Abstract:
A variable magnetization machine control system comprising a controller configured to adjust a d-axis current waveform and a q-axis current waveform in accordance with an operating condition of a variable magnetization machine to generate an adjusted d-axis current waveform and an adjusted q-axis current waveform that provide a driving voltage to drive the variable magnetization machine at a predetermined speed while maintaining the driving voltage below a predetermined maximum magnitude.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    Related subject matter is disclosed in International Application No. PCT/US2013/048562, filed on Jun. 28, 2013, and an International Application entitled “Variable Magnetization Machine Controller,” (Attorney Docket No. NS-WO145202), filed concurrently herewith, the entire contents both of these International Applications being incorporated by reference herein. 
     
    
     BACKGROUND 
     Field of the Invention 
       [0002]    The present invention generally relates to a variable magnetization machine controller. More particularly, the present invention relates to a controller that is able to reduce the voltage induced by a pulse current to control a variable magnetization machine, such as an electric motor or other type of variable flux machine, that is employed in an electric or hybrid electric vehicle, at the reduced voltage. 
       Background Information 
       [0003]    Electric vehicles and hybrid electric vehicles (HEV) include an electric motor that operates as a drive source for the vehicle. In a purely electric vehicle, the electric motor operates as the sole drive source. On the other hand, an HEV includes an electric motor and a conventional combustion engine that operate as the drive sources for the vehicle based on conditions as understood in the art. 
         [0004]    Electric vehicles and HEVs can employ an electric motor having variable magnetization characteristics as understood in the art. For example, the magnetization level of the motor can be increased to increase the torque generated by the motor. Accordingly, when the driver attempts to accelerate the vehicle to, for example, pass another vehicle, the motor control system can change the magnetization level to increase the torque output of the motor and thus increase the vehicle speed. 
         [0005]    In a typical motor control system, an inverter applies the control voltage to the motor. As understood in the art, as the speed of the motor is increased, the amplitude of the current pulse, such as the D-axis current pulse, will increase. Naturally, this current pulse will affect the voltage induced in the control system. 
       SUMMARY 
       [0006]    However, at a high motor speed, the voltage induced in the control system by the pulse current can increase to a high voltage level. At this high voltage level, the inverter may be no longer capable of providing enough voltage to drive the motor at the desired speed. Accordingly, it is desirable to provide an improved motor control system for a variable magnetization machine, such as a variable magnetization motor or other type of variable flux machine for a vehicle, that is capable of reducing the voltage induced by a pulse current to a low enough level so that the inverter can provide a sufficient voltage to drive the variable magnetization machine even at a high speed. 
         [0007]    In view of the state of the known technology, one aspect of a variable magnetization machine control system according to the disclosed embodiments comprises a controller configured to adjust a d-axis current waveform and a q-axis current waveform in accordance with an operating condition of a variable magnetization machine to generate an adjusted d-axis current waveform and an adjusted q-axis current waveform that provide a driving voltage to drive the variable magnetization machine at a predetermined speed while maintaining the driving voltage below a predetermined maximum magnitude. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]    Referring now to the attached drawings which form a part of this original disclosure: 
           [0009]      FIG. 1  is a partial cross-sectional schematic view of a variable magnetization machine according to a disclosed embodiment; 
           [0010]      FIGS. 2 through 4  are diagrammatic views illustrating an example of components, including a controller according to the disclosed embodiments, that are employed in a vehicle to control a variable magnetization machine such as that shown in  FIG. 1 ; 
           [0011]      FIGS. 5 and 6  are graphs which illustrate an example of the relationship between the magnetization state (M/S) of the variable magnetization machine and the d-axis current pulse that are applied to the variable magnetization machine by the configuration shown in  FIGS. 2 through 4  during a magnetization process and a demagnetization process; 
