Motor inrush current
Motor inrush current
(OP)
Hi, Fellows,
I am having trouble with Commissining.Whenever we started a motor (very small 25HP) it always tripped our main breaker, instead of branch breaker. From TCC curve, Main(SR750) shouldn't trip while motor starting. What is pazzling us that what could be the source of inrush current beside motor starting and energzing transformer in a compressor station. Is it possible that branch breaker refused to operate for some reason?
I am having trouble with Commissining.Whenever we started a motor (very small 25HP) it always tripped our main breaker, instead of branch breaker. From TCC curve, Main(SR750) shouldn't trip while motor starting. What is pazzling us that what could be the source of inrush current beside motor starting and energzing transformer in a compressor station. Is it possible that branch breaker refused to operate for some reason?





RE: Motor inrush current
Is the motor connected directly yo the breaker or there is a transformer after the main breaker?
Is the main breaker delay correctly setted? Moto can have a starting current of 6 or more times the rated current
Try to be more detailed
RE: Motor inrush current
There is only one LTG Transformer(30KVA)in parallel with motor feeder.
RE: Motor inrush current
RE: Motor inrush current
RE: Motor inrush current
Sounds like time for good recording meter to gather voltage and currents while the motor is starting and the breaker is tripping.
RE: Motor inrush current
Starting current is 500 - 800% of motor FLA depending on the motor design, and can last several seconds or longer depending on load. Again, if your feeder relay does not take that into account, it will trip as well. The settings you described sound like the long time I2t settings for thermal current.
Eng-Tips: Help for your job, not for your homework Read FAQ731-376
RE: Motor inrush current
The problem can be one of hte two mentioned
I can be the saturation of the CTs or also the not so good precision of them which cause a false ground current alarm.
Bu als can be the magnetizing current flowing in the motor at the moment you are closing the breaker.
Does the breacker open immediately or after some seconds?
I think the best action is going on the field and record the current waveforms.
RE: Motor inrush current
Size of main? Voltage? is there intervening trasnformer between the main and the motor?
I dont think you posted enuff information...
RE: Motor inrush current
Thank you for the feedbacks. I already solved this puzzle.
i phoned th site and they told me they were trying to satrt three 25HP Motors simutaneously, causing a huge asymetrical inrush current which tripped Main.
I suggested them to inrease the IOC od SR750 connected to Main. Hopefuly that will solve the problem.