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CHAPTER 3

INDUCTION MOTOR AND DIFFERENT SPEED CONTROL

METHODS

Page No

3.1 Introduction 58

3.2 Three Phase Induction Motor 58

3.2.1 Stator 60

3.2.2 Rotor 60

3.2.3 Working principle of three phase induction motor 62

3.2.4 Equivalent Circuit 63

3.2.5 Slip 65

3.3 Different Speed Control Methods 66

3.3.1 Stator voltage control method 66

3.3.2 Frequency control method 68

3.3.3 Volts Hertz (V/F) control method 68

3.4 Conclusion 71

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CHAPTER 3

INDUCTION MOTOR AND DIFFERENT SPEED CONTROL

METHODS

3.1 Introduction

This chapter describes the construction, principal of operation, start up consideration

and the basic speed control methods for induction motor.

3.2 Three Phase Induction Motor (IM)

The three-phase induction motors are also called as asynchronous motors, which are

most commonly used type of motor in industrial applications. In particular, the squirrel-Cage

induction motors are widely used electric motor in home and industrial applications, because

these machines are very economical, rugged and reliable. They are available in the ranges of

Fractional Horse Power (FHP) to multi-megawatt capacity. Fractional Horse Power motors

are available in single-phase as well as poly-phase (three-phase).The three phase machines

are used most often in variable-speed drives where the torque requirement is more.

An induction motor or asynchronous motor is a type of alternating current motor

where power is supplied to rotor by means of electromagnetic induction. An electric motor

rotates because of magnetic force exerted between a stationary electromagnet called the stator

and rotating electromagnet called the rotor. The current in stator side creates an

electromagnetic field which interacts with the secondary to produce a resultant torque,

transforming electrical energy into mechanical energy. The cross sectional view of induction

motor and its various parts [4] are as shown in Fig 3.1

Construction:

In this section, the construction details of induction motor are discussed. A three

phase induction motor mainly consists of two parts, stator and rotor. Stator is the stationary

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part while the rotor is the rotating part of the motor and they are separated by a small air gap

depending on the rating of the motor.

Fig. 3.1: Squirrel-cage three-phase Induction motor

Fig. 3.2 Stator

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3.2.1 Stator

The stator is as shown in Fig 3.2, which consists of a steel frame which encloses a

hollow cylindrical core made up of thin laminations of silicon steel to reduce eddy current

and hysteresis loss. A large number of uniform slots are cut on the inner periphery of the

core. The stator conductors are placed in these slots which are insulated from one another and

also from the slots. These conductors are connected as a balanced three phase star or delta

winding. The windings are wound for a definite number of poles depending on the

requirement of speed. It is wound for more number of poles, if speed required is less and vice

versa. According to the relation

Ns = 120f / P - - - - - - - - - - - - - - - (3.1)

Where Ns is the synchronous speed in RPM

f is the supply frequency

P is the number of poles

When a three phase supply is given to stator winding a magnetic field of constant

magnitude and rotating at synchronous speed is produced. This rotating magnetic field is

mainly responsible for producing the torque in the rotor, so that it can rotate at the rated

speed.

3.2.2 Rotor

The rotor is the rotating part of the induction motor and is mounted on the shaft of the

motor to which any mechanical load can be connected. Based on the construction of the

rotor, induction motors are broadly classified in two categories; squirrel cage motors and slip

ring motors. The stator construction is the same in both motors.

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Squirrel cage motors:

Fig. 3.3: Squirrel cage type rotor

Almost 90% of induction motors are squirrel cage motors. This is because the squirrel

cage motor has a simple and rugged construction. Fig 3.3 shows the squirrel cage type rotor,

it consists of cylindrical laminated core with axially placed parallel slots for carrying rotor

conductors. The rotor conductors are heavy bars of copper or aluminium. Each slot carries a

copper, aluminium or alloy bar. If the slots are semi closed, then these bars are inserted from

the ends. These rotor bars are permanently short-circuited at both ends by means of the end

rings, thus short circuiting themselves at both the ends. As rotor bars are short circuited on

themselves, it is not possible to add any external resistance in series with rotor circuit during

starting. The slots are slightly skewed, which helps in two ways i.e.

It reduces noise due to magnetic hum and makes motor run quietly.

It reduces locking tendency between rotor and stator.

Advantages of squirrel cage rotor:

1. It is simple in construction, rugged and can withstand rough handling.

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2. Maintenance cost is low.

