Abstract:
An electrical device has a capacitive storage element and first and second switches. The capacitive storage element and first and second switches are interconnected such that when interconnected with a direct current (dc) voltage supply and first and second windings of an electrical machine: (1) a first operational state exists in which conductive states of the first and second switches cause the dc voltage supply to conduct current through the first winding and the first switch and conduct current through the first and second switches and the second winding, respectively, thereby storing energy within the first and second windings, and (2) a second operational state exists in which non-conductive states of the first and second switches cause each of the first and second windings to discharge stored energy by conducting current through the capacitive storage element, thereby storing energy in the capacitive storage element.

Description:
This application claims priority to U.S. provisional application 61/409,638, filed on Nov. 3, 2010, the content of which is incorporated herein by reference. 
     BACKGROUND OF THE RELATED ART 
     Power factor is defined as the cosine of the phase angle between an instantaneous alternating current (ac) voltage and current, considering only fundamental values of the ac voltages and currents. Unity power factor is achieved when the phase angle between the voltage and current is zero. It is desired to have a unity power factor when drawing power from a utility, so that only real power is drawn, and no reactive power. A utility&#39;s investment for generation and distribution equipment will be minimized only if real power is drawn by users, which amounts to the user&#39;s power factor being unity. 
     Converters in the form of rectifiers distort ac current drawn from the utility supply, leading to non-sinusoidal current waveforms that introduce harmonics, other than the fundamental, that are undesirable to the operation of the utility, as well as contribute to additional losses that do not exist when only sinusoidal currents are drawn from the utility. Therefore, power factor correction is of importance for motor-drive applications in many countries because of regulations and incentives for manufacturers of electronics to build into their systems a capability for unity power factor operation and sinusoidal current draw. 
     Common ways to incorporate unity power factor operation and sinusoidal current draw are: (1) to provide a separate unity power factor correction (UPF) circuit, which is an expensive approach and takes additional space and volume for installation, and (2) operating a full-bridge controlled rectifier in boost mode, which is also an expensive solution. A solution was patented by Krishnan Ramu (U.S. Pat. No. 7,271,564, issued Sep. 18, 2007) for addressing these challenges; this solution employed a single transistor for controlling a two-phase machine. A disadvantage of the patented solution pertains to the limited torque-generating region of a two-phase machine. More specifically, when phase A is conducting, phase B must conduct also. The torque productions of these two phases are of opposite polarity, some of the time. Therefore, the torque production in a two-phase machine has a reduced output. 
     Although the currents in each phase of a two-phase machine may be made unequal, so the torque contributions from the two phases are unequal in magnitude, the net torque is also reduced. Moreover, the reduced torque is produced in every alternate torque-generation region of either phase A or B, whichever can produce the maximum torque compared to the other phase. 
     Assume the phase-B winding of a two-phase machine has less turns than the phase-A winding. The unequal number of turns between the phases makes phase B the auxiliary phase, with smaller torque generating capability compared to phase A. The net torque produced by the two is less in the machine with unequal number of turns compared to a machine with equal number of turns in the phase windings. 
     SUMMARY OF THE INVENTION 
     The invention disclosed herein overcomes the disadvantages of related-art devices with respect to unity power factor. 
     This and other objects of the invention may be achieved, in whole or in part, by an electrical device having a capacitive storage element and first and second switches. The capacitive storage element and first and second switches are interconnected such that when interconnected with a direct current (dc) voltage supply and first and second windings of an electrical machine: (1) a first operational state exists in which conductive states of the first and second switches cause the dc voltage supply to conduct current through the first winding and the first switch and conduct current through the first and second switches and the second winding, respectively, thereby storing energy within the first and second windings, and (2) a second operational state exists in which non-conductive states of the first and second switches cause each of the first and second windings to discharge stored energy by conducting current through the capacitive storage element, thereby storing energy in the capacitive storage element. 
     Additionally, the objects of the invention may be achieved, in whole or in part, by an electrical device having a capacitive storage element and a first switch. The capacitive storage element and first switch are interconnected such that when interconnected with a dc voltage supply and first and second windings of an electrical machine: (1) a first operational state exists in which a conductive state of the first switch causes the dc voltage supply to conduct current through the first winding and first switch, (2) a second operational state exists in which a non-conductive state of the first switch causes the first winding to discharge stored energy by conducting current through the capacitive storage element, thereby storing energy in the capacitive storage element, and (3) a third operational state exists in which the capacitive storage element discharges stored energy by conducting current through the second winding, thereby storing energy in the second winding. 
