Abstract:
A method and apparatus of determining the position of a rotor at standstill relative to a stator in a synchronous motor elevator machine includes injecting an AC current having a predetermined single frequency and a predetermined initial phase angle into a stator coil of the stator, and sampling the injected current and resultant voltage a predetermined number of times per period of the frequency. Subsequently the method calculates a stator inductance from the sampled voltages and currents using a DFT. By incrementing the initial phase angel a predetermined number of times over a 360 degree cycle, and repeating the injecting, sampling, and calculating with each incremented phase angle, the algorithm provides a predetermined number of calculated stator inductances. The position of the d axis relative to the stator is then determined from the minimum of the calculated stator inductances.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to angular position methods and devices for motors. More specifically, the present invention relates to a technique and apparatus to calculate the absolute angular position of a synchronous motor elevator machine at standstill by detecting stator iron saturation. 
     BACKGROUND OF THE INVENTION 
     Permanent magnet synchronous machines for elevator systems offer advantages over conventional induction elevator machines in the size required for a given duty. However, elevator systems utilizing synchronous motor elevator machines must be capable of detecting absolute angular rotor position, i.e., rotor magnetic flux d axis position and direction, relative to the stator pole windings to be able to achieve maximum torque. 
     This is particularly significant when the rotor position is lost due to circumstances such as a power failure. When an elevator experiences a power loss, the elevator brake is engaged to hold the elevator car in position. Once power is reestablished, torque to the elevator machine must be available and controlled when the machine brake is lifted to ensure controlled motion of the elevator car under unbalanced load conditions. 
     An incremental encoder with one index pulse has been used to establish absolute rotor position on prior art elevator machines. However, this may require up to one full revolution of the elevator machine to locate the index pulse after power loss. In larger elevator systems, one revolution of the elevator machine may result in as much as a one meter drop in the elevator car. 
     A technique that makes use of the saturation effect of the stator iron to detect the rotor position of a permanent magnet synchronous motor is disclosed in an article titled “Initial Rotor Angle Detection Of A Non-Salient Pole Permanent Magnet Synchronous Machine”, published in the Conference Records of the IEEE-Industry Applications Society Annual Meeting, New Orleans, La. Oct. 5-9, 1997 (the article). The article describes a method whereby a broad frequency band voltage pulse, of appropriate magnitude and width, is applied to each phase winding of the stator. A single sample of stator peak current is then measured in the time domain for each winding and used to calculate inductance. Since the inductance will vary with the partial saturation of the stator iron and the flux due to the position of the rotor&#39;s magnets, the algorithm can discern between a north pole and a south pole, and subsequently, the absolute position of the rotor. 
     However, this technique has inherent sampling issues in a noisy environment, such as an elevator system, that limits the repeatability of the results. This is because the voltage pulse generated is inherently composed of a broad band of frequencies. Therefore, any noise within the frequency band of the voltage pulse, e.g., the switching rate of the transistors in the elevator&#39;s Alternating Current Variable Frequency drive, or any harmonics thereof, effects the accuracy of the readings. Also, with this technique, rotor position is calculated from a single inductance measurement. Therefore one bad sample due to noise can dramatically impact the inductance calculation. The irregular curves of the experimental results shown in the article&#39;s FIG. 6 demonstrate the inherent errors in the inductance measurements, since the expected curves should be smooth sinusiods. 
     Additionally, in order to obtain an appropriate signal/noise ratio using this technique, significantly large magnitude voltage pulses and peak currents, e.g., at or near the rated current values of the motor, are required. This imposes an undesirable amount of torque on the braking system. In order to compensate for the torque, immediately following the voltage pulse for one phase a voltage pulse in the opposite direction is fired to force the phase currents back to zero. This drives the free wheeling current to zero and helps to minimizes the time torque is applied to the motor. 
     There is therefore a need for an improved method of detecting absolute angular rotor position relative to the stator windings for a synchronous motor. 
