Talk:Rotary converter

Two commutators
Wasn't that unusual? Were's most converters doing frequency conversions, thus using slip rings at both input and output?


 * This article is wrong and needs to be cleaned up. Many people confuse the rotary converter with the Motor generator or rotary phase converter. All three of those devices are separate and should be treated as such. The rotary converter was an DC to AC or AC to DC converter. It could be started from the DC or AC side but DC starting was preferred as it allowed a smooth and gradual start using a resistive motor starter. I have never heard of a frequency converter that has a common shared set of rotor windings. The only other device that might have shared rotor windings would be the dynamotor (not dynamo) which was a DC only motor-generator which served as a DC transformer, boosting or bucking the DC voltage. I have to check my books but I dynamotors might have had seperate rotor windings and was a motor generator set. A frequency converter for AC would have been a motor generator setup which could share a common shaft and housing but have a completely separate set of windings for the motor and generator side ie each side is electrically isolated. Thaddeusw (talk) 14:41, 5 July 2009 (UTC)

Can this be proven?
Rotary converter refers to a class of electrical machinery that was used to convert one form of electrical power into another form.

Wouldn't 'that is' or deleting 'that was' altogether be better? Surely it cannot be guaranteed that none are in use anywhere in the world anymore. —Preceding unsigned comment added by 85.211.159.106 (talk) 14:34, 20 January 2008 (UTC)


 * It is definitely in the past tense, because before transistors or vacuum tubes existed, the rotary converter was the only option available for voltage/phase conversion. They were everywhere in the power grid and were a fundamental component of many power systems. Now they are gone, replaced by quiet nonmoving solid state hardware that needs no maintenance and spends all day just humming to itself. DMahalko (talk) 04:51, 19 May 2008 (UTC)


 * Nope, they are still used to this day. Every aircraft stand in many airports throughout the world use them to convert the local mains frequency to the 400 Hz required by the aircraft.  Although there are more modern solid state converters they are not particularly favoured. 20.133.0.13 (talk) 15:01, 10 February 2009 (UTC)


 * Hi, The system you are describing is called a motor generator set or MG set. The rotary converter was a rotating piece of machinery that converted AC power to DC and vice-versa. It was neither motor nor generator but a sort of bi-directional rotating rectifier/inverter. Its armature windings were shared for both AC and DC power, its stator windings were wired like a DC motor (compound/field/series/shunt wound). The rotor had a set of three or six slip rings for AC power on one side and a commutator for DC on the other. A motor generator set always has two separate and distinct rotor windings for the motor and generator head. The device might share a single shaft and housing or be two separate devices connected together by belt or shaft coupling. An MG set is not exclusively for AC<->DC conversion which is the case for the rotary converter. An MG set and rotary converter are two totally separate devices. Thaddeusw (talk) 14:27, 5 July 2009 (UTC)


 * You are wrong. Rotary converters can also (and are) used to convert one frequency to another (although the frequencies do have to be harmonically related).  The aircraft application given is by far the most common use these days, but historically, they were also used to derive the 16 2/3 Hz supplies for some railway systems.  For aircraft use at least, the converter itself is a single machine with seven sliprings.  Three slip rings are for the input 50 Hz three phase supply, and the remaining four are for the three phase and neutral 400 Hz output.  The machine has no shaft extensions.  Solid state converters are not favoured in the aircraft business because the output is not as purely sinusoidal as is desired - or at least that is the official position.  I have not encountered an aircraft rotary converter with anything remotely approaching a clean 400 Hz sinewave output.  86.157.171.34 (talk) 13:35, 21 January 2013 (UTC)


 * I am not at all wrong. This article refers to the "Synchronous Rotary converter". You are thinking of a motor generator set, a completely different machine. Please read the Motor-generator article. If you are familiar with obsolete electrical machinery, the term "rotary converter" was used to refer to the "synchronous rotary converter". Today, the usage of the term has changed and can now refer to a variety of electrical machinery which converts one source of electric power to another. Many people simply do not understand this older terminology which causes confusion. Today, people hear the term rotary converter and think of a rotary phase converter (used to create 3 phase from single phase) or motor generator sets. And as far as solid state systems go, modern DSP driven IGBT switched PWM converters can get very close to pure sine, if not achieve pure sine. MG sets are preferred because they are rugged, simple and have no sensitive electronics. Though in some cases, solid state systems are used as they can be cheaper, light weight, easily portable and work off wide range of input voltages/frequencies without having to adjust anything. Thaddeusw (talk) 20:17, 23 January 2013 (UTC)
 * Synchronous rotary generators can change AC frequency too, although as noted, the frequencies have to have a simple harmonic relation. AFAIK, all of the railway 16 2/3Hz systems used them. I've not seen aircraft kit that wasn't an MG, but then these were just doing AC/DC conversion, as aircraft are almost exclusively either 400Hz or DC. There's just no need for AC frequency changing on aircraft. Andy Dingley (talk) 20:31, 23 January 2013 (UTC)


