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 ?

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.

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,

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.

Regards,
GGOSS