     In another embodiment of the invention a DC offset current is injected with the AC current into the stator windings. The direction of the d axis is then determined from the minimum of the calculated stator inductances. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic perspective view of an elevator system having a synchronous motor elevator machine in accordance with the present invention; 
     FIG. 2 is a diagrammatic representation of the synchronous motor of FIG. 1 with magnetic flux along the positive d axis locked in alignment and linked with the phase A stator winding; 
     FIG. 3 is a diagrammatic representation of the synchronous motor of FIG. 1 with the magnetic flux along the q axis locked in alignment and linked with the phase A stator winding; 
     FIG. 4 is a diagrammatic representation of the synchronous motor of FIG. 1 with the magnetic flux along the negative d axis locked in alignment and linked with the phase A stator winding; 
     FIG. 5 is a plot of the variation of the stator inductance as a function of the electrical phase angle of the rotor with no DC offset current applied to the stator windings in accordance with the present invention; 
     FIG. 6 is a plot of the variation of the stator inductance as a function of the electrical phase angle of the rotor with a DC offset current applied to the stator windings in accordance with the present invention; 
     FIG. 7 is a flow diagram of an algorithm for determining absolute angular rotor position (rotor d axis) relative to the stator in accordance with the present invention; 
     FIG. 8 is a diagrammatic representation of another embodiment of a synchronous motor with the magnets embedded in the rotor iron in accordance with the present invention; 
     FIG. 9 is a diagrammatic representation of another embodiment of a synchronous motor with the magnets centered on the q axis rather than the d axis in accordance with the present invention; and 
     FIG. 10 is a diagrammatic representation of another embodiment of a synchronous motor with a salient pole electrically wound rotor in accordance with the present invention. 
    
    
     SUMMARY OF THE INVENTION 
     The present invention offers advantages and alternatives over the prior art by providing a method of determining the absolute angular position of a synchronous motor elevator machine after a power loss. An appropriate AC current at a predetermined frequency is injected into the stator windings of the motor in order to determine the stator inductance. The saturation of the stator back iron due to the magnetic flux caused by the permanent magnets enables the algorithm to determine the position and direction of the rotor magnetic flux d axis, and subsequently, the absolute position of the rotor relative to the stator of the synchronous motor. By using Fourier analysis to calculate the stator inductance, this method has a high degree of immunity to both repetitive and random noise often generated in such noisy environments as an elevator system. Additionally, the high signal to noise ratio of method allows the injected current to be relatively small compared to the rated currents of the motor, therefore imposing minimally low torque on the braking system of the elevator. 
     These and other advantages are accomplished in an exemplary embodiment of the invention by providing a method of determining the position of a rotor relative to a stator in a synchronous motor. The method comprises injecting an AC current having a predetermined single frequency and a predetermined initial phase angle into a stator coil of the stator, and sampling the injected current and resultant voltage a predetermined number of times per period of the frequency. Subsequently the method calculates a stator inductance from the sampled voltages and currents using a Discrete Fourier Transform (DFT). By incrementing the initial phase angel a predetermined number of times over a 360 degree cycle, and repeating the injecting, sampling, and calculating with each incremented phase angle, the algorithm provides a predetermined number of calculated stator inductances. The position of the d axis relative to the stator is then determined from the minimum of the calculated stator inductances. 
     In an alternate embodiment of the invention a DC offset current is injected into the stator windings after the stator inductances have been calculated. A single stator inductance is then recalculated, and the direction of the d axis is determined from the recalculated stator inductance. 
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to FIG. 1, an exemplary embodiment of an elevator system in accordance with the present invention is shown generally at  10 . The elevator system comprises an elevator hoistway  12 , having an elevator car  14  positioned therein for vertical movement. The elevator car  14  is suspended and coupled to a counterweight  16  for relative movement therewith through a set of elevator ropes  18 . Car guide rails  20  and counterweight guide rails  22  provide T-shaped tracks which guide the elevator car  14  and counterweight  16  respectively throughout the hoistway  12 . An elevator machine  24  driven by a 3 phase 4 pole permanent magnet synchronous motor  26  is located in elevator machine room  28  and provides the mechanical power to hoist the elevator car  14  and passengers. 
     An elevator control system  29  includes an Alternating Current Variable Frequency (ACVF) drive  30  and an elevator control unit  34 . The electrical power source for motor  26  is supplied by the ACVF drive  30  through transmission line  32 . The speed of the synchronous motor  26  is therefore controlled by varying the output frequency of the ACVF drive  30 . The elevator control unit  34  receives data through transmission line  36  indicative of elevator functions, e.g., elevator car load, speed and hoistway position, required to control the elevator car  14 . The control unit  34  processes the data and supplies control signals to the ACVF drive  30  through transmission line  38 . The ACVF drive  30  includes a solid state power source and associated control circuits, and may include memory circuits for storing an executable program for determining the position of a rotor relative to a stator in a synchronous motor. The control circuits of drive  30  are also able to accurately determine applied motor voltage and sensed motor current. An encoder (not shown) connected to the shaft of the motor  26  also transmits shaft position data to the ACVF drive  30  to enable the elevator control system  29  to precisely track the position of the rotor during normal operation. 
