Abstract:
An overheating protection control apparatus for an inverter driving a rotating electric machine comprising: a temperature sensor for measuring the temperature of a power control element in the inverter, and a control device restricting the load factor of the rotating electric machine when the temperature measured by the temperature sensor reaches a threshold value. The control device modifies the threshold value based on a parameter affecting heat radiation or cooling of the inverter. Preferably, the inverter includes a plurality of power control elements. The temperature sensor detects the temperature of one or more, but not all of the plurality of power control elements. The parameter is a physical quantity affecting the temperature difference between the one or more power control elements and another power control element included in the inverter. Preferably, the inverter is cooled by a coolant medium. The parameter is the temperature of the coolant medium.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to an overheating protection control apparatus for an inverter, and an overheating protection control method for an inverter. 
       BACKGROUND ART 
       [0002]    Japanese Patent Laying-Open No. 03-003670 (PTL 1) discloses the technique of performing output current restriction control and associated reduction of output power when the value of a temperature sensor corresponding to an element or the like exceeds a predetermined value, as overheating protection control for an inverter. 
       CITATION LIST 
     Patent Literature 
       [0000]    
       
         PTL 1: Japanese Patent Laying-Open No. 03-003670 
         PTL 2: Japanese Patent Laying-Open No. 2008-072818 
         PTL 3: Japanese Patent Laying-Open No. 2007-129801 
         PTL 4: Japanese Patent Laying-Open No. 2009-171766 
         PTL 5: Japanese Patent Laying-Open No. 2010-124594 
         PTL 6: Japanese Patent Laying-Open No. 2009-189181 
       
