Talk:Doubly fed electric machine

Merge
Support - I agree that one article collecting all the stubs is better than a bunch of short, necessarily very repetitive articles. This is a general encyclopedia not a textbook on machine design; the individual variations are better illustrated if they can be compare in the same article. As usual, pictures and diagrams would be nice. What about disadvantages, too? Importance of doubly-fed machines in variable-speed wind turbines? --Wtshymanski 16:32, 11 August 2007 (UTC)

Merged content from Talk:Brushless wound-rotor doubly-fed electric machine
This page appears to be nothing more than an advert for some product of the "Best Electric Machines" company. It gives hardly anything in the way of technical description of how the device works (what is supposed to be in an encyclopedia) but does focus lots on the supposed benefits of the device, though without any explanation of why those benefits should arise (which should appear in the company's sales documents, rather than Wikipedia). I would suggest that the page therefore needs a major edit to add more information, or should be substantially reduced in length. (131.111.200.200 17:07, 20 October 2007 (UTC))
 * I agree 100%. I will add 'advertisement' tags Midlandstoday 21:55, 31 October 2007 (UTC)
 * I also agree, in regard to some of the statements being more like marketing-speak rather than an effort to explain how doubly-fed machines work. I find this statement particularly troublesome:
 * The wound-rotor doubly-fed electric machine incorporates the most optimum electromagnetic design of any electric machine but requires a slip ring assembly and very responsive electronic control, which together is its Achilles' Heel; otherwise, the wound-rotor doubly-fed electric machine (including electronic control) would surpass all electric machine systems, if efficiency, cost, and size of the system were the combined issue.
 * This argument is highly debatable and is probably not true, although I am not an expert to say so definitively. In many applications that do not require variable speed, such as utility AC generators, I believe a singly-wound synchronous machine would be far more efficient and compact than any doubly wound device, as well less costly, because much less electronic control is required.  Utility generators may be an extreme example but they suggest the fallacy of the statement. Theophilus Reed (talk) 21:28, 16 July 2008 (UTC)


 * The statement is not totally false, though market use exists of slip-ring induction motors in cranes, and potentially the new Siemens Aircraft motor with 5kW/kg power density announced recently is a Doubly-Fed device. The cost statement is false, as Switched Reluctance devices may be, via control system and motor construction savings, the most cost efficient inverter fed device for high efficiency and power dense operation (COI announcement, I am in the SR and DFEM markets, NOT ASSOCIATED WITH 'Best Electric Machine').  The cost may be and typically would be lower in production than a PM motor, as the magnets typically exceed the cost of copper windings.  DFEM are able to achieve (theoretically) higher efficiencies through substantial reduction of loss, as well as the double-speed function which improves power density with like power electronics.  Higher efficiencies are necessary for the power density to be sustained as continuous power, as loss heat must be removed from the motor.  Typical passive rotor devices over-cool the rotor, or the rotor produces little to no heat (as in SR or very well designed PM) and so there is over-cooling; This allows for a like efficiency DFEM to exist with higher power density without changes to internal cooling, though external overall motor cooling would need to reject the additional heat.  Either way, I agree that COI should be removed by a neutral party.  It hindered my initial pre-design research and it severely damages the credibility of the technology to uninformed customers and investors who try to look it up on wikipedia.  SeanCFountain (talk) 14:23, 13 July 2015 (UTC)

V and OR
Other than the tail end, this article is almost completely without inline citations and proper sourcing. I am concerned there could be some original research; some citations for verification would be most helpful. //Blaxthos ( t / c ) 20:38, 26 November 2009 (UTC)

HOW TO MODIFY THE EXISTING ASYNCHRONOUS INDUCTION MOTOR IN A W.E.G TO A D.F.I.G FOR REACTIVE CONTROL?
All the articles under D.F.I.G are describing the derivation to convert 3 axis into d q transformation ( 2 axis) only. How to modify the existing asynchronous induction motor in a Wind Electric Generator to a Doubly Fed Induction Generator for Reactive control or improve the Reactive Power?117.193.134.154 (talk) 16:18, 26 December 2010 (UTC)

Edit needed for writing quality
This single sentence:

"One company,[6] has patented and is selling a wound-rotor doubly-fed electric machine with a brushless means of independently exciting the wound rotor without relying on slip (induction) with the stator excitation and as a result, is a truly synchronous brushless wound-rotor doubly-fed electric machine with symmetrical quality of fully stable motoring or generating, even at synchronous speed where induction no longer exists."

needs to be broken up into at least 4 sentences. The article is full of similar constructs and could really use an editor in addition to the author. 192.91.173.36 (talk) 21:03, 17 March 2011 (UTC)

I was wondering if anyone could help me the research that this article is talking about done at the University of Wisconsin
From the Electronic Control section: Pioneering work of Drs. Albertson, Long, Novotny, and Schmitz from the engineering department of the University of Wisconsin realized this must be overcome with instantaneous control. Like any synchronous machine, losing synchronism will result in alternating torque pulsation and other related consequences.

Does anyone know where you could find any papers, or textbooks with the research that has been done? I am doing research on DFIG control and would love to see what they have done, but can't seem to find anything. —Preceding unsigned comment added by 76.23.44.183 (talk) 05:49, 28 March 2011 (UTC)

Link for a section inside this article is missing
In the "Power Density" and "cost" section, link of "compared" for same page's topic "Comparing_electric_machine_systems" doesn't work for there is no such section exist. Please create it.

