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.

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if you have any question. Thank you!