the rotation movement (theload is said to be resistive), the device providesa torque and operates as a motor.

v If the load is such that it tends to make thedevice rotate (the load is said to be driving) or itopposes the slow-down (stopping phase of a

load with a certain inertia), the device provideselectrical energy and operates as a generator.

Various types of DC motor (see Fig. 16 )

c Parallel excitation (separate or shunt)The coils, armature and field coil are connectedin parallel or supplied via two sources withdifferent voltages in order to adapt to thecharacteristics of the machine (e.g.: armaturevoltage 400 volts and field coil voltage180 volts).

Fig. 15: Torque/speed curves for a separate field excitation motor

Fig. 16: Diagrams of the various types of DC motor

Torque

Un

; Φn

0 Nmax

-Tn

-Tmax

TnI = In

I = Imax

I = -In

I = -Imax

Tmax

Speed

Operation at:constant constanttorque power

b: At constant powera: At constant torque

Torque

U=

-Un

U=

0

U=

-0.8

Un

U=

-0.6

Un

U=

-0.4

Un

U=

-0.2

Un

U=

0.2U

n

U=

0.4U

n

U=

0.8U

n

U=

Un

U=

0.6U

n

Speed0 Nn

M

M M

M

Supply 1 Supply

a: Separate field excitation motor

b: Series wound motor

c: Shunt wound motor

d: Compound wound motor

Supply 2

SupplySupply


The direction of rotation is reversed by invertingone or other of the windings, generally byinverting the armature voltage due to the muchlower time constants. Most bidirectional speeddrives for DC motors operate in this way.

c Series woundThe design of this motor is similar to that of theseparate field excitation motor. The field coil isconnected in series to the armature coil, henceits name.The direction of rotation can be reversed byinverting the polarities of the armature or the fieldcoil. This motor is mainly used for traction, inparticular on trucks supplied by battery packs. Inrailway traction the old TGV (French high-speed

train) motor coaches used this type of motor.More recent coaches use asynchronous motors.

c Compound wound (series-parallel excitation)This technology combines the qualities of theseries wound motor and the shunt wound motor.This motor has two windings per field coil pole.One is connected in parallel with the armature. Alow current (low in relation to the workingcurrent) flows through it. The other is connectedin series.It is an added flux motor if the ampere turns ofthe two windings add their effects. Otherwise it isa negative flux motor. But this particularmounting method is very rarely used as it leadsto unstable operation with high loads.


3 Operation of asynchronous motors

3.1 Squirrel cage motors

Consequences of a voltage variation

c Effect on the starting currentThe starting current varies with the supplyvoltage. If the supply voltage is higher during thestarting phase, the current drawn at the momentof power-up increases. This current increase isaggravated by the saturation of the machine.

c Effect on the speedWhen there are voltage variations, thesynchronous speed is not modified, but for amotor under load, an increase in the voltageresults in a slight decrease in the slip. Inpractical terms, this property cannot be used as,due to the saturation of the stator's magneticcircuit, the current drawn increases significantlythat may cause an abnormal temperature rise ofthe machine, even during operation with a lowload. On the other hand, if the supply voltagedecreases, the slip increases, and the currentdrawn increases to provide the torque, with theresulting risk of temperature rise. Moreover,since the maximum torque decreases as thesquare of the voltage, the motor may stall if thereis a significant decrease in the voltage.

Consequences of a frequency variation

c Effect on the torqueAs with any electrical machine, the torque ofthe asynchronous motor is of the typeT = K I Φ(K = constant coefficient depending on themachine)In the equivalent diagram in figure 17 , coil L isthat which produces the flux and Io is themagnetizing current.

At first approximation, disregarding theresistance ahead of the magnetizing inductance(that is, for frequencies of a few Hertz) current Iois expressed as:Io = U / 2π L fand the flux will be expressed as:Φ = k IoThe machine torque is therefore expressed as:T = K k Io IIo and I are the nominal currents for which themotor is designed.To avoid exceeding the limits, Io must be kept atits nominal value, which can only be achieved ifthe U/f ratio remains constant.

Fig. 17: Equivalent diagram of an asynchronous motor

Consequently, it is possible to obtain nominaltorque and currents as long as the supplyvoltage U can be adjusted according to thefrequency.When this adjustment is no longer possible, thefrequency can always be increased, but thecurrent Io decreases and the useful torque alsodecreases, as it is not possible to continuallyexceed the nominal current of the machinewithout risking a temperature rise.To achieve constant torque operation whateverthe speed, the U/F ratio must be kept constant.This is what a frequency inverter does.

c Effect on the speedThe speed of rotation of an asynchronous motoris proportional to the frequency of the supplyvoltage. This property is often used to makespecially designed motors operate at very highspeed, for example with a 400 Hz supply(grinding machines, laboratory and surgicalequipment, etc). It is also possible to achievevariable speed by adjusting the frequency, forexample from 6 to 50 Hz (conveyor rollers, liftingequipment, etc).

