Urgent Slip Ring Question...

[Riche007]

Hi Folks:
I've always found this forum as a good source of knowledge. I just wanted to ask a question with regards some issues we've experienced in recent weeks.

A little background....
We used to have wound rotor motors that were fed directly "across the line" with resistor starters. This method of starting was changed out last year to VFDs and the rotors in the motors were shorted. We've run like this for a few mths now and have had some relative success, however, we have lost two motors. Although we recognise the fact that the life spans of these motors are severely shortened, but what has perplexed us so far is the way both have failed. Both motors had one of its slip rings severely "burnt" off or let me say corroded. The even more perplexing thing is that the regular PMs were done within a cpl of weeks of the failures and the slip rings looked very fine. This means that this is like an "instantaneous" failure. Has anyone observed this before? Any theories out there?

 

[504bloke]

Are the slip rings still in use ?

How old were the slip ring motors? ie what class of insulation was the windings ?

If we do a slipring job with an invertor we normally have the stator/rotor rewound to class F (sometimes H) and thermistors fitted to the stator. We also have the rotor shorted at the windings so the sliprings are no longer used.

 

[swhite65]

How many brushes are in contact with the slip rings? Usually the brush yokes will allow from 1 to ? brushes per ring. Make sure you have the maximum capacity of brushes installed.

 

[Riche007]

Are the slip rings still in use ?

How old were the slip ring motors? ie what class of insulation was the windings ?

If we do a slipring job with an invertor we normally have the stator/rotor rewound to class F (sometimes H) and thermistors fitted to the stator. We also have the rotor shorted at the windings so the sliprings are no longer used.

The motors are about 15yrs old but they've undergone several rebuilding over the years.....so they are as good as "fairly new".

Like I said, I woundnt be suprised if the failures where due to the insulations on the windings breaking down, etc...in my case, one arm of the slip ring corrodes.

 

[Riche007]

How many brushes are in contact with the slip rings? Usually the brush yokes will allow from 1 to ? brushes per ring. Make sure you have the maximum capacity of brushes installed.

We have the optimum number of brushes on the ring....it's been in service for several years. The only thing that has changed is the shorting of the rotors and the VFD control.

 

[504bloke]

Do away with the sliprings and brushes. Just short the rotor out directly.

 

[Riche007]

Do away with the sliprings and brushes. Just short the rotor out directly.

I'd do that in a heartbeat, but that doesnt solve the problem. What are the odds that I wont be translating the "problem" to the rotors?

 

[504bloke]

I'd do that in a heartbeat, but that doesnt solve the problem. What are the odds that I wont be translating the "problem" to the rotors?

How do you know it doesnt solve the problem ?

 

[MDavidson]

A couple of points to bring to your attention-

1) If you do not have load reactors in the system, consider installing them, does anyone else remember the rash of "first turn failures in non VFD rated motors when VFD's first hit the market? Remember that anything that happens in the stator will be mirrored in the rotor, the pulsed dc used to create an ac waveform in a VFD creates current spikes. Carbon brushes are a natural resistance point and will heat up with these spikes.

2) You mentioned that these failures happened recently. Would these happen to be air handlers for building air conditioning? Possibly working much harder and drawing more current and thus amplifying point 1)?

 

[DickDV]

If this stator has been rewound a couple of times, it surely has Class F insulation which should be good enough. And anyway, this is a rotor problem, not a stator problem.

I strongly recommend shorting the rotor at the slip rings and removing the brushes and rigging. This is standard procedure with these types of conversions and the results are almost always very good.

And, in general, use the lowest carrier frequency on the drive that you can tolerate.

 

[RAJATPALLAV]

This link will definitely provide clue you are looking for.

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