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Static vs Rotary Phase Converter [Which One to Use?]


Phase converters have been around for decades, transforming a single-phase power source into a 3-phase. The two most popular types of phase converters are:

  • Static Phase Converter
  • Rotary Phase Converter

Static Phase Converter typically consists of one or several capacitors and a relay switch. Rotary Phase Converter is the same Static Phase Converter only with a small idler motor (or generator) and run-capacitors, that give you a continuous 3-phase power while you use your equipment.

The main difference between Static and Rotary Phase Converters is that Static Phase Converter will disconnect itself after your engine has started and you will be left with 2 legs worth of power (which will not allow you to use the full HP rating of your equipment). On the other hand, Rotary Phase Converter will continue providing you with a three-phase power supply and you can use your equipment to the maximum of its capacity.

With many options of three-phase motors out there, now you don’t have to pass up a good deal if you don’t have 3-phase power coming into your shop. All you have to do is find the right phase converter that suits your needs and get working!

Static Phase Converters

Static Phase Converter is the simplest and cheapest way to start your 3-phase motor. It is mostly suited for medium to light size loads and single machine use.

They are not as expensive as other types of phase converters and are the easiest to wire. The only thing that you will have to give up is some of your horsepower.

This type of converter is not suitable for ALL motors, especially if they require full HP in order to work. It could be used for some machines if the motor’s pulley size is reduced in half or 1/3 in diameter.

Typical applications of Static Phase Converters include:

  • Drills
  • Grinders
  • Mills
  • Lathes
  • Saws
  • Conveyor Belts
  • Irrigation pumps
  • Sewing machines
  • Vacuum pumps
  • Ventilation systems
  • Small Air compressors
  • Metalworking Equipment
  • Woodworking Equipment
  • Printing Equipment
  • Food Processing Equipment
  • Food Mixers
  • Food Blenders

For other uses, refer to the manufacturer of the equipment that you are considering buying. Regardless of how many times you have to call them, make sure that your equipment is compatible with that specific phase converter.

Here are some companies that offer Static Phase Converters:


They have two models:

  • ECONO-PHASE® Shifter. Static Phase Converters for light or medium-size loads. Uses run capacitors to maintain the third leg of power. This item is UL listed.
  • ADD-A-PHASE®. This is also a UL-listed device that comes in several models. It is designed to provide you with balanced power to full load rating and is suitable for multiple motor applications.

North America Phase Converter Co.

We also found 2 phase converters here and they do NOT recommend using them for the following applications (see more information on their website):

  • Pumps
  • CNC Machines
  • Plasma Cutters
  • Fans & Blowers
  • Air Compressors
  • Refrigeration Trucks

Their models include:

  • Standard (SPC) Static Converters. Regular Static Phase Converter that will run your motor at 2/3 of power. Light and medium-size loads.
  • High-Low (HL) Static Converters. This model incorporates high or low horsepower settings and a toggle switch.

Phase-A-Matic, Inc

This converter will run your equipment at approximately 2/3 of its nameplate horsepower.

Here is a link to their website.

Sizing a Static Phase Converter

Before sizing a Static Phase converter, you need to figure out the following:

  • Motor Winding. Static Phase Converters are primarily designed to work with Wye wound motors and this is how they give you 2/3 of power. If you use them with Delta wound motors, you will get only half of HP out of them.
  • HP Rating. This is the simple one. Static Phase Converters generally need to match the HP rating on the nameplate of your motor.

Static Phase Converter variations

Static Phase Converter could actually be used as a base for different types of phase converters. Idler motor and Run capacitors could be added to your Static Phase converter.

Adding the Idler motor

In order to get your motor to full (or close to full) horsepower, you may want to consider adding a three-phase idler motor to generate the third leg of power. PHASE-A-MATIC™ gives you specific instructions (included here) on how to do that.

If you decide to use the idler motor-generator make sure that:

… its HP rating falls within converter’s HP range

This way, you can also run several machines at the same time, as long as the idler is large enough to handle the largest load. It is recommended for the idler motor to be:

… twice the size of the largest load

Just make sure that your Static Phase Converter has:

… same HP rating as to your idler motor, NOT your load

After your largest motor has started, you can start adding other motors, as long as:

… their combined HP does NOT exceed 1/2 of idler motor’s HP rating

Adding the Run capacitors

Here are some wiring diagrams with Static Phase Converter and Run capacitors:

There is more information about it in this article (4th International Conference).

Adding the Run capacitors and an Idler motor

Well, this variation is also known as Rotary Phase Converter (which will be discussed below). What is a Static Phase Converter? It is a system of start capacitors and a relay switch.

If you use Static Phase Converter or just start capacitors and wire them properly together with Idler motor, you will generate 3-phase power, just as a Rotary Phase Converter would! You can find some wiring diagrams for Rotary Phase Converters in this article: “Rotary Phase Converter Wiring Diagram“.

If you ever decide to do one yourself (as long as you are qualified to do this work, which could be dangerous), here is a short video to help you out:

How to build a rotary phase converter

Rotary Phase Converters vs Static Phase Converters

So what do we have here? Here are some pros and cons of Static and Rotary Phase Converters:

Why Static Phase Converter is better than Rotary

  • It is less expensive
  • It is easier to wire

Why Rotary Phase Converter is better than Static

  • Rotary Phase Converter includes a system of start capacitors that works similar to Static Phase Converter.
  • It does NOT drop the third leg and can power your motor up to 90% of their nameplate HP power (Static Phase Converter will only give you about 2/3).

In conclusion, I would like to mention that everything comes down to your load and how heavy you are planning to use your equipment. If you have a small to medium-size load and you are doing this as a hobby, the Static Phase Converter may suit your needs perfectly.

If you have a small enterprise, large machines, or a very busy shop, you may want to go with a Rotary model.

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