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FIELD  
       [0001]    The present invention relates to a control device and a method for controlling an electric motor that can be used for a working machine including a construction machine such as an excavator, a bulldozer, a dump truck, and a wheel loader. 
       BACKGROUND 
       [0002]    There has been a working machine in which an operator thereof can set, at his/her will, an operation mode such as a power mode (P mode) which places emphasis on working efficiency and an economy mode (E mode) which places emphasis on fuel economy. On the other hand, a hybrid working machine including an electric motor for actuating a swing body, a traveling body and the like of the working machine has a structure in which an electric drive system and a hydraulic drive system are loaded together, the electric drive system driving the electric motor by electric energy, and the hydraulic drive system driving a work implement, a traveling unit and the like by actuating a hydraulic cylinder and a hydraulic motor by using hydraulic fluid for a hydraulic pump directly connected to an engine. Thus, when no output limit corresponding to the operation mode is imposed on the electric motor in spite of the output limit imposed on the engine in accordance with the operation mode being set, the operator who operates the hybrid working machine may perceive the difference in operability thereof as compared with the operability of the working machine, which is loaded only with the hydraulic drive system, as operational incongruity. Moreover, when no output limit corresponding to the operation mode is imposed on the electric motor in spite of the output limit imposed on the engine in accordance with the operation mode being set, there would be discordance generated between operating speeds of the electric drive system and the hydraulic drive system, which would also be perceived as incongruity by the operator who operates the hybrid working machine. 
         [0003]    In order to eliminate such operational incongruity, there exists a hybrid working machine in which a torque limit is imposed on the electric motor according to the operation mode being set. For example, the torque limit corresponding to the operation mode is imposed on a swing electric motor which swings an upper swing body of a hybrid excavator.  FIG. 7  illustrates a torque line graph of swing torque that can be exerted with respect to a swing speed in the hybrid excavator. A curve LP 1  is a maximum torque line representing the maximum torque that can be output by the swing electric motor when the P mode is set. A curve LE 1  represents a maximum torque line when the E mode is set. Under such torque limit, the maximum torque being output at the swing speed equal to or less than a predetermined swing speed is smaller when the E mode is set than when the P mode is set. 
       CITATION LIST 
     Patent Literature 
       [0004]    Patent Literature 1: Japanese Laid-open Patent Publication No. 2009-68197 
       SUMMARY 
     Technical Problem 
       [0005]    However, the torque limit set for the E mode as illustrated in  FIG. 7  would cause work efficiency to decrease since the maximum torque is limited low, which causes the swing power to decrease when the upper swing body swings at a low speed. Specifically, high swing power cannot be obtained when the upper swing body is to be swung from standstill to level off a mound with a side of a bucket of the work implement, thereby causing the work efficiency to decrease. 
         [0006]    In order to solve such problem, as illustrated in  FIG. 8 , the torque limit can be imposed as represented by a curve LE 2  in which, when the E mode is set, the torque limit is released at a selected swing speed in a low speed area so that the same torque can be obtained as when the P mode is set. 
         [0007]    In this case, however, the output limit imposed on the swing electric motor would be the same for the P mode and the E mode at the swing speed other than the swing speed at which the P mode and the E mode have the different maximum torques. This would cause a difference in the acceleration performance between the electric drive system and the hydraulic drive system when the P mode or the E mode is set, thereby still possibly imparting the operational incongruity to the operator. 
         [0008]    The present invention has been proposed in consideration of the aforementioned problems, and an object thereof is to provide the device and the method for controlling the electric motor that can reduce the operational incongruity including the acceleration performance in the working machine. 
       Solution to Problem 
       [0009]    To achieve the object mentioned above, according to the present invention, a control device of an electric motor comprises: an operating state setting unit configured to set an operating state; a maximum output acquiring unit configured to acquire maximum output of the electric motor that is preset according to the operating state set; a speed detecting unit configured to detect a speed of the electric motor; a torque limit value calculating unit configured to calculate a torque limit value based on the speed and the maximum output; and a torque limiting unit configured to limit torque of the electric motor by the torque limit value when accelerating the electric motor. 
