Abstract:
An electric motor system including: a rotor ( 9 ); a stator ( 8 ); the rotor and the stator being mounted for movement relative to one another; the rotor having multiple magnetic poles ( 11 ); the stator having multiple salient poles ( 12 ) facing the magnetic poles; a first coil set ( 1 A) and a second coil set ( 1 B); the first coil set having three phases, each of the phases comprising a first coil (u 1,  v 1,  w 1 ) and a second coil (u 1  v 1′,  w 1′ ) connected in series and wrapped around different of the salient poles in different directions relative to the rotor; the second coil set having three phases, each of the phases comprising a first coil (u 2,  v 2,  w 2 ) and a second coil (u 2′,  v 2′,  w 2′ ) connected in series and wrapped around different of the salient poles in different directions relative to the rotor; a driver ( 2 ) configured to drive the first coil set with a first three-phase voltage (U 1,  V 1,  W 1 ) and to drive the second coil set with a second three-phase voltage (U 2,  V 2,  W 2 ); the driver configured to drive the first phase of the first three-phase voltage and the first phase of the second three-phase voltage with a relative phase shift.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates generally to electric motor systems and, more particularly, to electric motor systems having a rotor with at least one permanent magnet, a stator with salient poles, a plurality of multi-phase coil sets with coils wound on salient poles of the stator, and a driver for supplying a plurality of multi-phase electric currents. 
       BACKGROUND ART 
       [0002]    Recently, the appearance of powerful rare-earth magnets on the market has enabled the development of small, high efficiency, high power permanent magnet-type synchronous motors that use rare-earth magnets in the rotors. These motors are used as motive sources and control driver sources in a variety of industries. In order to use such motors at high power, there have been increases in the electric current capacities of inverters for motors. 
         [0003]    One example of a conventional electric motor system is illustrated in  FIG. 7 . This electric motor system has two three-phase coil sets with distributed windings with branched connections, and two inverters, connected in series with the individual coils. Each of the inverters is provided with six insulated gate bipolar transistors (IGBTs) that are in complementary connection. The respective IGBTs are driven by gate drivers controlled using a PWM method, to provide three-phase AC power from the IGBTs through electric current parallel reactors L to the two coils of the electric motor. Another inverter is structured in the same manner, to drive in parallel, through two inverters, the respective three-phase coil sets that are connected in parallel. That is, in an electric motor with this type of structure, two inverters are connected in parallel to two three-phase coil sets. In this type of structure, an electric current parallel reactor L is indispensable for mitigating imbalance between the respective electric currents from the two inverters. 
         [0004]      FIG. 8  illustrates another conventional electric motor system. This electric motor system has an electric motor having a single three-phase coil set that has a branched connection, and inverters  71 ,  72 , and  73 , that have IGBTs that are connected in parallel. This electric motor system enables the supply of a large electric current through connecting in parallel relatively inexpensive below-current IGBTs. However, because the IGBTs are connected in parallel, balancing the electric current between the IGBTs is difficult, and typically it is necessary to have circuit designs that take into account a derating of between 10 and 30% (as it is necessary to use a device that has a rating that has a margin relative to the power use). Because of this, it is necessary to use IGBTs that have large rated powers relative to the maximum electric current values required by the electric motor, increasing the cost of manufacturing the inverters. 
         [0005]    Japanese Patent Application Publication JP-A-9-331694 discloses an induction motor wherein a high number of multiply split coils are formed by splitting coils for each phase, and multiple inverter primary circuits that are capable of applying multi-phase alternating current power individually to the split multi-phase coils are provided. This motor provides a high power inverter motor without requiring high power switching elements, which are relatively expensive when compared to low power ones. 
         [0006]    Japanese Patent Application Publication JP-A-7-298685 discloses a system for driving a 6-phase induction motor using two three-phase PWM inverters. The 6-phase induction motor has six phase coils u 1 , y 1 , w 1 , x 1 , v 1 , and z 1 , where the coils u 1 , w 1 , and v 1  form a three-phase winding W 1 , and the coils y 1 , x 1 , and z 1  form a three-phase winding W 2 . The two three-phase PWM inverters produce voltages with waveforms with a 180° phase difference, which are connected to the respective three-phase windings W 1  and W 2 . Thus a high power driving system is disclosed that provides either an in-phase or anti-phase symmetrical voltage waveform to each winding, where two different windings are connected to multiple inverters by forming a six-phase induction motor wherein the windings that are formed on opposing poles for a single phase in a three-phase induction motor are separated. 
