Decelerate 2 Speed Motor by Connecting to Low Speed Winding

[eemotor]

Hello All,
An interesting idea was raised today by my colleague. Let's say you have 3 phase, 2 speed (3600/1800 RPM) consequent winding motor and you're running it at no load at 3600 RPM. Shut the power off, reconnect for the lower speed and energize. At point of energizing the low speed, let's say the motor was still spinning at 3000 RPM but 1 minute has already passed. What happens to the motor. Does it rapidly slow down to the low speed (1800 RPM)? Is there any power generated back into the power lines as the motor is slowing down because the motor was spinning faster than synchronous speed at the point when it was energized?
Thank you,
eemotor

 

[davidbeach]

Supersynchronous speed = generation.

 

[waross]

I have worked on several machines with this feature.
When reconnected for low speed operation they slow down rapidly. The deceleration time from 3600 to 1800 is not identical but is similar to the acceleration time from 1800 to 3600.
As David has stated, the motor is regenerating during this period.
respectfully

 

[HCBFlash]

It seems to me that all the factors relating to "slip" will follow through perfectly for purposes of predicting the speed-current characteristic.

"Generating power"? That's not likely. The work being done (torque being produced to change the rotational speed) is simply in the opposite direction.

 

[waross]

Hi HCBFlash;
Think "Induction generator".
respectfully

 

[DickDV]

Generation under these conditions? Absolutely! That's no different than regen or snubber braking on VFD's.

Negtive torque on the motor shaft equals power flowing back into the source. That is assuming, of course, that the motor has excitation voltage on it.

 

[dpc]

Generation will happen. If the transition is timed just right, you can also have an out-of-synch condition. The motor slows down just enough to be badly out of phase with the system when the contactor closes back in at low speed. Two-speed starters are often provided with a time-delay relay to delay transition from high-to-low.