     As is typical for all synchronous motors, rotational speed or mechanical frequency (ω R ) of motor  26  is equal to, or is an integer submultiple of the frequency of the electrical power source (ω S ). The number of poles P (P=4 for motor  26 ) in a synchronous motor is related to the ratio of electrical frequency ω S  to mechanical frequency ω R  as follows: P=2*(ω S /ω R ). By way of example, in the 4 pole synchronous motor  26 , if the electrical frequency of the source is 60 cycles per second or 3600 cycles per minute, than the actual rotational speed of the motor will be 1800 revolutions per minute. Consequently, there is a distinction between electrical degrees and mechanical degrees for the synchronous motor. For the 4 pole synchronous motor  26  in this case, 90 electrical degrees represents only 45 mechanical degrees of rotation of the motor. 
     Referring to FIG. 2, the synchronous motor  26  includes a stator  40  having stator windings wired in 3 phases A, B, and C as is well known. Though the motor  26  is a 3 phase motor, only the single phase A stator winding  42  is shown for purposes of clarity. The direction of a DC current through the phase A winding  42  is represented by the point of an arrow  44  being indicative of current traveling out of the page toward the reader, and by the tail of an arrow  46  being indicative of current traveling into the page away from the reader. The stator also includes a stator back iron  48  around which the phase A stator winding  42  is wound. The stator back iron  48  has a high magnetic permeability which provides a conductive path for the magnetic flux  50  (represented by the dashed arrows) produced by the DC current in the phases of the coil windings  42 , i.e., the stator current flux. 
     The motor  26  also includes a rotor  52  mounted concentric and internal to the stator  40 , with an air gap  54  therebetween. The rotor includes an iron rotor core  56  having a pair of permanent magnet south poles  58  and a pair of permanent magnet north poles  60  mounted on its outer surface. The north pole magnets also produce a magnetic flux which conducts radially outward from the north poles as represented by the outwardly directed solid arrows  62 . The flux crosses the air gap  54 , is conducted through the stator back iron  48  and conducts radially inwardly toward the south poles  58  as represented by the inwardly directed solid arrows  64 . 
     Each north pole pair  60  has a positive direct axis (d axis)  66 , and each south pole pair  58  has a negative d axis  67 , which is aligned with the direction of the combined magnetic flux of each pole respectively, i.e., the magnetic phasors. Each pole pair  58  and  60  also has a quadrature axis (q axis)  68  which is aligned with the direction of the least amount of magnetic flux generated from the magnets. The q axis  68  may be defined as being exactly 90 electrical degrees for the d axis. In most, but not all, cases the d axis passes through the exact center of the magnets and the q axis passes through the spaces between the north and south pole magnets. 
     During normal operation, the ACVF drive  30  accurately tracks the position of the rotor  52  relative to the stator  40  by monitoring an index pulse generated from an encoder mounted on the motor shaft. However, after a power failure, the position of the rotor  52  is temporarily lost and the rotor  52 , along with its associated d axis  66 , can be locked by the elevator&#39;s  10  safety brakes anywhere within 360 degrees of its rotation. Since the maximum torque capability of the motor  26  varies with the position of rotor  52  relative to the stator  40 , it is important to be able to detect this position before the safety brakes are released. 
     Elevator motor  26  has a torque capability at zero speed which is typically at least twice that required to support the maximum load imbalance in order to accelerate hoistway inertias. At zero speed, peak torque capability for a given drive current limit is achieved with a torque angle of 90 electrical degrees between stator current flux  50  and the direction of maximum airgap flux, i.e., the d axes  66 ,  67 . In other words, with the stator current flux  50  aligned along the q axis  68  and the airgap magnetic pole flux  62  and  64  aligned along the d axes  67  and  66  respectively. Any deviation in position, θ, from the peak torque angle of 90 electrical degrees, degrades the peak torque capability by the factor of sin(90−θ)=cos(θ) where θ is the absolute position error in electrical degrees. Limiting θ to ±30 electrical degrees ensures that the peak torque capability will not fall below sin(60)=0.866, or a 13% reduction. 