     Still further, the objects of the invention may be achieved, in whole or in part, by an electrical device having a capacitive storage element and first and second switches. The capacitive storage element and first and second switches are interconnected such that when interconnected with a dc voltage supply and first and second windings of an electrical machine: (1) a first operational state exists in which a conductive state of the first switch causes the dc voltage supply to conduct current through the first winding and first switch, (2) a second operational state exists in which a non-conductive state of the first switch causes the first winding to discharge stored energy by conducting current through the capacitive storage element and dc voltage supply, thereby storing energy in the capacitive storage element, (3) a third operational state exists in which a conductive state of the second switch causes the capacitive storage element to discharge stored energy by conducting current through the second winding, thereby storing energy in the second winding, and (4) a fourth operational state exists in which a non-conductive state of the second switch causes the second winding to discharge stored energy by conducting current through the first winding, thereby storing energy in the first winding. 
     Still further, the objects of the invention may be achieved, in whole or in part, by an electrical machine having: (1) a dc voltage supply that has first and second electrical terminals; (2) first and second windings of the electrical machine, each of the first and second windings having first and second electrical terminals; (3) a capacitive storage element that has first and second electrical terminals; (4) first and second switches that each has first and second electrical terminals; and (5) first, second, and third unidirectional current devices that each conducts current unidirectionally and has first and second electrical terminals. A first electrical node connects the first terminal of the dc voltage supply and first terminal of the first winding. A second electrical node connects the second terminal of the first winding, first terminals of the first switch and first unidirectional current device, and second terminal of the second unidirectional current device. A third electrical node connects the second terminal of the first unidirectional current device and first terminals of the capacitive storage element and second switch. A fourth electrical node connects the second terminal of the second switch, first terminal of the second winding, and second terminal of the third unidirectional current device. A fifth electrical node connects the second terminal of the second winding and first terminal of the second unidirectional current device. A sixth electrical node connects the second terminals of the dc voltage supply, first switch, and capacitive storage element and the first terminal of the third unidirectional current device. 
     Still further, the objects of the invention may be achieved, in whole or in part, by an electrical machine having: (1) a dc voltage supply that has first and second electrical terminals; (2) first and second windings of the electrical machine, each of the first and second windings having first and second electrical terminals; (3) a capacitive storage element that has first and second electrical terminals; (4) first and second switches that each has first and second electrical terminals; and (5) first and second unidirectional current devices that each conducts current unidirectionally and has first and second electrical terminals. A first electrical node connects the first terminal of the dc voltage supply and first terminal of the first winding. A second electrical node connects the second terminal of the first winding, first terminals of the first switch and first unidirectional current device, and second terminal of the second winding. A third electrical node connects the second terminal of the first unidirectional current device and first terminals of the capacitive storage element and second switch. A fourth electrical node connects the second terminal of the second switch, first terminal of the second winding, and second terminal of the second unidirectional current device. A fifth electrical node connects the second terminal of the dc voltage supply, second terminals of the first switch and capacitive storage element, and first terminal of the second unidirectional current device. 
     Still further, the objects of the invention may be achieved, in whole or in part, by an electrical machine having: (1) a dc voltage supply that has first and second electrical terminals; (2) first and second windings of the electrical machine, each of the first and second windings having first and second electrical terminals; (3) a capacitive storage element that has first and second electrical terminals; (4) first and second switches that each has first and second electrical terminals; and (5) first and second unidirectional current devices that each conducts current unidirectionally and has first and second electrical terminals. A first electrical node connects the first terminal of the dc voltage supply, first terminal of the first winding, and second terminals of the capacitive storage element and second winding. A second electrical node connects the second terminal of the first winding and first terminals of the first switch and first unidirectional current device. A third electrical node connects the second terminal of the first unidirectional current device and first terminals of the capacitive storage element and second unidirectional current device. A fourth electrical node connects the second terminal of the second unidirectional current device and first terminal of the second winding. A fifth electrical node connects the second terminal of the dc voltage supply and second terminal of the first switch. 
