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Unbalanced Rotor Currents ?

Unbalanced Rotor Currents ?

Unbalanced Rotor Currents ?

(OP)
We recently overhauled & tested a 600 HP slipring motor with following nameplate:

Output - 600 HP    Speed - 990 RPM  (50 Hz)    
Stator Volts - 3300 V    Rotor Volts - 720 V
Stator Current - 96 A    Rotor Current - 390 A

During open shaft run, with the rotor energized at 400 V, we found unbalanced and hunting rotor currents of 70-75, 85-95 and 44-46 Amps. Suspecting rotor winding problem, I did a surge comparison test but it did not reveal any rotor winding problem (no phase unbalance or reversed connections etc.)

On open shaft run with stator energized at 400 V, the stator currents were equal and steady at 5.5 Amps.

The open shaft start up with stator energized was quicker and smoother as compared to start up with rotor energized (when it should have been the other way about) thus confirming that the unbalanced rotor currents were producing negative sequence torque resulting in net lower starting torque when rotor is energized.

Apparently both the stator & rotor were rewound some 12 years back and it was running without any problem. The motor came to me for conversion of sleeve bearings to anti-friction type.

What could cause these unbalanced and swinging rotor currents ? If the rotor winding is wrongly connected (reversed connections or unbalanced winding etc.) should not the surge test reveal it ?

I welcome any opinions on this strange phenomenon.


RE: Unbalanced Rotor Currents ?

I'm not that familiar with wound rotor motors.   Are you saying you did a no-load run with 3-phase voltage applied to the rotor and with stator windings shorted together?  If that is the case I would not be surprised that the torque-speed characteristic might be much different and slow or uneven acceleration might be normal.

On the subject of unbalance I have seen 10-20% unbalance in unloaded current (stator of squirrel cage motor) go down to 1-2% when load is added.

I have not heard of any standard being applied to this no-load current balance measurement (stator or rotor) and I don't have enough feel to know what is normal.

RE: Unbalanced Rotor Currents ?

(OP)
pete,

Yes, I ran the motor on no-load open shaft with voltage applied to rotor and stator shorted (and also the other way about). For a slipring motor with nearly equal stator and rotor voltages, torque production with stator or rotor energized is nearly the same.

In this case, the rated rotor volts of 72O is nearer to the applied voltage of 400 V than the stator volts of 3300 V. So, the start up with the rotor energized should be quicker than the start up with the stator energized. But it was not so is due to the fact that the unbalanced rotor currents produced negative sequence currents and torque which prevented a faster acceleration.

No-load unbalanced currents in this case was more than 50% which is quite severe regardless of what standards say.

The puzzle is that the surge test did not reveal any problem with the rotor winding or stator winding and also the fact the motor has been running well for the past 12 years or so.

RE: Unbalanced Rotor Currents ?

1)Never short the slip rings of a WRIM and start it across the line. The motor was most likely never designed with enough inductance in the rotor circuit to stant the currents induced.
2) The torque developed by the rotor is 99% done by the external rotor resistance.
3) When you  ran the motor with the rotor and the stator energized, was the rotation of the rotor and stator magnetic fields in the same or diferent directions???
4) The use of the "surge test" on rotors or stators is almost useless because the attenuation of the surge when applied to the windings in a magnetic core is VERY SEVERE.
The surge test was originally designed to test rotor or stator windings prior to their installation into the respective cores. It was never designed to be applied to a motor/generator already wound. Yes, I know that there are companies that make equipment to do this "surge test" but spend some time looking at the large signal analysis Vs small signal analysis and the effect of magnetic materials nearby when a large impulse is applied.
The swing rotor currents would come from the direction of the 2 magnetic fields interacting. Whatever rotation you gained was using the reluctance of the rotor and the resultant air gap flux.

RE: Unbalanced Rotor Currents ?

(OP)
doug,

1. The motor was first run with rotor energized and stator shorted through the LRS. Then, the motor was stopped and the stator was energized witn rotor connected to LRS. Both were not energized together at any point.

2. The rotor & stator were surge tested (at 1.5 times their rated voltages) before they were put together and no problem was seen.

