Patent Publication Number: US-2019173406-A1

Title: Inverter device

Description:
INCORPORATION BY REFERENCE 
     The disclosure of Japanese Patent Application No. 2017-232921 filed Dec. 4, 2017 including the specification, drawings and abstract, is incorporated herein by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an inverter device. 
     2. Description of Related Art 
     Japanese Patent Application Publication No. 2012-99612 (JP 2012-99612 A) discloses a semiconductor device including a latent heat storage material that absorbs the heat generated in a semiconductor element (switching element). The latent heat storage material transitions between a solid phase region corresponding to a solid state, a phase transition region where a solid state and a liquid state co-exist, and a liquid phase region corresponding to a liquid state, in accordance with the temperature thereof. The latent heat storage material absorbs heat energy when transitioning from the solid state to the liquid state, and releases heat energy when transitioning from the liquid state to the solid state. In the semiconductor device disclosed in JP2012-99612 A, the latent heat storage material having such physical properties is used to control the temperature of the semiconductor element. 
     However, the latent heat storage material has a problem with supercooling. Supercooling is a phenomenon in which, in the case where the temperature of the latent heat storage material is in the liquid phase region, even when the temperature of the latent heat storage material is lowered below its freezing point, the latent heat storage material is not recrystallized and is maintained in the liquid state although the temperature is supposed to be located in the solid phase region. When the latent heat storage material is supercooled, the temperature of the switching element cannot be appropriately controlled. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide an inverter device capable of suppressing supercooling of a latent heat storage material, and appropriately controlling the temperature of a switching element. 
     An inverter device according to an aspect of the present invention includes: a motor drive circuit unit that includes a plurality of arm circuits each including a plurality of switching elements, and that converts a direct current (DC) signal supplied from a DC power supply into a motor current formed of an alternating current (AC) signal; a radiator that includes a latent heat storage material transitioning between a solid phase region corresponding to a solid state, a phase transition region where the solid state and a liquid state co-exist, and a liquid phase region corresponding to the liquid state, in accordance with a temperature of the latent heat storage material, and that absorbs heat generated in the plurality of switching elements; a temperature sensor that detects the temperature of the latent heat storage material; and a control device connected to the plurality of switching elements and the temperature sensor; wherein the control device includes a determining unit that determines whether a condition is satisfied, the condition being that the temperature of the latent heat storage material is close to a melting point of the latent heat storage material, and that the temperature of the latent heat storage material is on a rising trend, and a supercooling suppressing unit that limits the motor current when the determining unit determines that the condition is satisfied. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and further features and advantages of the invention will become apparent from the following description of example embodiments with reference to the accompanying drawings, wherein like numerals are used to represent like elements and wherein: 
         FIG. 1  is a schematic diagram illustrating the general configuration of an electric power steering system to which an inverter device according to a first embodiment of the present invention is applied; 
         FIG. 2  is a block diagram schematically illustrating the electrical configuration of an ECU; 
         FIG. 3  is a plan view illustrating the general configuration of the inverter device; 
         FIG. 4  is a cross-sectional view taken along the line IV-IV of  FIG. 3 ; 
         FIG. 5  is a graph illustrating the characteristics of a latent heat storage material; 
         FIG. 6  is a flowchart illustrating exemplary operations of a limiter control unit; 
         FIG. 7  is a graph illustrating the characteristics of the latent heat storage material; 
         FIG. 8  is a block diagram schematically illustrating the electrical configuration of an ECU to which an inverter device according to a second embodiment of the present invention is applied; 
         FIG. 9  is a cross-sectional view of a part corresponding to the part illustrated in  FIG. 4 , illustrating the general configuration of the inverter device of  FIG. 8 ; and 
         FIG. 10  is a flowchart illustrating exemplary operations of a relay section control unit. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the exemplary embodiments, an inverter device is applied to an electric power steering system.  FIG. 1  is a schematic diagram illustrating the general configuration of an electric power steering system  2  to which an inverter device  1  is applied according to a first embodiment of the present invention. The electric power steering system  2  includes a steering wheel  3 , a steering operation mechanism  5 , and a steering assist mechanism  6 . The steering wheel  3  is a steering member for steering the vehicle. The steering operation mechanism  5  steers steered wheels  4  in conjunction with rotation of the steering wheel  3 . The steering assist mechanism  6  assists the driver in steering. 
     The steering wheel  3  and the steering operation mechanism  5  are mechanically coupled to each other via a steering shaft  7  and an intermediate shaft  8 . The steering shaft  7  includes an input shaft  9  coupled to the steering wheel  3 , and an output shaft  10  coupled to the intermediate shaft  8 . The input shaft  9  and the output shaft  10  are relatively rotatably coupled to each other via a torsion bar  11 . A torque sensor  12  is disposed near the torsion bar  11 . The torque sensor  12  detects a steering torque T applied to the steering wheel  3 , based on the amount of relative rotational displacement between the input shaft  9  and the output shaft  10 . 
     For example, the torque sensor  12  detects the steering torque T for steering to the right as a positive value, and detects the steering torque T for steering to the left as a negative value. The greater the absolute value of the steering torque T is, the greater the amount of steering torque T is. The steering operation mechanism  5  includes a rack-and-pinion mechanism. The rack-and-pinion mechanism includes a pinion shaft  13 , and a rack shaft  14  that serves as a steered shaft. The steered wheels  4  are coupled to the opposite ends of the rack shaft  14  via tie rods  15  and knuckle arms (not illustrated). 
     The pinion shaft  13  is coupled to the intermediate shaft  8 . The pinion shaft  13  rotates in conjunction with steering of the steering wheel  3 . A pinion  16  is coupled to an end (lower end in  FIG. 1 ) of the pinion shaft  13 . The rack shaft  14  extends linearly in the lateral direction of the vehicle. A rack  17  that meshes with the pinion  16  is formed at an axially intermediate portion of the rack shaft  14 . 
