Abstract:
A method of operating an electrical machine having first and second phase windings. The method includes: (1) applying positive first current to the first phase winding while the first phase winding&#39;s back electromotive force (emf) is positive; (2) applying negative second current to the first phase winding while the first phase winding&#39;s back emf is negative; and (3) applying positive third current to the second phase winding while the second phase winding&#39;s back emf is positive. The first current is conducted through a circuit composed of a battery, the first phase winding, and a first switch. The second current is conducted through a circuit composed of a first capacitive storage element, the battery, the first phase winding, and a second switch, and the third current is conducted through a circuit composed of the battery, the second phase winding, and a third switch.

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 
     Low cost motor drives in vehicle applications, such as electric bikes (hereafter referred to as E-Bikes) operated with battery-stored energy, are sought after because of their positive impact on the environment, the existing mass market of electric bikes, and the limited financial resources of the user community in countries such as China, India, and other developing nations. One of the significant cost elements in a motor drive is the power converter circuit, particularly in the number of power devices such as transistors and power diodes. Economy in the use of power devices translates into reduced control circuit components, such as gate drives, logic power supplies, and device protection circuits; such economy also leads to reduced printed circuit board area, heat-sink volume, and weight. Fewer power devices also leads to lower cost of the power electronic system for the motor drive. 
     SUMMARY OF THE INVENTION 
     The invention disclosed herein provides control techniques for a low-cost power converter applied to vehicle applications, including E-Bikes. The power converter has a small number of power devices and operates from energy stored in a battery. Batteries may be connected in series, in parallel, or both in series and in parallel to obtain a certain desired direct-current (dc) voltage, and such an arrangement is designated, herein, as a battery bank. The power converter: (1) may charge a battery without a separate power electronic circuit, using its own components for this purpose, and (2) does not require a separate inductor (or choke) to charge the batteries; instead, the windings of a switched reluctance machine or permanent magnet brushless de machine are used for smoothing a controlled, rectified voltage for charging the batteries. 
     The control techniques relate to the operation of the power converter during motoring and provide enhanced electromagnetic torque operation of the machine. One control technique provides ripple-free operation of the electric machine. 
     These and other 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. The method includes; (1) applying positive first current to the first phase winding while the first phase winding&#39;s back electromotive force (emf) is positive; (2) applying negative second current to the first phase winding while the first phase winding&#39;s back emf is negative; and (3) applying positive third current to the second phase winding while the second phase winding&#39;s back emf is positive. The first current is conducted through a circuit consisting of a battery, the first phase winding, and a first switch that is operating in a conductive state. The second current is conducted through a circuit consisting of a first capacitive storage element, the battery, the first phase winding, and a second switch that is operating in a conductive state, and the third current is conducted through a circuit consisting of the battery, the second phase winding, and a third switch that is operating in a conductive state. 
     Additionally, the objects of the invention may be achieved, in whole or in part, by a method of operating an electrical machine having a stator, a rotor, and first and second phase windings. The method includes: (1) applying positive first current to the first phase winding while the rotor is within a first range of rotation with respect to the stator; (2) applying negative second current to the first phase winding while the rotor is within a second range of rotation with respect to the stator; and (3) applying positive third current to the second phase winding while the rotor is within a third range of rotation with respect to the stator. The first current is conducted through a circuit consisting of a battery, the first phase winding, and a first switch that is operating in a conductive state. The second current is conducted through a circuit consisting of a first capacitive storage element, the battery, the first phase winding, and a second switch that is operating in a conductive state, and the third current is conducted through a circuit consisting of the battery, the second phase winding, and a third switch that is operating in a conductive state. 
