Does the slip-ring induction motor has a bigger NO-LOAD CURRENT, than the squirrel cage?

[elinBG]

title?

 

[desertfox]

Google?

 

[LionelHutz]

It could be higher or lower. Depends on the 2 motors in question.

 

[elinBG]

I mean two motors with the same power, number of poles, frame size and the wound rotor is not connected to resistance.

 

[electricpete]

Rotor electrical parameters (L2 and R2) would likely be different, but don't affect no-load current.

I suspect perhaps WRIM would tend to have physically larger rotor than SCIM of same rating. That means slightly longer and bigger diameter of stator bore, which would tend to increase no-load current slightly in general.

That's a guess about generalities based on theory. I may have missed some aspects. I agree with Lionel, generalities don't buy you much for this type of thing. There are plenty of variations possible for motors of similar ratings. Checking OEM data is worthwhile.

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(2B)+(2B)' ?

 

[elinBG]

So, at all?

"The NO-LOAD current of the WRIM is not much different than the NO-LOAD current of the squirrel cage IM"

How about the STARTING-CURRENT? If no additional resistance is connected to the WRIM?

My questions are because I am arguing with а colleagu.
Before 2 months we did Direct-on-line starting of a non-loaded squirrel cage induction motor 22kW/4poles.
Yesterday we had an 22kW/4poles Wound-rotor motor and he said that "its current is much higher" and we cannot start it directly.

I told him that I dont believe this, but however we didn't try, because he is much older and with bigger experience.

 

[waross]

The no-load current is not the same as the starting current.????

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[electricpete]

I assume we have moved from original question about no-load current to a new question about locked rotor current/

http://www.tecowestinghouse.com/PDF/woundrotor.pdf

The figure at the bottom of page 2 suggests "typical" locked rotor current with no resistance cut in (I assume that is curve C1) might be on the order of 400% of FLA. That is close enough to a normal SCIM (considering variabilities of these numbers) that I would have to think there is not a tendency for LRC to be higher or lower comparing WRIM to SCIM... we have to look at the specifi motors involved.

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(2B)+(2B)' ?

 

[waross]

Hi Pete. I would be more concerned with over current on the slip rings and brush gear, particularly at rest.
The low starting torque (it looks like less than 40%) with low resistance will be a concern, particularly with many of the high inertia loads that are typically driven by WRIMs.
This may lead to overheating due to extended acceleration times.
Nice link Pete. Thanks.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[electricpete]

Good point - I'd be nervous about that also. Could that initial surge of current cause damage to slip ring before movement starts? Beats me, but I wouldn't want to try without checking further.

Another aspect to mention, the link shows starting torque very low with no resistors - on the order of 35% for this particular illustration.


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(2B)+(2B)' ?

 

[LionelHutz]

Overall it's rather dumb to attempt any type of line power starting (ATL, soft-starter. Y-Delta, autotransformer etc) of a WRIM without rotor resistance. You just get rather high currents and very low torque. The WRIM was likely chosen because starting the load is difficult so you're just setting yourself up for failure.

 

[elinBG]

I mean starting current without load

http://www.eldvigateli.com

 

[LionelHutz]

WRIM starting current will be > SCIM.

 

[jraef]

A WRIM with a shorted rotor bar is always going to have greater starting current than SCIM. In fact you will have the highest possible current with the lowest amount of torque per amp, meaning most of the current will end up wasted as heat. WRIMs should never be started with a shorted rotor unless you have a very good VFD that can be programmed to optimize the current control toward torque production.

But a WRIM with the proper rotor resistance can be less that SCIM starting current and is generally considered the optimal starting method of an induction motor, albeit of course it must have a wound rotor.

No load running current of any AC motor is essentially a meaningless value for anything other than perhaps tracking long term maintenance needs, and only if you maintain accurate records of periodic measurement.

"Will work for (the memory of) salami"