     When the steering wheel  3  is operated (rotated), the rotation is transmitted to the pinion shaft  13  via the steering shaft  7  and the intermediate shaft  8 . The pinion  16  and the rack  17  convert rotation of the pinion shaft  13  into axial movement of the rack shaft  14 . In this way, the steered wheels  4  are steered. The steering assist mechanism  6  includes an electric motor  18  for assisting in steering, and a speed reduction mechanism  19  that transmits an output torque of the electric motor  18  toward the steering operation mechanism  5 . The electric motor  18  is a three-phase brushless motor in this embodiment. 
     The electric motor  18  is provided with a rotation angle sensor  20  that detects a rotation angle of the electric motor  18 . The rotation angle sensor  20  may include a resolver. The speed reduction mechanism  19  includes a worm gear mechanism. The worm gear mechanism includes a worm shaft  21 , and a worm wheel  22  that meshes with the worm shaft  21 . The worm shaft  21  is rotationally driven by the electric motor  18 . 
     The worm wheel  22  is coupled to the steering shaft  7  so as to be rotatable with the steering shaft  7 . The worm wheel  22  is rotationally driven by the worm shaft  21 . When the worm shaft  21  is rotationally driven by the electric motor  18 , the worm wheel  22  is rotationally driven, and the steering shaft  7  is rotated. Then, the rotation of the steering shaft  7  is transmitted to the pinion shaft  13  via the intermediate shaft  8 . 
     The rotation of the pinion shaft  13  is converted into axial movement of the rack shaft  14 . In this way, the steered wheels  4  are steered. That is, the electric motor  18  rotationally drives the worm shaft  21 , and thus can assist in steering. The vehicle is provided with a vehicle speed sensor  23  for detecting a vehicle speed V. The steering torque T detected by the torque sensor  12 , the vehicle speed V detected by the vehicle speed sensor  23 , signals output from the rotation angle sensor  20 , and so on are input to an electrical control unit (ECU)  24 . The ECU  24  controls the electric motor  18 , based on these input signals. 
       FIG. 2  is a block diagram schematically illustrating the electrical configuration of the ECU  24 .  FIG. 3  is a plan view illustrating the structure of the inverter device  1 .  FIG. 4  is a cross-sectional view taken along the line IV-IV in  FIG. 3 . Referring to  FIG. 2 , the ECU  24  includes the inverter device  1  that supplies electric power to the electric motor  18 . The inverter device  1  includes a motor drive circuit unit  26  and a microcomputer  25  (control device). The motor drive circuit unit  26  converts a direct current (DC) signal supplied from a DC power supply (not illustrated) into a motor current formed of an alternating current (AC) signal. The microcomputer  25  is connected to the motor drive circuit unit  26 , and controls the motor drive circuit unit  26 . 
     The electric motor  18  includes a rotor and a stator. Although not illustrated, the stator includes a U-phase field winding, a V-phase field winding, and a W-phase field winding. The motor drive circuit unit  26  includes a U-phase arm circuit  27 , a V-phase arm circuit  28 , and a W-phase arm circuit  29  respectively corresponding to the U-phase field winding, the V-phase field winding, and the W-phase field winding of the electric motor  18 . The U-phase arm circuit  27  includes a series circuit of a pair of switching elements  33  and  34  corresponding to a U phase of the electric motor  18 . The series circuit of the pair of switching elements  33  and  34  is connected in parallel between a DC power supply and a reference voltage (for example, ground voltage). The switching element  33  is provided as an upper arm. The switching element  34  is provided as a lower arm. 
     The V-phase arm circuit  28  includes a series circuit of a pair of switching elements  35  and  36  corresponding to a V phase of the electric motor  18 . The series circuit of the pair of switching elements  35  and  36  is connected in parallel between the DC power supply and the reference voltage. The switching element  35  is provided as an upper arm. The switching element  36  is provided as a lower arm. The W-phase arm circuit  29  includes a series circuit of a pair of switching elements  37  and  38  corresponding to a W phase of the electric motor  18 . The series circuit of the pair of switching elements  37  and  38  is connected in parallel between the DC power supply and the reference voltage. The switching element  37  is provided as an upper arm. The switching element  38  is provided as a lower arm. 
     In this embodiment, each of the switching elements  33  to  38  includes an n-channel metal insulator semiconductor field effect transistor (MISFET). Drains of the switching elements  33 ,  35 , and  37  of the upper arms are connected to the DC power supply. Sources of the switching elements  33 ,  35 , and  37  of the upper arms are connected to drains of the switching elements  34 ,  36 , and  38  of the lower arms. Sources of the switching elements  34 ,  36 , and  38  of the lower arms are connected to the reference voltage. 
     A regeneration diode may be connected in parallel to each of the switching elements  33  to  38 . The regeneration diode may be connected in parallel such that a forward current flows toward the drain of each of the switching elements  33  to  38 . In the U-phase arm circuit  27 , a U-phase connection point  39  between the paired switching elements  33  and  34  is connected to one end of the U-phase field winding of the electric motor  18  via a U-phase power supply line. 
     In the V-phase arm circuit  28 , a V-phase connection point  40  between the paired switching elements  35  and  36  is connected to one end of the V-phase field winding of the electric motor  18  via a V-phase power supply line. In the W-phase arm circuit  29 , a W-phase connection point  41  between the paired switching elements  37  and  38  is connected to one end of the W-phase field winding of the electric motor  18  via a W-phase power supply line. 
     The other ends of the U-phase field winding, the V-shape field winding, and the W-phase field winding of the electric motor  18  are connected to each other. The U-phase power supply line is provided with a U-phase current sensor  45 . The W-phase power supply line is provided with a W-phase current sensor  46 . Although not illustrated in  FIG. 2 , gate drive circuits for generating gate signals to the respective switching elements  33  to  38  are connected to the gates of the respective switching elements  33  to  38 . Referring to  FIGS. 3 and 4 , the inverter device  1  includes a substrate  51 , a radiator  52 , and a temperature sensor  53 , in addition to the motor drive circuit unit  26 . In  FIG. 3 , the radiator  52  and the temperature sensor  53  are indicated by the dashed lines. 
