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star delta motors


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I have a star delta 75 KW motor , i always have wondered what would happen if I modified this star delta contactors to be a direct on line control circuit ?

Is there a risk on the motor winding or any element in the circuit ?, please advice .....:)

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Welcome to the forum.


If you change from a star delta starter to a DOL starter, the motor will draw Locked rotor current to almsot full speed and will develop a much higher start torque.

This will result in a shorter starting time and may put additional strain on the mechanical system although from my experience, most damage occurs at the transition from star to delta due to the "auto reclose" effect.

The severe current and torque transients at the changeover will be eliminated and so the peak current and peak torque will be less than with the star delta starter.


The questions to ask are why was the star delta starter fitted in the first place? Why do you wish to make the change? If it was fitted to reduce the start current, or to reduce the start torque, then you would be better off to fit a soft starter to the application.


What is the application? let us know what the machine is and perhaps we can comment further.


Best regards,

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the star delta was fitted in the first place by the manufacturer , as here is a batch plant for mix concrete , this motor is to rotate the hands of the mixer to mix the concrete.( this is the load):)

when I had a short circuit for two consecutive times and I did not figure out the problem , I assumed it may be in the control circuit of the star delta , so i modified it to work DOL , and now it is working good for one weak without the problem occur again.

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Hi emad


If it works well DOL and is not causing any problems, I would definitely leave it alone.

I bel;ieve that there is more damage caused by star delta starters than by DOL and there are only three advantages for the star delta starter.

1. they statisfy the reduced voltage start requirements in some locations.

2. They use more bits so the control gear manufacturers sell more.

3. They cause more damage so the maintenance people have more work. - create more employment!!


Best regards,

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