Abstract:
An electric machine that includes a stator and a rotor, wherein the rotor is adapted to rotate relatively to the stator, and wherein the rotor comprises a rotor-switching-device which is arranged at the rotor.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    This invention relates to an electric machine or generator or motor and to a method to operate an electric machine or generator or motor or AC generator. 
         [0002]    In the prior art different types of electric machines (motors and generators) with wound or cage rotors are known. Such electric machines are divided into two major groups: synchronous machines and induction machines. Induction machines are quite flexible in terms of rotation speed and torque. In this context it is also known to vary the stator pole pitch and thus to further increase the flexibility of such induction machines. However, the main disadvantage of induction machines is the relatively low efficiency compared to synchronous electric machines. Although synchronous machines are more efficient, the efficiency is limited to only one rotation speed: the synchronous speed of the rotor with the AC frequency supplied to the machine. Thus, synchronous machines are not flexible enough. 
         [0003]    Therefore, it is an object of the current invention to provide an electric machine and methods to operate an electric machine with high efficiency at different rotation speeds. 
       SUMMARY OF THE INVENTION 
       [0004]    According to the invention, there is provided an electric machine, comprising a stator and a rotor, wherein the rotor is adapted to rotate relatively to the stator, and wherein the rotor comprises a rotor-switching device which is arranged at the rotor. 
         [0005]    Expediently, the rotor-switching device comprises a rotor-control-unit and at least one switch which is adapted to electrically connect rotor-coils and/or to provide or disconnect an electric connection to a power source. In particular, the rotor-control-unit is adapted to activate preselected electrical switches at a predefined moment of time so that for a given rotor coil one of the following events occur:
       Electrically connect the rotor coil(s) to a DC power source so that polarity is also selectable;   Electrically connect the rotor coil(s) to a selected phase of a multiphase AC power source so that polarity is also selectable;   Short-circuit the rotor coil(s);   Connect the rotor coil(s) to an electrical load;   Leave the rotor coil(s) electrically disconnected.       
 
         [0011]    In a preferred embodiment, the switches are mechanical switches such as relays or semiconductor switches. As already mentioned, the switches are adapted to electrically connect different rotor-coils. However, the switches are also adapted to provide a connection to the power supply. In one embodiment, a number of switches is the same as the number of (rotor) coils. Preferably, as the switches are located in or at the rotor-switching-device, they also rotate together with the rotor-switching-device. 
         [0012]    In one embodiment, the rotor comprises means to provide or conduct electric power, in particular a slip ring or a rotary transformer. A slip ring is an electrical connector designed to carry current or signals from a stationary wire into a rotating device. Typically, it consists of a stationary graphite or metal contact (brush) which rubs on the outside diameter of a rotating metal ring. As the metal ring turns, the electrical current or signal is conducted through the stationary brush to the metal ring making the connection. Additional ring/brush assemblies are stacked along the rotating axis if more than one electrical circuit is needed. Either the brushes or the rings are stationary and the other component rotates. A rotary (rotatory) transformer is a specialized transformer used to couple electrical signals between two parts that rotate in relation to each other. Preferably, typical rotary transformers provide longer life than slip rings. Expediently, the rotor comprises a plurality of slip rings or rotary transformers, dependent on the power supply or the current(s) and voltage(s) that have to be conducted. Preferably, these means are arranged at a rotor axis of the rotor. Preferably, the power source is an AC- or DC-power source. 
         [0013]    Expediently, the rotor-switching device may comprise at least one electric inverter, such as a DC-DC-inverter, a DC-AC-inverter, an AC-AC-inverter and/or an AC-DC-inverter. The same applies to the stator-switching-device. 