     The substrate  51  is a circuit board with the motor drive circuit unit  26  mounted thereon, and is formed in a circular shape in plan view in this embodiment. The substrate  51  may be a ceramic substrate, a glass epoxy substrate and the like. The substrate  51  includes a first main surface  54  on one side, and a second main surface  55  on the other side. The plurality of switching elements  33  to  38 , the plurality of regeneration diodes, and other elements are mounted on the first main surface  54  of the substrate  51 . In  FIGS. 3 and 4 , the regeneration diodes and other elements are not illustrated for purposes of simplicity. 
     A wiring pattern for connecting the switching elements  33  to  38  may be formed on the first main surface  54  of the substrate  51 . A control circuit (for example, the microcomputer  25 ) for controlling the switching elements  33  to  38  may be mounted on the second main surface  55  of the substrate  51 . Each of the switching elements  33  to  38  is formed in the shape of a rectangular parallelepiped having a rectangular shape elongated in one direction in plan view. Each of the switching elements  33  to  38  has a mounting surface  56  on one side, and a non-mounting surface  57  on the other side. 
     In this embodiment, the non-mounting surface  57  is a non-electrode surface with no electrode mounted thereon. That is, each of the switching elements  33  to  38  is a so-called lateral semiconductor device. A plurality of drain electrodes  58 , a plurality of source electrodes  59 , and one gate electrode  60  are formed on the mounting surface  56  of each of the switching elements  33  to  38 . Each of the drain electrodes  58  and the source electrodes  59  has a rectangular shape elongated in the short-side direction of the mounting surface  56  in plan view. 
     The plurality of drain electrodes  58  and the plurality of source electrodes  59  are alternately arranged at intervals in the long-side direction of the mounting surface  56 . The gate electrode  60  has the shape of a rectangle close to a square in plan view, and is arranged at the center in the width direction on one end side of the mounting surface  56 . The layout, the arrangement, and the number of drain electrodes  58 , source electrodes  59 , and gate electrodes  60  may be arbitrarily determined in accordance with the specifications of the switching elements  33  to  38 . For example, the drain electrode  58  and the source electrode  59  may be formed in a comb teeth shape to face each other. 
     Each of the switching elements  33  to  38  is joined to the first main surface  54  of the substrate  51 , with its mounting surface  56  facing the first main surface  54 . The drain electrodes  58 , the source electrodes  59 , and the gate electrode  60  are connected to the wiring pattern formed on the first main surface  54  of the substrate  51  by soldering. The first switching element  33  and the second switching element  34  may be arranged in a line at intervals along the periphery of the first main surface  54  of the substrate  51  such that their long-side directions coincide in plan view. 
     The third switching element  35  and the fourth switching element  36  may be arranged in a line at intervals along the periphery of the first main surface  54  of the substrate  51  such that their long-side directions coincide in plan view. The fifth switching element  37  and the sixth switching element  38  may be arranged in a line at intervals along the periphery of the first main surface  54  of the substrate  51  such that their long-side directions coincide in plan view. 
     A pair of the first switching element  33  and the second switching element  34 , a pair of the third switching element  35  and the fourth switching element  36 , and a pair of the fifth switching element  37  and the sixth switching element  38  may be arranged at positions respectively corresponding to three of the four sides of the square inscribed in the periphery of the substrate  51  in plan view. The radiator  52  covers all the plurality of switching elements  33  to  38 , and is connected to all the non-mounting surface  57  of the plurality of switching elements  33  to  38 . The radiator  52  may be connected to the non-mounting surfaces  57  of the plurality of switching elements  33  to  38  with metallic or insulating adhesive, a radiation sheet, or thermal grease interposed therebetween. 
     In this embodiment, the radiator  52  may be formed in a circular shape having a diameter less than or equal to the diameter of the substrate  51  in plan view. The radiator  52  may be formed in a circular shape having a diameter greater than the diameter of the substrate  51  in plan view. The radiator  52  may be arranged concentrically with the substrate  51  in plan view. The shape of the radiator in plan view is not limited to a circular shape. The radiator  52  may be formed in a polygonal shape such as a quadrilateral in plan view. 
     In this embodiment, the radiator  52  includes a casing  62  with a latent heat storage material  61  sealed therein. The latent heat storage material  61  may contain at least one of erythritol, paraffin, and inorganic hydrated salt (for example, sodium acetate). More specifically, the casing  62  includes a container part  63  filled with the latent heat storage material  61 , and a lid part  64  closing the container part  63 . The container part  63  and the lid part  64  may contain aluminum. The container part  63  includes a plate-shaped (in this embodiment, a circular plate shaped) bottom wall  65 , and a cylindrical (in this embodiment, a circular cylindrical) side wall  67 . The side wall  67  extends upright from the periphery of the bottom wall  65  so as to define an opening  66  on the side opposite to the bottom wall  65 . 
     The lid part  64  includes a plate-shaped (in this embodiment, a circular plate shaped) top wall  68 , and a cylindrical (in this embodiment, a circular cylindrical) side wall  70 . The side wall  70  extends downward from the periphery of the top wall  68  so as to define an opening  69  on the side opposite to the top wall  68 . The lid part  64  is press-fitted to the container part  63  in a manner such that the side wall  70  is externally fitted to the side wall  67  of the container part  63 . The lid part  64  may be press-fitted to the container part  63  in a manner such that the side wall  70  is internally fitted to the side wall  67  of the container part  63 . 
     The latent heat storage material  61  fills the space defined by the bottom wall  65  and the side wall  67  of the container part  63 . The properties of the latent heat storage material  61  will be described with reference to  FIG. 5 .  FIG. 5  is a graph illustrating the properties of the latent heat storage material  61 . When the temperature rise rate of the latent heat storage material  61  is greater than the natural cooling rate, a temperature T A  of the latent heat storage material  61  rises. On the other hand, when the natural cooling rate is greater than the temperature rise rate of the latent heat storage material  61 , the temperature T A  of the latent heat storage material  61  drops. 
