Electric Motors

Introduction:Introduction:

Electric motors are used to efficiently convert electrical

energy into mechanical energy. Magnetism is the basis of

their principles of operation. They use permanent magnets,

electromagnets and exploit the magnetic properties of

materials in order to create these amazing machines.

There are several types of electric motors available

today. The following outline gives an overview of several

popular ones. There are two main classes of motors: AC

and DC. AC motors require an alternating current or

voltage source (like the power coming out of the wall

outlets in your house) to make them work. DC motors

require a direct current or voltage source (like the voltage

coming out of batteries) to make them work. Universal

motors can work on either type of power. Not only is the

construction of the motors different, but the means used to

control the speed and torque created by each of these

motors also varies, although the principles of power

conversion are common to both.

They range in power ratings from less than 1/100 hp to

over 100,000 hp.  The rotate as slowly as 0.001 rpm to over

100,000 rpm. They range in physical size from as small as

the head of a pin to the size of a locomotive engine.

Classification of motors:Classification of motors:

D.C Motors

Construction:Construction:

A DC machine can operate as a motor or as a

generator. This kind of machine is usually realized as an

internal rotor/external pole machine. The ring coat shaped

housing of the machine is also used as a magnetic yoke for

the magnetic field through the armature and poles.

The excitation winding (field winding) is located directly

on the main poles of the stator. A current that flows in this

winding generates the main field. Since the machine is

operated with DC current, the magnetic field in the stator is

constant and so all iron parts of the stator can be made of

massive material. Nevertheless the main poles and the

commutating poles are often laminated because of easier

manufacture.

Modern DC machines, used in closed-loop controlled

drives, with a fast change in armature current and main

field consist of one completely laminated magnetic circuit. A

massive iron construction would strongly influence the

dynamics and the efficiency of the machine due to the

appearance of eddy currents. The rotating part of the

machine holds on its shaft the armature with the

commutator.

Since the alternating flux flows through the armature,

iron parts must be built from laminated, mutually insulated

and slotted magnetic steel sheets. The coils of the

armature winding are placed in the slots; their ends are

connected to the commutator segments. The current is fed

into the commutator by carbon brushes. As the rotor

revolves, conductors revolve with it. The brushes contact

the commutator segments.

Basic ConstructionBasic Construction The relationship of the electrical

components of a DC motor is shown in the following

illustration. Field windings are mounted on pole pieces to

form electromagnets. In smaller DC motors the field may

be a permanent magnet. However, in larger DC fields the

field is typically an electromagnet. Field windings and pole

pieces are bolted to the frame. The armature is inserted

between the field windings. The armature is supported by

bearings and end brackets (not shown). Carbon brushes

are held against the commutator.

ArmatureArmature The armature rotates between the poles of the field windings.

The armature is made up of a shaft, core, armature windings,

and commutator. The armature windings are usually for

Wound and then placed in slots in the core.

BrushesBrushes ride on the side of the commutator to provide supply

voltage to the motor. The DC motor is mechanically complex

this can cause problems for them in certain adverse

environments. Dirt on the commutator, for example, can

inhibit supply voltage from reaching the armature. A certain

amount of care is required when using DC motors in certain

industrial applications. Corrosives can damage the

commutator. In addition the action of the carbon brush

against the commutator causes sparks which may be

problematic in Hazardous environments.

Basic DC Motor Operation:Basic DC Motor Operation:

Magnetic FieldsMagnetic Fields

You will recall from the previous section that there are two

electrical elements of a DC motor, the field windings and

armature. The armature windings are made up of current

carrying conductors that terminate at a commutator. DC

voltage is applied to the armature windings through carbon

brushes which ride on the commutator. In small DC motors,

permanent magnets can be used for the stator. However, in

large motors used in industrial applications the stator is an

electromagnet. When voltage is applied to stator windings an

electromagnet with north and south poles is established. The

resultant magnetic field is static (no rotational).

For simplicity of explanation, the stator will be represented

by permanent magnets in the following illustrations.

Magnetic Fields A DC motor rotates as a result of two

magnetic fields interacting with each other. The first field

is the main field that exists in the stator windings. The

second field exists in the armature. Whenever current

flows through a conductor a magnetic field is generated

around the conductor.

Right-Hand Rule for Motors A relationship, known as

the right-hand rule for motors, exists between the main

field, the field around a conductor, and the direction the

conductor tends to move.

If the thumb, index finger, and third finger are held at

right angles to each other and placed as shown in the

following illustration so that the index finger points in the

direction of the main field flux and the third finger points

in the direction of electron flow in the conductor, the

thumb will indicate direction of conductor motion. As can

be seen from the following illustration, conductors on the

left side tend to be pushed up.

