Thermal Protection Relay - tripping

[Splitrings]

We recently installed a single phase, 10hp, vacuum pump. It has a Lavato thermal overload relay. It has a amperage range of 37-50 amps. The adjustment dial is set for the FLA rating of 50 amps. The relay keeps tripping. The first time you start the motor it will run for 4 or 5 mins and then trips. The second time it starts it will run for serval hours before tripping. The motor is only drawing anywhere from 28-33 amps. The motor was oversized to 10hp, the pump it runs was intended to have a 7.5 hp motor. Does it matter that we are under the thermal rating of the relay? It is rated for 37 amps and we are drawing only 33 or so. The relay is a Lavato 11RFN95350 or 11RF95350(couldn't read the number to be sure). At one point I did notice unbalanced current, about 23 amps on one leg and nearly 30 on the other. Any help would be appriciated. Thanks!

 

[Splitrings]

I only measured the imbalance once. I have not seen it since. If I recall correctly, the motor stalled shortly after I measured the imbalance.

I don't know what a megger test is. My background is structural engineering. I had a little electrical training in college but not much.

 

[ScottyUK]

OK, if the motor was in trouble when the readings were taken then the current draw might not have been stable at the time resulting in difference in values between readings.

A 'megger test' is an insulation test which is designed to find, among other things, weaknesses in the insulation system which lead to earth faults. Megger are well-known manufacturer of insulation testers and other test equipment, and their name has become synonymous with insulation test.


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If we learn from our mistakes I'm getting a great education!

 

[jraef]

Meh... Kirchoff was an effite Prussian egghead anyway... his so called "laws" don't mean doodley squat.

 

[OperaHouse]

That motor is capable of pulling way over 50 amps in some situations. No one has talked about line voltage. I gather this is a remote area. I've seen wide variences of line voltage in rural areas. The right HP motor would have likely worked better. Initial kickout of motor probably warmed it up enough so increased winding resistance lowered starting current enough to work on second try. I think an occasional voltage sag is causing the current to go way up ( brown out situation) and trip the protector. Come back with some voltage readings when the motor is running. The wiring to this building may be undersized or possibly some connection along the way is a problem.

 

[Splitrings]

It is not a electrical service related problem. The motor is 10' from a newly installed 100 amp pole mounted service. If the total length of wire between the transformer and motor is 50', I'd be surprised.

Each leg is at 119.5 volts with the motor running, 239 volts across the two.

 

[Splitrings]

One of the problems was restricted oil flow to this liquid ring pump. It was causing the pump to overheat and stalling the motor. That has been corrected.

I am still wondering if there is something else that caused the unbalanced current...bad run capacitor maybe? Either way the motor ran 40 hours straight Thursday and Friday without a problem. I'll watch and see if there are problems when it is restarted today.