     The latent heat storage material  61  transitions between a solid phase region  71  corresponding to a solid state, a phase transition region  72  where a solid state and a liquid state co-exist, and a liquid phase region  73  corresponding to a liquid state, in accordance with a rise in the temperature T A . In the case where the temperature T A  of the latent heat storage material  61  is in the solid phase region  71 , when the temperature T A  of the latent heat storage material  61  rises to a melting point T M  or above (T A  T M ), the latent heat storage material  61  transitions from the solid phase region  71  to the phase transition region  72 . 
     The latent heat storage material  61  absorbs heat energy when transitioning from the solid state to the liquid state. Therefore, when the latent heat storage material  61  is in the phase transition region  72  and is transitioning from the solid state to the liquid state, the rise in the temperature T A  of the latent heat storage material  61  is suppressed. Then, when the latent heat storage material  61  is completely in the liquid state, the temperature T A  of the latent heat storage material  61  rises again. On the other hand, in the case where the temperature T A  of the latent heat storage material  61  is in the liquid phase region  73 , when the temperature T A  of the latent heat storage material  61  drops to a freezing point T F  or below (T A ≤T F ), the latent heat storage material  61  transitions from the liquid phase region  73  to the phase transition region  72 . In this embodiment, for example, the melting point T M  and the freezing point T F  of the latent heat storage material  61  are substantially the same. 
     The latent heat storage material  61  releases heat energy when transitioning from the liquid state to the solid state. Therefore, when the latent heat storage material  61  is in the phase transition region  72  and is transitioning from the liquid state to the solid state, the drop in the temperature T A  of the latent heat storage material  61  is suppressed. Then, when the temperature T A  drops below the freezing point T F  (T A &lt;T F ) and the latent heat storage material  61  is completely in the solid state, the temperature T A  of the latent heat storage material  61  drops again. 
     The heat generated in the switching elements  33  to  38  are absorbed by the latent heat storage material  61  having the physical properties described above. The temperature sensor  53  is disposed in the casing  62  of the radiator  52 , and detects the temperature T A  of the latent heat storage material  61 . The temperature sensor  53  may be a thermocouple. The temperature sensor  53  is disposed at the center of the casing  62  in plan view. The temperature sensor  53  may be disposed at the center of gravity of the latent heat storage material  61 . 
     A part of the latent heat storage material  61  at the center of the casing  62  (the center of gravity of the latent heat storage material  61 ) is located at a position away from the switching elements  33  to  38 . Therefore, the part of the latent heat storage material  61  at the center of the casing  62  (the center of gravity of the latent heat storage material  61 ) is not easily heated, and does not easily transition from the solid state to the liquid state. Since the temperature sensor  53  is disposed on the part of the latent heat storage material  61  at the center of the casing  62  (the center of gravity of the latent heat storage material  61 ) and detects the temperature T A  of the latent heat storage material  61 , it is possible to monitor whether a part of the latent heat storage material  61  is remaining in the solid state. 
     In particular, in an embodiment in which the plurality of switching elements  33  to  38  are arranged at intervals along the periphery of the first main surface  54  of the substrate  51  (the periphery of the radiator  52 ) in plan view, the temperature sensor  53  is easily positioned. That is, in such an embodiment, by only arranging the temperature sensor  53  at the center of the casing  62  (the center of gravity of the latent heat storage material  61 ), the temperature sensor  53  can be located away from the switching elements  33  to  38 . Accordingly, compared with an embodiment in which the plurality of switching elements  33  to  38  are irregularly arranged, the temperature sensor  53  can easily be positioned. 
     A part of the latent heat storage material  61  that does not easily transition to the liquid state (that is, a region that is not easily heated) may be determined from an experiment in advance, and the temperature sensor  53  may be disposed on the part. The temperature T A  of the latent heat storage material  61  detected by the temperature sensor  53  is output to the microcomputer  25  (more specifically, a limiter control unit  83  that will be described below). Referring again to  FIG. 2 , the microcomputer  25  includes a central processing unit (CPU) and memories (a read-only memory (ROM), a random access memory (RAM), a non-volatile memory, and the like), and executes predetermined programs to serve as a plurality of function processing units. 
     The plurality of function processing units include an assist current value setting unit  81 , a limiter  82 , the limiter control unit  83 , a current command value setting unit  84 , a current deviation calculating unit  85 , a proportional-integral (PI) control unit  86 , a two-phase to three-phase converting unit  87 , a pulse width modulation (PWM) control unit  88 , a phase current calculating unit  89 , a three-phase to two-phase converting unit  90 , and a rotation angle calculating unit  91 . The limiter  82  and the limiter control unit  83  serve as a supercooling suppressing unit that suppresses supercooling of the latent heat storage material  61 . Supercooling is a phenomenon in which, in the case where the temperature T A  of the latent heat storage material  61  is in the liquid phase region  73 , even when the temperature T A  of the latent heat storage material  61  is lowered below the freezing point T F , the latent heat storage material  61  is not recrystallized and is maintained in the liquid state although the temperature T A  is supposed to be located in the solid phase region  71 . 
     The assist current value setting unit  81  sets an assist current value I a     0   * based on a steering torque T detected by the torque sensor  12  and a vehicle speed V detected by the vehicle speed sensor  23 , and outputs the assist current value I a     0   * to the limiter  82  at a predetermined constant current control cycle AT. The assist current value I a     0   * corresponds to a motor current for driving the electric motor  18 . That is, the assist current value I a     0   * is set to a positive value when the electric motor  18  needs to generate a steering assist force for steering to the right, and is set to a negative value when the electric motor  18  needs to generate a steering assist force for steering to the left. The assist current value I a     0   * is positive when the steering torque T has a positive value, and negative when the steering torque T has a negative value. 
     The assist current value I a     0   * is set such that its absolute value increases as the absolute value of the steering torque T increases. The assist current value I a     0   * is set such that its absolute value decreases as the vehicle speed V detected by the vehicle speed sensor  23  increases. A dead zone may be provided in which the assist current value I a     0   * is set to zero when the absolute value of the steering torque T is a very small value less than or equal to a predetermined value. 
