3 Phase Motor - Extractor

[HVACRSA]

Just a quick question -

I came across a 3 phase induction motor today that is used for a extract system in a bakery. The motor starts up but about 2 - 3 seconds later it trips the RCD. I never had time to megga the motor but I did have voltage going to all 3 phases.
Any suggestions?

[BrazilRIG]

Just a quick question -

I came across a 3 phase induction motor today that is used for a extract system in a bakery. The motor starts up but about 2 - 3 seconds later it trips the RCD. I never had time to megga the motor but I did have voltage going to all 3 phases.
Any suggestions?

Get a clamp on to see what each phase is drawing. Check all the electrical connections for good contact or replace the breaker.

[B G Scott]

As you have already indicated in your question, go and check the insulation value to earth with a good meter with the capability of testing at 500V or better.
The tripping RCD indicates a fault to earth, keep us informed with your progress.

[Frikkie]

If you've visually checked all the obvious like moisture and physical damage you have no choice but to use a mega tester for this type of fault. If the motor runs for a few seconds the primary cause may be mechanical. Possibly bearing or bearing housing wear that is allowing the rotor to come into contact with the stator windings. If you can't immediately see the fault at 500 or 1000 volts with a mega tester try testing whilst the fan is rotating after you spin it fast by hand.

If you follow the previous advice and replace the RCD i might suggest you test it first to save yourself unnecessary expense. I think it is only a very slim chance the RCD is the fault.

[HVACRSA]

Yes I checked for moisture and physical damage, spun the fan blade by hand and everything felt fine. Thanks for advice guys, I hope to get out there in the next couple of days. Pretty busy at the moment with temps hitting 40deg C!