Induction motor ratings

[thinker]

How to explain the fact that AC induction motors for dynamometer applications typically have HP (or KW) ratings for absorbing mode greater than for a motoring mode? At the same time some of motors have identical ratings for both modes of operation. Is there a difference in motor design?

 

[Skogsgurra]

Answer to your first question: The name-plate power is shaft power. It is electric power that heats the motor/generator and limits available power in the machine.

So, when motoring, you have a certain electric power - say P1 - and the shaft power is P1 times efficiency. So shaft power will be around 0.9*P1 (assuming 90 percent efficiency).

But, when braking, the shaft power is always greater than P1 (because of losses in the motor). So, shaft power is around 1.11*P1 (P1/.9)

Answer to your second question: Sometimes, people do not bother about the difference between motoring and braking.

There is a lot more to this and you can read about it in most text books on electric machines.