phase converters for big iron

[Mike Gemeny]

You can build your own rotary phase converter from an old 3 phase motor. And 3 phase motors can be some of the cheapest to get second hand.

It seems that a three phase motor will not start if a phase is missing, but if it is running it will continue to run if a phase is removed. The combination of the energized windings and the spinning rotor will have the effect of creating the third phase, although this is not an efficient state of affairs.

In my home machine shop I first used a bank of capacitors to help synthesize the third phase. The bank of WWII tin caps was adjusted until the lathe motor was running with the least buzz.

Then I added a three phase chipper. The first time I slammed the lathe from forward to reverse with the chipper I was surprised that the lathe reversed and the strain of reversing it could be heard in the chipper motor.

Then I added a three phase grinder to the setup. But when the grinder was used alone the cap bank was the wrong size and the motor had a terrible buzz in it. So I released the drive belt on the lathe and turned on the lathe motor. Bingo! The buzz in the grinder was gone!

So in this configuration I am in essence using the caps to start the lathe motor as a rotary phase converter for the grinder.

Hope this helps,

Mike Gemeny