         [0007]    Japanese Patent Application Publication JP-A-2004-64893 discloses an induction motor wherein two three-phase inverters and two three-phase windings are respectively split and connected. The phases of the two three-phase inverters are 180° out of phase with each other. 
         [0008]    Japanese Patent Application Publication JP-A-2006-203957 discloses an induction motor wherein two three-phase inverters and two three-phase windings are each split and connected. A single coil is connected to each phase, where coils that structure single three-phase connections are disposed at 120° angles on the stator, and two three-phase connections are disposed shifted 60° from each other. Two three-phase inverters of an identical phase provide power to the respective three-phase connections. 
         [0009]    However, the motors disclosed in the references described above are induction motors, and thus have fundamentally different structures from synchronous motors that use permanent magnets in the rotors. 
       SUMMARY OF THE INVENTION  
       [0010]    With parenthetical reference to the corresponding parts, portions or surfaces of the disclosed embodiment, merely for purposes of illustration and not by way of limitation, the present invention broadly provides an improved electric motor system comprising: a rotor ( 9 ); a stator ( 8 ); the rotor and the stator being mounted for movement relative to one another; the rotor having multiple magnetic poles ( 11 ); the stator having multiple salient poles ( 12 ) facing the magnetic poles; a first coil set ( 1 A) and a second coil set ( 1 B); the first coil set having three phases, each of the phases comprising a first coil (u 1 , v 1 , w 1 ) and a second coil (u 1  v 1 ′, w 1 ′) connected in series and wrapped around different of the salient poles in different directions relative to the rotor; the second coil set having three phases, each of the phases comprising a first coil (u 2 , v 2 , w 2 ) and a second coil (u 2 ′, v 2 ′, w 2 ′) connected in series and wrapped around different of the salient poles in different directions relative to the rotor; a driver ( 2 ) configured to drive the first coil set with a first three-phase voltage (U 1 , V 1 , W 1 ) and to drive the second coil set with a second three-phase voltage (U 2 , V 2 , W 2 ); the driver configured to drive the first phase of the first three-phase voltage and the first phase of the second three-phase voltage with a relative phase shift. 
         [0011]    The phase shift may be 30 degrees. The coils may be wrapped around adjacent salient poles and configured with the driver such that the adjacent salient poles have a relative magnetic flux phase shift of about 210 degrees. The first coil (u 1 ) of the first phase of the first coil set ( 1 A) and the second coil (u 2 ′) of the first phase of the second coil set ( 1 B) are wrapped around adjacent salient poles and in different directions relative to the rotor such that the adjacent salient poles have a relative magnetic flux phase shift of about 210 degrees. The number of magnetic poles may be 14×n and the number of salient poles may be 12×n, where n is a positive integer, or the number of magnetic poles may be 10×n and the number of salient poles may be 12×n, where n is a positive integer. The first coil of the first phase of the first coil set and the second coil of the first phase of the second coil set may be wrapped around adjacent salient poles in different directions relative to the rotor. 
         [0012]    The magnetic poles may be formed on a surface of the rotor. The magnetic poles may be embedded in the rotor. The first coil set and the second coil set may be connected to the driver independently of each other. The stator and the rotor may be mounted for rotational movement relative to one another about a common axis. The driver may comprise a plurality of insulated gate bipolar transistors ( 511 - 515 ), an AC to DC inverter ( 3 ) which is made up of an AC to DC rectifier ( 3 A), a rectifying capacitor ( 41 ), a CPU ( 611 ), a pulse-width-modulation module ( 612 ), and a gate driver ( 613 ). The system may further comprise a current sensor in communication with the CPU arranged to measure a current flow in an output line of the driver. The driver may not include a reactor (L) on one of its output lines. The integer n may be 1 or 2. The system may further comprise a third coil set ( 1 C) and a fourth coil set ( 1 D). The driver may comprise six IGBTs arranged to produce the first three-phase voltage. 
