Abstract:
An electrical power tool includes a motor, a load detecting unit, a trigger switch, and a power supplying unit. The load detecting unit detects a load applied to the motor. The trigger switch receives an instruction. The power supplying unit starts supplying of a driving electrical power to the motor when the trigger switch receives the instruction. The power supplying unit changes an amount of the driving electrical power based on the load detected by the load detecting unit.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priorities from Japanese Patent Application No. 2010-171707 and Japanese Patent Application No. 2010-172318 each filed Jul. 30, 2010. The entire content of each of these priority applications is incorporated herein by reference. 
       TECHNICAL FIELD 
       [0002]    The present invention relates to an inverter device and an electrical power tool. 
       BACKGROUND 
       [0003]    Japanese Patent Application Publication No. 2009-219428 discloses an electrical power tool, such as, a mower provided with a motor driven with an electrical power. 
       SUMMARY 
       [0004]    However, since a constant voltage is supplied to the motor in the above mower, an electrical power is wasted when the mower runs idle without mowing a lawn. 
         [0005]    In view of the foregoing, it is an object of the invention to provide an electrical power tool capable of reducing a waste of an electrical power. 
         [0006]    In order to attain the above and other objects, the invention provides an electrical power tool including: a motor; a load detecting unit that detects a load applied to the motor; a trigger switch that receives an instruction; and a power supplying unit that starts supplying of a driving electrical power to the motor when the trigger switch receives the instruction. The power supplying unit changes an amount of the driving electrical power based on the load detected by the load detecting unit. 
         [0007]    Preferably, the power supplying unit determines a driving status of the motor based on the load detected by the load detecting unit, and changes the amount of the driving electrical power based on the determination. 
         [0008]    Preferably, the power supplying unit reduces the amount of the driving electrical power when determining that the motor runs idle. 
         [0009]    Preferably, the motor is driven with an AC electrical power. The power supplying unit includes: a controller that generates an inverter PWM signal based on the load detected by the load detecting unit; and an inverter circuit having an inverter switch element that is connected to the motor and performs an ON/OFF operation based on the inverter PWM signal to convert a DC electrical power supplied from a DC electrical power to an AC electrical power and supply the AC electrical power to the motor as the driving electrical power, the amount of the driving electrical power changing in accordance with the ON/OFF operation of the inverter switch element. 
         [0010]    Preferably, the electrical power tool further includes a transformer switch element electrically connected to the inverter circuit. The controller generates a transformer PWM signal. The DC electrical power is supplied from a battery pack to the transformer switch element, the transformer switch element performing an ON/OFF operation based on the transformer PWM signal, the DC electrical power being converted to the AC electrical power with the ON/OFF operation of the transformer switch element and outputted to the power supplying unit. The power supplying unit further includes: a transformer that transforms the AC electrical power outputted from the transformer switch element; and a rectifying/smoothing unit that rectifies and smoothes the transformed AC electrical power. The inverter circuit converts the rectified and smoothed AC electrical power to the AC electrical power. The controller changes at least one of the inverter PWM signal and the transformer PWM signal based on the load detected by the load detecting unit. 
         [0011]    Preferably, the controller generates an inverter PWM signal having a maximum duty and a transformer PWM signal having a maximum duty when the load detected by the load detecting unit is greater than a first threshold. The controller generates an PWM signal having a duty smaller than the maximum duty when the load detected by the load detecting unit is smaller than a second threshold smaller than the first threshold, the PWM signal including at least one of the inverter PWM signal and the transformer PWM signal. 
         [0012]    Preferably, the power supplying unit stops supplying of the driving electrical power supplied to the motor when an overdischarge signal is inputted from the battery pack. 
         [0013]    Preferably, the load detecting unit detects the load based on a current flowing into the motor. 
         [0014]    Another aspect of the present invention provides an electrical power tool including: a motor; a load detecting unit that detects a load applied to the motor; a trigger switch that receives a first instruction; a power supplying unit that starts supplying of a driving electrical power to the motor when the trigger switch receives the first instruction; and a setting unit that receives a second instruction. The power supplying unit changes an amount of the driving electrical power when the setting unit receives the second instruction. 
