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Star-delta Contactors Rating?


chris1373

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I have to put together a star delta starter, for a 150 HP motor, it will start without load!

 

What will be the KW contactors you will pick?

 

I am not sure if i use new ones ore used ones!

 

If i use new, will they be for 132KW enought? If i use some siemens used ones, but in very good condition, shoul they be biger in KW?

 

Thanks for your answers!!!

 

Chris!

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Hi Chris

 

The star contactor should be rated at 1/3 of the motor rating, i.e. DOL rated for 50HP and the other two contactors should be rated at 58% of the motor rating provided that both are connected to the input lines. If the input three phase is connected to the main contactor only, and the delta contactor is taken from the output of the main contactor, then the main contactor is carrying the full line current and should be rated at 150HP.

 

Note: the star and the delta contactor must be mechanically and electrically interlocked and a special star delta timer must be used. There must be a small delay between the star contactor opening and the delta contactor closing to allow the arc to extinguish.

 

Best regards,

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What will be the use of the line contactor if i conect the delta contactor from the output of the line contactor?

 

Chris.

 

PS: Is there any abbreviation for "contactor"? :D

 

Hi Chris

 

The star contactor should be rated at 1/3 of the motor rating, i.e. DOL rated for 50HP and the other two contactors should be rated at 58% of the motor rating provided that both are connected to the input lines. If the input three phase is connected to the main contactor only, and the delta contactor is taken from the output of the main contactor, then the main contactor is carrying the full line current and should be rated at 150HP.

 

Note: the star and the delta contactor must be mechanically and electrically interlocked and a special star delta timer must be used. There must be a small delay between the star contactor opening and the delta contactor closing to allow the arc to extinguish.

 

Best regards,

 

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The common connection of a star delta starter connects the incoming supply to the input of both the main and the delta contactors.

This configuration minimises the size of the contactors, but requires two contactors to be opened to disconnect the motor for the supply.

If the delta contactor is connected to the output of the main contactor, the size of the main contactor must be increased to the DOL rating for the motor, and only the main contactor needs to be opened to disconnect the motor from the supply.

 

Best regards,

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