Slip Ring Motor operating like Squirrell cage Motor

[petronila]

Hello,

I would like to know your pros and count when the winding of a Slip Ring Wound Rotor is short circuited and the motor is used like a squirrell cage type.

Thanks

Petronila

 

[LionelHutz]

It works well if you use it with a VFD. You can still get lots of torque from the motor and keep the current low.

If you full-voltage or soft-start it though you'll require very large currents. You'll also really hurt the starting torque characteristics and since the load probably requires the high starting torque it usually doesn't work well. It's not recommended just because of the high currents.

 

[edison123]

If starting is not an issue, running should be fine.

*Even inside a hard-boiled egg, there is a golden heart!!!*

 

[jraef]

http://www.lmphotonics.com/slipring.htm

In around the middle of the age there is a good description of the issues.

 

[Marke]

Hello petronila

When you short out the rotor on a slip ring motor, it will behave as an induction motor with a very high Locked Rotor current (LRC) and a low Locked Rotor Torque (LRT).

The LRC of an induction motor is typically in the range of 550 - 850%. A shorted rotor slip ring motor would exhibit a LRC as high as 1200 - 1400%

The LRT of an induction motor will generally fall in the range of 120 - 260% (some large machines can be as low as 60%) The Slipring motor with a shorted rotor may have a LRT in the rnage of 30 - 60%.

The high LRC and low LRT result in very low starting efficiency with any form of starter. Even DOL, there is often insufficient torque to start the load.The rotor winding is not designed to absorb energy, so the maximum load inertia that can be started (except with a VSD) is very low.

Bottom line, there are very few applications where you can use a slip ring motor with a permanently shorted rotor due the starting difficulties except where used with a VFD.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[electricpete]

Good information. Seems like there's a typo.

"Shorting out the slip rings and attempting any form of reduced voltage start in the stator supply, will result in a much reduced start torque at a much higher start current. Effectively, the motor could exhibit a Locked Rotor Current in excess of 1000% and a Locked Rotor Current less than 100%. If we reduce the start current down to say 400%, then the start torque would be less than 100 x (400/1000) x (400/1000) or less than 16%!"

Shouldn't "current" be "torque".

If it were me, I would have duoble-checked it before I posred it.



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[electricpete]

Correction:
If it were me, I would have double-checked it before I posted it.

Hahaha...

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[motorspert]

Just one comment on used with a VSD. Check if there are insulated bearings fitted to the machine, if not it could be risky

regards

 

[GGOSS]

Yes, it should read torque electricpete.