Mmh-duke (talk) 03:19, 21 October 2014 (UTC)

this article is disappointing
This is very unfortunate, i always look up in wikipedia first, in any topic that i need to research about. Same thing i did about this topic, and as the heading suggest, this is purely an advertisement of the BDWSM. I even looked up in ieee transactions about this machine and there was a paper published in conference with out any single data, neither simulation nor an experimental verification. To my surprise no mathematical deduction either was given, and now that makes me wonder what value to extract from that paper except for some bunch of text. And now i can relate that paper with this wiki article, and its pretty clear that same person wrote both the articles. It is misleading, and not verifiable even remotely. I suggest to take off this article and have it written by some neutral and verified source. Sorry if i sound a little raged right now, but i needed some heads up for my research, and it really ruined some of my time, and worst of it is that the brushless wound rotor part is majorly misleading. — Preceding unsigned comment added by Hellboy1234567 (talk • contribs) 18:08, 15 March 2015 (UTC)

The opening diagram shows two AC-DC converters, which is confusing. Because one should be AC-DC, while the other one should be DC-AC. One would expect that the diagrams should be checked many times before uploading. This is mainly because these articles are meant to teach people things - and definitely not supposed to confuse people.KorgBoy (talk) 05:45, 6 March 2020 (UTC)


 * Hello KorgBoy. We're all volunteers here.  The position of diagram checker is unfilled.Constant314 (talk) 20:20, 6 March 2020 (UTC)

Conflict of interest editing
This article has been heavily edited by who has a COI as clear as the day is long. The tag needs to stay on until independent reviewers check the article for NPOV and sourcing; if you remove the tag please leave a note here. Thanks Jytdog (talk) 04:31, 4 July 2015 (UTC)

The information added by F. Klatt is referencing his own white papers and patents, which I have reviewed and were reviewed by an independent Doubly-Fed expert who I will not name without his permission. Our conclusion was that the articles and patents do not actually say anything about his device other than unsupported claims of capability and function, which are now introduced into this article. I arguably have a COI as I am in the Doubly-Fed Synchronous motor market, and so I will not edit the article directly until such point as I have thorough sourcing and the time to reference it. SeanCFountain (talk) 14:10, 13 July 2015 (UTC)

This article contains several disagreements of terminology and is not accurate to the technology
DISCLAIMER:: This comment contributed by someone who does not have a neutral stance, in that I am designing and constructing Doubly-Fed Synchronous devices for market sale; I have no interest or contact with the editor from 'Best Electric Machine', nor any interest in competing with 'Best Electric Machine' as we are not in competing markets; This article however is not just a poorly written one, but also contains incorrect or inconsistently used terminology and information. ::DISCLAIMER

Not pointed out in the article (not well at least) is that Doubly-Fed devices may benefit from two large reduction in losses:

'Copper' loss; Due to I^2R, by splitting current ideally in half to stator and half to rotor, for a DFEM with identical coil characteristics, I^2R loss is 1/4 a singly-fed device (assumes that DFEM has twice the copper, 1/2 loss is achieved at same copper). Or as pointed out in the NASA article linked below, a 40% or greater reduction in copper and other mass may be achieved for like losses.

Eddy Current losses; Eddy current losses are a function of frequency^2, and in a DFEM of similar speed, rotor and stator each see as little as half the frequency as a Singly Fed device, so Eddy current losses may be as little as 1/2 that of a Singly Fed device, due to two cores at 1/4 loss each.

Hysteresis losses; Although hysteresis losses are generally a linear function of frequency, and frequency observed is half, there are two cores, so the loss change is none for a similar device. There may be an opportunity for improvements, but the comparison is fair as no change in hysteresis loss.

Information from recently republished NASA article found at http://ntrs.nasa.gov/search.jsp?R=19820012523 needs to be incorporated. The recent editor from 'Best Electric Machine' does not describe the function of the machine or his claimed different device in a consistently interpretable manner.

As pointed out in the NASA article, a Doubly-Fed device does not use slip-induction to function. Upon study of published slip-induction based control works (not exhaustive study) it seems that the slip-induction viewpoint is a construct to allow simulation and control of the device as an induction machine. Control as an induction machine is not accurate to the behavior of true Doubly-Fed machines, which have independent and similar waveforms on both rotor and stator.

In so called DF Induction Machines (DFIM) the 'slip' is actually operation at higher or lower than synchronous speed. Induction device experts have argued to me passionately that if there is slip between the stator field rotation and the rotor mechanical speed, then it is an Induction device. Again as shown in the NASA article, because the field on the rotor is also rotating, 'synchronous' speed is not stator field to rotor mechanical, but stator field to rotor field.

Additionally Doubly-Fed devices encompass the potential for a general theory of all electric machines, since Singly-Fed devices may be treated as Doubly-Fed machines where the rotor field velocity is 0 (synchronous devices) or is rotating at stator field velocity, as in an Induction device. In the sense of field velocities, all electric machines are synchronous (even Induction) and may be controlled as such. In an Induction device (research needed perhaps to demonstrate) synchronous control methods may be employed if rotor field strength becomes a function of stator field rotor mechanical slip. This is an important distinction of control which exists only in inverter fed devices, since slip is not a constant relation to torque in inverter fed Induction devices, as it is in fixed frequency devices (known, need source; Field weakening in Induction devices demonstrates this).

Lastly (so far last that has occurred to me), the Brushless Doubly-Fed as described implies that University research demonstrated a device which supplied inverter fed power without slip-rings or slip-ring replacements, which is not accurate. To date, so far as I can determine, the only practical methods which may be used to transfer power to the rotor in a controlled wave-form are slip-rings or rotary transformers. Optical power transfer has power density and efficiency issues. The 'Best Electric Machine' contributor implies in patent and white papers that there exists a method of wireless power transfer which also acts as a commutator (as in Brushed DC motors). There may exist such a thing which does not introduce parasitic torques into the device, but to the best of my research the only method for doing so is another wound rotor motor, which necessarily introduces a regenerative torque onto the rotating assembly when producing rotor frequencies for mechanical super-synchronous (or typically twice mechanical synchronous) speeds, unless a higher field velocity is achieved in the transfer device/motor which results in a positive torque. In this system the transfer motor must have a power capacity greater than the primary motor.