     
       SUMMARY OF INVENTION 
     Technical Problem 
       [0009]    In accordance with the technique disclosed in the aforementioned Japanese Patent Laying-Open No. 03-003670, the load factor will be restricted with no exception when the value of the temperature sensor exceeds a predetermined threshold value. 
         [0010]    Since an inverter has a certain size and the spot of the inverter where the temperature sensor can measure is only a representative spot, the measured spot will not necessarily match the spot of the inverter where the temperature is highest. In order to eliminate the occurrence of an overheated site from anywhere in the inverter regardless of the various changes in the operating state of the inverter, sufficient margin must be set for the threshold value. 
         [0011]    This means that the load factor may be restricted even in the case where the operation is actually allowed without having to restrict the load factor. There may be a case where the performance of the inverter is not exhibited sufficiently. 
         [0012]    An object of the present invention is to provide an overheating protection control apparatus for an inverter and an overheating protection control method for an inverter that allows the performance of the inverter to be exhibited sufficiently. 
       Solution to Problem 
       [0013]    The present invention is directed to an overheating protection control apparatus for an inverter driving a rotating electric machine. The overheating protection control apparatus includes a temperature sensor for measuring the temperature of a power control element in the inverter, and a control device restricting the load factor of the rotating electric machine when the temperature measured by the temperature sensor reaches a threshold value. The control device modifies the threshold value based on a parameter that affects heat radiation or cooling of the inverter. 
         [0014]    Preferably, the inverter includes a plurality of power control elements. The temperature sensor detects the temperature of one or more but not all of the plurality of power control elements. The parameter includes a physical quantity affecting a temperature difference between the one or more power control elements and another power control element in the inverter. 
         [0015]    More preferably, the inverter is cooled by a coolant medium. The parameter is the temperature of the coolant medium. 
         [0016]    More preferably, the parameter includes any of a DC power supply voltage and a carrier frequency of the inverter. 
         [0017]    More preferably, DC power supply voltage boosted by a boost converter is supplied to the inverter. The parameter includes any of a DC power supply voltage of the inverter, a carrier frequency of the inverter, a power supply voltage prior to being boosted by the boost converter, and a flowing current of the inverter. 
         [0018]    According to another aspect, the present invention is directed to an overheating protection control method for an inverter driving a rotating electric machine. The method includes the steps of measuring the temperature of a power control element in the inverter, measuring a parameter differing from the temperature of the power control element in the inverter and that affects heat radiation or cooling of the inverter, modifying a threshold value based on the parameter, and restricting a load factor of the rotating electric machine when the measured temperature of the power control element in the inverter reaches the threshold value. 
       Advantageous Effects of Invention 
       [0019]    Since the load factor is restricted in accordance with the operating state of the inverter system in the present invention, the performance of the inverter can be exhibited sufficiently. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0020]      FIG. 1  is a circuit diagram representing a configuration of a vehicle  100  in which an inverter overheating protection control apparatus is mounted. 
           [0021]      FIG. 2  is a circuit diagram representing a detailed configuration of inverters  14  and  22  in  FIG. 1 . 
           [0022]      FIG. 3  is a circuit diagram representing a detailed configuration of a voltage converter  12  in  FIG. 1 . 
           [0023]      FIG. 4  represents the arrangement of IGBT elements and temperature sensors of a PCU  240 . 
           [0024]      FIG. 5  is a block diagram in association with motor control of a control device  30  in  FIG. 1 . 
           [0025]      FIG. 6  is a flowchart to describe a determination process of a load factor restriction start temperature Tps and motor drive control executed at a PM-ECU  32  and a MG-ECU  34  in  FIG. 5 . 
           [0026]      FIG. 7  represents an exemplified study when load factor restriction start temperature Tps is set at a fixed value. 
           [0027]      FIG. 8  is a diagram to describe a study of improving load factor restriction start temperature Tps. 
           [0028]      FIG. 9  represents an improved load factor restriction start temperature Tps. 
           [0029]      FIG. 10  represents an example of modifying load factor restriction start temperature Tps based on a carrier frequency fsw. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0030]    Embodiments of the present invention will be described in detail hereinafter with reference to the drawings. In the drawings, the same or corresponding elements have the same reference characters allotted, and description thereof will not be repeated. 
         [0031]      FIG. 1  is a circuit diagram representing a configuration of a vehicle  100  in which an inverter overheating protection control apparatus is mounted. Vehicle  100  is exemplified as a hybrid vehicle also incorporating an internal combustion engine. The present invention is also applicable to an electric vehicle and fuel cell vehicle, as long as the vehicle has an inverter mounted. 
         [0032]    [Description of Vehicle Driving System] 
         [0033]    Referring to  FIG. 1 , vehicle  100  includes a battery MB that is a power storage device, a voltage sensor  10 , a power control unit (PCU)  240 , a driving unit  241 , an engine  4 , a wheel  2 , and a control device  30 . Driving unit  241  includes motor generators MG 1  and MG 2 , and a power split mechanism  3 . 
