Alternate tests to hi-pots

[pugap]

Looking for alternate ways of verifying a hi-pot failure, aside from tearing the motor down or rehipoting. When a hipot failure is found, I'd like to verify if it the findings. You could tear into the motor to look for stray wires and such, but you can get into warranty issues with the motor manufacturer. With the hi-pot test being somewhat destructive, we want to avoid repeat tests.

 

[Snoball]

Easiest first test would be lowest setting meggar pass/fail.

 

[edison123]

Low voltage (110 or 230 V) single phase, bulb test. Apply low voltage between the winding & the bulb in series and the stator frame.

If the bulb glows, winding has failed. Often, if you hold the test for more than a minute, you see a plume of smoke from the failed area.

 

[OperaHouse]

I've never tried it, but it would be interesting to see what a TDR would do with a failed motor. It would be a subjective test and only useful if you had a large number of identical motors.

 

[stardelta]

Surge testing is a very good and non-destructive way of evaluating windings. See the link.

http://www.electro-wind.com/Supplier/baker.htm

 

[aolalde]

I will Prefer Insulation Resistance (IR) (with a megger) according to IEEE std 43.

 

[oldfieldguy]

Failure is a rather non-specific term. Did the hipot trip off during a test, or did you just get unacceptably low readings.

Was the motor considered serviceable before testing, was it in storage, was it having problems?

If the hipot tripped off, then a subsequent test with a megger (megohmmeter. Megger is a brand name)will likely indicate a ground, thus verifying the failure.

Disassembly and visual inspection may reveal a failure point, but generally speaking, a failure induced during hipot testing is a low-energy event and does not leave much of a visible indication.

In my experience, you're likely headed for a rewind if it failed and you cannot find a problem in the power leads.

For future reference, it might be prudent to perform testing as indicated in NETA Maintenance Test Specification 2001 paragraph 7.15 and Table 10.1 before proceeding to an overpotential (hipot) test.

A questionable result for the megger test would indicate that further testing may damage a sick motor. Often, an in-service motor with a poor megger (polarization index) result can be remediated by cleaning and drying, whereas one which has failed during hipot testing is likely a candidate for more expensive work or replacement.

old field guy

 

[pugap]

It is a when the motor is not passing the leakage current spec during installation into the final oem product. The motor manufacturer and the oem have the same leakage spec, but the oem "fails" motors that the manufacturer has passed. So the issue is to find another means of testing to verify the "failure" which is not as destructive as the hipot test.

 

[oldfieldguy]

pugap--

That brings a different perspective. What size motor are we discussing here?

Is the OEM testing ONLY the motor or is he adding external equipment to the test?

It is not likely that the hipot test will locate the "failure" since there is really not a "failure" in the usual sense that the equipment no longer is able to function, but rather that it does not meet specification.

Properly stored (temperature, humidity, etc.)new equipment should meet manufacturer tests before placing into service.

Insulation (leakage) testing is interesting stuff, though, and if the measured resistance is quite high, in the gigohm range, tiny variations in conditions at the time of test will produce numerical results that are apparently quite dissimilar but that may not be of real significance in actual operation.

For instance, does a 480-volt motor really care if its insulation resistance is 40 gigohms at 500 volts, or 20 gigohms? Yet it is not unusual to see this magnitude of change due to temperature and humidity variations at the time of testing.

old field guy

 

[pugap]

48 frame PSC, low (230V) and high (575) voltage applications. No where near gigaohms, more like 40-80 ohms per winding (depending on HP, voltage, etc).
OEM testing is being done with the end unit as part of the final unit inspection. Hipot failures are typically assumed to be from the motor, but to verify requires further hipot testing of the motor to verify. Issue is to get away from this practice.

 

[oldfieldguy]

pugap--

Are we absolutely sure we're not talking about winding resistance? Winding insulation is generally measured to a ground reference using a megohmmeter and typical measurements on new motors are in the gigohm range. It may be measured with what is referred to generically as a "hipot".

For field maintenance purposes, no motor should be put back in service with less than one megohm resistance, with a rule of thumb being one megohm of resistance per thousand volts of operating voltage. New equipment should be MUCH higher.

Winding resistance, however, is measured from lead to lead on the available motor windings. Measurement of resistance of copper is temperature-sensitive, but should be not too critical with equipment at ambient temperatures in normal maunufacturing facilities. However, being of lower ohmic value, it is not unlikely that the addition of external connections and wiring will make a measurable change to the measured values as aopposed to measurements taken directly at the motor leads.

If I am incorrect and you ARE referring to insulation tests, and your measured insulation resistance to ground for the motor and external circuitry is in the 40-80 ohm range at the voltages you reference, there are some investigations to make: first, disconnect and test the motor. It should be in the high megohm range to ground. If so, then investigate the wiring and motor controller.

Unless I'm missing something in this long distance discussion, no part of the motor power circuit should have this low an insulation resistance to ground.

old field guy

 

[electricpete]

The main tools you have without disassembling the motor are:
1 - Determ everything possible to eliminate possibilities other than the motor
2 - insulation resistance test (megger)
2A - If you are lucky enough to have access to the neutral brought out to the terminal box, then break the neutral and repeat megger of each phase with the other two grounded.
3 - winding resistance test (bridge)
4 - inspect accessible areas of terminal box and vent duct for any anomalies.
5 - Apply heat and check for improvement in insulation resistance.

I'd say those are the main ones. Surge test and hi-pots are possibilities but are both in the category of "potentially destructive" tests which should not be performed if the megger doesn't pass (unless you are prepared to sacrifice the winding during the test).


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[pugap]

oldfieldguy
I misread your post and was referring to winding resistance. There's no questions or issues there.