3. It has better efficiency and power factor.

4. A simple star delta starter is sufficient to start the rotor.

5. It is explosion proof as there are no slip rings and brushes.

Slip ring motors:

In the slip ring motor, the windings on the rotor are terminated to three insulated slip

rings mounted on the shaft with brushes resting on them. This allows an introduction of an

external resistor to the rotor winding. The external resistor can be used to boost the starting

torque of the motor and change the speed-torque characteristic. When running under normal

conditions, the slip rings are short-circuited, using an external metal collar, which is pushed

along the shaft to connect the rings. In normal conditions, the slip ring motor functions like a

squirrel cage motor.

3.2.3 Working principle of three phase induction motor

When a three phase supply is given to the three phase stator winding, a magnetic field

of constant magnitude and rotating at synchronous speed Ns is produced. This rotating

magnetic field sweeps across the rotor conductors and hence an electromagnetic force (EMF)

is induced in rotor conductors. As the rotor conductors are short circuited on themselves the

induced EMF sets up a current in the rotor conductors in such a direction as to produce a

torque, which rotates the rotor in same direction as magnetic field so that relative speed

decreases. The speed of rotor gradually increases and tries to catch up with the speed of

rotating magnetic field, but it fails to reach synchronous speed, because if it catches up with

speed of magnetic field, relative speed becomes zero and hence no EMF will be induced in

the rotor conductors, the torque becomes zero. Hence, rotor will not be able to catch up with

the speed of magnetic field but rotates at a speed Nr which is slightly less than the

synchronous speed [3].

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3.2.4 Equivalent Circuit

Fig.3.4 Per phase equivalent circuit of induction motor

The equivalent circuit [3] is as shown in Fig. 3.4. From the equivalent circuit diagram the

various power expressions can be written as follows:

Input power = 3VsIscos Φ --------------- (3.2)

Stator copper loss = Pls = 3 I2SRs --------------- (3.3)

Core Loss = -------------- (3.4)

Power across air gap = pg = 3 -------------- (3.5)

Rotor copper loss: Plr = 3 Rr -------------- (3.6)

Output power: Po = Pg - Plr = 3 Rr ( ) -------------- (3.7)

Since the output power is the product of developed torque Te and speed ωm, Te can be

expressed as

Te =

Te = Rr ( ) = 3 ------------- (3.8)

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From the equivalent circuit, the approximate equivalent circuit can be obtained as

shown in Fig 3.5, where the core loss resistor Rm has been dropped and the magnetizing

inductance Lm has been shifted to the input. This approximation is easily justified for an

integral horsepower machine, where (Rs + j Lls )<< Lm[5].

The performance prediction by the simplified circuit typically varies within 5 percent

from that of the actual machine

Fig. 3.5 Approximate per phase equivalent circuit of IM

In Figure 3.5, the current Ir is figured out by:

Ir = ---------------- (3.9)

Substituting Equation (3.9) in (3.8) yields

Te = 3 ---------------- (3.10)

A further simplification of the equivalent circuit of Fig 3.4 can be made by neglecting

the stator parameters Rs and Lls. This assumption is not unreasonable for an integral

horsepower machine, particularly if the speed is typically above 10 percent. Then, the

equation (3.10) can be simplified as

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Te = 3 ) ------------------ (3.11)

where, ωsl = sωe

The air gap flux can be given by

---------------- (3.12)

in a low-slip region, equation (3.11) can be approximated as

Te = 3 ------------------ (3.13)

where, >> Equation (3.11 ) is important because it indicated that at

constant flux , the torque Te is proportional to slip frequency , or at constant slip

frequency , torque Te is proportional to Ψm2.

3.2.5 Slip

The slip can be defined as the difference between the synchronous speed and actual

speed of the machine. It can be expressed in the percentage. Based on this slip speed, the

voltage induced in the rotor winding changes, which in turn changes the rotor current and

also the torque. As slip increases, the rotor current and the torque also increases. The rotor

moves in the same direction as that of the rotating magnetic field to reduce the induced

current (Lenz’s law). The slip can be expressed as given below [1][2]:

---------------- (3.14)

or Slip S = = --------------- (3.15)

or rotor speed Nr = Ns (1-S) -------------- (3.16)

Synchronous speed is given by Ns = ------------- (3.17)

Where P represents the number of poles and f is stator frequency in Hz

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Therefore equation 3.16 becomes,

Rotor speed Nr= (1-S) ------------------- (3.18)

Thus, the speed of an induction motor depends on slip ‘S’, stator frequency ‘f’ and the

number of poles ’P’ for which the windings are wound.

