Rotary Converter Application Notes

The Rotary Converter is designed to supply full running current to a three-phase motor normally providing it with full running torque. However, most motors will draw five times their running current during start-up. When used at its maximum HP rating the Rotary Converter cannot deliver the full (5 times) starting current to the motor and therefore cannot provide full starting torque. For heavy start-up loads a larger converter should be used.

NOTE: You can always use a larger Rotary Converter than the HP of the motor. There is no minimum load requirements for the Rotary Converters. Some customers will install a Rotary Converter larger than they need to accommodate any future additions to their equipment. Below are the minimum size recommendations for various applications.


Motor Load Types



May be used up to the HP rating of the converter.* 
Many restrictions apply.
Most applications require sizing the converter a
minimum of 50% larger (see all load types).
Contact GROMAX to
verify load type.
For instant reversing (as for rigid tapping), size according to



These include domestic & European lathes without a clutch, some pumps, wheel balancers, paper cutters, flywheel driven equipment, air conditioners, blowers, woodworking band saws, dough mixers, meat grinders, motors rated below 1000 RPM, etc. Use a converter with HP rating of at least 50% larger than HP of the motor.



These include Design "E" motors, Taiwanese, Chinese, Brazilian, Mexican motors, pumps starting under load, etc. Use a converter with twice the HP rating of the motor.



These include laundry extractors, hoists, elevators, etc. For these start-up loads use a converter with three times the HP rating of the motor.



Often hydraulic pumps, which come under a momentary load during use will be loaded well beyond their rated HP for the brief period of maximum PSI. Examples includes bailers, compactors, paper cutters, shears, pumps, etc. The HP of the converter must be at least as high as the actual HP developed by the motor. To calculate the HP developed, you must first find the actual amperage drawn during maximum PSI. This is different from the rated amps of the motor. Next you would divide the maximum amperage by 2.8 to find the actual HP being developed by the motor. That figure is the minimum size of converter to be used. Example: A 10 HP compactor with a motor rated at 28 amps but draws a peak of 40 amps momentarily at maximum compression. Divide 40 by 2.8 = 14.3 HP being developed, use model R-15 Rotary Converter.


Resistive Loads

Resistive loads must use the Rotary type converter, the Static type should never be used because it would be damaged. There are two methods to determine the HP of the converter to be used. One method is to take the amperage rating of the equipment and divide by 2.8 to find the equivalent HP. The other method is to take the KW rating and multiply times 1.34 or divide by .75 to find the equivalent HP of the equipment.


Computer, Rectifier & Transformer Loads

Transformers and electric equipment (welders, lasers, EDM machines, CNC equipment, computers, plating rectifiers, power supplies, etc.) can operate on the Rotary Converter. Use the same formula as for resistive loads to determine the proper size converter to use.

If a 4-wire wye input is required (all lines equal voltage to ground), a three phase delta-to-wye isolation transformer must be installed between the converter and the equipment to change the Delta power to wye power.


Multiple Motor Applications

In some applications it is possible for the Rotary Converter to run up to 3 times its rated HP provided the motors are not running heavily loaded. Always contact the factory for help before attempting to do this.