3. My problem was the swinging as well as very unbalanced rotor currents when rotor was energized and the resulting longer start up time.

All these were under open shaft no-load condition.

RE: Unbalanced Rotor Currents ?

Suggestion to edison123 (Electrical) Jan 17, 2004 marked ///\\\
We recently overhauled & tested a 600 HP slip ring motor with following nameplate:
Output - 600 HP    Speed - 990 RPM  (50 Hz)    
Stator Volts - 3300 V    Rotor Volts - 720 V
Stator Current - 96 A    Rotor Current - 390 A
During open shaft run, with the rotor energized at 400 V, we found unbalanced and hunting rotor currents of 70-75, 85-95 and 44-46 Amps.
///What was the voltage across the motor stator terminals?\\\
 Suspecting rotor winding problem, I did a surge comparison test but it did not reveal any rotor winding problem (no phase unbalance or reversed connections etc.)

On open shaft run with stator energized at 400 V, the stator currents were equal and steady at 5.5 Amps.
///How was the rotor winding connected and loaded?\\\
The open shaft start up with stator energized was quicker and smoother as compared to start up with rotor energized (when it should have been the other way about) thus confirming that the unbalanced rotor currents were producing negative sequence torque resulting in net lower starting torque when rotor is energized.

Apparently both the stator & rotor were rewound some 12 years back and it was running without any problem. The motor came to me for conversion of sleeve bearings to anti-friction type.

What could cause these unbalanced and swinging rotor currents ?
///Have you checked slip rings and motor rotor loading resistor?\\\
 If the rotor winding is wrongly connected (reversed connections or unbalanced winding etc.) should not the surge test reveal it?
///Yes, perhaps for the loaded motor by some mechanical shaft load.\\\
I welcome any opinions on this strange phenomenon.

 

RE: Unbalanced Rotor Currents ?

If open circuit test is done with supply to the stator and rotor opened defects if any in the connection or the winding can be found out.
The contact resistances between sliprings and brushes and the shorting cables may be checked.
The rotor current can fluctuate only if the rotor voltage fluctuates which depends on the speed and stator voltage.

RE: Unbalanced Rotor Currents ?

It is an interesting setup. As I said I have no experience with that, but I like to throw out  ideas.

As gsimson point out possible slip ring problem or loose connection in LRS.  I know you have thought of that.

One thing I see in this setup is the possibility of saturation of the stator iron when rotor is at full speed. Stator is designed to see rated flux at rated frequency. But frequency of air-gap field and stator field in your setup will be very low, only slip frequency. Frequency decreases, flux magnitude relatively constant,  and therefore volts/hz increases causing saturation..  Maybe this saturation causes unbalance.  After all the no-load currents are primarily limited by reactance.  I don't know exactly why the no-load current in normal motor tends to be more unbalanced than load current... I suspect the reactances are sensitive to small assymetries.  Maybe in your motor with saturated stator, that sensitivity is magnified.  Then again maybe I am just full of hot air.  If you had time it would be interesting to check whether there are heavy harmonics (evidence of saturation) in the rotor and stator currents.

=====================================
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RE: Unbalanced Rotor Currents ?

By the way the term "slip speed" as used above is same as normal: sync speed minus rotor speed.

As rotor approaches sync speed, the slip speed, air-gap flux frequency, and stator flux frequency will all aproach zero in your inside-out motor.

Now I go back to the normal motor at no-load, why does it seem to have higher unbalance?  The rotor is saturated.

And when either core is saturated, the currents drawn will be very sensitive to even a very small voltage imbalance.  I think the unbalance current we see at no-load in normal motor and your inside-out motor are hyper-sensitivity to voltage unbalance because of saturation.  (not small assymetries as I said above).

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RE: Unbalanced Rotor Currents ?

It's 11:20pm here and I am turning into a pumpkin, so you guys tell me tomorrow if I am off my rocker.

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Eng-tips forums: The best place on the web for engineering discussions.

RE: Unbalanced Rotor Currents ?

One other thing... the source of your small rotor supply voltage unbalance may include normal (small) variations in brush/ring contact resistance.