Conductors on the right side tend to be pushed down.

This results in a motor that is rotating in a clockwise

direction. You will see later that the amount of force

acting on the conductor to produce rotation is directly

proportional to the field strength and the amount of

current flowing in the conductor.

CEMF Whenever a conductor cuts through lines of flux a

voltage is induced in the conductor. In a DC motor the

armature conductors cut through the lines of flux of the

main field. The voltage induced into the armature

conductors is always in opposition to the applied DC

voltage. Since the voltage induced into the conductor is in

opposition to the applied voltage it is known as CEMF

(counter electromotive force). CEMF reduces the applied

armature voltage.

The amount of induced CEMF depends on many

factors such as the number of turns in the coils, flux

density, and the speed which the flux lines are cut.

Armature Field An armature, as we have learned, is

made up of many coils and conductors. The magnetic

fields of these conductors combine to form a resultant

armature field with a north and South Pole.

The north pole of the armature is attracted to the south

pole of the main field. The south pole of the armature is

attracted to the north pole of the main field. This attraction

exerts a continuous torque on the armature. Even though

the armature is continuously moving, the resultant field

appears to be fixed.

This is due to commutation, which will be discussed

next.

Commutation In the following illustration of a DC motor

only one armature conductor is shown. Half of the

conductor has been shaded Black, the other half white.

The conductor is connected to two Segments of the

commutator.

In position 1 the black half of the conductor is in

contact with the negative side of the DC applied

voltage. Current flows away from the commutator on

the black half of the conductor and returns to the

positive side, flowing towards the commutator on the

white half.

In position 2 the conductor has rotated 90°. At this

position the conductor is lined up with the main field.

This conductor is no longer cutting main field magnetic

lines of flux; therefore, no voltage is being induced into

the conductor. Only applied voltage is present. The

conductor coil is short-circuited by the brush spanning

the two adjacent commutator segments. This allows

current to reverse as the black commutator segment

makes contact with the positive side of the applied DC

voltage and the white commutator segment makes

contact with the negative side of the applied DC

voltage.

As the conductor continues to rotate from position 2 to

Position 3 current flows away from the commutator in

the white half and toward the commutator in the black

half.

Current has reversed direction in the conductor. This is

known as commutation.

Wiring types:Wiring types:

The dynamic behavior of the DC machine is mainly

determined by the type of the connection between the

excitation winding and the armature winding including the

commutation and compensation winding:

1. Separately excited DC machine:

Excitation and armature winding supplied at separate

voltages

2. Shunt DC machine:

Excitation and armature winding are connected in

parallel (i.e. fed by the same source)

2. Series-wound machine:

The excitation and the armature winding connected in

series; if the stator is laminated, series-wound machines

can operate at AC current

3. Compound machine:

This is a combination of 2 and 3 (both shunt and series

winding are available)

Types of DC MotorsTypes of DC Motors

The field of DC motors can be a permanent magnet, or

electromagnets connected in series, shunt, or compound.

1. Permanent Magnet Motors are use permanent

magnets rather than windings in the field section. DC

power is supplied only to the armature.

Permanent magnet motors are not expensive to

operate since they require no field supply. The magnets,

however, lose their magnetic properties over time and

this effect less than rated torque production. Some

motors have windings built into the field magnets that re-

magnetize the cores and prevent this from happening.

Permanent magnet motors produce high torque at low

speed, and are self-braking upon disconnection of

electrical power.

Permanent magnet motors cannot endure continuous

operation because they overheat rapidly, destroying the

permanent magnets.

2. Series Motors In a series DC motor the field is

connected in series with the armature. The field is

wound with a few turns of large wire because it must

carry the full armature current. An increase in load

results in an increase in both armature and field

current. As a result, the armature flux and field flux

increase simultaneously. Since the torque developed in

DC motors is dependent upon the interaction of

armature and field flux, torque increases by the square

of current increase.

Characteristic of series motors is the motor

develops a large amount of starting torque. However,

speed varies widely between no load and full load.

Series motors cannot be used where a constant speed

is required under varying loads.

Additionally, the speed of a series motor with no

load increases to the point where the motor can

become damaged. Some load must always be

connected to a series-

connected motor.