         [0034]    PCU  240  includes a voltage converter  12 , smoothing capacitors C 1  and CH, voltage sensors  13  and  21 , and inverters  14  and  22 . Vehicle  100  further includes a positive bus line PL 2  and a negative bus line SL 2  feeding power to inverters  14  and  22  driving motor generators MG 1  and MG 2 , respectively. 
         [0035]    Voltage converter  12  is provided between battery MB and positive bus line PL 2  for voltage conversion. Smoothing capacitor C 1  is connected between positive bus line PL 1  and negative bus line SL 2 . Voltage sensor  21  detects a voltage VL across the terminals of smoothing capacitor C 1  and provides the detected voltage to control device  30 . Voltage converter  12  boosts the voltage across the terminals of smoothing capacitor C 1 . 
         [0036]    Smoothing capacitor CH smoothes the voltage boosted by voltage converter  12 . Voltage sensor  13  detects voltage VH across the terminals of smoothing capacitor CH for output to control device  30 . 
         [0037]    Inverter  14  converts the DC voltage applied from voltage converter  12  into 3-phase AC voltage for output to motor generator MG 1 . Inverter  22  converts the DC voltage applied from voltage converter  12  into 3-phase AC voltage for output to motor generator MG 2 . 
         [0038]    Power split mechanism  3  is coupled to engine  4  and to motor generators MG 1  and MG 2  to split the power therebetween. For example, a planetary gear mechanism including three rotational shafts of a sun gear, a planetary carrier, and a ring gear may be employed as the power split mechanism. In the planetary gear mechanism, when the rotation of two of the three rotational shafts is determined, the rotation of the remaining one rotational shaft is inherently determined. These three rotational shafts are connected to each rotational shaft of engine  4 , motor generator MG 1  and motor generator MG 2 , respectively. The rotational shaft of motor generator MG 2  is coupled to wheel  2  by means of a reduction gear and/or differential gear not shown. Furthermore, a reduction gear for the rotational shaft of motor generator MG 2  may be additionally incorporated in power split mechanism  3 . 
         [0039]    Vehicle  100  further includes a system main relay SMRB connected between the positive electrode of battery MB and positive bus line PL 1 , and a system main relay SMRG connected between the negative electrode (negative bus line SL 1 ) of battery MG and negative bus line SL 2 . 
         [0040]    System main relays SMRB and SMRG have their conducting/non-conducting state controlled by a control signal applied from control device  30 . Battery MB and converter  12  are connected by system main relays SMRB and SMRG. 
         [0041]    Voltage sensor  10  measures a voltage VB of battery MB. A current sensor  11  detecting a current  1 B flowing to battery MB is provided for the purpose of monitoring the charging state of battery MB together with voltage sensor  10 . For battery MB, a secondary battery such as a lead battery, a nickel-metal hydride battery or a lithium ion battery, or a capacitor of large capacitance such as an electrical double layer capacitor may be employed. 
         [0042]    Inverter  14  is connected to positive bus line PL 2  and negative bus line SL 2 . Inverter  14  receives a voltage boosted from voltage converter  12  to drive motor generator MG 1  for the purpose of, for example, starting engine  4 . Furthermore, inverter  14  returns the power generated at motor generator MG 1  by the power transmitted from engine  4  to voltage converter  12 . At this stage, voltage converter  12  is under control of control device  30  so as to operate as a down-converting circuit. 
         [0043]    Current sensor  24  detects the current flowing to motor generator MG 1  as a motor current value MCRT 1 , which is output to control device  30 . 
         [0044]    Inverter  22  is connected to positive bus line PL 2  and negative bus line SL 2  in parallel with inverter  14 . Inverter  22  converts DC voltage output from voltage converter  12  into 3-phase AC voltage for output to motor generator MG 2  driving wheel  2 . Furthermore, inverter  22  returns the power generated at motor generator MG 2  to voltage converter  12  in accordance with regenerative braking. At this stage, voltage converter  12  is under control of control device  30  so as to operate as a down-converting circuit. 
         [0045]    Current sensor  25  detects the current flowing to motor generator MG 2  as a motor current value MCRT 2 , which is output to control device  30 . 
         [0046]    Control device  30  receives each torque command value and rotational speed of motor generators MG 1  and MG 2 , each of the values of current IB and voltages VB, VL and VH, motor current values MCRT 1  and MCRT 2 , and an activation signal IGON. Control device  30  outputs to voltage converter  12  a control signal PWU to effect a voltage boosting command, a control signal PWD to effect a voltage down-conversion command, and a shut down signal to effect an operation prohibition command. 
         [0047]    Furthermore, control device  30  outputs to inverter  14  a control signal PWMI 1  to effect a drive command for converting DC voltage that is the output from voltage converter  12  into an AC voltage directed to driving motor generator MG 1 , and a control signal PWMC 1  to effect a regenerative command for converting the AC voltage generated at motor generator MG 1  into DC voltage to be returned towards voltage converter  12 . 
         [0048]    Similarly, control device  30  outputs to inverter  22  a control signal PWMI 2  to effect a drive command for converting the DC voltage into AC voltage directed to driving motor generator MG 2 , and a control signal PWMC 2  to effect a regenerative command for converting the AC voltage generated at motor generator MG 2  into DC voltage to be returned towards voltage converter  12 . 
         [0049]    [Description of Vehicle Cooling System] 
         [0050]    Vehicle  100  includes, as the cooling system for cooling PCU  240  and driving unit  241 , a radiator  102 , a reservoir tank  106 , and a water pump  104 . 
         [0051]    Radiator  102 , PCU  240 , reservoir tank  106 , water pump  104  and driving unit  241  are connected in series in a circular manner through a water channel  116 . 
         [0052]    Water pump  104  serves to circulate a coolant such as anti-free fluid in the direction illustrated by the arrow. Radiator  102  receives the coolant subsequent to cooling voltage converter  12  and inverter  14  in PCU  240  from the water channel to cool the received coolant. 