     By varying the magnitude and/or phase angles of the phases A, B, and C in the 3 phase windings of the motor  26 , the ACVF drive  30  can align stator current and stator current flux anywhere within the 360 degrees of its cycle. Therefore knowing the absolute position error θ within ±30 electrical degrees would ensure that there would always be sufficient torque to allow controlled motion of the system to locate the index pulse after a power loss. Once the index pulse is located and power is restored, the precise position of the rotor can then be tracked by the control system  29 . 
     In FIG. 2, the rotor  52  is shown after a power failure with one of the north poles  60  locked in alignment with the phase A stator winding  42 . Since the positive d axis  66  of that pole is substantially aligned with the center of the winding  42 , the majority of the north pole flux  62  links to the winding  42  and conducts through the stator back iron  48  within the winding  42 . In this rotor  52  position the flux  62  from the north pole magnet  60  is large enough to magnetically saturate the stator back iron  48 . 
     Referring to FIG. 3, the rotor  52  is shown locked in a position where equal amounts of flux  62  from a north pole and flux  64  from a south pole link to the winding  42 . In this case the q axis  68  is substantially aligned with the center of the winding  42 . Consequently, the north pole flux  62  and the south pole flux  64  work to oppose each other and the stator back iron  48  is not saturated. 
     Referring to FIG. 4, the rotor  52  is shown locked in a position where one of the south poles  58  is aligned with the winding  42 . In this position, the negative d axis of that pole is substantially aligned with the center of the winding  42  and the majority of the south pole flux  64  links to the winding  42 . In this case the flux  64  from the south pole magnet  58  is large enough to magnetically saturate the stator back iron  48 . Since the inductance of the winding  42  is reduced when the stator iron  48  is saturated, the inductance of the coil is smallest in FIGS. 2 and 4 and the inductance is largest in FIG.  3 . 
     Referring to FIG. 5, a plot  70  of the variation of the stator  40  inductance as a function of the electrical angle of the rotor  52  may be obtained by injecting a small AC current into the stator windings and measuring the resultant voltage. In this case no DC stator current is applied, i.e., current indicating arrows  44  and  46  would be removed from FIGS. 2,  3 , and  4 , and therefore no stator current flux  50  is present. The relative rotor position of FIGS. 2 and 4 are represented by the minimum points  72  and  74  of FIG.  5  and the rotor position of FIG. 3 is represented by the maximum  76  point in between. Since there is no DC current to induce a stator flux  50 , the minimum points  72  and  74  of plot  70  are of equal value. The stator  40  inductance (measured as a function of electrical angle) thus has a constant component plus a periodic component comprising primarily a second harmonic frequency. This second-harmonic component is precisely aligned with the magnetic axes of the rotor  52 , with the maxima  76  in the positive and negative q-axes  68  and the minima  72 ,  74  in the positive and negative d-axes  66 ,  67 . Identifying the angular displacement of this second harmonic component relative to the stator electrical reference frame gives the desired location of the rotor d axis  66 ,  67 . 
     Referring to FIG. 6, for proper control of a permanent magnet motor, it is also desirable to distinguish between the positive  66  and negative  67  d axis directions. By way of example, the minimum points on FIG. 5 can not distinguish whether the winding  42  is aligned with a north pole magnet  60  or a south pole magnet  58 . To accomplish this an additional step of applying a DC offset current to the winding  42  is performed (as indicated by the directional arrows  44  and  46  in FIGS. 2,  3 , and  4 ) to provide a stator current flux  50 . 
     As plot  80  shows, when a north pole  60  is aligned with the winding  42 , the flux  50  produced by the DC current in the winding  42  adds to the flux  62  from the north pole magnet  60  and increases stator saturation. This slightly decreases the inductance at which was present with no stator current flux  50  as shown in FIG. 5 to provide minimum point  82 . When a south pole  58  is aligned with the winding  42 , the DC current flux  50  from the winding  42  opposes the south pole flux  64  and decreases stator saturation. This slightly increases the inductance that was present with no stator current flux  50  to provide minimum point  84  at a different value than minimum point  82 . Since the inductance of the winding  42  is different for north  60  and south  58  poles, one can distinguish the polarity of the rotor pole that is aligned with the winding  42 , and therefore discern the positive  66  and negative  67  d axes directions. 