SeanCFountain (talk) 14:11, 13 July 2015 (UTC)

Request edit on 13 July 2015
Addition of information from NASA article found here: http://ntrs.nasa.gov/search.jsp?R=19820012523

Removal of statements about the capacity of 'Best Electric Machine' and 'F. Klatt' designs, as they have not been demonstrated in simulation or experiment. I think reference to the existence of his company is legitimate, even though I and my company motor expert adviser don't agree about the plausibility of his unique ideas. F. Klatt has every right to prove us wrong through market application or simulation/experiment, it just doesn't belong in the article.

Addition of information regarding loss reduction (theoretical) in Doubly Fed Electric Machines (DFEM);
 * I2R with I reduced by half without change in conductor size (split between rotor and stator) allows 1/4 I2R loss in a converted induction device. Or in an optimized one, according to NASA article referenced, 40% or greater reduction in material.
 * Eddy current loss reduction to 1/2, as the frequency observed by rotor and stator cores is 1/2 the same frequency as the same singly fed synchronous device, and Eddy current scales by f^2 (1/4 loss, 2 core sets seeing loss = 1/2 loss) SeanCFountain (talk) 14:33, 13 July 2015 (UTC)


 * I'm going to close this out for a few reasons. Some of the request is not very specific (what information from the NASA article? Which text about capacity?) and it sounds like most/all? of their inquiry was addressed by the user above. If I'm missing something, please feel free to re-submit with a "replace X with Y" format. CorporateM (Talk) 06:28, 26 July 2015 (UTC)

Turgid writing
I'm a bit concerned by this sequence of edits. Some of the sentences are so long and turgid to be incomprehensible. For example: "Due to the voltage and current behavior described above the rotor will either require or generate active power from sub-synchronous to super-synchronous speeds with the potential torque rating of an armature winding set but only with the invention of stable synchronized bi-directional excitation that does not rely on induction, which ceases to exist at synchronous speed, for true synchronous operation; otherwise, the doubly fed electric machine is an induction (or asynchronous) electric machine producing torque and operating as a motor with the rotor generating power at speeds below synchronous speed (subsynchronous operation)." I'm not even sure that is a valid English sentence. John Nagle (talk) 07:45, 2 January 2016 (UTC)
 * The first sentence of the lede says "Doubly fed electric machines are electric motors or electric generators that have two armature winding sets, which is the most possible before the circuit topology duplicates, and as a result, both armature windings transfer significant active power between shaft and electrical system." Two armature winding sets? Is that right? Here's a paper from MIT on the subject..  That seems to indicate that a doubly-fed machine has separate external connections to both the armature and field windings, not two armature windings. This is useful for wind turbines, because you can electrically rotate the field and change the output frequency and phase of the generator somewhat. Without that, synchronous wind turbines have to be physically locked to the grid frequency, which is hard on the machinery as the wind changes. John Nagle (talk) 08:02, 2 January 2016 (UTC)

Field, armature, and all that
I think that the idea the Fklatt is trying to get across is that, with some types of doubly-fed electrical generator, power can be generated from both the rotating and stationary windings. Is that right? This paper from MIT shows a doubly fed wind turbine generator, but from Fig. 5, the field windings only consume power; they don't generate it. That's the way generators usually work; power goes into the field windings to create a magnetic field, and power comes out of the armature, rotating relative to the field. With a doubly-fed machine, the field is rotated electrically to allow for some speed variation in the wind turbine. (The MIT paper says +- 33%.) In that system, the doubly-fed system was just used to deal with sudden wind changes that sped up or slowed down the wind turbine. Pitch control on the big turbine blades, which is slow, would then automatically be adjusted until the turbine came back to synchronous speed.

But compare this article from MathWorks.. This shows a system where both windings produce power. (I think). Note the illustration "The Power Flow", with the arrows to the right through the AC-DC-AC converter. That's a neat result. Am I reading that right? This article from the University of Strathclyde seems to agree, and has a more readable introduction.

So I now think I see the problem on Wikipedia. Almost all ordinary electrical machines, both motors and generators, have a field winding (or a permanent magnet) and an armature winding. For generators, a little power goes into the field coils to create a magnetic field, and useful power comes out of the armature. But for these new doubly-fed machines with their big electronic AC-DC-AC converters and elaborate control systems, power can come out of both windings. This clashes with the traditional field/armature nomenclature for generators found in most textbooks. Fklatt seems to be trying to revise the traditional naming to accommodate this new thing, leading to changes in armature (electrical engineering). That's why we're in this mess. Is this correct?

The MIT paper deals with the language problem by just using the phrases "stator winding" (doesn't move) and "rotor winding" (rotates). They avoid using the terms "field" and "armature" at all. I suggest we resolve this by using the traditional definitions in the other articles, and following the MIT paper's terminology in the doubly-fed article. This means removing all mentions of "armature" and "field" from the doubly fed electric machine article. Comments? John Nagle (talk) 00:24, 18 January 2016 (UTC)
 * Hi Nagle, I think you nailed it. Jytdog (talk) 18:31, 18 January 2016 (UTC)
 * Working on a new, more readable intro section: User:Nagle/sandbox/doublyfedintro. John Nagle (talk) 20:42, 18 January 2016 (UTC)
 * Added intro section with simple drawing. John Nagle (talk) 06:38, 25 January 2016 (UTC)
 * Good picture. I suggest adding double head arrows to show power flows both ways from both rotor and stator.Constant314 (talk) 16:40, 25 January 2016 (UTC)

Classification section
I cut this from the article and am pasting it, collapsed, here on Talk. This is almost all unsourced and is promotional for Best Electric Machines.