         [0053]    As will be described afterwards with reference to  FIG. 4 , a temperature sensor  300  measuring the temperature of the coolant, temperature sensors  301  and  302  detecting the temperature of voltage converter  12 , and temperature sensors  303  and  304  detecting the temperature of inverters  14  and  22 , respectively, are provided in the configuration of  FIG. 1 . 
         [0054]    Based on an output from the temperature sensors, control device  30  generates a signal SP directed to driving water pump  104 , and provides the generated signal SP to water pump  104 . Based on the output of the temperature sensors, control device  30  executes overheating protection control such that voltage converter  12  and inverters  14  and  22  are not overheated. 
         [0055]      FIG. 2  is a circuit diagram representing a detailed configuration of inverters  14  and  22  in  FIG. 1 . 
         [0056]    Referring to  FIGS. 1 and 2 , inverter  14  includes a U-phase arm  15 , a V-phase arm  16 , and a W-phase arm  17 . U-phase arm  15 , V-phase arm  16  and W-phase arm  17  are connected in parallel between positive bus line PL 2  and negative bus line SL 2 . 
         [0057]    U-phase arm  15  includes IGBT elements Q 3  and Q 4  connected in series between positive bus line PL 2  and negative bus line SL 2 , and diodes D 3  and D 4  connected in parallel with IGBT elements Q 3  and Q 4 , respectively. Diode D 3  has its cathode connected to the collector of IGBT element Q 3 , and its anode connected to the emitter of IGBT element Q 3 . Diode D 4  has its cathode connected to the collector of TGBT element Q 4 , and its anode connected to the emitter of TGBT element Q 4 . 
         [0058]    V-phase arm  16  includes IGBT elements Q 5  and Q 6  connected in series between positive bus line PL 2  and negative bus line SL 2 , and diodes D 5  and D 6  connected in parallel with IGBT elements Q 5  and Q 6 , respectively. Diode D 5  has its cathode connected to the collector of IGBT element Q 5  and its anode connected to the emitter of IGBT element Q 5 . Diode D 6  has its cathode connected to the collector of IGBT element Q 6 , and its anode connected to the emitter of IGBT element Q 6 . 
         [0059]    W-phase arm  17  includes IGBT elements Q 7  and Q 8  connected in series between positive bus line PL 2  and negative bus line SL 2 , and diodes D 7  and D 8  connected in parallel with IGBT elements Q 7  and Q 8 , respectively. Diode D 7  has its cathode connected to the collector of IGBT element Q 7 , and its anode connected to the emitter of IGBT element Q 7 . Diode D 8  has its cathode connected to the collector of IGBT element Q 8 , and its anode connected to the emitter of IGBT element Q 8 . 
         [0060]    The intermediate point of each phase arm is connected to each phase end of each phase coil of motor generator MG 1 . Specifically, motor generator MG 1  is a 3-phase permanent magnet synchronous motor. The three coils of the U, V and W-phase have each one end connected together to the neutral point. The other end of the U-phase coil is connected to a line UL drawn out from the connection node of IGBT elements Q 3  and Q 4 . The other end of the V-phase coil is connected to a line VL drawn out from the connection node of IGBT elements Q 5  and Q 6 . The other end of the W-phase coil is connected to a line WL drawn out from the connection node of IGBT elements Q 7  and Q 8 . 
         [0061]    Inverter  22  of  FIG. 1  is similar to inverter  14  as to the internal circuit configuration, provided that it is connected to motor generator MG 2 . Therefore, detailed description thereof will not be repeated. For the sake of simplification,  FIG. 2  is depicted with control signals PWMI and PWMC applied to the inverter. Different control signals PWMI 1  and PWMC 1 , and control signals PWMI 2  and PWMC 2  are applied to inverters  14  and  22 , respectively, as shown in  FIG. 1 . 
         [0062]      FIG. 3  is a circuit diagram representing a detailed configuration of voltage converter  12  of  FIG. 1 . 
         [0063]    Referring to  FIGS. 1 and 3 , voltage converter  12  includes a reactor L 1  having one end connected to positive bus line PL 1 , IGBT elements Q 1  and Q 2  connected in series between positive bus line PL 2  and negative bus line SL 2 , and diodes D 1  and D 2  connected in parallel with IGBT elements Q 1  and Q 2 , respectively. 
         [0064]    Reactor L 1  has the other end connected to the emitter of IGBT element Q 1  and the collector of IGBT element Q 2 . Diode D 1  has its cathode connected to the collector of IGBT element Q 1  and its anode connected to the emitter of IGBT element Q 1 . Diode D 2  has its cathode connected to the collector of IGBT element Q 2  and its anode connected to the emitter of IGBT element Q 2 . 
         [0065]      FIG. 4  represents an arrangement of IGBT elements and temperature sensors of PCU  240 . 
         [0066]    Referring to  FIG. 4 , a coolant flows into the cooling channel of the casing of PCU  240 , as indicated by the top right arrow, and flows out as indicated by the bottom left arrow after passing through the cooling channel of the casing of PCU  240 . 
         [0067]    PCU  240  has temperature sensor  300  provided in the neighborhood of the inlet of the coolant. Temperature sensor  300  outputs a coolant temperature Tw to control device  30 . The PCU casing has arranged, from the coolant inlet towards the outlet, IGBT elements Q 1  and Q 2  and diodes D 1  and D 2  of voltage converter  12 , IGBT elements Q 3   g -Q 8   g  and diodes D 3   g -D 8   g  of inverter  14 , and IGBT elements Q 3   m -Q 8   m  and diodes D 3   m -D 8   m  of inverter  22 , in the cited order. PCU  240  also has temperature sensors  301 - 304  provided. For voltage converter  12 , temperature sensor  301  is provided in proximity to IGBT element Q 1  whereas temperature sensor  302  is provided in proximity to IGBT element Q 2 . For inverters  14  and  22 , temperature sensor  303  is provided in proximity to IGBT element Q 6   g , whereas temperature sensor  304  is provided in proximity to IGBT element Q 6   m.    
