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.
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480 ac motor with soft start fails at startup 2
- Thread starter jdenison
- Start date
need lots more info....what are the phases drawing for amps. is the input voltage balanced. what type of soft start? what are the motors driving? how long do they take to reach full speed? does the breaker trip instantly or after ten seconds? did you just meg the motor or did you meg the conductors leaving the soft start as well?
and... What is the shortest time between restarts?
Keith Cress
kcress -
Keith Cress
kcress -
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- #4
Do the breakers have a thermal magnetic release or just a magnetics release? Sounds to me like they may havea thermal release also, but I'm unsure from the information you posted!
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.
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.
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- #5
I agree that lots more information is needed. In addition to what was mentioned above, please state the make and model of the soft starter. Although not common, some manufacturers use a "2 phase" soft starting scheme even for larger motors (although yours is only 125HP, not want most of us consider "large"). 2-phase starting is where they use SCRs in only 2 of the 3 phases, the 3rd phase is a piece of bus bar. This can cause an imbalance in the current which would result in an increase in winding temperature. Usually it's relatively insignificant on lightly loaded applications that accelerate in a few seconds such as centrifugal pumps and fans, but on systems with high inertia or friction where the acceleration time is longer, it can show up in winding temperature readings and possibly aid in causing premature breaker trips.
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.
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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
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- Thread starter
- #6
Here is more info on 480 motors w/ soft starts that keep tripping. Motors are Baldor Super-e hp. 150, class f, rpm 3570, des b, amps 164, code G, hz 60, rating 40c amb. cont., nema nom eff 95.4, pf 90, ser. f. 1.15 they are driving a spencer blower mizer blower #c534t41a1, with an Allen Bradley SMC FLEX soft start model# 150f201nbd ser. a and a head of this is a Cutler Hammer HMCP 250 amp series C breaker cat # hmcp250j5 style# 1491d88g16 with adjust trip setting A875, B=900, C=1090, D=1200 AND SO ON up to 1750. the breaker was set at A which equals 875 amp, which I expect is low for the in-rush current. These have ran, but are now given me this trouble. I hope this is enough info for someone to help me w/ this issue. I can not get them to run long enough to get a good amp draw. I may be able to look up in SCADA, these keep tripping with in 10 sec or so. One blower shows a fault no load on C also the other does not there are two of the same blowers. The one that does not show that fault has trip setting set at B which equals 900A. I dont want this to get to long so I hope I've supplied enough info
That is a magnetic only breaker, there are no thermal elements. Its purpose is only to protect against short circuits. If it was set to low for inrush, it would have tripped instantly, not in 10 seconds. Besides with a soft starter, the kind of "inrush" this breaker is meant to trip on would not be an issue here, unless after 10 seconds your motor suddenly draws 875A or more. Tripping after 10 seconds is a dead giveaway that you have some sort of serious problem here, i.e. a bad soft starter, a bad motor winding, a bad motor lead, something that is not showing up when the soft starter is still ramping. One of the unsung problems with soft starters is that they can artificially mask deeper problems with other equipment in the system. Turning the trip setting up if there is a serious problem will only serve to increase the colateral damage.
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
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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
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reminds me of a a/b smc plus i had a problem with.i did all the resistance tests in the manual...then all the tests allen bradley suggested. i checked and rechecked the motor and conductors and couldnt find anything wrong. All resistance tests of the smc plus were within tolerance according to the manual. ended up being a bad power pole in the drive. i my case i figured it out by testing a known good smc plus of the same rating..even though the resistance was within tolerance stated in the manual it was alot lower than the known good drive..had to do a swap out through ab so i never really got to find out what the problem was. allen bradley just said " it must have a bad powerpole"
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