 * Methinks you missed the point. While you are correct that there is no need to change frequency within the aircraft power system itself, nevertheless the aircraft needs to be supplied 400 Hz power while it is parked on the ground when the engines and APU are not running.  This power is often supplied from the local 50 or 60 Hz mains power via a converter.  Whilst such a machine could, of course, be a motor generator set, most machines in the 50 Hz world are not.  They are a single rotary converter with no external shafts that convert 50 Hz to 400 Hz (a ratio of 8:1 which is easily achieved).  In the 60 Hz world the ratio is not simple enough (20:3) that a rotary converter can be used, and a motor-generator set is required.  86.157.171.34 (talk) 16:23, 25 January 2013 (UTC)

Improving this article
As with many wikipedia topics involving old technology no longer in common use that nobody knows much about, this article has not been very useful for explaining the technology. I am working to add high-quality public domain illustrations and to improve the overall technical quality of this article. DMahalko (talk) 04:48, 19 May 2008 (UTC)

history is important —Preceding unsigned comment added by 24.146.23.84 (talk) 02:44, 6 June 2008 (UTC)


 * Unfortunately this and the frequency converter article contain a circular reference with neither article explaining how a rotary frequency converter works. 20.133.0.13 (talk) 15:02, 10 February 2009 (UTC)

We use them in Sweden

In Sweden we still use them for AC/AC power-conversion, powering our separate railway grid. We use them to convert 50Hz power to 16 2/3 Hz power keeping both grids in sync. All of them is workhourses from about 1940s, some updated with some powerelectronics to reduce maintenance. A nice feature with theese is that you can largely overload it for a hour or so without destroying it. —Preceding unsigned comment added by 83.253.102.120 (talk) 13:17, 20 July 2008 (UTC)

Oppose merge with motor generator
While a motor generator and a rotary convert both convert electrical power from 1 voltage/frequency to another, they do it in 2 entirely different manners.

A motor generator converts the input electrical power, to mechanical power at the shaft, and then back to an electrical output.

A rotary convertor functions as a transformer, with mechanical rectification by means of the commutator, the synchronous rotation of the machine serving only to maintain proper phasing of this rectification. Wuh Wuz  Dat  05:01, 27 June 2009 (UTC)

Article needs a major cleanup
First off I would like to point out four different types of rotating electrical machinery that people often mix up. They are:


 * Synchronous Rotary Converter
 * Motor Generator Set
 * Rotary Phase converter
 * Dynamotor

Each of the four above machines are completely different except for the dynamotor which might technically be a Motor Generator.

The Synchronous Rotary Converter is a device that has a single shared rotor winding which is used to convert AC -> DC or DC -> AC. On one side there would be a commutator for DC power and the other side would have three or six slip rings for three or six phase AC. The stator windings were commonly connected in series with the rotor windings with field windings to regulate the speed, this is called compound wound. The AC side has three slip rings for three phase AC, and six slip rings for six phase AC which was created by a set of center tapped transformers. Single phase could be produced but due to the way the rotary converter is constructed single phase AC power would have a heavily distorted waveform. Two phase could be produced via the four wire system but was obsolete before it became popular. They could be started from either side but DC starting was preferred as it allowed for a smooth and gradual start using a resistive starter.

The Motor Generator set (sometimes abbreviated MG set)is another type of electrical machinery. It has a separate and distinct set of both motor and generator windings. It might have a single shared shaft with two separate housings for each side or it might share a single housing and shaft. Commonly today the motor and generator are usually separate from each other and are commonly joined directly together using a shaft coupling or sometimes a belt system. An MG set can convert any kind of power to another. DC->AC, AC->DC, DC->DC or AC->AC. Today they are commonly used for frequency conversion such as in aviation shore power systems that provide 400Hz AC power to planes when the engines are not running. Another use is to convert DC to AC power where a solid state inverter might be too sensitive to varying loads and voltage spikes.

A rotary Phase converter is a three phase squirrel cage motor that is wired to convert 240V single phase AC to 240V three phase AC. Commercial units are sold with no shaft protruding from the device along with an automatic capacitive starter. They are simple to make as you connect 240V to two of the three motor leads. Then a capacitor is connected from one of the input leads to the third motor leg with a momentary switch. To start the converter the 240V AC power is applied while holding the button down. Once the motor is started the button is released and a three phase load can be connected across the converters three legs. Starting can also be done with a single phase motor connected via belt coupling to the three phase motor to spin the converter up and then apply AC to the converter.

The dynamotor is an odd device that was used as a DC transformer. They could be used to step up or step down DC voltages or convert two wire DC to three wire DC power in which case it was referred to as an "equalizer". The Equalizer would be used to convert two wire 240V DC to three wire 120/240V DC. I can not find any real details of the dynamotor's construction except a simple diagram of one wired as an equalizer and a definition in a 1940 Audel book titled Practical Electricity. The Audel definition of a dynamotor is given as: "A dynamotor is a combination dynamo and motor having both windings on one core". It had two commutators on each end and might have had separate or shared field/rotor windings. I would appreciate it if anyone else could dig up more details. Dynamotors were used in very old military radio sets to boost the motor vehicle's 14V or 24V to 240V DC for tube plate voltage. Some sites showing these radios also mislabel motor generator sets as a dynamotor. A dynamotor appears to be a DC only machine.