         [0068]    Since PCU  240  has a certain size and the spots where temperature sensors  301 - 304  can measure are only representative spots, the measured spots will not necessarily match the spot of PCU  240  where the temperature is highest. Therefore, the temperature threshold value to initiate load factor restriction is determined such that none of the elements attains an overheated state regardless of the various changes in the operating state of inverters  14  and  22  as well as voltage converter  12 . However, if the margin provided is too great between the element heat-resisting temperature and the temperature threshold value, load factor restriction will occur frequently such that the performance of the inverter cannot be exhibited sufficiently. 
         [0069]    The present embodiment is directed to modifying the temperature threshold value based on the operating state of the inverter and/or voltage converter. 
         [0070]      FIG. 5  is a block diagram associated with motor control of control device  30  in  FIG. 1 . 
         [0071]    Referring to  FIG. 5 , control device  30  includes a power management ECU (hereinafter, PM-ECU)  32 , and a motor generator control ECU (hereinafter, MG-ECU)  34 . MG-ECU  34  includes a control circuit for inverter  22  driving motor generator MG 2  that is a driving motor, a control circuit (not shown) for inverter  14  driving motor generator MG 1 , and a drive control unit  430  for controlling the drive of water pump  104 . 
         [0072]    The inverter control circuit includes a 3-phase/2-phase conversion unit  424 , a load factor control unit  426 , a current command conversion unit  410 , subtracters  412  and  414 , PI control units  416  and  418 , a 2-phase/3-phase conversion unit  420 , and a PWM generation unit  422 . 
         [0073]    3-phase/2-phase conversion unit  424  receives motor currents Iv and Iw from two current sensors  25 . 3-phase/2-phase conversion unit  424  calculates motor current Iu=−Iv−Iw based on motor currents Iv and Iw. 
         [0074]    3-phase/2-phase conversion unit  424  converts the three phases of motor currents Iu, Iv and Iw into 2 phases using a degree of rotation θ from a rotation speed sensor not shown. In other words, 3-phase/2-phase conversion unit  424  converts the 3-phase motor currents Iu, Iv and Iw flowing through each phase of the 3-phase coils of motor generator MG 2  into current values Id and Iq flowing to the d axis and q axis using a degree of rotation θ. 3-phase/2-phase conversion unit  424  outputs the calculated current values Id and Iq to subtracter  412  and subtracter  414 , respectively. 
         [0075]    PM-ECU  32  receives element temperature Td and coolant temperature Tw from temperature sensors  300 - 304  provided at PCU  240  described with reference to  FIG. 4  to provide a load factor restriction command of motor generator MG 2  to load factor control unit  426 , and a driving command of water pump  104  to drive control unit  430 . 
         [0076]    When inverter element temperature Td is higher than load factor restriction start temperature Tps, PM-ECU  32  outputs a load factor restriction command to load factor control unit  426  to restrict the driving current supplied to motor generator MG 2  from inverter  22 . In response to receiving a load factor restriction command from PM-ECU  32 , load factor control unit  426  sets load factor LDR of motor generator MG 2 . Load factor control unit  426  outputs the set load factor LDR to current command conversion unit  410 . 
         [0077]    Current command conversion unit  410  receives a torque command value TR 2  from an external ECU, and receives a signal NRST or load factor LDR from load factor control unit  426 . Current command conversion unit  410  generates, in response to receiving signal NRST from load factor control unit  426 , current commands Id* and Iq* to output torque specified by torque command value TR 2 . 
         [0078]    Upon receiving load factor LDR from load factor control unit  426 , current command conversion unit  410  multiplies torque command value TR 2  by load factor LDR to calculate a restriction torque command value TRR. Current command conversion unit  410  generates current command Id* and Iq* to output the torque specified by restriction torque command value TRR. Current command conversion unit  410  outputs the generated current commands Id* and Iq* to subtracters  412  and  414 , respectively. 
         [0079]    Subtracter  412  calculates the deviation between current command Id* and current value Id (=Id*−Id), and provides the calculated deviation to PI control unit  416 . Subtracter  414  calculates the deviation between current command Iq* and current value Iq (=Iq*−Iq) to provide the calculated deviation to PI control unit  418 . 
         [0080]    PI control units  416  and  418  calculate the voltage control amounts Vd and Vq for adjusting the motor current using the PI gain to the deviations Id*−Id, Tq*−Tq, and provides the calculated voltage control amounts Vd and Vq to 2-phase/3-phase conversion unit  420 . 
         [0081]    2-phase/3-phase conversion unit  420  converts the voltage control amounts Vd and Vq from PI control units  416  and  418  into 3-phase signals from 2-phase signals using degree of rotation θ from the rotational speed sensor. In other words, 2-phase/3-phase conversion unit  420  converts voltage control amounts Vd and Vq applied to the d axis and q axis into voltage control amounts Vu, Vv and Vw applied to the 3-phase coils of motor generator MG 2  using degree of rotation θ. 2-phase/3-phase conversion unit  420  provides voltage control amounts Vu, Vv and Vw to PWM generation unit  422 . 
         [0082]    PWM generation unit  422  generates a signal PWMI 2  based on voltage control amounts Vu, Vv, and Vw, and input DC current voltage VH of inverter  22  to provide the generated signal PWMI 2  to inverter  22 . 
         [0083]      FIG. 6  is a flowchart to describe a determination process of load factor restriction start temperature Tps and motor drive control executed at PM-ECU  32  and MG-ECU  34  of  FIG. 5 . The process of this flowchart is invoked from the main routine to be executed at a predetermined interval or every time a predetermined condition is met. 
         [0084]    When the process of  FIG. 6  is started, temperature sensor  300  of  FIG. 4  measures coolant temperature Tw at step S 1 . At step S 2 , PM-ECU  32  determines load factor restriction start temperature Tps. Load factor restriction start temperature Tps is determined by the following equation (1). 
         [0000]        Tps=Tcri−ΔTerr   (1)
 
