Answer MET Question 16

Question: How does the change in rotor-frequency affect the induction motor when the load is changed from no-load to full-load?

Answer: we know the relation f'=sf.

where,

f' =rotor current frequency

s = slip

f = stator current frequency

From slip-torque graph:

from right side of the graph we observe with increase speed of rotor or slip decrease, the torque is also increasing. The torque of motor reaches a maximum value when $\displaystyle \small \mathrm{R_2 = sX_2}$ but as we proceed more left on the graph the torque starts to reduce and slip approaches to zero and speed to 100%.

It can be concluded the rotor frequency varies from zero to a maximum value equals to stator frequency. But this is not a practical case as the slip can never be zero in an induction motor.

Second thing in the graph to be noted is the Load-Torque curve. It shows that as the speed of motor is accelerating the load torque is increasing. This implies the slip is reducing with the load torque, so the rotor frequency  will decrease with load torque.