Unbalanced Rotor Currents ? 1

[edison123]

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.

 

[electricpete]

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.

 

[edison123]

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.

 

[DougMSOE]

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.

 

[edison123]

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.

 

[jbartos]

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.

 

[gsimson]

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.

 

[electricpete]

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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[electricpete]

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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[electricpete]

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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[electricpete]

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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[edison123]

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

 

[electricpete]

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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[aolalde]

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).

 

[edison123]

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.

 

[jbartos]

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

 

[Shortstub]

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

 

[Shortstub]

I retract my original query.

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

 

[Shortstub]

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

 

[electricpete]

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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