     The limiter  82  is switched between an operating state and a non-operating state. Under normal conditions, the limiter  82  is controlled in the non-operating state. When the limiter  82  is in the operating state, the limiter  82  limits the assist current value I a     0   * to an assist current limit value I a     1   *. The limiter  82  may limit the assist current value I a     0   * to a value between a predetermined upper limit (positive value) and a predetermined lower limit (negative value). 
     On the other hand, when the limiter  82  is in the non-operating state, the limiter  82  simply outputs the assist current value I a     0   * set by the assist current value setting unit  81  to the current command value setting unit  84 . The limiter control unit  83  switches the limiter  82  between the operating state and the non-operating state, based on the assist current value I a     0   * set by the assist current value setting unit  81  and the temperature T A  of the latent heat storage material  61  detected by the temperature sensor  53 . The operations of the limiter control unit  83  will be described in detail below. 
     The current command value setting unit  84  sets the value of a current to be applied to the coordinate axes of a dq coordinate system as a current command value, based on the assist current value I a     0   * or the assist current limit value I a     1   *, and outputs the current command value to the current deviation calculating unit  85 . More specifically, the current command value setting unit  84  generates a d-axis current command value I d * and a q-axis current command value I q * (hereinafter may be collectively referred to as “two-phase current command values I dq *”). 
     More specifically, the current command value setting unit  84  sets the q-axis current command value I q * to a significant value, and sets the d-axis current command value I d * to zero. The two-phase current command values I dq * set by the current command value setting unit  84  are output to the current deviation calculating unit  85 . The rotation angle calculating unit  91  calculates the rotation angle (electrical angle) of the rotor of the electric motor  18 , based on the output signal of the rotation angle sensor  20 , and outputs the rotation angle to the two-phase to three-phase converting unit  87  and the three-phase to two-phase converting unit  90 . 
     The phase current calculating unit  89  calculates a U-phase current I U , a V-phase current I V , and a W-phase current I W  (hereinafter may be collectively referred to as “three-phase detection currents I UVW ”), based on the phase currents of the two phases detected by the U-phase current sensor  45  and the W-phase current sensor  46 , and outputs the three-phase detection currents I UVW  to the three-phase to two-phase converting unit  90 . The three-phase to two-phase converting unit  90  converts the three-phase detection currents I UVW  in the UVW coordinate system calculated by the phase current calculating unit  89  into a d-axis detection current I d  and a q-axis detection current I q  (hereinafter may be collectively referred to as “two-phase detection currents I dq ”) in the dq coordinate system, and outputs the two-phase detection currents I dq  to the current deviation calculating unit  85 . For the coordinate transformation, the rotor angle θ calculated by the rotation angle calculating unit  91  is used. 
     The current deviation calculating unit  85  calculates deviations between the two-phase current command values I dq * set by the current command value setting unit  84  and the two-phase detection currents I dq  output from the three-phase to two-phase converting unit  90 . More specifically, the current deviation calculating unit  85  calculates a deviation of the d-axis detection current I d  from the d-axis current command value I d *, and a deviation of the q-axis detection current I q  from the q-axis current command value I q *. These deviations are output to the PI control unit  86 . 
     The PI control unit  86  executes a PI calculation on the current deviation calculated by the current deviation calculating unit  85 , and thereby generates a d-axis voltage command value V d * and a q-axis voltage command value V q * (hereinafter may be collectively referred to as “two-phase voltage command values V dq *”) to be applied to the electric motor  18 . The two-phase voltage command values V dq * are output to the two-phase to three-phase converting unit  87 . The two-phase to three-phase converting unit  87  converts the two-phase voltage command values V dq * to three-phase voltage command values V UVW *. For the coordinate transformation, the rotor angle θ calculated by the rotation angle calculating unit  91  is used. 
     The three-phase voltage command values V UVW * include a U-phase voltage command value V U *, a V-phase voltage command value V V *, and a W-phase voltage command value V W *. The three-phase voltage command values V UVW * are output to the PWM control unit  88 . The PWM control unit  88  generates a U-phase PWM control signal, a V-phase PWM control signal, and a W-phase PWM control signal respectively having duty ratios corresponding to the U-phase voltage command value V U *, the V-phase voltage command value V V *, and the W-phase voltage command value W W *. 
     The PWM control unit  88  generates gate control signals for the respective switching elements  33  to  38  in the inverter device  1 , based on these PWM control signals, and supplies the gate control signals to the gate drive circuits (not illustrated) connected to the gates of the respective switching elements  33  to  38 . The gate drive circuits generate gate signals corresponding to the gate control signals supplied from the PWM control unit  88 , and supplies the gate signals to the gates of the corresponding switching elements  33  to  38 . Thus, voltages corresponding to the three-phase voltage command values V UVW * are applied to the field windings of the respective phases of the electric motor  18 . 
     The current deviation calculating unit  85  and the PI control unit  86  serve as a current feedback control unit. The motor current flowing to the electric motor  18  is controlled by the current feedback control unit so as to approach the two-phase current command values I dq * set by the current command value setting unit  84 .  FIG. 6  is a flowchart illustrating exemplary operations of the limiter control unit  83 .  FIG. 7  is a graph illustrating the properties of the latent heat storage material  61 . 
     Referring to  FIG. 6 , the limiter control unit  83  resets an integrated amount P n  of the assist current value I a     0   * (described below) to set the operation state back to the initial state (step S 1 ). Then, the limiter control unit  83  determines whether the following condition is satisfied: the temperature T A  of the latent heat storage material  61  is close to the melting point T M  of the latent heat storage material  61 , and the temperature T A  of the latent heat storage material  61  is on a rising trend (step S 2 ). 
     Referring to  FIG. 7 , the limiter control unit  83  may determine that the temperature T A  of the latent heat storage material  61  is close to the melting point T M  when the temperature T A  of the latent heat storage material  61  is in a threshold temperature range R th  that is determined in advance with reference to the melting point T M  of the latent heat storage material  61 . The threshold temperature range R th  may have a value T M −T th  obtained by subtracting a threshold temperature T th  (first threshold temperature) from the melting point T M  as the lower limit, and may have a value T M +T th  obtained by adding a predetermined threshold temperature T th  (second threshold temperature) to the melting point T M  as the upper limit. 
