Abstract:
An apparatus for warm up control of an autonomous inverter-driven hydraulic unit has a warm up control section and a switch section. The warm up control section performs a predetermined process using a current speed and current pressure as input information and outputs a first switch command. The switch section is controlled by the first switch command so as to control on/off of power supplied to a fan motor. The apparatus causes an oil temperature to quickly rise to a level where a hydraulic unit driven by a hydraulic pump can work in an optimum condition.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This nonprovisional application claims priority under 35 U.S.C. §119(a) to Japanese Patent Application Nos. 2002-331568, filed in Japan on Nov. 15, 2002, the entire content of which is hereby incorporated by reference. 
   1. Technical Field 
   The present invention relates to a method and apparatus for warm up control of an autonomous inverter-driven hydraulic unit. More particularly, the present invention relates to an autonomous inverter-driven hydraulic unit which controls a fixed displacement pump coupled to a motor so as to control an oil pressure and a flow rate, the unit being used in a hydraulic driving apparatus such as a machine tool which is driven using pressured oil as driving source. 
   2. Background Art 
   A conventional hydraulic unit employs an arrangement, as is illustrated in  FIG. 1 , for example, for start-stop a hydraulic pump directly coupled to a motor by turning a power source on/off. And, for simplification in control, a radiator fan is interlocked with the start-stop. 
   It can be thought that a flow rate is increased by increasing a revolution, oil is returned to a tank from a relief valve or a hydraulic driving apparatus, and an oil temperature is raised by a pressure drop. 
   When the former arrangement is employed, the radiator fan is unconditionally started upon turning the power source on. Therefore, a necessary time period for rising the oil temperature becomes extremely longer when the oil temperature is low, and when the oil temperature is required to rise rapidly. 
   When the latter arrangement is employed, and when a hydraulic pump is a variable volume type pump, an additional hydraulic circuit for rising temperature is required because a flow rate is automatically varied in response to a load pressure. Also, the flow rate is decreased following the rising in load pressure due to the characteristics of the variable volume pump, so that a set value in pressure of a relief valve within a temperature rising circuit cannot be determined to be too higher value. Therefore, the rising in oil temperature due to the losses in pressure at the relief valve cannot be greatly expected, and as a result, a necessary time period for rising the oil temperature becomes longer due to the multiplier effect with the radiator fan. Further, a number of components such as temperature sensor and the like, becomes greater so that increase in cost is realized. 
   When the latter arrangement is employed, and when a hydraulic pump is a fixed volume type pump, and when the hydraulic pump is driven by a motor in which permanent magnets are mounted to a rotor, the hydraulic pump is difficult to be driven at high speed because a motor current cannot be flowed into the motor when a speed electromotive force becomes greater than a power source voltage. Therefore, the rising in oil temperature due to the losses in pressure at the relief valve cannot be greatly expected, and as a result, a necessary time period for rising the oil temperature becomes longer due to the multiplier effect with the radiator fan, similarly to that of the variable volume pump. 
   SUMMARY OF THE INVENTION 
   The present invention was made in view of the above problems. 
   It is an object of the present invention to provide a method and apparatus for warm up control of an autonomous inverter-driven hydraulic unit which can rapidly rise an oil temperature to an oil temperature at which a hydraulic unit driven by a hydraulic pump can work in an optimum condition. 
   A method for warm up control of an autonomous inverter-driven hydraulic unit of a first aspect according to the present invention, the autonomous inverter-driven hydraulic unit driving a fixed volume hydraulic pump using a motor controlled by an inverter, having a heat radiation section provided at a predetermined position of a circulation flow path of an oil which is discharged by the fixed volume hydraulic pump, the method comprises 
   detecting a temperature of the oil flowing the circulation flow path, 
   judging whether or not the detected oil temperature is equal to or less than a predetermined reference temperature, and 
   heating-up the heat radiation section in response to the judgment that the oil temperature is equal to or less than the predetermined reference temperature. 
   A method for warm up control of an autonomous inverter-driven hydraulic unit of a second aspect according to the present invention employs a radiator as the heat radiation section, and carries out heating-up the heat radiation section by controlling a radiator fan so as to lower the heat radiation efficiency of the radiator. 
   A method for warm up control of an autonomous inverter-driven hydraulic unit of a third aspect according to the present invention carries out the detection of an oil temperature by estimating an oil temperature from a revolution of the motor when a pressure is controlled. 