     Still further, the objects of the invention may be achieved, in whole or in part, by an electrical machine having: (1) a dc voltage supply that has first and second electrical terminals; (2) first and second windings of the electrical machine, each of the first and second windings having first and second electrical terminals; (3) a capacitive storage element that has first and second electrical terminals; (4) first and second switches that each has first and second electrical terminals; and (5) first and second unidirectional current devices that each conducts current unidirectionally and has first and second electrical terminals. A first electrical node connects the first terminal of the dc voltage supply, first terminal of the first winding, and second terminal of the second unidirectional current device. A second electrical node connects the second terminal of the first winding and first terminals of the first switch and first unidirectional current device. A third electrical node connects the second terminal of the first unidirectional current device and first terminals of the capacitive storage element and second winding. A fourth electrical node connects the first terminals of the second switch and second unidirectional current device and second terminal of the second winding. A fifth electrical node connects the second terminals of the dc voltage supply, first and second switches, and capacitive storage element. 
     Still further, the objects of the invention may be achieved, in whole or in part, by a method of operating an electrical machine having first and second phase windings and a common winding. According to the method, a determination is made: (1) whether each of the first and second phase windings is being energized by the flow of current and (2) whether current flow through a capacitive storage element, which stores energy discharged by the de-energization of the first and second phase windings, is increasing, decreasing, or positive. Energy stored by the capacitive storage element is discharged through the common winding if the current flow through the capacitive storage element is determined to be increasing. And energy stored by the capacitive storage element is discharged through the common winding if the current flow through the capacitive storage element is determined to be a positive current flow and neither of the first and second phase windings is being energized. The discharge of energy stored by the capacitive storage element through the common winding is discontinued if the current flow through the capacitive storage element is determined to be decreasing and at least one of the first and second phase windings is being energized. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Preferred embodiments of the invention are described in the following paragraphs of the specification and may be better understood when read in conjunction with the attached drawings, in which: 
         FIG. 1  illustrates a two-phase switched reluctance machine (SRM) having eight stator poles and ten rotor poles; 
         FIG. 2  illustrates common windings disposed on the stator of an SRM at locations where flux reversal does not occur; 
         FIG. 3  illustrates a first embodiment of a power converter defined by the invention; 
         FIG. 4  illustrates a second embodiment of the power converter defined by the invention; 
         FIG. 5  illustrates a third embodiment of the power converter defined by the invention; 
         FIG. 6  illustrates a fourth embodiment of the power converter defined by the invention; 
         FIG. 7  illustrates a fifth embodiment of the power converter defined by the invention; 
         FIG. 8  illustrates a sixth embodiment of the power converter defined by the invention; 
         FIG. 9  illustrates a seventh embodiment of the power converter defined by the invention; and 
         FIG. 10  illustrates a control method for implementing desired operational features of the power converter illustrated by  FIG. 9 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  illustrates a two-phase switched reluctance machine (SRM) having eight stator poles and ten rotor poles. SRM  100  has: (1) two stator poles  110  that are separated by 180 mechanical degrees and have phase-A windings  115  around them and (2) two stator poles  120  that are separated by 180 mechanical degrees and have phase-B windings  125  around them. Four windings  140  are disposed around four common poles  130  and constitute power factor correction (PFC) windings of the SRM; PFC windings  140  are substituted for an inductor within a PFC circuit. Common poles  130  carry flux, due to currents in the PFC or common windings  140 , and thereby augment the flux provided by phase windings  115  and  125 , thus generating increased torque. PFC windings  140  provide a physical inductor for PFC and enhance torque generation due to the flux they create on common poles  130 . Additionally, with SRM  100 : (1) no additional PFC inductor is required, as is required for conventional power PFC circuits and (2) the cooling of phase windings  115 ,  125  and PFC inductor  140  is accomplished by the cooling of SRM  100 , thus replacing cooling management of two separate entities (i.e., motor and inductor) with only one entity, that of the motor. 
       FIG. 2  illustrates common windings disposed on the stator back iron of an SRM at locations where flux reversal does not occur during operation. Stator  200  has four stator poles  212 . Phase windings  203  and  204  compose one phase, and phase windings  205  and  206  compose the second phase of the two-phase SRM. Phase windings  203 - 206  are wound around stator poles  212 , and PFC windings  201  and  202  are wound around back iron  210  of stator  200  where the polarity of the flux does not reverse as phases switch during operation. PFC windings  201  and  202 , when excited, will add to the flux generated by phase windings  203 - 206  and enhance torque production in the SRM. 
