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Phase Converter Sizing [Static & Rotary]


If you have a woodworking shop or some kind of machine-related hobby, you probably realized that you need a 3-phase power supply for some of your equipment. Sizing rotary phase converters is not easy, so I created this useful guide to help you out.

Phase Converters are designed to convert single-phase electricity into 3-phase power. Each of these converters uses a different method of conversion and therefore needs to be sized in a different way.

In order to avoid oversizing and spending more money than you really have to, you need to find out which device is better for your specific application. There are two basic phase converter types:

  1. Static Phase Converter (SPC)
  2. Rotary Phase Converter (RPC)

Now, let’s get into details:

Sizing a Static Phase Converter (SPC)

This is a basic phase converter that is designed as a startup device for 3-phase motors. Three-phase power will be generated until the motor starts and once it does, the converter will be disconnected and continue to run on one-phase power.

The horsepower of these motors will be greatly reduced since only 2 out of 3 windings will be receiving power (you are looking at one-third or one-half in HP power reduction).

These converters are more cost-effective and are commonly used to operate a single machine. This phase converter is only suitable for applications that can run on reduced horsepower.

Following machines are known for working on a static phase converter because the operator was able to control the load:

  • Drill press
  • Milling Machine
  • Table Saw
  • Valve actuator
  • Oil pump jack
  • Piston type compressor

Following machines are known to work after the motor’s pulley was reduced in half:

  • Air compressor
  • Belt-driven fan

Following machines will NOT work with static phase converter because they use full horsepower on their motors:

  • Water pump
  • Hydraulic car lift
  • Dust collector


A static phase converter sizing is based on the horsepower of the motor that you are trying to start (one to one). These loads should be carefully selected as mentioned above to avoid problems.

It is not recommending to oversize phase converter since it may give your machine more startup energy than it can handle and it will simply not start. Static phase converters are generally known for working with up to 240-volt motors.

You can run multiple motors on your static phase converter, as long as you start them separately (one after another).

Sizing a Rotary Phase Converter (RPC)

Unlike the static phase converter, the rotary phase converter will keep on producing 3-phase power while your engine is running and that means you get to keep its full HP. It’s a clean and reliable 3 phase power source as long as you get it from a reputable company that cares about its products and customers.

As with anything that you purchase, don’t forget to check a product warranty which will give you a clue about its reliability (usually about 10 years). Ratings are also important since the company has to give their products in for testing and if they pass it, they can proudly display a quality certificate.

If you are like many shop owners, you probably anticipate future growth and would like to set yourself up with a larger-size phase converter. Some rotary converters out there, like American Rotary, allow you to safely run smaller motors and maintain proper voltage from any size of equipment.

But you definitely don’t want to undersize it! So here are some guidelines:

1. Single-phase power supply

The first step in sizing your phase converter is to make sure you actually have enough single-phase power coming in. The formula goes as follows:

3-phase amps x 1.6 x 1.2 (20% above of what your load is)

= Required single phase amps with 20% (most cities require)

2. Load type

The next step is to understand what kind of load you have:

  • Type 1 load (easy to medium). General-purpose machines (non-computer) with little or medium resistance, no flywheel. Examples: milling machine, drill press, table saw, clutched lathe, bandsaw, etc.
  • Type 2 load (hard). Machines that start against load and develop full HP during use. Examples: gearhead lathe, ironworker, air conditioner, etc.
  • Type 3 load (very hard). High inertia machines that have a long startup time. Examples: air compressor, HVAC, band saw, wide belt sander, refrigeration, hydraulic pump, flywheel, hoist, elevator, foreign motor, etc.
  • Type 4 load (CNC turning centers). Machines that are operated by computers. Examples include CNC & VFD machines, PLCs, EDM, etc.

3. Power requirements

After that, check the power requirement for your machine:

  • Horse Power (HP).
  • Amps.
  • Kilowatts.

4. Enclosure

Are you placing it indoors or outdoor? If the housing of your converter is not suited for the environment you will be working in, it will be damaged rather quickly.

Phase converters could be made weatherproof, as well as dustproof (depending on your needs). Consult your manufacturing company for their specific ratings.

5. Quality

Which company you are buying it from? Make sure you buy your 3-phase converter from a reputable company that had its equipment tested and certified.

You may want to consider buying from UL listed company. Here is a nice video from an American rotary company with 5 sizing tips:

5 Things You Should Know Before Buying a Phase Converter

6. Time to size it!

American Rotary recommends sizing your load two to one:

Your 3 phase needs (Amps) x 2 = Single phase required (Amps)

This means if your load requirement is 5HP then the needed models would be sized at 10, see charts below (at 240v).

Sizing RPC by load type

Load type 1 (non-CNC, non-resistive)


  • Easy Load. One size up of what HP of your load is.
  • Medium load. Multiply HP by 1.5 and round it up.
  • Hard load. Multiply HP by 2.0 and round it up.
  • Very hard load. Multiply HP by 2.5 and round it up.

Sizing from American Rotary:

Load type 2 (resistive)

Check the table below for American Rotary sizing:

Load type 3 (resistive, high inertia)

If this is your type of load, check the American Rotary table below to find your proper phase converter:

Load type 4 (CNC, non-resistive)


Multiply your HP by 2


Multiply kW by 2.7 and round it up

Check for needed unit type (American Rotary) according to the load:

** Important notes:

  • You can increase the phase converter size if your equipment does not fall within standard parameters.
  • Make sure your equipment is not taking more than 60% of the power allowed by your phase converter.
  • If you have a foreign or high-efficiency motor, it is recommended to go one size up.

Multiple load sizing

It is also important to figure out if your application will be single or multiple loads. If you need to run multiple devices on your equipment, just make sure you are starting the largest load.

In order to run multiple loads on your Rotary Phase Converter (RPC), you need to make sure that all motor loads are non-resistive, non-CNC, and not hard starting. All you have to do is to find the appropriate type of equipment for each of your loads (phase converters) and add the numbers up.

Your load capacity is only limited to the size of the phase converter that you get. For resistive and heavy loads you will need a separate converter or a special wiring diagram.

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