Testing damper winding in a salient pole rotor

[petronila]

Dear All,

I am looking for a reliable test method/procedure for testing the damper windings in a salient pole rotor. See attached sketch.

The idea is to test the condition of the bars (detecting cracked or damaged bars or any abnormal condition of the whole damper winding) without dismantling. All ideas are welcome.

Thanks in advance

Petronila

 

[waross]

First test, quick and simple.
Energize one phase of the motor with a low voltage. Rotate the rotor while carefully monitoring the current.
This test is based on the growler test for a DC armature. Any broken bars or loose connections will cause a drop in exiting current while rotated past the energized poles.

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Ohm's law
Not just a good idea;
It's the LAW!

 

[electricpete]

What you described Bill sounds like a single phase test for an induction motor rotor. It's not obvious to me that it would work in this situation.

Typically for induction motor the test needs about 25% rated voltage to get enough flux crossing the airgap to be meaningful. And you have to do the test relatively quickly to avoid overheating. For salient pole sync generator or motor, it seems even more of a challenge to supply the excitation to pump flux across that non-uniform airgap without overheating.

There is also a difference in geometry. A perfect squirrel cage rotor will give a uniform response, which a single bar defect in a squirrel cage rotor will show a deviation at a rotation repeating once every pole pitch (let's say a quarter of a rev on 4 pole machine) In contrast the salient pole machine even when perfect already has a deviation repeating once every pole pitch (due to reluctance variation of the pole piece). So you've got a built in variation repeating every pole pitch (and pole pitch is now smaller let's say 1/10 of a revolution for a 10 pole salient pole machine). Current would go low when the rotor pole aligns with the energized stator pole phase group and high in between. I guess you'd have to be looking for the finer variations associated with individual bars within a pole pitch during the low period? And those individual variations are spread out due to the distributed nature of the stator winding. It's not obvious to me whether there's enough spatial resolution to be able to see that.

If you've seen it done, I withdraw my comment, but at first glance it seems questionable to me.

Then again, I don't have any alternative suggestions in mind. Here is a good EASA document describing a whole slew of testing methods for squirrel cage rotor bars. There might be something in there.
Squirrel Cage Rotor Testing (EASA)



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(2B)+(2B)' ?

 

[waross]

Hi Pete;
At rest, the motor may be considered as a transformer with the damper winding forming a sort circuited secondary.
As the rotor is turned, I would expect to see a regular variation in the exciting current. As a high resistance area passes the excited area of the stator, I would expect an irregular variation in the exciting current.
I admit that I have not used this test on an AC machine.
However, I have used a similar technique to trouble shoot DC machines with armature faults.
The flux in the DC machine had no problem passing the air gap.
You should be able to run full load current for a reasonable time on a cold stator. (Cooling will be an issue on a stationary rotor so a full load current test should be limited to several minutes.)
It is worth a try. If the Original Poster does try this test, I hope that he will share his results with us.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!

 

[petronila]

Thanks for the inputs.

The poles are dismantled and disconnected so it is necessary to test one by one. It is a synchronous machine

Thanks

Petronila

 

[Hoxton123]

We used to follow similar procedures to those suggested by others.

If the poles are disconnected, you could try crack testing the bars etc. Ultrasonic or dye penetrant, for example. if you have a local electrical repair shop, they may have some ideas.

Is it a motor or a generator? Motors have the most arduous duty, usually, for pole face windings.

Please let us know the outcome as we all have to lern from each other!

 

[petronila]

Thanks for the replies,

Hoxton, This is a generator. I will share any outcome.

Petronila

 

[Gr8blu]

Petronila: To be complete, I'll give you a few options for an assembled machine as well as a disassembled machine.
Assembled:
With enough knowledge of the internal geometry a current-signature approach can be used where a known voltage is applied to the stator winding and the rotor rotated through at least 360 mechanical degrees while current waveform is monitored. Different fault conditions will exhibit different current signatures. Be warned - there's a lot of detail required of the original design/construction/geometry to make truly accurate conclusions on the rotor condition. The same method can also be used for determining stator faults as well.
Detail necessary includes: number and geometry of both stator and rotor slot, stator coil turns/coil, stator coil pitch, rotor bar pitch, number bars-per-pole, number poles, whether rotor bar slot is "open" or "closed", rotor bar material - if multiple materials are used, method of joining bar to shorting ring, etc.

Disassembled:
If bars remain shorted (which may be possible for generator applications, but is less likely for motor applications), do a modified "growler" test. Modified in this case means applying enough current to create a noticeable magnetic field - which, due to the fewer number of bars, is going to be higher than a comparable induction rotor test. If bars are not shorted, the growler test still applies - but a sensitive enough instrument can read the impedance of individual bars and realistically determine the condition of the bar integrity (cracks and other deformations will show up as higher impedance, usually). Given the large cross-section of the bar and the fact that it is (often) a single conductor, the device needs to be very sensitive to make accurate measurements. If measuring impedance, remember to correct for ambient temperature as well!

Also if disassembled - ultrasound may or may not pick up a problem, but die penetrant most likely will - at least for visible sections of the bar extending past the pole piece. To be honest, I've only used the die penetrant approach when the bar and ring is still connected and I suspect the joint itself is the issue.

Converting energy to motion for more than half a century

 

[waross]

If I had a ductor available I may do a comparison test between poles.
If I did not have a ductor available, I would not expend any money or time finding one.
I may use available components to do a ductor type test. That is, pass a high current across the damper segments and measure the voltage drop across the connection points to determine the impedance. I would do diagonal and straight across tests on each segment.
If you are able to use a high DC current, your results may be clearer.

--------------------
Ohm's law
Not just a good idea;
It's the LAW!