How big of a rotary converter do I need?

A guide to sizing a rotary converter from the American Rotary website.


     I am printing this out here because of some issues that need to be clarified when sizing a rotary converter.

     American Rotary posted a video of them starting an air compressor from one of their rotary converters and a competitors.

     They claim it’s a 20 hp compressor running from a 20 hp rotary. Assuming that is correct, then this is a big break through in the market place. Normally that can’t be done. They don’t say whether there is air in the tank and at what pressure, which makes a huge difference depending on the type of unloader system.

     In  one of my videos I show the struggle a homemade 20 hp rotary converter has starting a 10hp air compressor when there is pressure in the tank.  I show the starting power is even worse when you run the output of the rotary through a transformer to raise the voltage. These videos and more are on my You Tube channel.   

     American Rotary is making claims of a new circuit that gives 200% more starting power.  

     But as typical there can be a difference between advertising claims and real life.

     Below is their guide on sizing a rotary converter. Taken from their website  on 10/2016

     Their text is in italics and I made my comments in purple. I am not disparaging them, as a matter of fact I recommend them if you are looking to purchase a rotary converter. Just want to make people aware of things you need to consider before a purchase.


 American Rotary sizing guide.

The following sections provide sizing guidelines for the 4 most common types of 3-phase motor loads.  On this page you will find “single motor loads” divided into 3 categories Typical, (CNC/Voltage-Sensitive, Resistive), and a final section on multiple motor loads.  Please call us toll free 1-888-743-6832 for a professional sizing recommendation or should you have any other questions.

For TYPICAL Single Motor Loads (not CNC or Resistive):
Including milling machines, blowers, band saws, lathes, pumps, etc.
The total running current should be less than 75% of full load 3-phase current.  Uh? what does that mean??

  • Easy – no flywheel, clutch, little resistance/inertia, 1-2 times full load current upon start-up
    Choose the rotary phase converter HP that is one size larger than the 3-phase load.
  • Medium– machine inertia, medium resistance, 3-4 times full load current upon start-up
    Choose the rotary phase converter size by multiplying HP by 1.5 and round up.
  • Hard – flywheel, no clutch, starts against load, 5-6 times, or locked rotor current upon start-up
    Choose the rotary phase converter size by multiplying HP by 2.0 and round up.
  • Very Hard / Frequent Start– elevator, hydraulic pump, equipment under continuous load etc
    Choose the rotary phase converter size by multiplying HP by 2.5 and then round up. 
  • Increase rotary phase converter by one extra size for motors that are foreign or high efficiency or that have locked rotor currents greater than 6 times full load current. 
  • Increase rotary phase converter size by the ratio of load RPM to 1800 (common for 2-speed motors).

As you can we are always increasing the size of the rotary idler. Sizing a converter is important.

For CNC / VOLTAGE-SENSITIVE Single Motor Loads (CNC, Voltage-Sensitive):
Including CNC mills, CNC lathes and turning centers, PLC, EDM, CNC machining, CNC woodworking, etc.

The total running current should be less than 60% of full load 3-phase current. Why is this 60% number here? I thought you could run at 100% power. 
Do not combine other loads with CNC applications unless they are small compared to the loads rating. 

  • Choose the CNC phase converter size by either:
    – multiply the HP (of the spindle) by 2.0 for all standard axis equipment, or
    – multiply the machine kW by 2.7, then round up to the nearest HP converter.
  • Increase CNC phase converter by one extra size for non-standard equipment (forth axis, bar feeder, etc.)
  • Increase CNC phase converter by one extra size for motors that are foreign or high efficiency or that have locked rotor currents greater than 6 times full load current. 
  • Increase CNC phase converter size by the ratio of load RPM to 1800 (common for 2-speed motors).

For RESISTIVE Single Motor Loads:
Including heaters, transformers, welders, power supply, etc.

If only kW are known, estimate the current by multiplying the kW by 3.75. kW x 3.75 ~ current (amps)

  • Divide the current rating of the load (use 230 Volt amperage of the equipment) by 0.6
  • Round up to the nearest full load current of the CNC phase converter to maintain the necessary voltage balance.

For MULTIPLE Motor Loads:
Common examples: a dust collector and table saw, two milling machines, a mill and a lathe, etc.

Our CNC rated rotary phase converters are uniquely balanced to run multiple machines, limited only by the current available. They are not limited to 3X’s the rated horsepower as long as the phase converter is sized for starting the largest load. If you are considering running multiple machines, please call 1-888-743-6832 for a sizing recommendation. Our applications engineers and computer simulation models will determine the correct rotary phase converter for your application.

All of the motor loads must be simple (no resistive, CNC, or hard starting loads).

  • All of the motors must be light to medium loads. These lightly loaded 3-phase motors become a part of the rotary phase converter system and assist in generation of the third line.
  • Use the Single Motor Loads formulas to determine the largest rotary phase converter needed for each piece of equipment. Your rotary phase converter cannot be smaller than this. For multiple motor starting, size each piece and add the sizes of rotary phase converters together (this method can usually be avoided).
  • The other equipment (totaling no more than 2 times the rated HP of the rotary phase converter) may be started and run, assuming the loads are not heavy.Note: Combined motor loads and resistive loads must be sized by subtracting the resistive current from the full load current rating of the rotary phase converter. Combinations exceeding the hp rating of a rotary phase converter should be wired using the multiple motor load wiring diagram.


Does this make sense? They tout the abilities of the CNC rated phase converter and now it says no CNC or hard starting loads. This must be a misquote or misprint. From this you could believe that you can run multiple motors on the CNC machine rated rotary, but not multiple CNC machines. That is a huge discrepancy that you would want to know about before purchase. 

It also says ‘All the motors must be light to medium loads’? Uh? Why are you buying this? My thought was if you want to have real three phase power, if you want voltages and amps to balance, and you want to run lots of big machines just make the rotary bigger. This leads us to believe that is not going to work. Also the paragraph under the headingFor Multiple Motor Loads’ seems to be confusing. It says (no resistive, CNC, or hard starting loads). This implies that we can not run multiple resistive, CNC, or hard to start loads?

I still say the manufacturers should break out the video cameras and do some real filming. Do all sorts of live tests, give us the numbers.

I believe as I have said on the other pages that American Rotary is looking like one of the best manufacturers for a rotary. They are serious about innovating and improving the industry and reputation of the rotary converter. The above information about sizing a rotary converter seems odd. Maybe it is older and they just have not updated the website to reflect their newer equipment. If that is the case they can contact me, I will be happy to update.

This info seems to proves that what I was saying for years is correct. I was giving the do it yourself crowd an alternative to the rotary. Something that would get their hard to start motors running with the advantages that Ronk talks about.

I am not understanding where the 75 and 60 percent numbers in the above guide come from, or what they apply to. Below is ad copy from the front end of the site where it says 100% and now in the fine print, they are telling you not 100% but 75 or 60.  


The above ad from American Rotary states right there 100%.

The best advice I can give you is you should call them and explain exactly what you are going to run. They can help you with sizing a rotary converter. After all they build them!

Getting the ad copy to match the real life performance has been a touchy subject for me.

When I first started selling information on building a transformer converter there was a lot of ridicule. People called me a scammer and liar and dishonest. There were people that would come on the forum and flat out say it would not work. 

Then over a couple year period I got over 900 positive feedbacks on eBay from people that bought the information. Lots of people in Australia, England, Ireland and Canada. All countries with more of a challenge with running three phase motors because of having to run all the motors at 380, 415, 480 or 500 volts and the limits on how much power they had to work with.

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