Electrical – AC motor to apply braking torque

accontrolinduction motor

I've got a project that involves lifting a heavy weight a few feet, holding it there mechanically, and then letting it down safely. (Assume I'm raising 6000# up 3 ft)

Could the same AC motors I use to lift the weight be used to slow the decent of the heavy weight when it is coming back down? (The weight should hit the ground at no more than 1 ft/sec, slower is better)

Assume the motor is geared so that 2000 stator rotations would take things from fully up to fully down, and no worm gears are involved. (You might notice that the max-allowable speed going down would drive the stator well above it's forward speed going down, so in free fall, the motor would act as a generator that could potentially regulate the attached load to control the descent rate. Plus, driveline friction is your friend during the descent.)


  1. Could a heavy-duty AC motor safely produce braking torque against it's stator that is rotating backwards? (Either by applying voltage or by loading it and using the generator effect)
  2. If #1 is plausible, how would I wire/control it? (Ideally, could the generator effect self-stablize the decent by creating a terminal velocity of the motor?)

Details, if you care:
If I had a big budget and no pre-existing hardware, a single-acting hydraulic cylinder would work nicely. (A single acting cylinder pushed in one direction only, and relies on gravity (or similar) to return it to it's starting position) However, in my parts bin, I have a couple of 1HP A.O.Smith AC120V Farm-Duty motors (Similar to this) that have enough power for the lifting. Using them instead of buying hydraulics would save $4-700.

Best Answer

You cannot do this with single phase motors, but you could do it with 3 phase motors and VFDs that have a Dynamic Braking function, assuming the braking functionality will allow the load you are wanting to stop.

But this is WAY more complex than you are thinking it is. You need a VFD that is capable of Flux Vector Control using a shaft encoder feedback loop plus the capability of providing what is called "Torque Proving" so that you do not release the mechanical brake until you KNOW that you have full negative torque at the motor shaft, which means before the motor shaft is moving. That is not something that inexpensive equipment is capable of.