     Still further, the objects of the invention may be achieved, in whole or in part, by a electrical machine system having an electrical machine with a stator, a rotor, and first and second phase windings. A processor generates a logical-TRUE condition for: (1) a first rotor range signal when the rotor is within a first range of rotation with respect to the stator, (2) a second rotor range signal when the rotor is within a second range of rotation with respect to the stator, and (3) a third rotor range signal when the rotor is within a third range of rotation with respect to the stator, and otherwise generates a logical-FALSE condition for the first, second, and third rotor range signals. A phase current controller generates a logical-TRUE condition for: (1) a first phase current signal when both a pulse width modulation (PWM) signal has a logical-TRUE condition and current conducted through the first phase winding is positive, (2) a second phase current signal when both the PWM signal has a logical-TRUE condition and current conducted through the first phase winding is negative, and (3) a third phase current signal when both the PWM signal has a logical-TRUE condition and current conducted through the second phase winding is positive, and otherwise generates a logical-FALSE condition for the first, second, and third phase current signals. Logic circuitry generates a logical-TRUE condition for: (1) a first gate signal when both the first rotor range signal and first phase current signal have a logical-TRUE condition, (2) a second gate signal when both the second rotor range signal and second phase current signal have a logical-TRUE condition, and (3) a third gate signal when both the third rotor range signal and third phase current signal have a logical-TRUE condition, and otherwise generates a logical-FALSE condition for the first, second, and third gate signals. A power converter: (1) conducts current from a first source through the first phase winding when the first gate signal has a logical-TRUE condition, (2) conducts current from a second source through the first phase winding when the second gate signal has a logical-TRUE condition, and (3) conducts current from either the first or second source through the second phase winding when the third gate signal has a logical-TRUE condition. 
    
    
     
       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 first embodiment of a power converter; 
         FIG. 2  illustrates a second embodiment of a power converter; 
         FIG. 3  illustrates voltage, current, phase torque, and total air gap torque applied to the power converter illustrated by  FIG. 1 , for a first embodiment of the control technique defined by the invention; 
         FIG. 4  illustrates voltage, current, phase torque, and total air gap torque applied to the power converter illustrated by  FIG. 1 , for a second embodiment of the control technique defined by the invention; 
         FIG. 5  illustrates a control circuit that generates gate signals for the transistors of the power converters illustrated by  FIGS. 1 and 2 ; and 
         FIG. 6  summarizes, for each angular rotor position, the current conduction state of each, transistor and the polarity applied to phases A and B for the power converter illustrated by  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  illustrates a first embodiment of a power converter. Power converter  100  receives a single-phase, alternating current (ac) supply voltage  102  provided by a utility grid. Supply voltage  102  is rectified through a single-phase, bridge rectifier  104  and filtered with a capacitor C 1    106 , so as to produce a direct current (dc) voltage, V dc , across capacitor  106 . V dc  is also referred to as a de link voltage. Power converter  100  provides operational modes of battery-bank charging, motoring, and regeneration. 
     During the charging mode of operation, V dc  charges a battery bank  116  via a buck converter, which comprises a transistor  110 , diode  122 , and phase-A winding  130 . The charging of battery bank  116  is accomplished by changing the on-time duration of transistor  110 , using pulse width modulation (PWM) control, so that current in phase-A winding  130  is regulated to a desired charging current of battery bank  116 . Transistor  110  is turned on and off with a duty cycle of d so that current conducted through transistor  110  charges battery bank  116  to a voltage V b  through phase-A winding  130 , which also serves as a voltage-smoothing inductor. Thus, during the charging operation, sonic energy from capacitor  106  is transferred to battery bank  116  and other such energy is transferred to phase-A winding  130 . 
     When current conveyed to battery bank  116  exceeds an established limit, transistor  110  is turned off. After transistor  110  is switched off energy stored within phase-A winding  130  will be discharged as a current flowing through phase-A winding  130 , battery bank  116 , and diode  122 . Thus, battery bank  116  continues to charge for some time after transistor  110  is switched off. The rate of charging battery bank  116  is determined by a reference current command in a feedback-current control loop of a buck power-conversion control system. 
     During the motoring operational mode of a phase-B winding  140 , a transistor  114  is switched on so that battery bank  116  discharges its stored energy by conveying a current through phase-B winding  140  and transistor  114 . Current regulation is achieved by controlling transistor  114  with an appropriate PWM signal. When transistor  114  is turned off, the energy stored in phase-B winding  140  is discharged by the conduction of current through phase-B winding  140 , a diode  124 , capacitor  106 , and a capacitor  118 ; as energy is discharged from phase-B winding  140 , the voltage applied across phase-B winding  140  is −(V dc −V b ). With V d , greater than V b , the commutation of current can be faster. 
     Phase-A winding  130  may be energized via a boost circuit comprising transistor  112  and battery bank  116 . Upon switching on transistor  112 , current is conveyed by battery bank  116  through phase-A winding  130  and transistor  112 . When the current conveyed through phase-A winding  130  exceeds an established limit or the energy stored within phase-A winding  130  needs to be discharged, transistor  112  is turned off and the stored energy within phase-A winding  130  is discharged by the conveyance of a current through phase-A winding  130 , diode  120 , capacitor  106 , and capacitor  118 . Energy discharged by phase-A winding  130  is stored by capacitor  106 . As phase-A winding  130  discharges its stored energy, a voltage −(V dc −V b ) is applied across phase-A winding  130 , which forces the current flowing through phase-A winding  130  to diminish to zero. Full power is applied to phase A using the boost operation mode. 
