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Copyright 2005
Smith Electric Motorworks

Appliction of Converters

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Application of Phase Converters
Selecting a Converter for Motor Loads

Three things come into basic consideration when sizing a converter to motor loads:

  1. The Hp of the largest single motor you wish to operate;
  2. The stresses placed on this largest single motor; and
  3. The total Hp of all the motors you wish to operate at one time.

Most applications for a rotary phase converter will have a number of small motors and one or two larger motors. The largest motor normally presents the most difficult starting load. We deal with choices concerning it immediately following, as well as the system capacity of the rotary.

Lightly- and Moderately-Loaded Motors

For lightly loaded machine tools (drills, mills, low-speed lathes fitted with a clutch, pedestal grinders, bandsaws, conveyors, shapers, motor-driven welders, dishwashers) you may wish to check our Autogen static phase converters.

On a multi--motor application with a broad HP range of lightly-loaded motors, some converter manufactures advocate using a rotary of equal HP to the largest motor operated. This is not a wise move, since on motor startup, the rotary has to pull itself while providing two-thirds of the power for the load motor. This is why we always recommend that you choose a rotary that is at least one size larger than the largest operated motor, even on these lightly-loaded machines. The multi--motor load may then be three times the HP of the largest motor you operate.

Normally Loaded Motors

For best all-around equipment performance, a rotary that is twice the size of the largest single motor in your system should be used. This will allow most load motors to be started and operated at up to 150% of their full-load torque. In this equation, a 5 Hp air compressor would be operated on a model 215 (10 Hp) Rotogen or AAMP converter. Other load motors, up to an average load of 13 to 15 Hp could be operated at the same time on this model. Applications that require a converter twice the size of the largest operated motor include; hydraulic ironworkers, A/C and refrigeration compressors, balancers, planers, shapers and edgers, sawmill equipment, grinder pumps, augers, grain drying fans, hoists, elevators, bank vacuum systems, door openers, hammermills, dumbwaiters, deep-well submersible pumps 15 Hp and smaller, car wash pumps, automotive lifts, and other such applications.

On multi-motor centrifugal pump and grain fan applications, it is often necessary to provide additional capacitor panels to balance line currents and voltage under load.

Difficult Motor Applications

Some motors operate under extreme starting/running loads, and require special consideration. A Rotary Converter that is at least two-and-one-half (2 1/2) times the size of the largest operated motor is used. Applications include; Submersible (deep-well) water pumps 20 Hp and larger, high-speed geared-head lathes manufactured in the Far East and Europe, and/or lathes with multi-speed motors, some fertilizer blenders and cement mixers, cement augers, soft ice-cream machines, most hydraulic brakes, shears, and compactors, pallet notchers, fire sirens and other tough loads.

Please note that GWM's Application Guide now includes complete information for your machines in an easy-to-follow guide keyed to our Rotary and Static Phase Converters.

Operating EPact and Other High-Efficiency Motors; Voltage Sensitive Foreign Motors

Special consideration must be given to EPact-rated motors and other high-efficiency designs manufactured after November 1997 that are to be operated on a phase converter. Many high-efficiency motor designs in fact will not operate on a static/capacitor-type phase converter. For best operating results, the minimum rotary converter to operate an EPact rated motor will have twice the frame HP rating of the EPact motor--i.e. a 10 HP EPact motor requires at minimum a 20 HP (model 256) Rotary Converter--even on lightly-loaded machines.

European motors--particularly on voltage-sensitive equipment such as crankshaft and surface grinders--are best operated on the Digi-Series Rotary Converter to prevent over voltage "saturation." High-efficiency motors often contain a higher silicon-content electrical steel which does not respond well to a capacitor-induced voltage, therefore a converter with a higher inductance-to-capacitance ratio, such as GWM's Digi-Series is more desirable. Some foreign (non-U.S.) built machine designs may require a "buck" (reduced)-voltage transformer connected ahead of the converter system to lower the basis at which 3-phase voltages are produced. Many European machines are designed for 200 or 220 volt-service, not the 240 to 250 volts usually found on U.S. single-phase lines.

Generally, one size larger rotary than we would normally apply to a given load is best for high-efficiency designs--that is, normal loads require a rotary 2 1/2 to 3 times the largest EPact motors HP rating, heavy loads, 4 times the motor's rating. Some poorly designed motors on very heavily-loaded machines have difficulty operating at all on a rotary phase converter--any manufacturer's rotary phase converter--unless the motor is oversized to ease the load. For more insight into this problem please refer back to "Applying Motors to Insure Good Converter Performance." See also, "Truths and Myths About Multi-Motor Ratings" for applications involving large, heavily-loaded motors.

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