         [0013]    In another aspect, the invention is directed to a electric motor system comprising: a rotor ( 9 ); a plurality of magnetic poles ( 11 ) on the outer periphery of the rotor ( 9 ); a stator ( 8 ) encompassing the rotor ( 9 ); a plurality of salient poles ( 12 ) formed on the inner periphery of the stator ( 8 ); multiple independently connected three-phase coil sets ( 1 A,  1 B); and a controlling means ( 2 ). Each phase (e.g. u) has a first coil (u 1 ) and a second coil (u 1 ′) which are connected in series, but are wrapped in mutually opposite directions on different salient poles. The controlling means ( 2 ) provides at least two different three-phase electric currents (U 1 , V 1 , W 1 ; U 2 , V 2 , W 2 ) having different phases to the multiple three-phase coil sets ( 1 A,  1 B). The controlling means ( 20 ) includes control units ( 61 ,  62 ,  63 ,  64 ) and inverter units ( 51 ,  52 ,  53 ,  54 ). 
         [0014]    One embodiment of the electric motor system is characterized by the number of magnetic poles being 14×n and the number of salient poles being 12×n, where n is a positive integer. 
         [0015]    Another embodiment of the electric motor system is characterized by the number of magnetic poles being 10×n and the number of salient poles being 12×n, where n is a positive integer. 
         [0016]    Yet another embodiment of the electric motor system is characterized by the controlling means providing, to multiple three-phase coil sets, two types of three-phase electric power having a mutual phase difference of 30°, where an adjacent coil that is supplied power with a phase that is different by 30° is wound in the opposite direction. 
         [0017]    Yet another embodiment of the electric motor system is characterized by the magnetic poles being formed on the surface of a rotor. 
         [0018]    Yet another embodiment of the electric motor system is characterized by the magnetic poles being embedded in a rotor. 
         [0019]    Thus several advantages of one or more aspects are to provide a high power, high efficiency electric motor system without increasing the power of the switching elements, such as IGBTs, used in the inverter, and to reduce the size and the manufacturing costs of the electric motor system. These and other advantages of one or more aspects will become apparent from a consideration of the ensuing description and accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS  
         [0020]      FIG. 1  is a diagram illustrating a first embodiment of the invention. 
           [0021]      FIG. 2  is a cross-sectional view of the 14-pole, 12-slot electric motor of the first embodiment. 
           [0022]      FIG. 3  is a graph illustrating the phases of the driving voltage of the first embodiment. 
           [0023]      FIG. 4  is a diagram illustrating a second embodiment of the invention. 
           [0024]      FIG. 5  is a cross-sectional view of the 28-pole, 24-slot electric motor of the second embodiment. 
           [0025]      FIG. 6  is a graph illustrating the phases of the driving voltage of the second embodiment. 
           [0026]      FIG. 7  is a prior art conventional electric motor system. 
           [0027]      FIG. 8  is an alternate prior art conventional electric motor system. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0028]    At the outset, it should be clearly understood that like reference numerals are intended to identify the same structural elements, portions or surfaces consistently throughout the several drawing figures, as such elements, portions or surfaces may be further described or explained by the entire written specification, of which this detailed description is an integral part. Unless otherwise indicated, the drawings are intended to be read (e.g., cross-hatching, arrangement of parts, proportion, degree, etc.) together with the specification, and are to be considered a portion of the entire written description of this invention. As used in the following description, the terms “horizontal”, “vertical”, “left”, “right”, “up” and “down”, as well as adjectival and adverbial derivatives thereof (e.g., “horizontally”, “rightwardly”, “upwardly”, etc.), simply refer to the orientation of the illustrated structure as the particular drawing figure faces the reader. Similarly, the terms “inwardly” and “outwardly” generally refer to the orientation of a surface relative to its axis of elongation, or axis of rotation, as appropriate. 