         [0000]    where Tcri represents the element heat-resisting temperature of the IGBT element, and ΔTerr represents the worst case value in the variation of the temperature increase between an IGBT element having the temperature measured and an IGBT element not having the temperature measured. Load factor restriction start temperature Tps will be described in detail hereinafter with reference to the drawing. 
         [0085]      FIG. 7  represents an exemplified study when load factor restriction start temperature Tps is set at a fixed value. 
         [0086]    In  FIG. 7 , element temperature Td is plotted along the vertical axis whereas coolant temperature Tw is plotted along the horizontal axis. Load factor restriction start temperature Tps is set at a value having a constant margin to element heat-resisting temperature Tcri. In  FIG. 7 , load factor restriction start temperature Tps takes the same value even if coolant temperature Tw changes. 
         [0087]    Only a representative element in the inverter has its temperature measured, and a determination is made as to whether load factor restriction is to be executed or not in view of a load factor restriction initiation condition based on the measured temperature. However, since the temperature of all the elements is not measured, as shown in  FIG. 4 , there is variation in the temperature difference between an element having its temperature measured and an element not having its temperature measured. A value taking into account the variation is subtracted from element heat-resisting temperature Tcri and the subtracted result is taken as load factor restriction start temperature Tps. Accordingly, the maximum value Tmax of the element temperature matches element heat-resisting temperature Teri or is in the range of Tcri to Tps. 
         [0088]    The variation factor between elements includes: a) element loss variation (caused by variation in each property of the gate threshold voltage, gate resistance, and switching time); b) variation in heat resistance (void in solder or the like, coolant flow, coolant temperature distribution and the like); c) degradation in heat resistance; and d) variation between temperature sensors. Among these variation factors, the absolute value of a, b and c varies depending upon the element temperature increase ΔT. The absolute values of a, b and c tend to become larger as ΔT increases. 
         [0089]    When load factor restriction start temperature Tps is determined based on coolant temperature Tw=T 0  as the reference in  FIG. 7 , ΔT=T 11  at coolant temperature Tw=T 1  and ΔT=T 21  at coolant temperature Tw=T 2 , resulting in a smaller ΔT. Accordingly, the aforementioned variation factors a, b and c become smaller. Representing the element highest temperature taking into consideration the element variation when the load factor is to be restricted, element variation Tmax-Tps becomes smaller as coolant temperature Tw becomes higher, as ΔT 12  and ΔT 22 . Therefore, the region indicated at ΔT 13  and ΔT 23  when coolant temperature Tw=T 1  and Tw=T 2 , respectively, is the excessive margin region. It is appreciated that the element performance is not exploited effectively when the coolant temperature is high. The present invention is devised to exploit the performance of the element to the maximum level, and prevent unnecessary load factor restriction. 
         [0090]      FIG. 8  is a diagram to describe a study of improving load factor restriction start temperature Tps. 
         [0091]      FIG. 9  represents an improved load factor restriction start temperature Tps. Referring to  FIG. 8 , at coolant temperature Tw=T 1 , element heat-resisting temperature Tcri minus variation ΔT 12  is set for Tps. At coolant temperature Tw=T 2 , element heat-resisting temperature Tcri minus variation ΔT 22  is set for Tps. Thus, region Ae represents the region where the introduction of load factor restriction can be avoided by applying the art of the present embodiment. 
         [0092]    The reason why such modification is allowed will be described hereinafter. The element heat-resisting protection requirement corresponds to the establishment of equation (2) set forth below. Teri represents the element heat-resisting temperature, Tps the load factor restriction start temperature, and ΔTerr the temperature variation between elements (worst case value). 
         [0000]        Tcri &gt;( Tps+ΔTerr )  (2)
 