V= Ia*(Ra+Rf) + E If=Ia

E= K*Φ*ω = K*Ia* ω T= K*Φ*Ia = K*Ia^2

3. Shunt Motors

In a shunt motor the field is connected in parallel (shunt)

with the armature windings. The shunt-connected motor

offers good speed regulation. The field winding can be

separately excited or connected to the same source as the

armature. An advantage to a separately excited shunt field is

the ability of a variable Speed drive to provide independent

control of the armature and field. The shunt-connected motor

offers simplified control for reversing. This is especially

beneficial in regenerative drives.

4. Compound Motors Compound motors have a field

connected in series with the armature and a separately

excited shunt field. The series field provides better

starting torque and the shunt field provides better

speed regulation. However, the series field can cause

control problems in variable speed drive applications

and is generally not used in four quadrant drives.

Hint:

To reverse the direction of rotation of d.c motor, it is

necessary to reverse the direction of current through the

armature with respect to the current of field circuit. This is

simply done by reversing either the armature circuit

connection with respect to the field circuit or vise versa.

Reversal of both circuit connections will produce the same

direction of rotation. Usually armature circuit selected for

several reasons:

First: the field is highly inductive circuit and frequent

reversal induces undesirable high emf.

Second: if the shunt field is reversed the series field must

also reversed, otherwise the motor will be differential

compounded.

Third: if the reversing switch is defective and field is fails

to close, the motor may "run away".

Advantages and disadvantages of D.C machinesAdvantages and disadvantages of D.C machines

Advantages:

Easy to understand design

Easy to control speed

Easy to control torque

Simple, cheap drive design

Disadvantages:

Armature reaction

Commutation process

Expensive to produce

High maintenance

Speed Control Of D.C Motor

Introduction:Introduction:

The speed of a DC motor is directly proportional to the

supply voltage, so if we reduce the supply voltage from 12

Volts to 6 Volts, the motor will run at half the speed. How

can this be achieved when the battery is fixed at 12 Volts?

The speed controller works by varying the average

voltage sent to the motor. It could do this by simply

adjusting the voltage sent to the motor, but this is quite

inefficient to do. A better way is to switch the motor’s supply

on and off very quickly. If the switching is fast enough, the

motor doesn't notice it, it only notices the average effect.

When you watch a film in the cinema, or the television,

what you are actually seeing is a series of fixed pictures,

which change rapidly enough that your eyes just see the

average effect - movement. Your brain fills in the gaps to

give an average effect.

The Motor drive divided into two categories:The Motor drive divided into two categories:

1. D.C-D.C converters

1.1. Rheostat

1.2. Choppers

1.2-1.Single quadrant

1.2-2.Two quadrant

1.2-3.Four quadrants

2. A.C-D.C converter (Thyristor Rectifiers)

2.1. Single quadrant

2.2. Two quadrant

2.3. Four quadrants

Methods for adjusting the machine speed:Methods for adjusting the machine speed:

1. Varying the flux, i.e. the excitation current

(concerning the saturation in the excitation circuit, only a

weakening of the flux is possible) the regulation of the

rotational speed at a constant armature voltage is possible

only to speed values above the rated rotational speed, i.e.

beyond the rotational speed at maximum flux. Maximum

permitted excitation current. Limit: mechanical stress

(centrifugal force) and commutation (brush fire, sparking).

2. Reducing the armature voltage Right arrow the

regulation of the rotational Speed is possible only to speeds

below the rated rotation speed, to avoid Possible fire on

the brushes at higher voltages; (voltage switching, e.g. from

220 V to 110 V or supply at DC motor controller, Leonard

set).

3. Increasing Rtot with an additional series resistance R

(starter) in the armature Circuit. This possibility is rarely

used due to the additional losses and strong load

Dependency of the speed.

D.C-D.C converters (Chopper)

Definition: A DC-to-DC converter is a device that accepts

a DC input voltage and produces a DC output with a

desired voltage level. In addition; DC-to-DC converters

are used to provide noise isolation, power bus regulation,

etc.

General block

diagram:

General out lines:

Vavg = (1/T).∫v(t)dt = (td/T).Vm =D.Vm

Where D is the duty cycle defined as (td/T)

single quadrant chopper:

We will start off with a very simple circuit (see the figure

below). The inductance of the field windings and the

armature windings has been lumped together and called La.

The resistance of the windings and brushes is not important

to this discussion, and so has not been drawn.

Q1 is the MOSFET. When Q1 is on, current flows

through the field and armature windings, and the motor

rotates. When Q1 is turned off , the current through an

inductor cannot immediately turn off, and so the inductor

voltage drives a diminishing current in the same direction,

which will now flow through the armature, and back through

D1 as shown by the red arrow in the figure below. If D1

wasn’t in place, a very large voltage would build up across

Q1 and blow it up.

