Spinning a de-energized AC motor

[sry110]

Good Afternoon all,

I am working on a drivetrain application where we will have an 11kW, 400V/3-ph/50Hz, 4-pole motor that is 'dragged along' at 120 rpm for a short time period, not to exceed 20 seconds, at initial startup of the system. The motor would be connected to its 400V power supply during this period. After the <20 second period, the 11kW motor would be energized and would spin up to its full rated speed of approx. 1475 rpm.

A colleague of mine is concerned about "re-gen", or the motor acting as a generator while it is being pulled along at approx. 8% speed while being de-energized. I have not been able to find any resources citing that this would be detrimental to the motor or its control circuit, or pose a danger to the user, especially for this relatively brief operating period. That said, I am admittedly out of my depth in this particular topic so I would be open to any and all education on the matter.

 

[davidbeach]

How is it "connected to its 400V power supply" but needing later to "be energized and ... spin up to its full rated speed"?

 

[waross]

Consider a short power outage on a running motor.
An immediate reconnection, before the residual magnetism decays may be disasterous.
Often 8 to 10 seconds is adequate time for the residual to decay.
I don't see a problem.
Anecdote:
A machine was driven by two motors belted together at different speeds.
Admittedly small motors, about 4 to 8 kW.
The speed was changed by switching power from one machine to the other instantly.
Never had a problem, despite that one motor would be turning at a very high speed when the switch was made from high speed (much higher than synchronous speed) to low speed.

 

[edison123]

There is no 're-gen' in a motor with no power supply. Since the motor is already spinning via the load, it will have a shorter inrush current period than when it is started from zero speed. Just make sure the motor rotates in the same direction as it is being spun by the load. Otherwise, the shaft will likely shear off with much higher inrush currents.

 

[waross]

Otherwise, the shaft will likely shear off with much higher inrush currents.

We used to regularly plug (DOL reverse to stop) and instantly reverse NEMA "U" frame motors.
Not much more current than locked rotor, just a longer duration.
The newer "T" frame motors do not have the thermal capacity to reliably support instant reversing or plugging.