To draw the torque slip characteristics of induction motor (three phase) following points are considered:
- At synchronous speed (Ns); slip, s = 0 and torque, T = 0.
- When rotor speed is very near to synchronous speed i.e. when the slip is very low the value of the term (sX2)2 is very small in comparison to R22 and is neglected. Therefore, torque is given by the expression:
- As the slip increases torque increases and attains its maximum value when s = R2/X2. This maximum value of torque is also known as break down or pull out torque.
- When a further increase in slip occurs due to increase in load beyond the point maximum torque
i.e. when slip is high, the value of term (sX2)2 is very large in comparison to R22.
Therefore, R22 is neglected as compare to (sX2)2 and torque is given by the expression:
T = ksE22R2 / R22
In the above expression, all quantities are constant except slip s.
Therefore, T α s
Torque Slip Characteristics of Induction Motor
Thus, at low values of slip, torque is approximately proportional to slip s and the torque slip characteristics of induction motor is a straight line as shown in the figure.
The region (from s = 0 to s = sm) is called the stable region of operation and operating point of the motor should be in this region.
In the stable region, the value of slip is small. Hence this region is also called as the low slip region.
T = ksE22R2 / (sX2)2
or T = kE22R2 / (sX22)
In the above expression, all quantities are constant except s.
Therefore, T α 1/s
Thus at higher values of slip (i.e. the slip beyond that corresponding to maximum torque) torque is approximately inversely proportional to slip, s and the torque slip characteristics of induction motor is rectangular hyperbola as shown in the figure.
The region (extending from s = sm to s = 1) is called unstable region. In this region with the increase in load, slip increases but torque decreases.
The result is that the motor could not pick up the load and slows down and eventually stops. In the unstable region, the value of slip is large so this region is also called as the high-slip region.
Effect of Rotor Resistance on Torque Slip Characteristics
To see the effect of rotor resistance on torque slip characteristics of induction motor, consider a slip ring induction motor in which additional resistance in the rotor circuit can be introduced through slip rings. The rotor reactance at stand still X2 remains constant.
The maximum value of the torque developed by an induction motor is independent of rotor resistance R2.
Therefore, the effect of change in rotor resistance is the change in slip at which this maximum torque takes place; greater the rotor resistance, greater the value of slip at which the maximum torque occurs since
s = R2/X2
The torque slip characteristics of induction motor (three phase) are shown in the figure for various values or rotor resistance R2 keeping rotor reactance X2 constant. The maximum value of the torque can be obtained even at the start by adding that much resistance in the rotor circuit so that R2 becomes equal to X2.
When R2 = X2; s = R2/X2 = 1 i.e. at start torque will be maximum.
Thanks for reading about torque slip characteristics of induction motor.
Induction Motors – 7 | Objective Type Question Answers
#1 The probable reason for an induction motor running too hot could be
any of the above
#2 Which of the following motors has the highest power to weight ratio?
#3 In a 3-phase induction motor, iron loss
in rotor is negligible as compared to that in stator
#4 Which of the following parameters for an induction motor varies as square of the supply voltage
Maximum running torque
#5 If an induction motor is to be run on unbalanced supply, then it should be run at
#6 In case single phasing occurs in delta connected motor
one phase will be seriously over-loaded and two others will be slightly overloaded
#7 If single phasing occurs when the motor is running, it should not be loaded beyond
50 per cent of rated load
#8 Which of the following is not determined by circle diagram ?
#9 What is the advantage of the slip ring induction motor over the squirrel cage induction motor?
It can be started with the help of rotor resistances.
#10 What is the advantage of starting a slip-ring induction motor with the help of rotor resistances as compared to other methods ?
The starting torque increases due to the rotor resistances.
#11 What is the advantage of the double squirrel-cage rotor as compared to the round bar cage rotor ?
The starting current of the motor is lower.
#12 Under average service conditions, motor bearings may be safely operated at temperatures up to
#13 The most important factor in motor maintenance is to
keep it clean and free from foreign materials.
#14 The most important factor in motor maintenance is to
keep it clean and free from foreign materials.
#15 Phase advancers are used with induction motors to
improve power factor
#16 In moist surroundings which frame of motor will be preferred ?
Totally enclosed fan cooled
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