Reversing

To reverse a DC motor, the supply voltage to the

armature must be reversed, or the magnetic field must be

reversed. In a series motor, the magnetic field is supplied

from the supply voltage, so when that is reversed, so is the

field, therefore the motor would continue in the same

direction. We must switch either the field winding’s supply,

or the armature winding’s supply, but not both.

One method is to switch the field coil using relays:

When the relays are in the position shown, current will

flow vertically upwards through the field coil. To reverse the

motor the relays are switched over. Then the current will be

flowing vertically downwards through the field coil, and the

motor will go in reverse.

However, when the relays open to reverse the direction,

the inductance of the motor generates a very high voltage

which will spark across the relay contact, damaging the

relay. Relays which can take very high currents are also

quite expensive. Therefore this is not a very good solution.

A better solution is to use what is termed a full-bridge circuit

around either the field winding, or the armature winding.

We will put it around the armature winding and leave the

field winding in series.

The bridge power converter:

As described in the previous, the speed of a series DC motor

can be altered by varying the voltage applied to its terminal.

One way of varying the applied voltage is by using the pulse-

width modulation (PWM) technique. Using this technique, a

fixed frequency voltage signal with varying pulse-width is

applied to the motor terminal. The following Figure shows an

example of a PWM signal where T is the signal period, td is

the pulse-width, and Vm is the signal amplitude. The average

voltage can be calculated from:

Vavg = (1/T).∫v(t)dt = (td/T).Vm =D.Vm

Where D is the duty cycle defined as (td/T)

From the previous equation it can be seen that the average

(DC component) of the voltage signal is linearly related to the

pulse-width of the signal, or the duty cycle of the signal since

the period is fixed. Therefore, varying the duty cycle of the

signal can alter the voltage applied to the motor terminal.

The PWM voltage waveforms for the motor can be

obtained using a special

power electronic circuit (DC chopper). A DC chopper

basically uses power

switching devices to switch a constant DC voltage on and

off according to a specified

switching scheme in order to obtain the required voltage and

current waveforms. There are various types of DC chopper

configurations.

In this section, we will discuss the DC chopper configuration

which called:

Bridge power converter also known as H-bridge converter.

The schematic diagram of this converter is shown in

following Figure, T1 to T4 are controlled switches that can be

implemented using power semiconductor devices such as

Power MOSFET.

These devices provide low resistance for the current flow

when they are turned on and very high resistance when

turned off. Diodes D1 through D4 provide a path for

preserving the continuity of the current flow when one or

more of the switches are turned off. This is necessary to

protect the power switches from excessive voltage spike due

to the inductive load presented by the DC motor. These

diodes are also known as

Freewheeling diodes.

The DC voltage supply Vm can be obtained from a

rectified ac signal or a DC voltage source such as a car

battery.

A full bridge circuit is shown in the diagram below. Each side

of the motor can be connected either to battery positive, or to

battery negative. Note that only one MOSFET on each side

of the motor must be turned on at any one time otherwise

they will short out the battery and burn out!

To make the motor go forwards:

Q4 is turned on, and Q1 has the PWM signal applied to it.

The current path is shown in the diagram below in red. Note

that there is also a diodes connected in reverse across the

field winding. This is to take the current in the field winding

when all four MOSFETs in the bridge are turned off.

Q4 is kept on so when the PWM signal is off, current can

continue to flow around the bottom loop through Q3's

intrinsic diode:

To make the motor go backwards:

Q3 is turned on, and Q2 has the PWM signal applied to it:

Q3 is kept on so when the PWM signal is off, current can

continue to flow around the bottom loop through Q4's

intrinsic diode:

For regeneration: when the motor is going backwards for

example, the motor (which is now acting as a generator) is

forcing current right through its armature, through Q2's diode,

through the battery (thereby charging it up) and back through

Q3's diode:

Four Quadrant Operation

A.C-D.C Converter(Thyristor rectifiers):

1- One quadrant

2- Two quadrant

3- Four quadrant

Closed Loop Control

A.C motorsThree phase induction motor

Introduction

The Induction motor is a three phase AC motor and is the most

widely used machine. Its characteristic features are-

o Simple and rugged construction

o Low cost and minimum maintenance

o High reliability and sufficiently high efficiency

o Needs no extra starting motor and need not be

synchronized

An Induction motor has basically two parts – Stator and Rotor

Construction

The Stator is made up of a number of stampings with slots to

carry three phase windings. It is wound for a definite number of

poles. The windings are geometrically spaced 120 degrees apart.

Two types of rotors are used in Induction motors - Squirrel-cage

rotor and Wound rotor.  

