Wound Rotor Induction Motor with Liquid Rheostat Arcing Slip Rings

[Concentrator]

Hi,

We operate ball mills with dual-coupled 3500hp 4160v wound rotor induction motors. We have recently experienced frequent arcing of the slip rings which seems to start on the center ring. The arcing damages the ring badly which requires us to shutdown the mill to sand it smooth again. The motors had run well for the past several years, but in the last few months have encountered these frequent breakdowns. We lost 2 motors on a particularly hot day (for Minnesota). We are getting excessive dust in the collector area which we clean ever 1-2 weeks. We are also getting brushes that heat and then expand and get stuck in the brush holders. We use 24 carbon brushes with a graphite inserts and 3lb springs on each motor. The brush manufacturer swears the material has been the same for years, tension on the springs seem consistent, and ventilation of the slip ring area with ambient air doesn’t seem to make a difference.

Something I noticed was that the brush wires “jump” and sometimes arcing is observed when the shorting contactor pulls in after the liquid rheostat is used to start the motor.

1. Is it normal to have such a high current surge when the shorting contactor pulls in?
2. Has anyone found solutions to similar sudden-frequent ring/brush failures?

 

[waross]

I suspect that dust is your culprit. I had a similar problem in a sawmill with a lot of fine dry dust in the air. The dust builds up in the brush holders and causes the brushes to stick. When the brushes are not free to "Ride" the slip rings or commutator they will slip. In the case of the saw mill, someone had to physically "snap" every brush in the plant several times every weekend. If a motor was missed on a weekend schedule the brushes would often start arcing about Tuesday or Wednesday.
Each brush was lifted about one quarter inch and then let snap down several times to clean out the dust buildup in the brush holder.
My motors and generators were DC with commutators rather than slip rings but brush holders are brush holders and a dust build-up will stick the brushes.

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

 

[gattie]

I suggest you check the liquid resistor solution concentration & the connections & contacts between the resistor & slipring shorting contactor.

 

[Concentrator]

Thanks for the advice - we will change our PM procedure to more thoroughly clean the brushes and holders. We had been pulling on the brush wire to be sure each one was free, and we blew the area out with an air lance but maybe that wasn't enough. We check our liquid rheostat concentration once per month, but we have not checked connections or contacts for quite some time.

The manual for this system discusses how the current surge is inherent to the process, but I believe there is SOMETHING we can do to reduce that surge of current a bit.

I have been in contact with 2 brush manufacturers who have offered to let us try new brushes in the motors to reduce the dusting problem and create “filming” of the ring, which we currently are not getting.

 

[7anoter4]

The dust it is obviously a source of brush damage. I think the dust also could damage the rings too. It could be a shaft eccentricity also.
I wander why the current is so elevated when the brushes are short circuited.
My opinion the current has to be no more than rated. Could be an overload or someway the slip is still big than rated.

 

[Dopler]

High rotor current could be the problem, from your explanation I infer that a step resistor is used for starting, verify correct connections in final short circuit contactors, does It has a rotor current amp? Verify against name plate.
Carbon dust could be a problem also or a symptom, a problem is when brush manufacturers changes the graphite composition and the ampacity for area unit is reduced because of that, temperature rises and brushes do not handle the current, you can notice this by inspecting damaged brushes they increase the size and are like exploited (too much amps/in2) “softer” brushes generate more dust, “harder” brushes generate more ring wear. Symptom, if the rotor is taking more current than rated brushes can fail, high current could be because problems in starting resistor
Ring eccentricity could damage brushes, can be verified with a magnetic dial and turning the rotor (if motor coupling allow it).

 

[waross]

Something I noticed was that the brush wires "jump" and sometimes arcing is observed when the shorting contactor pulls in after the liquid rheostat is used to start the motor.

