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Sizing Rotary Phase Converters

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Phase Converters...
     TURN SINGLE PHASE INTO THREE PHASE ELECTRICIT

Phase Converters convert single phase power to industrial 3-phase power. Different types of phase converters exist, and they each have a certain method to achieve the conversion. Because American Rotary offers every type of phase converter available...Rotary, Digital, CNC, and Static...you can always be sure that we will recommend the method that is preferred for your specific application!

Rotary Phase Converters are a cost-effective and efficient means to start and run 3-phase motors on single phase power at 100% of their rated horsepower. Rotary phase converters can be used with heavy loads, resistive loads, multiple motor loads, multiple speed loads, and CNC equipment.

Sizing Rotary Phase Converters

Can American Rotary Phase Converters be oversized?

Customers often buy larger rotary CNC phase converters than are required for their application. Many shops anticipate adding additional equipment in the future. American Rotary phase converters are balanced from a no load condition, which means that there is no minimum size to a motor that can safely be run with any of our rotary phase converters, even if you have a very large rotary phase converter. For example, a 2HP mill can be safely run on our 40HP rotary phase converter and still maintain a properly balanced voltage. This is an important distinction in the marketplace. Not all rotary phase converters are balanced from a no-load condition!

How Do I Size Rotary Phase Converters?

Sizing rotary phase converters can be very simple or somewhat complicated depending on the application it will run. The sizing information provided here can be used as a guideline to help you determine what size rotary phase converter your application will require.

Please call toll free 1-800-458-3687 for assistance.

Determine the load type of the application:

  • Easy - no flywheel, clutch, little resistance/inertia, 1-2 times full load current upon start-up
  • Medium- machine inertia, medium resistance, 3-4 times full load current upon start-up
  • Hard - flywheel, no clutch, starts against load, 5-6 times, or locked rotor current upon start-up
  • Very Hard- elevator, hydraulic pump, equipment under continuous load etc
  • Frequent Start- instant reversing, heavy running loads
  • CNC - turning centers, PLC's, EDM, CNC machining
  • Resistive - heaters, transformers, welders, power supplies etc.

CNC Rotary Phase Converters For Single Motor Loads (CNC, Non-Resistive)

Including: CNC machining, turning centers, PLC's, EDM, etc.

  • Choose the CNC phase converter size by either:
    • multiplying 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 size for non-standard equipment (forth axis, bar feeder, etc.)
  • Make sure the total running current is no more than 60% of full load 3-phase current.
  • Go up one extra CNC phase converter size for any motors that are foreign, high efficiency, or have locked rotor currents that are greater than 6 times full load current.
  • Increase CNC phase converters size by the ratio of load RPM to 1800 (common for 2-speed motors). Note: Do not combine other loads with CNC applications unless they are small compared to the loads rating.

Rotary Phase Converters For Single Motor Loads (Non-Resistive, Non-CNC)

Including: milling machines, blowers, band saws, lathes, pumps, etc.

  • Determine the load type - Easy, Medium, Hard, or Very Hard
    • Easy Load: Choose the converter HP that is one size larger than the load.
    • Medium Load: Choose the CNC converter size by multiplying HP by 1.5 and round up.
    • Hard Load: Choose the converter size by multiplying HP by 2.0 and round up.
    • Very Hard/Frequent Start Load: Choose the CNC phase converter size by multiplying HP by 2.5 and then roundup.
  • Make sure the total running current is no more than 75% of full load 3-phase current.
  • Go up one extra CNC phase converter size for any motors that are foreign, high efficiency, or have locked rotor currents that are 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).

CNC Rotary Phase Converters For Single Motor Loads (Resistive)

Common examples: heaters, transformers, welders, power supply, etc.

  • Divide the current rating of the equipment (Use 230 Volt amperage of the equipment) by 0.6
  • Round that number up to the nearest full load current of the CNC phase converter. This maintains a 5% voltage balance.

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

Rotary Phase Converters For Multiple Motor Loads

Common examples: 2 milling machines, a dust collector and table saw, etc.

  • Use the Single Motor Loads formulas to determine the largest CNC phase converter needed for each piece of equipment. Your CNC phase converter cannot be smaller than this. For multiple motor starting, size each piece and add the sizes of CNC phase converters together (this method can usually be avoided).
  • The other equipment (totaling no more than 2 times the rated HP of the CNC 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 CNC phase converter. Combinations exceeding the hp rating of a CNC converter should be wired using the multiple motor load wiring diagram.

Note: Our CNC rotary phase converters are rated to run multiple machines, only limited by the current available. They are not limited to 3X's the rated horsepower as long as CNC phase converters are sized for starting the largest load. If you are considering running multiple machines, please call 1-800-458-3687 for a sizing recommendation. Our applications engineers and computer simulation models will determine the correct rotary phase converter for your application. Running multiple machines on a single CNC phase converter is a common, yet situational scenario where specific conditions must be satisfied:

  • 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 CNC phase converter system and assist in generation of the third line.

Determine the load type of the application, and then consult the chart below. 
Please call us toll free at 1-800-458-3687 for sizing assistance and/or a professional recommendation.

  • Easy - no flywheel, clutch, little resistance/inertia, 1-2 times full load current upon start-up
  • Medium- machine inertia, medium resistance, 3-4 times full load current upon start-up
  • Hard - flywheel, no clutch, starts against load, 5-6 times, or locked rotor current upon start-up
  • Very Hard- elevator, hydraulic pump, equipment under continuous load etc
  • Frequent Start- instant reversing, heavy running loads
  • CNC - turning centers, PLC's, EDM, CNC machining
  • Resistive - heaters, transformers, welders, power supplies etc.
Rotary Phase Converter Largest HP Motor Load Maximum CNC Load Max Total Load Combined Equip Operating Cost
HP kW Easy Med Hard Amps kW HP* Amps kW ($ per/hr)
3 2.2 2 2 1.5 0-5 0-1.5  1.5  9  2.7  $.01
5 3.7 3 3 2 0-8 0-2.4  2.5  15  4.5  $.015
7.5 5.6 5 5 3 0-11 0-3.3  3.75  22  6.6  $.02
10 7.4 7.5 6 5 0-15 0-4.5  5  28  8.4  $.03
15 11.2 10 10 7.5 0-20 0-6  7.5  42  12.6  $.045
20 14.9 15 12 10 0-25 0-7.5  10  54  16.2  $.06
25 18.6 20 15 10 0-30 0-9  12.5  68  20.4  $.07
30 22.4 25 20 15 0-42 0-12.6  15  80  24.0  $.09
40 29.8 30 25 20 0-55 0-16.5  20  104  31.2  $.12
50 37.3 40 35 25 0-70 0-21  25  126  37.8  $.15
60 44.7 50 40 30 0-80 0-24  30  150  45.0  $.18
80 58.7 60 50 40 0-110 0-33  40  208  62.4  $.24
100 73.3 80 70 50 0-140 0-42  50  252  75.6  $.30
120 88.0 100 80 60 0-160 0-48  60  300  90.0  $.36
* total spindle horsepower: total kW of the machine must be less than max CNC load kW.
† combined loads do not include CNC or voltage sensitive loads.
‡ cost based on national average of $.10/kWhr.
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