         [0029]      FIG. 1  illustrates a first embodiment of the electric motor system. As shown, the electric motor system broadly comprises electric motor  1  and driving circuit  2 . Electric motor  1  has stator  8  and rotor  9 . Two three phase coil sets,  1 A and  1 B, are wound around poles  12  of stator  8 . Rotor  9  has permanent magnets  11 . Driving circuit  2  produces two three-phase driving voltages for driving electric motor  1 . 
         [0030]    Driving circuit  2  contains converter unit  3  (for converting three-phase alternating current power source  5  into direct current), rectifying capacitors  41  and  42 , control units  61  and  62  (for generating respective three-phase PWM signals), and first and second inverter units  51  and  52  (for converting the direct current into the desired alternating current through switching operations as controlled by control units  61  and  62 ). 
         [0031]    Control unit  61  provides inverter unit  51  the proper switch signals to generate three-phase driving voltages U 1 , W 1 , and V 1  at the proper frequency and phase. Control unit  61  comprises CPU  611 , PWM circuit  612 , and gate driver  613 . CPU  611  provides signals to PWM circuit  612 , which generates a three-phase PWM signal based on the calculation results of CPU  611 . Gate inverter  613  performs current amplification of the PWM signal provided to it from PWM circuit  612 . CPU  611  is provided with a memory into which a predetermined control program is written, a calculating unit, and an interrupt port for inputting a sensor value measuring the driving voltage W 1 . Using the driving voltage W 1  as a reference, CPU  611  is able to control the generation of driving voltages U 1  and V 1  with precise phase differences. PWM circuit  612  generates a three-phase PWM signal based on the calculation results by CPU  611 . Gate driver  613  performs current amplification of the three-phase PWM signal as well as an inverse PWM signal, and outputs them to the inverter unit  51 . 
         [0032]    Control unit  62  is structured identically to control unit  61 . However, CPU&#39;s  611  and  621  are synchronized to generate driving signals with a desired phase difference between first and second inverter units  51  and  52  through synchronized calculations. Control units  61  and  62  may alternatively be controlled through a shared CPU. Furthermore, control such as PAM (Pulse Amplitude Modulation) may be used instead of PWM control. 
         [0033]    First inverter  51  is structured from IGBTs  511  and  512  for the U 1  phase, IGBTs  513  and  514  for the V 1  phase, and IGBTs  515  and  516  for the W 1  phase. Each of the pairs of IGBTs are connected in series between the direct current positive output and the direct current negative output of converter unit  3 . The gates of IGBP&#39;s  511  through  516  are connected to gate driver  613 , and switching operations are performed by a three-phase pulse signal. The two IGBTs for a given phase, for example IGBTs  511  and  512  for phase U 1 , are driven by gate signals that are mutual inverses of each other. Doing so produces the proper driving voltage U 1 , at the emitter of IGBT  511  and the collector of IGBT  512 . The driving voltage V 1  is outputted similarly from IGBTs  513  and  514 , and the driving voltage W 1  is outputted similarly from IGBTs  515  and  516 . 
         [0034]    As will be described below, three-phase driving voltages U 1 , V 1 , and W 1  have phase differences of 120° from each other. Second inverter unit  52  is also structured with six IGBTs  521  through  526 , in the same manner as first inverter unit  51 , and outputs three-phase driving voltages U 2 , V 2 , and W 2 . Additionally, it is noted that while IGBTs are used in this embodiment as the switching elements for first and second inverter units  51  and  52 , different switching element types, such as power MOSFETs, bipolar transistors, and the like, may be used instead. 
         [0035]    Three-phase driving voltages U 1 , V 1 , and W 1  are supplied from inverter unit  51  to three-phase coil set  1 A of electric motor  1 , and three-phase driving voltages U 2 , V 2 , and W 2  are provided from inverter unit  52  to three-phase coil set  1 B. Three-phase coil sets  1 A and  1 B are connected independently of each other, and are driven by individual inverter units  51  and  52 , respectively. 