         [0014]    In a preferred embodiment, the electric machine comprises a stator-switching-device which comprises a stator-control-unit, wherein the stator-switching-device comprises at least one switch which is adapted to electrically connect stator-coils and/or to provide or disconnect an electric connection to the power source. Preferably, these switches are of the same type and design as the switches of the rotor-switching-device. Thus, the same features and advantages apply. In particular, the stator-switching-device is adapted to activate preselected electrical switches at a predefined moment of time so that for a given stator coil one of the following events occur:
       Electrically connect the stator coil(s) to a selected phase of a multiphase AC power source so that polarity is also selectable;   Electrically connect the stator coil(s) to a DC power source so that polarity is also selectable;   Short-circuit the stator coil(s);   Connect the stator coil(s) to an electrical load;   Leave the stator coil(s) electrically disconnected.       
 
         [0020]    Expediently, the stator comprises at least one stator-pole-pair, wherein the rotor comprises at least one rotor-pole-pair, wherein each pole-pair is formed by at least three coils. The rotor-switching-device is adapted to provide the rotor with DC and/or AC. In the same manner, the stator-switching-device is adapted to provide the stator with DC and/or AC. Generally, the electric machine can be operated in all its operation modes as generator and as motor. In one embodiment the electric machine can be operated as double fed machine. Double fed electric machines are such electric machines in which the rotor also takes active part in energy conversion process. The magnetic field created by the rotor is not static, but it is rotating relative to the rotor itself and electric energy is either “pumped” to the rotor or taken out of the rotor depending on current operation mode. In the present invention the rotor may create a variable pole magnetic field which rotates relative to the rotor itself. The rotor as well as the stator can generate rotating magnetic fields (using e.g. three-phase AC). Preferably, the stator-switching-device is adapted to vary a number of stator-pole-pairs by electrically connecting appropriate stator-coils, wherein the rotor-switching-device is adapted to vary a number of rotor-pole-pairs by electrically connecting appropriate rotor-coils. In one embodiment, the electric machine can be a single fed synchronous machine. In this case, the stator-coils are provided with AC and the rotor-coils are provided with DC. The synchronous machine is an AC rotating machine whose speed under steady state condition is proportional to the frequency of the current in its armature, respectively, in its rotor. This is also named the “synchronous speed”. The synchronous speed N is given by 
         [0000]        N=f/p· 60  s /min 
         [0000]    where,
 
f is the frequency of the AC supply current (power frequency) in Hz, and p is the is the number of pole pairs.
 
         [0021]    Each pole pair comprises two poles, a north- and a south-pole. Thus, the smallest pole pair number is “1” (e. g. 1 north pole and 1 south pole). Preferably, the number of stator-pole-pairs is the same as the number of rotor-pole-pairs. They correspond to each other. In other words, by varying the number of stator-pole-pairs, respectively, the number of rotor-pole-pairs, the number of magnetic poles and in consequence the synchronous speed N is changeable. Advantageously, the electric machine is adapted to change the pole number, respectively, the pole angular pitch while working (the pole pitch is defined as a peripheral distance between centres of two adjacent poles and the pole angular pitch is defined as the angle formed by the centres of two adjacent poles to the rotor&#39;s rotation axis). Thus, the electric machine can be operated at highest efficiency at different rotation speeds and a given power frequency. Another embodiment of the invention is capable to operate as double fed electric machine. In such case rotation speed N is a product of pole pair number and two frequencies—one supplied to the rotor and the other supplied to the stator. By varying pole pair numbers of the rotor and the stator the present invention increases functionality of a double fed machine too, as there will be at least two different synchronous rotation speeds for every two given supply frequencies. The electric machine can be operated as AC motor and as AC generator at a variety of output frequencies at a given rotation speed. The rotor is adapted to create a variable magnetic field which can vary on command in terms of direction and magnitude, relative to the rotor itself. The rotor comprises a rotor or rotation axis, respectively, and is supported inside the stator. Both, the stator and the rotor, extend along the rotation axis. The stator of the electric machine consists preferably of a laminated electrical steel core that comprises the stator-coils. The stator-switching-device preferably connects the stator-coils with a polyphase system. Polyphase systems have three or more energized electrical conductors carrying alternating currents with a definite time offset between the voltage waves in each conductor. With polyphase power a rotating magnetic field can be generated. Preferably, three coils are connected to form a three-phase winding, respectively, system. Advantageously, this connection is not fixed. The connection is provided by the stator-switching-device and so different coils and also a different number of coils can be connected to each and every AC phase to vary the number of poles, respectively, pole pairs of the stator and in consequence, the synchronous rotation speed of the electric machine. The same is valid for the rotor-switching-device and the rotor-coils. To further improve rotor rotation speed flexibility, the rotor-coils are preferably provided with AC by the rotor-switching-device, thus operating the electric machine as double fed machine. This means that both, the stator and the rotor, generate a rotating magnetic field using AC current. In one embodiment, the rotating magnetic fields of the stator and the rotor rotate in different directions. 
