ROTOR END RINGS 4

[cryovette]

Anyone recall if the rotor end rings has one that is constantly positive and the other is constantly negative?

 

[dpc]

So it might help if you told what type of machine you are talking about....

 

[cryovette]

Sorry...AC Induction...thanks for pointing that out,dpc.

 

[rovineye]

AC synchronous motor, exciter field slip rings, one is always positive, other always negative, even when reversing. Exciter polarity stays the same.

 

[dpc]

For an induction motor, the induced voltage and current in the rotor are ac at the slip frequency.

 

[cryovette]

Thank you dpc and rovineye. So if I have it right, the polarity alternates (besides the synchorous motor/wound motor)in the end rings as it does in the fields of the rotor so verdigris shouldn't be an issue in this machine.

 

[electricpete]

I had to look up verdigris, but I guess I should have recognized verdi for green (like green copper (or cuprous or whatever it is) oxide

http://en.wikipedia.org/wiki/Verdigris

I guess some forms of galvanic corrosion would be accelerated by a dc potential (which doesn't exist in induction motor as you say).

I don't think dc potential is required for oxidation of copper. But I certainly don't ever remember seeing any green on any of our large fabricated copper bar rotors (has anyone else?). I would guess that keeping the rotor relatively warm / dry prevents copper from oxidizing ?

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[cryovette]

Thanks for the reply, electricpete. Here is the direct quote that I read...the first time around I must have read slip rings and was thinking end rings.

"Electrolytic action can occur at slip ring surfaces
producing formation of verdigris. Verdigris is a greenish coating that forms on non-ferrous metals.
Electrolytic deterioration can be prevented by reversing the polarity of the slip rings once or twice a year."

I would imagine that you are right about conditions being dry and warm in preventing this. I assume that perhaps this is seen more with motors that are installed inside pump houses for water supplies or large irrigation applications(where it is cooler and damp)?

 

[jraef]

Might I point out a small error in your 2nd posting above? Although it has slip rings, a Wound Rotor motor IS an AC induction motor; the rotor voltage is AC on that just as it is on a Squirrel Cage Induction Motor. Only a Synchronous motor uses DC across the slip rings.

From the "Kilowatt Classroom" website on synchronous motors (taken from an old Electric Machinery book I used to have, but can no longer find):

"Slip ring polarity is not critical and should
be periodically reversed to equalize the
wear on the slip rings. The negative polarity
ring will sustain more wear than the positive
ring due to electrolysis."