         [0015]    Another aspect of the present invention provides an electrical power tool including: an AC motor driven with an AC electrical power; a trigger switch that receives an instruction; an inverter circuit that converts a DC electrical power supplied from a battery pack to an AC electrical power, and supplies the AC electrical power to the AC motor; a controller configured to control the inverter circuit; and a power switch, a driving electrical power being supplied to the controller when the power switch is turned ON. The controller controls the inverter circuit to start converting the DC electrical power to the AC electrical power after the trigger switch receives the instruction. 
         [0016]    Preferably, the electrical power tool further includes: a transformer switch element connected between the battery pack and the inverter circuit, the DC electrical power being supplied from the battery pack to the transformer switch element and converted to an AC electrical power by an ON/OFF operation of the transformer switch element; a transformer that transforms the AC electrical power outputted from the transformer switch element; a rectifying/smoothing unit that rectifies and smoothes the transformed AC electrical power, the inverter circuit converting the rectified and smoothed AC electrical power to the AC electrical power; and a transmitting unit that transmits the DC electrical power supplied from the battery pack to the controller via the AC motor when the trigger switch receives the instruction. The controller controls the inverter circuit to start converting the DC electrical power to the AC electrical power after the DC electrical power is transmitted via the AC motor. 
         [0017]    Preferably, the power switch is disposed between the battery pack and the controller, and the transmitting unit is disposed between a connecting point between the power switch and the controller and the AC electrical motor. 
         [0018]    Preferably, the electrical power tool further includes a trigger detecting unit having a plurality of resistors connected to the AC motor in series, the DC electrical power supplied from the battery pack being divided by the plurality of resistors and outputted to the controller. The controller controls the inverter circuit to start converting the DC electrical power to the AC electrical power after the divided DC electrical power is inputted. 
         [0019]    Preferably, the electrical power tool further includes: a transformer switch element connected between the battery pack and the inverter circuit, the DC electrical power being supplied from the battery pack to the transformer switch element and converted to an AC electrical power by an ON/OFF operation of the transformer switch element; a transformer that transforms the AC electrical power outputted from the transformer switch element; and a rectifying/smoothing unit that rectifies and smoothes the transformed AC electrical power, the inverter circuit converting the rectified and smoothed AC electrical power to the AC electrical power. The inverter circuit includes a plurality of inverter switch elements connected between the rectifying/smoothing unit and the AC motor, the rectified and smoothed AC electrical power being converted to an AC electrical power by ON/OFF operations of the plurality of inverter switch elements. The controller controls the inverter circuit to start converting the DC electrical power to the AC electrical power after the DC electrical power is inputted via the AC motor. The controller controls one inverter switch element to turn ON and the other inverter switch element to turn OFF until the DC electrical power is inputted via the AC motor. 
         [0020]    Preferably, the electrical power tool further includes a trigger detecting unit having a plurality of resistors connected to the AC motor in series, the DC electrical power supplied from the battery pack being divided by the plurality of resistors and outputted to the controller. The controller controls the inverter circuit to start converting the DC electrical power to the AC electrical power after the divided DC electrical power is inputted. The plurality of inverter switch includes a first switch, a second switch connected to the first switch, a third switch, and a fourth switch connected to the third switch, the first switch and the third switch being connected to a positive terminal of the battery pack, the second switch and the fourth switch being connected to a negative terminal of the battery pack, the AC motor being connected between a connecting point between the first switch and the second switch and a connecting point between the third switch and the fourth switch, the trigger detecting unit being connected to the fourth switch in parallel. The controller controls the first switch to turn ON and the second switch, the third switch, and the fourth switch to turn OFF until the DC electrical power is inputted via the AC motor. 