         [0022]    Preferably, the stator comprises a plurality of stator-slots and the rotor comprises a plurality of rotor-slots, wherein each stator-coil is arranged in at least two stator-slots, and wherein each rotor-coil is arranged in at least two rotor-slots. Preferably, the rotor-coils as well as the stator-coils extend substantially parallel to the rotation axis. However, it is also possible that the rotor-coils and/or the stator-coils extend at least partly transverse or oblique to the rotation axis. In addition, the rotor-coils and/or the stator-coils can have at least partly a corrugated form along the rotation axis. 
         [0023]    In a single fed electric machine embodiment there are preferably four rotor slots and six stator slots. The number of rotor coils is preferably four and the number of stator coils is preferably six. Each coil occupies two adjacent slots, thus a stator coil annular pitch is 60° and a rotor coil angular pitch is 90°. The rotor is expediently supplied with DC through two slip rings. 
         [0024]    In a double fed electric machine embodiment there are preferably six rotors slots and preferably six stator slots. The number of rotor coils is preferably six and the number of stator coils is preferably six. Each coil occupies two adjacent slots, thus a stator coil angular pitch is 60° and a rotor coil angular pitch is 60. The rotor is expediently supplied with AC through slip rings. 
         [0025]    It shall be understood that embodiments with different number of coils and slots, e. g.  18 ,  24  or  48  etc. are also possible and preferred. 
         [0026]    Expediently, the electric machine, in particular the rotor, can be operated with DC and/or AC, wherein AC comprises at least 3 phases. In other words, a minimal number of phases is preferably 3 (phase A, phase B and phase C). It goes without saying that also polyphase systems with more phases are possible, e. g. 6 phases. 
         [0027]    Advantageously, the electric machine can be operated as synchronous machine and as induction machine. In induction operation mode the rotor switching device switches rotor-coils in short and not to any of the AC phases or the DC power source. The rotating magnetic field created by the stator will induce electric currents in the rotor-coils which is the principle behind induction motors. By selecting a combination of coils and polarities to excite the rotor, the rotor-switching-device is capable of changing the magnetic pole number of the rotor. It is adapted to connect rotor-coils in short thus operating the electric machine in induction mode. This is useful when starting the electric machine and also during transition between two synchronous speeds. Electric power is delivered to the rotor via e. g. two slip rings or brushless via rotary transformers and rectifiers. 
         [0028]    The rotor may be also connected to a multiphase AC source to operate as double fed machine to create a rotor rotating magnetic field. Multiphase AC is brought to the rotor switching device via slip rings (at least 3) or via a set of rotating transformers. By selecting a combination of coils and polarities to be connected to a certain phase, the magnetic pole number of the rotating magnetic field can be beneficially changed. A communication connection between the rotor switching device and the stator switching device or its control units, respectively, is used to coordinate both devices so that the pole pair number and the rotating magnetic field of the rotor and the pole pair number and the rotating magnetic field of the stator are for example the same. In particular, if the rotor-coils are connected in short by the rotor switching device, the electric machine will operate as single fed induction machine. If the frequency of the multiphase AC source supplied to the rotor drops to zero then the electric machine will operate as single fed synchronous machine. Further benefits can bring the ability to connect sensors to the rotor-switching-device to monitor key characteristics of the rotor during operation. For instance temperature, rotor coil electric currents and other important readings can be monitored inside the rotor and information passed to the stator-switching-device via the information channel or the communication connection, respectively, between the two switching devices, This way malfunctions can be anticipated before a major accident occurs. It is also possible a set of gyroscopic sensors inside the rotor to determine exact position of the rotor at a certain moment of time. This may eliminate the need of complicated other sensors mounted outside the rotor with the same function. 
