Rotary phase converter

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.

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 the trains.

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 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.

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.)

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 at 50 Hz, have standardised on single-phase AC at 15 kV 16+2⁄3 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.[2]

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.

The popularity of the Variable-frequency drive (VFD) has increased in the last decade, especially in the home-shop market. This is because of their relative low cost and ability to generate three-phase output from single phase input. A VFD converts AC power to DC and then converts it back to AC through a transistor bridge, a technology that is somewhat analogous to that of a switch-mode power supply. As the VFD generates its AC output from the DC bus, it is possible to power a three-phase motor from a single-phase source. Nevertheless, commercial-grade VFDs are produced that require three-phase input, as there are some efficiency gains to be had with such an arrangement.

A typical VFD functions by rapidly switching transistors on and off to "chop" the voltage on the DC bus through what is known as pulse-width modulation (PWM). Proper use of PWM will result in an AC output whose voltage and frequency can be varied over a fairly wide range. As an induction motor's rotational speed is proportional to input frequency, a change in the VFD's output frequency will cause the motor to change speed. Voltage is also changed in a way that results in the motor producing a relatively constant torque over the useful speed range.

The output of a quality VFD is an approximation of a sine wave, with some high frequency harmonic content. Harmonic content will elevate motor temperature and may produce some whistling or whining noise that could be objectionable. The effects of unwanted harmonics can be mitigated by the use of reactive output filtering, which is incorporated into better quality VFDs. Reactive filtration impedes the high frequency harmonic content but has little effect on the fundamental frequency that determines motor speed. The result is an output to the motor that is closer to an ideal sine wave.

In the past, VFDs that have a capacity greater than 3 hp (2.2 kW) were costly, thus making the rotary phase converter (RPC) an attractive alternative. However, modern VFDs have dropped considerably in cost, making them more affordable than comparable RPCs. Also working in the VFD's favor is its more compact size relative to its electrical capacity. A plus is many VFDs can produce a "soft start" effect (in which power is gradually applied to the motor), which reduces the amount of current that must be delivered at machine start-up.

Use of a VFD may result in motor damage if the motor is not rated for such an application. This is primarily because most induction motors are forced-air cooled by a fan or blower driven by the motor itself. Operating such a motor at a lower-than-normal speed will substantially reduce the cooling airflow, increasing the likelihood of overheating and winding damage or failure, especially while operating at full load. A manufacturer may void the warranty on a motor powered by a VFD unless the motor is "inverter-rated." As VFDs are the most popular method of powering motors in new commercial installations, most three-phase motors sold today are, in fact, inverter-rated.