A squirrel-cage rotor consists of thick conducting bars

embedded in parallel slots. These bars are short-circuited at both

ends by means of short-circuiting rings.

A wound rotor has three-phase, double-layer, distributed

winding. It is wound for as many poles as the stator. The three

phases are wired internally and the other ends are connected to

slip-rings mounted on shaft with brushes resting on them. The

brushes are connected to an external resistance that does not

rotate with the rotor and can be varied to change the N-T

characteristics. In fact an Induction motor can be compared with a

transformer because of the fact that just like a transformer it is a

singly energized device which involves changing flux linkages with

respect to a primary (stator) winding and secondary (rotor)

winding.

Basic equations and equivalent circuit diagram

The stator and rotor of the induction machine both are equipped

with a symmetrical Three phase winding. Because of the symmetry

it is sufficient to take only one phase.

Every phase of the stator and the rotor winding has an active

resistance of R1 and R2, As well as a self-inductance of L1 and

L2.

The windings of the stator and the rotor are magnetically

coupled through a mutual Inductance M.

Since the current flowing in the stator winding has the frequency f1

and the current Flowing in the rotor winding has the frequency f2,

then at the rotor speed n.

• Currents induced from the stator into the rotor have f = f2

• Currents induced from the rotor into the stator have f = f1.

According to this, voltage equations for the primary and

secondary sides can be derived.

The equivalent circuit diagram after the conversion of the rotor

parameters on The stator side is presented

The voltage and current equations are:

With this equivalent circuit diagram, the operational performance of

an induction machine can be completely described. This diagram

is purposely used for the operation with a constant stator flux

linkage, as well as for the operation on network with constant

voltage and frequency.

For normal machines with the network frequency f1 = 50 Hz, the

stator resistance R1Can be neglected:

R1 = 0

At normal operation the windings of slip ring rotor are also short -

circuited through Slip rings and brushes like the squirrel cage rotor.

As far as the skin effect in squirrel Cage rotor is neglected, the

operational performance for both types of the rotor

Theory of operation

As the stator connected to three phase balanced supply, a

balanced current will flow; as a result a rotating magnetic field will

be set up rotating at speed defined as

Ns=60F/pWhere P= # of pairs of poles of stator winding

This field travels past rotor conductors, inducing a voltage in

each conductor. As the rotor winding is short circuited a current will

flow in it. The interaction between stator rotating flux and rotor

currents will set up a torque tending to rotate the rotor in the same

direction of the stator flux rotation. The rotor will flow the stator flux

at a speed Nr which must be kept lees than Ns to maintain torque.

Practically Nr is near Ns during normal operating condition. For

the observer on the rotor surface the stator flux will slip past him.

The slip is defined as (Ns-Nr) and in per unit is

S = (Ns-NR)/NsThe frequency of the voltage induced in rotor winding depends

on the difference between Ns and Nr and is given by

Fr = p (Ns-Nr)/60 = S*Fs

Where Fr = frequency of induced rotor voltage

Fs = frequency of applied stator voltage

Classes of Polyphase Induction motor

The rotor of a polyphase induction machine may be one of two

types; the squirrel cage-rotor, with alternatives for motor classes A,

B, C, D and the wound rotor.

The polyphase induction motor has a squirrel-cage rotor with a

winding consisting of conducting bars embedded in slots in the rotor

iron and short-circuited at each end by conducting end rings. The

extreme simplicity and ruggedness of the squirrel-cage construction

are outstanding advantages of this type of induction motor and

make it by far the most commonly used type of motor in sizes

ranging from fractional horsepower on up.

Design Class A:

Normal Starting Torque, Normal Starting Current, Low Slip

This design usually has a low-resistance, single-cage rotor. It

emphasizes good running performance at the expense of starting.

The full-load slip is low and the full-load efficiency is high. The

maximum torque usually is well over 200 percent of full-load torque

and occurs at a small slip (less than 20 percent). The high starting

current (500 to 800 percent of full-load current when started at

rated voltage) is the principal disadvantage of this design.

Design Class B:

Normal Starting Torque, Low Starting Current, Low Slip

This design has approximately the same starting torque as the

class-A design with but 75 percent of the starting current. Full-

voltage starting, therefore, may be used with larger sizes than with

class A. The starting current is reduced by designing for relatively

high leakage reactance, and the starting torque is maintained by

use of a double-cage or deep-bar rotor. The full-load slip and

efficiency are good, about the same as for the class A design.

However, the use of high reactance slightly decreases the power

factor and decidedly lowers the maximum torque (usually only

slightly over 200 percent of full-load torque being obtainable).