         [0021]    Preferably, the electrical power tool further includes: a transformer switch element connected between the battery pack and the inverter circuit, the DC electrical power being supplied from the battery pack to the transformer switch element and converted to an AC electrical power by an ON/OFF operation of the transformer switch element; a transformer that transforms the AC electrical power outputted from the transformer switch element; and a rectifying/smoothing unit that rectifies and smoothes the transformed AC electrical power, the inverter circuit converting the rectified and smoothed AC electrical power to the AC electrical power. The controller controls the transformer switch element to start the ON/OFF operation after the trigger switch receives the instruction. 
         [0022]    Preferably, the controller controls the inverter circuit to stop converting the DC electrical power to the AC electrical power when an overdischarge signal is inputted from the battery pack. 
         [0023]    Another aspect of the present invention provides an inverter device including; a main body; a load detecting unit that detects a load applied to a motor which is connected to the main body; and a power supplying unit that starts supplying of a driving electrical power to the motor. The power supplying unit changes an amount of the driving electrical power based on the load detected by the load detecting unit. 
         [0024]    Another aspect of the present invention provides an inverter device including: a main body; an inverter circuit that converts a DC electrical power supplied from a battery pack to an AC electrical power, and supplies the AC electrical power to a AC motor which is connected to the main body; a controller configured to control the inverter circuit; and a power switch, a driving electrical power being supplied to the controller when the power switch is turned ON. The controller controls the inverter circuit to start converting the DC electrical power to the AC electrical power after a trigger switch connected to the AC motor in series is operated. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0025]    The particular features and advantages of the invention as well as other objects will become apparent from the following description taken in connection with the accompanying drawings, in which: 
           [0026]      FIG. 1  is a side view of a mower according to a first embodiment of the present invention; 
           [0027]      FIG. 2  is a circuit diagram of the mower according to the first embodiment; 
           [0028]      FIG. 3  is a flowchart of a voltage control performed by a microcomputer according to the first embodiment; 
           [0029]      FIG. 4  is an explanation diagram of the voltage control performed by the microcomputer according to the first embodiment; 
           [0030]      FIG. 5  is a flowchart of a voltage control performed by the microcomputer according a variation of the first embodiment; 
           [0031]      FIG. 6  is a circuit diagram of a mower according to a second embodiment of the present invention; 
           [0032]      FIG. 7  is a flowchart of a voltage control performed by a microcomputer according to the second embodiment; 
           [0033]      FIG. 8  is an explanation diagram of a voltage control performed by the microcomputer according to a first variation according to the second embodiment; 
           [0034]      FIG. 9  is a flowchart of a control of a voltage control performed by the microcomputer according to a second variation according to the second embodiment; 
           [0035]      FIG. 10  is a circuit diagram of a mower according to a third embodiment of the present invention; 
           [0036]      FIG. 11  is a flowchart of a control of a voltage control performed by a microcomputer according to the third embodiment; and 
           [0037]      FIG. 12  is a circuit diagram of a mower according to a fourth embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0038]    A mower  1  (one example of an electrical power tool) according to a first embodiment of the present invention will be described with reference to  FIGS. 1-4 . 
         [0039]      FIG. 1  is a side view of the mower  1 . The mower  1  is provided with a main body  3 , an inverter  2  detachable from the main body  3  via a latch  2 A, and a battery pack  4 . When a trigger switch  31  is operated by a user, an electrical power is supplied from the battery pack  4  to an AC motor  32  via the inverter  2 . In the following explanation, it is assumed that the inverter  2  is connected to both the main body  3  and the battery pack  4 , though the inverter  2  is detachable from the main body  3  and the battery pack  4 . Further, a handle  5 , front wheels  6 , and rear wheels  7  for allowing the mower  1  to move are provided on the main body  3 . A lawn bag  8  for accommodating a lawn mowed by a rotating blade (not shown) connected to the AC motor  32  is detachably provided at a rear side of the main body  3 . 
         [0040]      FIG. 2  is a circuit diagram of the mower  1 . As described above, the mower  1  includes the inverter  2  and the main body  3 . When the trigger switch  31  is operated, the inverter  2  converts a DC electrical power supplied from the battery pack  4  into an AC electrical power, and outputs the AC electrical power to the AC motor  32  of the main body  3 . 
