Tripping MCC breaker

[Wormfood]

Working with a 10HP motor with reversing contactors that intermittently trips the MCC bucket. Not blowing any fuses or heaters dropping out starter, just trips the MCC circuit breaker.

Service: 460, 3-phase, 30-AMP with 150Amp instanteous trip in MCC bucket, goes to a 30-Amp fused disconnect near machine. #1 starter with proper heaters reside in MCC bucket. Reversing contactors are in the machine's electrical panel.

Motor: 460V, 3-phase, 10HP, Code G
Locked Motor Amps @ 80 Amps


Does not always happen on the starting of the motor. Double tapped one direction button once and had the breaker trip. The only thing I've seen similar to this is a short in a motor, here the breaker would trip in a intermittent fashion. Anyone have any other ideas?

 

[mull982]

Working with a 10HP motor with reversing contactors that intermittently trips the MCC bucket. Not blowing any fuses or heaters dropping out starter, just trips the MCC circuit breaker.

Service: 460, 3-phase, 30-AMP with 150Amp instanteous trip in MCC bucket, goes to a 30-Amp fused disconnect near machine. #1 starter with proper heaters reside in MCC bucket. Reversing contactors are in the machine's electrical panel.

Motor: 460V, 3-phase, 10HP, Code G
Locked Motor Amps @ 80 Amps


Does not always happen on the starting of the motor. Double tapped one direction button once and had the breaker trip. The only thing I've seen similar to this is a short in a motor, here the breaker would trip in a intermittent fashion. Anyone have any other ideas?

Sounds to me like you are trying to start the motor out of phase with the decaying field in the motor which can lead to high current transients. When a motor is stopped there will be a period of a couple of seconds or so where the magnetic field in the motor is decaying however while it it decaying, the motor is now in generating mode and is generating a voltage that becomes quickly out of phase with the source voltage in the MCC. If you try to restart the motor in the same direction or change directions before the motors field dies down and the motor stops generating than the source voltage of the MCC will be out of phase with the motors internal voltage when the contactor shuts and this can cause high current transients that will trip circuit breakers as well as put mechanical stress on the motor.

Rapidly starting a motor several times by pressing the start button consecutively as you described is a perfect example of this happening.

When does this problem happen? When starting? When reversing directions?

 

[Wormfood]

I've seen it happend several different ways. Machine is a vertical cardboard bailer.

1. Motor was engaged in forward in automatic, the hydraulic shaft reaches the end of the stroke and the breaker trips.
2. Double tapped the forward button twice in less than a second (finger slip), motor barely had time to engage and breaker trips.
3. Motor was reversing and reached top of the hydraulic shaft where the motor is to turn off and breaker trips.

I can't make it repeatable. Seems to do it at will.

 

[Jraef]

I concur. The other possibility, if that is not the issue, is an actual short in the cables or the motor. Some shorts are of high enough resistance to not cause a trip every time. Have you meggered the circuit?

Another possibility: Have you recently changed the motor itself? Some newer High Efficiency motor designs are pulling extremely high inrush current now, higher than some breakers can be set for inst. trip. That's why the NEC was amended a few years ago to allow for upwards of 1700% FLA for I.T. if it is demonstrated that the breaker will nuisance trip without being set that high. unfortunately you are probably somewhere near the top of that breaker's setting range, you may have to change the breaker.

 

[RoberteFuhr]

I recommend monitoring the current with a high speed power analyzer. Compare the recorded values to the manufacturer's time current curve and see if the breaker is tripping per the TCC. If the breaker is not tripping per the TCC, then replace the breaker. If the breaker appears to be functioning properly, then focus on the why the current is getting high enough to trip the breaker.