     Alternatively, by measuring the inductance with a DC offset current injected in addition to an AC current in the stator, both the location and direction of the d axes  66 ,  67  can be determined in one step. With this additional DC excitation, the iron will saturate more or saturate less depending on the direction of the flux axes  66 ,  67 , and therefore the inductance will exhibit a deeper minimum in the positive than in the negative flux axis direction. 
     The stator inductance (measured as a function of electrical angle) thus has a constant component plus a periodic component comprising primarily of the first and second harmonic of the electrical angle. The second harmonic component can be used to determine the location of the flux axis as previously described. The first harmonic component can be used to determine the positive direction of the flux axis. 
     Another significant advantage of imposing a DC current in addition to the AC component needed for the inductance measurement is to prevent the dead time effects in the inverter from affecting the inductance measurement. If only an AC current component is used to measure inductance, the phase current of the motor is crossing zero current at the measurement frequency. The dead time of the switching devices adds an additional voltage component from the commanded voltage at the frequency of the zero crossings. This additional voltage component can have a serious impact on the inductance measurement if the voltage command is used in the impedance calculation (which is desirable). By adding a DC component to the current command, the phase current can be made to not cross zero, which greatly improves the accuracy of the inductance measurement. 
     With the ACVF drive  30  connected to the PM synchronous motor  26 , the drive power section and control electronics can be used to measure the motor stator inductance and determine the location of the rotor flux axis, i.e., d axes  66  and  67 . One method (algorithm) for doing this is to apply a small sinusoidal current perturbation and to observe the voltage required to product the current. As will be explained in greater detail hereinafter, it is important to choose a single predetermined frequency which will yield signal levels high enough for accurate measurements. A Discrete Fourier Transform (DFT) can then be used to calculate the complex stator impedance, of which the imaginary part is the inductive reactance. By measuring the stator inductance at a number of points (e.g., 20) within an electrical period, a DFT method can be used to accurately extract the phase of the second harmonic of inductance, even in the presence of noise or higher harmonic components. These inductance measurements can be made at low excitation levels and without offset bias so that no net torque is produced which might move the machine through the brake. Then the additional step of applying a DC bias current can be used to determine d axis direction. 
     Alternatively, the inductance measurements can be made with a DC offset current imposed on the AC current to determine both location and direction of the d axis in one step. It is important that the DC component be substantially equal to or larger than the amplitude of the AC component in order to prevent the phase current from crossing zero. It is also important that the DC component be relatively small so that the motor torque produced by the DC current will not unduly load the brake or cause any motion of the rotor. Typically the AC current and the DC current each have an amplitude of about 10% of motor peak rating, resulting in a combined injected AC and DC peak current of approximately 20% of motor peak rating. However, due to the noise immunity of the DFT method of computing impedance, inductance measurements can be made at low excitation levels and still yield accurate results. 
     Referring to FIG. 7, a flow diagram of an algorithm for determining the rotor flux axis (d axis) is shown. The algorithm may be implemented as a program within the memory of the elevator control system  29 , i.e., the ACVF drive  30  or the control unit  32 . After starting the program in block  100 , the phase angle θ of the small sinusoidal current to be generated from the ACVF drive  30  is initialized to 0 in block  102 . By setting the phase angle θ to zero and incrementing it through 360 electrical degrees, the measurement of inductance can be sweep around the circumference of the stator back iron  48 . 
     In block  104 , the ACVF drive  30  is commanded to inject a small AC current at a single predetermined frequency ω and electrical angle θ for a two step determination of d axis position and d axis direction. Optionally, an additional DC offset current may simultaneously be injected if d axis position and direction is to be determined in one step. 
     Choosing the appropriate frequency ω, it is important to consider the frequencies at which repetitively generated noise may exist. By way of example, the switching frequency of the transistors within the ACVF drive  30  will likely be a source of repetitive noise. If a predetermined frequency ω is chosen such that there are no harmonics or aliasing effects from the transistor switching frequency or other noise generating frequencies, than signal to noise ratios will be higher and inductance measurements will be more accurate. 
     Proceeding to block  106 , the injected current and the resultant voltage are sampled N times per period. Then the complex magnitude and direction of the applied current at the injected frequency ω, i.e., the current phasor I(ω), and the complex magnitude and direction of the resultant voltage, i.e., the voltage phasor V(ω), are measured using a Fourier analysis such as a DFT. Though a DFT is used in this embodiment, other Fourier analysis techniques may also be used, e.g., a Fourier Transform or a Fast Fourier Transform. 