       FIG. 3  illustrates a first embodiment of a power converter defined by the invention. Power converter  300  has one transistor  306 ,  307  for each phase winding  302 ,  303  and a third transistor  308  for a common winding  304 . A full-diode-bridge rectifier  301  rectifies an alternating current (ac) voltage supply. 
     Phases A  302  and B  303  have the same number of windings. The phase- and common-coil arrangements disclosed in FIGS. 3, 4, and 6 of U.S. provisional patent application 60/955,661, the content of which is incorporated herein by reference, are suitable for use with power converter  300 . 
     Transistors  306  and  308  are switched on to energize phase-A winding  302  and common winding  304 . Transistor  308  is switched on and off to maintain a sine-wave current drawn from the ac supply through diode bridge  301  and common winding  304 . When transistor  308  is switched off, current through phase-A winding  302  is freewheeling and a capacitor  305  is charged by the discharge of energy from windings  302  and  304 . To commutate current from phase-A winding  302  rapidly, transistors  306  and  308  have to be switched off; current from phase-A winding  302  is conducted through diodes  311  and  312 , capacitor  305 , and diode  309 , resulting in capacitor  305 &#39;s voltage being applied to phase A, negatively. If transistor  308  is switched on while transistor  306  remains off, the applied voltage across phase A goes to zero because diode  311  stops conducting. In a state in which phase-A winding  302  is conducting current while transistor  306  is switched off and transistor  308  is switched on, current in phase-A winding  302  will freewheel and decay slowly. The flux generated by common winding  304 , when energized, is always additive with the flux from both phases and aids torque generation. 
     The operation of phase B is similar to that of phase A. Phase-B winding is energized by the flow of current through common winding  304 , diode  311 , transistor  307 , phase-B winding  303 , diode  313 , and transistor  308 . Phase-B winding discharges stored energy through diodes  313 ,  311 , and  310  and capacitor  305 . 
       FIG. 4  illustrates a second embodiment of the power converter defined by the invention. Power converter  400  is identical to that of power converter  300  with diodes  312  and  313  removed, resulting in only three-freewheeling diodes and three transistors. The operation of power converter  400  is similar to that described for power converter  300 . 
       FIG. 5  illustrates a third embodiment of the power converter defined by the invention. Power converter  500  only requires one transistor per phase winding and has no direct current (dc) link capacitor. Power converter  500  has three freewheeling diodes  509 - 511 , and common winding  304  does not have a transistor to control current through it. 
     Transistor  506  is switched on to build current in phase-A winding  302 , which generates torque in a motor. Transistor  506  is also switched on to draw sinusoidal current from the ac supply. When transistor  506  is switched off, current in phase-A winding  302  commutates through diodes  509  and  511 , charging a capacitor  520  and energizing common winding  304 . After current has commutated from phase-A winding  302 , energy stored in capacitor  520  is discharged through common winding  304 . 
     The operation of phase-B winding  303  is similar to that of phase-A winding  302 . Phase-B winding  303  is energized by the conduction of current through dc voltage supply  301 , phase-B winding, and switch  507 . Phase-B winding discharges stored energy by conducting a current through diode  510  and  511 , capacitor  520 , and common winding  304 , thereby energizing capacitor  520  and common winding  304 . Common winding  304  cannot be controlled independently of phases A and B. 
       FIG. 6  illustrates a fourth, embodiment of the power converter defined by the invention. Power converter  600  is similar to power converter  500  but includes an additional transistor  608  to regulate the simultaneous conduction of current through common winding  304  and either of phase-A winding  302  or phase-B winding  303 . Transistor  608  controls the energization of common winding  304  independently of the energization of phase-A winding  302  or phase-B winding  303 . Power converter  600  also includes an additional diode  611  for discharging energy stored by common winding  304 . 
     Transistor  506  is switched on to energize phase-A winding  302  and draw a sinusoidal current from the ac supply. When transistor  506  is switched off, current in phase-A winding  302  commutates through diode  509  and charges capacitor  520 . If transistor  608  is also switched on, energy from phase-A winding  302  will not only charge capacitor  520  but will also energize common winding  304 . If transistor  608  is switched off while common winding  304  is energized, the current in common winding  304  will freewheel through diode  611  and decay slowly, if current in phase-A winding  302  is fully commutated while transistor  608  remains on, energy from capacitor  520  will discharge through common winding  304 . 
     Similarly phase-B winding  303  is energized by switching transistor  507  on and off to draw sinusoidal current from the ac supply, while generating the desired torque in the SRM. Phase-B winding discharges stored energy by conducting current through diode  510  and capacitor  520 . When transistor  608  is switched on while capacitor  520  is charged, this charge is discharged through the conduction of current through switch  608  and common winding  304 . 
     The anode of common winding  304 &#39;s freewheeling diode  611  is connected to the positive or upper rail of the output of rectifier  301 . Diode  611  prevents energy within common winding  304  from being discharged into phase-A winding  302  or phase-B winding  303 . 
