Megger vs. Meter

[iant]

You are right Dick,
In Australia - Megger tesating of appliances and instalations is compulsory - allowing for sensitive electronic components of course.
the testing voltage is manditory to twice the working voltage.
For Us any voltage below 1000V ac us considered as low voltage
Ausie qualified electricians dont work above this voltage.
The higher voltages have the different tests you mention.
I am qualified to operate up to 220KV but do not carry out testing.

 

[yam785]

Going beyond megger testing, motor rewinders use another method to test insulation involving placing a high voltage on the wiring and leaving it there for (as I recall) around 10 minutes. It seems that insulation that initially tests good can start to leak and fail after a few minutes of stress. If memory serves, that is called a PI or potential index test.

DickDV
I believe this is call highpot testing or something similar. It is great for new motors, but where I used to work, we believe they killed a 2400V motor, that ran fine, when shut down. It was a good idea to find the failure ahead of time, but problem was, it kept tripping after that. They were not prepared to deal with not being able to run this chiller.

Am I right in my thinking, that this high voltage test, actually caused the motor to fail, and make it un-runable?

Thanks for any insight.

 

[allscott]

As to insulation resistance values at the reject point, I've always heard 1Meg as being the pass/fail point. I've seen AC motors running at 200K but that's just asking for a flash and probably destroying the motor. Incidently, motor insulation that tests unacceptably low will often go back up to very good values once the windings are steam cleaned and then baked dry in an oven.

ANSI has guidelines for testing power line insulation and it is half of the voltage listed above. I've always thought that they were too low on motors, especially motors running on inverter power.

Going beyond megger testing, motor rewinders use another method to test insulation involving placing a high voltage on the wiring and leaving it there for (as I recall) around 10 minutes. It seems that insulation that initially tests good can start to leak and fail after a few minutes of stress. If memory serves, that is called a PI or potential index test.

Not sure what you are refering to as power line insulation but as a manufacturer of medium voltage power cable (15KV to 45KV) we do withstand tests on our cable for 5 minutes at 2 to 3 times the insulation value. We also test for partial discharge PD test at this voltage. During this test we are measuring for discharge in the pico coloumb range (less than 5pC).

I've never heard of a PI test but if you are refering to HiPot testing the theory is very similar to a megger except instead of measuring resistance you are measuring leakage current, as I'm sure your aware.

 

[allscott]

Am I right in my thinking, that this high voltage test, actually caused the motor to fail, and make it un-runable?

Thanks for any insight.

I would be surprised but...

 

[DickDV]

A megger test on a motor that is "on the brink" of failure can be made to fail completely will being meggered.

Also, if you try to megger a 120V motor with a 1000V megger or a 480V motor with a 4000V megger, you could easily damage the motor

 

[iant]

Many motors are designed to operate at their maximum voltage.
The HI-POT test as with the Megger test is primarily for insulation.
Both to frame and between phases. The original post questioning the validity of megger testing, i believe, has been answered.
Motor Insulation is only rated for that voltage range.
It is posible to blow up a motor using a VSD above 100% because the voltage between the phases can go higher than the maximun design.

 

[allscott]

Many motors are designed to operate at their maximum voltage.
The HI-POT test as with the Megger test is primarily for insulation.
Both to frame and between phases. The original post questioning the validity of megger testing, i believe, has been answered.
Motor Insulation is only rated for that voltage range.
It is posible to blow up a motor using a VSD above 100% because the voltage between the phases can go higher than the maximun design.

So you megger between phases do you? lol

 

[iant]

yes - we even turn the power off
loose more meggers that way

 

[allscott]

yes - we even turn the power off
loose more meggers that way

Not sure what you are trying to say but to avoid any confusion from anyone reading this thread, you do not megger a motor between phases. They are electrically connected.

 

[iant]

in STAR they are connected - Disconect star point if you can

They are not connected in delta
u-u1
v-v1
w-w1

Potential voltage 415
Test voltage 1000V

 

[allscott]

My apologies, you are correct.

 

[leitmotif]

I think things are getting fuzzed due to lingual differances. To keep someone who does not know from making mistake:
1. NEVER megger on energized equipment. Can destroy megger and electrocute you.
2. NEVER megger solid state
3. Yes you can megger phase to phase but a more approprite meter would be low range resistance. The only reason to megger phase to phase would be to check for insulation breakdowns common to two motor / transformer windings. Would have to open up all connections. Could be done on 6 or 12 lead motors - for others 9 lead or 3 lead kinda waste of effort.
4. The general use for megger is to check insulation resistance between a conductor in motor transformer or in a power run (conduit or cabled). There are some that can be used to get a resistance reading as the one shown above can do but read the manual or see the Biddle website for info on this application.
5. NEVER megger a person - they produce a lethal voltage but GENERLLY not enough current to kill. After he picks himself off floor he will be very irritated with you and just might impose body damage on you. If you megger yourself well at the minimum you will know why not to do it again.

Dan Bentler

 

[iant]

you attach teh megger to a urinal wall