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?

 

[edison123]

I have not seen any wound rotor currents being measured. But slip power recovery systems measure the current and power, I think.

Muthu

http://www.edison.co.in

 

[roydm]

As a temporary measure you could perhaps use one of the old moving iron cipon ammeters, they will measure all the way down to DC or a Hall Effect clipon.
For a permenent fix how about Hall Effect type CTs
Roy

 

[rockman7892]

This is a great question for I've wondered this for some time.

It is my understanding that at full load all the rotor current is flowing through the slip rings and through the shorting contacts in the Rheostat and this can be upwards of 1000A depending on the size of the motor. I believe the voltage on the rotor will be small, but the product of the voltage and current on the rotor will give a power that is similar to the input power on the stator minus any losses.

I had someone put a clamp on meter on the rotor cables once to prove that there was no current and although I always thought that there was current on these cables I had to believe what I was seeing with the clamp on. It now makes sense that the reason it was reading no current, was due to the fact that the frequency of the current was too low for the meter.

 

[jschwei314]

I have used DC shunts in the three rotor leads at the slip tank. The mV drop across these shunts, through rated voltage isolation amps, looked at currents with an ocilliscope.

I have always wanted to rectify the volatge output and compare the three signals for an unbalance. I never did. I never had a call for problems with unbalance. I have always set the unbalance trip setting in the motor protection relay tight for this reason.

 

[esee135]

I agree with schweitzereaton. Shunts are the way to go with voltage-iso amps. I've had too many incidents of so called dc-ct's having hysteresis. The shunt size's aren't too bad, I've had a few which are into the 2000A range, not that much bigger than the bus it's bolted on.

I guess it all depends on how much the piece of machinery is worth to you relative to the cost of instrumentation and your own labour. A couple of shunts and iso amps could be worth a few thousand, but that motor may be worth that same amount per day, for several days.

 

[sreid]

I would use LEM Hall Effect Current Sensors.

http://www.lem.com/

 

[OhioAviator]

Thanks everyone. Some good ideas, and the LEM products are worth further investigation. But I also like the good ol' reliability of DC shunts as schweitzereaton and esee135 suggest. What I haven't heard, though, is a suggestion on the appropriate current/voltage balance relaying that might be appropriate (or even available).

I suppose I might be able to use three of those LEM sensors with a DC or mA output and send the three signals to a PLC. Once there, I can do some mathematical manipulation to look at absolute values of rotor phase current and/or current balance. Other suggestions, please! Thanks!

 

[LionelHutz]

You can get hall effect type current sensors which output a 0-10V or 4-20mA true RMS signal. Here's a link to some examples I knew about to give you an idea. Obviously, you'll have to track down something similar but with a higher current range.

http://www.crmagnetics.com/products/Current-C6.aspx

 

[rockman7892]

We have a similar issue with a 6500Hp wound rotor motor on which the slip ring compartment becomes extremely hot. It may be due to poor ventilation.

The datasheet shows 900A rotor current, so I'm assuming that at full load this 900A is flowing through the slip rings, rotor cables, and shorting contactor back to the rotor.

 

[OhioAviator]

rockman7892... when you say that you have a similar issue with your 6,500HP wound rotor motor, are you referring to my original issue of excessively high brush wear on one ring only? Not clear from your post.

At any rate, yes, at full load the 900A is indeed flowing out of the slip rings, brushes, rotor cables, to the shorting contactor and back. Which means, in your particular case, at full load your wound rotor motor is operating pretty much as a squirrel cage motor.

In our application, we use a liquid rheostat, which consists of a tank that's filled with approx. 1,500 gallons of salt water (plain water with fine soda ash mixed in). The rotor cables coming from the rotor are connected via feed-thru insulators to three fixed electrodes in the bottom of the tank. At the top of the tank (always in the brine solution) are three matching electrodes mounted on a common beam, effectively shorting the three top electrodes together. This constitutes the 'wye' or 'star' point of the rotor circuit. When the motor is first started the top electrodes are all the way at the top of the brine solution, placing the maximum amount of distance (and salt water) between the top and bottom electrodes. This is maximum starting resistance. As the motor accelerates, the top electrodes are gradually lowered down toward the bottom electrodes, eventually reaching a point where the top and bottom electrodes are meshed together although they never actually touch each other. This is the normal operating position, and also the point of minimum rotor resistance. At this point the wound rotor motor acts much like a squirrel cage motor. In our particular application, as the motor encounters a load greater than 1.5x full load amps, the motor controls automatically begin to raise the top electrodes away from the lower electrodes, thereby increasing rotor resistance. This increased rotor resistance allows the motor to maintain maximum torque at lower rpm while at the same time limiting the stator line current until the load decreases, thereby allowing the motor rpm to recover back to full rpm. As the motor recovers, the top electrodes are automatically lowered back down to the minimum resistance (normal operation) position.

