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Star-delta starter connection

ACPdream

Aug 14, 2022
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Hello everyone. Do you know if when sizing the contactors for a star-delta starting motor connection, the power of the motor is also considered? Please, help me :3
 

Bluejets

Oct 5, 2014
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Definetely ......or should i say, obviously.
Line and delta contactor usually the same size and star can be smaller in relation to those.
Also any overload unit will need to be calibrated/set to reflect star/delta.

Most these days would buy an off-the-shelf starter to suit the hp range the motor fits within and then the contactors would be the correct size.
If DIY then there should be reference to contactor/overload sizes in the manufacturers spec sheets.
 

ACPdream

Aug 14, 2022
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Yep. Thanks. :D and one more question, if I need a 67A contactor (main contactor and 67A is the current calculated based on the line current) and it has a power of 25kW, now it is designed with a 75kW motor.
The choice of contactor is not correct, right?
 

Bluejets

Oct 5, 2014
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main contactor and 67A is the current calculated based on the line current
The current is based on the motor specs.
and it has a power of 25kW,
now it is designed with a 75kW motor.
Cannot follow what you are on about here........ however, a 25kw motor full load would be around 50A or more, depending.
A 75kw motor would be around 180A or more.
So I don't know where you are going with this.
Obvious smaller one would not suit the other.

It would be common practice for the larger motor to use an auto transformer starter.
 

ACPdream

Aug 14, 2022
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ok... i'm getting the point of this conversation, anyway, thanks man.
 

ACPdream

Aug 14, 2022
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I don't think so, i'm designing a star-delta starter for a 100hp motor, for this, I need to size the elements such as contactors, switches, relays... so I had doubts about choosing them.
 

Bluejets

Oct 5, 2014
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I don't think so,

Yes, well, you would say that wouldn't you.
To "design" such an animal would require previous knowledge which, given you have to ask about contactor size, is obvious you have very little.
In addition there is much more to starter design than just contactor size.
Any starter/ motor combination connected to a supply authority source would need to have not only design certification to connect in the first instance but also for insurance purposes and supply authority inspection and approval to connect.
Not to mention required electrical licencing.
Especially given 100hp motor.

You may get away with an old washing machine motor plugged into a power outlet but not what you are suggesting.
 
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