Rotor Interturn Fault 1

[maypot]

Hello,

We suspect interfault in the rotor of one 150 k VA generator set. It`s a brushless machine.The exciter is rated 82 V 6 Amp.
The insulation resistance is fine and we wish to know basically how to test the rotor for interfault fault.
We do not have in hand dedicated tool for this purpose.
Grateful to have any input from forum members.

Bob

 

[Skogsgurra]

Hard to do from outside without stopping machine. What are your symtoms? Vibration? Uneven voltage? Wrong voltage?

A Baker test is normally very conclusive. But you need to disconnect any electronics, even diodes etc, before doing it. Most good winder shops can help you with that.

If there's no Baker or similar available, you can feed each winding with AC (50 or 60 Hz) and check current. If there is a lot more in one winding, then you have an interwinding short. You can use almost any voltage level as long as you do not overstress or overheat the winding.

Gunnar Englund

http://www.gke.org

 

[maypot]

Thks skogsgurra. The generator hardly developed the full rated voltage,at no load, with full excitation. Basically, what does the Baker test do ?

Bob

 

[ScottyUK]

You might consider an RSO test on the rotor. This is a very sensitive test which will detect a rotor inter-turn or earth fault even if it hasn't formed a 'hard' fault. This is a popular test on large utility-scale generators, but in principle it should be transferrable to a much smaller set. The RSO requires the unit to be offline in order to perform the test. Some sources say that an RSO is not possible on a set with a rotating rectifier: this is not strictly true, but the test does need to be modified slightly.

The link below gives some good guidance on fault detection methods used on larger machines - some may not be practical or economical on a small machine.

http://www.epri.com/attachments/224581_ShortedTD.pdf

Doble Powertest in the UK bought out Powermann in the last year or two, a company who have been in the business of RSO tests and other generator diagnostics for a long time. Worth a call if you are in Europe.


----------------------------------

One day my ship will come in.
But with my luck, I'll be at the airport!

 

[edison123]

maypot,

If you have access to pole to pole connections, then you can do an ac drop test.

Apply a low voltage (say 100 to 200 V ac) to the field winding terminals (after disconnecting them from rotating rectifier assembly). Then measure the voltage drop across each pole winding. The drops must be within 10%.

One thing though, you mentioned with full excitation, your stator volts is zero (am I correct ?). If so, did you check whether the neutral terminals are bonded together ?

* Anyone who goes to see a psychiatrist ought to have his head examined *

 

[jplinn]

Did you check the Diodes and fuses to insure the the bridge is working correctly? I believe on a rotating exciter you use an indirect measurtement for field current. A blown diode or open fuse may be limiting the field current.

If the bridge is working correctly. perfrom an open circuit saturation test and compare the manufacturer data to your test results. They should be close.

On two or four pole machines a flux probe is the good method to detect shorted turns. With a flux probe you can identify which pole has shorted turns.

As suggested by edison123 a pole drop also works well. However, many time the shoeted turns only show up at speed and as such will be missed by a pole drop.

If possible to perfrom a running impedance test will also identify speed sensitive shorted turn.



Reference

http://www.generatortech.com

jpl

 

[olicg]

Baker is the name of company who provides high voltage test tools. I think skogsgurra is talking of Surge tester from Baker Instruments Inc.

The surge test consists of applying a short, fast rise time, high current impulse to a winding. This high rise time impulse will induce, via Lenz’s Law, a voltage difference between adjacent loops of wire within the winding. If the insulation between the two loops of wire is damaged or somehow weakened, and if the voltage difference between the wires is high enough there will be an arc between the wires. The arc is detected by observing a shift in the surge waveform.

The Surge test is performed with an impulse generator and an oscilloscope type display to observe the “surge waveform” in progress. The surge waveform is a representation of the voltage present across the test leads of the Baker tester during a test. The indication of a turn-to-turn fault is a shift to the left, and/or a decrease in amplitude of the surge test waveform as the test voltage is increased.

RSO - Recuurent Surge Oscillation uses low voltage HF source for injection at both ends of windings. Its based on TDR principle. Doble is the best source for this to me.

Olicg