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A simple homemade phase converter

A rotary phase converter, abbreviated RPC, is an electrical machine that produces three-phase electric power from single-phase electric power. This allows three phase loads to run using generator or utility-supplied single phase electric power.


Types of RPC

A rotary phase converter may be built as a Motor-generator set. A motor-generator set has the advantage that it isolates the generated three phase power from the single phase supply and that the three phase voltages can be balanced. However, because of weight, cost, and efficiency, most RPC's are not built this way.

Instead, they are built out of a three-phase induction motor or generator - called an idler - on which two of the terminals (the idler inputs) are powered from the single phase line. The rotating flux in the motor produces a voltage on the third terminal. A voltage is induced in the third terminal that is shifted by 120 degrees from the voltage between the first two terminals. In a three-winding motor, two of the windings are acting as a motor, and the third winding is acting as a generator.

Power quality

A common measure of the quality of the power produced by an RPC or any phase converter is the voltage balance, which may be measured while the RPC is driving a balanced load such as a three phase motor. Other measures of the quality of an RPC are the harmonic content of the power produced and the power factor of the RPC motor combination as seen by the utility. The best phase converter for any application depends upon the sensitivity of device being run to all of these factors. Three phase induction motors are very sensitive to voltage imbalance.

The quality of three phase power generated by such a phase converter depends upon a number of factors including:

  • Capacity of the phase converter (idler horsepower rating)
  • Power level demands of the equipment being supplied. For instance, "hard starting" loads such as heavily loaded machinery or well pumps may have higher requirements than other loads rated at the same horsepower.
  • Power quality demands of the equipment being supplied (CNC equipment may have more stringent power quality requirements than a welding machine )
  • Use of techniques to balance the voltage between the three legs.

Quality improvement

RPC manufacturers use a variety of techniques to deal with these problems. Some of the techniques include,

  • The insertion of capacitors between the terminals to balance the power at a particular load.
  • The use of idlers with higher power ratings than the loads.
  • The construction of special idler motors with more windings on the third terminal to boost the voltage and compensate for the sag caused by the load.
  • The use of electronics to switch in capacitors, during start up or otherwise, based on the load.
  • The use of filters.

How RPCs work

The main principles of RPC operation are as follows:

  • Three phase induction motors have three terminals called "legs", usually numbered (arbitrarily) L1, L2, and L3.
  • A three phase induction motor can be run at two-thirds of its rated horsepower on single phase power applied to any pair of legs, once spun up by some means.
  • A three phase induction motor that is spinning under single phase power applied to legs L1 and L2, generates an electric potential (and can deliver power through) leg L3, although without some form of current injection, special windings in the idler, or other means the voltage will sag when a load is applied.
  • Power factor correction is a very important consideration when building or choosing an RPC. Power factor correction is desirable because an RPC that has power factor correction will consume less amperage from the single-phase service supplying power to the phase converter and its loads.

Balanced voltage between the three legs of power is important for operational life of the equipment receiving that power. Unbalanced three phase power can damage the equipment that it is meant to operate.

Use of RPCs


RPCs may be used anywhere three phase devices need to be used, but only single phase power is available. Three phase motors cannot run on single phase power without a device to generate three phase power. Since prices (and quality) of used three phase motors are usually more favorable than those of their single phase counterparts, demand exists for phase converters. This is also true because single phase electric motors generally are not available in sizes over 15 hp (11 kW) due to their complexity, starting requirements and relative high expense and low efficiency compared to three phase motors. In fact, single phase motors larger than 5 hp (3.7 kW), though available, are rarely seen in use for these reasons. RPCs are sold by various vendors, but often also are home made.

Electric railways

Rotary phase converters are used in the opposite direction (three-phase to single-phase) for electric railways. In Europe, electricity is normally generated as three-phase AC at 50 Hertz. Five European countries, Germany, Austria, Switzerland, Norway and Sweden have standardised on single-phase AC at 15 kV 16⅔ Hz for railway electrification. Phase converters are, therefore, used to change both the phase and the frequency.

Alternatives to rotary converters

Alternatives exist to rotary phase converters for operation of three-phase equipment on a single-phase power supply. All techniques have potential problems.

  • Static phase converters require de-rating of the motor being run, typically to 60% of its three-phase rating. A 10 hp (7.5 kW) motor can be run only at 6 hp (4.5 kW). The power balance here is extremely poor.
  • Three phase inverters and related Variable frequency drives, may produce large amounts of harmonic distortion in their output, and the efficiency of the inverter-motor combination can be poor. Early inverter drives could damage some motors not rated for use with an inverter and shortened their lifespan. Motor manufacturers may void warranties on non-rated motors if they are run at a variable speed or with an inverter, but most three phase motors over 10 hp (7.5 kW) sold today are rated for use with an inverter drive.

See also

External links


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