         [0029]    In both cases (rotor supplied with AC or DC), the stator may be designed or operated similarly. The stator-switching-device electrically connects the stator-coils to a multiphase AC source to create a rotating magnetic field. By selecting a combination of coils and polarities to be connected to a certain phase, the magnetic pole number of the rotating magnetic field can be beneficially changed. A communication connection between the stator switching device and the rotor switching device or its control units, respectively, is used to coordinate both devices so that rotor pole numbers and stator pole pair numbers are the same. If rotor speed is very different from synchronous speed then induction mode is activated (rotor switching device connects rotor-coils in short). When the rotor reaches speed close to synchronous speed then rotor-switching-device excites proper combination of rotor-coils to create rotor magnetic field with the same number of poles as the number of poles of momentary stator rotating magnetic field. 
         [0030]    Another advantage presents the energy storage device inside the rotor. It can be a battery, capacitor or other device which stores electric energy. It can be used to supply power to the rotor switching device in case of power breakdown to the rotor. Using the supply from the energy storage device, the rotor switching device can perform a controlled emergency stop procedure in coordination with the stator switching device. 
         [0031]    According to the invention, a method to operate an electric machine, comprising a stator and a rotor, wherein the rotor is adapted to rotate relatively to the stator, and wherein the rotor comprises a rotor-switching-device which is arranged at the rotor, comprises the steps:
       providing the stator with AC using a stator-switching device;   providing the rotor with AC or DC or short-circuiting the rotor using the rotor-switching-device.       
 
         [0034]    Advantageously, the method comprises also the steps:
       selecting a (proper) stator pole pair number using the stator-switching device; selecting (proper) corresponding operational mode for the rotor using the rotor-switching-device such as induction, synchronous or double fed.       
 
         [0036]    The method according to the invention can include the features and advantages of the electric machine according to the invention and vice versa. 
         [0037]    Additional advantages and features of the current invention are shown in the following description of preferred embodiments of the current invention with reference to the attached drawings. Single features or characteristics of respective embodiments are explicitly allowed to be combined within the scope of the current invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0038]      FIG. 1 : shows a perspective view of an exemplary single fed embodiment of an electric machine; 
           [0039]      FIG. 2   a : shows a stator and rotor design of the electric machine of  FIG. 1 ; 
           [0040]      FIG. 2   b : shows a communication connection between a rotor-switching device and a stator-switching-device of the electric machine of  FIG. 1   
           [0041]      FIG. 3 : shows a perspective view of a double fed embodiment of an electric machine; 
           [0042]      FIG. 4   a : shows a stator and rotor design of the electric machine of  FIG. 3 ; 
           [0043]      FIG. 4   b : shows a communication connection between a rotor-switching-device and a stator-switching-device of the electric machine of  FIG. 3 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0044]    Referring now to  FIG. 1 , a perspective view of an electric machine  20  is shown. The electric machine  20  comprises a stator  40  and a rotor  60 . A rotor-switching-device  64  is arranged at a rotor axis  61  of the rotor  60 . Means for providing or conducting electric power  62  such as slip rings or rotary transformers are also arranged at the rotation axis  61 . In this embodiment, they conduct current or voltages DC+V or DC−V, respectively, to the rotor-coils r of the rotor  60 . The electric machine  20  furthermore comprises a stator-switching-device  44  that is electrically connected to an AC power source or to phases A, B and C, respectively. Thus, the stator-coils s of the stator  40  are provided with alternating current. This type of electric machine is named single fed machine.  FIGS. 2   a  and  2   b  show the design of the electric machine  20  more detailed. 