So in conclusion the term rotary converter is often misused and the issue needs to be clarified. Thaddeusw (talk) 15:50, 5 July 2009 (UTC)


 * This article discusses what is generally an obsolete technology and so widespread general knowledge of it is fading as the engineers and mechanics that worked on it get old and die. At this point researching some of these for Wikipedia may need to become a form of engineering archaeology, trying to hunt down the relevant devices and methods either on eBay or in the ownership of collectors.


 * What is shown as the two-phase rotary converter appears to be the base of the dynamotor / equalizer. The only missing parts are the two balancing transformers connected in a cross arrangement to form the floating half-voltage neutral wire. There is apparently an example of a huge two-phase rotary converter sitting at WMSTR in Rollag, MN that is more likely to have been an equalizer. It was used in the Soo Line railroad repair shops, and is located in the Pabst engine building if you want to look at it.


 * For more research help I suggest you access the 1917 Hawkins Electrical Guide via Google Books (see that article for links) to find more detailed information.


 * DMahalko (talk) 03:16, 6 July 2009 (UTC)


 * After some further research the proper term for what I am calling a rotary converter is actually called a synchronous converter or synchronous rotary converter. It was common to call it a rotary converter even though the term rotary converter can be applied to a much broader class of electrical machinery. This article is showing diagrams and pictures of a synchronous converter. Under applications it is also being called a frequency converter which would imply its is an AC -> AC machine which it clearly is not. Under the Obsolescence section it also says it survives as a way to convert 50 or 60 cycle power to 400Hz which is not what a synchronous converter does. A motor generator set is used to convert 50 or 60Hz to 400Hz. If this article is indeed about the Synchronous converter then it needs to be cleaned up and even renamed to synchronous converter to better distinguish it.


 * The Two phase synchronous rotary converter is in no way related to the dynamotor or equalizer. It was designed to produce two phase AC power from a DC power source or DC current from a Two phase power source. Two phase current was used before Three phase but did not enjoy the wide spread use of three phase (which was clearly superior.) Two phase AC was mainly delivered via a four wire system each set of two for each phase. The waveforms were electrically separated by 90 degrees instead of the 120 degrees that separates three phase. I believe you might be confusing two phase with split phase AC power. Two phase power could be used in a three wire configuration but was not as common as the four wire system. The dynamotor is a conjunction of the two words dynamo (A commutating DC generator) and motor. The dynamotor equalizer system I described was for a DC system which cannot be balanced with transformers. DC needed to be balanced with mechanical machinery to create a DC "neutral" wire. This enabled two wire 240V DC customers to use 120 volt lighting in their buildings along with other 120V electrical equipment. The dynamotor properly balanced the load between the two 240V legs and the DC "neutral" wire. It might have been a simple MG set that was for DC only and thus called a dynamotor. Thaddeusw (talk) 16:38, 6 July 2009 (UTC)

I have moved the article merge tag. This article should remain separate from the motor generator article. I moved the tag down to the Frequency converter section. The Alexanderson alternator is a motor generator set which produces a high frequency AC current for radio transmission. The motor side was completely separate from the alternator side. —Preceding unsigned comment added by Thaddeusw (talk • contribs) 17:01, 6 July 2009 (UTC)

How were they started (for AC-to-DC)
Mechanically, from AC end, or by temporary DC input ? Were there different designs for different start-up methods ? - Rod57 (talk) 15:39, 5 November 2015 (UTC)
 * There were probably examples of every possible method. Three main approaches seem to have been used, separate starting motors, spinning them up with AC, or with DC. Note also that the field coils need to be energised, so it's hard to start one by simply barring it over slowly by hand.
 * Large converters were spun by separate motors. See File:500kW Westinghouse rotary converter (Rankin Kennedy, Electrical Installations, Vol II, 1909).jpg
 * For AC starting there was a large multi-pole changeover switch for "Start" and "Run". This starts with the field coils arranged across the AC supply as a normal AC motor. Once spinning, the switch is moved to Run and the fields are then connected in series across the DC output. There's a a description here: http://ir.library.oregonstate.edu/xmlui/bitstream/handle/1957/45097/HinrichsMA1907.pdf?sequence=3
 * Railway systems seem to have favoured DC starting, where these were often started twice a day to bring more converters in a sub-station on line to cope with rush hour traffic. One converter that had been running all day fed across to another, turning it as a DC motor.
 * Note that the large "steering wheel", often with a visible leadscrew, isn't there to start it, it's to adjust the phasing between brush position and field poles, to optimise for best performance. This may have been adjusted for a best start or run position, but that's more than I know. Andy Dingley (talk) 16:34, 5 November 2015 (UTC)

What voltage regulation ?
When used for AC to DC, how does the output voltage vary with load ? Does output voltage retain original no-load relation to the input AC voltage ? Were there mechanisms to manually or automatically adjust the output voltage ? - Rod57 (talk) 15:48, 5 November 2015 (UTC)