           [0012]      FIG. 7  is a block diagram illustrating an example of components of a controller employed in the configuration shown in  FIGS. 2 through 4  according to a disclosed embodiment; 
           [0013]      FIG. 8  is a graph illustrating an example of d-axis and q-axis currents provided by the controller to operate the variable magnetization machine at a low speed according to a disclosed embodiment; 
           [0014]      FIG. 9  is a graph illustrating an example of d-axis and q-axis currents provided by the controller to operate the variable magnetization machine at a high speed according to a disclosed embodiment; 
           [0015]      FIG. 10  is a graph illustrating an example of the d-axis and q-axis currents, and d-axis and q-axis voltages, provided by the controller to operate the variable magnetization machine, in relation to the magnetization state and torque of the variable magnetization machine, a combination of the d-axis and q-axis voltages, and a maximum voltage according to a disclosed embodiment; 
           [0016]      FIG. 11  is a graph illustrating an example of d-axis and q-axis currents provided by the controller to operate the variable magnetization machine at a low voltage according to a disclosed embodiment; 
           [0017]      FIG. 12  is a graph illustrating an example of d-axis and q-axis currents provided by the controller to operate the variable magnetization machine at a high voltage according to a disclosed embodiment; 
           [0018]      FIG. 13  is a graph illustrating an example of d-axis and q-axis currents having a smoother pulse tip as provided by the controller to operate the variable magnetization machine according to a disclosed embodiment; 
           [0019]      FIG. 14  is a block diagram illustrating an example of components of a controller employed in the configuration shown in  FIGS. 2 through 4  including a low pass filter arrangement according to another disclosed embodiment; 
           [0020]      FIG. 15  is a graph illustrating an example of d-axis and q-axis currents having a smoother pulse shape as provided by the controller shown in  FIG. 14  to operate the variable magnetization machine according to a disclosed embodiment; 
           [0021]      FIG. 16  is a graph illustrating an example of d-axis and q-axis currents having a sinusoidal waveform as provided by the controller as discussed herein to operate the variable magnetization machine according to a disclosed embodiment; 
           [0022]      FIG. 17  is a graph illustrating an example of d-axis and q-axis currents provided by the controller to operate the variable magnetization machine at a low speed according to another disclosed embodiment; 
           [0023]      FIG. 18  is a graph illustrating an example of d-axis and q-axis currents provided by the controller to operate the variable magnetization machine at a high speed according to another disclosed embodiment; 
           [0024]      FIG. 19  is a graph illustrating an example of d-axis and q-axis currents provided by the controller to operate the variable magnetization machine at a low voltage according to another disclosed embodiment; 
           [0025]      FIG. 20  is a graph illustrating an example of d-axis and q-axis currents provided by the controller to operate the variable magnetization machine at a high voltage according to another disclosed embodiment; 
           [0026]      FIG. 21  is a graph illustrating an example of d-axis and q-axis currents provided by the controller to operate the variable magnetization machine according to a further disclosed embodiment; and 
           [0027]      FIG. 22  is a block diagram illustrating an example of components of a controller employed in the configuration shown in  FIGS. 2 through 4  arrangement according to a further disclosed embodiment. 
       
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
       [0028]    Selected embodiments will now be explained with reference to the drawings. It will be apparent to those skilled in the art from this disclosure that the following descriptions of the embodiments are provided for illustration only and not for the purpose of limiting the invention as defined by the appended claims and their equivalents. 
         [0029]    As shown in  FIG. 1 , a variable magnetization machine  10 , which can also be referred to as a variable magnetization motor or other type of variable flux machine, includes a rotor  12  and a stator  14 . As discussed herein, the terms variable magnetization machine and variable flux machine can be used synonymously to refer to the same type of machine. The variable magnetization machine  10  can be employed in any type of electric vehicle or HEV such as an automobile, truck, SUV and so on, and in any other type of apparatus as understood in the art. The rotor  12  and the stator  14  can be made of metal or any other suitable material as understood in the art. 