         [0010]    According to the present invention, the torque limiting unit limits the torque of the electric motor by a torque limit value for deceleration when decelerating the electric motor, regardless of the operating state set. 
         [0011]    According to the present invention, the torque limiting unit is capable of outputting the maximum torque at a predetermined speed or less, regardless of the operating state set. 
         [0012]    According to the present invention, the operating state setting unit is an operation mode selection unit and/or a throttle dial. 
         [0013]    According to the present invention, a method for controlling an electric motor comprises: setting an operating state; acquiring maximum output of the electric motor that is preset according to the operating state set; detecting a speed of the electric motor; calculating a torque limit value based on the speed and the maximum output; and limiting torque of the electric motor by the torque limit value when accelerating the electric motor. 
         [0014]    According to the present invention, the limiting includes limiting the torque of the electric motor by a torque limit value for deceleration when decelerating the electric motor, regardless of the operating state set. 
         [0015]    According to the present invention, the limiting includes being capable of outputting the maximum torque at a predetermined speed or less, regardless of the operating state set. 
         [0016]    According to the present invention, the setting includes operation mode selection setting and/or throttle dial value setting. 
         [0017]    The present invention is adapted to obtain the maximum output of the electric motor that is preset according to the operating state set and to calculate the torque limit value based on the speed of the electric motor and the maximum output obtained, whereby the operational incongruity including the acceleration performance can be reduced when operating the working machine. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0018]      FIG. 1  is a perspective view illustrating an overall structure of a hybrid excavator according to a first embodiment of the present invention. 
           [0019]      FIG. 2  is a schematic diagram illustrating the structure of a control system of the hybrid excavator illustrated in  FIG. 1 . 
           [0020]      FIG. 3  is a torque line graph illustrating the overview of torque limit imposed on a swing motor by a controller. 
           [0021]      FIG. 4  is a diagram illustrating a control flow of the torque limit imposed on the swing motor by the controller. 
           [0022]      FIG. 5  is a diagram illustrating a control flow of torque limit imposed on an electric motor by a controller according to a second embodiment of the present invention. 
           [0023]      FIG. 6  is a diagram illustrating a control flow of torque limit imposed on an electric motor by a controller according to a third embodiment of the present invention. 
           [0024]      FIG. 7  is a torque line graph illustrating an example of torque limit imposed on a conventional electric motor. 
           [0025]      FIG. 8  is a torque line graph illustrating another example of the torque limit imposed on the conventional electric motor. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0026]    Embodiments of the present invention will be described below with reference to the accompanying drawings. 
       First Embodiment 
     [Overall Structure] 
       [0027]      FIGS. 1 and 2  illustrate the overall structure of a hybrid excavator  1  as an example of a working machine. The hybrid excavator  1  includes a vehicle body  2  and a work implement  3 . The vehicle body  2  includes a lower traveling body  4  and an upper swing body  5 . The lower traveling body  4  includes a pair of traveling units  4   a.  Each traveling unit  4   a  includes a crawler belt  4   b.  Each traveling unit  4   a  drives the crawler belt  4   b  by a right travel motor and a left travel motor (a travel motor  21 ) to make the hybrid excavator  1  travel. 
         [0028]    The upper swing body  5  is swingably mounted on the lower traveling body  4  and swings by the driving of a swing motor  24  (an electric motor). The upper swing body  5  is also provided with an operator&#39;s cab  6 . The upper swing body  5  includes a fuel tank  7 , a hydraulic fluid tank  8 , an engine compartment  9 , and a counter weight  10 . The fuel tank  7  stores fuel for driving an engine  17 . The hydraulic fluid tank  8  stores a hydraulic fluid discharged from a hydraulic pump  18  to a hydraulic cylinder such as a boom cylinder  14  or hydraulic equipment such as the travel motor  21 . The engine compartment  9  houses parts such as the engine  17  and the hydraulic pump  18 . The counter weight  10  is arranged posterior to the engine compartment  9 . 
