Measuring Rotor Current In Wound Rotor Motor

[OhioAviator]

Hello all,

I recently had a problem with a 4.16 Kv, 4,000 HP wound rotor induction motor where the motor started eating brushes like crazy on one slip ring, and greatly accelerated brush wear on a second slip ring.

The surfaces of all three slip rings looked fine, so it wasn't a ring pitting or abrasion problem. Brush manufacturer and part number has not changed in several years.

This motor uses a liquid rheostat (soda ash & water) for it's rotor resistance and what we found was the bridge that carries the upper (movable) electrodes had tilted on one end, making one end of the bridge about an inch lower than the other. This placed the two electrodes of one phase very much closer to each other, almost touching in fact, than the electrodes of the other two phases. Once we corrected the electrode alignment problem the excessive brush wear went away.

Question:
Is there a way to measure or monitor the rotor phase currents, or more specifically the rotor phase current balance, in the rotor circuit? I don't imagine using CT's will work due to the rotor current's very low frequency when the motor is at or near full speed. I had thought about using shunts, but shunts of this size (1200-1500 Amps) are expensive, large, and unwieldy. Then there is the problem of what kind of voltage balance relaying would work in this situation.

Any thoughts? Is anyone else measuring rotor currents in large HP wound rotor motors? If yes, how?

 

[electricpete]

Good discussion.

I never thought about using the concept volts/hz for CT’s, but makes sense considering the relevant voltage would be on the CT secondary voltage (product of secondary current and burden impedance).

Another interesting aspect – even in the absence of saturation, there is an L/R decay constant associated with a CT and many clamp-ons that attenutates the low frequency components as was discussed in thread238-217182

Needless to say spec sheet is a good starting point. There were some spec sheets linked in that thread as well as suggestions from Scottty for Hall effect probes that might reach high current ranges.


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[rockman7892]

OhioAviator

I actually have (2) of these 6500HP WRIM's at my plant. They are both connected to a Rheostat consisting of a salt solution. It is the newer one that we just commissioned that has extremely high temperatures on the slip ring enclosure. Temperaturs upwards of 170deg F have been measured on the enclosure and the associated ductwork connected to the enclosure. The initial thought is that we are not getting enough cooling air through this enclosure from the forced air motor cooling system.

On our existing motor however we have recently noticed pitting of the slip rings. Even ones that we have replaced fairly recently have been severly pitted or chipped and if I recall correctly this was occuring on two of the three phases.

Skogsgurra

I'm curious about your respones in regards to measuring the current by means of measuring the voltage on the rotor cables. Where would you take the voltage measurements, at the slip compartment? Would you measure between phases?

 

[OhioAviator]

Skogsgurra,

With temps of 170deg F and higher on the surface of the collector ring enclosure, you definitely have a problem. Inadequate ventilation might certainly be an issue, but I'd also be curious to know at what temperatures are the collector rings operating. Might not be surprised to see 275F temps and higher. If that's the case, maybe adding several additional brushes to each collector ring, to get the current density down, might help. How do the collector rings and brush faces look? Are the rings displaying any pitting and/or burning markings? Do the brush faces look like they've been operating too hot (charcoal gray color or a faint rainbow coloration)? If yes, then it warrants further investigation and possibly additional brushes or a change in brush composition. If you can get your hands on an infrared camera to take some photos of the brushes and collector rings while in operation that might point you in the right direction. Wear the proper arc-flash PPE. IR cameras won't see infrared through plexiglass, so you'll have to open the inspection doors or covers to take the IR photos.

As for your existing motor and the pitting & chipping... has anything obvious changed lately that might be causing this to happen on the collector rings? Again, how do the brushes look? Are they also cracked, pitted, chipped, or grooved? Excess vibration might be something to look at, especially on motors with overhung collector rings (rings that are located out beyond the non-drive end bearing). Are the rings contaminated by something such as oil or grease vapors? Are any brushes getting stuck in the brush holders? This would cause severe sparking and result in burned or pitted collector rings, with corresponding burn markings on the faces of the brushes. Without seeing the actual motor it's difficult to diagnose the problem. If you can't track down the cause of the collector ring pitting problem, then maybe you'll just have to get someone in to machine the collector rings back to a clean, true, concentric surface. DO NOT TRY STONING THE RINGS. Machining the rings is the single best method of getting rid of collector ring surface problems. It's a pain in the rear end, but well worth it. Hope this helps.

