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Question about star & delta starter


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Hi I'm new on this forum and thank you for the designer

and I hve this question :


if i want to use start and delta with motor it must writting

on the motor the voltage is (380vA /660vY.. A mean delta)

and the supply voltage is 380v or equale the voltage of delta


so what is mean the voltage 660 and 380

and as i know the voltage of star equale : UL /1.7

and delta voltage equale : UL


dose mean the UL in star 660/1.7 and in delta 380

and thank you for help


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Hello Nader


Welcome to the forum. I hope that you will find our group interesting and of value.


There is another recent thread about this very question. you could have a look at http://www.lmpforum.com/inforum/index.php?showtopic=1145

Basically, provided that the insulation in the motor is good enough, you can operate the motor at it's rated output in star or in delta provided that the flux in the iron is equal to the design flux.

This requires that the voltage across a winding is the same in star or in delta.

If the motor is designed to operate in delta at 400V 50Hz, then when you connect the motor in star, you must have 400 volts across each winding. That means you now need a supply that has 400Volts between phase and neutral rather than phase to phase.

At 400volts phase to neutral, you have a phase to phase voltage of 660 Volts.


If you connect the motor in star but to a 400 volt supply, you have reduced the voltage across each winding from 400 volts to 230 volts, so you have reduced the flux in the iron. This reduces the start current and the start torque and is a means of making a reduced voltage starter. If you operate the motor under load while still in star, you must reduce the shaft load to one third of it's rating or you will damage the motor.


Best regards,



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