Question

With the increase of output power, analyze the changes of mechanical loss, stator core loss and rotor core loss of an induction motor.

Answer 1

Lets First Derive a relation between the Output Power and the losses, in an Induction Motor.

The various losses in the stator of an induction motor are stator core loss or iron loss and stator copper loss. whereas in rotor, the different losses are rotor copper loss and rotor iron loss or core loss and other friction loss or mechanical losses.

Here we are considering only the stator iron loss , rotor iron loss and mechanical losses only. we know that the stator iron loss or core loss depends on two factors namely, flux density and supply frequency.

The equation for the stator core loss is given by,

Stator Core Loss , Pc = 3 * ( V² ) / Rfe

Therefore from the above formula we can come to a conclusion that when the output power of the induction motor is increased, then the stator core loss is also increased. ( since the output power is directly proportional to the Stator Voltage ).

Now considering the rotor core loss,

The rotor core loss depends upon the frequency of the rotor current, whereas the frequency of the rotor current depends on the slip or the speed of the rotor.

Since the Rotor speed is almost equal to the Rotating magnetic Field (RMF), thus the frequency of the rotor current is very small (since the difference between the rotor speed and the RMF is very small).

Therefore the Rotor Iron loss or the rotor core loss is negligible. and almost remains constatnt with increase in output power of the motor.

Mechanical power developed = Rotor input power - rotor copper losses

To observe the changes in mechanical loss lets consider the rotor gross output as,

Rotor gross output = rotor input - rotor copper losses.

we know that rotor copper loss = slip * rotor input

thereby we get,

Rotor gross output = ( rotor input - slip ) * rotor input

= ( 1 - slip ) * rotor input

we also know that slip = Rotor speed - synchronous speed / synchronous speed

therefore,

Motor output power = Rotor output power - mechanical losses

then we can say that when the output power is increased , the mechanical losses are almost constant for the given induction motor .