         [0041]    The inverter (inverter device)  2  includes a main body  2 ′ that is an outer frame. The main body  2 ′ accommodates a battery voltage detecting unit  21 , a power source  22 , a transforming unit  23 , a rectifying/smoothing circuit  24 , a transformed voltage detecting unit  25 , an inverter circuit  26 , a current detecting resistor  27 , a PWM signal outputting unit  28 , and a microcomputer  29 . 
         [0042]    The battery voltage detecting unit  21  includes resistors  211  and  212  connected in series. The voltage supplied from the battery pack  4  is divided by the resistors  211  and  212 , and outputted to the microcomputer  29 . In the present embodiment, the battery pack  4  includes four lithium battery cells. Since each lithium battery cell has 3.6V of rated voltage, the battery pack  4  has 14.4V of rated voltage. 
         [0043]    The power source  22  includes a power switch  221  and a voltage regulator circuit  222  connected in series between the battery pack  4  and the microcomputer  29 . The voltage regulator circuit  222  includes a three-terminal regulator  222   a  and capacitors  222   b  and  222   c  for preventing an oscillation. When the power switch  221  is turned ON by a user, the voltage regulator circuit  222  transforms 14.4V of voltage supplied from the battery pack  4  to a predetermined voltage (for example, 5V), and outputs the predetermined voltage to the microcomputer  29  as a driving power. Note that when the power switch  221  is turned OFF, the inverter  2  is halted since the driving power is not supplied to the microcomputer  29 . 
         [0044]    The transforming unit  23  includes a transformer  231  and an FET  232 . A primary side of the transformer  231  and the FET  232  are connected in series between the battery pack  4  and a GND. A gate of the FET  232  is connected to the microcomputer  29 . The FET  232  is turned ON/OFF in accordance with a first PWM signal (described later) outputted from the microcomputer  29  to the gate of the FET  232 . When the FET  232  is turned ON/OFF, the DC electrical power supplied from the battery pack  4  is outputted to the primary side of the transformer  231  as an AC electrical power. The AC electrical power is transformed by the transformer  231  and outputted from a secondary side of the transformer  231 . 
         [0045]    The rectifying/smoothing circuit  24 , the transformed voltage detecting unit  25 , the inverter circuit  26 , and the current detecting resistor  27  are connected to the secondary side of the transformer  231 . 
         [0046]    The rectifying/smoothing circuit  24  includes diodes  241  and  242  and a capacitor  243 . The AC voltage transformed by the transformer  231  is rectified by the diodes  241  and  242 , and the rectified voltage is smoothed to a DC voltage (for example, 141V) by the capacitor  243 . 
         [0047]    The transformed voltage detecting unit  25  includes resistors  252  and  252  connected in series. The DC voltage outputted from the rectifying/smoothing circuit  24  is divided by the resistors  211  and  222 , and outputted to the microcomputer  29 . 
         [0048]    The inverter circuit  26  includes four FETs  261 - 264 . The FETs  261  and  262  connected in series and the FETs  263  and  264  connected in series are connected to an output terminal A of the rectifying/smoothing circuit  24  in parallel. Specifically, a drain of the FET  261  is connected to the output terminal A, and a source of the FET  261  is connected to a drain of the FET  262 . In a similar manner, a drain of the FET  263  is connected to the output terminal A, and a source of the FET  263  is connected to a drain of the FET  264 . 
         [0049]    The source of the FET  261  and the drain of the FET  262  are connected to a first terminal  32   a  of the AC motor  32  of the main body  3  via the trigger switch  31 . The source of the FET  263  and the drain of the FET  264  are connected to a second terminal  32   b  of the AC motor  32 . Gates of the FETs  261 - 264  are connected to the PWM signal outputting unit  28 . The FETs  261 - 264  are turned ON/OFF in accordance with second PWM signals (described later) outputted from the PWM signal outputting unit  28 . When the FETs  261 - 264  are turned ON/OFF, the DC electrical power outputted from the rectifying/smoothing circuit  24  is outputted to the AC motor  32  of the main body  3  as an AC power. 