     Phase-A winding  130  may also be powered from the energy stored in capacitor  106  using the buck circuit comprising capacitor  106 , transistor  110 , phase-A winding  130 , and battery bank  116 . The operation of powering phase-A winding  130  is similar to the operation of charging battery bank  116 , described above. Therefore, phase-A winding  130  can be powered in the motoring mode either from the energy stored in capacitor  106  or that stored in battery bank  116 . 
     The conduction of current through phase B winding  140  is initiated by switching on transistor  114  and is controlled by PWM of transistor  114 . The PWM current control is similar to that employed for charging battery bank  116 . When transistor  114  is switched off while phase B winding  140  is energized, phase B winding  140  conducts current to capacitor  106  through diode  124 , which provides a transfer of energy from phase B winding  140  to capacitor  106 . PWM control of phase B winding  140  causes capacitor  106  to be charged multiple times and this energy transferred to capacitor  106  is used to energize phase-A winding  130 . 
       FIG. 2  illustrates a second embodiment of a power converter. Power converter  200  has transistors  110  and  112  with anti-parallel diodes  120  and  122 , respectively, across them. Transistors  110  and  112  and diodes  120  and  122  constitute an inverter phase leg, as do transistors  210  and  212  and diodes  220  and  222 . Together, these two sets of phase legs of an inverter constitute a single-phase H-bridge inverter, which is available in the form of an intelligent power module with gate drivers and protection circuits for over-current, under-voltage, and over-voltage operation; they are compact in size and very cost effective for mass production. 
     Transistor  110 , phase-A winding  130 , battery bank  116 , capacitor  106 , and diode  122  are used as a buck power circuit for charging battery bank  116  from the dc source voltage, V dc . Similarly transistor  210 , phase-B winding  140 , battery bank  116 , capacitor  106 , and diode  222  serve as another buck power converter circuit to charge battery bank  116  from capacitor  106 . Both avenues for battery charging through phase-A winding  130  and phase-B winding  140  present an opportunity for fast charging. 
     Rectifier  104  rectifies an ac voltage provided by ac voltage source  102 . The rectified voltage is applied across capacitor  106  as V dc . When transistor  110  is switched on, capacitor  106  conveys current through transistor  110 , phase-A winding  130 , and battery bank  116 . When transistor  110  is switched off, phase-A winding  130  discharges its stored energy by conducting a current through battery bank  116  and diode  122 . 
     Similarly, transistor  212  is switched on to discharge energy stored in battery bank  116  by the conduction of current through phase-B winding  140  and transistor  212 , thereby energizing phase-B winding  140 . When transistor  212  is switched off, energy stored by phase-B winding  140  is discharged by the conduction of current through diode  220 , capacitor  106 , and capacitor  118 , thereby charging capacitor  106 . Transistor  210  is switched on to conduct current from capacitor  106  through transistor  210 , phase-B winding  140 , and battery bank  116 , thereby energizing phase-B winding  140  and charging battery bank  116 . When transistor  210  is switched off, energy stored by phase-B winding  140  is discharged by conducting a current through battery bank  116  and diode  222 , thereby charging battery bank  116 . 
     When energization of phase-A winding  130  is in a positive half-cycle (i.e., its induced electromotive force (emf) is in opposition to voltage V b  across battery bank  116 ), transistor  112  regulates the current in phase-A winding  130 . When transistor  112  is turned off to lower the current in phase-A winding  130  or to extinguish the current, phase-A winding  130  discharges its stored energy by conveying current through diode  120 , capacitor  106 , and capacitor  118 . 
     When the induced emf of phase-A winding  130  is in an additive state, the machine is ready for negative current operation. Switching on transistor  110  causes capacitor  106  to convey current through transistor  110 , phase-A winding  130 , and battery bank  116 . 
     Injecting positive current when induced emf is negative, and vice versa, enables regenerative braking in a permanent magnet brushless direct current motor (PMBDCM). When an induced emf of phase-A winding  130  is positive (i.e., its polarity is in opposition to V b ), transistor  110  is switched on so that a negative current flows through phase-A winding  130 , so as to transfer energy from the machine to battery bank  116 . Likewise, when induced emf is negative (i.e., when it is additive to V b ), transistor  112  is switched on to convey positive current from battery bank  116  through transistor  112 . In each instance, regenerative braking slows the speed of the machine. 
