OT - Questions on Insulation Tester

Good afternoon all, hope everyone is doing well.

When using an insulation tester on a powercable and motor that's fully disconnected from a servo drive, the readings should be OL from each phase to ground to ensure cable insulation is good and windings in motor are good.

However, when testing between phases, I have found some discrepancies and wanted clarification. Some of our maintenance guys say it's okay to use an insulation tester between phases to see if a motor is bad or not. I have read online that I should only use a regular ohm meter to test between phases. What is the correct method to test b/w phases?

I tested this on a huge 100lb Fanuc spindle motor (a06b-0831-b230, 240VAC 3phase) we are shipping for repair for mechanical issues. Using a megger on both 250V and 500V, between any two phases shows >250 Megaohms and >550 megaohms respectively, however using an ohm meter, I see 0 ohms between any two phases. The guy from Fanuc says it's possible to damage the windings using an insulation tester b/w phases, so the repair shop will check on that too for me.

However, I distinctly remember testing a 480VAC 3phase allen bradley motor for a grinder infeed wheel with a megger between any two phases showed 0 ohms using 500V option.

So both of my experiences b/w Fanuc and AB motors are conflicting, and the feedback I have received from my colleagues and OEMs is also conflicting, so hoping someone could help clarify for me? Thanks a bunch.

Insulation tester for line to ground, should read 1 Million ohms plus an insulation factor, typically 500 000 ohms per 100V. So, a standard 120V household circuit should have 1.5 M ohms from line to ground.

When testing a motors windings, without really advanced windings testers, your basic digital multi-meter continuity check is sufficient to find open windings. Use of an insulation is not necessary, and may damage windings depending on their insulation ratings. Volts applied with low resistance will draw high current! Limited in this case by the meters power source.

Yeah, OL would be overload or going towards infinity, so similar to your 1 Million ohms, no?

Sounds good, yeah, I'll start using basic multimeters for phase-to-phase and then insulation tester for phase-to-ground. I think the megger we have can do both.

Thanks,

to check motor winding
Low Ohms motor lead T1 to motor Lead T2
high Ohms an Lead to ground or chassis
the feed wires should be disconnected from both the starter and the motor
and each lead should be checked separately (Lead to Ground)
motor windings should be only a few Ohms < 5 or so depending on the motor
ever ever use a high pot tested on a motor the can blow a good motor ( I have seen it happen )
It takes some experience to do it right
the best place for this information is AEMC
they make the best I have used many of their test interments over the years

alive15 said:

Yeah, OL would be overload or going towards infinity, so similar to your 1 Million ohms, no?

Sounds good, yeah, I'll start using basic multimeters for phase-to-phase and then insulation tester for phase-to-ground. I think the megger we have can do both.

Thanks,

Click to expand...

I thought OL is Open Leads, which is, as you stated, going towards infinite resistance.

Read the PDF from megger a "stitch in time"
https://www.testequipmentdepot.com/megger/pdf/electrical-insulation-testing_guide.pdf

MasterBlaster said:

I thought OL is Open Leads, which is, as you stated, going towards infinite resistance.

Click to expand...

Yeah, that's correct. Overload is not the correct term. Open leads or open line.

Thanks everyone else for the info!