Doubly-fed electric machine


Doubly-fed electric machine

Doubly-fed "electric machines" (i.e., electric motors or electric generators) belong to a category of electric machines that incorporate two multiphase winding sets of similar power rating that have independent means of excitation. As a result, doubly-fed electric machines are synchronous electric machines by nature but with both winding sets actively participating in the energy conversion process (i.e., doubly-fed or dual armature).

Classification

All electric machines are categorized as either "Singly-Fed" with one winding set that actively participates in the energy conversion process or "Doubly-Fed" with two active winding sets. Although sometimes described as doubly-fed, the wound-rotor induction machine (either Krämer drive where the slip power from rotor is lost in resistors or subsynchronous Scherbius drive where the slip power is fed back to AC line) and the field-excited synchronous machine are singly-fed machines because only one winding set actively participates in the energy conversion process.

The Wound-Rotor Doubly-Fed Electric Machine, the Brushless Wound-Rotor Doubly-Fed Electric Machine, and the so-called Brushless doubly-fed induction electric machines are the only examples of doubly-fed electric machines.

Features of doubly fed machines

The wound-rotor doubly-fed electric machine is the only electric machine that is able to operate with rated torque to twice synchronous speed for a given frequency of excitation (i.e., 7200 rpm @ 60 Hz and one pole-pair versus 3600 rpm for singly-fed electric machines). Higher speed for a given frequency of excitation is a quantifying metric that shows lower cost, higher efficiency, and higher power density.

As do all electromagnetic electric machines, doubly fed machines need current to produce the torque. Because there are no permanent magnets in the doubly fed machine, current is also needed for the magnetic flux production. Like wound rotor synchronous machines, the magnetic flux can be produced by the stator current, rotor current or by the combination of the both. It is common to run the machine completely magnetized from the rotor and thus having power factor of the stator at unity. At synchronous speed the rotor current has to be DC, as in ordinary synchronous machines. However, the rotor current must be AC current, if the speed of the machine differs from the synchronous speed. Thus reactive power is needed to be fed in the rotor winding when it is used to magnetize the machine in non-synchronous operation.

Rotor current is also needed to produce torque in addition to magnetization. Thus active power is present in the rotor in addition to reactive power.

The frequency and the magnitude of the rotor voltage is proportional to the slip, that is, the difference between the speed of the machine and the synchronous speed. At standstill the frequency will be the same as the frequency in the stator and the voltage 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 than the stator. One can easily see that 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 active power or generate it out depending the speed and torque. If the machine is producing torque, that is, it is operating as a motor, the rotor will generate power if the speed is below synchronous speed (subsynchronous operation). At standstill all power fed in the stator (excluding losses) is returned via the rotor. The magnitude of the active power depends on the torque of the motor. Thus if the motor has rated torque, rated power is circulating through the stator and rotor. Like all electric machine, the efficiency of the machine is not very good at low speeds because current is required to produce torque but no or little mechanical power is produced (i.e., power is the product of torque and speed).

If the machine is operating as a motor at speeds over the synchronous speed (supersynchronous operation), the mechanical power is fed in both through the stator and rotor. As a consequence the efficiency is now better than with singly fed motors. For example, at maximum speed 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 produced power, is better than singly-fed electric machines. Naturally one has yet 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.

For operation as a generator 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.

Doubly-fed electric machines outperform the others in supersynchronous speeds. They can operate at constant torque to twice synchronous speed if each active winding is rated at half the total power of the machine (i.e., contiguous operation between sub-synchronous through supersynchronous speed range).

It is important to note, however, that doubly fed machines do not produce more rated torque per volume than singly fed machines. The bigger power rating is due to the higher speed attainable without weakening the magnetic flux. The short time maximum torque of a doubly fed machine is, however, much higher than with ordinary induction machines and in practice limited only by the temperature of the windings and the maximum current capability of the rotor frequency converter.

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.

Further note, that 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 turn ratio 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. 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 in the operating speed range.

