Got called to a lathe yesterday and found that the main motor contactor contacts where welding closed, thus not allowing the operators to stop the motor. Anyhow I changed the contactor with new one and installed a new overload (AB E1 plus 18-90 amp range) Started the motor and the overload tripped right away.
I then tong tested the motor phases out of the contactor and they are only pulling between 12-18 amps. BTW the motor is rated for 30 amps. I set the overload trip current up to 50 amps, it ran for approx. 5 minutes then tripped. AT restart it tripped right away. Anyhow the day ended and i then went home.
I went back there again this morning and the operator stated that the motor ran for approx 30 minutes then tripped out. He set the overload up to 80 amps and it tripeed right away. I arrived a little later and set the overload back to 50 amps and the motor ran for about 30 minutes and then tripped. I cranked the OL up to 80 and it tripped right away. I tong tested the phase cables again and none of the windings pulled more than 18 amps fulled loaded, but the OL was tripping even when set to 80 amps.
I suspect that the overload is faulty, even though it is brand new. I have ordered another for the customer, so I am hoping that is the problem.
Does anyone else have any input?
Bottom line: If i tong test the motor and its only pulling 18 amps max, there should be no reason why the OL would trip. The motor cables are the only thing coming out of the bottom of the contactor. BTW the contactor is a 85 amp contactor, due to alot of jogging action. but the ol was tripping during normal running operation.