         [0045]      FIG. 2   a  shows a communication connection  50  between the rotor-switching-device  64  and the stator-switching device  44 . In particular, a rotor-control-unit  66  is connected to a stator-control-unit  46 . The rotor-switching-device  64  also comprises a sensor  92  and a rotor energy storage unit  90 . 
         [0046]      FIG. 2   b : Switches l 15 , l 16 , l 17  and l 18  ( FIG. 2   b ) are operated by a rotor-control-unit  66  so that their switch position is the same and changed simultaneously from 1 to 4. Switches l 19 , l 20  and l 21  are operated by the stator-control-unit  46  so that their switch position is the same and changed simultaneously from 1 to 2. All switches depicted on  FIG. 2   a  are shown in position 1. Below, different operation modes are described: 
         [0047]    Four pole synchronous mode operation: Switches l 19 , l 20  and l 21  are switched to position 1. Stator-coil s 10  is connected to phase A. Stator-coil s 11  is connected to phase B. Stator-coil s 12  is connected to phase C. Stator-coil s 13  is connected to phase A. Stator-coil s 14  is connected to phase B. Stator-coil s 9  is connected to phase C. This forms a familiar four pole stator-coil configuration—ABCABC. Rotor switches l 15 , l 16 , l 17  and l 18  are switched to position 2. This way the rotor-coils r 5 , r 6 , r 7  and r 8  will create a  4  pole magnetic field SNSN (south-north-south-north). When rotor speed is synchronized with stator rotating 4 pole field the rotor will follow synchronously the stator rotating field. 
         [0048]    Two pole synchronous mode operation: Switches l 19 , l 20  and l 21  are switched to position 2. Stator-coil s 10  is connected in reverse to phase A. Stator-coil s 11  is connected to phase B. Stator-coil s 12  is connected in reverse to phase C. Stator-coil s 13  is connected to phase A. Stator-coil s 14  is connected in reverse to phase B. Stator-coil s 9  is connected to phase. This forms a familiar two pole stator-coil configuration—A B- C A- B C-. Rotor switches l 15 , l 16 , l 17  and l 18  are switched to position 3. This way the rotor-coils r 6  and r 8  will create a 2 pole magnetic field SN. Rotor-coils r 5  and r 7  are connected in short to work in induction mode to assist the rotor to reach synchronous speed. When rotor speed is synchronized with stator rotating 2 pole field the rotor will follow synchronously the stator rotating field. 
         [0049]    Four pole induction mode operation: Switches l 19 , l 20  and l 21  are switched to position 1. Stator-coil s 10  is connected to phase A. Stator-coil s 11  is connected to phase B. Stator-coil s 12  is connected to phase C. Stator-coil s 13  is connected to phase A. Stator-coil s 14  is connected to phase B. Stator-coil s 9  is connected to phase C. This forms a familiar four pole stator-coil configuration—ABCABC. Rotor switches l 15 , l 16 , l 17  and l 18  are switched to position 4. This way the rotor-coils r 5 , r 6 , r 7  and r 8  are connected in short. Stator rotating field will induce currents in rotor-coils making the rotor follow stator field at sub synchronous speed. 
         [0050]    Two pole induction mode operation: Switches l 19 , l 20  and l 21  are switched to position 2. Stator-coil s 10  is connected in reverse to phase A. Stator-coil s 11  is connected to phase B. Stator-coil s 12  is connected in reverse to phase C. Stator-coil s 13  is connected to phase A. Stator-coil s 14  is connected in reverse to phase B. Stator-coil s 9  is connected to phase C. This forms a familiar two pole stator-coil configuration—A B- C A- B C-. Rotor switches l 15 , l 16 , l 17  and l 18  are switched to position 4. This way the rotor-coils r 5 , r 6 , r 7  and r 8  are connected in short. Stator rotating field will induce currents in rotor-coils making the rotor follow stator field at sub synchronous speed. 
