Megging motor
Megging motor
(OP)
A subject came up today about what an "acceptable" reading to ground is for a three phase AC motor (480 V and 4160V)when taking a megger reading. Many people say to consult the manufacturer because the actual construction of the motor will affect this. Also, a lot of times, it is clear when taking the reading if the motor is almost shorted to ground or not. However, I have heard that some of the Navy guys use specific guidelines or equations to make this determination. I was just wondering what guidelines were used in industries other than mine to make this determination.





RE: Megging motor
For old machines at 480 Volts line voltage, 2 MegOhms at 40 C is OK.
For new windings you could expect at least 50 Megohms.
Details of the procedure, test voltage and explanation of the temperature influence are in that standard.
RE: Megging motor
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RE: Megging motor
This is what I refer to when asked. There may be changes (Rev's) but pretty standard for Rotating Equipment and Cables. Corrections welcome.
IEEE STANDARD 43-1974 (29) ACCEPTANCE CRITERION FOR INSULATION
For Rotating Equipment
•IR is equal to or greater than 1 Megohm plus rated equipment KV.
MEGGER
IEE STANDARD NO. 43.1974
ACCEPTANCE CRITERION:
•IR IS EQUAL TO OR GREATER THAN 1 MEGOHM + RATED KV.
MOTOR RATING VOLTAGE DC MEGOHMETER (Test Voltage)
460/575 500 or 1000
2300/2400 1000 or 2500
4000/41602 2500
6600/6900 5000
POLARIZATION INDEX
PI = IR Measurement at 10 minutes
(Divided by)IR Measurement at 1 minute
ACCEPTANCE CRITERION:
Usually 2
IEEE STANDARDS: 690-1984(1), 422-1986(43), 43-1974 (29)
(for cable Insulation testing)