NoJolt! Zen and the Art of Home Improvement

How to Build a Rotary Phase Converter

Home > Site Map > Safety!> Contact Me ! >

How to Build a DIY Rotary Phase Converter

If you are reading this then it is most likely because you have acquired (or are thinking about getting) a piece of decommissioned industrial shop equipment that is powered by a 3 phase motor, but your shop only has single phase power. If you don't really understand what the difference between Three phase and single phase is then look here. You have probably heard that you can somehow hook up a 3 phase motor to single phase current to generate 3 phase power. This is possible, because an induction motor and an induction generator are basically the same things. The motor that you are using as a generator (rotary phase converter) is called an idler, and needs to have a 20-30% higher horsepower rating than the largest equipment motor that you will be using, and needs to be rated for 220-240 volts.

To make a simple rotary phase converter out of a 3 phase motor

  • Connect 230 volt single phase power to the T1 and T2 supply terminals (or wires) of the motor that you are using as a converter.
  • Get it Spinning (with a rope wrapped around the motor shaft for example) to get it going - it won't start on its own.
  • Take 3 phase power off of the T1, T2, and T3 terminals to power your three phase shop equipment.

    It really is as simple as that as long as you understand the limitations of such a simple device.

  • 1) The three legs of power that you are supplying are not balanced, because of their different sources.
  • 2) The phase separation which is supposed to be 120 degrees between all legs also won't be just right, because the single phase line voltage to L1 and L2 is 180 degrees apart.
  • 3) Because of the imperfect nature of the power supplied to both the equipment motor and the idler neither of them will run at full efficiency - they will be down on power, noisier, produce more vibration and heat, will consume more energy, and won't last as long as they would if supplied with proper three phase power.
  • 4) More complex electronics (such as CNC machinery) might be damaged by such a crude power supply.
  • 5) Your local codes inspector may or may not approve of this kind of device.
  • All that being said, it will still work, and there are things that a determined diy (do-it-yourselfer) can do to make the system work better, such as using run capacitors between the L1-L3 and L2-L3 legs to even out the voltage balance. You should also use a Starting capacitor so that you don't have to manually spin the idler up when you start it. If you will only be using 3 phase power occasionally, and aren't concerned with the other limitations of this method then it may be just what you want to do.

    Making your Rotary Phase Converter Better

  • Add a magnetic motor starter switch - A magnetic switch contains an electromagnet that holds the switch in the on position once current is flowing, and is much safer for both operator and equipment in the event of a power failure, because once power is restored the equipment will remain off until you reactivate it. This is obviously safer for shop personnel, but also helps to protect equipment, and prevent fires. A proper motor starter also protects the rotary phase converter from over current - A circuit breaker is not intended to act as an on/off switch or to protect motors.

  • Add a starting capacitor - A starting capacitor should be rated at least 250 volts and between 50-100 microfarads per rated horsepower of your idler. The starting capacitor(s) goes between the T1 and T3 idler connections. You can wire the starting capacitor through its own momentary switch or use the momentary terminals of a magnetic switch to energize it, or you can use a self starting configuration. In any event you only want the starting capacitor to be in the circuit until the idler starts spinning. There are configurations that use the same capacitor as both a start and run capacitor.

  • Add run capacitors - A phase converter will work just fine without run capacitors, but they will improve performance and efficiency to some degree. Run capacitors must be rated for continuous duty at high voltages (330-370 Volts) and are permanently connected between the T1-T3 and T2-T3 connections. Ideal voltage balance is difficult to achieve without some kind of dynamic adjustment, because different load states will require different configurations of run capacitors. But it really doesn't matter that much in most cases for motor loads. Just use about 12-16 microfarads per rated horsepower of the idler as a general rule.

  • Safety first, it should go without saying that you can be injured or killed by high voltage electric equipment or you could burn your shop down or damage equipment that you plug into a beast like this. If this guide isn't enough information for you to figure the details out by yourself, then you should probably reconsider this project.


  • The larger the horsepower rating of your idler motor the better it will all work, but also the system will draw more current and thus be more expensive to run.

  • If you have multiple pieces of equipment in your shop that are powered by three phase motors, all of the motors that are running at any one time will act as rotary phase converters, and will improve the quality of the power. Just wire them all including your idler through a single 3 phase sub panel, and power two legs of the sub panel with single phase 240 - the other leg will be powered by the phase converter and any other motors that you have idling. Breaker everything (including the idler) with the normal size breaker for the individual motors. Then if you have one piece of equipment that draws a lot of current, you can start your phase converter, and then start another motor and let it idle in addition to the phase converter, while you use the high draw equipment. Needless to say you need to address all safety implications of having multiple machines powered up at one time.

  • There is no such thing as a free ride. You have to supply enough single-phase current to power the equipment that you are using and to satisfy the parasitic energy consumption of the rotary phase converter(s). Size wires and breakers accordingly.

  • If you are going to go to the trouble of building a rotary phase converter make it out of a high quality motor so that it will give good service for a long time. If at all possible you want a nice big TEFC motor with high quality sealed bearings.

  • Capacitors are expensive, but you can wire them up in parallel to get the value you need - 3 20mf caps in parallel is equivalent to one 60 mf.

  • Used Capacitors are fine if they are of the high quality oil filled variety. The cheap electrolyte caps break down over time, so avoid used ones of that type.

  • Three phase motors, capacitors, housings etc. (and also machinery) can often be bought from industrial recyclers for a TINY fraction of the new price.

    Related Articles

  • What is the difference between Three Phase and Single Phase electrical service?
  • How to Build a Phase Converter
  • How to Build a self starting Phase Converter
  • Another article on Building a self starting Phase Converter - very clearly written and illustrated.
  • Jim Hanrahan's classic paper on making homemade RPCs
  • Motor Starter Wiring Diagrams - Old Wood Working Machines Knowledge base article on Motor Starter Switches
  • Practical Machinist RPC forum - Lots of pictures and user experiences with home built phase converters

  • Home > Site Map > Safety!> Contact Me ! >