# 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=s_{mT }by solid line. The dotted region from s=1 to s=s_{mT} 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 T_{L1} to T_{L2}. As we the motor torque is now more than the load torque T_{L2}, 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=s_{MT}are not as stable as operating point between s=0 to s= s_{MT}.

Now, we increase the load torque, again from T_{L2} to T_{L1}, 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= s_{MT} 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!

Dear the writer,

I have wondered some points in Case 2 for long time and has no one fullfils it.

My problem is that at standstill ,the starting torque of induction motor is more than that of the load torque TL1, thus the motor will start to rotate resulting in speed increase. According to your explanation, the operating point of the system is certainly at point A. As can be from the figure, there are three intersection points of load-torque characteristic i.e in the middle and point C of the graph, respectively. Why both of them are not the operating point as point A.

I am looking forward to hearing your response soon.

Kind Regards,

Chaiwat Tongchoi

First of all I would like to thank you for asking your doubt. Let me explain: Just start from the beginning. The motor is first at standstill condition with load coupled to its shaft. When you start the motor, as the starting torque is more than the starting load torque, the motor will accelerate till a balance in between the motor torque and load torque is arrived at. This balance between motor torque and load arrives first at point A. After this point the motor will continue with the speed corresponding to point A as there is a perfect balance of load torque and motor torque. The motor will now only change its speed if there is any disturbance in load torque. This is the reason, the operating point is A not middle or point C as these points will never be reached. Did you understand?

Please ask if still in doubt. You voice will never be lost at this blog.

Dear admin,

Thanks so much for your clear explanation. I do not have any question.

Best Regards,

C.Tongchoi

Faculty of Industrial Technology, Lopburi, Thailand.