         [0036]    As illustrated in  FIG. 1 , first three-phase coil set  1 A is structured from coils u 1 , u 1 ′, v 1 , v 1 ′, w 1 , and w 1 ′, and is branch-connected centered on node N 1 . Respective coil pairs are connected in series. For example, coils u 1  and u 1 ′ are connected in series. Similarly, coils v 1  and v 1 ′, and coils w 1  and w 1 ′ are also connected in series. Driving voltage U 1  is applied to the end of coil u 1  as shown in  FIG. 1 . Similarly, driving voltage V 1  is applied to coil v 1 , and driving voltage W 1  is applied to coil w 1 . Similarly, second three-phase coil set  1 B is also branch-connected, centered on node N 2 . As shown in  FIG. 1 , three-phase driving voltages U 2 , V 2 , and W 2  are applied to coils u 2 , v 2 , and w 2 . 
         [0037]      FIG. 2  illustrates a cross-section of the electric motor  1  of the first embodiment. Electric motor  1  is a 14-pole/12-slot surface permanent magnet synchronous electric motor. There are  14  permanent magnets  11  of alternating polarities provided on the outer periphery of rotor  9 . There are  12  salient poles  12  on the inner periphery of stator  8 , protruding towards the center of rotation of rotor  9 . Salient poles  12  are arranged at 30° intervals. 
         [0038]    Each coil u 1 , u 2 ′, w 1 ′, w 2 , v 1 , v 2 ′, u 1 ′, u 2 , w 1 , w 2 ′, v 1 ′, and v 2  is wrapped onto its own salient pole as shown in  FIG. 2 . Additionally, coils u 1 , v 1 , w 1 , u 2 , v 2 , and w 2  have winding directions that are the same as each other, and opposite from coils u 1 ′, v 1 ′, w 1 ′, u 2 ′, v 2 ′, and w 2 ′. Because of this, the magnetic field vector that is generated in each of the aforementioned coil pairs (such as coils u 1  and u 1 ′) are of opposite polarities relative to rotor  9 &#39;s center of rotation. By sequentially winding the same wire onto different salient poles in different directions, it is possible to obtain magnetic forces having a 180° phase difference from a single driving current. As shown in  FIG. 2 , the winding direction of the coil alternates every two sets of slots moving clockwise around the stator. For example, coils v 2  and u 1  are wound in the same direction, which is opposite from coils u 2 ′ and w 1 ′. 
         [0039]    In the 14-pole, 12-slot electric motor system of the first embodiment, the electric angles of adjacent salient poles are 210° (14×180°/12) from each other. That is, there is a 210° difference between the phases of magnetic fluxes that are produced by adjacent coils. 
         [0040]      FIG. 3  illustrates the phases of the three-phase driving voltages in the first embodiment. The horizontal axis of  FIG. 3  indicates the phase of the driving voltage, and the vertical axis indicates the driving voltage (the relative voltages). Driving voltages U 1 , V 1 , and W 1  from first inverter unit  51  have, respectively, 0°, 120°, and 240° phase differences when using driving voltage U 1  as the reference. Furthermore, driving voltages U 2 , V 2 , and W 2  from second inverter unit  52  have phase differences of 30°, 150°, and 270°, respectively, when using driving voltage U 1  as the reference. Consequently, as illustrated in  FIG. 3 , driving voltages U 1 , V 1 , and W 1  from first inverter unit  51  and driving voltages U 2 , V 2 , and W 2  from second inverter unit  52  have a 30° phase difference from each other, respectively. For example, U 1  and U 2  are separated 30° in phase, with U 2  being 30° earlier than U 1 . 
         [0041]    Adjacent coils have a magnetic flux difference that is a result of a phase difference in their driving voltages, and possibly an added phase difference if the coils are wound in opposite directions. Referring to  FIGS. 1-3  in unison, it will be described how each coil is driven with a voltage that causes its resulting magnetic flux to have a phase delay of 210° relative to the next clockwise coil. For example, the phase of the magnetic flux of coil u 1  is 210° later than the phase of the magnetic flux of coil u 2 ′, and similarly, the magnetic flux phase of coil u 2 ′ is 210° later than coil w 1 ′. 