       FIG. 7  illustrates a fifth embodiment of the power converter defined by the invention. Power converter  700  is similar to power converter  600  but replaces diode  611  with a diode  711  that has its cathode connected to common winding  304  and its anode connected to the negative or lower rail of the output of rectifier  301 . Diode  711  supports the transfer of energy stored within common winding  304  back to phase-A winding  302  and transistor  506  or phase-B winding  303 , transistor  507 , and diode  711 . 
     Transistor  506  is switched on to energize phase-A winding  302  and draw a sinusoidal current from the ac supply. When transistor  506  is switched off, current in phase-A winding  302  flows through diode  509  and charges capacitor  520 . If transistor  608  is also switched on, energy from phase-A winding  302  will not only charge capacitor  520  but will also energize common winding  304 . If transistor  608  is switched off while common winding  304  is energized, the current in common winding  304  will need an alternative path to flow. Thus, when common winding  304  is energized and transistor  608  is switched off, either transistor  506  or  507  needs to be switched on to provide a path for the current in common winding  304 . A failure to switch on one or both of transistors  506  and  507  while transistor  608  is switched off and a current is flowing in common winding  304  will result in a failure of power converter  700 . 
     Three objectives for maximizing torque and power output in SRM drives are: (1) quickly commutating phase currents; (2) applying energy stored in a capacitor, which recovers energy from phase windings (not the dc link capacitor), to a common winding so as to maximize flux for a torque generating phase; and (3) charging the capacitor, which recovers energy from the phase windings (not the dc link capacitor), without interference from the common winding or any other extraneous factor. To achieve these objectives, power converter  700  must has the operational features described below. 
     When either of transistors  506  and  507  are switched off, transistor  608  is switched off so that capacitor  520  receives the energy stored in phase-A winding  302  or phase-B winding  303 , rather than common winding  304 . By turning off transistors  506 ,  507 ,  608  in this fashion, the current commutation of phases is achieved without any hindrance from common winding  304 . 
     During one phase&#39;s current commutation, a path for the flow of current in energized common winding  304  is always available through the phase that is being initiated for current conduction. For example, if phase-A winding  302  is being commutated, then transistor  507  is switched on to energize phase-B winding  303  while transistors  506  and  608  are turned off; because transistor  507  is switched on energized common winding  304  can be discharged by conducting current through phase-B winding  303 , transistor  507 , and diode  711 . 
     When phase-A winding  302  is being commutated, phase-B winding  303  is being initiated for current conduction and transistor  608  is turned off. During this time, transistor  507  has to be modulated to control current in phase-B winding  303  by turning transistor  507  off for some duration in each pulse width modulation cycle (PWM cycle). If transistor  507  is switched off, there is no path available for common winding  304 &#39;s current. By turning on transistor  608 , a path for current in common winding  304  is available, through capacitor  520 . Therefore, the turn-off duration of transistor  507  and turn-on duration of transistor  608  are coordinated so that there is always a path for the current in common winding  304 . This control step is very critical for the functioning of the circuit and also applies to phase-B winding  303 &#39;s commutation. More specifically, when the current through phase-B winding  303  is being commutated, transistors  506  and  608  have to be coordinated to ensure a path is available for current in common winding  304 . 
       FIG. 10  illustrates a control method for implementing desired operational features of power converter  700 . Method  1000  compares  1002  a measured or estimated speed of a machine&#39;s rotor with a speed command to generate a speed error. The speed error is processed  1004  by a proportional integrator and limiter to generate a torque command. The torque command is processed  1006  with respect to a rotor-position feedback signal to generate a current command. Both the current command and rotor-position feedback signal are processed  1008  to generate signals for enabling the energization of the phase-A and phase-B windings. For each of phases A and B, a phase current feedback signal is compared  1010  with a current command signal to generate a phase-error current signal. Each of the phase-error current signals is processed  1012  by a proportional integrator and limiter to generate respective phase voltage commands, which are modulated  1014  with a carrier PWM signal to generate phase-A and phase-B gating signals G A  and G B , respectively. Gating signals G A  and G B  are processed  1016  to generate phase-A and B commutation gating signals G AG  and G BC . And gating signals G A , G B , G AG , and G BG  are processed  1018  to generate a gating signal for energizing a common winding. 
     Table 1 summarizes the application of control signals, generated by method  1000 , to power converter  700 . In Table 1, G A , G B , and G C  indicate a gating signal for transistors  506 ,  507  and  608 , respectively, to energize phase-A winding  302 , phase-B winding  303  and common winding  304 , respectively; G AG  and G BG  indicate a whether a commutation condition exists for transistors  506  and  507 , respectively. For gating signals G A , G B  and G C , in Table 1, a “0” indicates an off condition and a “1” indicates an on condition. For signals G AG  and G BC , a “0” indicates that the commutation condition does not exist and an “X” indicates that the condition does exist. Parameter i A  is the current through phase-A winding  302 , and parameter i C  is the current through common winding  304 . The upwards and downwards arrows indicate whether current is increasing or decreasing. 
     