         [0050]    The current detecting resistor  27  is connected between sources of the FETs  262  and  264  and the GND. A high-voltage side terminal of the current detecting resistor  27  is also connected to the microcomputer  29 . With this construction, the current flowing into the current detecting resistor  27 , that is, the current flowing into the AC motor  32  is outputted to the microcomputer  29  as a voltage. 
         [0051]    The microcomputer  29  controls the ON/OFF operation of the FET  232  based on the transformed voltage detected by the transformed voltage detecting unit  25 , so that an AC voltage having a target effective voltage is outputted from the transformer  231 . Specifically, the microcomputer  29  generates a first PWM signal based on the transformed voltage detected by the transformed voltage detecting unit  25 , and outputs the first PWM signal to the gate of the FET  232  to turn ON/OFF the FET  232 . 
         [0052]    Further, the microcomputer  29  controls the ON/OFF operations of the FETs  261 - 264  based on the current flowing into the AC motor  32  detected by the current detecting resistor  27 , that is, based on the load applied to the AC motor  32 , so that an AC voltage suitable to the load is outputted from the inverter circuit  26 . Specifically, the microcomputer  29  generates second PWM signals based on the current (load) detected by the current detecting resistor  27 , and outputs the second PWM signals via the PWM signal outputting unit  28  to the gates of the FETs  261 - 264  to turn ON/OFF the FETs  261 - 264 . 
         [0053]    In the present embodiment, when the current (load) detected by the current detecting resistor  27  is equal to or greater than a predetermined value, the microcomputer  29  determines that the main body  3  mows a lawn, and alternately turns ON a set of the FETs  261  and  264  (hereinafter called “first set”) and a set of the FETs  262  and  263  (hereinafter called “second set”) at 100% of duty by second PWM signals. Thus, since a greater voltage is supplied to the AC motor  32  when the main body  3  mows a lawn, it becomes possible to effectively mow a lawn. 
         [0054]    On the other hands, when the current (load) detected by the current detecting resistor  27  is smaller than a predetermined value, the microcomputer  29  determines that the main body  3  runs idle, and alternatively turns ON the first set and the second set at a duty (for example, 40%) lower than 100% by second PWM signals. Thus, since a smaller voltage is supplied to the AC motor  32  when the main body  3  runs idle, it becomes possible to reduce a waste of an electrical power. 
         [0055]    Further, the microcomputer  29  determines an occurrence of an overdischarge in the battery pack  4  based on the battery voltage detected by the battery voltage detecting unit  21 . Specifically, when the battery voltage detected by the voltage detecting unit  21  is equal to or smaller than a first overdischarge threshold, the microcomputer  29  determines that an overdischarge is occurring in the battery pack  4 , and stops the ON/OFF operation of the FET  232  by a first PWM signal and the ON/OFF operations of the FET  261 - 264  by second PWM signals. 
         [0056]    Further, the battery pack  4  includes a protecting IC or a microcomputer (not shown) that have an overdischarge detecting function. The protecting IC or the microcomputer outputs an overdischarge signal to the microcomputer  29  via a LD terminal, when the battery voltage is equal to or smaller than a second overdischarge threshold larger than the first overdischarge threshold. When also receiving the overdischarge signal, the microcomputer  29  stops the ON/OFF operation of the FET  232  by a first PWM signal and the ON/OFF operations of the FET  261 - 264  by second PWM signals. With this construction, the life of the battery pack  4  is prevented from being shorten. The protection IC or the microcomputer outputs an overdischarge signal to the microcomputer  29  via the LD terminal, when the at least one cell voltage of the battery pack  4  is equal to or smaller than a third overdischarge threshold of the cell. 
         [0057]    Next, a voltage control performed by the microcomputer  29  will be described with reference to  FIG. 3 . 
