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.
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.
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:
RPC manufacturers use a variety of techniques to deal with these problems. Some of the techniques include,
The main principles of RPC operation are as follows:
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.
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.
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 exist to rotary phase converters for operation of three-phase equipment on a single-phase power supply. All techniques have potential problems.