Understanding Induction Motor Stability


The concept of stability of Induction Motor is very important and vital for interview point of view or from selection of a particular Induction Motor for a given load. In all Motors, the speed drops if we increase the load torque as the power of Induction Motor is constant (Power = TorquexSpeed).

For getting a operating point for a given load torque, the point of intersection of load torque characteristics and Motor speed Torque characteristics determines the point of stable operation of Motor. But this does not guarantee that the point of intersection will be stable one.


Suppose the Motor characteristics is such that as the load torque increases the speed of Motor increases then obviously this Motor and load combination will not result into stable operation.

Thus it is very important for stable operation of the system consisting of Motor and Load that as the Load Torque increases Motor speed decreases and vice versa.

Normally an Induction Motor is designed to operate at low full load slip ranging from 0.02 to 0.05 under normal operating condition.



Figure above shows the slip-torque characteristics of an Induction Motor. Normal operating region is shown from s=0 to s=smT by solid line. The dotted region from s=1 to s=smT is not used for the operation.

Now we will consider some cases to have full understanding of stability of Induction Motor.

Case1:

Consider the slip-torque and load torque curve as shown in figure below.



As can be seen from the figure above, the load curve and the slip-torque characteristics intersect at point C, so this point C will be the operating point but will this operating point be stable operation? To answer this, we increase the load torque, as shown by the dotted line. As we see, as the load torque increases, the new operating point becomes D where the slip is more when compared with point C which in turn means that speed of Induction Motor has decreased. Hence we can say from the definition of Stability of Induction Motor that operating point C is stable.

Case2:

Consider the slip-torque and load torque curve as shown in figure below.



Carefully observe that, the point of intersection of load torque curve and the slip-torque characteristics is A. Now we decrease the load torque from TL1 to TL2. As we the motor torque is now more than the load torque TL2, the motor will accelerate till it reaches the operating point B. Mind that there is a far variation in the speed of Motor at point A and B. At point A the slip is near 1 while at point B, slip is near zero. Therefore we can say that operating point between s=1 to s=sMTare not as stable as operating point between s=0 to s= sMT.

Now, we increase the load torque, again from TL2 to TL1, the operating point shifts to point C. As the speed of motor at C is less than that of speed at B, hence operating point B is stable one. Also note that in this case, the variation in speed at B and C is less, that is why we say as operating point between s=0 to s= sMT is more stable.

Case3:

Consider the slip-torque and load torque curve as shown in figure below.



In this case the load torque requirement is more than the Induction Motor starting torque so the Motor will not start at all and hence no question for stability analysis of operating point.


Please write in comment box if you have any question. Thank you!

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