 

[Skogsgurra]

I have used the cables themselves as 'current transducers'. It works quite well: measure voltage drop along the three cables, make sure that you measure along the same length. The slip is usually such (one ortwo percent) that you can follow the voltage (i.e. current) variations on an analogue meter (if you can find one - they have a semicircular or square scale and a little pointer ). You can also use a DMM with peak detector. Any Fluke DMM will work.

I would not use a mains connected scope for this, but a battery scope is fine. Fluke, TEK, Agilent etcetera.

This measurement is OK if you want to compare currents in the sliprings. But it can also be used if you calculate current from voltage, resistance, length, area, resistivity and temperature. Your error should be well under 5 % if you do it right.

Gunnar Englund

http://www.gke.org

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

 

[OhioAviator]

Thanks Skogsgurra. Your suggestion makes perfect sense and sounds fine for taking a one-off voltage (current) measurement. However, I'm looking for a solution employing permanently installed equipment to allow for continuous rotor current measurement and/or monitoring. The more I think about my PLC inputs idea the more I'm warming up to it. Hopefully somebody will come up with a better idea.

Incidentally, I'm old enough to not only own a Simpson 260, I actually use it! That and my trusted 'Wiggy'

 

[Skogsgurra]

I got a 260 Simp, too! We should start a club! I also got an AVO 8.

Gunnar Englund

http://www.gke.org

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

 

[jraef]

Thought I had posted this earlier, maybe I never hit the "submit" button...

I think it's necessary to point out what I think is a minor flaw in some of the earlier postings regarding DC shunts, Hall Effect etc. You said this was a Wound Rotor INDUCTION motor, not a Synchronous motor. In a WRIM, the rotor current will be AC, not DC as it is in a Synchronous exciter system. A simple AC CT will work just fine, then use a Current Transducer to give your analog signal to the PLC, or you can now buy CTs that give a 4-20ma or 0-10VDC output directly. I'm not sure of the range of current that comes from a WRIM rotor circuit as the resistance and speed changes, but if it's high, a Rogowski Coil style transducer would be a better, albeit more expensive, solution.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[davidbeach]

Ah, Jeff, what frequency would you expect in the rotor of an induction motor running at 0.03 slip? It may well all be AC (except at synchronous speed), but it won't be 60Hz.

 

[jschwei314]

The current one wants to measure has varying frequency. At start the frequency will be 60hz. At running speed the frequency will be slip frequency. That is why we suggested shunts and DC amp meters.

 

[jraef]

OK, I see the point and bow to the experts. I've never had to think about measuring the current of a WRIM rotor, but I just wanted to make sure a common error wasn't being made because of the "slip ring motor" issue. I see that a lot.

But would a regular old CT care if the frequency was 60Hz or 2Hz? I know they saturate at higher-than-design frequency, but do they fail to couple at low frequency? I've never pondered that before, but I can't see why they would.


"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> faq731-376

 

[Skogsgurra]

Yes, Jeff. They do fail below a certain frequency.

It is about the volt-seconds available in the core. The burden is mostly ohmic and constant. Then, if you have a 1000/5 CT with a 1 ohms burden and is running 1000 A primary, the secondary voltage will be 5 V rms at 60 Hz and it corresponds to a certain area under the sine curve, i.e. a certain flx (or Vs) area. If you keep everything the same, but reduce frequency to 6 Hz, your voltage should still be 5 V rms, but the area under the sine will be ten times larger. That corresponds to ten times more Vs, which will have saturated the core long before you get down to 6 Hz. Most CTs have a range that stops somewhere between 40 and 45 Hz when running nominal (maximum) burden. Can be taken lower if burden is decreased.

Gunnar Englund

http://www.gke.org

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

 

[ScottyUK]

The same V/Hz rules which apply to motors apply equally to transformers, whether they are CTs or VTs. In all cases it is core saturation which determines the limit.


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