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{{Short description|Electrical machine}}
[[File:500kW Westinghouse rotary converter (Rankin Kennedy, Electrical Installations, Vol II, 1909).jpg|thumb|1909 500&nbsp;kW Westinghouse rotary converter]]

A '''rotary converter''' is a type of [[electrical machine]] which acts as a mechanical [[rectifier]], [[Power inverter|inverter]] or [[frequency converter]]. 

Rotary converters were used to convert [[alternating current]] (AC) to [[direct current]] (DC), or DC to AC power, before the advent of chemical or [[Solid state (electronics)|solid state]] power rectification and inverting. They were commonly used to provide DC power for commercial, industrial and railway electrification from an AC power source.<ref>{{Cite journal |last=Owen |first=Edward L |date=1996-01-01 |title=History |url=https://ieeexplore.ieee.org/document/476602 |access-date=2022-11-15 |website=[[IEEE]]|doi=10.1109/2943.476602 }}</ref>
==Principles of operation==

[[Image:Rotary Converter - Single Phase to DC.png|thumb|400px|Wiring schematic for a simplified [[bipolar field]] [[Gramme ring]] [[Single-phase electric power|single-phase]] to [[direct current]] rotary converter. (In actual use, the converter is [[drum-wound]] and uses a [[multipolar field]].)<ref>Hawkins Electrical Guide, 2nd Ed. 1917, p. 1459, fig. 2034</ref>]]

[[Image:Rotary Converter - Two Phase to DC.png|thumb|400px|Wiring schematic for a simplified [[Two-phase electric power|two-phase]] to [[direct current]] rotary converter, with the second phase connected at right angles to the first.<ref>Hawkins Electrical Guide, 2nd Ed. 1917, p. 1460, fig. 2035</ref>]]

[[Image:Rotary Converter - Three Phase to DC.png|thumb|400px|Wiring schematic for a simplified [[Three-phase electric power|three-phase]] to [[direct current]] rotary converter, with the phases separated by 120 degrees on the commutator.<ref>Hawkins Electrical Guide, 2nd Ed. 1917, p. 1461, fig. 2036</ref>]]

The rotary converter can be thought of as a [[motor-generator]], where the two machines share a single rotating [[armature (electrical engineering)|armature]] and set of [[field coil]]s. The basic construction of the rotary converter consists of a DC generator (dynamo) with a set of [[slip ring]]s tapped into its rotor windings at evenly spaced intervals. When a dynamo is spun the electric currents in its rotor windings alternate as it rotates in the magnetic field of the stationary field windings. This alternating current is rectified by means of a [[Commutator (electric)|commutator]], which allows [[DC current|direct current]] to be extracted from the rotor. This principle is taken advantage of by energizing the same rotor windings with AC power, which causes the machine to act as a synchronous AC motor. The rotation of the energized coils excites the stationary field windings producing part of the direct current. The other part is alternating current from the [[slip ring]]s, which is directly rectified into DC by the [[Commutator (electric)|commutator]]. This makes the rotary converter a hybrid dynamo and mechanical rectifier. When used in this way it is referred to as a synchronous rotary converter or simply a [[synchronous converter]]. The AC slip rings also allow the machine to act as an alternator. 

The device can be reversed and DC applied to the field and commutator windings to spin the machine and produce AC power. When operated as a DC to AC machine it is referred to as an '''inverted rotary converter'''.

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 [[Rectifier|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.

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 adjustable [[voltage regulation]], which can compensate for [[voltage drop]] in the supply network; it also provided complete [[power isolation]], harmonics isolation, greater surge and transient protection, and [[voltage sag|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:<ref>Hawkins Electrical Guide, 2nd Ed. 1917, p. 1461</ref>
* 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 [[Brushed DC electric motor|commutated electric motor]] and an alternating current can be taken out of the collector rings. This is called an ''inverted rotary converter'' (see [[Power inverter|inverter]]).
* 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 synchronism 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''.

===Self-balancing dynamo===
{{Unreferencedsection|date=September 2015}}
The self-balancing dynamo is of similar construction to the single- and two-phase rotary converter. It was commonly used to create a completely balanced three-wire 120/240-volt AC electrical supply. The AC extracted from the slip rings was fed into a transformer with a single center-tapped winding. The center-tapped winding forms the DC neutral wire. It needed to be driven by a mechanical power source, such as a steam engine, diesel engine, or electric motor. It could be considered a rotary converter used as a double current generator; the alternating current was used to balance the DC neutral wire.