Design Class C:

High Starting Torque, Low Starting Current. This design uses a

double-cage rotor with higher rotor resistance than the class-B

design. The result is higher starting torque with low starting current

but somewhat lower running efficiency and higher slip than the class-

A and class-B designs.

Design Class D:

High Starting Torque, High Slip This design usually has a single-

cage, high-resistance rotor (frequently brass bars). It produces

very high starting torque at low starting current, high maximum

torque at 50 to 100 percent slip, but runs at a high slip at full load (7

to 11 percent) and consequently has low running efficiency.

On the other hand, a wound rotor is built with a polyphase winding

similar to, and wound with the same number of poles as, the stator.

The terminals of the rotor winding are connected to insulated slip

rings mounted on the shaft. Carbon brushes bearing on these rings

make the rotor terminals available external to the motor

Modes of operation

An induction machine has three operation modes:

• Motor (the rotor rotates slower than the rotation field):

M > 0, n > 0, 0 < s < 1

• Generator (the rotor rotates faster than the rotation field):

M < 0, n > n1, s < 0

• Braking operation (the rotor rotates in reverse direction to the

rotating field:

M > 0, n < 0, s > 1

EfficiencyBy neglecting the copper losses in the stator R1 = 0 the efficiency

of an induction

Machine at rated operation is:

To obtain a higher rated efficiency, the rated slip Sn should be as

small as possible. In Practice, under the consideration of the stator

copper losses and the iron losses, the Efficiency reaches a value

between 0.8 - 0.95.

Single-Phase Induction Motor

Single-Phase Theory

Because it has but a single alternating current source, a single-

phase motor can only produce an alternating field: one that pulls

first in one direction, then in the opposite as the polarity of the field

switches. A squirrel-cage rotor placed in this field would merely

twitch, since there would be no moment upon it. If pushed in one

direction, however, it would spin.

The major distinction between the different types of single-

phase AC motors is how they go about starting the rotor in a

particular direction such that the alternating field will produce rotary

motion in the desired direction. This is usually done by some

device that introduces a phase-shifted magnetic field on one side

of the rotor.

Split-Phase Motors

The split phase motor achieves its starting capability by having

two separate windings wound in the stator. The two windings are

separated from each other. One winding is used only for starting

and it is wound with a smaller wire size having higher electrical

resistance than the main windings. From the rotor's point of view,

this time delay coupled with the physical location of the starting

winding produces a field that appears to rotate. The apparent

rotation causes the motor to start. A centrifugal switch is used to

disconnect the starting

winding when the motor

reaches approximately 75% of

rated speed. The motor then

continues to run on the basis

of normal induction motor

principles.

Capacitor-Start Motors

Capacitor start motors form the largest single grouping of

general purpose single phase motors. These motors are available

in a range of sizes from fractional through 3HP. The winding and

centrifugal switch arrangement is very similar to that used in a split

phase motor. The main difference being that the starting winding

does not have to have high resistance. In the case of a capacitor

start motor, a specialized capacitor is utilized in a series with the

starting winding. The addition of this capacitor produces a slight

time delay between the magnetization of starting poles and the

running poles. Thus the appearance of a rotating field exists.

When the motor approaches running speed, the starting switch

opens and the motor continues to run in the normal induction

motor mode.

This moderately priced motor produces relatively high starting

torque, 225 to 400% of full load torque. The capacitor start motor is

ideally suited for hard to start loads such as conveyors, air

compressors and refrigeration compressors. Due to its general

overall desirable characteristics, it also is used for many

applications where high starting torque may not be required. The

capacitor start motor can usually be recognized by the bulbous

protrusion on the frame

where the starting capacitor

is located

Capacitor start capacitor run

These motors have a run capacitor and an auxiliary winding

permanently connected in parallel with the main winding. In

addition, a starting capacitor and a centrifugal switch are also in

parallel with the run capacitor. The switch disconnects as the

motor accelerates. It should be noted that the capacitor start-

capacitor run motor utilizes the same winding arrangement as the

permanently split capacitor motor when running a full load speed

and the same winding arrangement as a capacitor-start Motor

during startup.

The advantage of the capacitor start-capacitor run design is

derived from the fact that the start winding and capacitor remain in

the circuit at all times (similar to PSC type motor) and produce an

approximation of two-phase operation at the rated load point, plus

with an additional capacitor in series with the start winding circuit

(similar to the capacitor-start type motor), the starting current now

leads the line voltage, rather than lagging as does the main

winding, dramatically increasing starting torque. Capacitor Start-

capacitor run motors feature a low running current due to an

improved power factor caused by the run capacitor.