         [0029]    The work implement  3  is mounted in the center of the anterior part of the upper swing body  5  and includes a boom  11 , an arm  12 , a bucket  13 , the boom cylinder  14 , an arm cylinder  15 , and a bucket cylinder  16 . The base end of the boom  11  is rotatably connected to the upper swing body  5 , and the tip end of the boom  11  is rotatably connected to the base end of the arm  12 . The tip end of the arm  12  is rotatably connected to the bucket  13 . The boom cylinder  14 , the arm cylinder  15 , and the bucket cylinder  16  are the hydraulic cylinders adapted to move telescopically by the hydraulic fluid discharged from the hydraulic pump. The boom cylinder  14 , the arm cylinder  15 , and the bucket cylinder  16  are adapted to swing the boom  11 , the arm  12 , and the bucket  13 , respectively. 
         [0030]    In  FIG. 2 , the hybrid excavator  1  includes the engine  17  as a driving source, the hydraulic pump  18 , and a generator  19 . A diesel engine is used as the engine  17 , and a variable displacement hydraulic pump (such as a swash plate hydraulic pump) is used as the hydraulic pump  18 . The hydraulic pump  18  and the generator  19  are mechanically joined to an output shaft of the engine  17  so that the hydraulic pump  18  and the generator  19  would be driven by driving the engine  17 . A hydraulic drive system includes a control valve  20 , the boom cylinder  14 , the arm cylinder  15 , the bucket cylinder  16 , the travel motor  21  and the like, which are driven by the hydraulic pump  18  as a hydraulic source. 
         [0031]    An electric drive system includes a capacitor  22 , an inverter  23 , and a swing motor  24 . The generator  19  and the capacitor  22  serve as a power source for the swing motor  24  to swing the upper swing body  5 . That is, the swing motor  24  exerts motor action by the electric energy supplied from the generator  19  or the capacitor  22  to perform swing acceleration on the upper swing body  5 , and exerts regenerative action when the upper swing body  5  experiences swing deceleration so that the electric energy is supplied (charged) to the capacitor  22 . An SR (switched reluctance) motor is used as the generator  19 , for example. The generator  19  is mechanically joined to the output shaft of the engine  17  so that a rotor shaft of the generator  19  is rotated by driving the engine  17 . An electric double layer capacitor is used as the capacitor  22 , for example. The capacitor  22  may be replaced with a nickel-hydrogen battery or a lithium-ion battery. The swing motor  24  is provided with a speed sensor  25  that detects a speed of the swing motor  24  and converts the speed into an electric signal to be output to a hybrid controller  23   a  provided in the inverter  23 . An interior permanent magnet synchronous motor is used as the swing motor  24 , for example. A resolver, a rotary encoder or the like is used as the speed sensor  25 . Here, the hybrid controller  23   a  includes a CPU (a calculation unit such as a numeric data processor), a memory (a storage unit), and the like. The hybrid controller  23   a  manages excessive temperature rise in each unit such as the capacitor  22  by receiving a signal of a detection value detected by a temperature sensor such as a thermistor and a thermocouple included in the generator  19 , the swing motor  24 , the capacitor  22  and the inverter  23 . Moreover, the hybrid controller  23   a  performs charge/discharge control on the capacitor  22 , assist control of power generated by the generator  19  and the engine, and powering/regenerating control on the swing motor  24 . 
         [0032]    The hydraulic drive system and the electric drive system are driven in response to the operation of a control lever  26  such as a work implement lever, a travel lever, and a swing lever that are provided in the operator&#39;s cab  6  of the vehicle body  2 . The control input of the control lever  26  is converted into an electric signal by a lever control input detection unit  27 . The lever control input detection unit  27  includes a pressure sensor. A pilot hydraulic pressure generated according to the operation of the control lever is detected by the pressure sensor, and voltage or the like output therefrom is converted to obtain the lever control input. When the control lever  26  is an electric lever, the lever control input detection unit  27  includes electric detection means such as a potentiometer, where the lever control input is obtained by converting the voltage or the like generated according to the lever control input. 
