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3ph compressor motor tripping


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I have just installed a second hand 3ph 7.5kw compressor, and it keeps tripping the breaker in its box on start up.

there are 3 contactors in its box, 2 of which pull in and the motor stars to turn. the motor will then trip, sometimes before the other contactor pulls in, sometimes after.

There is a timer (0-30 secs) in the box with an led which sometimes flashes if it pulls the 3rd contactor in.

The compressor ran ok at another site, all i have done is connect 2 phases and a neutral to the existing wires. Both phases are 240v.


Any ideas ?

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Hello autopaint


Welcome to the forum


It sounds to me as though you are trying to run a three pahse motor on two phases. The starter that you are using is a star/delta starter.

You must connect a three phase motor to three phases. If you only use two phases, you will damage the motor and it will not start correctly.


Best regards,

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Hello autopaint,


Certainly sounds like you have a star/delta starter installed. That being the case (as indicated by Marke) they need a 3-phase supply to operate correctly. The fact that you are connecting 2 phases and a neutral is more than likely the cause of your CB trip conditions.




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