         [0042]    Coil u 1  is driven by driving voltage U 1 . Coil u 2 ′ is driven by driving voltage U 2 . As shown in  FIG. 3 , driving voltage U 2  is leading driving voltage U 1  by 30°. In other words, U 1  has a phase delay of 30° relative to U 2 . As shown in  FIG. 2  and described earlier, coils u 1  and u 2 ′ are wound in opposite directions around their respective slots. The opposite winding direction causes an opposite magnetic flux to be generated in the coils given an identical driving voltage. The effect of opposite winding directions is equivalent to a phase difference of 180°. The 30° phase delay between the driving voltages, combined with a 180° phase difference due to opposite winding directions, causes coils u 1  and u 2 ′ to have a 210° phase difference in their magnetic flux (u 1  being delayed by 210° relative to u 2 ′). 
         [0043]    The phase difference between coil u 2 ′ and coil w 1 ′ is also 210°. However, the origination of the delay is slightly different. Since coils u 2 ′ and w 1 ′ are both “prime” coils, their windings are in the same direction. Thus, there is no relative magnetic flux phase shift due to winding direction. However, referring to  FIG. 3 , it can be seen that driving voltage U 2  and driving voltage W 1  have a phase difference of 210° and U 2  is 210° delayed relative to W 1 . The 210° driving voltage delay directly results in a 210° magnetic flux delay between coils u 2 ′ and w 1 ′. 
         [0044]    The pattern of adjacent coils having either a 30° driving voltage shift and a winding reversal, or a 210° driving voltage shift and no winding reversal, repeats clockwise around the stator. For example, the 30° driving voltage phase shift and opposite winding directions is observed between the adjacent coil pairs: u 1  and u 2 ; w 1 ′ and w 2 ; v 1  and v 2 ; u 1 ′ and u 2 ; w 1  and w 2 ′; and v 1 ′ and v 2 . Alternatively, the 210° driving voltage shift and same direction windings are found in the adjacent coil pairs: u 2 ′ and w 1 ′; w 2  and v 1 ; v 2 ′ and u 1 ; u 2  and w 1 ; w 2 ′ and v 1 ′; and v 2  and u 1 . This pattern produces the effect that any two adjacent coil pairs will have a magnetic flux phase shift of 210°. Referring to  FIG. 2 , each subsequent coil in the clockwise direction around the rotor has an increasing magnetic flux phase shift of 210° more than the previous coil. For example, coil u 2 ′ has a magnetic flux phase shift of 210° relative to coil u 1 , coil w 1 ′ has a magnetic flux phase shift of 420° (420°−360°=60°) relative to u 1 , w 2  has a phase shift of 630° (630°−360°=270°) relative to u 1 , v 1  has a phase shift of 840° (840°−360°=120°) relative to u 1 . 
         [0045]    By having two three-phase driving power supplies that have a relative phase difference of 30° as described, it is possible to produce electric angles that are 210° different between adjacent coils for each of the 12 coils. This enables synchronized driving of the 14-pole, 12-slot electric motor as set forth in the first embodiment. 
         [0046]    Furthermore, in this embodiment, since not only are the electric currents nearly identical phases (electric currents wherein the phases are different by 30°) as applied to adjacent coils u 1  and u 2 ′, but also the coils are wound in opposite directions, the magnetic coupling between adjacent coils is strong. For example, when there is a large electric current in coil u 1  the induced magnetic field lines will flow through the armature and into coil u 2 ′, reinforcing the magnetic flux generated by coil u 2 ′. The result is that the strong magnetic coupling between adjacent arms makes it possible to achieve increased flux density and thus increased motor performance. In contrast, in the case of typical distributed winding structures, the magnetic coupling between adjacent coils is weak. 
         [0047]    Typically, in an electric motor with distributed windings, wherein the coils are wound bridging multiple slots, the winding wires that are wound onto another slot would have to go around to the tip portion of the salient pole, and thus when compared to an electric motor with concentrated windings wherein the coils are wound onto a single salient pole, the length of the coils are longer by the length of the wiring to the coil end portions of the stator. Furthermore, because, in the distributed windings, the winding wire that is wound in another slot at the end portion of the salient pole wraps around and is layered, so, when compared to the case of the concentrated windings, the coil end portions of the stator are fatter. In the electric motor system as set forth in the present invention, the coils are wound as concentrated windings, and so it is possible to reduce the thickness of the coil end portion of the stator, enabling the coil portion to have a compact design. Additionally, the resistance of the winding wires can be reduced because the length of the winding wires is reduced through the concentrated windings, and thus there is the benefit of being able to reduce the size of the driving circuit. 