         [0033]    Provided in the operator&#39;s cab  6  are a fuel regulation dial (a throttle dial)  28  and a mode switchover unit  29 . The fuel regulation dial (the throttle dial)  28  is a switch for setting the amount of fuel supplied to the engine  17 , and a set value of the fuel regulation dial (the throttle dial)  28  is converted into the electric signal to be output to an engine controller  30 . 
         [0034]    The engine controller  30  includes a calculation unit such as a CPU (a numeric data processor), and a memory (a storage unit). The engine controller  30  creates a signal for a control command based on the set value of the fuel regulation dial (the throttle dial)  28 , and a common rail control unit  32  receives a control signal and regulates the amount of fuel injected to the engine  17 . That is, the engine  17  is an engine that can be subjected to common rail electronic control, by which a target output can be achieved by appropriately controlling the fuel injection amount, and the torque that can be output by an engine speed at a given instant can be set freely. 
         [0035]    The mode switchover unit  29  sets an operation mode of the hybrid excavator  1  to a power mode (P mode) or an economy mode (E mode) and includes, for example, an operation button or switch, or a touch panel provided in the operator&#39;s cab  6 . The operator of the hybrid excavator  1  operates the operation button or the like to switch the operation mode. The power mode is the operation mode for performing engine control and pump control in which the fuel economy is held down while maintaining a heavy workload. On the other hand, the economy mode is the operation mode for performing the engine control and the pump control such that an operating speed of the work implement  3  is secured in light-load work while further holding down the fuel economy. Once the operation mode is set by the mode switchover unit  29  (switchover of the operation mode), an electric signal corresponding to the setting is output to the engine controller  30 , a pump controller  33 , and the hybrid controller  23   a.  Note that in the power mode, the output torque of the engine  17  is matched with the absorption torque of the hydraulic pump  18  in a region where the engine output (the speed and the output torque) of the engine  17  is relatively high. In the economy mode, this matching occurs when the engine output is lower than the case in the power mode. 
         [0036]    The pump controller  33  receives the signal output from the engine controller  30 , the mode switchover unit  29 , and the lever control input detection unit  27 , and generates the signal for the control command to regulate the amount of the hydraulic fluid ejected from the hydraulic pump  18  by performing tilting control on a swash plate angle of the hydraulic pump  18 . Here, a signal from a swash plate angle sensor  18   a  that detects the swash plate angle of the hydraulic pump  18  is input into the pump controller  33 . The pump capacity of the hydraulic pump  18  can be calculated by detecting the swash plate angle thereof by the swash plate angle sensor  18   a.  Provided in the control valve  20  is a pump pressure detection unit  20   a  that detects the pump ejection pressure of the hydraulic pump  18 . The detected pump ejection pressure is converted into the electric signal and input into the pump controller  33 . Note that the engine controller  30 , the pump controller  33 , and the hybrid controller  23   a  are connected via an in-vehicle LAN such as a CAN (Controller Area Network) in order to exchange information with one another. 
         [0037]    [Overview of Torque Limit] 
         [0038]    An overview of torque limit on the swing motor  24  will now be described.  FIG. 3  illustrates the overview of the torque limit according to a first embodiment, the torque limit being imposed on the swing motor  24  by the engine controller  30 . It is a torque line graph illustrating limit characteristics of the swing torque with respect to the swing speed. In  FIG. 3 , the swing torque generated when powering (accelerating) is indicated by a positive, and the swing torque generated when regenerating (decelerating) is indicated by a negative. When the P mode is set by the mode switchover unit  29 , the torque limit is imposed in accordance with a torque limit curve LP illustrating the maximum torque that can be output by the swing motor  24 . When the E mode is set by the mode switchover unit  29 , on the other hand, the torque limit at the time of powering (accelerating) is imposed in accordance with a torque limit curve LE by which the maximum swing output that is preset to the E mode is limited. That is, when the operation mode is set, the torque limit is imposed in accordance with an equivalent horsepower curve corresponding to the maximum swing output for the operation mode being set. Therefore, a decrease in the maximum swing output for the operation mode is represented by the equivalent horsepower curve of the swing motor  24  that is a torque limit curve formed by shifting the entire curve to the lower left, as illustrated in  FIG. 3 . As a result, the swing operation with acceleration performance corresponding to the hydraulic drive system can be performed, thereby eliminating the operational incongruity. 
