Insulation Test Results - Can't Wrap My Head Around It

Hello,

Hopefully someone here can help me out because I am not understanding something.

I did an insulation test on one of our motors (a06b-1411-b153) and I got back some results that I need help with.

Insulation Results March 9 2022
Cable and Motor @ 1000V
U-G 13.7 MOhm
V-G 13.2 MOhm
W-G 13.4 MOhm
Cable and Motor @ 500V
U-G 15.8 MOhm
V-G 15.2 MOhm
W-G 15.1 MOhm
Cable Only 1000V (disconnected from amp and terminal strip)
U-G 125 MOhm
V-G 152 MOhm
W-G 170 MOhm
Cable Only 500V (disconnected from amp and terminal strip)
U-G 125 MOhm
V-G 167 MOhm
W-G 177 MOhm
Motor Only 1000V (disconnected from terminal strip)
U-G 1 GOhm
V-G 1.5 GOhm
W-G 1.4 GOhm
Motor Only 500V (disconnected from terminal strip)
U-G >550 GOhm
V-G >550 MOhm
W-G >550 MOhm
Cable and Motor @ 1000V (after all previous tests)
U-G 49.5 MOhm
V-G 49.5 MOhm
W-G 49.8 MOhm
Cable and Motor @ 500V (after all previous tests)
U-G 53.4 MOhm
V-G 53.2 MOhm
W-G 53.1 MOhm


My questions are:

Why did my cable and motor insulation results jump up from around 13-14 MOhm to 50MOhm after reconnecting everything?

Why is my insulation results lower when cable and motor connected versus higher when disconnected?

I'm not exactly sure what your numbers mean, but I'm pretty sure you're not supposed to put a1000V Megger into a Fanuc motor. Keep it at 500V.

Quote:

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Originally Posted by the_gentlegiant

I'm not exactly sure what your numbers mean

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I'm not either. Measuring hundreds of megaohms in a workshop environment seems pretty meaningless to me. A speck of dust or a puff of moist air will bounce the numbers all over the place. And what do you do if your cable rings 50 MOhms instead of 100 - throw it away and buy a new one?

I say dig through all the Fanuc Servo Amp and Servo Motor Maintenance manuals and find out what good Megger readings should be. That or google it.

Most the numbers look fairly even. I'd say that's a good sign.

1st question. Are you seeing an issue in performance or simply running a test and curious on results.

2nd question. and this is based solely on the way you listed the tests.
Did you test while connected to the drive?


Cable and Motor @ 1000V <- Here you do not list any conditions.
U-G 13.7 MOhm
V-G 13.2 MOhm
W-G 13.4 MOhm
Cable and Motor @ 500V
U-G 15.8 MOhm
V-G 15.2 MOhm
W-G 15.1 MOhm
Cable Only 1000V (disconnected from amp and terminal strip) <- Here you specifically list "disconnected from amp".
U-G 125 MOhm
V-G 152 MOhm
W-G 170 MOhm

Hi,
measuring insulation resistance with a high voltage source is extremely common in industrial practice. Every time an electricain does a job he/she is supposed to check with a Megger the
insulation resistance (and earth loop impedance) befrore he/she may certify the work is safe.

If you are working on single phase equipment 500V satisfies the safety regs but 1000V is required for 3 phase 400V equipment.

If a Fanuc cannot handle 1000V insulation test its junk anyway. Insulation gets a real hard time in servos and spindles because of the high voltage transients caused by high frequency VFD currents.
In fact most good servos like Fanuc have 'vacuum impregnated' windings, an expensive process to thoroughly impregnate the entire winding volume with epoxy insulation goo.....all just to improve
the insulation resistance.

A Megger is effectively a sensitive ammeter and it measures the leakage current when 1000V (or 500V) between to nominally isolated components. If the insulation resistance is 10MOhm the current is:

I_leakage=1000 /10,000,000
=100uA

Thus any form of moisture in the air or dust or grime on the surface is going to effect the leakage current considerably, and thence you insulation resistance measurement. The variations you are seeing
are quite normal.

Legally all devices are required to display 1 MOhm or greater insulation resistance. For your servo I would say that anything over 5 MOhm is good, anything under requires
further investigation, under 1 MOhm is legally faulty.

Craig

Hi,
if the servo/spindle is still connected to the servo/spindle drive it would unwise to apply 1000V between windings as that would place voltage stress on the output IGBTs/MOSFETs of the drive.
Test the insulation of the servo by all means, but disconnect from the drive.

There is a procedure for insulation testing VFDs but best to follow the procedure in the manual for the drive.

Craig

Here's what Fanuc says about testing with megohmeter:

- - - Updated - - -

OOOPS...

Attachment 477196