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RE: Unbalanced Rotor Currents ?

(OP)
pete,

appreciate your posts when your are trying to stay awake.

in this case, rotor saturation does not arise since i am applying only 400 V as against the rated rotor volt of 720 V.

btw, both the stator and rotor are fractional slot windings (not that this would create any unbalanced currents).

My guess is that the last motor repair shop screwed up the rotor winding connections but I am puzzled why this did not show up in the surge test.

kumar

RE: Unbalanced Rotor Currents ?

I think I was incorrect on the saturation.  If we energize one winding (stator or rotor), then at no-load the other winding sees close to 0 hz.  However it also sees close to zero volts.  No saturation

=====================================
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RE: Unbalanced Rotor Currents ?

Edison123:

Do a transformer test with locked rotor.

Feed first the stator and measure rotor induced voltage. It should be according to the nameplate ratio.
Now feed the rotor and measure the stator induced voltage. Same ratio should result.
If not the rotor winding is wrong.
Surge test is a comparison impedance test, as far as you screw symmetrically a winding it tests Right. (Same problem in all three phases).

RE: Unbalanced Rotor Currents ?

(OP)
aolalde,

Thx for your post. I already did that and found that for a balanced stator applied voltage, rotor induced voltage (though nearly equal to name plate ratio) is varying by nearly 7 to 10%.

btw, in transfromer test (or open circuit test as we call it), the turns ratio with stator energized will be more than the turns ratio with the rotor energized. This is due to winding leakages. Both these raios will not be the same.

RE: Unbalanced Rotor Currents ?

Suggestion: The rotor and/or stator may be experiencing interturn short(s).

RE: Unbalanced Rotor Currents ?

Edison123,
Does the rotor-current unbalance reduce as the motor is loaded?

RE: Unbalanced Rotor Currents ?

I retract my original query.

Running the stator @ 8% (400/3300V)precludes development of a stable magnetizing flux wave.  This will affect rotor current.

RE: Unbalanced Rotor Currents ?

Re my last post:
Oops, arith"O"... should be ~12%, not 8%!

RE: Unbalanced Rotor Currents ?

ss - When the unbalance occurred was not with stator energized at 400v, but with rotor energized at 400v.  400V/700V ~ 65% of full voltage.

=====================================
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RE: Unbalanced Rotor Currents ?

ep,
Thanks for the advisory.  The problem now worsens.  If you model the motor with the stator and rotor parameters interchanged, the rotor as the armature connected to a supply voltage of 400V, and the stator leads shorted, the magnetizing current is even less stable.

My point is... if, prior to the mechanical work, there was no noticable problem with the electrics, then the test stated showing rotor current variation is... a witch hunt.  Especially if there is no prior comparable data.  

RE: Unbalanced Rotor Currents ?

I agree I personally wouldn't do that test because I don't know anything about it. I wouldn't call it a witchhunt... all edison asked is why it acts that way.  

Can you explain more why we would expect unstable magnetizing current?

I agree on the basis of different equivalent circuit parameters we expect different torque speed. Edison has already responded to that comment by pointing out that the pu voltage on the rotor is much higher, which should certainly make the average torque speed higher when energized by the rotor (since torque ~ Vpu^2), even though we don't know the exact shape.

There is one other aspect: harmonic torques. As we know long ago motors had problem with locking, cogging, syncronous torques etc. Careful selection of rotor and stator slots takes care of that. I am positive that there is very substantial change in the harmonic  fluxes when we energize from rotor. This may undo the careful design of the OEM and reintroduce torque cogging, locking, etc

=====================================
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RE: Unbalanced Rotor Currents ?

(OP)
ss,

Wound rotor motors can be run with rotor energized and stator shorted and it behaves exactly the same way if it is energized other way about. No load currents (with stator or rotor energized) are always equal for balanced voltages. I have tested many WRIM's with rotor energized without any problem. This is the first time I am seeing a rotor, which passed a surge test, showing unbalanced no-load rotor currents.

pete,

cogging, crawling, harmonic induction and synchronous torques etc. are all connected with cage rotors and not with WRIM's.