This results in better efficiency, better

power factor, increased starting torque

and lower 120 Hz torque pulsations than

in equivalent capacitor-start and split-phase designs. The capacitor

start-capacitor run motor is basically a combination of the

capacitor-start and PSC motor types and is the best of the single-

phase motors.

Permanent-Split Capacitor Motors

The capacitor of this motor is left in series with the starting

winding during normal operation. The starting torque is quite low,

roughly 40% of full-load, so low-inertia loads such as fans and

blowers make common

applications. Running

performance and speed

regulation can be tailored by

selecting an appropriate

capacitor value. No centrifugal

switch is required.

Shaded-Pole Motors

The shaded pole motor is the simplest of all single phase

starting methods. In the shaded pole motor, the stator poles are

notched and a copper short circuiting ring is installed around a

small section of the poles.

As a result of the alteration of the filed pole configuration, the

build-up of the magnetic field is delayed in the portion of the pole

surrounded by the copper shorting ring. From the rotor's point of

view, this makes the magnetic field seem to rotate from the main

pole toward the shaded pole. This slight appearance of field

rotation is adequate to start the rotor moving and, once started, it

will accelerate up to full speed.

The shaded pole motor is simple and inexpensive, but has low

efficiency and a very low starting torque. Speed regulation is poor,

and it must be fan-cooled during normal operation. Shaded-pole

motors are thus used in shaft-mounted fans and blowers, and also

small pumps, toys, and intermittently used household items.

Advantages & Disadvantages

Advantages:

- Simple & robust construction

- Can run directly from the main supply

- Power electronic may be applied to improve the performance of

the motor

- Brushless

- Low cost and minimum maintenance

- High reliability and sufficiently high efficiency

Disadvantages:

- Difficult model to understand and complicated to compute

simulation

- Cogging & crawling phenomenon

- Its complicate to apply speed control

Speed control of induction motor

techniques

Pulse Width Modulated (PWM)

Figure shows a block diagram of the power conversion unit in a

PWM drive. In this type of drive, a diode bridge rectifier provides

the intermediate DC circuit voltage. In the intermediate DC circuit,

the DC voltage is filtered in a LC low-pass filter. Output frequency

and voltage is controlled electronically by controlling the width of

the pulses of voltage to the motor.

Essentially, these techniques require switching the inverter

power devices (transistors or IGBTs) on and off many times in

order to generate the proper RMS voltage levels.

This switching scheme requires a more complex regulator than

the VVI. With the use of a microprocessor, these complex

regulator functions are effectively handled. Combining a triangle

wave and a sine wave produces the output voltage waveform.

The triangular signal is the carrier or switching frequency of the

inverter. The modulation generator produces a sine wave signal

that determines the width of the pulses, and therefore the RMS

voltage output of the inverter.

AC drives that use a PWM type schemes have varying levels

of performance based on control algorithms. There are 4 basic

types of control for AC drives today. These are Volts per Hertz,

Flux Vector Control, and Field Oriented Control.

V/Hz control is a basic control method, providing a variable

frequency drive for applications like fan and pump. It provides fair

speed and torque control, at a reasonable cost.

Sensor-less Vector control provides better speed regulation, and

the ability to produce high starting torque.

Flux Vector control provides more precise speed and torque

control, with dynamic response.

Field Oriented Control drives provide the best speed and torque

control available for AC motors. It provides DC performance for AC

motors, and is well suited for typical DC applications.

Volts/Hertz

Volt/Hertz control in its simplest form takes a speed reference

command from an external source and varies the voltage and

frequency applied to the motor. By maintaining a constant V/Hz

ratio, the drive can control the speed of the connected motor.

Typically, a current limit block monitors motor current and

alters the frequency command when the motor current exceeds a

predetermined value. The V/Hz block converts the current

command to a V/Hz ratio. It supplies a voltage magnitude

command to the voltage control block. The angle of this tells the

voltage where it should be with respect to current. This determines

flux current to the motor. If this angle is incorrect, the motor can

operate unstable. Since the angle is not controlled in a V/Hz drive,

low speeds and unsteady states may operate unsatisfactorily. An

additional feature in newer drives, a “slip compensation” block, has

improved the speed control. It alters the frequency reference when

the load changes to keep the actual motor speed close to the

desired speed.

While this type of control is very good for many applications, it

is not well suited to applications that require higher dynamic

performance, applications where the motor runs at very low

speeds, or applications that require direct control of motor torque

rather than motor frequency.