   A method for warm up control of an autonomous inverter-driven hydraulic unit of a fourth aspect according to the present invention carries out the detection of an oil temperature by estimating an oil temperature from a pressure when a flow rate is controlled. Wherein, the flow rate controlling may be performed by revolution controlling. 
   A method for warm up control of an autonomous inverter-driven hydraulic unit of a fifth aspect according to the present invention employs decreasing a revolution of the radiator fan or stopping the radiator fan as the controlling of the radiator fan which is carried out for lowering the heat radiation efficiency. 
   A method for warm up control of an autonomous inverter-driven hydraulic unit of a sixth aspect according to the present invention, the autonomous inverter-driven hydraulic unit being arranged to drive a fixed volume hydraulic pump using an integral-type motor controlled by an inverter, the method comprises 
   detecting a temperature of the oil flowing a circulation flow path, 
   judging whether or not the detected oil temperature is equal to or less than a predetermined reference temperature, and 
   shifting a current phase from an optimum current phase for increasing heat generation of the motor in response to the judgment result indicating that the oil temperature is equal to or less than the predetermined reference temperature. 
   A method for warm up control of an autonomous inverter-driven hydraulic unit of a seventh aspect according to the present invention employs a brushless DC motor as the motor, and carries out the processing for shifting a current phase from an optimum current phase by shifting a current phase in leading phase side with respect to a speed electromotive force so as to increase a maximum revolution. 
   An apparatus for warm up control of an autonomous inverter-driven hydraulic unit of an eighth aspect according to the present invention, the autonomous inverter-driven hydraulic unit driving a fixed volume hydraulic pump using a motor controlled by an inverter, having a heat radiation section provided at a predetermined position of a circulation flow path of an oil which is discharged by the fixed volume hydraulic pump, the apparatus comprises 
   an oil temperature detection section for detecting a temperature of the oil flowing the circulation flow path, 
   a judgment section for judging whether or not the detected oil temperature is equal to or less than a predetermined reference temperature, and 
   a warm up section for heating-up the heat radiation section in response to the judgment by the judgment section that the oil temperature is equal to or less than the predetermined reference temperature. 
   An apparatus for warm up control of an autonomous inverter-driven hydraulic unit of a ninth aspect according to the present invention employs a radiator as the heat radiation section, employs a radiator fan controlling section for controlling a radiator fan as the warm up section, and controls the radiator fan so as to lower the heat radiation efficiency of the radiator in response to the judgment result obtained by the judgment section indicating that the oil temperature is equal to or less than the predetermined reference temperature. 
   An apparatus for warm up control of an autonomous inverter-driven hydraulic unit of a tenth aspect according to the present invention employs a section which carries out the detection of an oil temperature by estimating an oil temperature from a revolution of the motor when a pressure is controlled, as the oil temperature detection section. 
   An apparatus for warm up control of an autonomous inverter-driven hydraulic unit of an eleventh aspect according to the present invention employs a section which carries out the detection of an oil temperature by estimating an oil temperature from a pressure when a flow rate is controlled, as the oil temperature detection section. Wherein, the flow rate controlling may be performed by revolution controlling. 
   An apparatus for warm up control of an autonomous inverter-driven hydraulic unit of a twelfth aspect according to the present invention employs a section which decreases a revolution of the radiator fan or stops the radiator fan for lowering the heat radiation efficiency, as the radiator fan controlling section. 
   An apparatus for warm up control of an autonomous inverter-driven hydraulic unit of a thirteenth aspect according to the present invention, the autonomous inverter-driven hydraulic unit being arranged to drive a fixed volume hydraulic pump using an integral-type motor controlled by an inverter, the apparatus comprises 
   an oil temperature detection section for detecting a temperature of the oil flowing a circulation flow path, 
   a judgment section for judging whether or not the detected oil temperature is equal to or less than a predetermined reference temperature, and 
   a current phase controlling section for shifting a current phase from an optimum current phase for increasing heat generation of the motor in response to the judgment result of the judgment section indicating that the oil temperature is equal to or less than the predetermined reference temperature. 
   An apparatus for warm up control of an autonomous inverter-driven hydraulic unit of a fourteenth aspect according to the present invention employs a brushless DC motor as the motor, and employs a section which carries out the processing for shifting a current phase from an optimum current phase by shifting a current phase in leading phase side with respect to a speed electromotive force so as to increase a maximum revolution, as the current phase controlling section. 