Classification
Electric machines are either Single Fed with one multiphase winding set that actively participates in the energy conversion process or Double Fed with two active winding sets. An active winding set (or armature winding set has at least two AC phases with independent electrical ports, which is necessary for the production of a rotating or moving magnetic field with an Electromotive force that actively participates in the energy conversion process. Since both winding sets of a doubly-fed electric machine actively participate in the energy conversion process, a doubly-fed electric machine operates to twice synchronous speed or twice the constant torque range with a given frequency of excitation and as a result, both armature winding sets contribute to electromechanical power production. Many confuse the singly-fed slip-energy recovery induction and the field-excited synchronous electric machines with two electrical ports as doubly-fed but only one port or winding set actively participates in the energy conversion process and as a result, these electric machines are not designed for operation to twice synchronous speed with a given frequency of excitation. A practical doubly-fed electric machine system must operate between sub-synchronous and super synchronous speed without control discontinuity.

Only practical with the evolution of control technology that avoids instability and discontinuity over the sub-synchronous to super-synchronous speed range, particularly at synchronous speed where induction ceases to exist, there are now three varieties of doubly-fed electric machine systems: 1) the Doubly-Fed Induction Electric Machine (DFIM), which is the conventional wound-rotor doubly-fed electric machine with an active winding set on the rotor and stator, respectively, and flux vector controlled rotor excitation through a multiphase slip-ring assembly; 2) the Brushless Doubly-Fed Induction Electric Machine (BDFIM), which is the brushless doubly-fed induction (or reluctance) electric machine with cascaded active winding sets of unlike pole-pairs on the stator assembly of which one is flux vector controlled and a flux focusing rotor assembly; and 3) Synchro-Sym electric machine system, which is the only Brushless Doubly Fed Synchronous Electric Machine (BDFSM). Unlike the doubly-fed electric machine topologies that always rely on a slip in speed between the Rotor (electric) and Stator (i.e., slip-induction) for operation with pronounced instability potential at synchronous speed where induction ceases to exist, the Synchro-Sym electric machine system has a brushless real time control method that eliminates reliance on slip-induction, multiphase slip ring assemblies, or any derivative of rotor flux vector control to provide stable synchronous operation to twice synchronous speed for a given frequency of excitation with active rotor and stator winding sets.

The symmetrical circuit topology and operational relationships of the wound-rotor doubly-fed synchronous electric machine cores with active winding sets on the rotor and stator, respectively, become the classic study for all other electric machines by de-optimizing their symmetry with asymmetry; for instance, by replacing the symmetrical circuit topology provided by the rotor active winding set with the asymmetrical circuit topology provided by a passive permanent magnet assembly, which has no active power port and as a result, cannot actively participate in the energy conversion process. A true doubly-fed electric machine must have two or more active winding sets (ports) excited with bi-directional power in order to produce a rotating or moving magnetic field for active contribution in the energy conversion process and for providing practical operation from sub-synchronous to super-synchronous speed without regions of discontinuity, such as about synchronous speed where induction ceases to exist.

The DFIM and BDFIM rely on speed-based asynchronism (or slip) between the rotor and stator windings to induce speed-synchronized current onto the rotor winding set. However at the low slip experienced about synchronous speed, the time critical measurement or excitation synthesis of shallow time-differential signals makes stability increasingly elusive. The BDFIM has eliminated the multiphase slip-ring assembly and partially improved stability by sacrificing size, cost, and efficiency with incorporation of a second stator Armature winding set. In contrast, the BDFSM as provided by the Synchro-Sym electric machine system brushlessly propagates instantaneously derived speed-synchronized multiphase excitation to the rotor winding set without discontinuity and without relying on slip-induction as exhibited by synchronous electric machines, although slip-induction may be experienced as in all singly fed and doubly-fed induction electric machines.

Doubly-fed electric machines can be categorized as either wound-rotor synchronous doubly-fed electric machines or a variety of induction doubly-fed electric machines. Under the same electric machine design constraints equally available to all but without considering the excitation control means of the rotor armature, the wound-rotor doubly-fed synchronous electric machine (WRSDFM) has at least the following attributes: 1) With similarly rated armature winding sets on the rotor and stator respectively, only the WRSDFM has a rotor assembly that actively participates in the energy conversion process with the stator assembly to provide twice the power density as all other electric machine systems by assuming the rotor and stator assemblies occupy the same physical volume; 2) With twice the power density as other electric machines, only the WRSDFM is half the cost as all other electric machine systems for a given power rating, by assuming the physical size of the electric machine for a given power rating determines the amount of materials with the WRSDFM showing twice the power density; 3) With efficiency normalized to power rating at a given frequency and voltage of excitation, the WRSDFM with twice the power rating at a given frequency and voltage of excitation has higher efficiency than all other electric machine systems; 4) With a dual ported (or symmetrical) transformer topology, which avoids saturating the core with increasing torque current, only the WRSDFM has multiply higher peak torque potential compared to all other electric machine systems.