     In this case, when the temperature T A  of the latent heat storage material  61  is in the range greater than or equal to the lower limit T M −T th  and less than or equal to the upper limit T M +T th  (T M −T th ≤T A ≤T M +T th ), the limiter control unit  83  determines that the temperature T A  of the latent heat storage material  61  is close to the melting point T M . The threshold temperature T th  in the lower limit T M −T th  and the threshold temperature T th  in the upper limit T M +T th  may be different values. 
     The threshold temperature range R th  may have the melting point T M  as the upper limit. In this case, when the temperature T A  of the latent heat storage material  61  is in the range greater than or equal to the lower limit T M −T th  and less than the melting point T M  (T M −T th ≤T A &lt;T M ), the limiter control unit  83  determines that the temperature T A  of the latent heat storage material  61  is close to the melting point T M . Referring again to  FIG. 6 , when the absolute value |I a     0   *| of the assist current value I a     0   * is greater than or equal to a predetermined threshold I th  (I th ≤|I a     0   *|), the limiter control unit  83  determines that the temperature T A  of the latent heat storage material  61  is on a rising trend. 
     If the limiter control unit  83  determines that the temperature T A  of the latent heat storage material  61  is less than the lower limit T M −T th  of the threshold temperature range R th  (T M −T th &gt;T A ), or determines that the absolute value |I a     0   *| is less than the threshold I th  (I th &gt;|I a     0   *|) (step S 2 : NO), the limiter control unit  83  resets the integrated amount P n  of the assist current value I a     0   * (described below) (step S 1 ), and executes step S 2  again. 
     If the limiter control unit  83  determines that the temperature T A  of the latent heat storage material  61  is in the threshold temperature range R th  (T M −T th ≤T A ≤T M  +T th ), and the absolute value |I a     0   *| is greater than or equal to the threshold I th  (|I a     0   *|≥I th ) (step S 2 : YES), the limiter control unit  83  determines whether the amount of heat required to reach the liquid phase region  73  is applied from the switching elements  33  to  38  to the latent heat storage material  61  (steps S 3  and S 4 ). 
     In step S 3 , the limiter control unit  83  calculates the integrated amount P n  of the assist current value I a     0   * during the period from the resetting of the integrated amount P n  to the present time. More specifically, this period is the period from the last resetting of the integrated amount P n  to the time when the procedure reaches step S 3  without the determination in step S 2  being negative. This period includes n current control cycles AT (n is a natural number), and n assist current values I a     0   * corresponding to the periods. Accordingly, the limiter control unit  83  calculates the integrated amount P n  of n assist current values I a     0n   *. 
     More specifically, the integrated amount P n  is the sum obtained by integrating each multiplied value |I a     0n   *|×ΔT that is obtained by multiplying the absolute value |I a     0n   *| of each of n assist current values I a     0n   * by the current control cycle ΔT. That is, the integrated amount P n  is P n-1 +|I a     0n   *|×ΔT. n=1, P n-1  is zero, and the integrated amount P n  is |I a     01   *|×ΔT. Then, the limiter control unit  83  determines whether the integrated amount P n  is greater than or equal to a predetermined threshold P th  (P n ≥P th ) (step S 4 ). The threshold P th  may be a design parameter corresponding to the heat capacity of the latent heat storage material  61  stored in the radiator  52 , and proportional to the mass. 
     When the integrated amount P n  is greater than or equal to P th  (P n ≥P th ), it means that the amount of heat required to reach the liquid phase region  73  is applied to the latent heat storage material  61 . On the other hand, when the integrated amount P n  is less than P th  (P n &lt;P th ), it means that the amount of heat required to reach the liquid phase region  73  is not applied to the latent heat storage material  61 . If the limiter control unit  83  determines that the integrated amount P n  is less than P th  (P n &lt;P th ) and the amount of heat required to reach the liquid phase region  73  is not applied to the latent heat storage material  61  (step S 4 : NO), the limiter control unit  83  executes step S 2 , step S 3 , and step S 4  again. 
     In this manner, the limiter control unit  83  repeats step S 2 , step S 3 , and step S 4  until the integrated amount P n  becomes greater than or equal to the threshold value P th  (P n ≥P th ). Then, the limiter control unit  83  calculates the integrated amount P n  of the assist current value I a     0   * during the period in which the determination in step S 2  continues to be affirmative without being negative after the resetting of the integrated amount P n . If the limiter control unit  83  determines that the integrated amount P n  is greater than or equal to P th  (P n ≥P th ), and the amount of heat required to reach the liquid phase region  73  is applied to the latent heat storage material  61  (step S 4 : YES), the limiter control unit  83  switches the limiter  82  from the non-operating state to the operating state (step S 5 ). Thus, the assist current value I a     0   * output from the assist current value setting unit  81  is limited to the assist current limit value I a     1   *. 
     Then, gate control signals generated based on the assist current limit value I a     1   * are output from the PWM control unit  88  to the motor drive circuit unit  26 . Thus, the amount of heat applied from the switching elements  33  to  38  to the latent heat storage material  61  decreases, so that the temperature T A  of latent heat storage material  61  drops. Note that the absolute value |I a     1   *| of the assist current limit value I a     1   * may be set to be less than the threshold I th  (I th &gt;|I a     1   *|) In this case, the temperature T A  of the latent heat storage material  61  shifts to a falling trend, so that the temperature T A  of the latent heat storage material  61  can be reliably lowered. 
     The absolute value |I a     1   *| of the assist current limit value I a     1   * may be set based on a subtracted value P n −P th  obtained by subtracting the threshold P th  from the integrated amount P n . For example, the absolute value |I a     1   *| of the assist current limit value I a     1   * may be set to be a value proportional to the subtracted value P n −P th . Even in this case, the temperature T A  of the latent heat storage material  61  can be reliably lowered. 