         [0030]    In this example, the rotor  12  is configured to include a plurality of pairs of flux barriers  16  and  18 , which can be configured as air gaps or can include any suitable type of insulating material as is conventional in the art. Although only one full pair and two partial pairs of the flux barriers  16  and  18  are shown, in this example, six pairs of flux barriers  16  and  18  can be spaced at 60 degree angles about the outer perimeter of the rotor  12 . Naturally, the rotor  12  can include as many pairs of flux barriers  16  and  18  as deemed appropriate for the environment in which the variable magnetization machine  10  is employed. Also, as shown in this example, a q-axis of the motor passes through the center of a pair of flux barriers  16  and  18 . However, the pairs of flux barriers  16  and  18  can be positioned at any suitable location with respect to the q-axis to achieve the operability of the embodiments discussed herein. 
         [0031]    As further shown, a surface bridge  20  of the rotor  12  is present between the radially outward boundary of each flux barrier  18  and the outer circumference  22  of the rotor  12 . Furthermore, a d-axis flux bypass  24  is present between each of the adjacent pairs of flux barriers  16  and  18 . In this example, the surface bridges  20  and d-axis flux bypasses are made of the same material as the rotor  12 . However, the surface bridges  20  and d-axis bypasses  24  can be made of any suitable type of material as known in the art. 
         [0032]    In addition, a plurality of low-coercive-force magnets  26  are spaced between adjacent pairs of flux barriers  16  and  18  about the circumference of the rotor  12 . As indicated, each of these magnets  26  extend longitudinally in a perpendicular or substantially perpendicular direction with respect to portions of adjacent flux barriers  16 . However, the magnets  26  can be configured in any suitable size and shape. Also, in this example, the rotor  12  includes 6 magnets  26  which are positioned between the 6 pairs of flux barriers  16  and  18  and spaced at 60 degree intervals in a circumferential direction about the rotor  12 . However, the number of magnets  26  can change with respect to a change in the number of pairs of flux barriers  16  and  18 . Furthermore, each magnet  26  can be configured as a plurality of magnets. In this example, a d-axis passes through a center of a magnet  26 . However, the magnets  26  can be positioned at any suitable location with respect to the d-axis to achieve the operability of the embodiments discussed herein. 
         [0033]    The stator  14  includes a plurality of stator teeth  28  and other components such as windings (not shown) which can be configured in any conventional manner. In this example, the stator teeth  28  are configured as wide stator teeth as known in the art. However, the stator teeth  28  can have any suitable size, and the stator  14  can include any number of stator teeth  28  to achieve the operability of the embodiments discussed herein. In this example, the stator teeth  28  are open to the inner circumference  30  of the stator  14 , but can be closed if desired. Also, an air gap  32  is present between the outer circumference  22  of the rotor  12  and the inner circumference  30  of the stator to enable the rotor  12  to rotate unrestrictedly or substantially unrestrictedly about an axis  34 . 
         [0034]      FIGS. 2 through 4  are diagrammatic views illustrating an example of the manner in which a controller  100  ( FIG. 4 ) according to the disclosed embodiments is employed in a vehicle  102  to control the variable magnetization machine  10 . The vehicle  102  can be an electric vehicle or HEV such as an automobile, truck, SUV or any other suitable type of vehicle. As understood in the art, when a driver presses the accelerator  104 , an acceleration signal is input to a controller  106 , such as an electronic control unit (ECU) or any other suitable type of controller. Also, a speed sensor  108 , such as a tachometer or any other suitable type of sensor, senses the rotational speed of, for example, a drive wheel  110  of the vehicle  102  and provides a vehicle speed signal to the controller  106 . 
         [0035]    The controller  106  includes other conventional components such as an input interface circuit, an output interface circuit, and storage devices such as a ROM (Read Only Memory) device and a RAM (Random Access Memory) device. It will be apparent to those skilled in the art from this disclosure that the precise structure and algorithms for the controller  106  can be any combination of hardware and software that will carry out the functions of the present invention. In other words, “means plus function” clauses as utilized in the specification and claims should include any structure or hardware and/or algorithm or software that can be utilized to carry out the function of the “means plus function” clause. Furthermore, the controller  106  can communicate with the accelerator  104 , the speed sensor  108  and the other components in the vehicle  102  discussed herein in any suitable manner as understood in the art. In addition, the components of the controller  106  need not be individual or separate components, and one component or module can perform the operations of multiple components or modules discussed herein. Also, each component can include a microcontroller as discussed above or multiple components can share one or more microcontrollers. 