     When current is positive, the induced emf plus V b  act so as to build up current when transistor  112  is turned off, such that the sum of induced emf and V b  drives a current into phase-A winding  130  for transferring energy to capacitor  106 . When current in phase-A winding  130  is negative, the induced emf has to be positive; that means induced emf opposes V b , with the result that the sum of V dc  and induced emf helps to build up the current when transistor  110  is turned on. When transistor  110  is turned off, to regulate the current, the induced emf assists the energy transfer from phase-A winding  130  to battery bank  116 . 
     Regenerative braking is achieved with phase-B winding  140  in a manner similar to that described above with respect to phase-A winding  130 . Positive currents in phase windings  130  and  140  are injected and controlled by transistor  122  and transistor  212 , respectively. Similarly negative currents in phase windings  130  and  140  are controlled by transistor  110  and transistor  210 , respectively. 
     Advantages provided by power converter  200  include the following. No external inductor is required for filtering because the machine phases are utilized for this purpose during battery charging from the grid. Both positive and negative currents can be injected into the machine phases, thus increasing the torque of the machine. Full regenerative braking is achieved. Full utilization of electronic devices and machine are achieved. The use of a single-phase, inverter bridge provides compact packaging of the power circuit, resulting in low cost and high reliability. Power converter  200  provides a low-cost solution for the control of a battery operated two-phase PMBDCM drive having high torque output. 
       FIG. 3  illustrates voltage, current, phase torque, and total air gap torque applied to power converter  100  for a first embodiment of the control technique defined by the invention. The following assumptions are made with regard to the application of the control technique to power converter  100 : (1) the torque generated by a two-phase PMBDCM is proportional to current in the phase windings; (2) phase-A winding  130  is operated with alternating current; (3) enough energy is available in capacitor  106  to drive phase-A winding  130 ; (4) the full voltage V b  of battery  116  is applied across each phase of the machine; only motoring operation is considered; (5) phase-B winding  140  is operated only in its positive half-cycle with positive current (i.e., for 180 electrical degrees); and (6) phase B&#39;s back emf is 90 electric degrees out of phase with respect to phase A. Within  FIG. 3 : (1) e a  and e b  are the induced emfs in phases A and B, respectively; (2) i a  and i b  are currents in phase A and B windings, respectively; (3) the air-gap torques generated by phase A and B excitations are T ea  and T eb , respectively; (4) the total instantaneous air-gap torque of the machine is T e ; (5) T av  is the average air-gap torque of the machine; (6) θ r  is the rotor position in electrical radians; (7) E and I are peak induced emf and current in the machine phases, respectively; and (8) T 1  is the torque associated with current in a phase. 
     When phase A&#39;s back emf e a  is positive, a positive current i a  is injected into phase-A winding  130 , thereby generating positive torque T ea . When phase A&#39;s back emf becomes negative, the polarity of current in phase-A winding  130  is reversed and a negative current is injected in phase-A winding  130 . Applying negative current when the back emf is negative will still generate a continuous positive torque. 
     When phase B&#39;s back eta e b  is positive, a positive current i b  is injected into phase B winding  140 , thereby generating positive torque T eb . Phase B&#39;s current is commutated when its back emf becomes negative. Thus, the net air-gap torque generated varies between T 1  and 2T 1  and always remains positive. As a result, the maximum average torque generated by the machine, using power converter  100  and full current energization, is 1.5T 1 . The maximum torque that may be obtained by applying alternating current to both phases is 2T 1 , which is not possible with power converter  100  but is possible with converter  200 . 
     With power converter  100 , phase A current cannot be maintained (the positive half cycle) at full value if capacitor  106  is not charged sufficiently by current commutation from phase B and during current-control intervals in phase B. Therefore, the average torque can be lower than 1.5T 1 . Further torque derating can occur with the reality of induced emfs departing from their ideal shapes of rectangles. 
     In summary, the first embodiment of the control technique: (1) applies positive current to a first phase winding of an electrical machine when the first phase winding&#39;s back emf is positive, so as to generate positive torque; (2) applies negative current to the first phase winding when the first phase winding&#39;s back emf is negative; (3) applies positive current to a second phase winding of the electrical machine when the second phase winding&#39;s back emf is positive; and (4) either: (a) discontinues the application of current to the second phase winding when the second phase winding&#39;s back emf is negative or (b) applies negative current to the second phase winding when the second phase winding&#39;s back emf is negative. 