         [0051]    Emergency stop: Rotor switches l 15 , l 16 , l 17  and l 18  are switched to position 1. All rotor-coils are open. The rotor will not produce any torque, regardless of presence of stator field and rotor power supply. In another embodiment single fed machine there may be a DC to DC inverter as part of the rotor switching device. This way high voltage DC can be supplied to slip rings and the built in rotor switching device DC to DC inverter to reduce the voltage to appropriate level. This way current over slip rings will be reduced bringing down losses and wear of the slip rings. Furthermore in another embodiment the brushes and slip rings can be replaced by a rotating transformer, having its primary coil stationary and its secondary coil mounted on the rotor and rotating along with it. This way AC will be supplied and integrated in rotor switching device AC to DC inverter should provide transition from AC to DC current needed to excite rotor-coils to produce magnetic field, static relative to the rotor. In this case brushes and slip rings are eliminated altogether. 
         [0052]      FIG. 3  basically shows the same concept as  FIG. 1 . However, a rotor  60  is provided with alternating current. This type of machine is named doubly fed machine.  FIG. 4   b  shows the design in detail. 
         [0053]      FIG. 4   a  shows basically the same design as  FIG. 2   b . However, according to the design of the electric machine of  FIG. 3 , a rotor-switching-device  64  is provided with alternating current C′, B′, A′. 
         [0054]    Switches l 15 , l 16 , l 17 , l 18 , l 34  and l 35  ( FIG. 4   b ) are operated by a rotor-control-unit  66  so that their switch position is the same and changed simultaneously from 1 to 4. Switches l 19 , l 20  and l 21  are operated by a stator-control-unit  46  so that their switch position is the same and changed simultaneously from 1 to 2. All switches depicted on  FIG. 4   a  are shown in position 1. 
         [0055]    Below, different operation modes are described: 
         [0056]    Four pole doubly fed mode operation: Switches l 19 , l 20  and l 21  are switched to position 1. Stator-coil s 10  is connected to phase A. Stator-coil s 11  is connected to phase B. Stator-coil s 12  is connected to phase C. Stator-coil s 13  is connected to phase A. Stator-coil s 14  is connected to phase B. Stator-coil s 9  is connected to phase C. This forms a familiar four pole stator-coil configuration—ABCABC. Rotor switches l 15 , l 16 , l 17 , l 18 , l 34  and l 35  are switched to position 2. Rotor-coil r 5  is connected to phase A′, rotor-coil r 6  is connected to phase C′, rotor-coil r 7  is connected to phase B′, rotor-coil r 8  is connected to phase A′, rotor-coil r 32  is connected to phase C′, rotor-coil r 33  is connected to phase B′. These connections form a four pole rotating field—A′C′B′A′C′B′. This field rotates relative to the rotor and its direction is opposite to the stator rotating field. This way the rotor will accelerate to an angular speed which the sum of the angular speeds of both stator and rotor rotating fields. 
         [0057]    Two pole doubly fed mode operation: Switches l 19 , l 20  and l 21  are switched to position 2. Stator-coil s 10  is connected in reverse to phase A. Stator-coil s 12  is connected in reverse to phase C. Stator-coil s 13  is connected to phase A. Stator-coil r 14  is connected in reverse to phase B. Stator-coil s 9  is connected to phase C. This forms a familiar two pole stator-coil configuration—A B- C A- B C-. Rotor switches l 15 , l 16 , l 17 , l 18 , l 34  and l 35  are switched to position 3. Rotor-coil r 5  is connected to phase A′, rotor-coil r 6  is connected in reverse to phase C′, rotor-coil r 7  is connected to phase B′, rotor-coil r 8  is connected in reverse to phase A′, rotor-coil r 32  is connected to phase C′, rotor-coil r 33  is connected in reverse to phase B′. These connections form a two pole rotating field—A′ C′- B′ A′- C′ B′-. This field rotates relative to the rotor and its direction is opposite to the stator rotating field. This way the rotor will accelerate to an angular speed which the sum of the angular speeds of both stator and rotor rotating fields. 