     Subsequently, the limiter control unit  83  determines whether the temperature T A  of the latent heat storage material  61  is sufficiently lowered (step S 6 ). In step S 6 , the limiter control unit  83  may determine whether the temperature T A  of the latent heat storage material  61  is less than the freezing point T F  of the latent heat storage material  61  (T A &lt;T F ). If the limiter control unit  83  determines that the temperature T A  is greater than or equal to the freezing point T F  (T A ≥T F ) (step S 6 : NO), the limiter control unit  83  executes step S 6  again. 
     If the limiter control unit  83  determines that the temperature T A  is less than the freezing point T F  (T A &lt;T F ) (step S 6 : YES), the limiter control unit  83  switches the limiter  82  from the operating state to the non-operating state (step S 7 ). Thus, the assist current value I a     0   * output from the assist current value setting unit  81  is output unchanged to the current command value setting unit  84 . Then, the limiter control unit  83  resets the integrated amount P n  of the assist current value I a     0   * (step  51 ), and executes step S 2  again. 
     In step S 6 , the limiter control unit  83  may determine whether the temperature T A  of the latent heat storage material  61  is less than the lower limit T M −T th  of the threshold temperature range R th  (T A &lt;T M −T th ). That is, if the limiter control unit  83  determines that the temperature T A  is greater than or equal to the lower limit T M −T th  of the threshold temperature range R th  (T A ≥T M −T th ) (step S 6 : NO), the limiter control unit  83  executes step S 6  again. 
     If the limiter control unit  83  determines that the temperature T A  is less than the lower limit T M −T th  of the threshold temperature range R th  (T A &lt;T M −T th ) (step S 6 : YES), the limiter control unit  83  switches the limiter  82  from the operating state to the non-operating state (step S 7 ). In this manner, in the inverter device  1 , if the limiter control unit  83  determines that the temperature T A  of the latent heat storage material  61  reaches the liquid phase region  73 , the limiter control unit  83  limits the assist current value I a     0   * to prevent the temperature T A  from reaching the liquid phase region  73 , thereby reducing the amount of heat to be applied to the latent heat storage material  61 . 
     Thus, it is possible to suppress a rise in the temperature T A  of the latent heat storage material  61 , and lower the temperature T A  of the latent heat storage material  61 . Therefore, the temperature T A  of the latent heat storage material  61  can be prevented from reaching the liquid phase region  73 . Further, the temperature T A  of the latent heat storage material  61  can be maintained in the solid phase region  71  or the phase transition region  72 , so that a part of the latent heat storage material  61  can always be maintained in the solid state. 
     As a result, when the temperature T A  of the latent heat storage material  61  in the phase transition region  72  drops, the liquid part of the latent heat storage material  61  can be recrystallized, using the solid part of the latent heat storage material  61  as the starting point of recrystallization. This makes it possible to suppress supercooling of the latent heat storage material  61 , and appropriately control the temperature of the switching elements  33  to  38 . 
       FIG. 8  is a block diagram schematically illustrating the electrical configuration of an ECU  24  to which an inverter device  101  according to a second embodiment of the present invention is applied.  FIG. 9  is a cross-sectional view of a part corresponding to the part illustrated in  FIG. 4 , illustrating the general configuration of the inverter device  101  of  FIG. 8 . Elements corresponding to those of the inverter device  1  described above are denoted by the same reference numerals, and will not be further described. 
     Referring to  FIGS. 8 and 9 , the ECU  24  includes the inverter device  101  in place of the inverter device  1 . The inverter device  101  differs in configuration from the inverter device  1 . That is, the inverter device  101  does not include the limiter  82  or the limiter control unit  83 , but includes a second radiator  102 , a relay section  103 , and a relay section control unit  108 . The second radiator  102  is disposed away from the radiator  52 . In this embodiment, the second radiator  102  is located at a position facing the substrate  51  with the radiator  52  interposed therebetween. A housing (not illustrated) made of metal (for example, made of aluminum) and accommodating the ECU  24  may be used as the second radiator  102 . 
     The relay section  103  is a transfer member that transfers the heat of the radiator  52  to the second radiator  102 . The relay section  103  is switched between a connecting state for thermally connecting the radiator  52  to the second radiator  102  and a disconnecting state for thermally disconnecting the radiator  52  from the second radiator  102 . In the following description, it is assumed that the relay section  103  includes a mechanical relay. The relay section  103  includes a support part  104 , a connection part  105 , a movable part  106 , and an electromagnetic part  107 . Each of the support part  104 , the connection part  105 , and the movable part  106  may be made of a metal material. 
     The support part  104  is attached to the casing  62  of the radiator  52  (the lid part  64  in this embodiment). The connection part  105  is attached to the second radiator  102 . The movable part  106  is pivotally attached to the support part  104 . The movable part  106  pivots in a range between the connecting state and the disconnecting state. In the connecting state, the movable part  106  abuts the connection part  105  to thermally connect the radiator  52  to the second radiator  102 . In the disconnecting state, the movable part  106  is spaced apart from the connection part  105  to thermally disconnect the radiator  52  from the second radiator  102 . 
     The movable part  106  is normally in the disconnecting state. The movable part  106  may be normally in the connecting state. However, in this case, consideration needs to be given on the temperature rise of the second radiator  102  and the influence of the temperature rise on other members. For example, if the housing accommodating the ECU  24  is used as the second radiator  102 , the temperature of the entire ECU  24  rises. Therefore, it is not always preferable that the movable part  106  is normally in the connecting state. 
     The electromagnetic part  107  is connected to the microcomputer  25 . The electromagnetic part  107  has magnetic properties such that the electromagnetic part  107  is magnetized to N pole or S pole in accordance with the direction of the current input from the microcomputer  25 , and generates an attractive force or a repulsive force to the movable part  106 . When the electromagnetic part  107  generates a repulsive force, the movable part  106  is switched from the disconnecting state to the connecting state. Thus, the radiator  52  is thermally connected to the second radiator  102 . Then, the heat of the radiator  52  is transferred to the second radiator  102  via the relay section  103 , so that the temperature T A  of the latent heat storage material  61  is lowered. 
