View Full Version : rotary phase converter question

10-21-2003, 11:26 AM
i know the difference between a rotary phase converter and a static phase converter. but it seems there are several variants of rotary phase converters.
i see complete rotary converters for sale. most have some type of control panel, and the phase converter portion that looks just like a big motor with no output shaft.
i have seen phase converter "control boxes" for sale and i've see kits to build your own box.
i've seen the plans to build your own entire converter, using various capacitors and motors.

here's where my confusion arises. in the online plans i've found, they show how to use a single-phase motor to turn the 3-phase motor and you use the output from the 3-phase motor to power your 3-phase equipment. in the control boxes and control box kits, they say all you need is a big 3-phase motor to complete the kit. so what drives the 3-phase motor? are only two phases driven and then once the motor starts spinning you use your 240V to power two of the phases on your machine and tap off the third phase of the converter motor to run the third phase of your machine? this sounds like it would have the same effect as running a static converter.

what am i missing?


andy b.

10-21-2003, 12:37 PM
By the time the power comes out the third wire the phase has been shifted to give a rotating field.


10-21-2003, 12:45 PM
A 3 phase motor will run quite happily on single phase power it just won't start.
The single phase motor (pony motor) is used to kick start the 3 phase motor (idler) which will then run and produce 3 phase power for your mill, lathe etc. I use a static convefter to start my 3 phase idler motor. The static converter - the control box on a rotary - consists of a bank of capacitors bith "run" and "start". The start caps produce a pseudo 2 phase signal to your idler which fools it into thinking it's being supplied with 3 phase power and thus it starts. Usually the start caps disengage after a short time. The run caps then act to balance out the 3 phase produced by your idler.
If you use a pony to start the idler you lose the benifit of the run caps.
I use an ancient *huge* static driving an ancient *huge* 5hp GE idler. Works great!

10-21-2003, 01:11 PM
There is another type, which has a big single-phase motor driving a 3 phase generator (OK, alternator).

This type is usually cobbled up by the user, but I think that some of the "for CNC" converters are made like that also. They may be in one housing instead of being separate pieces.

10-21-2003, 02:08 PM
A cheaper/simpler alternative to the 1-ph 'pony' (for starting the 3-ph generator/motor) that I have seen used is just a pulley on the shaft with a pull cord wrapped around it to jerk start that motor. The cord is pulled while simultaneously applying the line power with a switch. Not as elegant... but it works.

10-21-2003, 02:13 PM
NOW i get it. i thought the commercial phase converters were a single-phase motor turning a 3-phase to generate the phases. i see it is basically a 3-phase that was started with a static converter, as kenc said.
well, the rope and pulley starting method is now looking more attractive. i think i can gather the contacts and caps to build the rest of it.


andy b.

10-21-2003, 07:56 PM
You don't have to use a rope either,with a little machine work you can use a commonly avalible centrifugal switch and a start cap. to do the work.

Alistair Hosie
10-21-2003, 08:42 PM
There is a good diagram in one of my woodworking books on this. Instead of a starting rope the guy had a small motor rigged so as to have a small wheel on one end this was brought into play by a simple foot operation with a little pedal. When lifted it made contact with the main motors pulley shaft so as to be brought up to speed. When the motor got up to speed it was dropped down out of the way and switched off till needed. Saves a lot of unnecessary pulling and seemed safer too Alistair

10-21-2003, 10:18 PM
A self starting phase converter can quit easily be built. I have found the following to be one of the best explanations of the theory and construction.


10-21-2003, 10:54 PM
I have a 5hp phase convertor wired into a single phase panel, I have a third "ground bar" mounted on a insulator, I feed the 3rd power from the phase convertor to the isolated bar, when I wire a starter, I use a two pole breaker, wire the 3rd wire direct. YOU must remember that the 3rd wire is hot while panel power main is on. NEC permits the breaking of two power legs to start and stop a motor. Dissconecting power is accomplished by shutting main down in panel, this removes power from phase convertor. All this is hooked in with panel.

The more motors that run on the panel the smoother all of them are. One carries the others. I installed it for the lathe I have, since then I have several other machines I got for nearly nothing.

Works better then just the phase convertor.
I do not kick start or have to start anything. The capacitor-inductors work.

WITH, my cnc, the phase convertor made the gear reduction rattle. I did not try adding in a idler motor. I converted to a single phase in, 3 phase out invertor.

THE phase convertor is a cube from some kind of appliance with capacitor. I wish I knew. The convertor is epoxy potted in to protect the wiring design. The stinkers.. or we would all know, I'd post it..

[This message has been edited by ibewgypsie (edited 10-21-2003).]

11-08-2003, 11:36 AM
When I built my converter I mounted the recoil from a old snowmobile engine to the shaft. Just give it a yank and flip the switch. It works great.