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Sizing Rotary Converters
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.
- 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).
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.
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.