         [0048]    Additionally, the three-phase coil sets  1 A and  1 B are connected independently (no direct electrical connection between them), and are driven by different inverter units  51  and  52 , respectively, making balancing reactors L (such as used in the prior art shown in  FIG. 7 ) unnecessary. Furthermore, since a single IGBT handles only a single phase, it is possible to design the driving circuit without derating the IGBTs. This makes it possible to reduce the size of the driving circuitry, with the effect of being able to reduce the manufacturing cost of the electric motor system. 
         [0049]    A second embodiment of the electric motor system is shown in  FIG. 4  through  FIG. 6 . Because the structure of the second embodiment is identical in many respects to that of the first embodiment, explanations regarding the identically structured portions are omitted, and only those components of the second embodiment that are different from those in the first embodiment are described in any detail below. 
         [0050]      FIG. 4  is a circuit diagram of the second embodiment of the electric motor system. In this embodiment, electric motor  10  has four three-phase coil sets  1 A,  1 B,  1 C, and  1 D, and a driving circuit  20  that produces four three-phase driving voltages (first three-phase driving voltages U 1 , V 1 , W 1 ; second three-phase driving voltages U 2 , V 2 , W 2 ; third three-phase driving voltages U 3 , V 3 , W 3 ; and fourth three-phase driving voltages U 4 , V 4 , W 4 ). Driving circuit  20  has, in addition to control units  63  and  64  from the first embodiment, third and fourth inverter units  53  and  54 . In control units  61 ,  62 ,  63 , and  64 , the CPUs  611 ,  621 ,  631 , and  641  perform mutually synchronized calculations to produce driving voltages with specific phase differences in inverter units  51  through  54 . Note that CPUs  611 ,  621 ,  631 , and  641  may be replaced with a single shared CPU to control units  61  through  64 . 
         [0051]    Third and fourth inverter units  53  and  54 , structured identically to first and second inverter units  51  and  52 , are each structured from six IGBTs,  531  through  536 , and  541  through  546 . Third inverter unit  53  drives three-phase driving voltages U 3 , V 3 , and W 3 . Similarly, fourth inverter unit  54  drives three-phase driving voltages U 4 , V 4 , and W 4 . As shown in  FIG. 6 , the third and fourth three-phase driving voltages, U 3 , V 3 , W 4  and U 4 , V 4 , W 4 , have phases that are 120° different from each other. The third three-phase driving voltages U 3 , V 3 , and W 3  are in the same phases as the first three-phase driving voltages U 1 , V 1 , and W 1 . Similarly, the fourth three-phase driving voltages U 4 , V 4 , and W 4  are in the same phases as the second three-phase driving voltages U 2 , V 2 , and W 2 . 
         [0052]    As shown in  FIG. 4 , electric motor  10 , in addition to having two three-phase coil sets  1 A and  1 B, as in the first embodiment, is provided with third and fourth three-phase coil sets  1 C and  1 D. Third three-phase coil set  1 C is structured from coils u 3 , u 3 ′, v 3 , v 3 ′, w 3 , and w 3 ′, and driving voltages U 3 , V 3 , and W 3  are applied to these coils as shown. Similarly, fourth three-phase coil set  1 D is structured from coils u 4 , u 4 ′, v 4 , v 4 ′, w 4 , and w 4 ′, and driving voltages U 4 , V 4 , and W 4  are applied to these coils a shown. 
         [0053]      FIG. 5  presents a cross-sectional diagram of electric motor  10 . Electric motor  10  is a 28-pole, 24-slot surface permanent magnet electric motor. That is, 28 permanent magnets  110  are disposed with alternating polarities on the outer periphery of rotor  90 , and 24 salient poles  120  are disposed at 15° intervals on the inner periphery of stator  80 , protruding towards the rotational center of rotor  90 . 