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==History==
[[Image:Rotary-Converter.jpg|thumb|Railroad Rotary Converter from [[Illinois Railway Museum]]]]

The rotary converter was invented by [[Charles S. Bradley (electrical engineer)|Charles S. Bradley]] in 1888.<ref>Hughes, Thomas Parke. Networks of Power: Electrification in Western Society, 1880–1930. Baltimore, Maryland: Johns Hopkins University Press - 1993, pp=120–121</ref> A typical use for this type of AC/DC converter was for [[Railway electrification system|railway electrification]], where utility power was supplied as alternating current. Trains were designed to work on direct current, since DC traction motors could be built with speed and torque characteristics suited to propulsion use, and could be controlled for variable speed. The AC induction motor was not as well suited to traction use when powered from a fixed frequency supply.  Before the invention of [[mercury arc rectifier]]s and high-power [[semiconductor]] [[rectifier]]s, this conversion could only be accomplished using [[motor-generator]]s or rotary converters.

Rotary converters soon filled the need to use all the competing [[electric power]] delivery systems that cropped up in the 1880s and early 1890s. These included single phase AC systems, poly-phase AC systems, low voltage incandescent lighting, high voltage arc lighting, and existing DC motors in factories and street cars.<ref>Garud, Raghu; Kumaraswamy, Arun; Langlois, Richard (2009). Managing in the Modular Age: Architectures, Networks, and Organizations. New York: John Wiley & Sons. p. 249</ref><ref>Hughes, Thomas Parke. Networks of Power: Electrification in Western Society, 1880–1930. Baltimore, Maryland: Johns Hopkins University Press - 1993, pp=120–121</ref> Most machinery and appliances at that time were operated by DC power, which was provided at the user level by rotary converter substations for residential, commercial and industrial consumption. Rotary converters provided high current DC power for industrial [[Electrochemistry|electrochemical]] processes such as [[electroplating]]. [[Steel mill]]s needed large amounts of on-site DC power for their main roll drive motors. Similarly, [[paper mill]]s and [[printing press]]es required direct current to start and stop their motors in perfect synchronization to prevent tearing the sheet.

===Obsolescence===
The stopgap of needing to use rotary converters was slowly overcome as older systems were retired or upgraded to match the newer AC universal system. AC to DC synchronous rotary converters were made obsolete by [[mercury arc rectifier]]s in the 1930s and later on by semiconductor rectifiers in the 1960s.<ref name="Payne">{{cite book |last=Payne |first=Christopher |title=New York's Forgotten Substations: The Power Behind the Subway |year=2002 |publisher=Princeton Architectural Press |isbn=978-1568983554}}</ref>{{rp|54}} Some of the original [[New York City Subway]] substations using synchronous rotary converters operated until 1999.<ref name="Payne" />{{rp|12}} Compared to the rotary converter, the mercury arc and semiconductor rectifiers did not need daily maintenance, manual synchronizing for parallel operation, nor skilled personnel, and they provided clean DC power. This enabled the new substations to be unmanned, only requiring periodic visits from a technician for inspection and maintenance. 

AC replaced DC in most applications and eventually the need for local DC substations diminished along with the need for rotary converters. Many DC customers converted to AC power, and on-site solid-state DC rectifiers were used to power the remaining DC equipment from the AC supply.

== See also ==
{{div col|colwidth=30em}}
* [[Cascade converter]]
* [[Frequency converter]]
* [[Motor–generator]]
* [[Rotary converter plant]]
* [[Rotary phase converter]]
* [[Three-phase electric power]]
* [[Traction current converter plant]]
{{div col end}}

==References==
{{Reflist}}
{{Refbegin}}
* {{cite book |last=Slichter |first=W.I. |chapter=Converters, Synchronous or Rotary |editor-last=Pender |editor-first=Harold |title=Handbook for Electrical Engineers |chapter-url=https://books.google.com/books?id=bOceAQAAIAAJ&pg=PA279 |year=1917 |publisher=John Wiley & Sons |location=New York |pages=279–291}}
* Greenberg, Bernard S. (1999). [http://www.nycsubway.org/tech/power/rotary.html "Rotary Converter Power Technology: AC, DC, and Subway Power."] nycsubway.org.
{{Refend}}

{{Commons category|Rotary converters}}

[[Category:Electric power systems components]]
[[Category:AC power]]
[[Category:Inverters]]
[[Category:Rectifiers]]