3 phase motor reversing itself

[Tiden Elec]

Hi I have a 3 phase 400v motor wired in star configuration it is controlled via a 3 pole 415v coil contactor which is switched simply via a stay put stop ... In for off Out for run.. ( Spanish design ) there is also a single phase fan connected between one phase and the star point of the motor. Got a number of these machines like this but this one.... sometimes when you release the stop button to start the motor it runs in reverse !!!!
In 50+ years in the industry Ive never seen this happen .... Any ideas?
Thanks
Dennis

 

[James]

I wonder if there is a phase missing or a faulty contactor. The missing phase possibly being replaced by the single phase motor.

 

[Rockingit]

I'm assuming that you've done all the usual supply polarity / rotation checks?

 

[Lucien Nunes]

Agree, likely intermittent contact or connection in one phase. If the fan is disconnected it probably won't budge, but with the fan there is enough of a phase-shifted current flowing to generate a modest starting torque. Presumably it doesn't have much load at start-up?

Clamp ammeter on each line in turn to spot the one that doesn't always have load.