V/Hz Speed vs. Torque

The plot above shows the steady state torque performance of

a Volts/Hertz drive. A torque transducer directly on the motor shaft

supplied the data that is plotted. The drive is given a fixed

speed/frequency reference. Then load on the motor is increased

and actual shaft torque is monitored.

Notice that the ability of the drive to maintain high torque

output at low speeds drops off significantly below 3 Hz. This is a

normal characteristic of a Volts/Hertz drive and is one of the

reasons that the operating speed range for Volts/Hertz drives is

typically around 20:1.

As the load is increased, the motor speed drops off. This is not

an indication of starting torque. This only shows the ability of the

drive to maintain torque output over a long period of time.

Sensor-less Vector

Sensor-less Vector Control, like a V/Hz drive, continues to

operate as a frequency control drive, with slip compensation

keeping actual motor speed close to the desired speed.

The Torque Current Estimator block determines the percent of

current that is in phase with the voltage, providing an approximate

torque current. This is used to estimate the amount of slip,

providing better speed control under load.

The control improves upon the basic V/Hz control technique by

providing both a magnitude and angle between the voltage and

current. V/Hz drives only control the magnitude. V-angle controls

the amount of total motor current that goes into motor flux enabled

by the Torque Current Estimator. By controlling this angle, low

speed operation and torque control is improved over the standard

V/Hz drive

Flux Vector

The flux vector control retains the Volts/Hertz core and adds

additional blocks around the core to improve the performance of

the drive. A “current resolver” attempts to identify the flux and

torque producing currents in the motor and makes these values

available to other blocks in the drive. A current regulator that more

accurately controls the motor replaces the current limit block.

Notice that the output of the current regulator is still a frequency

reference.

The early versions of Flux vector required a speed feedback

signal (typically an encoder) and also detailed information about

the motor in order to properly identify the flux and torque currents.

This led to the requirement for “matched motor/drive”

combinations.

While there is nothing inherently wrong with this approach, it does

limit the users motor choices and does not offer independent

control of motor flux and torque.

Flux vector control improves the dynamic response of the

drive and in some cases can even control motor torque as well as

motor speed. However, it still relies on the basic volts/Hertz core

for controlling the motor.

Recently, flux vector control has been enhanced to allow the

drive to operate without the use of a speed feedback device,

relying instead on estimated values for speed feedback and slip

compensation. Again, the basic Volts/Hertz core is retained.

Field Oriented Control

What distinguishes a product using Field Oriented Control

from a traditional vector product is its ability to separate and

independently control (or regulate) the motor flux and torque.

Notice that in the definition of Field Oriented Control we did not say

“currents in an AC motor”. That’s because the concept applies

equally well to DC motors and is the reason we can demonstrate

“DC like” performance using Field Oriented Control on AC drives.

Force Technology uses patented, high bandwidth current

regulators in combination with an adaptive controller, to separate

and control the motor flux and torque. This is a fundamental

difference between Force Technology and other vector control

techniques.

A high bandwidth current regulator that separates and controls

the components of stator current replaces the Volts/Hertz core.

The high bandwidth characteristics of this control eliminate

nuisance trips due to shock loads and continuously adapt to

changes in the motor and load characteristics.

A separate adaptive controller uses information gained during

auto tuning, actual reference information, and motor feedback

information to give independent torque and flux control. This allows

continuous regulation of the motor speed and torque.

Also notice that Force Technology generates separate flux

and torque references to improve the overall control of those

quantities.

Sensor-less Field Oriented Control

As with flux vector products the newest versions of Force

Technology allow users to control the motor without the use of a

speed-sensing device. A major difference is that the drive

continues to operate with Field Oriented control, instead of

reverting back to Volts/Hertz control.

This provides significant benefits with dynamic performance,

trip less operation, and torque regulation.

Below is a plot of a drive using the Sensor-less version of

Force Technology. Notice that the torque output is consistent from

no load to full load over a very wide speed range.

You can also see that the motor has a speed/torque

characteristic that is very similar to its DC counterpart, even when

operating above base speed.

Performance Comparison

The graph below shows a drive using Force Technology

operating with and without an encoder, and a Volts/Hertz drive.

Notice that there is very little difference in operation with or without

an encoder. You can

clearly see the response

to the step load and the

recovery time. The

same can be seen when

the load is removed.

Comparison

between D.C

&A.C Drives

D.C A.C

Weight Heavy Light

Size Large Small

Cost Expensive Less expensive

Starting torque High Low

High speed Not used Suitable

Control Simple Complex

Feed back signal Available Complicated

Types Separately,

series,

shunt

Induction,

Synchronous