Operating region of Induction Motor for hoisting down.

[drspanda]

Hello friends,

This is a generic discussion regarding operating region of Induction Motor(IM) used in hoist. For the sake of convention, if I suppose when IM is lifting a load up then it runs in a first quad of Torque/Speed with ABC phase sequence absorbing power from the system. Speed close to rated speed, assuming clockwise rotation.
Now, when IM is lowering the same load with ACB phase sequence, at first instance it appears that due to gravity and rotation of motor the speed in counter clockwise direction shall become very high. But it is not so. My doubt is "Is the motor operating in other quad of Torque/Speed? Speed is found to be the same.
For same speed in either directions I am not able to explain this phenomenon?
Can anyone throw light upon this subject?

 

[just the cowboy]

As long as it's not overloaded.

The speed is set due to the field at a rated load. If you were to overload the motor, then the speed can increase in the down direction, and current will go up due to the fields not matching. This is seen in blowers all the time when they are run unloaded, they pass their synchronized field.

 

[jraef]

When lowering, the load is attempting to overhaul the motor, meaning spin it faster than the grid frequency wants to. But the slip goes negative and the opposite torque / speed curve applies to regeneration (4th quadrant) in that the motor, so it is applying NEGATIVE torque to the system. Think of it this way; when in regen mode, the motor is a generator, the load is attempting to speed it up. But being connected to the grid, that motor would need to speed up the ENTIRE GRID, which is not going to happen. So although it will try, the motor is the limiting factor. Current is still flowing the other direction, so the same thermal limits apply to that motor and if the motor overloads and the contactor opens, the retarding effect is immediately lost so the load runs away, crashing to the floor. the same thing happens if grid power is lost. That's why you need mechanical brakes capable of stopping the load in free-fall.

 

[DickDV]

I think its useful to look at a common NEMA B torque-speed characteristic curve. If you locate the zero torque, synch speed point on the graph and then picture a mirror image of the load curve reflected and inverted over the zero load point, you essentially have the front face of the load curve going below the zero torque point as a nearly straight line with slight curvature similar to the load side curvature.

The portion of the curve below the zero torque line is braking torque with the slip going up in speed rather than down and current increasing similar to the load side current only upside down. This region is often thought of as regenerative braking but you can view the induction motor as an induction generator just as well.

Amazing machines---those induction motors!

As jraef points out, the generation only occurs because the motor is overdriven to push against the grid frequency. Without that frequency standard, the motor would simply freewheel since there is no stator excitation.