Megging / Ohming a Motor

[adamscb]

Forum,

From everything I've read / heard, when checking the insulation of a three-phase motor you use a Megger going from each phase to ground, but then a regular continuity tester for phase-to-phase checks. Is there a particular reason for that? Could Megging phase-to-phase possibly damage the motor, or is it safety-related in that you want to avoid messing with high DC voltages as much as possible? Please advise

 

[Ingenieur]

Forum,

From everything I've read / heard, when checking the insulation of a three-phase motor you use a Megger going from each phase to ground, but then a regular continuity tester for phase-to-phase checks. Is there a particular reason for that? Could Megging phase-to-phase possibly damage the motor, or is it safety-related in that you want to avoid messing with high DC voltages as much as possible? Please advise

ph to gnd is an insulation test, should be very high, x Mohms
ph to ph is testing coil continuity, should be < a few ohm range

 

[Cow]

I'm not an expert on this but I have megged a fair amount of motors, mostly 480v. Standard procedure for me, is 1000v tested phase to ground and phase to phase. Brian John and Zog will be the first to point out that you need to temperature correct your readings. I do not do this, it's generally a go-no go test for me, it usually tests very good or almost zero in my experience.

So far I've found two motors out of the couple hundred or so I've megged that have shown infinite resistance between two of the motor leads when megged phase to phase. You're looking for 0 megaohms. Since I already have the megger and test leads right there, it only takes a few more seconds to verify one winding isn't open which is why I test phase to phase.

I do have a 289 I use with a low ohm setting to test between the windings and verify they are all close to the same reading. Generally, I don't get the 289 out unless I've got a motor giving me issues that the megger isn't picking up on.

 

[ActionDave]

I hardly ever use a megger for anything other than a quick and dirty test to ground.

The only reason I can think of to test "phase to phase" is if the labels on the leads are missing, and it would be far better to pray to God this never happens to you.

 

[kwired]

Phase to phase test should have a low resistance and there is no need for the high test voltage used for meg-ohm testing. If your usual digital meter with a low test voltage can't read any continuity through the winding they are open-circuited.

 

[ptonsparky]

The Flukes we use have a pretty good low ohm scale that can catch shorted windings. Depending on the size of the motor.

Most of ours are pass/fail and we do not do predictive maintenance, or haven’t for some years now. It’s a good idea if you can schedule down time and keep proper records.

On occassion we open all the leads and meg for winding to winding faults if it has passed a ground fault test.

Dont ignore the basic smell test.

 

[retirede]

I think by “phase to phase” he actually means winding to winding.

 

[junkhound]

On the subject of measuring motor or generator winding resistance:

Was aboard ship 200 mi NW of HI. Problematic 3 MW 11 kV generator was suspected of having a shorted turn on the rotating field winding (turned out to be a degraded Basler regulator due to thermall hot 'fast-on' connector).
No fancy 4 terminal dc meter for measuring the spec 84 milliohm winding resistance at room temp. No inductance bridge either.

Soldered 10 AWG to 6 parallel D cell. Applied across the field winding. Gen set had been off for a few days so at room temp. measured voltage drop (using 4 term type connections) with Fluke 77 and 10 AWG wire current also with a Fluke (about 9 amps for 6 D cells in paralled driving) . Calculated 84 mohms, so all was good.
Usefull technique for low value dc measurements when a fancy 4 terminal meter not available. Can measure down to a few hundred microohms.

 

[ActionDave]

On the subject of measuring motor or generator winding resistance:

Was aboard ship 200 mi NW of HI. Problematic 3 MW 11 kV generator was suspected of having a shorted turn on the rotating field winding (turned out to be a degraded Basler regulator due to thermall hot 'fast-on' connector).
No fancy 4 terminal dc meter for measuring the spec 84 milliohm winding resistance at room temp. No inductance bridge either.

Soldered 10 AWG to 6 parallel D cell. Applied across the field winding. Gen set had been off for a few days so at room temp. measured voltage drop (using 4 term type connections) with Fluke 77 and 10 AWG wire current also with a Fluke (about 9 amps for 6 D cells in paralled driving) . Calculated 84 mohms, so all was good.
Usefull technique for low value dc measurements when a fancy 4 terminal meter not available. Can measure down to a few hundred microohms.

Thanks. I'm gonna write that down.

 

[zog]

Meg phase to ground for 10 minutes, Record values at 30 sec, 1 min, and 10 minutes so you can calculate DAR and PI. Temp correct to 40C. A proper winding resistance test would be done with a winding resistance test set or a DLRO.

 

[Tony S]

 

[Ingenieur]

On the subject of measuring motor or generator winding resistance:

Was aboard ship 200 mi NW of HI. Problematic 3 MW 11 kV generator was suspected of having a shorted turn on the rotating field winding (turned out to be a degraded Basler regulator due to thermall hot 'fast-on' connector).
No fancy 4 terminal dc meter for measuring the spec 84 milliohm winding resistance at room temp. No inductance bridge either.

Soldered 10 AWG to 6 parallel D cell. Applied across the field winding. Gen set had been off for a few days so at room temp. measured voltage drop (using 4 term type connections) with Fluke 77 and 10 AWG wire current also with a Fluke (about 9 amps for 6 D cells in paralled driving) . Calculated 84 mohms, so all was good.
Usefull technique for low value dc measurements when a fancy 4 terminal meter not available. Can measure down to a few hundred microohms.

clever