       
         
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 G A   
                 G B   
                 G AC   
                 G BC   
                 G C   
                 Comments 
               
               
                   
               
             
             
               
                 1 
                 1 
                 0 
                 0 
                 1 
                 i c  inceasing (i c  ↑) 
               
               
                 0 
                 1 
                 X 
                 0 
                 1 
                 i c  ↑ 
               
               
                 0 
                 1 
                 X 
                 0 
                 0 
                 i c  ↓ (i c  decreasing) 
               
               
                 1 
                 0 
                 0 
                 0 
                 1 
                 i c  ↑ 
               
               
                 1 
                 0 
                 0 
                 0 
                 0 
                 i c  ↓ 
               
               
                 0 
                 1 
                 0 
                 0 
                 1 
                 i c  ↑ 
               
               
                 0 
                 1 
                 0 
                 0 
                 0 
                 i c  ↓ 
               
               
                 0 
                 0 
                 0 
                 0 
                 1 
                 i c  ↑ (i c  &gt; 0) 
               
               
                 1 
                 0 
                 0 
                 X 
                 1 
                 i c  ↑ 
               
               
                 1 
                 0 
                 0 
                 X 
                 0 
                 i c  ↓ 
               
               
                 0 
                 0 
                 0 
                 X 
                 1 
                 i c  &gt; 0 
               
               
                 0 
                 0 
                 X 
                 0 
                 1 
                 i c  &gt; 0 
               
               
                   
               
             
          
         
       
     