A current surge is normal when the shorting contactor pulls in. But, check that the motor has accelerated sufficiently before the contactor pulls in. If the contactor is pulling in too soon, the current surge will be greater. Is your liquid rheostat travelling as far as it should before the contactor pulls in?
As an example;
A motor rated to run at 1760 RPM will draw full load current when the motor is loaded so that the speed is pulled down to 1760 RPM.
If the motor is loaded down to (1760 - 40) = 1720 RPM the current will be close to double. Close the contactor at 1680 RPM and the current will be about triple. If you close the contactor when the motor has accelerated to 1680 RPM, even with no load on the motor, there will be a current surge of about 300% that will diminish as the motor accelerates to equilibrium speed for whatever the actual load is.
Your rated slip is synchronous speed minus rated speed. The slip when the contactor closes may be two, three, four or more times rated slip with a current surge to match.
You may be able to reduce the surge by tuning your timing settings and adjusting your liquid rheostat travel but you may not be able to eliminate it.

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

 

[jraef]

A current surge is normal when the shorting contactor pulls in. But, check that the motor has accelerated sufficiently before the contactor pulls in. If the contactor is pulling in too soon, the current surge will be greater. Is your liquid rheostat travelling as far as it should before the contactor pulls in?

That's exactly where my mind went on this.

It also addresses the issue brought up by gattie of the liquid resistor electrolyte solution concentrations. It could affect the final resistance value remaining too high when the contactor pulls in, so the motor is not at maximum acceleration compared to what it used to be / should be.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[macmckim]

Was reading the original comments about the brushes and saw Concentrator comment about the slips rings " The arcing damages the ring badly which requires us to shutdown the mill to sand it smooth again"

Had experience couople of years ago with WRM brushes being chewed up on the slip ring and we found the user was sanding (emery sand paper)the slip ring, that under magnitication the slip ring had pits and score marks on it. The rings themselves were eating the brushes.
After machining the rings to a mirror finish - the client problem was solved as have not hear a peep from them in 3-4 years on the subject.

 

[OhioAviator]

Was reading through this thread and thought I'd offer my two cents. We operate quite a few WR motors ranging in sizes 3,000 HP up to 7,000 HP. We use liquid rheostats on all of them, not only to start, but also during operation to control incoming line current when the motor sees heavy loads (automobile shredding).

A couple of years ago I had one motor that continually ate brushes on one phase (middle ring) and also experienced several damaging arc flashes. Thank goodness the arc detection system shut down the motor before catastrophic failures occurred. Once we got tired of cleaning up the motor and resurfacing the collector ring, we meggared the rotor circuit and found the middle ring was only a few thousand ohms away from being solidly grounded. We were fortunate enough to be able to change the collector rings in place and haven't had any problems since.

As for the liquid rheostat brine concentration... that normally won't be a problem unless someone either (a) puts additional soda ash in the tank which raises the conductivity or (b) forgets to add water periodically to keep the tank at the proper level. As the water evaporates out the brine solution becomes more concentrated thereby increasing conductivity.

Lastly, do not ignore motor load. Brush problems don't develop for no apparent reason. Something has changed. Is the motor being operated significantly beyond its rated load? Brush sticking in the holders is a classic symptom of brush overload.

Hope this helps.

 

[Concentrator]

Thank you for your help. I have only been with the company for 4 months and these motors have proved themselves to be quite a challenge. I think we need a visit from a proven technical expert that is highly knowledgeable about wound-rotor induction motors and liquid rheostats. Does anyone have contact info for someone who could make a visit to our site in MN?

Another motor failed on us today. When the operator started it he heard a loud growl. By the time it was investigated and shut down, the brush area of the motor had caught on fire. Some of the brush boxes blew apart, the brushes blew off the shunts, and the rings are now pitted. We had just PM'd this motor 2 days ago.

 

[OhioAviator]

Hello Concentrator,
Sorry to hear about your latest motor failure today. There are a couple of people you can contact for help:

Sabine Mueller
VP, Metso Minerals/Mueller Engineering
2300 South 170th Street
New Berlin, WI 53151
Phone: 262-796-1900

J.P. Boucher'
Gen. Mgr.- American Industrial Motor Service (AIMS)
235 West Oklahoma Avenue
Milwaukee Wisconsin 53207
Toll Free: 888.818.4200
Phone: 414.482.4200

I would try Sabine first, then try J.P. at AIMS.

Interesting that you folks PM'd the motor two days ago. What were the meggar readings on the rotor? Question: Do you have an arc detection system installed on your motors to detect arcing at the collector rings? If not, you might consider installing such a system. Sabine at Metso Mueller Engineering can fix you right up.
Hope this helps.