         [0039]    The torque limit is imposed in accordance with the torque limit curve (LP or LE) corresponding to the equivalent horsepower curve. Now, in a region R 1  where the swing speed of the swing motor  24  is low, the maximum torque TPmax is output so that no obstacle such as a sense of insufficient power would be in the way of performing the swing operation. When the E mode is set, the amount of torque that can be output is decreased in a region where the swing speed is high. However, in the region where the swing speed is high, the output necessary for the swing operation can be obtained sufficiently since the swing speed is high even when the torque is low. Moreover, when either operation mode is set, there is one torque limit curve for regenerating (decelerating) as illustrated in  FIG. 3 , with no difference in the torque limit. As a result, the stopping performance of the swing motor  24  can be sufficiently delivered. 
         [0040]    [Detail of Torque Limit] 
         [0041]    Next, specific torque limit control imposed on the swing motor  24  will be described with reference to  FIG. 4 . As illustrated in  FIG. 4 , input into the engine controller  30  directly or via the hybrid controller  23   a  are: a swing lever stroke (control input of the control lever  26 ) by the control lever  26 ; a swing motor speed of the swing motor  24  detected by the speed sensor  25 ; and the electric signal indicating the operation mode set by the mode switchover unit  29 . 
         [0042]    The swing lever stroke is input into a swing lever stroke/target swing speed conversion table TB 1 . The swing lever stroke/target swing speed conversion table TB 1  outputs to a computing unit  101  a target swing speed Sm corresponding to the input swing lever stroke based on a preset relationship between the swing lever stroke and the target swing speed. The computing unit  101  subtracts a current swing motor speed Sn detected by the speed sensor  25  from the target swing speed Sm and inputs the subtracted speed deviation ΔS to a PID control unit  102 . The PID control unit  102  calculates torque Ta from the speed deviation ΔS. This torque Ta is then multiplied by a gain K by a computing unit  103  to be output to a minimum value selection unit (MIN selection)  106 . Here, the gain K is set to 1 when the swing motor speed Sn is determined to be positive by a positive/negative determination unit  104 , and set to −1 when it is determined to be negative. 
         [0043]    On the other hand, the signal indicating the operation mode set by the mode switchover unit  29  is input into an operation mode/maximum swing output conversion table TB 11 . The operation mode/maximum swing output conversion table TB 11  outputs a maximum swing output Plim preset for each operation mode to a torque limit value calculation unit  105 . The torque limit value calculation unit  105  calculates, based on the absolute values of the input maximum swing output Plim and the swing motor speed Sn, a torque limit value Tlim by the following formula. 
         [0000]        T lim=( P lim×1000)/((2π/60)× Sn )
 
         [0000]    The calculated torque limit value Tlim is input to the minimum value selection unit  106 . 
         [0044]    The minimum value selection unit  106  selects the smaller of the torque (the target swing torque) input from the computing unit  103  and the torque limit value Tlim input from the torque limit value calculation unit  105 , and outputs it to a minimum value selection unit (MIN selection)  107 . The minimum value selection unit  107  outputs the smaller of the torque input from the minimum value selection unit  106  and a motor powering maximum torque TPmax that is a preset fixed value to a maximum value selection unit (MAX selection)  108 . The minimum value selection unit  107  operates so as to output the torque that does not have a greater value on the positive side than the motor powering maximum torque TPmax at the time of powering, especially in the region R 1  where the swing speed is low. 
