3 phase motor wiring and earthing ...help

[TMJ]

I am know the textbook advice is 'seek qualified advice' but...
I have restored/resurrected a couple of old woodwork machines and rewired with new cable etc etc.
Some motors have an earth point on them and some do not although earth exists somewhere else on the frame.
I am now trying to make an extractor set up and the old motor I have found has no earth point on it - just 3 places to wire in the 3 phases. I think there may have been an attachment point for earth on the conduit gland entering the motor? But it is not clear.
So if I used this motor would it be 'appropriate' to fix a earth somewhere on its frame?

....or wait till my electrician next visits to shake his head at my antics!
Tom

 

[HappyHacker]

Unless it is double insulated with the double square marking (most unlikely on an old three phase motor) it will need to be earthed. It may have been earthed by its mounts to an earthed metal frame originally

 

[TMJ]

Thanks
The motor says class 'A' insulation
This was what the housing looks like and the external connection.

I guess if one of those fixing screws in the housing has continuity with the motor frame I could attach an earth there.

 

[MikeK]

The external connection with the bare copper wire in your second image is the earthing point. If you can remove the compression screw and clean off any corrosion inside the tunnel for the new earthing wire, you should be fine. The Class A insulation means the maximum operating temperature of the motor is 105 degrees C. Depending on where the motor is located, it could overheat and the insulation of the internal windings could start to degrade or melt if there is not enough air flowing through the motor.

As with all three-phase motors, pay attention to the rotation of the shaft when you power it up. If the motor is turning the opposite direction, change any two of the input phase connections and the motor will spin in the opposite direction. Also, make sure the overload protection for the motor will open all phases if any phase overloads. This will prevent the motor from "single phasing", which can substantially overheat the windings.