Features of doubly-fed machines
The wound rotor doubly-fed synchronous electric machine as provided by the Synchro-Sym electric machine system is the only electric machine that operates with rated torque to twice synchronous speed for a given frequency of excitation and without control discontinuity at synchronous speed where slip-induction ceases to exist (i.e., 7,200 rpm with one pole-pair doubly-fed machine when both stator and rotor are fed with 60 Hz versus 3,600 rpm for otherwise similar singly-fed electric machine). In high power applications two or three pole-pair machines are common. Higher speed with rated torque under the same pole count, voltage, and excitation frequency as any other electric machine, means that doubly-fed machines have lower cost per kW, higher efficiency, and higher power density as other electric machines, including rare-earth permanent magnet synchronous electric machines.

In concept, any multiphase electric machine can be converted to a wound-rotor doubly-fed electric motor or generator by changing the rotor assembly to a multiphase wound rotor winding set. If the rotor winding set can transfer bi-directional active or working power to the electrical system, the conversion result is a wound-rotor doubly-fed electric motor or generator with higher speed and power rating than the original singly-fed electric machine. These advantages can be achieved without core saturation, all by electronically controlling half or less of the total motor power for full variable speed control.

As do all electromagnetic electric machines, doubly-fed machines need torque current and magnetic flux to produce torque. Because there are no permanent magnets in the doubly-fed machine, magnetizing current is needed to produce magnetic flux. Magnetizing current and torque current are orthogonal vectors and do not add directly. Since the magnetizing current is much smaller than the torque current, it is only significant in the efficiency of the machine at very low torque. Furthermore, magnetizing current of the wound rotor doubly-fed electric machine can be shared between the stator and rotor windings. If all magnetizing current is supplied by the rotor windings, the stator will only have torque current and so unity power factor. However, by optimal current sharing the total I2R loss can be minimized.

At synchronous speed the rotor current has to be DC, as in ordinary synchronous machines. If the shaft speed is above or below synchronous speed, the rotor current must be AC at the slip frequency. Thus the rotor winding requires reactive power when it is used to magnetize the machine in non-synchronous operation.

Torque production requires that rotor current also has a torque producing component in addition to magnetization. Thus active power is present in the rotor in addition to reactive power.

The frequency and the magnitude of the voltage is proportional to the difference between the speed of the machine and the synchronous speed (the slip). At standstill, the frequency will be the same as the frequency in the stator; the voltage magnitude is determined by the ratio of the stator and rotor winding turns. Thus if the number of turns is equal, the rotor has the same voltage as the stator. The doubly-fed machine is a transformer at standstill. The transformer-like characteristics are also present when it is rotating, manifesting itself especially during transients in the grid.

Due to the voltage and current behavior described above the rotor will either require or generate active power from sub-synchronous to super-synchronous speeds with the potential torque rating of an armature winding set but only with the invention of stable synchronized bi-directional excitation that does not rely on induction, which ceases to exist at synchronous speed, for true synchronous operation; otherwise, the doubly-fed electric machine is an induction (or asynchronous) electric machine producing torque and operating as a motor with the rotor generating power at speeds below synchronous speed (subsynchronous operation). At standstill all active power fed in the stator (excluding losses) is returned via the rotor. If the motor has rated torque, rated active power is circulating through the stator, rotor and frequency converter and only losses are taken from the grid. The mechanical power being the angular speed multiplied by the torque of the motor is zero at standstill. Thus like all electric machines, the efficiency of the machine is not very good at low speeds because losses depend on the current that is required to produce torque but little or no mechanical power is produced.

If the machine is operating as a motor at speeds over the synchronous speed (supersynchronous operation), the mechanical power is fed through both the stator and rotor for higher rated power than a singly-fed electric machine; otherwise, the machine acts as a generator over the same speed range but only with stable synchronized excitation that does not rely on induction. As a consequence the efficiency is now better than with singly-fed motors, since efficiency is normalized to rated power. For example, the doubly-fed electric machine with equal stator and rotor turns produces same torque at double speed (and thus twice the power) as a singly-fed electric machine. The losses, being roughly proportional to the torque, are quite the same. Thus efficiency, which is the power taken divided by the total power produced, is better than singly-fed electric machines. Naturally one has to take into account the loss of the power electronic control equipment. However, the frequency converter of the doubly-fed machine has to control only 50% or less of the power of the machine, and thus has about half of the loss of the singly-fed machines' frequency converter that has to pass through 100% of the power.

Since efficiency is the ratio between the output power to the input power, the magnetic core efficiency of a wound rotor doubly-fed machine, which has just two winding sets of loss but shows twice the power for a given frequency and voltage of operation, is comparable to the magnetic core efficiency of permanent magnet machines with just one winding but without magnetizing current. Coupled with the low power electronic controller, the wound-rotor doubly-fed electric machine system would be more efficient than permanent magnet machine systems without magnetizing current.

For operation as a generator a similar situation exists. At subsynchronous speeds the stator is generating the power but part of it has to be fed back to rotor. At supersynchronous speeds both the rotor and stator are producing power to the grid.

Thus the current rating of the rotor converter is defined by the maximum active current required by the torque production and the maximum reactive current required to magnetize the machine.

With synchronous speed determined by the excitation frequency, only Doubly-fed electric machines can operate to supersynchronous speeds. They can operate with the same constant torque as singly-fed electric machines to twice synchronous speed if each active winding (i.e., armature) is rated at half the total power of the doubly-fed electric machine (or the same rated power of the armature of a singly-fed electric machine) and can provide continuous operation between sub-synchronous through supersynchronous speed range. As a result, for a given frequency and voltage of excitation (and pole count), the doubly-fed electric machine (with two armature) is rated at twice the rated power of singly-fed electric machine (with a single but similarly rated armature). For the reasons provided, specific power (KW/Kg) or power density (KW/L) has no comparative relevance unless excitation frequency, excitation voltage, and pole count are similar in the comparison, which determines the speed of operation, and as a result, the core of a Brushless wound-rotor doubly-fed electric machine should show nearly twice the power density under the same design constraints equally available to all, such as the same torque, same air-gap area, same air-gap flux density (which is determined by the saturation limit of the electrical steel core), etc.

