Electric hoist motors
Electric hoist motors
(OP)
Do electric hoist/lift motors apply a constant force to the weight they are lifting? What would happen if the force translated from the motor torque was the same as the weight of the object at a position off the ground? In other words, the force pointing up is equal to the weight pointing down but the motor is still receiving current from the controller and trying to lift the object.
Thanks, just wondering
Thanks, just wondering
RE: Electric hoist motors
Recognise that equation?
F is the net force. Gravity down, hoist cable up, in this case.
Note that this equation does not contain V, only its rate of change. If A = 0 that implies V is a constant. I said nothing of whether that constant is zero or not.
More information, better answers.
RE: Electric hoist motors
RE: Electric hoist motors
RE: Electric hoist motors
It is good to know that the motor wouldn't like moving something slowly or just holding it in place. That is what I was thinking but it is good to have clarified.
RE: Electric hoist motors
I'm surmising that the question you wanted to ask is something akin to "How does a hoist hold a load stopped without burning up the motor" or perhaps "What's the minimum speed at which a hoist can raise a load" or something of that sort.
A normal everyday cheap simple hoist doesn't rely on the motor to hold it stopped. The motor probably has a mechanical, spring-applied, holding brake built into it that engages whenever the power to the motor is off (or below some threshold very low voltage). Or, maybe, it has a gearbox with sufficient friction characteristics inherent in its design that it's self-locking. (Worm gears.)
If you are using the motor output torque to balance the load, or moving the load at a very low speed (up or down), then you can't use the holding brake or gearbox self-locking. And in that case ... The motor needs to be supplied current to magnetise its innards in order to develop torque, even if it isn't developing revs. It will, effectively, be stalled. If it is an AC induction motor, then it will have to be supplied a small AC frequency that is just enough to offset the induction motor's inherent "slip" characteristics. In a normal AC induction motor with an integral cooling fan driven by the motor shaft itself, that fan won't be spinning if the motor shaft isn't spinning (and if the motor is spinning very slowly, same goes for the cooling fan). Do that with enough torque output (= magnetising current supply) and the various resistances inside the motor will be making heat. Do that for long enough ... the magic smoke comes out, and then it don't work no more!
RE: Electric hoist motors
Always provide ample excess hoisting torque to handle the code required test weight (125% design load). You really really do not want to loose control of the load.
Some hoist are provided with various kinds of mechanical load brakes so the motor never has to get rid of the excess energy of lowering a load. Some of these include worm gears.
Some hoists are provided with dynamic or regenerative load brakes.
All are provided with some sort of holding brake.
If you need a hoist, either use an off the shelf model or engage an expert, designing from scratch is not a simple job.
RE: Electric hoist motors
Generally speaking, once stopped you want the brakes to be applied and the motor shut off. Then, you generate the torque again and release the brakes when you again want to lift or lower the load.