PowerFlex 753/755 regen on power loss

I've heard conflicting things and want some expertise here. If a 753/755 were to lose power while motor is running, will it regen on the line side of the VFD?

phuz said:

I've heard conflicting things and want some expertise here. If a 753/755 were to lose power while motor is running, will it regen on the line side of the VFD?

Click to expand...

No. Not possible.
For an AC induction motor to become a generator, you need a minimum of these two things:
1) the motor must remain energized so that the magnetic fields are maintained. There may be a small amount of residual magnetism in the iron cores, but not enough to sustain any appreciable energy.

2) the load must be spinning FASTER than the frequency that is applied to the stator.

When line power fails, the drive will shut down, so neither of the above conditions exist. Even if you have the drive set for decel (instead of coasting), to where the DC bus caps keep the VFD alive for a few more seconds after the line side drops, you would need to have your Decel rate set to be faster than the natural decay of the speed so that condition 2 is met, which might make the motor regenerate into the DC bus of the drive, but that’s it. From there, the energy hits the brick wall of the diode bridge rectifier. That’s a one way bridge.

The only possible way ANY VFD can put energy back into the line is for you to have bought an Active Front End (AFE) drive. In Rockwell world, that’s the 755TR drive. Even then, you can simply program the drive to NIT allow regeneration on loss of line side power, since as an AFE drive, it is always monitoring the line side.

This is a common misconception that is kept alive by power system software that determines fault levels and will consider the contribution of spinning loads. Someone in that industry decided that AFE drives CAN contribute, but really, you can make that decision in setup and most people say No.

That's what I thought. Thank you!
My AB supplier said otherwise.