         [0036]    As further shown in  FIG. 2 , the controller  106  outputs signals to control the speed and the torque of the variable magnetization machine  10  to reach the appropriate machine operating state to achieve the desired vehicle acceleration as understood in the art. For instance, the controller  106  can access an appropriate loss map from among a plurality of previously prepared loss maps that can be stored in a memory  112 . Each loss map can indicate respective loss characteristics for a respective magnetization state (M/S) as indicated. The controller  106  can then, for example, generate a loss plot which represents an amount of loss for each respective M/S and derive a minimal loss point as indicated. The controller  106  can therefore output a signal to control the variable magnetization machine  10  to achieve that ideal M/S. 
         [0037]    As shown in  FIG. 3 , the signal representing the ideal M/S is input to an M/S selection strategy module  113  which performs hysteresis control and, as discussed in more detail below, outputs a signal representing the actual M/S signal (also referred to as a target magnetization state signal M* as discussed with below with regard to  FIG. 7 ) and an M/S change flag signal Q. As shown in  FIG. 4 , the controller  100 , which can be an M/S and torque controller, receives the signal representing the actual M/S signal and the M/S change flag signal Q, and outputs an M/S and torque control signal, such as a pulse width modulated (PWM) signal, to control the variable magnetization machine  10 . That is, the controller  100  is coupled to an e-powertrain which includes, for example, a battery  116 , an inverter arrangement  118 , and the variable magnetization machine  10 . In this example, inverter arrangement  118  can be, for example, a pulse width modulator (PWM) voltage inverter, or any other suitable type of inverter configuration as understood in the art. 
         [0038]    As further shown in  FIG. 3 , the M/S selection strategy module  113  includes a sample and hold circuit  120  that includes a switch  122  and a z-transform component  124 . The M/S selection strategy module  113  further includes a subtractor  126 , a proportional-integral (PI) compensator  128 , an absolute value circuit  130 , a comparator  132  and a comparator input component  134 . 
         [0039]    The ideal M/S signal is input to the switch  122  of the sample and hold circuit  120  and the subtractor  126 . The subtractor  126  subtracts a feedback signal from the ideal M/S signal and outputs and error signal to the PI compensator  128 . As understood in the art, the PI compensator  128  removes a steady state error from the error signal and provides the error signal with the steady state error removed to the absolute value circuit  130  as a modified error signal. The absolute value circuit  130  outputs an absolute value of the modified error signal to the comparator  132 . The comparator  132  also receives an input signal from the comparator input component  134 . In this example, the input signal represents a value “1” but can be set to any suitable value to achieve the effects discussed herein. 
         [0040]    The comparator  132  provides an output based on the modified error signal and the input signal to control switching of the switch  122  of the sample and hold circuit  120 . The comparator  132  also provides the output as a reset signal to the PI compensator  128  as understood in the art. The comparator  132  further provides the output as an M/S change flag signal Q to the M/S changing current trajectory control module  114  as discussed in more detail below. 
         [0041]    As further shown, the z-transform component  124  provides a feedback of the actual M/S signal output by the sample and hold circuit  120  as a second input to the switch  122 . The switch  122  outputs either the ideal MIS signal or the feedback signal from the z-transform component  124  as the actual M/S signal based on the state of the output signal provided by the comparator  132 . Therefore, the components of the M/S selection strategy module  113  discussed above operate as a hysteresis control component that is configured to receive an ideal magnetization state signal, output an actual magnetization signal based on the ideal magnetization state signal for control of a variable magnetization machine, and modify the actual magnetization state signal in accordance with an error value between the ideal magnetization state signal and the actual magnetization state signal. That is, when the error value results in the comparator  132  outputting a signal having a value that controls the switch  122  to output the modified signal from the z-transform component  124  as the actual M/S signal, the controller  100  in effect modifies the actual M/S signal in accordance with an error value between the ideal magnetization state signal and the actual magnetization state signal. Thus, the sample and hold circuit  120  (sample and hold component) that is configured to output the actual magnetization state signal and to modify the actual magnetization state signal in accordance with the error value. The M/S selection strategy module  113  configured to operate as the hysteresis control component is further configured to output the M/S change flag signal as a pulse signal in synchronization with the actual M/S signal such that the variable magnetization machine  10  is further controlled in accordance with this pulse signal. 
