Megging motor 1

[jpwillard]

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.

 

[aolalde]

The main procedure for insulation testing is IEEE Std 43.
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.

 

[machmech]

I found this with Google.

http://www.ecmweb.com/mag/electric_review_polarization_index/

 

[54494060]

JP Willard, I provided this info on/to the same type of Question you have in the IEEE (electrical) Code issues of the Eng. Tips Forum which is pretty standard for determining the insulation value of rotating equip. Hope it helps.

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)