RE: Unbalanced Rotor Currents ?

If you say torque cusps etc are not a problem on wound rotor motors, I will take your word for it (I don't know much about that).

What about if there were a rotor turn fault far from the slip ring connection down in the middle of the winding. Is it possible surge test would miss it?  Maybe a rotor winding resistance test would be worthwhile?  I don't think it's a likely (for a rotor turn fault to persist without escalating rapidly to phase or ground fault when energized at rotor side) but you never know.

=====================================
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RE: Unbalanced Rotor Currents ?

Question: Have slip rings, brushes, connections, etc. that may be producing unbalanced and hunting motor rotor currents, been checked?

RE: Unbalanced Rotor Currents ?

(OP)
pete & jb,

The rotor winding resistances are equal both at sliprings and at the winding take off leads (which were newly provided due to the bearing alteration). Brushes are all seated well on the sliprings. Connections from the brushes to the external terminals were also found ok.

RE: Unbalanced Rotor Currents ?

Edison123

Do you have winding data and lamination details for stator and rotor? 600 HP regularly had a wave winding rotor can you provide the connection details?

Why do you affirm that wound rotors do not have harmonic, cogging or noise problems? Do those motors develop a perfect sinusoidal flux distribution and the air gap reluctance is totally constant in spite of slots?

RE: Unbalanced Rotor Currents ?

Back to the original problem!
    Regardless of which winding is used as the armature, and which the "rotor", and although I agree either condition will result in rotation, you are really comparing apples to oranges.
    If either the stator or rotor is connected to the source, its current and voltage measurements are at line frequency.  The other measurements, ignoring startup, are at slip frequency.  So my questions...

1) What was the test setup for each condition!

2) Was the instrumentation adequate for the frequency of the circuit in which they were connected?

  
 

RE: Unbalanced Rotor Currents ?

SS
#2 is answered by the original post.  
Rotor currents measured when rotor was externally powered.  
Stator currents measured when stator was externally powered.
=> All measured currents are at 50hz. There is no reason to suspect current measurement error associated with frequency.

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RE: Unbalanced Rotor Currents ?

I thought that may be a short circuit develops in the rotor winding only when it spins, due to centrifugal forces. Static tests will not detect such a condition.

RE: Unbalanced Rotor Currents ?

Suggestion: Magnetic circuits should not be overlooked. There may be some problems with the magnetic circuits, e.g. poles, gaps, etc.

RE: Unbalanced Rotor Currents ?

EP,

You misunderstood my caveat.  Line frequency parameters are only present in the "rotor" at standstill, otherwise they are at a much lower frequency.  The following will explain my "inductive" reasoning (excuse the pun):

My first engineering job was in an industry that had numerous WRIM's.  Most were GE, Westinghouse, and Western Electric (yes, I said Western Electric) machines, manufactured in the early 20th century (as I was).  (An aside... they were monstrous in size compared to today's designs!)  I remember several salient points:

1) The reverse open-circuit, blocked-rotor, test, i.e., the rotor as the primary winding, and the stator as the secondary winding, could uncover iron damage, if significant.  However, a more useful aspect of this test was that turns-ratio could be determined with greater accuracy than possible with the conventional test, i.e., stator as primary, rotor as secondary.

2) Either test was more useful if power measurements were included.

3) If rotation was a requirement for the reverse test, then short-circuiting the stator winding was done, but only if the stator was wye-connected.  If the winding was delta-connected, the terminals were not shorted!

4) If rotation was a requirement for either test, the instrumentation on the secondary side had to be capable of working at slip frequency, i.e., sf, where f represents line frequency.  Then, for no-load tests, secondary current and voltage parameters have an operational frequency less than 1 Hz. I know of no portable instrumentation with such a capability, unless it is of the moving iron-vane type.  A standing joke at the time was that it took two men to carry the tong-type ammeter.  Today's instrumentation with a "true" RMS capability may be able to carryout such a low frequency measurement, but only if so specified by the manufacturer.    

ED123,
What say you regarding your finding/conclusion!  Is the machine in service?
 

RE: Unbalanced Rotor Currents ?