   When the method for warm up control of an autonomous inverter-driven hydraulic unit of a first aspect according to the present invention is employed, the autonomous inverter-driven hydraulic unit drives a fixed volume hydraulic pump using a motor controlled by an inverter, has a heat radiation section provided at a predetermined position of a circulation flow path of an oil which is discharged by the fixed volume hydraulic pump, and the method detects a temperature of the oil flowing the circulation flow path, judges whether or not the detected oil temperature is equal to or less than a predetermined reference temperature, and heats-up the heat radiation section in response to the judgment that the oil temperature is equal to or less than the predetermined reference temperature. Therefore, an oil temperature can be rapidly risen by heating-up the heat radiation section. 
   When the method for warm up control of an autonomous inverter-driven hydraulic unit of a second aspect according to the present invention is employed, a radiator is employed as the heat radiation section, and heating-up the heat radiation section is carried out by controlling a radiator fan so as to lower the heat radiation efficiency of the radiator, Therefore, operation and effect similar to those of the first aspect are realized by carrying out simple controlling which controls the radiator fan. 
   When the method for warm up control of an autonomous inverter-driven hydraulic unit of a third aspect according to the present invention is employed, the detection of an oil temperature is carried out by estimating an oil temperature from a revolution of the motor when a pressure is controlled. Therefore, operation and effect similar to those of the first aspect or second aspect are realized with no oil temperature sensors. 
   When the method for warm up control of an autonomous inverter-driven hydraulic unit of a fourth aspect according to the present invention is employed, the detection of an oil temperature is carried out by estimating an oil temperature from a pressure when a flow rate is controlled. Therefore, operation and effect similar to those of the first aspect or second aspect are realized with no oil temperature sensors. 
   When the method for warm up control of an autonomous inverter-driven hydraulic unit of a fifth aspect according to the present invention is employed, decreasing a revolution of the radiator fan or stopping the radiator fan is employed as the controlling of the radiator fan which is carried out for lowering the heat radiation efficiency. Therefore, operation and effect similar to those of the second aspect are realized. 
   When the method for warm up control of an autonomous inverter-driven hydraulic unit of a sixth aspect according to the present invention is employed, the autonomous inverter-driven hydraulic unit is arranged to drive a fixed volume hydraulic pump using an integral-type motor controlled by an inverter, and the method detects a temperature of the oil flowing a circulation flow path, judges whether or not the detected oil temperature is equal to or less than a predetermined reference temperature, and shifts a current phase from an optimum current phase for increasing heat generation of the motor in response to the judgment that the oil temperature is equal to or less than the predetermined reference temperature. Therefore, heat generation of the motor is increased by increasing wattless current, the oil temperature is rapidly risen, consequently. 
   When the method for warm up control of an autonomous inverter-driven hydraulic unit of a seventh aspect according to the present invention is employed, a brushless DC motor is employed as the motor, and the processing for shifting a current phase from an optimum current phase is carried out by shifting a current phase in leading phase side with respect to a speed electromotive force so as to increase a maximum revolution. Therefore, flow rate is increased by driving the brushless DC motor at a higher revolution, the oil temperature is more rapidly risen, consequently. 
   When the apparatus for warm up control of an autonomous inverter-driven hydraulic unit of an eighth aspect according to the present invention is employed, the autonomous inverter-driven hydraulic unit drives a fixed volume hydraulic pump using a motor controlled by an inverter, and has a heat radiation section provided at a predetermined position of a circulation flow path of an oil which is discharged by the fixed volume hydraulic pump, and the apparatus detects a temperature of the oil flowing the circulation flow path using the oil temperature detection section, judges whether or not the detected oil temperature is equal to or less than a predetermined reference temperature using the judgment section, and heats-up the heat radiation section in response to the judgment by the judgment section that the oil temperature is equal to or less than the predetermined reference temperature, using the temperature rise section. 
   Therefore, an oil temperature can be rapidly risen by heating-up the heat radiation section. 
   When the apparatus for warm up control of an autonomous inverter-driven hydraulic unit of a ninth aspect according to the present invention is employed, a radiator is employed as the heat radiation section, a radiator fan controlling section for controlling a radiator fan is employed as the warm up section, and the apparatus controls the radiator fan so as to lower the heat radiation efficiency of the radiator in response to the judgment result obtained by the judgment section that the oil temperature is equal to or less than the predetermined reference temperature. Therefore, operation and effect similar to those of the eighth aspect are realized by carrying out simple controlling which controls the radiator fan. 
