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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.)

Summary:

  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.

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    \$\begingroup\$ My Tesla primarily slows itself down using the same heavy-duty AC motors it uses to speed up. So yeah, you can do that. \$\endgroup\$
    – Samuel
    Jan 4, 2017 at 20:31
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    \$\begingroup\$ @Samuel: But the Tesla isn't trying to do it with a capacitor-start single-phase induction motor. \$\endgroup\$
    – Dave Tweed
    Jan 4, 2017 at 20:48
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    \$\begingroup\$ You can do it with a 3 phase AC motor and a VFD. Also consider using a servo-motor plus servo-drive. \$\endgroup\$
    – user57037
    Jan 4, 2017 at 20:49
  • \$\begingroup\$ @DaveTweed Ah, my reading jumped to EOF on the if-statement on the details line. \$\endgroup\$
    – Samuel
    Jan 4, 2017 at 20:51
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    \$\begingroup\$ Can you perhaps just fit a traditional disk brake to the motor? That's how it's done on a friend of mine's 3 tonne lathe, no hydraulics, no fancy-pants VFDs, just a plane old disk brake. \$\endgroup\$
    – Sam
    Jan 6, 2017 at 0:01

2 Answers 2

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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.

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  • \$\begingroup\$ Thanks! I assumed the answer was either this, or "simple as hooking up some big resistors" as @Sam said. No matter how many times I work with them, my mind never fully groks the intricacies of how AC motors work. Looks like this isn't going to help me avoid spending >> half my project budget on hydraulics. :( I might look into some alternative braking methods for slowing the decent that would let me still use the existing motors for the lifting. \$\endgroup\$
    – SvdSinner
    Jan 5, 2017 at 17:55
  • \$\begingroup\$ I can't see a reason why DC injection wouldn't work on a single phase motor. \$\endgroup\$ Dec 2, 2017 at 11:53
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Induction motors, when connected to the supply, will generate back into the supply if pushed faster than synchronous speed. If the decent speed you want is about the same as the ascent speed, you can just reverse the motor and run it down. Another way is to put a low voltage DC through the stator. That will make the motor act as a break, and is completely controllable with a small power supply. The energy will be dissipated in the rotor, but for a short time that won't overheat it.

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