nirvic23

i have a 3 phase motor,need to check insulation resistance.can you please tell me how to connect probes and what reading should i get if the motor is good?

5volts

A megger is used to read from phase (winding) to ground. This checks for breakdown of insulation of motor windings. The rule of thumb is 1000ohms per volt. Then .5 or half a meg to ground would be acceptable for a 480 volt motor.

Zog

Be careful of "Rules of thumb", they are usually wrong.

Here is the NEMA spec, all valkues need to be temperature corrected to 40 degrees C (Very important, otherwise your readings are meaningless)

Minimum IR readings:
· IR 1 min = 100MW for DC armature and AC winding built after 1970
· IR 1 min = 5MW for most machines and random-wound stator coils and form wound coils rated below 1kV
· IR 1 min = kV+1 for machines made before 1970, all field windings, and others not listed above

As you can see 0.5M would never be considered acceptable. You also need to calulate the DAR or PI ratios, depending on the size of the motor.

wptski

Nice breakdown! How about minimums for cables below 600V like NM or even simple cords? I've searched and never found anything for cables.

__________________
Bill

Zog

600V rated cables should be tested at 1000VDC and be at least 100M corrected to 20 degrees C.

Control wiring IR tests performed at 500V for 300V rated wires and 1000V for 600V rated wires. Minimum spec is 2M

5volts

For the OP who is asking how do i connect the probes on a megger its safe to say follow the 1000ohms per volt. A "Rule of Thumb" should give him a good understanding of what to look for when he's meggering. A three phase 480 volt motor at .5 meg should be acceptable when trouble shooting a Faulty circuit such as a VFD/mag/motor problem. If I read more than .5 I am looking somewhere else on the circuit. It is a judgement call and I don't pull motors based on the no less than "One Megohm Rule". Assuming he is not installing new cables or working on switch gear if he were than he must follow specs for workmanship/Safety reasons/liability. I work mostly with 480 volt AC induction motors where we run till fail unless we have some downtime. Usually the VFD will pick up the Faulty motor before a megger will. Personally I think "Rule of Thumbs" are great because they will lead you in the right direction as opposed to a wild goose chase when troubleshooting. When a system is down and thousands of dollars per minute is drainning away I have no time to look at specs, or "google". I have to use the tools I've been taught and make the judgement call myself.

A megger can also be used for trending purposes if you have a decent preventitive maintenance program. When you install you megger the motor and from there you continue to monitor the results regularly. Then you can get a idea when a breakdown is going to occur.

Thanks for the list!

Zog

It is not that hard to carry the specs with you. As a certified NETA test tech I am required to follow these specs. Anything less and I need the customer to sign a waiver to energize or I could lose my certification. Only takes a few seconds to look it up. I also carry all the specs in PDF format on my smartphone.

Excellent point! I would even go as far as saying trending is the most valuable part of megger testing, even more so that the actual value itself. Knowing when something is going to be below the spec in the future allows for planning a replacement or repair during the next shutdown and having the right equipment ready for it. We call that predictive maintenence.

TheRick

That's fine, but as a maintenance electrician in an industrial facility, I am required to keep production rolling. To that end, the bottom line is that the 1K ohm per volt rule WORKS.

I could not agree more! Many do not understand that there are 3 differerent types of maintenance! Preventative, Predictive, and Corrective. The fact is corrective is always the more expensive way to go. Regular testing, and the trending of those tests is very valuable information that can definitely save lots of $$ in lost production time.

However, at 2:00am if the megger reads .5M ohm or better, I am moving on in my troubleshooting, because the motor is not my problem, and I need to get prodcution going again!

__________________
Rick -

"fool-proof systems do not take into account the ingenuity of fools"

acro

Without a megger on hand - only a DMM, is there much more than you can do than check for a short to ground on any of the motor phases?

Checking phase to phase doesn't really tell much other than to check for an open circuit, correct?


Say two of the phases shorted together somewhere in the windings. How could you find that?

frenchelectrican

Not always able catch the short to ground with DMM or DVM due the DVM or DMM only crank couple volts in ohm function and not very much to determed if you have actual short in the winding to the frame or arc short.

Oui either closed or open circuit with DVM or DMM.

Only way you can do that is use the megger it will tell the Megaohms and you can able tell the differnce when you engerized the megger and go thru the test prodcures.

And use Zog's specs that is very clearly stated the best methold and time to run the megger most case in a minuite the most.

Just be aware if you did actually meggered few different size motours it will show differnt result so there are few ways to dealt with it.

And there are few other threads have allready have megger discussion so you may want to hit the search function for this purpose.

Merci,
Marc

__________________
Bleu est beau.

acro

10-4

CheapCharlie

Actually winding resistance is a very important test on larger motors. (Phase to phase with a DVM). You have to be careful that the motor is not spinning at all (a fan for example) or your readings will be all over the place. But even a difference of .5 an ohm can be significant.

Cram Renraff

I have a 3 phase, 480 volt 20 hp motor running a conveyor which usually has standing freight on it. If they use more than half of the belt, it begins to overload (and is real close at half). All three phases read 100 megohms to ground. I'm thinking the motor is undersized for the job but since I am a vendor working at a different companies facility and they have their own engineering department, I am unlikely to convince the powers that be of this. Could I be wrong? Should I meg between the windings? Oddly, perhaps, of the five conveyors in this facility with 20 hp motors, this one pulls the lowest amperage without any freight on it.