   When the apparatus for warm up control of an autonomous inverter-driven hydraulic unit of a tenth aspect according to the present invention is employed, a section which carries out the detection of an oil temperature by estimating an oil temperature from a revolution of the motor when a pressure is controlled, is employed as the oil temperature detection section. Therefore, operation and effect similar to those of the eighth aspect or ninth aspect are realized with no oil temperature sensors. 
   When the apparatus for warm up control of an autonomous inverter-driven hydraulic unit of an eleventh aspect according to the present invention is employed, a section which carries out the detection of an oil temperature by estimating an oil temperature from a pressure when a flow rate is controlled, is employed as the oil temperature detection section. Therefore, operation and effect similar to those of the eighth aspect or ninth aspect are realized with no oil temperature sensors. 
   When the apparatus for warm up control of an autonomous inverter-driven hydraulic unit of a twelfth aspect according to the present invention is employed, a section which decreases a revolution of the radiator fan or stops the radiator fan for lowering the heat radiation efficiency, is employed as the radiator fan controlling section. Therefore, operation and effect similar to those of the ninth aspect are realized. 
   When the apparatus for warm up control of an autonomous inverter-driven hydraulic unit of a thirteenth aspect according to the present invention is employed, the autonomous inverter-driven hydraulic unit is arranged to drive a fixed volume hydraulic pump using an integral-type motor controlled by an inverter, the apparatus detects a temperature of the oil flowing a circulation flow path using the oil temperature detection section, judges whether or not the detected oil temperature is equal to or less than a predetermined reference temperature using the judgment section, and shifts a current phase from an optimum current phase for increasing heat generation of the motor in response to the judgment result of the judgment section that the oil temperature is equal to or less than the predetermined reference temperature using the current phase controlling section. 
   Therefore, heat generation of the motor is increased by increasing wattless current, the oil temperature is rapidly risen, consequently, 
   When the apparatus for warm up control of an autonomous inverter-driven hydraulic unit of a fourteenth aspect according to the present invention is employed, a brushless DC motor is employed as the motor, and a section which carries out the processing for shifting a current phase from an optimum current phase by shifting a current phase in leading phase side with respect to a speed electromotive force so as to increase a maximum revolution, is employed as the current phase controlling section. Therefore, flow rate is increased by driving the brushless DC motor at a higher revolution, the oil temperature is more rapidly risen, consequently. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1  is a schematic diagram illustrating an arrangement of a conventional hydraulic unit; 
       FIG. 2  is a schematic diagram illustrating an arrangement of an autonomous inverter-driven hydraulic unit which is applied a method for warm up control of an embodiment according to the present invention; 
       FIG. 3  is a block diagram illustrating an arrangement of a warm up controlling section in detail; and 
       FIG. 4  is a schematic diagram illustrating an arrangement of an autonomous inverter-driven hydraulic unit which is applied a method for warm up control of another embodiment according to the present invention, 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Hereinafter, referring to the attached drawings, we explain a method and apparatus for warm up control of an autonomous inverter-driven hydraulic unit of an embodiment according to the present invention. 
     FIG. 2  is a schematic diagram illustrating an arrangement of an autonomous inverter-driven hydraulic unit which is applied a method for warm up control of an embodiment according to the present invention. 
   In the autonomous inverter-driven hydraulic unit, oil is sucked from a tank  1  through an oil filter  2  by a fixed displacement pump  3 , and pressured oil is discharged through a discharge opening. A relief valve  4  is provided for preventing a discharge pressure from exceeding a predetermined pressure, and a partial part of the discharged pressured oil is returned to the tank  1  through a restrictor  5  and a radiator  6 . Further, oil is returned to the tank  1  through the radiator  6 . Furthermore, a radiator fan  7  is provided for promoting cooling of the oil in the radiator  6 . The reference numeral  7   a  represents a fan motor for driving the radiator fan  7 . 
