Rotary Converter refers to a class of electrical machinery that was used to convert one form of electrical power into another form. A very typical use would have been for railway electrification where utility power was supplied as Alternating Current (ac) but the trains were designed to work on Direct Current (dc). Prior to the existence of large-scale semiconductorrectifiers, this could only be accomplished through the use of a motor-generator or a rotary converter.
In concept, 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. In fact, the usual practice 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; instead, some of the electrical energy 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.
(One way to envision what is happening 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.)
Rotary converters have essentially been made obsolete by smaller, cheaper, far more reliable semiconductorrectifiers. 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 completely eliminating the need for rectification or frequency conversion.
Such 3 phase converters may also allow the frequency (see also frequency converters) to be varied allowing for different equipment frequency requirements (50Hz, 60Hz, 400Hz, etc.) and also for motor speed control (VFDs).
Some locomotives are driven by 3-phase motors with 3 phase convertersconverted from the incoming supply of either DC or 1 phase AC.
Note: In the U.S. Phase Converter Standards Organization's research they found that some companies called a product that they sold a "Rotary Phase Converter" but, in fact, their product is made with just a control box (or a Static Converter) and an electric motor.
A rotaryconverter (click photo for enlargement), or synchronous converter, is a large, rotating electromechanical device, like a motor or a generator, formerly used to convert alternating current (AC) to direct current (DC).
Rotaryconverters were gradually replaced in the mid-century by the new technology of the mercury arc rectifier, in wholly new, unmanned, totally windowless "monolith"-looking substations.
Synchronous converters could be started and brought up to speed (synchrony) as AC induction motors (with the help of additional "starting" windings), as DC motors back-fed from siblings or the third rail, or with the aid of a smaller "auxiliary" motor attached, which latter was standard in the IRT substations.
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