Rotary converter

Rotary converter

A rotary converter is a type of electrical machine used to convert one form of electrical power into another form. There are several types:

* Rotary Phase Converter (RPC) - for converting single-phase power to three-phase power. See Rotary phase converter.
* Rotary Frequency Converter - for converting the frequency of an AC supply: for example, between a 50 Hz distribution network and a 25 Hz industrial plant power system, or to connect a 40 Hz generating plant to a 60 Hz distribution system. See Frequency converter.
* Rotary AC to DC Converter - for converting AC to DC power or DC to AC. See below.

Applications

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-generators or rotary converters.

This technique was also used for providing DC voltages to radio transmitters and to telephone exchanges before 1935.

Rotary frequency changers were important during the growth of electric power distribution systems. Low frequencies such as 25 Hz were used for large motor loads, but 50 or 60 Hz generation became more common for lighting. The use of frequency converters allowed industrial and lighting loads to share the same generation and distribution system, providing important benefits by increasing the average load on the generation system. For example, a steel mill that initially used 25 Hz power for its heavy motor loads as it grew in size might have found it less costly to buy additional energy from a 50 Hz or 60 Hz electrical utility instead of increasing its investment in its own 25 Hz generating capacity, by using a frequency converter to import power.

Principles of operation

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.

In this first illustration of a single-phase to direct-current rotary converter, it may be used five different ways: [Hawkins Electrical Guide, 2nd Ed. 1917, p. 1461]
* If the coil is rotated, alternating currents can be taken from the collector rings, and it is called an alternator.
* if the coil is rotated, direct current can be taken from the commutator, and it is called a dynamo.
* If the coil is rotated, two separate currents can be taken from the armature, one providing direct current and the other providing alternating current. Such a machine is called a "double current generator".
* If a direct current is applied to the commutator, the coil will begin to rotate as a commutated electric motor and an alternating current can be taken out of the collector rings. This is called an "inverted rotary converter".
* If the machine is brought up to synchronous speed by external means and if the direction of the current through the armature has the correct relationship to the field coils, then the coil will continue to rotate in sychronism with the alternating current as a synchronous motor. A direct current can be taken from the commutator. When used this way, it is called a "rotary converter".

One way to envisage 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.

Obsolescence

AC to DC rotary converters have essentially been made obsolete by smaller, cheaper, more reliable semiconductor rectifiers. For railway electrification by 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 phase converters are still used when three-phase motors must be operated on a single-phase power supply, although usually only for small loads under a few dozen horsepower (tens of kilowatts).

One area where these machines have survived is in converting the 50 or 60Hz utility power to 400Hz for ground powering of aircraft and their systems.

High-frequency machines

An Alexanderson alternator can be considered a form of rotary converter for producing radio frequency power. While electromechanical converters were regularly used for long wave transmissions in the first three decades of the 20th century, electronic techniques were required at higher frequencies. A surviving long-wave rotary converter for radio frequency is kept at the SAQ in Grimeton historic site.

See also

* Cascade converter
* Motor-generator
* Rotary phase converter
* Frequency converter
* Three-phase

References

External links

* [http://www.nycsubway.org/tech/power/rotary.html An article about AC-to-DC rotary converters used for traction power]


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