         [0054]    As shown in  FIG. 5 , coils u 1 , u 2 ′, w 1 ′, w 2 , v 1 , v 2 ′, u 1 ′, u 2 , w 1 , w 2 ′, v 1 ′, v 2 , u 3 , u 4 ′, w 3 ′, w 4 , v 3 , v 4 ′, u 3 ′, u 4 , w 3 , w 4 ′, v 3 ′, and v 4  are arranged clockwise along the stator, wrapped onto corresponding salient poles  120 . Additionally, coils u 1 , v 1 , w 1 , u 2 , v 2 , w 2 , u 3 , v 3 , w 3 , u 4 , v 4 , and w 4 , have winding directions that are the same as each other, and opposite from coils u 1 ′, v 1 ′, w′, u 2 ′, v 2 ′, w 2 ′, u 3 ′, v 3 ′, w 3 ′, u 4 ′, v 4 ′, and w 4 ′. Because of this, the magnetic field vector that is generated in each of the aforementioned coil pairs (such as coils u 1  and u 1 ′) are of opposite polarities relative to the center of rotation of rotor  90 . 
         [0055]      FIG. 6  illustrates the phases of the three-phase driving voltages in the second embodiment. In this figure, the horizontal axis shows the phases of the driving voltages and the vertical axis shows the driving voltages (the relative voltages). Driving voltages U 3 , V 3 , and W 3  of third inverter unit  53  have phase differences of 0°, 120°, and 240°, respectively, when driving voltage U 1  is used as the reference. Furthermore, driving voltages U 4 , V 4 , and W 4  of fourth inverter unit  54  have phase differences of 30°, 150°, and 270°, respectively, when driving voltage U 1  is used as the reference. Consequently, as shown in  FIG. 6 , driving voltages U 1 , V 1 , and W 1 , output from first inverter unit  51 , and driving voltages U 3 , V 3 , and W 3 , output from third inverter unit  53 , have substantially identical phases. Similarly, driving voltages U 2 , V 2 , and W 2 , output from second inverter unit  52 , and driving voltages U 4 , V 4 , and W 4 , output from fourth inverter unit  54 , have substantially identical phases. Driving voltages U 3 , V 3 , and W 3 , output from third inverter unit  53 , and driving voltages U 4 , V 4 , and W 4 , output from fourth inverter unit  54 , have respective phase differences of 30°. Adjacent coils that are supplied with electric currents having phase differences of 30° are wound in opposite directions. 
         [0056]    Even in the 28-pole, 24-slot electric motor system configuration of the second embodiment, the electric angles of adjacent salient poles will differ from each other by 210° (28×180°/24). That is, the phases of the magnetic fluxes formed by adjacent coils will differ from each other by 210°. As in the first embodiment, having two three-phase driving power supplies with a phase difference of 30° enables the electric angle between adjacent coils to be 210°. Similarly, synchronous driving of the 28-pole, 24-slot electric motor of the second embodiment is possible. The structure of the second embodiment has the same effects as the electric motor system of the first embodiment. 
         [0057]    The first embodiment (a 14-pole, 12-slot electric motor), and the second embodiment (a 28-pole, 24-slot electric motor), are described as illustrative examples. Note that the first embodiment is a (14×1)-pole, (12×1)-slot electric motor, and the second embodiment is a (14×2)-pole, (12×2)-slot electric motor. The electric motor system is not limited to the previous examples, but rather, the same effects can be obtained in a (14×n)-pole, (12×n)-slot electric motor system (where n is a positive integer). Furthermore, the same effects can be obtained even in a (10×n)-pole, (12×n)-slot electric motor (where n is a positive integer). 
         [0058]    Furthermore, while the presented embodiments are described as having a surface permanent magnet electric motor, the electric motor system disclosed is not limited thereto, but rather can be applied also to embedded magnet-type electric motors, and it should be noted that the same effects can be obtained therein. 
         [0059]    The present invention contemplates that many changes and modifications may be made. Therefore, while a number of embodiments of the electric motor system have been shown and described, and a number of alternatives discussed, persons skilled in this art will readily appreciate that various additional changes and modifications may be made without departing from the nature of the invention, as defined and differentiated by the following claims.