   The autonomous inverter-driven hydraulic unit further comprises a P-Q control section  8 , a speed control section  9 , an inverter section  10 , a motor  11 , a pulse generator  12 , a speed detection section  13 , a pressure sensor  14 , a warm up control section  15 , and a switch section  16 . The P-Q control section  8  outputs a speed command based upon discharge pressure-discharge flow rate characteristic (P-Q characteristic). The speed control section  9  carries out the speed controlling operation based upon the speed command and the current speed, and outputs a current command. The inverter section  10  receives a supplied power source, and works based upon the current command. The motor  11  works by receiving an alternate voltage from the inverter section  10 , and drives the fixed displacement pump  3 . The pulse generator  12  is connected with the motor  11 , and outputs a pulse signal, The speed detection section  13  detects a speed of the motor  11  by measuring an interval between pulse signals which is output from the pulse generator  12  and is received by the speed detection section  13 , The pressure sensor  14  detects a pressure of discharged pressured oil. The warm up control section  15  carries out a predetermined processing based upon a current speed and current pressure so as to output a first switch command. The switch section  16  is controlled by the first switch command so as to control on/off of the supplied power source to the fan motor  7   a.    
     FIG. 3  is a block diagram illustrating an arrangement of the warm up control section  15  in detail. 
   The warm up control section  15  comprises a first oil temperature estimation section  21 , a second oil temperature estimation section  22 , a pressure control mode judgment section  23 , a switch section  24 , and a warm up control and judgment section  25 . The first oil temperature estimation section  21  receives a current speed, and estimates an oil temperature under constant pressure control. The second oil temperature estimation section  22  receives a current pressure, and estimates an oil temperature under constant flow control, The pressure control mode judgment section  23  receives a current pressure, judges based upon P-Q characteristic whether it is constant pressure control condition or constant flow control condition, and outputs a second switch command in response to the judgment result, The switch section  24  works based upon the second switch command so as to select the estimated oil temperature from the first oil temperature estimation section  21  or the estimated oil temperature from the second oil temperature estimation section  22 , The warm up control and judgment section  25  receives the selected oil temperature, judges whether the oil temperature is greater or smaller than the reference temperature, and outputs a first switch command in response to the judgment result. 
   Operation in the first oil temperature estimation section  21  is as follows. 
   When an oil pressure is controlled to be a constant pressure, a revolution of the fixed displacement pump  3  is controlled so as to make a pressure to be a set pressure unless a condition of an oil pressure circuit of load is not changed. When an oil temperature becomes lower, viscosity of the oil becomes lower, pump efficiency becomes increased, so that the revolution becomes lower. Therefore, an oil temperature can be estimated from a revolution under an operating condition with a constant pressure. 
   Operation in the second oil temperature estimation section  22  is as follows. 
   When an oil pressure is controlled to be a constant pressure, a revolution of the fixed displacement pump  3  is controlled so as to make a flow rate to be a set flow rate unless a condition of an oil pressure circuit of load is not changed. When an oil temperature becomes lower, viscosity of the oil becomes lower, pump efficiency becomes increased, so that the pressure becomes higher. Therefore, an oil temperature can be estimated from a pressure under an operating condition with a constant flow rate. 
   Operation of the autonomous inverter-driven hydraulic unit having the above arrangement is as follows. 
   The speed control section  9  carries out the speed control based upon a difference between the speed command output from the P-Q control section  8  which maintains P-Q characteristic and the current speed, and outputs a current command so as to control the inverter section  10 . An AC voltage output from the inverter section  10  is supplied to the motor  11  which drives the fixed displacement pump  3 . The fixed displacement pump  3  sucks oil from the tank  1  through the oil filter  2 , and discharges the oil. A partial part of the oil flows in a serial circuit made of the restrictor  5  and the radiator  6 . 
   To the warm up control section  15 , current speed and a current pressure are supplied, and by the pressure control mode judgment section  23 , it is judged whether it is a constant pressure control condition or a constant flow control condition, and a second switch command corresponding to the judgment result is output. Consequently, the estimated oil temperature from the first oil temperature estimation section  21  or the estimated oil temperature from the second oil temperature estimation section  22  is selected by the switch section  24 . 
   When the selected estimated temperature is higher than the reference temperature, a first switch command instructing the switch section  16  to be turned on is output from the warm up control and judgment section  25 . Therefore, for this case, the radiator fan  7  works so as to accelerate heat radiation from the radiator  6 . 
   On the contrary, when the selected estimated temperature is lower than the reference temperature, a first switch command instructing the switch section  16  to be turned off is output from the warm up control and judgment section  25 . Therefore, for this case, the radiator fan  7  stops so as to suppress heat radiation from the radiator  6 . As a result, the warm up of oil can be made rapidly. In other words, a required time period for warm up to a predetermined temperature can be shortened. 
   The radiator fan  7  may be lowered its revolution instead stopping the radiator fan  7 , in a case where the required time period for warm up may become longer to some degree. 