Doubly-fed machines do not produce more continuous rated torque per volume than singly-fed machines, since torque is a function of at least the air-gap area and air-gap flux density (which is determined by the saturation limit of the electrical steel core) equally available to all. With electric machine torque and size virtually the same with the same air-gap area and flux density, the bigger power rating is due to the higher speed attainable without weakening the magnetic flux. The short time maximum torque of a wound rotor doubly-fed electric machine is, however, much higher than all other electric machines, including induction or permanent magnet machines, because increasing torque current does not directly increase air-gap flux, which quickly leads to core saturation for all other electric machine types. In practice, torque current increase is only limited by the temperature of the windings and the maximum current capability of the rotor frequency converter.

With one of the two armature winding sets residing on the rotor and stator body, respectively, the rotor real estate of the wound-rotor doubly-fed machine actively participates in the energy conversion process, which is different from all other electric machines, including permanent magnet synchronous machines. As a result, the magnetic core of the wound-rotor doubly-fed electric machine shows highest power density.

Changing of the direction of the rotation requires the swap of two stator phases near zero speed if symmetrical speed range in both directions is required.

It is common to dimension the doubly-fed machine to operate only at a narrow speed range around synchronous speed and thus further decrease the power rating (and cost) of the frequency converter in the rotor circuit.

Typical applications of doubly-fed machines have been high power pumps and fans, hydro and wind generators, shaft generators for ships etc. where operating speed range has been quite narrow, less than ±30% of the synchronous speed and only small power is required in the subsynchronous range.

Due to the high rotor to stator winding turns ratio that is typical in these applications and the high voltage thus induced in the rotor at standstill, the starting of this kind of restricted operating speed range motor drive is usually done with rotor resistors in induction motor mode. When speed is in the operating speed range, the resistors are disconnected and the frequency converter is connected to the rotor. If the starting torque is low enough it is also possible to short circuit the stator and use the frequency converter in the induction motor control mode to accelerate the motor to the operating speed range. Generators, naturally, don't usually need any additional starting means because wind or water is used to accelerate the machine to the operating speed range.

Electronic control
The controller, a frequency converter, conditions bi-directional (four quadrant), speed synchronized, and multiphase electrical power to at least one of the winding sets (generally, the rotor winding set). Due to the lack of damper windings used in synchronous machines, the wound-rotor doubly-fed electric machines are susceptible to instability without stabilizing control because torque is a function of position. Pioneering work of Drs. Albertson, Long, Novotny, and Schmitz. from the engineering department of the University of Wisconsin realized this must be overcome with instantaneous control. Like any synchronous machine, losing synchronism will result in alternating torque pulsation and other related consequences.

Doubly-fed electric machines require electronic control for practical operation and should be considered an electric machine system or more appropriately, a variable-frequency drive.

There. Jytdog (talk) 00:31, 18 January 2016 (UTC)

Wound-rotor doubly-fed electric machine section
This section too was unsourced and promotional.

Construction
Two multiphase winding sets with similar pole-pairs are placed on the rotor and stator bodies, respectively. Since the rotor winding set actively participates in the energy conversion process with the stator winding set, utilization of the magnetic core real estate is optimized in contrast to all other electric machine types.

The doubly-fed machine operation at unity stator power factor requires higher flux in the air-gap of the machine than when the machine is used as wound rotor induction machine. It is quite common that wound rotor machines not designed to doubly-fed operation saturate heavily if doubly-fed operation at rated stator voltage is attempted. Thus a special design for doubly-fed operation is necessary.

A multiphase slip ring assembly is traditionally used to transfer power to the rotating winding set and to allow independent control of the rotor winding set. The slip ring assembly requires maintenance and compromises system reliability, cost and efficiency. Attempts to avoid the slip ring assembly are constantly being researched with limited success (see Brushless doubly-fed induction electric machines).

Control
Although the multiphase slip ring assembly reduces reliability and requires regular maintenance, it allows easy control of the rotor (moving) winding set so both multiphase winding sets actively participate in the energy conversion process with the electronic controller controlling half (or less) of the power capacity of the electric machine for full control of the machine.

This is especially important when operating at synchronous speed, because then the rotor current will be DC current. Without slip rings the production of DC current in the rotor winding is only possible when the frequency converter is at least partly located in the rotor and rotating with it. This kind of rotor converter naturally requires its own winding system (preferably using high frequency in the 10 kHz range for compact size) for power transfer out of or into the rotor. Furthermore, there are thermal and mechanical constraints (for example centrifugal forces) of the power electronic assembly in the rotor. However, high speed alternators have had electronics incorporated on the rotor for many years. Furthermore, high frequency wireless power transfer is used in many applications because of improvements in efficiency and cost over low frequency alternatives.

Efficiency
Neglecting the slip ring assembly, the theoretical electrical loss of the wound-rotor doubly-fed machine core in supersynchronous operation is comparable to the most efficient electric machine systems available (the synchronous electric machine with permanent magnet assembly) under similar operating metrics. The efficiency is similar because the total current is split between the rotor and stator winding sets while the electrical loss is proportional to the square of the current flowing through the winding set. Further considering that the electronic controller handles less than 50% of the power of the machine, the wound-rotor doubly-fed machine theoretically shows nearly half the electrical loss of other machines of similar rating.

