Rotary phase converter
A basic three-phase induction motor will have three windings, each end connected to terminals typically numbered (arbitrarily) as L1, L2, and L3 and sometimes T1, T2, T3.
A three-phase induction motor can be run at two-thirds of its rated horsepower on single-phase power applied to a single winding, once spun up by some means.
However, without current injection, special idler windings, or other means of regulation, the voltage will sag when a load is applied.
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.
As a general guideline, unbalanced three-phase power that exceeds 4% in voltage variation can damage the equipment that it is meant to operate.
All of them needed a separated supply system or converters to take power from the standard 50 Hz electric network.
The asynchronous traction motor can run on a single RPM determined by the frequency of the feeding current and the loading torque.
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.
[citation needed] 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.
[2] Alternatives exist to rotary phase converters for operation of three-phase equipment on a single-phase 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.
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.
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.
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.