3.3 Different Speed Control Methods

From equation 3.18, the speed of IM can be varied by varying the slip ‘S’ or number

of poles ‘p’ or frequency of supply. The different methods of speed control of induction

motor can be broadly classified in to scalar and vector control methods. In this work, scalar

control methods are used. Hence only details of scalar methods are discussed here. The

explanation of vector control method is beyond the scope of this thesis. The scalar methods

of speed control [4] can be classified as

Stator voltage control

Frequency control

Stator voltage and frequency control i.e Volts Hertz control

Rotor voltage control

The first three methods are the basic methods of speed control and are explained in

detail as follows:

3.3.1 Stator voltage control method

A very simple and economical method of speed control is to vary the stator voltage at

constant supply frequency. The three-phase stator voltage at line frequency can be controlled

by controlling the switches in the inverter. As seen from the equation (3.10) the developed

torque is proportional to the square of the stator supply voltage and a reduction in stator

voltage will produce a reduction in speed. Therefore, continuous speed control may be

obtained by adjustment of the stator voltage without any alteration in the stator frequency.

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Te = 3

---------- (3.19)

The Torque speed curves with variable stator voltage [3] control are shown in Fig 3.6.

Fig.3.6: Speed-Torque characteristics with variable stator voltage

The salient features of stator voltage control method are:

For low-slip motor, the speed range is very low.

Not suitable for constant-torque load.

Poor power factor.

Used mainly in low power applications, such as fans, blowers, centrifugal

pumps, etc.

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3.3.2 Frequency control method

The torque and speed of induction motors can be controlled by changing the supply

frequency but keeping the voltage constant. If the frequency is decreased keeping voltage

constant, then saturation of air-gap flux takes place. At low frequency, the reactance will

decrease and the motor current may be too high. If the frequency is increased above its rated

value, then the air gap flux and rotor current decreases correspondingly, the developed torque

also decreases. Due to these reasons this method of control is rarely used.

The torque speed characteristics with frequency control [3] are shown in Fig.3.7.

Fig.3.7 Speed-Torque characteristics with variable frequency control

3.3.3 Volts Hertz (V/F) control method

The constant V/F control method is the most popular method of Scalar control. If an

attempt is made to reduce the supply frequency at the rated supply voltage, the air gap flux

Ψm will tend to saturate, causing excessive stator current and distortion of flux wave.

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Therefore, the region below the base or rated frequency should be accompanied by the

proportional reduction of stator voltage so as to maintain the air gap flux constant. If the ratio

of voltage to frequency is kept constant, the flux remains constant. By varying the voltage

and frequency the torque and speed can be varied. The torque is normally maintained

constant while the speed is varied. This arrangement is widely used in the locomotives and

industrial applications. The purpose of the volts hertz control scheme is to maintain the air-

gap flux of AC Induction motor constant in order to achieve higher run-time efficiency. The

magnitude of stator flux is proportional to the ratio of stator voltage & the frequency. If ratio

is kept constant the stator flux remains constant & motor torque will only depends upon slip

frequency. In variable-frequency, variable-voltage operation of a drive system, the machine

usually has low slip characteristics (i.e low rotor resistance), giving high efficiency. In spite

of the low inherent starting torque for base frequency operation, the machine can always be

started at maximum torque as indicated in Fig 3.8.The absence of high in-rush starting current

in a direct-start drive reduces stress and therefore improves the effective life of the machine.

By far the majority of variable-speed ac drives operate with a variable-frequency, variable-

voltage power supply.

Fig 3.8 shows the torque-speed characteristics [3] of the machine with constant V/F

control method.

Other than the variation in speed, the torque-speed characteristics of the V/F control

reveal the following:

The starting current is low.

The stable operating region of the motor is increased. Instead of simply running at its

base/ rated speed (NB), the motor can be run typically from 5% of the synchronous

speed (NS) up to the base speed. The torque generated by the motor can be kept

constant throughout this region.

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Since almost constant rated torque is available over the entire operating range, the

speed range of the motor becomes wider. User can set the speed as per the load

requirement, thereby achieving the higher efficiency.

One of the most advantages is soft start capability in which motors are ramped up to

speed instead of being abruptly thrown on line. This useful feature reduces

mechanical stresses on the motor and leads to lower maintenance cost as well as a

longer motor life.

Because of above reasons V/F control method is proposed in this work.

Fig.3.8 Speed-Torque curves with constant V/F ratio

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3.4 Conclusion

The construction, different parts of induction motor i.e stator and rotor, different

types of induction machines, advantages of squirrel-cage induction motor over wound-rotor

induction motor, the working principle, equivalent circuit of induction motor, various power

expressions, slip, the different speed control methods of induction motor and the speed v/s

torque characteristics are discussed in this chapter.