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A simple homemade phase converter

A rotary phase converter, abbreviated RPC, is an electrical machine that converts power from one polyphase system (including frequency) to another, converting through rotary motion. Typically, single-phase electric power is used to produce three-phase electric power locally to run three-phase loads (any industrial machinery with three-phase motors) in premises (often residential or consumer) where only single-phase is available.



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. This is desirable because an RPC that has power factor correction will consume less current 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.


The first locomotive with a phase converter (only for demonstration purposes)
The Kandó phase converter (1933)
The "Kandó" locomotive, the first locomotive using a phase converter system

At the beginning of the 20th century, there were two main principles of electric railway traction current systems:

  1. DC system
  2. 16⅔ Hz single phase system

These systems used series-wound traction motors. All of them needed a separated supply system or converters to take power from the standard 50 Hz electric network.

Kandó synchronous phase converterEdit

Kálmán Kandó recognized that the electric traction system must be supplied by single-phase 50 Hz power from the standard electric network, and it must be converted in the locomotive to three-phase power for traction motors.

He created an electric machine called a synchronous phase converter, which was a single-phase synchronous motor and a three-phase synchronous generator with common stator and rotor.

It had two independent windings:

  • The outer winding is a single-phase synchronous motor. The motor takes the power from the overhead line.
  • The inner winding is a three-phase (or variable-phase) synchronous generator, which provides the power for the three- (or more) phase traction motors.

Single-phase supplyEdit

The direct feed from a standard electric network makes the system less complicated than the earlier systems and makes possible simple recuperation.

The single-phase feed makes it possible to use a single overhead line. More overhead lines increase the costs, and restrict the maximum speed of train.

Speed controlEdit

The asynchronous traction motor can run on a single RPM determined by the frequency of the feeding current and the loading torque.

The solution was to use more secondary windings on phase converter, and more windings on motor different number of magnetic poles.


A rotary phase converter (RPC) may be built as a motor-generator set. These completely isolate the load from the single-phase supply and produce balanced three-phase output. However, due to weight, cost, and efficiency concerns, most RPCs 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° 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 qualityEdit

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 quality measures include the harmonic content of the power produced and the power factor of the RPC motor combination as seen by the utility. Selection of the best phase converter for any application depends on the sensitivity of the load to these factors. Three-phase induction motors are very sensitive to voltage imbalances.

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

  • Power 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 improvementEdit

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.



Demand exists for phase converters due to the use of three-phase motors. With increasing power output, three-phase motors have preferable characteristics to single-phase motors; the latter not being available in sizes over 15 hp (11 kW) and, though available, rarely seen larger than 5 hp (3.7 kW). (Three-phase motors have higher efficiency, reduced complexity, with regards to starting, and three-phase power is significantly available where they are used.)

Electric railwaysEdit

Rotary phase converters are used to produce a single-phase for the single overhead conductor in electric railways. Five European countries (Germany, Austria, Switzerland, Norway, and Sweden), where electricity is three-phase AC as 50 Hz, have standardised on single-phase AC at 15 kV ​16 23 Hz for railway electrification; phase converters are, therefore, used to change both phases and frequency. In the Soviet Union, they were used on AC locomotives to convert single phase, 50 Hz to 3-phase for driving induction motors for traction motor cooling blowers, etc.[1]

Alternatives to rotary convertersEdit

Alternatives exist to rotary phase converters for operation of three-phase equipment on a single-phase power supply.

Static phase convertersEdit

These may be an alternative where the issue at hand is starting a motor, rather than polyphase power itself. The static phase converter is used to start a three-phase motor. The motor then runs on a single phase with a synthesised third pole. However, this makes the power balance, and thus motor efficiency, extremely poor, requiring de-rating the motor (typically to 60% or less). Overheating, and quite often destruction of the motor, will result from failing to do so. (Many manufacturers and dealers specifically state that using a static converter will void any warranty.) An oversized static converter can remove the need to de-rate the motor, but at an increased cost.

Three-phase invertersEdit

Three-phase inverters and related variable frequency drives, may, under certain circumstances, produce large amounts of harmonic distortion in their output, and the efficiency of the inverter-motor combination can be poor.

When poor efficiency in a VFD powered motor has been investigated, in every case documented to date, the root cause for the loss in efficiency was either improper installation or a failure to adhere to best practices. Almost universally, a VFD increases the efficiency of a system and reduces operating costs.

Inverter drives (VFDs)Edit

Inverter drives (VFDs) can damage some motors not rated for use with an inverter, but this is not a concern when motor voltages are less than 440 V AC. Motor manufacturers may void warranties on non-inverter 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.

This damage can be prevented by the use of load and line reactors or in Europe a DU/DT filter. This reactor or filter reduces the voltage stack up that can occur on older high impedance motors such as wound rotor motors. The filter prevents the windings of the motor form insulation breakdown caused by voltage stack up.

VFDs that have a capacity greater than 3 hp (2.2 kW) can be costly, thus making the RPC an attractive option.

See alsoEdit


  1. ^ Захарченко Д.Д., Ротанов Н.А. "Тяговые электрические машины" (Traction electrical мachinery) Москва, Транспорт, 1991 - 343 p., p.231
  • Sitkei Gyula: A magyar elektrotechnika nagy alakjai. (Energetikai Kiadó Kht. 2005)
  • "Az egyfázisú, 50 periódusú, fázisváltós vontatási rendszer újabb fejlődése (The newest developments of single-phase 50 Hz traction systems)" (PDF) (in Hungarian).

External linksEdit