Adjusting the speed of 3-phaseasynchronous motors(subject described in detail in "CahierTechnique" no. 208)

For many years, there were very few possibilitiesfor adjusting the speed of asynchronous motors.

Stator Rotor

Leakage inductance

Energy losses

Magnetic flux inductance

Iron losses

Active power

Io

L


Squirrel cage motors were mostly used at theirnominal speed. In practice only pole-changingmotors or motors with separate stator windings,which are still frequently used nowadays, couldprovide several fixed speeds.

Nowadays, with frequency inverters, squirrelcage motors are controlled at variable speed,and can thus be used in applications previouslyreserved for DC motors.

Pole-changing motors

As we have already seen, the speed of a squirrelcage motor is dependent on the frequency of theline supply and the number of pairs of poles. It istherefore possible to obtain a motor with two ormore speeds by creating combinations of coils inthe stator that correspond to different numbers ofpoles.This type of motor only allows speed ratios of 1to 2 (4 and 8 poles, 6 and 12 poles, etc). It hassix terminals (see Fig. 18 ).For one of the speeds the line supply isconnected to the three corresponding terminals.For the second, the terminals are linked to oneanother, as the line supply is connected to theother three terminals.More often than not, at both high and low speed,starting is carried out by connecting directly to

Fig. 18: Different types of Dahlander connection

the line supply without using any special device(DOL starting).In some cases, if required by the operatingconditions and permitted by the motor, thestarting device automatically performs thechange to low speed before initiating the changeto high speed or before stopping.Depending on the currents drawn during the LowSpeed -LSP- or High Speed -HSP- connections,protection may be provided by one thermaloverload relay for both speeds or by two relays(one for each speed).In general, these motors have a low efficiencyand a fairly low power factor.

Motors with separate stator windings

With this type of motor, which has two electricallyindependent stator windings, two speeds can beobtained in any given ratio. However theirelectrical characteristics are often affected by thefact that the LSP windings must withstand themechanical and electrical stresses that resultfrom operating the motor at HSP. Thus, this typeof motor operating at LSP sometimes draws ahigher current than at HSP.It is also possible to create three and four speedmotors by coupling the poles to one or both ofthe stator windings. This solution requiresadditional connectors on the coils.

Ph1

2U 2W2V

1U 1W1V

2U 2W2V

1U 1W1V

Ph2 Ph3

High speedHigh speed

Ph1 Ph2 Ph3

Low speed

Ph1

2U 2W2V

1U 1W1V

2U 2W2V

1U 1W1V

Ph2 Ph3

High speed

Ph1 Ph2 Ph3

Low speedHigh speedLow speed

∆/Y Dahlander connection (for constant torque)

Y/YY Dahlander connection (for quadratic torque)

Low speed

Ph2

Ph1

2V

2U 2W

1V1W

1U

Ph3 Ph2

Ph1

2U

2W 2V

1U 1W

1V Ph3

Ph1

2V 2W

2U

1W 1V

1U

Ph3 Ph2

Ph1

2U

2W 2V

1U 1W

1V

Ph3 Ph2


3.2 Slip-ring motors

Use of the rotor resistorThe rotor resistor for this type of motor is used todefine:v Its starting torque (see section 1)v Its speedConnecting a permanent resistor to the rotorterminals of a slip-ring motor lowers its speed(the higher the resistance the lower the speed).This is a simple solution for varying the speed.

3.2.2. Adjusting the speed by the slip

These rotor or "slip" resistors can be short-circuited in several notches to obtain either adiscontinuous adjustment of the speed, orgradual acceleration and complete starting of themotor. They must withstand the period ofoperation, in particular when they are intended tovary the speed. Hence, they are often quitesizeable and fairly costly.This extremely simple process is being used lessand less as it has two major disadvantages:v During operation at low speed, a largeproportion of the energy taken from the linesupply is dissipated in straightforward losses inthe resistors.v The speed obtained is not independent of theload, but varies with the resistive torque applied

Fig. 19: Speed/torque curve with "slip" resistor

on the motor shaft by the machine (see Fig. 19 ).For a given resistance, the slip is proportional tothe torque. Thus, for example, the reduction inspeed obtained by a resistor can be 50% at fullload and only 25% at half load, while the no-loadspeed remains virtually unchanged.

