Molded case circuit breaker trips when motor shut off ?????

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Davebones

Senior Member
Location
North Of Tampa Fl
I can't verify this as I was out on vacation last week . We have a test stand that has 2 ( 75 hp ) driven pumps on it . This is across the line starters . The operators claim that when they shut the pumps off ( yes they claim when they shut them off ) that the 400 amp molded case circuit breaker " SQ D LA36400" that feeds the busway that feeds the stand tripped . The trip settings are at mid range .They say this happened 3 times last week . My boss has ordered a replacement breaker but I have doubts that the breaker is the problem . Any thoughts on this ...
 
What are they doing to shut them off? In other words, any chance that the control circuit can "bounce" and give them a very rapid Off-On-Off kind of sequence? That might do it, because if too fast, the motor is still in re-gen when reconnected briefly, which would cause a massive current spike.

I would THOROUGHLY check out the entire operation before just assuming you have a bad breaker.
 
The best I can get out of them is they say they stopped the # 1 boost pump,then the #1 main pump . Then they stopped the # 2 boost pump , then the #2 main pump . They say they then tryed to start a few hours later and found the breaker feeding the bus tripped . They claimed that this happened at least 3 times ....
 
The only thing I could think of is this: When the power is removed the motor keeps spinning for a little while until it comes to a full stop. But during this time the motor acts like a generator and therefore generates power. This could be tripping the breaker. How are you turning the motor off? Is the motor completely disconnected from the circuit breaker in the turn-off process?
 
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