         [0000]    ΔTerr is represented by the following equation (3), where α represents the part in accordance with ΔT (=the increase of the element temperature from coolant temperature), and β represents a constant. 
         [0000]      Δ Terr=α+β   (3)
 
         [0093]    Therefore, when ΔT is small (high coolant temperature), ΔTerr is small since a, becomes smaller. Therefore, equation (2) is established even if Tps is increased. As a result, as shown in  FIG. 9 , load factor restriction start temperature Tps is to be determined as a function of coolant temperature Tw, such as Tps=f (Tw). More specifically, load factor restriction start temperature Tps is determined to become higher as the coolant temperature rises. 
         [0094]    α and β in equation (3) can be represented as set forth below according to the aforementioned element variation factors of a) element loss variation, b) heat resistance variation, c) degradation in heat resistance, and d) variation between temperature sensors. “A” represents a coefficient. 
         [0000]      α= A ( a+b+c )×Δ T   (4)
 
         [0000]      β= d   (5)
 
         [0095]    By equations (2)-(4), Tps corresponding to the boundary condition of Equation (2) is obtained. 
         [0000]    
       
         
           
             
               
                 
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                       T 
                     
                     - 
                     d 
                   
                 
               
             
           
         
       
     
         [0096]    By further inserting ΔT=Tps−Tw, 
         [0000]        Tps=Tcri−A ( a+b+c )×( Tps−Tw )− d.  
 
         [0000]    By solving this equation for Tps, the following equation (6) can be derived. 
         [0000]        Tps =( Tcri+A ( a+b+c )× Tw−d )/(1 +A ( a+b+c ))  (6)
 
         [0097]    Referring to  FIG. 6  again, following the determination of load factor restriction start temperature Tps at step S 2 , control proceeds to step S 3  where element temperature Td is measured. Element temperature Td is determined based on the outputs from temperature sensors  301 - 304  of  FIG. 4 . The output of any one temperature sensor may be used as a representative thereof, or an average value and the like may be used. 
         [0098]    At step S 4 , a determination is made as to whether element temperature Td exceeds load factor restriction start temperature Tps. When Td&gt;Tps is met at step S 4 , control proceeds to step S 5 , otherwise, control proceeds to step S 6 . At step S 6 , a determination is made that load factor restriction is not performed. 
         [0099]    In this case, motor generator MG 2  is driven based on torque command value TR 2  at step S 7 . In  FIG. 5 , signal NRST is output from load factor control unit  426 , and current command conversion unit  410  generates a motor current command based on torque command value TR 2 . 
         [0100]    In contrast, at step S 5 , a determination is made that load factor restriction is to be performed. In this case, control proceeds to step S 7  where a motor current command is generated based on a value (restriction torque command value TRR) corresponding to torque command value TR 2  multiplied by load factor LDR, as described for current command conversion unit  410  in  FIG. 5 . The torque restriction executed at step S 7  may be carried out by another way as long as the restriction to prevent exceeding element heat-resisting temperature Teri is effected, such as lowering the upper limit of the torque command. 
         [0101]    Following the execution of motor drive control at step S 7 , control proceeds to step S 8  for transition to the main routine. 
         [0102]    In the present embodiment, load factor restriction start temperature Tps is variable, and set based on coolant temperature Tw, as set forth above. Accordingly, the performance of the inverter can be exhibited sufficiently, increasing the operable range without the load factor being restricted at high temperature. The frequency of load factor restriction occurring is reduced, allowing an operation in which the performance of the vehicle is exhibited sufficiently. 
       Another Modification Example 
       [0103]      FIGS. 7-9  have been described based on the case where load factor restriction start temperature Tps is set based on coolant temperature Tw. Load factor restriction start temperature Tps may be set based on another parameter. This parameter includes various items as long as it is a physical quantity affecting heat radiation or cooling of the inverter. For the parameter, carrier frequency fsw of the inverter, inverter voltage VH (voltage after boosting), converter input voltage VL (voltage before boosting), and flowing current Irms (battery current IB, inverter currents MCRT  1  and MCRT 2 , or the like) can be cited. 
         [0104]      FIG. 10  represents an example of load factor restriction start temperature Tps being modified based on carrier frequency fsw. 
         [0105]    In  FIG. 10 , element temperature Td is plotted along the vertical axis whereas the inverter carrier frequency fsw is plotted along the horizontal axis. The heat radiation from an IGBT element becomes greater as carrier frequency fsw becomes higher. The variation between elements is also increased as the heat radiation becomes greater. Therefore, the margin with respect to element heat-resisting temperature Tcri must be increased as the carrier frequency becomes higher from fsw 1  to fsw 2  and to fsw 3 . Thus, load factor restriction start temperature Tps is set lower as the carrier frequency becomes higher in  FIG. 10 . 
         [0106]    For the purpose of taking into consideration other parameters, a function with VH, VL, fsw and Irms as parameters may be determined such as load factor restriction start temperature Tps=f 1 (VH, VL, fsw, Irms). 
         [0107]    α and β in Equation (3) can be set as set forth below. For a-d, various variations are indicated, likewise with Equation (4). A 1  represents a coefficient. 
         [0000]      α= A 1( a+b+c )× f 1( VH,VL,fsw,Irms )  (7)
 