         [0042]      FIGS. 5 and 6  are graphs which illustrate an example of the relationship between the M/S and the d-axis current pulse that the controller  100 , along with the battery  116  and the inverter arrangement  118 , applies to the variable magnetization machine  10  during a magnetization process ( FIG. 5 ) and a demagnetization process ( FIG. 6 ). An example of components of the controller  100  will now be described with regard to  FIG. 7 . As will be appreciated from the description of this embodiment and the other embodiments set forth herein, the q-axis current is reduced online using feedback, and by adding a regulated amount to the q-axis current, the torque of the variable magnetization machine  10  can be maintained constant or substantially constant. 
         [0043]    As shown in  FIG. 7 , the controller  100  in this example includes a total loss minimizing current vector command module  200 , a current regulator  202 , a rotary frame/stationary frame component  204 , and a stationary frame/rotary frame component  206 . In this example, the output of the rotary frame/stationary frame component  204  is coupled to the e-powertrain and, in particular, to the inverter arrangement  118  which provides power to the variable magnetization machine  10 . 
         [0044]    As can be appreciated by one skilled in the art, the controller  100  preferably includes at least one microcomputer with a control program that controls the components of the controller  100  as discussed below. Thus, the microcomputer or microcomputers can be configured and program to embody any or all of the total loss minimizing current vector command module  200 , the current regulator  202 , the rotary frame/stationary frame component  204 , and the stationary frame/rotary frame component  206  The controller  100  includes other conventional components such as an input interface circuit, an output interface circuit, and storage devices such as a ROM (Read Only Memory) device and a RAM (Random Access Memory) device. It will be apparent to those skilled in the art from this disclosure that the precise structure and algorithms for the controller  100  can be any combination of hardware and software that will carry out the functions of the present invention. In other words, “means plus function” clauses as utilized in the specification and claims should include any structure or hardware and/or algorithm or software that can be utilized to carry out the function of the “means plus function” clause. Furthermore, the controller  100  can communicate with the variable magnetization machine  10  in any suitable manner as understood in the art. In addition, although several of the components of the controller  100  are described as modules, these components need not be individual or separate components, and one component or module can perform the operations of multiple components or modules discussed herein. Also, each module can include a microcontroller as discussed above or multiple modules can share one or more microcontrollers. 
         [0045]    As further shown in  FIG. 7 , the total loss minimizing current vector command module  200  receives a torque command rem and a sensed or estimated rotation signal ω of the rotor  12  from, for example, a controller (not shown) in response to, for example, a driver of the vehicle attempting to accelerate the vehicle  102 . In response, the total loss minimizing current vector command module  200  outputs a d-axis current signal i* ds  and a q-axis current signal i* qs  for selecting the optimum d-axis current i d  and the optimum q-axis current i q . That is, in this example, the total loss minimizing current vector command module  200  outputs the d-axis current signal i* ds  to an adder  208  and the q-axis current signal i* qs  to an adder  210 . 
         [0046]    The M/S changing current pulse control module  114  receives the sensed or estimated rotational speed signal ω, as well as magnetization signal M* (also referred to as the actual M/S signal as discussed above with regard to  FIG. 3 ), magnetization signal M̂ and the M/S change flag signal Q. An adder  208  adds output provided by the M/S changing current pulse control module  104  to the d-axis current signal i* ds , and an adder  210  adds the output provided by the M/S changing current pulse control module  200  to q-axis current signal i* qs . The adders  208  and  210  provide their outputs to the current regulator  202 , which provides d-axis current voltage signal V r * ds  and q-axis current voltage signal V r * qs  to the rotary frame/stationary frame component  206 . In this example, the rotary frame/stationary frame component  206  provides the voltage signals to the inverter arrangement  118 , which provides voltages V a , V b  and V c  to the three poles of the variable magnetization machine  10 . 
