How to Size a Rotary Phase Converter

As easy as 1, 2, 3

Sizing a rotary phase converter properly cannot be over stated.   Proper sizing will insure years of trouble free performance for your equipment.

If at any time you have questions about sizing a rotary phase converter, please call one of our application engineers at 877-545-2926.

Note - You can never over size a phase converter; there are no minimum load requirements for a North America Rotary Phase Converter.

Step 1: Determine Load Type

Classifications for Loads 

  • Easy – Very little rotating force on motor start up.  Relatively low starting torque.  Usually draws @1-2 the Full Load Amperage (FLA) when starting.

Examples of Easy Loads – drill press, milling machine, wood shaper, and table saw

  • Medium – A medium/moderate amount of force during start up.  Medium amount of starting torque. Usually draws 3-4 times the FLA when starting.

Examples of Medium Loads – wide belt sander, lathe with clutch, grinder, dough mixer

  • Hard/ Heavy- Typically starting under load.  High inertia loads.  Heavy rotational force in the motor during shaft rotation.  Usually draws 5-6 times FLA when starting.  

Examples of Hard / Heavy Loads – lathe without clutch, dust collector, fans, ironworker

  • Very Hard / Heavy-– High peak loads.  Very heavy twisting and turning force produced by motor during start up.  Very high inertia loads.  Usually draws 6+ times FLA when starting.  

Examples of Very Hard Loads – air compressors, submersible pumps, elevator, large flywheel machines 

  • CNC – Computer controlled machines.  Electronically or digitally controlled.  Tend to be very voltage sensitive. 

Examples of CNC Loads – CNC milling machines, CNC router, CNC lathe, EDM, variable frequency drive (VFD) 

  • Resistive – An electrical load where voltage and current are converted to energy in the form of heat. 

Examples of Resistive Loads – heater, kiln, welder, oven

  • Inductive – A load that has a coil of wire as the current drawing element. Pulls large amounts of inrush current when first energized and then settles down. 

Examples of Inductive loads – battery charger, transformer, lighting ballast 

Unsure what your load type is?  Visit our Motor Load Chart to find out.

Step 2: Determine Power Requirements of Load 

  • Motor Loads – Find horsepower (HP) , kilowatts (kW) or amperage
  • CNC Loads – Find total horsepower (HP), total kilowatts (kW), total kilo-volt-amps (kVA) or total amperage of machine
  • Resistive and Inductive Loads – Find total kilowatts (kW), total kilovolt-amperes (kVA) or total amperage of machine

Helpful formulas:

 Amperage / 2.8 = HP

kW X 1.342 = HP

kVA X 1000 / Volts X 1.732 = Amps

Step 3Pick rotary phase converter using chart (available below)

  1. Find classification for your load (Easy, Medium, Hard or power requirements – Amps, kVA etc.)
  2. Scroll down column to your power requirements
  3. Follow row to right (move to blue column)
  4. Congratulations!  This is the correct size rotary phase converter for your needs.
Pro-Line Model & Sizing Chart
Motor Loads (Horsepower) CNC, Resistive & Inductive Loads Pro-Line
Easy Med Hard Very
3 2.5 2 1.5 1 N/A N/A N/A N/A PL-3
5 3 3 2 1.5 N/A N/A N/A N/A PL-5
7.5 5 5 3 2.5 12 5 2.5 3.5 PL-7
10 7.5 6 5 3 16 6 3.7 5 PL-10
15 10 7.5 7.5 5 25 8 5.6 7.5 PL-15
20 15 12 10 7.5 32 10 7.7 10 PL-20
25 20 15 12 10 40 12 9.5 12 PL-25
30 25 20 15 10 49 21 11.8 15 PL-30
40 30 25 20 15 61 25 15.5 20 PL-40
50 40 30 25 20 76 30 21.5 25 PL-50

Still have sizing questions?  Please call our application engineers for assistance at 877-545-2926.

****Additional Rotary Phase Converter Sizing Rules to Consider****

1. Add one (1) additional size to rotary phase converter for European, Brazilian and Taiwan made motors.

2. Add one (1) additional size to rotary phase converter for motor built between 1952 and 1965.

3. Add two (2) additional sizes to phase converter for motors older than 1952.

4. For 2-speed motors, add horsepower together of two speeds and size from that horsepower.

General Sizing Rules for Rotary Phase Converters

For Easy LoadsShould be at least one size larger than load motor.

 For Medium LoadsShould be at least 1.5 times larger than load motor.

 For Hard / Heavy LoadsShould be double (2 times) the size of load motor.

 For Very Hard / Heavy LoadsShould be 2.5 times larger than load motor.

 For Multiple Easy Loads (not starting together) – Could be sized 75 – 95% of total loads.

 For Multiple Medium Loads (not starting together) – Could be sized between 60 – 75% of total loads.

 For Multiple Hard / Heavy Loads (not starting together) – Could be sized between 50 – 65% of total loads.

 For Multiple Very Hard / Heavy Loads (not starting together) – Could be sized between 40 – 60% of total loads.

 For All CNC, Resistive and Inductive Loads – Should be double (2 times) the size of the load.

Please call or email a NAPCES application engineer if you have any questions about sizing at 877-545-2926.  Our expertly skilled engineering team will be glad to work with you to answer any questions you may have.