         [0045]    The maximum value selection unit  108  selects and outputs the greater of the torque input from the minimum value selection unit  107  and a motor regenerating maximum torque TMmax (a negative value) that is a preset fixed value. The maximum value selection unit  108  selects, at the time of powering, the torque input from the minimum value selection unit  107  as the maximum value because the motor regenerating maximum torque TMmax is a negative value. At the time of regenerating when the negative torque is output from the computing unit  103 , however, the maximum value selection unit  108  outputs the torque that does not have a smaller value on the negative side than the motor regenerating maximum torque TMmax. That is to say, a swing motor torque command Tc is output within a limit range of the motor regenerating maximum torque TMmax (the lower limit of the swing torque) according to the swing lever stroke. Moreover, when either operation mode is set, the torque limit value Tlim calculated by the torque limit value calculation unit  105  at the time of regenerating is a positive value, whereas the torque Ta output from the PID control unit  102  to the computing unit  103 , which is provided with the gain K as −1 from the positive/negative determination unit  104 , is a negative value. As a result, the negative torque Ta is selected by the minimum value selection unit  106 . Thus, at the time of regenerating (decelerating), the swing motor command value Tc is calculated and output regardless of the operation mode. A computing unit  109  then multiplies the torque output from the maximum value selection unit  108  by the gain K corresponding to the sign of the swing motor speed Sn, and outputs the multiplied torque Tc as the swing motor torque command to the hybrid controller  23   a.  By the aforementioned method, the torque limit can be performed as represented by the torque line graph in  FIG. 3 . 
         [0046]    Note that the aforementioned process of obtaining the swing motor torque command may be performed by the hybrid controller  23   a  in place of the engine controller  30 , or by a controller in which the engine controller  30  and the hybrid controller  23   a  are integrated. 
       Second Embodiment 
       [0047]    In the aforementioned first embodiment, the torque limit value Tlim is calculated by using the maximum swing output Plim obtained according to the operation mode. In a second embodiment, a torque limit value Tlim is calculated by using a maximum swing output Plim that is obtained by a set value of a throttle dial (a fuel regulation dial  28 ) instead of an operation mode. This is because, in a hybrid excavator  1 , the throttle dial can also perform engine output control and set the operating state. 
         [0048]      FIG. 5  illustrates a torque limit control flow according to the second embodiment. In this torque limit control flow, a signal corresponding to the set value of the throttle dial (the fuel regulation dial  28 ) is used in place of the signal corresponding to the operation mode in  FIG. 4 , and a throttle dial/maximum swing output conversion table TB 12  is used in place of the operation mode/maximum swing output conversion table TB 11 . The rest of the structure is the same as that in  FIG. 5 . 
       Third Embodiment 
       [0049]      FIG. 6  illustrates a torque limit control flow according to a third embodiment. In the third embodiment, the torque limit control is performed by the combination of the first and second embodiments. That is, a signal corresponding to an operation mode as well as a signal corresponding to a set value of a throttle dial (a fuel regulation dial  28 ) are to be input, each signal being input into an operation mode/maximum swing output conversion table TB 11  and a throttle dial/maximum swing output conversion table TB 12 , respectively, and each table converting the signals and outputting maximum swing outputs Plima and Plimb, respectively. A minimum value selection unit (MIN selection)  201  then inputs the maximum swing output Plim that is the smallest of the maximum swing outputs Plima and Plimb to a torque limit value calculation unit  105 . The rest of the structure is the same as those in  FIGS. 4 and 5 . 
       Fourth Embodiment 
       [0050]    In a fourth embodiment, a signal corresponding to an operation pattern is input in place of the signal corresponding to the operation mode in the first embodiment. The signal corresponding to the operation pattern is an operation pattern that is determined by a determination process of the operation pattern among a plurality of operation patterns and is input as a signal indicating the operation pattern. Once being input, the signal indicating the operation pattern is converted to a maximum swing output Plim preset for every operation pattern by an operation pattern/maximum swing output conversion table and is output. 
