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Measuring Rotor Current In Wound Rotor Motor

Measuring Rotor Current In Wound Rotor Motor

Measuring Rotor Current In Wound Rotor Motor

(OP)
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?
 

RE: Measuring Rotor Current In Wound Rotor Motor

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

Muthu
www.edison.co.in

RE: Measuring Rotor Current In Wound Rotor Motor

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

RE: Measuring Rotor Current In Wound Rotor Motor

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.

RE: Measuring Rotor Current In Wound Rotor Motor

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.

RE: Measuring Rotor Current In Wound Rotor Motor

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.

RE: Measuring Rotor Current In Wound Rotor Motor

I would use LEM Hall Effect Current Sensors.

http://www.lem.com/

RE: Measuring Rotor Current In Wound Rotor Motor

(OP)
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!
 

RE: Measuring Rotor Current In Wound Rotor Motor

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
 

RE: Measuring Rotor Current In Wound Rotor Motor

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.   

RE: Measuring Rotor Current In Wound Rotor Motor

(OP)
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.
 

RE: Measuring Rotor Current In Wound Rotor Motor

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 smile ). 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
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Measuring Rotor Current In Wound Rotor Motor

(OP)
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' smile
 

RE: Measuring Rotor Current In Wound Rotor Motor

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

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Measuring Rotor Current In Wound Rotor Motor

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  
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RE: Measuring Rotor Current In Wound Rotor Motor

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.

RE: Measuring Rotor Current In Wound Rotor Motor

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.

RE: Measuring Rotor Current In Wound Rotor Motor

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: Eng-Tips.com Forum Policies  

RE: Measuring Rotor Current In Wound Rotor Motor

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

RE: Measuring Rotor Current In Wound Rotor Motor

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.
  

----------------------------------
  
If we learn from our mistakes I'm getting a great education!
 

RE: Measuring Rotor Current In Wound Rotor Motor

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: ability of clamp-on probe to see exponentially-decaying dc current  

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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RE: Measuring Rotor Current In Wound Rotor Motor

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?   

RE: Measuring Rotor Current In Wound Rotor Motor

(OP)
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.
 

RE: Measuring Rotor Current In Wound Rotor Motor

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

RE: Measuring Rotor Current In Wound Rotor Motor

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

RE: Measuring Rotor Current In Wound Rotor Motor

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?

RE: Measuring Rotor Current In Wound Rotor Motor

(OP)
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.
 

RE: Measuring Rotor Current In Wound Rotor Motor

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.

RE: Measuring Rotor Current In Wound Rotor Motor

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

RE: Measuring Rotor Current In Wound Rotor Motor

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.

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