 

[OhioAviator]

Sorry, I meant to address the above post to rockman7892. My apologies.

 

[Skogsgurra]

rockman

You need to find a piece of cable where the three cores are exposed and then measure voltage along a stretch of each core. Sometimes, it is difficult to find an exposed length that is long enough to give meaningful readings.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[rockman7892]

OhioAviator

The new motor has not been shut down to where I would have a chance to look at the brushes. I am interested however in shooting the actual collector rings and brushes with a temp gun. Are you saying that adding more brushes will disperse less of the total rotor current (900A) through each brush and therefore reduce the heating at each brush? As soon as the motor is shutdown I am going to check the brushes.

Of course we will wear PPE if we ever shoot the slip rings directly. There was always fear of doing this as others thought there was a high voltage on the rotor, although I am trying to explain at full load there is little voltage on this rotor although we still need to take caution.

On our existing motor there has been nothing unusual that may have caused this chipping or pitting on the brushes. We decided to go ahead and machine the collector rings, and I'm going to watch how this effects the wear on the brushes.

Your statment about using the rheostat when the motor current gets over 1.5FLA was interesting. When this happens does the extra resistance shift the motor speed torque curve to the left, and thus the load torque curve will intersect this motor curve at a lower point and thus use less current and at the same time the load torque will slow down to match where the load torque intersects the motor torque at the lower speed?

Obviously this is assuing that the load torque is kept the same. So essentially lower toruqe will be required at the lower rpm and thus current will decrease allowing the motor to recover. Once recovered does the process need to adjust before the resistors are removed to prevent from overloading again?

 

[OhioAviator]

rockman7892,

Yes, if you add more brushes it will increase the total brush sq. inch surface area, which will decrease the current density (amps/sq. in.) per brush. That will have limited benefit however, as each brush type has an optimum current density that they 'like' to operate at. Less than optimum and you usually get excessive brush dusting. More than optimum and you get localized heating and other bad things. Helwig and Morganite are great technical resources for all brush related matters. And, of course, all of this assumes that the collector rings were designed with sufficient mass for adequate heat dissipation to begin with. If the rings have been operating satisfactorily for months and years, then collector ring design isn't a factor as far as heat dissipation is concerned.

You can certainly use a heat gun to shoot the rings and that might point out an unusually hot area. However, as they say, a picture is worth a thousand words; and in this case, that's doubly true. An IR image will give you a MUCH clearer picture (no pun intended) of exactly what's getting hot and what isn't. If you get your hands on a color IR camera, use the 'ironbow' setting as that seems to bring out detail much more clearly. I've attached a sample IR photo of one of our motors as an example (hopefully I did this right).

Liquid rheostat.... yes, when the motor encounters a heavy load, the extra resistance in the rotor circuit shifts the speed-torque curve to the left, thereby causing a corresponding reduction in stator line current. If the load torque isn't too great, the motor will work through the load at reduced rpm until the load begins to diminish. At that point, the rpm will begin to pick back up with a corresponding decrease in stator line current. The motor controls will see that rpm increase/stator amps decrease and automatically begin to bring the rheostat electrodes back together until they reach minimum resistance.

However, if the load is too great, the motor will continue to lose rpm. At that point, the way our motor controls are set up, the rheostat will begin to close back up (decreased rotor resistance), which will cause a corresponding increase in motor torque albeit with a corresponding increase in stator line current. If the load isn't too great, the motor will recover. If not, the motor will trip out on overcurrent.

 

[roydm]

With all the overheating and brush issues there's a lot to be said for the old style wound rotor motors I worked on as an apprentice. Once up to speed you threw a lever that shorted out the sliprings.

Can I join your AVO 8 and Wiggy club.
I also have a galvanohmeter that the linemen used for troubleshooting railway signals when they used LeClanche cells.

 

[OhioAviator]

Anyone who owns a galvanometer and knows what "LeClanche cells" are is an OFFICIAL member of the Simp 260/AVO 8/Wiggy Club!

 

[roydm]

Yes the signal guys loved those old cells, they were in a glass jar with the positive electrode wrapped in some cloth material and a zinc rod for negative. They just replaced individual parts and electrolyte.
The cells were part of a tablet exchange system between stations, the engineer wasn't allowed to travel without the tablet. The machine wouldn't issue one unless the machine at next station sent a signal to say the line was clear.