Meggar Tests and Procedures 1

[Fraser]

I am asking about a procedure at work they call meggaring motors. I am instructed to remove the 3 phase conductors from the ac/dc drives. Then I am instructed to meggar each conductor to ground. Time is not considered. Distance to motor is not considered. The leads may or may not go through junction boxes. The surronding atmosphere is salt air(I work at a port). Most motors are connected Delta. Last night the abb drive tripped on earth fault,overcurrent,and over temperature. The "meggar test was done as described above. On the next shift the 92 amp /460 volt delta connected motor was removed for rewind after they tried to run it again. I believe that a meggar test should be done at the motor with all connections disassembled. If one wants to test the cables that is a seperate procedure. They are of the opinion you do the meggar test at the drive and you can test both the cable and the motor at the same time. If the ohms is equal across all windings then the motor is ok. some where as stated by them in the value of 2 ohms. Is there published literature on these procedures?<br>

 

[umn]

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I feel it is always better to check the motor at the winding terminals rather than the cable end. Ideally, this should not matter- but taking into consideration the working environment (moist and salty), I guess there is a probability that the insulation can become bad, and over many feet of cable length, there could be so much leakage if the cabling is old.<br>
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I've come across many instances where there is a cable fault. So, if there is a bad cable, we may be suspecting that the poor motor is guilty if we don't check the motor at the winding terminals.<br>
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Hence what we usually do is:<br>
1. Disconnect the conductors from the drive control end, and megger between cables and each cable to ground. Between cables, it should give almost equal value, in the range if 1-3 ohms for medium rating motors. Between each conductor and ground, megger value should be relatively high, around 1 meg ohm or above. (More exact values will be specified by the manufacturer; it will also depend on the working environment and type of motor enclosure)<br>
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2. If there is any problem, go to the motor, disconnect the cable - thus isolate the motor- and check the motor terminals again. Now, we can identify the problem to either the cable or the motor.<br>
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Let me see if more info is available on the web. I'll let you know.<br>
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umn@ieee.org