         [0047]    As further shown in  FIG. 7 , current sensors  212  sense the currents associated with V a , V b  and V c  being applied to the variable magnetization machine  10 . The current sensors  212  provide the sensed current signals I a , I b  and I c  to the stationary frame/rotary frame component  206 . The stationary frame/rotary frame component  206  thus provides a detected d-axis current signal i r   ds  and a detected q-axis current signal i r   qs  to the current regulator  202 . As understood in the art, the current regulator  202  regulates the d-axis current voltage signal V r * ds  and q-axis current voltage signal V r * qs  based on the d-axis current signal i r   ds  and the detected q-axis current signal i r   qs  that are fed back from the stationary frame/rotor frame component  206 . 
         [0048]    Examples of operations performed by the controller  100  for reducing the voltage induced by a pulse current to a low enough level so that the inverter arrangement  118  can provide a sufficient voltage to drive the variable magnetization machine  10  even at a high speed will now be described. 
         [0049]    As understood in the art, the v-axis voltage V D  and the q-axis voltage V Q  of the variable magnetization machine  10  can be defined by the following matrix: 
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         [0050]    The controller  100  can therefore reduce the v-axis voltage V D  and the q-axis voltage V Q  by reducing the values of certain terms in the matrix. In this example, the controller  100  reduces the values of the terms sL D  and sφ PM  by reducing the rate of increase (e.g., the ramp or slope) of the d-axis current i d  waveform. That is, as shown in  FIG. 8 , at a low speed of the variable magnetization machine  10 , the pulse of the d-axis current i d  can be relatively short. In other words, the ramp of the current id can be relatively steep because the maximum value of the d-axis current i d  is a relatively low value. Likewise, the maximum value of the q-axis current i q  is also a relatively low value. 
         [0051]    However, at a high speed of the variable magnetization machine  10 , the terms −ωL Q , ωL D  and ωφ PM  can increase to values much higher than those during the low speed operation. To deal with this drastic increase in the values, the controller  100  reduces the rate of increase of the d-axis current i d  as shown in  FIG. 9 . For example, the current regulator  202  controls the d-axis current i d , the q-axis current i q , or both, to insure that the voltages v q  and v d  do not exceed certain prescribed values as shown in  FIG. 10 . Therefore, as further shown in  FIG. 10 , the absolute value of the combined voltage |v dq | of the d-axis voltage v d  and the q-axis voltage v q  does not exceed the maximum combined voltage Max |v dq |. As a result, the inverter arrangement  118  can still provide a sufficient voltage to drive the variable magnetization machine  10  even at a high speed. This also allows the variable magnetization machine  10  to have more degrees of freedom balance between less voltage, less torque ripple and less loss. 
         [0052]    As discussed above,  FIGS. 8 through 10  illustrate examples of waveforms representing the d-axis current i d  and the q-axis current i q  provided by the controller  100  to control the variable magnetization machine  10  at low and high speeds.  FIGS. 11 and 12  illustrate examples of waveforms representing the d-axis current id and the q-axis current i q  provided by the controller  100  to control the variable magnetization machine  10  at low and high voltages. As can be appreciated from  FIGS. 8, 9, 11 and 12 , the relationships between the d-axis current i d  and the q-axis current i q  and their respective pulse lengths are similar for a low voltage that the controller  100  provides to the variable magnetization machine  10  to operate at a low speed ( FIGS. 8 and 11 ) and a high voltage that the controller  100  provides to the variable magnetization machine  10  to operate at a high speed ( FIGS. 9 and 12 ). Thus, the controller  100  prevents the absolute value of the combined voltage |v dq | of the d-axis voltage v d  and the q-axis voltage v q  from exceeding the maximum combined voltage Max |v dq | even when the controller  100  provides a high voltage to the variable magnetization machine  10  to cause the variable magnetization machine  10  to operate at a high speed. 
