Building A Rotary Phase Converter

Hosts  Tool Craft & Marty Escarcega

June 24th 2006

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Members in Attendance

Tom Davis  

Russ Huffman 

Neil Peters 

John Lea

David Rabenius

Eugene Neigoff

Chuck LaRue 

Tony Cassinet 

Bob Sanders 

Doug Endrud

Gerald Goldberg

Bill Van Orden 

John Fleming 

Owen Jeffers 

Tim Coppage

Neil Butterfield 



Members sated with doughnuts and seated, ready to go!

Home shops are faced with the problem of running equipment that contain 3 phase electric motors in shops with only single phase power. The solutions available to the home shop are:

Marty recommended a small publication as reference for the home shop:

How To Run Three Phase Motors On Single Phase Power
T. J. Lindsay, Lindsay Publications
Bradley, Il 60915
Copyright 1977
Current availability is unknown.

Replacing the motors can be a cost effective solution for small horsepower applications. The problem comes if there are special mounting configurations, machine interfaces, etc. As motor size goes above 2 horsepower, other solutions are more cost effective.

The Variable Frequency Drive is becoming a cost effective solution for some applications. In applications of 1 to 3 horsepower VFD's should be considered. In standard motor speed applications, the VFD digitally switches the power at 120 degree intervals. This approximates a sine wave power source for the motor. Changing the frequency of the switching will cause the speed of the motor to change up or down. 

We discussed some specific applications to equipment that have variable speed controls built into them. The best answer to this question is get with  a competent VFD person that can understand specifically how your machine is built.

Cautions in using these drives are:


Marty Escarcega our demonstrator for this event shows what's inside a static phase converter.  I for one was surprised when I opened mine up a few years ago to find pieces of cardboard.

Static Phase Converters.    Static Phase Converters cause the equipment motor to develop only 50 to 60% of rated power. They are easy to install utilizing a 220V single phase power source. The units are sold in specific motor horsepower sizes that must be matched to the motor size in the equipment.  They are essentially a motor start capacitor, and a relay to switch the capacitor out of  the circuit once the motor starts. Once the motor is started, it runs on 2 phases thus the reduction in rated horsepower. Note: Using a higher horsepower rated unit does not increase the usable horsepower of the 3 phase motor.

The Rotary Phase Converter uses a 3 phase motor sized to be slightly larger than the motor load to be connected to it as an idler motor. The idler motor is brought up to rated speed using 2 phase power. Once the idler motor is at speed, it will induce power in the third phase leg. These 3 phase legs are then tapped as the power source for the 3 phase motors in the shop equipment. Using this configuration, the resulting power is over 90% of the rated horsepower of the idler motor

A great feature of the Rotary Phase Converter is you can run up to 4 pieces of equipment on one Rotary Phase Converter. The combined horsepower of the equipment load may even be slightly higher than the rating of the idler motor. Club members using Rotary Phase Converters report starting the Rotary Phase Converter as they enter the shop. The unit runs until they are  ready to leave the shop. If there is no load on the Rotary Phase Converter, the current draw is very low. Rotary Phase Converters can be built on a cart, or wiring distribution to shop equipment from the Rotary Phase Converter can be used. 

The Rotary Phase Converter is a cost effective solution for the home hobby shop. Used  3 phase motors for the idler motor can be scrounged easily. Be sure to have the voltage of the idler motor be the voltage you want from the Rotary Phase Converter. The balance of the parts should run under $100. With a little time in the home shop, you can make a very reliable converter.

We demonstrated 2 methods of making a Rotary Phase Converter

The electronic version is essentially the equivalent of a static phase converter connected to a 3 phase idler motor. The pictures of Russ Huffman's unit we built today shows how this unit would look. Russ's unit is built around a 3 horsepower motor. See the schematic posted on the club web page for the typical wiring diagram. Russ' unit was completed and brought up to speed and generated the 3 phase power. We also demonstrated how to use an oil filled run capacitor to balance the power in the 3 phases.

For the basic Electronic Rotary Phase Converter you will need:


Potential Relay Grainger # 6X550

Construction Steps:

Some general notes about wiring:

The standard wire color code for 3 phase 230V wiring is:

T1 = Black
T2 = Red
T3 = Blue

Swapping any 2 of these wire positions will reverse the direction of motor rotation

Many home shops use a sub panel to distribute power in the shop.  Use special care to connect a dedicated ground wire from the main panel to the sub panel for proper wiring protection. The neutral feed wire between the main panel and sub panel in this application does not meet electrical  code.  Please consult an electrician or the National Electrical Code Handbook if you have any doubts about what you are doing!  

If in doubt, DON'T!

The inside of Tom's Biggee* phase converter


Marty directs Russ Huffman on the wiring of the phase converter.  While ###, ### and ### look on.


Russ Huffman and  ### install the parts while Marty supervises.


Marty tests Russ's phase converter.  He is measuring the voltage on the three legs and adjusting them to be equal with a different sized run capacitors.


On to Tom Davis's monster converter.  Marty wires while Russ Huffman, Bob Sanders (hidden), Tom and #### observe and help.  It was discovered that the big motor actually would spool up without the assistance of the pony motor!


Tom Davis's large phase converter which uses  big 7.5 hp idler motor.  This second version is recommended for larger loads and idler motors (above 6 horsepower). This version uses a pony motor (single phase fractional horsepower) to get the  3 phase idler motor turning at some rpm. In the example in the pictures, we have approximated half the rated speed for the idler motor. 220V single phase power is then applied to the idler motor and it comes up to rated rpm. The idler motor then induces power in the third phase leg.  The pictures of Tom Davis' unit shows a 7.5 horsepower unit in this version. The wiring was done for demo purpose to show the application. The unit was not fully completed at the meeting, it did start and run to show proof of concept.

* Apologies to Dave Thomas and Wendy's Inc.

Updated 6/28/2006