phase converters

Single-phase power can be obtained from any three-phase electrical system by connecting any two phase leads. However, it is not possible to obtain three-phase power from a single-phase power source without recourse to either a rotary or static phase converter

A rotary phase converter is a machine that can convert single-phase power to three phase power. More accurately termed a phase generator than a converter, it generates a voltage that, when paralleled with two voltages obtained from single-phase line power, produces three-phase power. Rotary converters can provide three-phase current that can power three-phase as well as inductive loads.

below Figure is a schematic diagram for a rotary converter. The electromechanical machine is connected to a single-phase source, and it generates a third phase output for powering three-phase loads and motors. Single-phase lines L1 and L2 are connected to a three-phase fused disconnect switch or magnetic starter. The outputs of the switch or starter are designated TI and T2. Parallel branches are fed to the rotary converter terminals, also designated TI and T2. The converter produces an output T3, which, after passing through the switch or starter, is combined with T1 and T2 to form the three-phase output (T1, T2, and T3) for driving motors or other three-phase loads.


When the rotary converter is running alone without a load, line T3 provides the highest voltage with respect to ground. This line should not be used to power any single-phase loads. The manufacturers of these machines caution that all electrical equipment should be grounded as required by the NEC, and that the rotary converter should always be started before energizing motor loads. All wire sizes, lengths, and voltage drops should be in accordance with the manufacturer’s recommendations.

Rotary converters can power metalworking and woodworking machines, farm equipment, pumps, compressors, elevators, lasers, battery chargers, plasma cutters,electro discharge machining (EDM) systems, heating elements, and variable-frequency drives. However, they are not suitable for powering heavy starting loads or instantly reversing or momentarily overloaded motors such as those that power laundry extractors, paper cutters, air conditioners, hoists, or high-speed lathes

Because of the high current required to start a motor (which can be from 5 to 10 times the normal running current), the horsepower of the largest motor or combination of motors started at exactly the same time should not exceed the manufacturer’s maximum rating for the rotary converter. However, once the motor or combination of motors has been started, it is acceptable to start additional smaller motors up to but not exceeding the total horsepower rating of the converter. For example, a 3-hp rotary converter could run motors whose combined rating is up to 9 hp if they are running lightly loaded.

Magnetic controls or single-phase loads must always be energized by lines T1 and T2. Phase converter manufacturers warn against connecting a ground or neutral to the T3 line, the artificially produced phase which can easily be identified as the line with the highest voltage to ground when the converter is running.

The manufacturers agree that the user must pay close attention to recommended wiring size and length to prevent slow starting caused by a voltage drop. They add that when starting a motor whose horsepower rating equals the rating of the rotary converter, lower starting torque can occur. This is most likely to happen when the motor is driving a heavy load, because of the higher starting current. However, full running torque usually can be obtained.

Rotary phase converters should be located in a clean, dry room with sufficient air circulation to provide adequate air cooling. Moreover, the converter should not be operated near flammable liquids, gasses, or dust, because these could ignite fires in the insulation of the converter and damage or destroy it.

Rotary phase converters that operate from 220 and 460 V, single-phase power are available. Almost all machine loads require that the converter be sized at least 50 percent higher than the largest horsepower rating of any motor that is to be driven. Where large horsepower loads are to be powered, additional rotary phase converters can be paralleled to drive those loads safely.

A static phase converter is another option for converting single-phase to three-phase power. These converters are suited only for operating motor loads. For all other applications, rotary converters must be used. The static phase converter includes capacitors to start the motor as is done in single-phase capacitor-start motors.

Static phase converters do not actually generate three-phase power continuously the way rotary phase converters do. Just as in capacitor-start motors, the capacitors are disconnected once the motor has started, and the motor continues to run on singlephase power. However, because only two of the three windings are powered while the motor is running, the power output of the motor is reduced to 66 percent of its rated value. For example, a 10-hp motor will start with a 10-hp output but will run only as a 6.6-hp motor.

Motor loads that have high starting torque but will run with reduced power are important characteristics in the selection of an appropriate static phase converter. These phase converters can be used with air compressors, for example, because of their high starting torque characteristics. However, in many applications the pulley diameter of the motor must be reduced to compensate for the loss of horsepower once the compressor is running.

Static phase converters are not recommended for driving motor loads that operate close to their maximum rated horsepower ratings. For example, a static phase converter might be acceptable for driving a three-phase motor in a lathe only if it will function satisfactorily at slow speeds. When higher speed is required, the converter can start the lathe turning, but the higher speed settings cannot be achieved because of diminished horsepower output. Rotary phase converters are better suited to these applications.

Another factor to take into account when considering the purchase of a static converter
is duty cycle. (This is defined as the percentage of time a motor runs fully or

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