Comments on " meggers ".
Posted 23 May 2006 - 03:44 PM
" Megger " is a general name for a meg - ohm meter, used to measure resistance of a motor, or compressor winding. ( By the way, there is also a company named megger, that makes the same. ) Generally, what you are doing, is imposing a voltage on the winding, to see if there are any breaks, or bad spots, which may arc to ground. These tests are done, with power off, of course. The meter I use , is capable of putting out 250 volt, 500 volts, and 1000 volts dc. You want to use a higher voltage, than the tested equipment normally runs at, so you can stress the winding and see if there are any bad spots. So if you are testing a 120 volt motor, 250 volt would probably suffice,....... on a 240 volt motor, 500 v,................ and a 600 volt motor would be 1000 volts. You would attach one test lead to ground wire, or casing of the motor / compressor, and the other lead to the one of the motor leads, compressor leads, or if a small compressor, then the pins on the compressor themself. Sometime if the service tech. does not have a megger, he will try with his multi meter, which puts out 9 volts. If he can read that the winding is grounded, by putting 9 volts, ( which is the meters output,) across the winding, ....... its pretty safe to say....yup, its shot. Now what should a good reading be ??
Motor manufacturers typically recommend that for each 1000 volt ( KV ) that the motor runs at, you have one meg ( meg - ohm ) of resistance, with 1 meg being the MINIMUM. The higher reading , the better.....so infinity, ( a symbol, looks like 8 turned sideways ) would be the ideal reading, meaning that no voltage was able to arc to ground. Excellent !! :)If I am testing a 120 volt motor, then the one meg rule applies. If I am testing a 4600 volt motor ( using a high - pot, which is like a high voltage megger ), then 4.6 meg would be my minimum.
However, taking one reading on a motor, and making a decision could be questionable, unless the reading is dead to ground. and there is no question that the winding is shot. A borderline motor could continue to run normally. Practice in industry, is to take a test, every so often, and " trend " these readings. if there is a downward trend in the resistance, then the winding is starting to fail, and should be replaced.
Compressor manufacturers will have their own recommendations as to what to their specs should be. However, there are some cautions to be wary of when testing compressors:
1 - Do not power up, or megger test a compressor while in a vacuum. :yikes:I have seen this caution many times, so its good practice to listen to. Apparently you can damage the winding while in a vacuum, and applying power.:shock:
Hoping this assists someone in their trouble shooting !!;)
Posted 29 May 2006 - 05:02 AM
For service manuals and lots of other goodies, become an Apprentice ==> Apprenticeship
Posted 29 May 2006 - 06:50 AM
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