460V Motor failed on DC Hi-Pot test 1

[HamidEle]

thread237-8885

we failed on some 460V Hi-pot test applying 1000V DC. To me 1000v is a proper DC voltage for 460V motor as per IEEE-43 table-1.
Wondering what your thoughts are.

Thanks in advance.

 

[edison123]

All LV machines should withstand 1 KV DC hipot easily. Did you megger the winding after 'the failure'? Sometimes, the DC test voltage creeps/tracks to the ground via the exposed end winding of random wound windings and trips the HV kit. Such a tracking failure is not a winding failure.

Muthu

http://www.edison.co.in

 

[HamidEle]

Edison123, Thanks for the response. The injected 675V where they failed the test in one minute. The voltage did not even go to 1000v DC. We did not megger it after the failure. Maybe there was a cracking failure which causes the injected to the ground. will find out.

 

[RRaghunath]

Could it be that the kit has a limitation that it could not feed the capacitance of the motor windings (and tripped)??
Once you do megger the motor, let us know.

 

[HamidEle]

Raghunath, Planning to do a megger test on 500V DC to see what the IRs are. what I am thinking is the motor insulations might be compromised due to the moisture or dirty conditions. will share with you guys later.

 

[waross]

With a 480 Volt supply running 5% overvoltage the peak voltage will be 713 Volts.
I would megger at 1000 Volts.
I think that 1000 Volts may be low for Hi-Potting a 460 Volt motor.
Has this motor failed in service?


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[che12345]

Dear Mr. HamidEle,
1. Please tell us more details:
a) Motor 460V is 3phase , kW rating. Winding is in "star" or "delta" formation? What was the (trip voltage) recorded?
2. Did you carry out the [insulation resistance test] at (500Vdc or 1000Vdc) with respect to earth/frame
a) (if it is wound in star, with six terminals accessible), of each individual winding ? What are the (three) readings?
b) (if it is wound in delta), only one measurement is required. What is the reading?
3. In general, a motor rated 460Vac with insulation resistance to earth >2M Ohm would be acceptable to proceed on with the next stage "high voltage" test. The test voltage may be dc or ac.
4. If your test set has a [current trip] setting, set it to [maximum] (temperately) . Attention: see 5 below.
5. Increase the test voltage slowly and smoothly. Observe the leakage current (if the test set is equipped with one). Otherwise, observe the volt meter. It should be smoothly increasing without fluctuation, up to the final test voltage and maintained for the duration. Attention: take note the voltage reading when the volt meter (starts to fluctuate) and when it [trips}.
6. For high voltage test, the value of the [withstand voltage (in kV) and the duration (in s)] are the governing factors. That is why some simple HV test sets are not equipped with the leakage current meter.
7. If [insulation resistance] is in order but failed on [high voltage dielectric test], look out factors that are affecting the "creepage" and the "clearance" distances or cracks in the insulation material.
Che Kuan Yau (Singapore)

 

[waross]

Mr. Che;
I believe that HamidEle is in Canada.
Most 460 Volt motors are nine lead, dual voltage, 230/460 Volt rated motors.
Some 460 Volt motors are three lead motors.
Most motors in Canada are star connected, delta connected motors are rare at 460 Volts.
The star point is buried.
The frequency is 60 Hz.
Note:
In Canada standard motor voltages are derived from multiples of 115 Volts: 115V, 230V, 460V, 575V.
In Canada the standard supply voltages are derived from multiples of 120 volts and a factor of root three: 120V, 208V, 240V, 277V, 480V, 347V and 600V.
I hope this helps you understand Mr. HamidEle's situation.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[HamidEle]

Waross and Che,

Thanks for your inputs. What Waross sated is correct.

Yes. I am in Canada. We carried out the test with Line to earth on 1000V DC. The reading is for Ph-G.

 

[edison123]

che - Winding connection has got nothing to do with AC or DC HV test to ground.

Muthu

http://www.edison.co.in

 

[che12345]

Dear Mr. edison123
1. " che - Winding connection has got nothing to do with AC or DC HV test to ground."
Agreed. My intention is if the three windings can be separated, then it is possible to test each winding individually, to detect which winding is causing the problem. If the windings are not separable, it is possible to test with any one of the leads.
It is preferred and would save time to connect [all terminals (irrespective 3, 6, 9 or 12 etc.) all together] and test [them] (with respect to frame).

2. Dear Mr. waross
" I hope this helps you understand Mr. HamidEle's situation. "
Disagreed. Your invaluable information on the location (Canada), voltage (120V, 208V, 240V, 277V ...) and frequency (60Hz) etc. do NOT help on how to solve Mr. HamidEle's problem. It shows the "location" NOT the "situation/problem".
Che Kuan Yau (Singapore)

 

[waross]

You are welcome Mr. Che. Glad to help.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[HamidEle]

Thank you guys for the valuable inputs. Just noticed the motor is on VFD and has to withstand at least 1000V reflected wave from the drive output. in this case, we may have to stick with 1000v dc. Otherwise, we would have a problem when running.

 

[edison123]

1 KV DC hipot for VFD duty motors is very low. In my shop, we do 2 KV AC, 1 min AC hipot even for non VFD 415 V motors.

Muthu

http://www.edison.co.in

 

[EdStainless]

I agree Edison, I used to build motors and our 3kV motors would be tested at 10kV. And even our lkV motors were never tested below 5kV. These were all run on VFDs.


= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy

 

[HamidEle]

Guys, I know most of you are motor experts. Just want to clarify: DC Hi-pot tests on motors are based on phase to ground/earth. For instance, if I use 500v on phase to ground, the phase to phase would be 866V. I just need to Megger the insulation impedance in the field, trying to put stress on the motor in the field. In my humble opinion, 500V phase to ground is way over 277V when running. The reflected waves caused by VFD are not continuously voltage, more like at Peak/ transients. These should be 3.1 time motor rated voltage. we should not use these value to determine test voltage.

 

[waross]

The injected 675V where they failed the test in one minute.

By what criterea did the motor fail the test?
Did the test set trip off or was the leakage judged to be unacceptable?
Did the motor impedance drop abruptly as the voltage was raised?
I hesitate to comment further on a failed test without knowing more about the testing method and the benchmark for failure.
I have always field tested 460 Volt motors with a 1000 Volt DC Megger.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[HamidEle]

Hi, Waross,

Initially they tested them on 100V DC and then 500v. it shows some leakage on 500V test. I know NETA allows to Meggar motors under 1000V below using 500v dc.

Thanks

 

[che12345]

Dear Mr. HamidEle
1. With due respect, please answer Mr.waross's post dated 17 Mar 20 14:45 ;request for further details:
" By what criteria did the motor failed the test?
Did the test set trip off or was the leakage judged to be unacceptable?
Did the motor impedance drop abruptly as the voltage was raised?
I hesitate to comment further on a failed test without knowing more about the testing method and the benchmark for failure."

2. Your reply " Initially they tested them on 100V DC and then 500v. it shows some leakage on 500V test. I know NETA allows to Meggar motors under 1000V below using 500v dc."
With due respect, your reply IS irrelevant. The term [Meggar (should be Megger), an instrument manufacturer) should be avoided]. It is better to use the term [insulation resistance test].

3. FYI: [Insulation resistance test measured in M/G.. Ohm] is usually conducted in dc, while [dielectric high-voltage test measured in kV usually duration of 60s] is conducted in ac/dc. These are two different tests. Both test are carried out, each detects defects of different nature.
Che Kuan Yau (Singapore)

 

[HamidEle]

FYI, the vendor came back and recommended to use 500v to Meggaer 460v motor as it is not dielectric withstand test.