Single Phase motors on 3 Phase

Can anyone help, please!

I know you can run a 3 ph motor on single phase but can you run a motor supplied a single phase on 3 phase?

I have a milling machine that has been converted to single phase. The motor for the cross travel has RYB and neutral connections and has a capacitor connected across RB. Now, my guess is that the mftr makes a

3ph motor as standard and converts to single phase by the use of the capacitor. I have 3ph so would like to convert back to 3ph. The plate on the motor does say single phse so it is a manufacturers conversion, not a home one.

Any comments.


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If you have a neutral as part of your three phase installation then you will have 240v between any phase and neutral so single phase motors (or anything else) can be run of it.

I don't know of a way to create an artificial neutral with a suitably low impedance to draw any useful power with just capacitors.


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Bob Minchin

One pont is that there may be a good reason why the manufacturer did this and without access to an original schematic, it's hard to tell if the motor is actually 3 phase and not some sort of split phase motor.

One reason might be to limit the torque, as you never get full power from a 3 phase motor using a cap to phase shift the third phase. In the absence of a shear pin, it could be a reasonable way to limit torue and thus avoid mech damage at the end stops.

Theat's for the feed motor - the spingle motor would never be converted to 1 ph from 3 using single cap by any sane manufacturer, as you would not get full power and such techniques are considered something of a bodge in electrical engineering circles...


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