     On the other hand, when the electromagnetic part  107  generates an attractive force, the movable part  106  is switched from the connecting state to the disconnecting state. Thus, the second radiator  102  is thermally disconnected from the radiator  52 , and is cooled naturally. Referring back to  FIG. 8 , in this embodiment, the plurality of function processing units of the microcomputer  25  include the relay section control unit  108 , in place of the limiter  82  and the limiter control unit  83 . In this embodiment, the relay section  103  and the relay section control unit  108  serve as a supercooling suppressing unit that suppresses supercooling of the latent heat storage material  61 . 
     The relay section control unit  108  switches the relay section  103  between the disconnecting state and the connecting state, based on the assist current value I a     0   * set by the assist current value setting unit  81  and the temperature T A  of the latent heat storage material  61  detected by the temperature sensor  53 .  FIG. 10  is a flowchart illustrating exemplary operations of the relay section control unit  108 . The operations of the relay section control unit  108  differ from the operations of the limiter control unit  83  in that step S 15  is executed in place of step S 5 , and in that step S 17  is executed in place of step S 7 . 
     The operations of the relay section control unit  108  in steps other than steps S 15  and S 17  are the same as those of the limiter control unit  83  in steps other than steps S 5  and S 7 , and will not be described herein. If the relay section control unit  108  determines that the integrated amount P n  is greater than or equal to P th  (P n ≥P th ), and the amount of heat required to reach the liquid phase region  73  is applied to the latent heat storage material  61  (step S 4 : YES), the relay section control unit  108  switches the relay section  103  from the disconnecting state to the connecting state (step S 15 ). 
     Thus, the radiator  52  is thermally connected to the second radiator  102 . Then, the heat of the radiator  52  is transferred to the second radiator  102  via the relay section  103 , so that the temperature T A  of the latent heat storage material  61  is lowered. Then, if the relay section control unit  108  determines that the temperature T A  of the latent heat storage material  61  is less than the freezing point T F  (T A &lt;T F ) (step S 6 : YES), the relay section control unit  108  switches the relay section  103  from the connecting state to the disconnecting state (step S 17 ). Thus, the second radiator  102  is thermally disconnected from the radiator  52 , and is cooled naturally. 
     In this manner, in the inverter device  101 , if the relay section control unit  108  determines that the temperature T A  of the latent heat storage material  61  reaches the liquid phase region  73 , the relay section control unit  108  switches the relay section  103  from the disconnecting state to the connecting state to prevent the temperature T A  from reaching the liquid phase region  73 , thereby transferring the heat of the radiator  52  to the second radiator  102 . Thus, it is possible to suppress a rise in the temperature T A  of the latent heat storage material  61 , and lower the temperature T A  of the latent heat storage material  61 . Therefore, the temperature T A  of the latent heat storage material  61  can be prevented from reaching the liquid phase region  73 . Further, the temperature T A  of the latent heat storage material  61  can be maintained in the solid phase region  71  or the phase transition region  72 . Accordingly, a part of the latent heat storage material  61  can always be maintained in the solid state. 
     As a result, when the temperature T A  of the latent heat storage material  61  in the phase transition region  72  drops, the liquid part of the latent heat storage material  61  can be recrystallized, using the solid part of the latent heat storage material  61  as a starting point of recrystallization. This makes it possible to suppress supercooling of the latent heat storage material  61 , and appropriately control the temperature of the switching elements  33  to  38 . 
     While exemplary embodiments of the present invention are described above, the present invention may be practiced in other embodiments. In the first embodiment described above, if the limiter control unit  83  determines that the temperature T A  of the latent heat storage material  61  is in the threshold temperature range R th  (T M −T th  ≤T A ≤T M +T th ), and the absolute value |I a     0   *| is greater than or equal to the threshold I th  (|I a     0   *|≥I th ) (step S 2 : YES), the limiter control unit  83  may skip steps S 3  and S 4 , and switch the limiter  82  from the non-operating state to the operating state (step S 5 ). 
     In the second embodiment described above, if the relay section control unit  108  determines that the temperature T A  of the latent heat storage material  61  is in the threshold temperature range R th  (T M −T th ≤T A ≤T M +T th ), and the absolute value |I a     0   *| is greater than or equal to the threshold I th  (|I a     0   *|≥I th ) (step S 2 : YES), the relay section control unit  108  may skip steps S 3  and S 4 , and switch the relay section  103  from the disconnecting state to the connecting state (step S 15 ). 
     An inverter device having a structure achieved by combining the first embodiment and the second embodiment may be employed. That is, the inverter device  1  of the first embodiment may further include the second radiator  102 , the relay section  103 , and the relay section control unit  108 . The inverter device  101  of the second embodiment may further include the limiter  82  and the limiter control unit  83 . 
     In the case of these structures, step S 5  of the limiter control unit  83  and step S 15  of the relay section control unit  108  may be performed in parallel. Also, step S 7  of the limiter control unit  83  and step S 17  of the relay section control unit  108  may be performed in parallel. In the embodiments described above, when the temperature T A  of the latent heat storage material  61  is in the range greater than or equal to the lower limit T M −T th  and less than or equal to the upper limit T M +T th  (T M −T th ≤T A ≤T M +T th ), the temperature T A  of the latent heat storage material  61  is determined to be close to the melting point T M . 
     However, when an absolute value |T A −T M | of the deviation between the temperature T A  of the latent heat storage material  61  and the melting point T M  of the latent heat storage material  61  is less than or equal to the threshold temperature T th  (|T A −T M |≤T th ), the temperature T A  of the latent heat storage material  61  may be determined to be close to the melting point T M . In the embodiments described above, each of the switching elements  33  to  38  may be a so-called vertical semiconductor device with an electrode formed also on the non-mounting surface  57 . In this case, the radiator  52  is connected to the switching elements  33  to  38  in a manner insulated from the switching elements  33  to  38 . 
     In the embodiments described above, the switching elements  33  to  38  may be devices other than MISFETs. Examples of devices other than MISFETs include high electron mobility transistor (HEMT), insulated gate bipolar transistor (IGBT), and bipolar junction transistor (BJT). Further, various modifications can be made to the present invention within the scope of the appended claims.