If an operator continuously monitors themachine, he can set the speed in a certain zonefor relatively high torques by modifying theresistance value on demand, but adjustment forlow torques is virtually impossible. In fact, if theoperator inserts a very high resistance in order toobtain a "low speed at low torque" point, thesmallest variation in the resistive torque causesthe speed to increase from zero to almost 100%.The characteristic is too unstable.

For machines with special variation of theresistive torque according to the speed,adjustment can also prove impossible.

Example of slip operation:For a machine which applies a resistive torque of0.8 Tn to the motor, different speeds can beobtained, represented by the sign • on thediagram in figure 19 .

At steady torque, the speed decreases while therotor resistance increases.

1

0.75

0.50

0.25

0 0.5 1 1.5 20.8 Torque

Speed

Slip operation zone

Acceleration zone

Natural curve of the motor

Curve with low rotor resistance

Curve with high rotor resistance


3.3 Other speed variation systems

Variable voltage regulatorThis device is only used for low powerasynchronous motors. It requires a resistivecage motor.

Speed variation is achieved by increasing themotor slip by reducing the voltage.

It is fairly widely used in ventilation, pump andcompressor systems, applications for which itsavailable torque characteristic providessatisfactory operation. However, as frequencyinverters are now becoming very competitive,they are gradually replacing this solution.

Other electromechanical systems

The use of the electromechanical speedadjustment systems described below is on thedecline since the generalization of electronicvariable speed drives.

c AC commutator motors (Schrage)These are special motors. Speed variation isachieved by varying the position of the brusheson the commutator in relation to the neutral line.

c Eddy current drivesThis consists of a drum directly connected to theasynchronous motor operating at constantspeed, and a rotor with a coil supplied with DC(see Fig. 20 ).

The movement is transmitted to the output shaftby electromagnetic coupling. The slip of thisassembly can be adjusted by adjusting theexcitation of this coil.A built-in tachogenerator is used to control thespeed with a high degree of accuracy.

A ventilation system is used to evacuate thelosses due to the slip.

This principle was widely used in liftingequipment and in particular cranes. Itsconstruction makes it a robust system with nowearing parts, which is suitable for intermittentoperation and for power levels up to a hundredor so kW.

c Ward Leonard setThis device, which was widely used in the past,consists of a motor and a DC generator whichsupplies a DC motor (see Fig. 21 ).

Speed control is achieved by regulating theexcitation of the generator. A low control currentenables powers of several hundred kW to becontrolled in all the torque-speed quadrants. Thistype of drive was used on rolling mills andelevators for mining.

Fig. 20: Schematic cross-section of an eddy currentvariable speed drive

This variable speed control solution was themost economical and provided the highestperformance levels prior to the appearance ofsemiconductors, which have now rendered itobsolete.

Mechanical and hydraulic speed drives

Mechanical and hydraulic drives are still usedtoday.

Many solutions have been devised formechanical drives (pulleys/belts, ball bearings,cones, etc). These drives have the disadvantage

Fig. 21: Diagram of a Ward Leonard set

Asynchronous motor

Tachogenerator

DC powered coilMetal drum

Output shaft

Rotor

DC motorGeneratorAC motor

Line supply


of requiring meticulous maintenance and aredifficult to use for servocontrol. These drives facestrong competition from frequency inverters.

Hydraulic drives are still very widely used forspecific applications. They are characterized bysignificant output powers and the ability to

develop high torques at zero speed continuously.In industry they are mainly to be found inservocontrol applications.

This type of drive will not be described here as itdoes not fall within the framework of this study.


The following table gives a quick overview of allthe available electric motors, together with theirmain characteristics and the areas in which theyare used.

We must stress the importance of 3-phasesquirrel cage asynchronous motors. The term

"standard" applied to this type of motor is noweven more appropriate since they are ideal foruses that have arisen from the development ofelectronic devices for variable speed control.

4 Conclusion

Type of motor Squirrel cage asynchronous Slip-ring Synchronous Stepper DC3-phase single phase asynchronous wound rotor rare earth

rotor

Motor cost Low Low High High High Low High

Dust and damp Standard Possible On request, On request, Standard Standard Possible,proof motor expensive expensive very expensive

DOL starting Easy Easy Special starting Impossible Not possible Not possible Not possibledevice above a few kW

Variable speed Easy Very rare Possible Frequent Always Always Alwayscontrol

Cost of variable Increasingly Very Economical Very Fairly Very Veryspeed control economical economical economical economical economical economicalsolution

Speed control Increasingly Very low Average High Very high Medium to high High to veryperformance higher high

Use Constant or Constant speed Constant or Constant or Variable speed Variable speed Variable speedvariable speed for the majority variable speed variable speed

Industrial use Universal For low power Decreasing High power Machine tools, Open loop Decreasingratings ratings at high dynamics position control,

medium voltage for low powers

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