480 ac motor with soft start fails at startup
480 ac motor with soft start fails at startup
(OP)
we have had 2 large 480 motor w/ soft starts installed both seem to be failng on start up they both are 164 amp. we have an adjust. trip setting breaker ahead of a soft start set at approx 900+ amp for inrush which you would think should be plenty. I see no faults in history of soft start and everything looks good physically load side fo soft start but it keeps tripping the breaker. We have on one motor a high winding temp on C PHASE and a high inboard bearing temp. We saw the fault of no load on C, but we did get the motor restarted it ran for quite some time and went back out on high winding temp. Is the inrush setting sufficent? What may cause this high winding temp if the motor megs out fine? we are running this motor at 95% to 100% which should be a happy point. C phase is approx 15 degrees higher than A or B according to our SCADA system.





RE: 480 ac motor with soft start fails at startup
RE: 480 ac motor with soft start fails at startup
Keith Cress
kcress - http://www.flaminsystems.com
RE: 480 ac motor with soft start fails at startup
What type of machines are the motors driving?
Is it possible the breakers are tripping on thermals due to the start current and start time exceeding the breaker thermal curve or due to insufficient cool down time between starts.
RE: 480 ac motor with soft start fails at startup
You also don't clearly describe what you mean by "failure". Is it only the fact that the breaker is tripping? If so, how long into the start sequence is it tripping? 1 second, 10 seconds, 100 seconds? That can help distinguish the mode of failure as well. If for example you are using a Magnetic Only breaker set at 900A and it trips 20 seconds after you start, that is clearly a failure in either the motor winding or the soft starter.
The more you describe, the more we can help.
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies
RE: 480 ac motor with soft start fails at startup
RE: 480 ac motor with soft start fails at startup
Get out your megger and start testing the motor. Work backwards from there and when you have eliminated everything else, start looking at the soft starter. The SMC Flex is a true 3 phase soft starter, but it has integral bypass contacts. Unlike those that use a real 3 pole contactor, A-B uses 3 separate 1` pole contacts in each of the SCR assemblies. I'm not exactly sure how they work internally, but if one of those contacts has failed to close at the end of ramp, and the soft starter shuts off the SCRs at the end of ramp as many do, you would get no current on that phase. If the motor was still accelerating when that happened, it's possible that the current on the other 2 phases would spike to Locked Rotor and trip that breaker (LRC on a motor with 164FLA would exceed 875A). Turning up the trip settings will not really help if that's the case. I think the SMC would detect and display a bad bypass contact, however nothing is perfect.
"If I had eight hours to chop down a tree, I'd spend six sharpening my axe." -- Abraham Lincoln
For the best use of Eng-Tips, please click here -> FAQ731-376: Eng-Tips.com Forum Policies
RE: 480 ac motor with soft start fails at startup