Electronic control

The electronic controller, a frequency converter, conditions bi-directional (i.e., four quadrant), speed synchronized, and multiphase electrical power to at least one of the winding sets (generally, the rotor winding set). Using four quadrant control, which must be continuously stable throughout the speed range, a wound-rotor doubly-fed electric machine with two poles (i.e., one pole-pair) has a constant torque speed range of 7200 rpm when operating at 60 Hz. However, in high power applications two or three pole-pair machines with respectively lower maximum speeds are common.

The electronic controller is smaller, less expensive, more efficient, and more compact than electronic controllers of singly-fed electric machine because in the simplest configuration, only the power of the rotating (or moving) active winding set is controlled, which is less than half the total power output of the electric machine.

Due to the lack of damper windings used in synchronous machines, the doubly fed electric machines are susceptible to instability without stabilizing 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, an adjustable-speed drive.

Wound-rotor doubly-fed

Construction

Two multiphase winding sets with similar pole-pairs are placed on the rotor and stator bodies, respectively. The wound-rotor doubly-fed electric machine is the only electric machine with two independent active winding sets, the rotor and stator winding sets, occupying the same core volume as other electric machines. 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.

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 woltage is attempted. Thus a special design for doubly fed operation is necessary.

A multiphase slip ring assembly (i.e., sliding electrical contacts) is traditionally used to transfer power to the rotating (moving) 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 compromises core real estate, reliability, cost, and efficiency, it allows independent electronic 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 own winding system (preferably using high frequency in the 10 kHz range for compact size) for power transfer out of or into the rotor. However, this kind of arrangement may have higher cost and lower efficiency than the slip ring alternative due to multiple power conversions and the thermal and mechanical constraints (for example centrifugal forces) of the power electronic assembly in the rotor. However, electronics have been incorporated on the rotor for many years (i.e., high speed alternators with brushless field exciters) for the improved reliability. Furthermore, high frequency power transfer is used in many applications because of improvements in efficiency and cost over low frequency alternatives, such as the DC link chokes and capacitors in traditional electronic controllers.

Efficiency

Neglecting the slip ring assembly, the theoretical electrical loss of the wound-rotor doubly-fed machine in supersynchronous operation is comparable to the most efficient electric machine systems available (i.e., the synchronous electric machine with permanent magnet assembly) with similar operating metrics because the total current is split between the rotor and stator winding sets while the electrical loss of the winding set is proportional to the square product of the current flowing through the winding set. Further considering the electronic controller conditions less than 50% of the power of the machine, the wound-rotor doubly-fed electric motor or generator (without brushes and with stable control at any speed) theoretically shows nearly half the electrical loss (i.e., winding set loss) of other electric motor or generator systems 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 constant-torque speed range is up to 7200 rpm @ 60 Hz with 2 poles compared to 3600 rpm @ 60Hz with 2 poles for other electric machines. In theory, the core volume is nearly half the physical size (i.e., winding set loss) of other electric motor or generator systems of similar rating.

Cost

Neglecting the slip ring assembly, the theoretical system cost is nearly 50% less than 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.

Conclusion

The wound-rotor doubly-fed electric machine incorporates the most optimum electromagnetic design of any electric machine but requires a slip ring assembly which 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. The wound-rotor doubly-fed electric machine has found commercial success in very large applications with limited speed range, such as Windmills, where efficiency and low cost power electronics outweigh reliability issues associated with the slip ring assembly and the control complexity.

Double fed induction generator

DFIG is an abbreviation for Double Fed Induction Generator, a generating principle widely used in wind turbines. It is based on an induction generator with a multiphase wound rotor and a multiphase slipring assembly with brushes for access to the rotor windings . It is possible to avoid the multiphase slipring assembly (see brushless doubly-fed electric machines), but there are problems with efficiency, cost and size. A better alternative is a brushless wound-rotor doubly-fed electric machine.

The principle of the DFIG is that rotor windings are connected to the grid via sliprings and back-to-back voltage source converter that controls both the rotor and the grid currents. Thus rotor frequency can freely differ from the grid frequency (50 or 60 Hz). By controlling the rotor currents by the converter it is possible to adjust the active and reactive power fed to the grid from the stator independently of the generators turning speed. The control principle used is either the two-axis current vector control or direct torque control (DTC) (US patent|6448735). DTC has turned out to have better stability than current vector control especially when high reactive currents are required from the generator [http://nwpc.vtt.fi/presentations/niiranen_slides.pdf] .