         [0058]    Four pole induction mode operation: Switches l 19 , l 20  and l 21  are switched to position 1. Stator-coil s 10  is connected to phase A. Stator-coil s 11  is connected to phase B. Stator-coil s 12  is connected to phase C. Stator-coil s 13  is connected to phase A. Stator-coil s 14  is connected to phase B. Stator-coil s 9  is connected to phase C. This forms a familiar four pole stator-coil configuration—ABCABC. Rotor switches l 15 , l 16 , l 17 , l 18 , l 34  and l 35  are switched to position 4. This way the rotor-coils r 5 , r 6 , r 7 , r 8 , r 32  and r 33  are connected in short. Stator rotating field will induce currents in rotor-coils making the rotor follow stator field at sub synchronous speed. Two pole induction mode operation: Switches l 19 , l 20  and l 21  are switched to position 2. Stator-coil s 10  is connected in reverse to phase A. Stator-coil s 11  is connected to phase B. Stator-coil s 12  is connected in reverse to phase C. Stator-coil s 13  is connected to phase A. Stator-coil s 14  is connected in reverse to phase B. Stator-coil s 9  is connected to phase C. This forms a familiar two pole stator-coil configuration—A B - C A - B C. Rotor switches l 15 , l 16 , l 17 , l 18 , l 34  and l 35  are switched to position 4. This way the rotor-coils r 5 , r 6 , r 7 , r 8 , r 32  and r 33  are connected in short. Stator rotating field will induce currents in rotor-coils making the rotor follow stator field at sub synchronous speed. 
         [0059]    Emergency stop: Rotor switches l 15 , l 16 , l 17 , l 18 , l 34  and l 35  are switched to position 1. All rotor-coils r 5 , r 6 , r 7 , r 8 , r 32  and r 33  are open. The rotor will not produce any torque as no current would pass through rotor-coils, regardless of presence of stator field and rotor AC power supply. In another embodiment of doubly fed machine there may be a DC to AC inverter as part of the rotor switching device. This way high voltage DC can be supplied to slip rings and the built in DC to AC inverter to supply rotor-coils with multiphase AC to produce rotating magnetic field. Furthermore in another embodiment the brushes and slip rings can be replaced by a rotating transformer, having its primary coil stationary and its secondary coil mounted on the rotor and rotating along with it. This way AC will be supplied and internal AC to AC inverter should provide transition from AC to AC current again to supply rotor-coils with multiphase AC to produce rotating magnetic field by the rotor-coils. In this case brushes and slip rings are eliminated altogether. In both cases of DC to AC inverter with slip rings or AC to AC inverter being used the inverter should be reversible so that when the rotor is spinning at sub synchronous speed electric energy induced in rotor-coils should be transported out of the rotor through the slip rings or rotating transformer. 
       REFERENCE NUMERALS 
       [0000]    
       
           20  electric machine 
           40  stator 
           44  stator-switching-device 
           46  stator-control-unit 
           50  communication connection 
           60  rotor 
           61  rotor axis 
           62  means to conduct/provide electric power 
           66  rotor-control-unit 
           64  rotor-switching device 
           90  rotor energy storage unit 
           92  set of sensors 
         AC alternating current 
         DC, DC +V, DC −V direct current 
         A, B, C . . . phases (stator) 
         A′, B′, C′ . . . phases (rotor) 
         V 1 , V 2 , V 3  . . . voltages 
         R rotation axis 
         s, s 1 , s 2 , s 3  . . . stator-coils 
         r, r 1 , r 2 , r 3  . . . rotor-coils 
         l, l 1 , l 2 , l 3 , . . . switches