         [0051]    The determination process of the operation pattern determines that it is the operation pattern of a heavy excavation when, for example, an arm lever for moving an arm  12  among work implement levers  26  is operated in the excavating direction, and when a pump ejection pressure of a hydraulic pump  18  is higher than a certain set value. In addition, the determination process determines that it is the operation pattern of a hoist swing when, for example, a swing lever  26  is operated and a boom lever for moving a boom  11  among the work implement levers  26  is operated in a direction of raising or lowering the boom  11 . As described, the determination process of the operation pattern is to estimate the operation being attempted by an operator at that moment by a specific input value. Note that the hoist swing is the operation where earth and sand excavated by a bucket  13  is released at a desired position at which an upper swing body  5  stops swinging while raising the boom  11 . 
         [0052]    More detailed torque limit control can be performed by adopting such determination process of the operation pattern. 
       Fifth Embodiment 
       [0053]    In a fifth embodiment, a hydraulic fluid temperature may be used as the input, and a hydraulic fluid temperature/maximum swing output conversion table may correct and convert a maximum swing output Plim that is output from each table according to the hydraulic fluid temperature. The hydraulic fluid temperature would be low and the viscosity of the hydraulic fluid would be increased when under the environment of low outdoor air temperature, when starting up a hybrid excavator  1 , or the like. Although the increase in the viscosity of the hydraulic fluid would incur a decrease in the operating speed of a work implement, a swing motor  24  that is electrically driven would operate regardless of the temperature of the hydraulic fluid. An operator would thus feel a sense of incongruity when the temperature of the hydraulic fluid is not considered in operating an upper swing body  5  by the swing motor  24 . Therefore, a swing operation can be performed in harmony with the operating speed of a work implement  3  by correcting and converting the value of the maximum swing output Plim according to the hydraulic fluid temperature, as described in this fifth embodiment. Note that, unlike the hybrid excavator, a normal hybrid excavator which swings by a hydraulic motor would experience a decrease in the maximum swing output with a decrease in the hydraulic fluid temperature. The operator would therefore feel no sense of incongruity when the control is performed by incorporating information on the hydraulic fluid temperature as described in the present embodiment. Here, the hybrid excavator  1  includes a sensor for detecting the hydraulic fluid temperature of a hydraulic pump  18 , and an engine controller  30 , a pump controller  33 , or a hybrid controller  23   a  monitors the hydraulic fluid temperature. 
         [0054]    By performing torque limit control in consideration of the operating state of the hydraulic drive system, there would be a better output balance and a torque balance between the operation of the electric drive system by the swing motor  24  and the operation of the hydraulic drive system, thereby reducing the operational incongruity. 
       REFERENCE SIGNS LIST 
       [0055]      1  hybrid excavator 
         [0056]      2  vehicle body 
         [0057]      3  work implement 
         [0058]      4  lower traveling body 
         [0059]      5  upper swing body 
         [0060]      11  boom 
         [0061]      12  arm 
         [0062]      13  bucket 
         [0063]      14  boom cylinder 
         [0064]      15  arm cylinder 
         [0065]      16  bucket cylinder 
         [0066]      17  engine 
         [0067]      18  hydraulic pump 
         [0068]      18   a  swash plate angle sensor 
         [0069]      19  generator 
         [0070]      20  control valve 
         [0071]      20   a  pump pressure detection unit 
         [0072]      21  travel motor 
         [0073]      22  capacitor 
         [0074]      23  inverter 
         [0075]      23   a  hybrid controller 
         [0076]      24  swing motor 
         [0077]      25  speed sensor 
         [0078]      26  control lever 
         [0079]      27  lever control input detection unit 
         [0080]      28  fuel regulation dial 
         [0081]      29  mode switchover unit 
         [0082]      30  engine controller 
         [0083]      32  common rail control unit 
         [0084]      33  pump controller 
         [0085]      105  torque limit value calculation unit

Summary:
A control device of an electric motor includes: an operating state setting unit configured to set an operating state; a maximum output acquiring unit configured to acquire maximum output of the electric motor that is preset according to the operating state set; a speed detecting unit configured to detect a speed of the electric motor; a torque limit value calculating unit configured to calculate a torque limit value based on the speed and the maximum output; and a torque limiting unit configured to limit torque of the electric motor by the torque limit value when accelerating the electric motor.