Power density
Neglecting the slip ring assembly and considering similar air-gap flux density, the physical size of the magnetic core of the wound-rotor doubly-fed electric machine is smaller than other electric machines because the two active winding sets are individually placed on the rotor and stator bodies, respectively, with virtually no real-estate penalty. In all other electric machines, the rotor assembly is passive real estate that does not actively contribute to power production. The potential of higher speed for a given frequency of excitation, alone, is an indication of higher power density potential. The continuous constant-torque speed range is up to 7200 rpm @ 60 Hz with 2 poles for a given design torque compared to 3600 rpm @ 60 Hz with 2 poles for other electric machines. In theory, the core volume is nearly half the physical size compared to other machines of similar rating.

Cost
Neglecting the slip ring assembly and the reduced material costs do to the higher power density, such as copper windings and electrical steel, the theoretical system cost of a wound-rotor doubly-fed electric machine is nearly 50% less compared to other machines of similar rating because the power rating of the electronic controller, which is the significant cost of any electric machine system, is 50% (or less) than other electric motor or generator systems of similar rating.

Peak Torque
With the symmetrical or dual-ported transformer topology of two active winding sets on the rotor and stator, respectively, the wound-rotor doubly-fed electric machine core produces nearly twice the peak torque of any electric machine by similarly increasing torque current without affecting air-gap flux density or saturating the magnetic core because the flux production with increasing torque currents on each side of the air-gap are neutralized. For all electric machines, peak torque current increases dissipation while reducing efficiency.

There. Jytdog (talk) 00:33, 18 January 2016 (UTC)

Brushless doubly-fed versions
This content too is mostly unsourced and promotional. Jytdog (talk) 00:35, 18 January 2016 (UTC)

Brushless doubly-fed induction electric machine
Brushless doubly-fed induction electric machine is constructed by adjacently placing two multiphase winding sets with unlike pole-pairs on the stator body. With unlike pole-pairs between the two winding sets, low frequency magnetic induction is assured over the speed range. One of the stator winding sets (power winding) is connected to the grid and the other winding set (control winding) is supplied from a frequency converter. The shaft speed is adjusted by varying the frequency of the control winding. As a doubly-fed electric machine, the rating of the frequency converter need only be fraction of the machine rating.

The brushless doubly-fed electric machine does not utilize core real-estate efficiently and the dual winding set stator assembly is physically larger than other electric machines of comparable power rating. In addition, a specially designed rotor assembly tries to focus most of the mutual magnetic field to follow an indirect path across the air-gap and through the rotor assembly for inductive coupling (i.e., brushless) between the two adjacent winding sets. As a result, the adjacent winding sets are excited independently and actively participate in the electro-mechanical energy conversion process, which is a criterion of doubly-fed electric machines.

The type of rotor assembly determines if the machine is a reluctance or induction doubly-fed electric machine. The constant torque speed range is always less than 1800 rpm @ 60 Hz because the effective pole count is the average of the unlike pole-pairs of the two active winding sets. Brushless doubly-fed electric machines incorporate a poor electromagnetic design that compromises physical size, cost, and electrical efficiency, to chiefly avoid a multiphase slip ring assembly.

Brushless wound-rotor doubly-fed electric machine
The brushless wound-rotor [synchronous] doubly-fed electric machine (BWRSDF) incorporates the electromagnetic structure of the wound-rotor induction (or asynchronous) doubly-fed electric machine but replaces the traditional multiphase slip ring assembly with a brushless real-time emulation control means that independently powers the rotor multiphase winding set with speed and phase synchronized AC excitation automatically and without delay as hypothesized by electric machine experts since the 1960s to avoid torque angle instability and slip induction discontinuity. Without experiencing control discontinuity between sub-synchronous and super-synchronous speeds while under stable instantaneously controlled excitation, the rotor multiphase winding set of only the BWRSDF becomes an equal power contributing (or active) participant in the electrical to mechanical power conversion process in conjunction with the stator multiphase winding set (or armature) and as a result, the rotor winding set provides the second armature for a dual-armature (or doubly-fed) electric machine system. In accordance with synchronous operating principles, the rotor armature cannot rely on induction (or asynchronous) principles as a result of a slip between the speeds of Rotor and Stator winding sets because slip-induction ceases to exist about either side of synchronous speed, which always leads to discontinuous operation. Without the reliance on slip-induction for practical operation, the brushless wound-rotor synchronous doubly-fed electric machine operationally differs significantly from wound-rotor doubly-fed induction electric generators or motors, which traditionally include a multiphase slip-ring assembly, and the so-called brushless doubly-fed induction or reluctance electric machines that rely on very different principles of unlike pole-pair slip-induction for operation. The doubly-fed induction electric machines have been relegated to high power generator applications, such as wind turbines, where their higher efficiency and lower cost as a result of doubly-fed prevail over instability, which is externally mitigated by the inertia of the prime mover.