The doubly-fed generator rotors are typically wound with from 2 to 3 times the number of turns of the stator. This means that the rotor voltages will be higher and currents respectively lower. Thus in the typical ± 30 % operational speed range around the synchronous speed the rated current of the converter is accordingly lower leading to a low cost of the converter. The drawback is that controlled operation outside the operational speed rage is impossible because of the higher than rated rotor voltage. Further, the voltage transients due to the grid disturbances (three- and two-phase voltage dips, especially) will also be magnified. In order to prevent high rotor voltages - and high currents resulting from these voltages - from destroying the IGBTs and diodes of the converter a protection circuit (called crowbar) is used.

The crowbar will short-circuit the rotor windings through a small resistance when excessive currents or voltages are detected. In order to be able to continue the operation as quickly as possible an active crowbar (for example US patent|7164562) has to be used. The active crowbar can remove the rotor short in a controlled way and thus the rotor side converter can be started only after 20-60 ms from the start of the grid disturbance. Thus it is possible to generate reactive current to the grid during the rest of the voltage dip and in this way help the grid to recover from the fault.

As a summary, a doubly fed induction machine is a wound-rotor doubly-fed electric machine and has several advantages over a conventional induction machine in wind power applications. Firstly, as the rotor circuit is controlled by a power electronics converter, the induction generator is able to both import and export reactive power. This has important consequences for power system stability and allows the machine to support the grid during severe voltage disturbances (low voltage ride through, LVRT). Secondly, the control of the rotor voltages and currents enables the induction machine to remain synchronized with the grid while the wind turbine speed varies. A variable speed wind turbine utilises the available wind resource more efficiently than a fixed speed wind turbine, especially during light wind conditions. Thirdly, the cost of the converter is low when compared with other variable speed solutions because only fraction of the mechanical power, typically 25-30 %, is fed to the grid through the converter, the rest being fed to grid directly from the stator. The efficiency of the DFIG is very good for the same reason.

Brushless doubly-fed versions

Brushless doubly-fed induction electric machines

Brushless doubly-fed induction "electric machines" (i.e., electric motors or electric generators) are 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. Although brushless doubly-fed electric machines have not seen commercial success since their conception in the early 1970s, the promise of a low cost, highly efficient electronic controller keeps the concept under perpetual study, research, and development.

Brushless wound-rotor doubly-fed electric machine

The Brushless wound-rotor doubly-fed "electric machine" (i.e., electric motor or electric generator) incorporates the electromagnetic structure of the wound-rotor doubly-fed electric machine, but replaces the traditional multiphase slip ring assembly with a brushless means to independently power the rotor winding set (i.e., doubly-fed) with multiphase AC power. The torque of the wound-rotor doubly-fed electric machine is dependent on both slip and position, which is a classic condition for instability. For stable operation, the frequency and phase of the multiphase AC power must be synchronized and fixed instantaneously to the speed and position of the shaft, which is not trivial at any speed and particularly difficult about synchronous speed where induction no longer exists. If these conditions are met, all the attractive attributes of the wound-rotor doubly-fed electric machine, such as high power density, low cost, ultra-high efficiency, are realized without the traditional slip-ring assembly and instability problems [http://www.bestelectricmachine.com/file_download/FAQ.pdf FAQ.pdf] . One company, [http://www.bestelectricmachine.com Best Electric Machine] , has patented and is selling a brushless, fully stable, synchronous wound-rotor doubly-fed electric machine with symmetric quality of motoring or generating. Another brushless wound-rotor construction invented by Lars Gertmar has been described in the patent application [http://v3.espacenet.com/origdoc?DB=EPODOC&IDX=SE514818&F=0&RPN=WO0067363&DOC=deb45b02a96017d4924c09c38b43eaba61] .

ee also

*Electrical motor
*Electrical generator
*Singly-fed electric machine

References


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