Motor inrush current 1

[SELEC]

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?

 

[spargher]

We need more info expecially abou your circuit
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

 

[SELEC]

the motor is not directly connected to the branch breaker.The max FLA is 800A, plus motor inrush, Max Current the main bus will draw is only 1100A. The coresponding time for Main( SR750) is ~60s, which is long enough to avoid trip the branch circuit. I am assuming there maybe some other source of inrush current.

There is only one LTG Transformer(30KVA)in parallel with motor feeder.

 

[davidbeach]

Does the main have a ground trip? It may be set too low and is responding to false residual during the motor start.

 

[SELEC]

Yes, the main has a residual ground trip, which is set same as phase current setting with a diffrent curve.Is the false residual value very high during motor starting?

 

[davidbeach]

The false residual is caused by saturation of the CTs. Somehow though, it seems that a 25HP motor shouldn't be causing that much inrush.

Sounds like time for good recording meter to gather voltage and currents while the motor is starting and the breaker is tripping.

 

[jraef]

Inrush current on a motor is different than starting current, although many people mix up the two terms. Inrush is a short duration spike that occurs when magnetizing the stator windings, and can be as high as 20 times the motor FLA depending on the motor design. It only lasts for a maximum of 2 cycles however. If your trip curve on the feeder relay does not take that into account, it may cause a trip.

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

 

[spargher]

I agree both with davidbeach and jraef.
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.

 

[rbulsara]

Does this happen when a particulalr motor is started or any motor?

Size of main? Voltage? is there intervening trasnformer between the main and the motor?

I dont think you posted enuff information...

 

[SELEC]

Hi, Fellows,

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.