Considering the speed (and voltage) of all electric machines is determined by the number of pole pairs and the frequency of excitation, electric machines with a 2 pole (i.e., one pole-pair) armature operate at 3600 RPM under 60 hertz of excitation but only a true BWRSDF can achieve stable and contiguous operation to the super-synchronous speed of 7200 RPM with the same torque performance, which is tantamount to twice the power density of all other electric machines, including permanent magnet and reluctance electric machines, by reasonably assuming the rotor and stator armature assemblies are equally power rated and occupy virtually the same physical volume as was qualified by at least pioneering electric machine experts from the 1960s, such as Professors D.W. Novotny, Norbert L. Schmitz, and Willis F. Long from the electrical engineering department of the University of Wisconsin. Since efficiency and cost are normalized to power rating, other ramifications are half the cost (e.g., half the material for a given power rating) and half the electrical loss (e.g., torque current is divided equally between the two armatures for lower electrical loss, which is a function of the square of the current (I2R), and the electronic controller need only control the power of one armature (or half the power) of the electric machine). Operationally comparable to a dual ported (i.e., symmetrical) transformer, the air-gap flux (i.e., flux linkage between the rotor and stator) remains virtually constant regardless of the level of flux producing torque current and as a result, the brushless wound-rotor [synchronous] doubly-fed electric machine (BWRSDF) can achieve multiply higher peak torque density than all other electric machines without saturating the magnetic core, which is an attractive attribute for transmission-less electric propulsion systems, such as electric vehicles. In addition, this same attribute allows the air-gap flux density of only the BWRSDF to be designed closer to the saturation limit of the magnetic steel core without operational concern and effectively, allowing a higher air-gap flux density than other electric machines. Like any synchronous electric machine with excess VAR capacity in conjunction with field control (i.e., field weakening) and torque angle control, the BWRSDF can be a Synchronous condenser with the ability to balance the electrical AC system, such as the Smart Grid, with leading to lagging or unity Power factor adjustment.

Without practical invention, pioneering experts have only theorized that stable excitation control of the rotor armature for a true BWRSDF must be a real-time (i.e., instantaneous) emulation control (RTEC) method that without delay, automatically and sensorlessly compensates for the destabilizing torque angle effects from at least external shaft perturbations further complicated by the effects of slip-induction. Very different from RTEC, today's state-of-art Field Oriented Control (FOC) rely on the imprecision and costly time delays of simulation control methods that sequentially measure speed and position, then estimate the rotor time constant, and finally, synthesize the excitation frequency and waveform, all by an offline digital electronic processor, which further aggravate the instability and slip-induction issues as a result of control delays and imprecisions that have prevented the realization of a true BWRSDF electric machine in the past. For brushless operation, rotor multiphase power cannot propagate through the traditional multiphase slip-ring assembly with its issues of additional cost, efficiency, size, maintenance, suitability, and reliability.

Without including the multiphase slip-ring assembly or RTEC, the basic symmetrical topology and relationships of the BWRSDF are the classic introductory textbook study of electric machines and by simply de-optimizing their high performance with asymmetry, the relationships of the BWRSDF become the study for all other electric machines, including permanent magnet, induction, and reluctance electric machines. So although the attractive performance attributes of the BWRSDF have been studied and understood for decades, the formidable invention of a practical brushless RTEC (BRTEC) has not been forthcoming and as a result, the BWRSDF has been kept from practical application until recently.

There Jytdog (talk) 00:35, 18 January 2016 (UTC)

I was the one of the article pioneers on doubly-fed electric machine for Wikipedia with over 30 years of experience. Now, I am pretty disappointed in Wikipedia. Technical ignorance in this area (electric machines) by your editors is overwhelming. Everything I have given Wikipedia is pertinent technical information. My information (or others) has never been advertisement, except to some wikipedia editors who still think the world is flat (and will not seek technical expertise to understand the subject matter that they are lacking). It is a real shame that much pertinent information of many professionals has been removed from the doubly-fed section because of pure technical obstinance (or ignorance) on the part of some editors. If you ban me, please return my donation or is wiki editors those that accept donations under false premises! It is pretty obvious the subjectivity when editors use pseudonyms instead of real names (as I do). I apologize for my ignorance in surfing wiki for directing my comment to the proper authorities, so please forward this to those in charge. 69.21.16.94 (talk) 14:47, 18 January 2016 (UTC)


 * I have read the sections removed. I agree that they are largely unsourced and very difficult to read and there are some statements that could be promotional such as “Synchro-Sym electric machine system is the only electric machine that …”.  Who says it is the only?  However, over all, they do not read like overly promotional material.  The biggest problem is jargon and sentence structure.  Because of the dependence on jargon, the only people likely to understand it are those who are already experts.Constant314 (talk) 18:16, 18 January 2016 (UTC)
 * Lack of sourcing is the biggest problem. If independent, secondary sources had been provided we could de-jargonize and we could make sure that the devices are described in general and that the content is not just about Best Machine's devices.  The content is un-useable as it stands. Jytdog (talk) 18:31, 18 January 2016 (UTC)

Frequency Converter Set
Doubly fed machines are also used as frequency converters in a motor generator setup. If the motor was to remain off and the rotor held stationary, the doubly fed machine would act as a transformer and no frequency conversion would take place. When the motor is started in a direction counter to the input frequency, the resultant frequency is calculated by the formula:

$$f_{out} = f_{line} + {p*n \over 120}$$

where: f = frequency; p = number of poles in the doubly fed machine; n = revolutions per minute of driving motor.

An example would be converting obsolete 25 cycle power to 60 cycle power using a 10 pole 25 cycle motor turning synchronously at 300RPM and a doubly fed machine with 14 poles. This information comes from an 1970 printing of the Audels Wiring Diagrams for Light and Power. The only bit of information that is missing is if the rotational direction will effect the frequency conversion, e.g. reversing the shaft will produce a subtractive effect. If so, my example of converting 25 to 60 cycle would instead output a 10 cycles with a 180 degree phase offset from input. Thaddeusw (talk) 13:55, 8 June 2016 (UTC)

External links modified
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most interesting?
“most interesting”? According to whom? 62.63.201.39 (talk) 13:41, 7 October 2022 (UTC)

Diagram in Introduction has a typo
The convertor on the left in the diagram should be a DC to AC.TrimmerinWiki (talk) 20:55, 18 December 2023 (UTC)