       FIG. 4  illustrates voltage, current, phase torque, and total air gap torque applied to power converter  100  for a second embodiment of the control technique defined by the invention. The second embodiment provides a smooth and uniform instantaneous torque, for ripple-free operation that is suitable for high-performance applications. 
     The following assumptions are made with regard to the application of the control technique to power converter  100 : (1) phase A has positive and negative currents for λ/2 radians (i.e., 90 degrees) and conducts current only for π radians and (2) phase B has positive current for π radians (i.e., 180 degrees). Within  FIG. 3 : (1) ‘a’ is a real number between 0 and 1, including 0 and 1 and (2) the stator phase-currents are regulated to be a multiple, a, of the nominal current I (i.e., aI). 
     Positive phase-A current is injected for the first 90 degrees of positive, induced emf within phase A, so as to generate a torque of aT 1 , where T 1  is the torque generated for nominal current in phase A. Positive current of magnitude aI is injected into phase B for 180 degrees of positive, induced emf, which generates a positive torque of aT 1 . Phase B cannot be supplied with negative current using power converter  100 , but phase A can be injected with negative current. Injecting a negative current, −aI, in phase A for the next 90 degrees, when the phase A induced emf is negative, generates a positive torque of aT 1 . After the 90 degrees of negative current injection within phase A, the induced emf of phase A changes from negative to positive and therefore, a positive current of aI is injected into phase A for the next 90 degrees, resulting in a positive torque of aT 1 . 
     Thus, for 360 degrees of torque generation: (1) phase A is injected with positive current aI during the first 90 degrees, so as to produce torque of aT I ; (2) phase B is injected with positive current aI during the next 180 degrees, so as to produce torque of aT 1 ; and (3) phase A is injected with negative current −aI during the last 90 degrees, so as to produce torque of aT 1 . As a result, uniform torque is generated with a magnitude of aT 1 .  FIG. 6  summarizes, for each angular rotor position, the current conduction state of each of power converter  100 &#39;s transistors and the polarity applied to phases A and B. 
     The two embodiments of the control technique have been described in direct reference to power converter  100 , but are applicable to power converter  200  and other power converters having features similar to those of power converters  100  and  200 . 
       FIG. 5  illustrates a control circuit that generates gate signals for transistors that energize the phase windings of an electrical machine based on the absolute position of the machine&#39;s rotor. Control circuit  500  includes a current controller  505 , processor  510 , logical-AND gate  521  for generating a gate signal G TA2 , logical-AND gate  522  for generating a gate signal G TA1 , and logical-AND gate  523  for generating a gate signal G TB . Gate signal G TA2  is applied to transistor T A2    112  to ensure positive current control in phase A. Negative phase A current is regulated by transistor T A1    110  and its gate signal is G TA1 . Positive phase B current is regulated by transistor T B    114  and its gate signal is G TB . 
     Gate signals G TA1 , G TA2 , and G TB  are produced in the following way. Current controller  505  modulates each of a phase-A positive current signal, phase-A negative current signal, and phase-B positive current signal by a carrier PWM signal so as to generate a variable duty cycle signal at the carrier PWM frequency. The variable duty cycle signal generated from each current control signal is compared, by a logical-AND gate, with a logic signal, generated by a processor  510 , indicating whether the rotor of the machine is within a particular range of angular positions. 
     Ideal current and induced emf waveforms have been considered in the two control techniques disclosed herein. In practice, currents and induced emfs deviate from their ideal shapes. Consideration has to be built into control techniques to cope with practical realities. For example, in current control, a finite time is allowed in the rise and fall of the currents. Accordingly, to fully harvest electromagnetic torque and power of a machine, current references are advanced in their onsets and turned off in advance of their ideal turn-off times. Advancing turn-on and turn-off for current control in practical implementations of the control techniques and power converters can make dwell periods (or conduction intervals) of phase currents greater than that of the ideal intervals. The turn-on and turn-off instances for the transistors can be programmed or compensated in the controller depending on the machine-phase inductances and current reference magnitudes, so that the torque output is smooth and equal to the limit of the desired torque magnitude. Compensation methods are described, for example, in R. Krishnan, “Switched reluctance motor drives”, CRC Press, 2001, the content of which is incorporated herein by reference. 
     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, drawings, 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.