Dear all,
I have a 3-phase, 400V, 50Hz 75kW submersible waste water evacuation pump that was installed two months ago in a sump. When the pump was delivered, we took measurement of the insulation resistance of its motor, and the values between phases and between the phases and earth was greater than 999mega-ohms. However, we observed that when the pump was installed and run, it was tripping on overload with the setting at 70A. We then increased it in steps until it stopped tripping when the setting was 80A. The starter system is star-delta.

However, we recently observed that the pump was tripping on overload and when we shut it off and took the insulation resistance measurement, the values we got were about 3 mega-ohms between phases and between phases and earth!!!!

Can anyone tell me what may have been the cause of this failure/tripping/low insualation resistance and recommend remedial action that I can take? Please treat as urgent. Best regards

Gidday there vmac,
What part of the world are you in?.
I note the 400V 50Hz in your post.
I take it that this pump is wired in delta, ie no neutral.
From my calculation, this pump will be carrying 129.7A per phase, and that is the run current, not the start current, which can be 4-5 times the run current.
That is assuming that we are talking about a 75kW pump motor here.
I think you may have your O/loads set a tad too low.
Same side of the coin, mate, I can't see how a submersible pump can drop it's Insulation Resistance that far unless there has been moisture ingress in a connection somewhere in the wiring.
I hope this is of help,
Cheers,
Mike.

One other thing vmac, how deep is this pump?.

Thanks for the prompt response Trumpy and you are quite correct with the running current. Been having a running battle with the guy in the field wrt the setting.

The sumps are 1.4m deep. any ideas?

One other thought is, have you got any phase failure protection on this pump?.
A 3 phase motor will run quite happily on 2 phases, but if you try and start it with only 2 phases of the 3, it will burn out the motor pretty quickly.

Won't overvoltages break down a motors insulation rather quickly?

There are phase-failure relays.I have even checked the polarization index, and it is about 1.9.

Not that deep at all there vmac,
I know what you mean now, with the depth, we had at work when I was an Industrial Electrician, what was known as an I-well.
Not deep but used to pump out sludge and other stuff.
What is the upstream protection on this pump?.
IE, in the way of a Circuit Breaker or the like?.
Dnk rightly mentions over-voltage, however the biggest problem with 400V closed delta is a dropped phase, the other two try and compensate for the lost one, meaning a slow cooking death.

vmac,
You reckon you got 3 Megohms?.

Hop into the Chat room, it's futile trying to talk here in the Forums.

[This message has been edited by Trumpy (edited 07-19-2006).]