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For Star Delta, How is the sizing of the contactors done?

In star-delta starter, the motor is initially connected in star configuration and then, after a preset time, the motor is disconnected from the supply and reconnected in delta configuration.
D.O.L. starter would switch starting current about 6x nominal current - see curve 1 in Attachment1.
However, current in the star configuration is 1/3 of the full voltage (delta) current and so the star contactor will switch just about 2x nominal current during starting - see curve 2 of Attachment1.
Therefore, sizing of the star contactor is 1/3 of sizing for D.O.L.
When motor is reconnected in delta configuration, current raises to about 3x nominal current - see peak on curve 2 of Attachment1. Therefore, sizing of the line and delta contactors is about 1/2 of sizing for D.O.L. (6/3=2)
However, Selection of contactors always depends also on required coordination and bigger contactors can be needed when particular type of coordination is requested.




Note that the Coordination tables is an experimental tables that are tested using the combinations available in the catalog, it is tested & validated document for products physically tested in our laboratories, thus using other components or changing the existing ones will not achieve any coordination
thus we recommend using the combinations available in the Tesys contactors Catalog.
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