         [0053]    In addition, the controller  100  can control the rising and falling of the d-axis current i d  and the q-axis current i q  so that their waveforms have a smoother pulse tip as shown in the graphs of  FIG. 13 . When current is large, the terms −ωL Q  and ωL D  are large, and thus the ramp rate is reduced only when the current is large. Furthermore, as shown in  FIG. 14 , a low pass filter (LPF) arrangement  214  including one or more low pass filters can be provided at the outputs of the magnetization current pulse control module  114  so that the output of the magnetization current pulse control module  114  is filtered before being received by the adders  208  and  210 . With the inclusion of the LFP arrangement  214 , the controller  100  can provide the d-axis current i d  and the q-axis current i q  with waveforms have a smoother pulse shape as shown in the graphs of  FIG. 15 . 
         [0054]    In addition, the controller  100  can be configured to provide the d-axis current i d  and the q-axis current i q  with any type of waveforms that are suitable for achieving the advantages discussed herein, namely, limiting the combined voltage |v dq | of the d-axis voltage v d  and the q-axis voltage v q  from exceeding the maximum combined voltage Max |v dq | even when the controller  100  provides a high voltage to the variable magnetization machine  10  to cause the variable magnetization machine  10  to operate at a high speed. For instance, the controller  100  can be configured to provide the d-axis current i d  and the q-axis current i q  with sinusoidal waveforms as shown in the graphs of  FIG. 16 . 
         [0055]    As shown in the graphs of  FIGS. 17 and 18 , the controller  100  can reduce the value of the q-axis current i q  before increasing the d-axis current i d  to achieve high speed/high torque operation of the variable magnetization machine  10 . By doing this, the q-axis voltage v q  is reduced during the high speed/high torque operation and thus, the combined voltage |v dq | of the d-axis voltage v d  and the q-axis voltage v q  does not exceed the maximum combined voltage Max |v dq | even during this high speed/high torque operation. The controller  100  can also reduce the value of the q-axis current i q  before increasing the d-axis current i d  during the high voltage operation of the variable magnetization machine  10  as shown, for example, in the graphs of  FIGS. 19 and 20  to achieve this same effect of limiting the combined voltage |v dq | of the d-axis voltage v d  and the q-axis voltage v q  from exceeding the maximum combined voltage Max |v dq | during this high voltage operation. The controller  100  can even reduce the value of the q-axis current i q  to zero or substantially zero as shown in the graph of  FIG. 21  if necessary to insure that the combined voltage |v dq | of the d-axis voltage v d  and the q-axis voltage v q  does not exceed the maximum combined voltage Max |v dq |. 
         [0056]    As shown in  FIG. 22 , the controller  100  can include an ordinary control module  220  instead of the total loss minimizing current vector command module  200  as discussed above. The controller  100  can further be configured to include a reducing current control module  222 , a torque calculator  224 , a stator flux linkage observer  226  and an Iq current trajectory selector  228 . The stator flux linkage observer  226 , which can also be referred to as a stator flux linkage estimator, can be configured to estimate the stator flux linkage by adding a compensation value that is obtained from an L(Y-Yh) reference in a Luenburger style observer for machine electrical state variables associated with the variable magnetization machine  10 . This can provide more accurate torque estimation, and reduce pulsating torque. In this example, the stator flux linkage observer  226  receives the d-axis current signal i r   ds  and the detected q-axis current signal i r   qs  and provides estimated stator flux linkage signals λ r   ds  and λ r   qs  to the torque calculator  224 . The torque calculator  224  calculates the value of a sensed or estimated torque T̂ based on the estimated stator flux linkage signals λ r   ds  and λ r   qs , the detected d-axis current signal i r   ds  and the detected q-axis current signal i r   qs  that are fed back from the stationary frame/rotor frame component  206 . 
