induction motor braking torque

[fhuang]

An induction motor is controlled with vector-control
algorithm. Keeping Id constant, if the polarity of Iq
is changed from '+' to '-', the motor torque should change
from motoring to regenerating.

Theoretically, if /Iq/ is the same, the torque, /T/, should
be the same. The polarity of Iq only changes the direction
of the torque.

In reality, the regerating torque is always higher, by 10%
than the motoring torque, WHY?

 

[wired1]

Could you define Iq and Id? When motoring, the current is used for 1. torque, 2. magnetization, 3. heat/losses. It's conceivable that these components have a different characteristic when "regening." Years ago I used some drives by Control Techniques. Their manual had some good engineering calcs on how to size regen resistors. Maybe you can get an App Eng to fax you that page.

 

Have you considered the losses, which subtract from the motor torque, but add to the regen torque? Is this a real motor or a model, or both?

 

[electricpete]

good comment from chris.

I'm betting he hit the nail on the head with the comment about losses, assuming that you are computing torque from electrical quantities and change in rotor kinetic energy, rather than measuring torque directly. 5% losses is not unreasonable.