CHAPTER 3
INDUCTION MOTOR AND DIFFERENT SPEED CONTROL
METHODS
Page No
3.1
Introduction
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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
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3.2.4
Equivalent Circuit
63
3.2.5
Slip
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3.3
Different Speed Control Methods
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3.3.1
Stator voltage control method
66
3.3.2
Frequency control method
68
3.3.3
Volts Hertz (V/F) control method
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3.4
Conclusion
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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
Where
- - - - - - - - - - - - - - - (3.1)
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
---------------
Stator copper loss = Pls = 3 I2SRs
(3.2)
--------------- (3.3)
Core Loss =
--------------
(3.4)
Power across air gap = pg = 3
--------------
(3.5)
Rotor copper loss: Plr = 3
--------------
(3.6)
Rr
Output power: Po = Pg - Plr = 3
Rr (
)
-------------- (3.7)
Since the output power is the product of developed torque T e 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
where,
constant flux
frequency
3.2.5
>>
------------------ (3.13)
Equation (3.11 ) is important because it indicated that at
, the torque Te is proportional to slip frequency
, or at constant slip
, torque Te is proportional to Ψm2.
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 =
or rotor speed
=
--------------- (3.15)
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 airgap 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, variablevoltage 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.
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