Motor starting problem

[MikeSimpson]

Hi,
We have installed a new star/delta starter panel for a 185KW motor which drives a compressor. Occasionally the main breaker trips on the transition from star to delta. We have experimented with the timer to try and achieve the optimum time to lessen the current and voltage effects on changeover. Any other tips would be helpful as we still seem to be having a problem. The loading of the compressor is timed after the motor has changed over to delta.

 

[waross]

If you have adjustable magnetic trips on the breaker, you can try a higher setting.

 

[mohanlal]

Check your instanteneous over current trip set value. It can be adjusted to a higher value so that tripping at the time of starting could be avoided.
As long as there is no fault in the motor,the setting can be increased.
subramanain

 

[hottip]

Check auxiliary contacts of relays/contactors (which are doing the star/delta sequence) and the control diagram of the star/delta panel (can be bad contacts in the control wiring). Sometimes there is a problem with the above mentioned items (at least I had the same problem like you and it was from the items I mentioned. Good luck!

 

[jraef]

MikeSimpson,
Do a keyword search of this forum on "Y Delta transition spike" , you will find that this has been discussed in detail many times. The condensed version is that this is very common, and has to do with the fact that most star/delta (Y-Delta) starters are open transition, and that introduces a transition spike that can be high enough to trip circuit breakers, blow fuses and even sometimes break motor shafts. The answer is to go with a closed transition starter, which is expensive, complicated and requires resistors, or change to a solid state soft starter.

Eng-Tips: Help for your job, not for your homework Read faq731-376

 

[Walden]

I multiply the FLC x 1.5 to find the rating of the MCCBs on these types of starters.

 

[Marke]

See faq237-613

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[MikeSimpson]

Thanks for the advice. The techs are ongoing with their investigations although they have reported another problem! On start up in star the current peaks at its maximum for around 3-4 seconds which I think is slightly to long. I know the motor will develop enough torque to overcome the load torque which I remember to be approx 85% synchronous speed at which point the current should decrease. I am thinking of a loading problem. Any ideas?

 

[Walden]

Are you certain that the compressor is started unloaded?

 

[MikeSimpson]

The loading is affected by a timer from the compressor controller which allows ample time for the motor to be connected in delta. However I am not convinced that there is not residual air or excessive oil in the compressor rotors which is given a light load condition. I think this is the cause of the extended current peak in star. I am wondering if anyone has experienced the same phenomena.

 

[waross]

Remember that with the reduced current of the Star connection the torque will be less and the acceleration time will be longer than you are used to with Motors that are started Across the Line.
Does the compressor have a lot of inertia? Is it reasonable to try a start with the belts off?

 

[GGOSS]

On open transition star delta starters, the current transient can be as high as twice the motor's locked rotor current.

The magnetic or instantaneous trip for many circuit breakers is around 12 times the base rating. Therefore if the motor has an LRC greater than 6 times FLC, nuisance tripping can be expected.

Hope the above assists you to make a decision on your best way forward.

Regards,
GGOSS

 

[MikeSimpson]

Thanks for all the advice. I had the techs wind up the magnetic protection of the local breaker to maximum and change the upstream breaker with one that gave the proper discrimination. So far so good! I will know in future to allow for transition transients with a star/delta starter. Thanks.

 

[Marke]

Hello MikeSimpson

A common problem with star/delta starters, in addition to the transition transients, is that often the torque available in star is not sufficient to accelerate the motor and load to full speed so the transition accurs at less than full speed. If the transition occurs below 85% speed, then the current will be close to LRA until the motor reaches around 85% speed when the current will begin to fall away. Actual values of current and speeds are very motor and load dependent, but the majority of star delta starters that I have seen do not achieve a worthwhile reduction in current because the torque in star can not get the load to full speed before the transition.

Best regards,

Mark Empson

http://www.lmphotonics.com