Leakage Current

[Zorab]

Hi to all,

Looking for recommendations. We have supply 1320 kW wound motor, brand new and TECO brand. Upon testing on step voltage @ 2500 VDC, they got a leakage current of 1000mA. Looking for suggestions or any test methods to detect the problem. Thanks.

 

[Skogsgurra]

Describe the step voltage. What is the risetime? What is the internal impedance of the voltage source - is it a Bruker kind of thing? How did you apply the step and how did you measure the resulting current?

Capacitance between winding and frame is huge in such machines and charging that capacitance can easily start with amperes for a few microseconds. I don't think that you have problem at all. But can't say until you are more specific.

What is the insulation resistance? I guess gigohms. Did you do any tan(delta) or PI tests? Results?

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[electricpete]

As skogsgurra said, you can expect a high current due to capacitive charging currents immediately after the step.
You would typically wait 60 seconds after a voltage step before recording current per IEEE 95.

Btw you didn’t tell us the machine voltage and whether you are testing the rotor or the stator.


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(2B)+(2B)' ?

 

[dArsonval]

Multiple paths to ground will always exist. It's not a problem, it's a fact.

The only test that counts is when "real world" power is applied to the machine.

Moisture,[the environment] in some form is likely the culprit for the readings.

As suggested by Gunnar, "... I don't think you have a problem."

John

 

[electricpete]

Based on the info available, I don't quite agree with your implication, John. It sounds like you're saying it doesn't matter what current is measured. Then why bother even measuring?

IF the reading were taken 60 seconds after applying voltage (that's a big IF, but it is standard in our plant procedure based on IEEE 95), then the calculated resistance is in the same general ballpark as the insulation resistance (which is also measured after 60 seconds). R = V / I = 2500V / 1A ~ 2500 ohms = 2.5 kohms.

2.5 kohms is not what I expect to see for a 1250kw machine (even if it's only 600 volt machine or so, and moreso if it's a higher voltage). I myself would be very careful about energizing it without further investigation: (Resolve the test method time question. Apply insulation resistance temperature correction. Ensure all cables and accessories have been determinated from the winding. Inspect brushes and rings, terminal box. Lift brushes while checking rotor. Check phase to phase winding resistance. Check humidity. Inspect any accessible portions of windings).

Again I'm assuming 60 second reading which op has not clarified. I think most insulation resistance test sets would trip if they delivered 1A+ for 60 seconds (which suggests Gunnar was right that it was a shorter-duration measurement), but it doesn't seem prudent to suggest ignoring a reading without more info/discussion.

Edit - op hasn't been back since May 23




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[dArsonval]

Certainly "test" and testing is necessary before applying [real world] potential to an apparatus.

I agree with you Pete. Based on the info presented, my implication is a poor one.

The initial post lacked necessary wording or apparatus nomenclature to fully describe the motor.
It was eluded the motor had a [wound ROTOR], "but"... [it] was described as a wound MOTOR.
All motors are "wound" in some way or fashion.

It was also noted the generated post was by someone who did-not actually do the test.
"They" got a leakage current of...

Okay.

What kind of equipment was used for the test? We don't know that either.

Perhaps my off-the-cuff remark stems from having tested motors with sophisticated equipment
only to discover during application of real world power to the item, it still blew up.

In testing, I often think about what Prof. Charles F. Scott wrote regarding transformers.

"The idea of connecting a primary coil having a resistance of only 1 ohm between the mains of a 500-volt
circuit was so palpably a violation of Ohm's law as to shock the convictions and experience of those who
were familiar only with direct-current phenomena. It was a bold idea of a daring inventor who did that which
all are now so familiar."

Hence, ... put some power to it, and witness what happens.

John

 

[electricpete]

Thanks John, that makes sense.


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