     The DFT is determined with the following formula: 
     
       
           DFT=X (ω)=1 /NΣx ( nT )* e   −jωnT (for  n =0 to  N −1)= a−jb,   
       
     
     where: 
     X(ω) is the current phasor I(ω), or voltage phasor V(ω) at the angle θ; 
     N is the number of times the infected current or resultant voltage is sampled per period of time for frequency ω; 
     T is the sample time; 
     n is an integer from 1 to N; 
     x(nT) is the magnitude of the current or voltage sample for that time nT; and 
     a and b are the real and imaginary Cartesian coordinates of the complex X(ω). 
     The DFT inherently contains a great deal of noise immunity, and therefore inductance measurements can be made at low excitation levels and still yield accurate results. This is because the DFT includes a plurality of N samples for each calculation. Consequently, no one bad sample due to noise can dramatically impact the inductance calculation. 
     Proceeding to block  108 , the stator inductance at the phase angle θ is determined. With locked rotor, the complex impedance behaves just as a resistance R(θ) and an inductance L(θ) where: 
     
       
           V (ω)/= R (θ)+ jωL (θ) wherein ω L (θ)=the imaginary part of { V (ω)/ I (ω)}. 
       
     
     Therefore L(θ)=(b  1 /ω)(a V b I −a I b V )/((a I ) 2 +(b I ) 2 ), where: 
     L(θ) is the inductance of the stator back iron as a function of the angle θ; a V  and b V  are the real and mechanical Cartesian coordinates of the complex V(ω); and a I  and b I  are the real and imaginary Cartesian coordinates of the complex I(ω). 
     In block  110 , the phase angle θ of the injected current is incremented and looped back to block  104  to repeat the process until θ=360 degrees. Therefore, measuring the inductance L(θ) around the circumference of the stator back iron  48 . The program will then proceed to either block  112  if a DC offset was not injected, or to block  116  if the DC offset current was injected. 
     Stepping to block  112 , if a DC offset was not injected with the AC sinusoidal current, than L(θ) will have two equal minima 180 degrees apart, located at the positive d axis and the negative d axis as best seen in FIG.  5 . The absolute minima can be calculated using time domain or calculated using frequency domain methods by computing the DFT at the first and second harmonic of electrical frequency. 
     Once the minima are determined, the program proceeds to block  114  which repeats a single inductance measurement with a DC offset current to determine the direction of the d axis. Inductance measured with a DC offset current (such as indicated by arrows  44  and  46  in FIGS. 2,  3 , and  4 ) will produce a DC offset current flux  50  (best seen in FIGS. 2,  3 , and  4 ), which will decrease the inductance when the DC offset current flux opposes the permanent magnet flux and increase the inductance when the DC offset current flux adds to the permanent magnet flux. From this information the direction of the d axis can be determined and the program can step to block  118  where it ends. 
     If a DC offset was infected with the sinusoidal currents above, the program steps to block  116  where the location of the d axis is determined from the minimum inductance L(θ) measured. This can be calculated through several techniques, e.g., using well known time domain method, or frequency domain methods such as computing the DFT at the first and second harmonic of electrical frequency. 
     Referring to FIGS. 8,  9 , and  10 , though the synchronous motor described in the above embodiments is a  4  pole surface mounted permanent magnet motor with the poles of the magnets centered on the d axis, the same methodology is applicable to other types of synchronous motors as well. By way of example FIG. 8 is an embodiment of a synchronous motor  130  with the permanent magnets  132  embedded in the rotor  134  iron rather than mounted on the surface of the rotor. In an alternate embodiment, FIG. 9 shows a synchronous motor  140  where the magnets  142  are centered on the q axis  144  rather than the d axis  146 . In this case the magnets  142  are oriented so that the magnetic flux  148 , which defined the magnitude and direction of the d axis  146 , is directed perpendicularly to the radial direction of the q axis  144 . 
     Additionally, the rotor may include electromagnets rather than permanent magnets, for example, FIG. 10 is a synchronous motor  150  having a salient pole wound rotor  152 . The rotor  152  includes salient poles  154  with wire coils  156 , which are wound on the poles  154  to provide the magnetic flux as required. 
     While preferred embodiments have been shown and described, various modifications and substitutions may be made thereto without departing from the spirit and scope of the invention. Accordingly, it is to be understood that the present invention has been described by way of illustration and not limitation.