     Within Table 1, the gating logic for transistor  608  is based on the following points. When transistors  506  and  507  are both conducting current, so as to energize both phase A and phase B of the machine, transistor  608  may also conduct current so as to energize common winding  304 . And when phases A and B are both conveying current through their respective transistors  506  and  507 , transistor  608  can be safely turned off. Similarly, when either one of transistors  506  and  507  is switched on to support the flow of current through its respective phase winding  302 ,  303 , transistor  608  can be switched on and off, conditioned by the current control requirements of common winding  304 ; the same is true for a situation in which one of transistors  506  and  507  is switched on and the other of the two transistors has a commutation signal common, in that transistor  608  may be switched on and off according to the current control demand for common winding  304 . 
     When both transistors  506  and  507  are turned off and when current in common winding  304  is greater than zero, transistor  608  cannot be turned off until one of the two transistors  506  or  507  is turned on. This is the most severest constraint to be enforced in the control of the motor drive system. 
     Consider the case where common winding  304  is an independent phase C and not a common winding. When phase C conducts for the same duration as phase A or B, the control switching logic is somewhat similar. The control differs in that phase C is turned on in a sequence of energization of the phases. For clockwise rotation, let the energization sequence be phase A, then B and then C, cyclically. Then, phase C is turned on while phase B is conducting or being commutated. In anti-clockwise rotation of the motor, the energization phase sequence is phase A, then phase C and then phase B, cyclically. 
     Thus, phase C will be energized during phase As conduction or current commutation. Regardless of whether transistors  506  or  507  are on or off, transistor  608  can be turned on during phase C&#39;s period for energization. 
     When the previously-conducting phase is commutated (A or B depending on the direction of rotation), its transistor  506  or  507  respectively, is turned off; during this mode, transistor  608  cannot be turned off as there is no path for the flow of current in phase C. Therefore, transistor  608  has to be on. Switching transistor  608  on in this fashion may conflict with the current control requirement of phase C. For example, the current in phase C may have to be reduced, but keeping transistor  608  on will increase the current in phase C&#39;s winding. 
     When such a conflict arises, it may be resolved by turning on phases A or B, using their respective transistors  506 ,  507 . However, switching on phase A or phase B at this time may interfere with the current control requirement of phase A or B. For example, phase A or B may be commutating its current. The priority as to commutation of phase A or B or control of current in phase C has to be assigned to resolve the conflict. 
     When phase A is commutating, phase C is starting to approach its energization region. Delaying phase A&#39;s commutation, to accommodate current control in phase C, can be accomplished without much penalty on phase A&#39;s commutation by advancing the commutation of phase A. Thus, the time available for phase A&#39;s commutation becomes longer and provides flexibility to control phase C&#39;s current. The same applies if phase B precedes phase C. 
     Consider the ease in which phase C has to be commutated. At this time, either phase A or B is being energized or already in conduction. Phase C&#39;s commutation can be advanced and that will provide flexibility to resolve the conflict between phase C&#39;s commutation and incoming phase A or B current control requirement. 
     The control strategy identified in Table 1 may be summarized as follows. If the current through common winding  304  is increasing, then switch transistor  608  on. If the current through common winding  304  is greater than zero and both of transistors  506  and  507  are switched off, then transistor  608  must be switched on. If the current through common winding  304  is decreasing and at least one of transistors  506  and  507  is switched on, then transistor  608  may be switched off. 
       FIG. 8  illustrates a sixth embodiment of the power converter defined by the invention. Power converter  800  provides power for a two-phase SRM having a common winding  304  and no dc link capacitor. Power converter  800  operates similarly to power converters  600  and  700 . Phase-A winding  302  and phase-B winding  303  are energized to draw sinusoidal current from ac supply  301  as they do in power converters  600  and  700 . When phase-A winding  302  is energized and transistor  506  is switched off, energy from phase-A winding  302  is commutated via diode  809  and through: (1) a first path comprising common winding  304  and a diode  811  and (2) a second path comprising a capacitor  820  and rectifier  301 ; capacitor  820  and common winding  304  are charged by the conduction of current through them. 
     By turning on transistor  608 , energy stored in capacitor  820  is transferred to common winding  304 . To discontinue the flow of current in common winding  304 , transistor  608  is switched off, and the energy stored in common winding  304  is discharged by the conduction of current through: (1) diode  811 , phase-A winding  302 , and a freewheeling diode  809 , (2) diode  811 , phase-B winding  303 , and a freewheeling diode  810 , or (3) both of the current paths. 
     If transistors  506  or  507  are switched on while common winding  304  has current flowing through it, then current from common winding  304  will flow through capacitor  820 , diode  811 , and phase windings A and B. Thus, energy stored in capacitor  820  or common winding  304  can be used to excite phase windings A and B. As a result, the voltage across capacitor  820  can be regulated by discharging its energy through phase winding A or B or both, so as to keep capacitor  820  operating within its voltage rating. 
       FIG. 9  illustrates a seventh embodiment of the power converter defined by the invention. Power converter  900  is identical to power converter  800  except that diode  811  in power converter  800  is replaced by a transistor  910  in power converter  900 . Transistor  910  provides an ability to energize phase-A winding  302  and phase-B winding  303  independently of common winding  304 . 
     Table 2 identifies modes of operating of power converter  900  with respect to phase-A winding  302 . In Table 2, i A  is the current through phase-A winding  302 , i C  is the current through common winding  304 , and v c  is the voltage across capacitor  820 . 
     As indicated in Table 2, when: (1) transistor  506  is switched on, (2) transistors  507 ,  608 , and  910  are switched off, and (3) no current is conducted through diodes  809  and  810 , then the current conducted through phase-A winding  302  is greater than zero and increasing so as to energize phase A. When transistors  506 ,  507 ,  608 , and  910  are switched off and current is conducted through diode  809  but not diode  810 , then the current conducted through phase-A winding  302  is greater than zero, phase A is de-energizing, and capacitor  820  is being charged by the energy from phase A. When: (1) transistors  506  and  608  are switched on, (2) transistors  507  and  910  are switched off, and (3) no current is conducted through diodes  809  and  810 , then current conducted through phase-A winding  302  is greater than zero and current conducted through common winding  304  is greater than zero. When: (1) transistors  506  and  910  are switched on, (2) transistors  507  and  608  are switched off, and (3) no current is conducted through diodes  809  and  810 , then current conducted through phase-A winding  302  is greater than zero and current conducted through common winding  304  decreases. When: (1) transistor  910  is switched on, (2) transistors  506 ,  507 , and  608  are switched off, and (3) current is conducted through diode  809  but not diode  810 , then current conducted through phase-A winding  302  decreases and current conducted through common winding  304  decreases. 
     For the modes of operation identified in Table 2 in which all transistors are switched off except transistor  910 , phase-B winding  303  may also conduct a current. This feature ensures there is a path for current from common winding  304  to flow at all times, so long as one of transistors  808  and  910  is on. 
     
