Rotary converter
From Wikipedia, the free encyclopedia
Rotary converter refers to a class of electrical machinery that was used to convert one form of electrical power into another form. There are several types of "rotary converter". They are:
- Rotary Phase Converter (RPC) - for converting the phase of electrical power; such as single phase to three phase power. See Rotary phase converter.
- Rotary Frequency Converter - for converting the frequency of an AC supply; e.g. converting 60Hz (US household frequency) to 50Hz (UK household frequency). See Frequency converter.
- Rotary AC to DC Converter - for converting AC to DC power or DC to AC. See below.
A typical use for an AC/DC Converter was for railway electrification, where utility power was supplied as alternating current (AC) but the trains were designed to work on direct current (DC). before the invention of mercury arc rectifiers and high-power semiconductor rectifiers, this conversion could only be accomplished using motor-generator or rotary converters.
This technique was also used for providing DC voltages to radio transmitters before 1935.
The rotary converter can be thought of as a motor-generator where the two machines share a single rotating armature and set of field coils. The usual practice, in fact, was to have two commutators, one at each end of the armature (or, for AC-to-DC machines, a set of slip rings and a commutator). The advantage of the rotary converter over the discrete motor-generator set is that the rotary converter avoids converting all of the power flow into mechanical energy and then back into electrical energy; some of the electrical energy instead flows directly from input to output, allowing the rotary converter to be much smaller and lighter than a motor-generator set of an equivalent power-handling capability. The advantages of a motor-generator set include complete power isolation, harmonics isolation, voltage output control, greater surge and transient protection, and sag (brownout) protection through increased momentum.
(One way to envision what is happening in an AC-to-DC rotary converter is to imagine a rotary reversing switch that is being driven at a speed that is synchronous with the power line. Such a switch could rectify the AC input waveform with no magnetic components at all save those driving the switch! The rotary converter is somewhat more complex than this trivial case because it delivers near-DC rather than the pulsating DC that would result from just the reversing switch, but the analogy may be helpful in understanding how the rotary converter avoids transforming all of the energy from electrical to mechanical and back to electrical.)
AC to DC Rotary converters have essentially been made obsolete by smaller, cheaper, far more reliable semiconductor rectifiers. For railway electrification via a catenary wire there has also been a tendency to switch from medium-voltage DC or low-frequency AC to high-voltage, mains-frequency AC, thus eliminating the need for any rectification or frequency conversion.
Rotary converters were in earlier days also used for producing radio frequency. Therefore different systems were developed. However this worked unsatisfactorily. The only workable rotary converter for radio frequency today is that of SAQ in Grimeton.