(OP)
ss,

The motor had been in service for nearly 12 years before it came to me for bearing conversion. Now the motor is in service again and the client has not reported any problem.

In re point 3 of your last post, why would not a delta wound stator (shorted across terminals) work in the reverse mode ? Am I missing something ?

RE: Unbalanced Rotor Currents ?

Comment on the Delta wound stator shorted: If the stator is Delta-Wound and rotor is Wye-wound, there will be voltage shifts affecting the reverse test results.

RE: Unbalanced Rotor Currents ?

For WRIM, Coupling between rotor and stator windings is purely magnetic. Then the windings connection (wye or delta) do not change any electrical performance nor for the stator neither for the rotor.

RE: Unbalanced Rotor Currents ?

ed,
I didn't say it wouldn't work for the delta connection.  It just isn't necessary.  A delta connection is already a closed circuit.  However, the jumpers used between the stator terminals could influence operation depending on their size.

Transform the delta to an equivalent wye.  Then the jumpers are analogous to the external resistors of a WRIM.  For no-load operation when slip is near zero, then the jumper resistance magnitude compared to the stator winding resistance magnitude, alters torque output.

Perhaps it was done as routine in your facility because the stator winding connection was unknown.  Do you know the wire size of the jumper?  BTW, measurement of stator current and voltage, would probably have been more revealing.

jb,
Anything is possible.  But a mix is not done as a rule for viable commercial design, unless the WRIM stator is wired for different speeds.  (doug, any input?)

aol,
Hopefully my response to ed's query, should suffice.

RE: Unbalanced Rotor Currents ?

(OP)
ss,

Just trying to understand your post.

Do you mean that if the stator winding is delta connected, then the motor will run (under open shaft) without shorting the line terminals ?

If yes, then no. The motor will not run because it will still be in open circuit state and will see only induced voltage across stator terminals.

If no, then I misunderstood your post (again!).

As Aolalde says, winding connection does not change the basic physics of the induction motors.

RE: Unbalanced Rotor Currents ?

(OP)
aolalde,

You had mentioned an interesting point about "dynamic" rotor winding short.

Suppose, if I had mechanically driven this motor with another motor and then applied surge testing to the rotor sliprings, do you think

(a) I can see the rotor short or

(b) I would need to buy another (expensive!) surge tester.

RE: Unbalanced Rotor Currents ?

Edison:

I think your proposed test a) must work, since the rotor connection will be seen steady by the surge tester through the brushes and if your pony motor is close to full speed that kind of short should be shown. I have seen wound rotor windings with faults or wrong connection working if not at optimum conditions.

As far as I know, other ‘expensive’ test equipment should be a current signature analyzer, but those work better with the motor loaded so the slip shows the rotor winding performance.

RE: Unbalanced Rotor Currents ?

With the rotor mechanically driven and stator deenergized/open-circuited, you could apply line frequency to the rotor and measure current to determine impedance. Repeat between each pair of slip rings.  Shorted turn should cause some unbalance (sort of similar to pole drop test).  As I'm sure you know inductance varies with turns-squared and you don't have as much interference from various contact resistances as you would with dc test or low-frequency rotor currents during operation.

Then again, this is now a helluva lot different than your original test powered from the rotor (only difference is stator open-circuited).

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RE: Unbalanced Rotor Currents ?

Comment on edison123 (Electrical) Jan 29, 2004 marked ///\\\
Wound rotor motors can be run with rotor energized and stator shorted and it behaves exactly the same way if it is energized other way about.
///Yes, somewhat. WRIM is not designed to be powered from the rotor and deliver the shaft rated HP. There an energy flow problem.\\\
No load currents (with stator or rotor energized) are always equal for balanced voltages. I have tested many WRIM's with rotor energized without any problem. This is the first time I am seeing a rotor, which passed a surge test, showing unbalanced no-load rotor currents.
///The reverse tests will always be somewhat off since the rotor side is energetically rated less than the stator side. Also, the rotor source of electromagnetic field is different from the stator source because of the air gap geometries. Perhaps, a motor simulation might reveal more about the differences.\\\

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