       
         
               
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                   
                   
                 D A  809 
                 D B  810 
                   
                   
                   
               
               
                 T A  506 
                 T B  507 
                 Current 
                 Current 
                 T C  608 
                 T D  910 
                 Outcome 
               
               
                   
               
             
             
               
                 On 
                 Off 
                 No 
                 No 
                 Off 
                 Off 
                 i A  &gt; 0, i A  ↑ 
               
               
                 Off 
                 Off 
                 Yes 
                 No 
                 Off 
                 Off 
                 i A  &gt; 0 
               
               
                 Off 
                 Off 
                 Yes 
                 No 
                 Off 
                 Off 
                 i A  &gt; 0, v c  ↑ 
               
               
                 On 
                 Off 
                 No 
                 No 
                 On 
                 Off 
                 i A  &gt; 0, i C  &gt; 0 
               
               
                 On 
                 Off 
                 No 
                 No 
                 Off 
                 On 
                 i A  &gt; 0, i C  ↓ 
               
               
                 Off 
                 Off 
                 Yes 
                 No 
                 Off 
                 On 
                 i A  ↓, i c  ↓ 
               
               
                   
               
             
          
         
       
     
     Features of the invention include: (1) a power converter having a dc link capacitor and a common winding that is used to draw a near sinusoidal current and drive a suitable SRM of two or more phases; (2) a common winding around the back iron of a stator or around common poles; (3) different numbers of winding turns for two-phase windings to provide reduced torque capability for power factor correction; (4) flux from a common winding or phase for power factor correction is additive with the another phase; (5) a common winding is disposed at a location of a stator back iron where no flux reversal occurs; (6) flux from a common winding is additive with that of another phase; (7) a common winding is a motor phase that has reduced torque capability and is used for power factor correction; (8) a machine having one main phase and an auxiliary phase or winding around the back iron that may or may not be used for power factor correction; (9) a machine having one main phase and an auxiliary phase or winding around the back iron that is used as a regular motor phase; (10) a phase with reduced torque capability; (11) a power converter with or without a dc link capacitor; and (12) a common winding or lesser machine phase that is only used for regular operation of the SRM, not power factor correction. 
     The foregoing has been a detailed description of possible embodiments of the invention. Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention. Accordingly, it is intended that this specification and its disclosed embodiments be considered as exemplary only, with the true scope and spirit of the invention being indicated by the following claims.