regen with bldc

[jratzing]

I am interested in using a BLDC motor drive both for driving a motor as well as for using the motor as a generator (regen). Some controllers mention specifically that they have regenerative features, others don't mention it it all. Looking at driving circuits for BLDCs, it looks to me that all controllers should be able to handle regeneration, as long as the back EMF is greater than the PWM duty cycle times the battery voltage. That means that even not at full throttle (or at speeds which generate a back EMF which is lower than the battery voltage), there is a possibility for regeneration. Is there anything I am overlooking here? Is there anyone with suggestions for controllers which allow regen at all speeds?

[Comatose]

Its not quite that easy - your typical airplane BLDC setup simply freewheels, with no regeneration whatsoever, and then just goes into a (chopped or hard) brake, also with no regeneration.

Regen is harder than not having it. Fundamentally, all you have to do is turn the bridge on backwards, but the devil is in the details. With a freewheeling controller, you only have to switch one mosfet per pwm cycle. Synchronous rectification requires at least two and full-time regeneration requires at least four. This is of course doable, but you get dramatically higher switching losses. So, you need bigger fet drive stages, more heatsinking, better fets or all three.

There's also basically no benefit in an airplane, boat, pump or fan controller, because stopping and starting is rare and the amount of energy recovered is miniscule. The places where you get major gains from it are applications which carry a lot of kinetic energy around, and which start and stop often - cars, robots, positioning systems, servos.