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3ph compressor motor tripping

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#1 autopaint


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Posted 30 July 2005 - 10:34 AM

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 ?

#2 autopaint


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Posted 30 July 2005 - 12:33 PM

newbie question relating to topic below.
Do I need to connect all 3 phases to my motor/starter or just 2 ? I currently have 2 ans the other red wire from motor/starter to neutral. There are 3 red wires and an earth coming from the compressor.

#3 marke


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Posted 30 July 2005 - 10:31 PM

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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Posted 31 July 2005 - 11:24 PM

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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