Electrical Machines: Part 2b (iii): Torque & speed characteristics of an induction motor
This is the second part, of topic 2 in the series of "Electric Machines”.
This is in continuation of our discussion on induction motors in the preceeding subparts, in which we discussed slip and equivalent circuit of
induction motors and in this part we will discuss
torque-speed curve, maximum or pullout torque, and
variations in torque speed characteristics of induction motor due to its rotor resistance.
So let’s First follow proceed with the torque speed curve of an induction motor. The figure shows the torque speed curve of an induction
motor which is a plot of induction motor’s synchronous speed, which is plotted on x axis vs its induced torque, expressed as a % of full
load torque, which is plotted on y axis.
Following this curve from the beginning:
we can see in that , the starting torque of the induction motor is almost 200% of its full load torque, or we can say, starting torque is slightly
greater than full load torque. This curve increases linearly up to the level of
pull out torque which is the value of torque that theoretically can not be exceeded by the motor. This pull out torque is around 2 to 3
times of full load torque. Then the curve dips down sharply and reaches
zero at synchronous speed because torque developed at synchronous speed is zero Carrying this further, if motor’s speed is
increased beyond synchronous speed then machine enters into generator region and direction of induced torque is reversed.
Now let’s look at maximum or pullout torque of an induction motor in a bit more detail.
The figure below shows the effect of rotor resistance on motor’s pullout speed and maximum torque. It
can be observed that if the rotor resistance is increased, the pullout speed of the motor decreases, but the maximum torque remains
constant. We can say that maximum induced torque occurs when power consumed by rotor resistor is maximum. Let’s now have a brief
look on variations in torque speed characteristics of induction motor due to its rotor resistance.
If rotor of induction motor is designed with high resistance then motor’s starting torque is very high, but also, at the same time, slip is very
high in normal operating conditions. This increased slip reduces converted power and so motor’s efficiency is reduced,
Opposite to this scenario, when rotor is designed with low resistance then motor’s starting torque is low and slip is also low in
normal operating conditions which increases motor’s efficiency.: Both of these scenarious are illustrated in the figure:
Now, There is a need to reach compromise between high starting torque and good efficiency which can be done by using different
techniques. One of them is to use wound rotor induction
motor with extra resistance inserted in the starting so as to have high starting torque, but it will need to be removed this extra resistance
during normal operation for achieving high efficiency. Problem is that, wound rotor induction motors are expensive and require
high maintenance. Also sometimes it is required to completely seal the motor if it is placed in hazardous environment which is
possible only in the case of completely self contained rotor.
That’s all from this part. In our next video we will discuss about synchronous motors.