Rotary phase converter - AbsoluteAstronomy.com

A rotary phase converter, abbreviated RPC, is an electrical machine

Electrical machine

An electrical machine is the generic name for a device that converts mechanical energy to electrical energy, converts electrical energy to mechanical energy, or changes alternating current from one voltage level to a different voltage level....

that produces three-phase electric power

Three-phase electric power

Three-phase electric power is a common method of alternating-current electric power generation, transmission, and distribution. It is a type of polyphase system and is the most common method used by grids worldwide to transfer power. It is also used to power large motors and other heavy loads...

from single-phase electric power

Single-phase electric power

In electrical engineering, single-phase electric power refers to the distribution of alternating current electric power using a system in which all the voltages of the supply vary in unison. Single-phase distribution is used when loads are mostly lighting and heating, with few large electric motors...

. 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

Motor-generator

A motor-generator is a device for converting electrical power to another form. Motor-generator sets are used to convert frequency, voltage, or phase of power. They may also be used to isolate electrical loads from the electrical power supply line...

set. These have the advantage that in isolating the generated three-phase power from the single phase supply and balancing the 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

Idler

An idler is a mechanical device such as an idler pulley or idler wheel that is secondary to the main transfer of power in a mechanical system. They are support rollers on which conveyor belts move...

, on which two of the terminals (the idler inputs) are powered from the single phase line. The rotating flux

Magnetic flux

Magnetic flux , is a measure of the amount of magnetic B field passing through a given surface . The SI unit of magnetic flux is the weber...

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 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 quality measures include the harmonic content of the power produced and the power factor

Power factor

The power factor of an AC electric power system is defined as the ratio of the real power flowing to the load over the apparent power in the circuit, and is a dimensionless number between 0 and 1 . Real power is the capacity of the circuit for performing work in a particular time...

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 motor

Induction motor

An induction or asynchronous motor is a type of AC motor where power is supplied to the rotor by means of electromagnetic induction. These motors are widely used in industrial drives, particularly polyphase induction motors, because they are robust and have no brushes...

s 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
Numerical control
Numerical control refers to the automation of machine tools that are operated by abstractly programmed commands encoded on a storage medium, as opposed to controlled manually via handwheels or levers, or mechanically automated via cams alone...

may have more stringent power quality requirements than a welding machine
Arc welding
Arc welding is a type of welding that uses a welding power supply to create an electric arc between an electrode and the base material to melt the metals at the welding point. They can use either direct or alternating current, and consumable or non-consumable electrodes...

)
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. 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.

General

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 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, 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

Railway electrification system

A railway electrification system supplies electrical energy to railway locomotives and multiple units as well as trams so that they can operate without having an on-board prime mover. There are several different electrification systems in use throughout the world...

. In Europe

Europe

Europe is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...

, electricity is normally generated as three-phase AC at 50 Hertz

Hertz

The hertz is the SI unit of frequency defined as the number of cycles per second of a periodic phenomenon. One of its most common uses is the description of the sine wave, particularly those used in radio and audio applications....

. Five European countries, Germany

Germany

Germany , officially the Federal Republic of Germany , is a federal parliamentary republic in Europe. The country consists of 16 states while the capital and largest city is Berlin. Germany covers an area of 357,021 km2 and has a largely temperate seasonal climate...

, Austria

Austria

Austria , officially the Republic of Austria , is a landlocked country of roughly 8.4 million people in Central Europe. It is bordered by the Czech Republic and Germany to the north, Slovakia and Hungary to the east, Slovenia and Italy to the south, and Switzerland and Liechtenstein to the...

, Switzerland

Switzerland

Switzerland name of one of the Swiss cantons. ; ; ; or ), in its full name the Swiss Confederation , is a federal republic consisting of 26 cantons, with Bern as the seat of the federal authorities. The country is situated in Western Europe,Or Central Europe depending on the definition....

, Norway

Norway

Norway , officially the Kingdom of Norway, is a Nordic unitary constitutional monarchy whose territory comprises the western portion of the Scandinavian Peninsula, Jan Mayen, and the Arctic archipelago of Svalbard and Bouvet Island. Norway has a total area of and a population of about 4.9 million...

and Sweden

Sweden

Sweden , officially the Kingdom of Sweden , is a Nordic country on the Scandinavian Peninsula in Northern Europe. Sweden borders with Norway and Finland and is connected to Denmark by a bridge-tunnel across the Öresund....

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

Frequency changer

A frequency changer or frequency converter is an electronic device that converts alternating current of one frequency to alternating current of another frequency. The device may also change the voltage, but if it does, that is incidental to its principal purpose.Traditionally, these devices were...

Alternatives to rotary converters

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

Static phase converters are actually not a converter at all. They start the motor spinning and then "single-phase" the motor. They also require de-rating of the motor being run, typically to 60% or less of its three-phase rating. A 10 hp motor can be run only at 6 hp. The power balance and thus motor efficiency is extremely poor. Failure to run a motor at reduced load will result in overheating and quite often destruction of the motor. Many machine manufacturers and dealers specifically state that the use of a static converter will render any warranty null and void.

Three phase inverters and related variable frequency drive
Variable frequency drive
A variable-frequency drive is a system for controlling the rotational speed of an alternating current electric motor by controlling the frequency of the electrical power supplied to the motor. A variable frequency drive is a specific type of adjustable-speed drive...

s, 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)can damage some motors not rated for use with an inverter but this is not a concern when motor voltages are less than 440 VAC. 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 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 can be costly thus making the RPC an attractive option.

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