         [0057]    The ordinary control module  220  and the reducing current control module  222  receive a torque command T* em  from, for example, a controller (not shown) in response to, for example, a driver of the vehicle attempting to accelerate the vehicle. In response, the ordinary control module  220  outputs a d-axis current signal i* ds  and a q-axis current signal i* qs  for selecting the optimum d-axis current i d  and the optimum q-axis current i q . Thus, the ordinary control module  220 , which can also be referred to as a current command module, computes a vector current command in a dq axis based on a torque command T* em . The reducing current module  222  in this example computes the reducing current based on a difference between the torque command T* em  and the estimated torque T̂ provided by the torque calculator  224 . As further shown, the reducing current control module  222  provides the reducing current to the Iq current trajectory selector  228 . Thus, the controller  100  can control the Iq current trajectory selector  228  to provide the output from the M/S changing current pulse control module  114  to the adder  208 , the reducing current from the reducing current control module  222  to the adder  208 , or a value of 0 to the adder  208  as indicated to adjust the value of the q-axis current signal i* qs  as desired to attain the type of current waveforms discussed herein. The Iq current trajectory selector  228  selects which input signal to be output according to the M/S change flag signal Q and the rotational speed signal ω indicating the rotational speed of the variable magnetization machine  10 . When the M/S change flag signal Q is low, that is, not during an M/S change period, the Iq current trajectory selector  228  outputs a zero since no additional q-axis current i q  is required for the variable magnetization machine  10 . When the M/S change flag signal Q is high and the rotational speed signal ω indicates that the rotational speed of the variable magnetization machine  10  is low, the Iq current trajectory selector  228  outputs the output that is provided by the reducing current control module  222 . When the M/S change flag signal Q is high and the rotational speed signal ω indicates that the rotational speed of the variable magnetization machine  10  is high, the Iq current trajectory selector  228  outputs the output that is provided by the M/S changing current trajectory control module  114  to reduce the voltage output by the current regulator  202 . Further details of the ordinary control module  220 , the reducing current control module  222 , the torque calculator  224  and the iq current trajectory selector  228  are disclosed in International Application No. PCT/US2013/048562 referenced above. 
         [0058]    As can be appreciated from the above, the embodiments of the controller  100  described herein that is capable of reducing the voltage induced by a pulse current to a low enough level so that the inverter can provide a sufficient voltage to drive the variable magnetization machine even at a high speed. 
       General Interpretation of Terms 
       [0059]    In understanding the scope of the present invention, the term “comprising” and its derivatives, as used herein, are intended to be open ended terms that specify the presence of the stated features, elements, components, groups, integers, and/or steps, but do not exclude the presence of other unstated features, elements, components, groups, integers and/or steps. The foregoing also applies to words having similar meanings such as the terms, “including”, “having” and their derivatives. Also, the terms “part,” “section,” “portion,” “member” or “element” when used in the singular can have the dual meaning of a single part or a plurality of parts. The terms of degree such as “substantially”, “about” and “approximately” as used herein mean a reasonable amount of deviation of the modified term such that the end result is not significantly changed. 
         [0060]    While only selected embodiments have been chosen to illustrate the present invention, it will be apparent to those skilled in the art from this disclosure that various changes and modifications can be made herein without departing from the scope of the invention as defined in the appended claims. For example, the size, shape, location or orientation of the various components can be changed as needed and/or desired. Components that are shown directly connected or contacting each other can have intermediate structures disposed between them. The functions of one element can be performed by two, and vice versa. The structures and functions of one embodiment can be adopted in another embodiment. It is not necessary for all advantages to be present in a particular embodiment at the same time. Every feature which is unique from the prior art, alone or in combination with other features, also should be considered a separate description of further inventions by the applicant, including the structural and/or functional concepts embodied by such features. Thus, the foregoing descriptions of the embodiments according to the present invention are provided for illustration only, and not for the purpose of limiting the invention as defined by the appended claims and their equivalents.