Induction motor regenerating back to inverters

[LMAO]

Assume two independent inverters feeding two INDUCTION motors interlocked by gears. The system normal operation is running one inverter as master and another as follower (slave) to share the load. However sometimes it is necessary to stop one inverter and run the other. This obviously results in other motor spinning as well. Assuming that these are induction motors, will that be a problem in terms of motor regenerating back into non-running inverter under following conditions?

1. Inverter completely de-energized
2. DC bus energized but IGBTs not firing (inverter stopped).

 

[iceworm]

My guess (and it is a guess)
I'm guessing the concern is will the non-powered, but spinning, motor act as an induction alternator and push power back into the non running inverter. I would say no:
The non-powered motor is spinning. But since it is not powered, it is not excited - so it won't generate.

 

[JoeStillman]

My guess (and it is a guess)
I'm guessing the concern is will the non-powered, but spinning, motor act as an induction alternator and push power back into the non running inverter. I would say no:
The non-powered motor is spinning. But since it is not powered, it is not excited - so it won't generate.

I'd call for an output contactor anyway.​

 

[Jraef]

My guess (and it is a guess)
I'm guessing the concern is will the non-powered, but spinning, motor act as an induction alternator and push power back into the non running inverter. I would say no:
The non-powered motor is spinning. But since it is not powered, it is not excited - so it won't generate.

This is correct. For an induction motor to regenerate, it must have excitation of the stator windings. If the inverter is off, there is no excitation, so no regen. There might be s tiny amount of residual magnetism in the stator core, which is sometimes used in the home power market to make induction motors generate, but that requires capacitors to build up that voltage to the point of self excitation.

The reason Inverter DRIVES make a motor regenerate is because the drive is purposely commanded to put a LOWER frequency on the motor compared to its spinning speed, which excites the windings. If you just turn a drive off, the motor doesn’t go into regen.

 

[winnie]

We do this in our lab on a regular basis. We have dynamometer test benches with two induction motors connected by an inline torque sensor. One is a standard off the shelf IM, the other is the unit being tested.

You need to drive the IM at a lower frequency than it is spinning at for it to generate. If the inverter is off, there is no magnetic flux, and the motor won't operate as a generator.

Now you still need to consider that you might accidentally get into a regen state, for example if one drive is off and the other is spinning the load, and then the first drive gets turned on and things that it needs to accelerate a stopped load. But if the drive is _off_ or not modulating then you can't get regen.

-Jon

 

[LMAO]

Thank's all. Considering induction motors can't regen without excitation I expected this to be case but I just wanted to make sure.