     FIG. 4  is a schematic diagram illustrating an arrangement of an autonomous inverter-driven hydraulic unit which is applied a method for warm up control of another embodiment according to the present invention. 
   The autonomous inverter-driven hydraulic unit comprises a converter section  31 , an inverter section  32 , and a fixed displacement pump  34 . The converter section  31  receives an AC power and outputs a DC voltage. The inverter section  32  receives the DC voltage and outputs an AC voltage which is supplied to a brushless DC motor  33 . The fixed displacement pump  34  is connected to the brushless DC motor  33 , and is driven by the brushless DC motor  33 . The brushless DC motor  33  is made in one body with the fixed displacement pump  34 . 
   The autonomous inverter-driven hydraulic unit further comprises a P-Q control section  35 , a revolution control section  36 , a current control section  37 , a phase map section  38 , a current phase for temperature rise output section  39 , a third switch section  40 , a pressure sensor  41 , a pulse generator  42 , a speed detection section  43 , and a warm up control section  44 , The P-Q control section  35  receives a set pressure, a set flow rate, and a set horsepower, generates discharge pressure-discharge flow rate characteristic (P-Q characteristic), and outputs a revolution command based upon a current discharge pressure and a current speed. The revolution control section  36  carries out the revolution controlling operation based upon the revolution command and the current speed, and outputs a current command, The current control section  37  carries out the current control operation based upon the current command and the current phase command, and outputs a duty command, The phase map section  38  holds a map of each of current phases with respect to a revolution of every current command, receives a current command and a current speed, and outputs a corresponding current phase date table. The current phase for temperature rise output section  39  receives a current command and a current speed, and outputs a current phase for temperature rise which is to be determined for the temperature rise (a current phase which is shifted from a current phase date table to some degree). The third switch section  40  selects one of a current phase date table and a current phase for temperature rise, and outputs the selected current phase as a current phase command. The pressure sensor  41  detects a discharge pressure from the fixed displacement pump  34 . The pulse generator  42  is connected to the motor  33 . The speed detection section  43  receives a pulse from the pulse generator  42 , and detects a current speed based upon an interval of pulses. The warm up control section  44  receives a current speed and a current pressure, estimates an oil temperature based upon the current speed and the current pressure, judges whether the estimated oil temperature is greater or smaller than the predetermined reference value, generates a switch command in response to the judgment result, and supplies the switch command to the third switch section  40 . 
   The arrangement of the warm up control section  44  is similar to that of the warm up control section  15 . The current phase for temperature rise output section  39  may output a previously determined current phase in response to a current command and a current speed. The current phase for temperature rise output section  39  may also calculates a current phase by carrying out a predetermined operation based upon a current command and a current speed, and outputs the calculated current phase. 
   Operation of the autonomous inverter-driven hydraulic unit having the above arrangement is as follows. 
   The P-Q control section  35  outputs a revolution command corresponding to a current pressure and a current speed. The revolution control section  36  carries out the revolution control operation based upon the revolution command and the current speed, and outputs a current command. The current control section  37  carries out the current control operation based upon the current command and a current phase command, and supplies a duty command to the inverter section  32 . Based upon the output from the inverter section  32 , the brushless DC motor  33  is driven, the fixed displacement pump  34  is driven so as to discharge pressured oil. 
   In this case, when the oil temperature is higher than the reference temperature, the third switch section  40  is operated so as to make a current phase date table to be a current phase command. Therefore, the brushless DC motor  33  can be driven so as to make an efficiency to be an optimum efficiency, consequently the heat generation of the motor is greatly suppressed. 
   On the contrary, when the oil temperature is lower than the reference temperature, the third switch section  40  is operated so as to make a current phase for temperature rise to be a current phase command. Therefore, wattles current is increased, the heat generation of the motor is accordingly increased, the heat is transmitted to the fixed displacement pump  34 , consequently an oil temperature is rapidly increased. 
   A motor current phase can be shifted in leading phase side with respect to a speed electromotive force. In this case, increase of a speed electromotive force can be suppressed, and high speed rotation can be realized, by carrying out the control such that a rotor flux of the brushless DC motor is weakened by a flux generated at the motor coil. As a result, a flow rate is increased, a partial part of oil is returned to the tank through the relief valve, an oil temperature is rapidly increased due to pressure drop. 
   Of course, the control of the radiator fan can be added to the embodiment. In this case, the oil temperature can more rapidly be increased. 
   The processing similar to that of the above embodiment can be performed using computer program and the like.