meggering a 3 phase motor controller

[qmed1]

I have a marine 440 volt motor controller I meggered and all 3 phases went to 0 when meggered. I megged the motor leads with them seperated from the power leads and the motor is god and there is no aparent electric cable damage from the controller to the actual motor.I have never seen this and appreciate any suggestions from the group.I just started an account and this looks good.Thanks everyone....

 

[Zogzog]

Are you meggering phase to phase or phase to ground?

 

[qmed1]

we always ground the earth line of the megger motor and go to 1 of the 3 phases at a time straight to the motor...

 

[dpc]

You have to break it down. Separate conductors running from starter to motor and megger just those conductors.

Any chance you are meggering something in the starter that doesn't like being meggered?

Surge arresters are a possible culprit.

 

[BJC]

Is the Y grounded in the starter? I have never heard of itb but they do strange things with shipboard systems.

 

[waross]

Some meggers have only one scale and you can't go wrong.
Some meggers have several scales and one of the scales may be a 6 volt scale for continuity tests. The 6 volt scale is often reversed so an open circuit looks like a short on the higher voltage ranges if the 6 volt scale is mistakenly used for insulation testing.
Some meggers start their scale at about 200,000 ohms. A motor may be below this value and still work, but the megger will indicate zero.
If you have had a flash over, you may have carbon tracking on your starter.
Check the starter and the leads seperately.
respectfully

 

[peebee]

Isn't B-phase grounding typical in some marine applications?

 

[waross]

Hi peebee;
Even if the system is grounded on "B" phase, the starter should be isolated from the system before grounding and should ring clear.
If there has been a flash over in the starter there may be carbon tracking to ground. Do you get ground on the load side terminals or the line side terminals?
respecfully