         [0000]      β= d   (8)
 
         [0108]    By Equations (2), (3), (7) and (8), Tps corresponding to the boundary condition of Equation (2) is obtained. 
         [0000]    
       
         
           
             
               
                 
                   Tps 
                   = 
                     
                    
                   
                     f 
                      
                     
                       ( 
                       
                         VH 
                         , 
                         VL 
                         , 
                         fsw 
                         , 
                         Irms 
                       
                       ) 
                     
                   
                 
               
             
             
               
                 
                   = 
                     
                    
                   
                     Tcri 
                     - 
                     
                       Δ 
                        
                       
                           
                       
                        
                       Terr 
                     
                   
                 
               
             
             
               
                 
                   = 
                     
                    
                   
                     Tcri 
                     - 
                     α 
                     - 
                     β 
                   
                 
               
             
             
               
                 
                   = 
                     
                    
                   
                     Tcri 
                     - 
                     
                       A 
                        
                       
                           
                       
                        
                       1 
                        
                       
                         ( 
                         
                           a 
                           + 
                           b 
                           + 
                           c 
                         
                         ) 
                       
                       × 
                       f 
                        
                       
                           
                       
                        
                       1 
                        
                       
                         ( 
                         
                           VH 
                           , 
                           VL 
                           , 
                           fsw 
                           , 
                           Irms 
                         
                         ) 
                       
                     
                     - 
                     d 
                   
                 
               
             
           
         
       
     
         [0109]    The value determined by the equation set forth above is to be taken as load factor restriction start temperature Tps. A map with VH, VL, fsw and Irms as parameters may be determined based on experimental results. Moreover, a combination of coolant temperature in addition to these parameters may be taken into account. 
         [0110]    It should be understood that the embodiments disclosed herein are illustrative and non-restrictive in every respect. The scope of the present invention is defined by the terms of the claims, rather than the description of the embodiments set forth above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims. 
       REFERENCE SIGNS LIST 
       [0111]      2  wheel;  3  power split mechanism;  4  engine;  10 ,  13 ,  21  voltage sensor;  11 ,  24 ,  25  current sensor;  12  voltage converter;  14 ,  22  inverter;  30  control device;  100  vehicle;  102  radiator;  104  water pump;  106  reservoir tank;  116  water channel;  241  driving unit;  300 - 304  temperature sensor;  410  current command conversion unit;  412 ,  414 ,  412 ,  414  subtracter;  416 ,  418 ,  416 ,  418  control unit;  420  2-phase/3-phase conversion unit;  424  3-phase/2-phase conversion unit;  422  PWM generation unit;  426  load factor control unit;  430  drive control unit; C 1 , CH smoothing capacitor; D 1 -D 8 , D 3   g -D 8   g , D 3   m -D 8   m  diode;  32  power management ECU;  34  motor generator control ECU; L 1  reactor; MB battery; MG 1 , MG 2  motor generator; PL 1 , PL 2  positive bus line; Q 1 -Q 8 , Q 3   g -Q 8   g , Q 3   m -Q 8   m  IGBT element; SL 1 , SL 2  negative bus line; SMRB, SMRG system main relay.