         [0058]    A flowchart shown in  FIG. 3  starts when the power switch  221  is turned ON in a state where the battery pack  4  has been connected to the inverter  2 , or when the battery pack  4  is connected to the inverter  2  in a state where the power switch  221  has been turned ON. 
         [0059]    First, the microcomputer  29  determines whether or not the trigger switch  31  has been turned ON (S 101 ). When the trigger switch  31  has been turned ON (S 101 : YES), the microcomputer  29  starts the ON/OFF operation of the FET  232 , that is, the transforming operation of the transformer  231  by a first PWM signal (S 102 ). 
         [0060]    Next, the microcomputer  29  determines, based on the transformed voltage detected by the transformed voltage detecting unit  25 , whether or not the transformed voltage is greater than a target voltage (for example, 141V) (S 103 ). When the transformed voltage is greater than the target voltage (S 103 : YES), the microcomputer  29  reduces the duty of the first PWM signal (S 104 ). On the other hands, when the transformed voltage is smaller than the target voltage (S 103 : NO), the microcomputer  29  increases the duty of the first PWM signal (S 105 ). 
         [0061]    Next, the microcomputer  29  sets the duty of second PWM signals to 40% to supply an AC voltage having 40V of effective voltage to the AC motor  32  (S 106 ). As described later, in the present embodiment, the duty of second PWM signals is set to one of 40% and 100%. 
         [0062]    Next, the microcomputer  29  determines which of 40% and 100% the duty of the second PWM signals is set to (S 107 ). When the duty is set to 40% (S 107 : 40%), the microcomputer  29  determines whether or not the current (load) detected by the current detecting resistor  27  is greater than a first threshold (S 108 ). When the current (load) is greater than the first threshold (S 108 : YES), the microcomputer  29  determines that the main body  3  mows a lawn, and changes the duty of the second PWM signals to 100% to supply an AC voltage having 100V to the AC motor  32  as shown in  FIG. 4  (S 109 ), and goes to S 112 . On the other hands, when the current (load) is equal to or smaller than the first threshold (S 108 : NO), the microcomputer  29  determines that the main body  3  runs idle, or the load applied to the AC motor  32  is small although the main body  3  mows a lawn, and goes to S 112  without going to S 109 . 
         [0063]    On the other hands, when the duty is set to 100% (S 107 : 100%), the microcomputer  29  determines whether or not the current (lead) detected by the current detecting resistor  27  is smaller than a second threshold smaller than the first threshold (S 110 ). When the current (load) is smaller than the second threshold (S 110 : YES), the microcomputer  29  determines that the main body  3  runs idle, and changes the duty of the second PWM signals to 40% to supply an AC voltage having 40V to the AC motor  32  (S 111 ), and goes to S 112 . On the other hands, when the current (load) is equal to or greater than the second threshold (S 110 : NO), the microcomputer  29  determines that the main body  3  mows a lawn, and goes to S 112  without going to S 111 . 
         [0064]    Next, the microcomputer  29  determines whether or not the battery voltage detected by the battery voltage detecting unit  21  is smaller than the first overdischarge voltage (S 112 ). When the battery voltage detected by the battery voltage detecting unit  21  is smaller than the overdischarge voltage (S 112 : YES), the microcomputer  29  determines that an overdischarge is occurring in the battery pack  4 , and stops the ON/OFF operation of the FET  232  by a first PWM signal and the ON/OFF operations of the FETs  261 - 264  by second PWM signals to stop the operations of the transforming unit  23  and the inverter circuit  26  (S 113 ). As the result, the power supply to the AC motor  32  is stopped. 
         [0065]    When the battery voltage detected by the battery voltage detecting unit  21  is equal to or greater than the overdischarge voltage (S 112 : NO), the microcomputer  29  determines whether or not the overdischarge signal has been inputted from the battery pack  4  (S 114 ). When the overdischarge signal has been inputted from the battery pack  4  (S 114 : YES), the microcomputer  29  stops the ON/OFF operation of the FET  232  by a first PWM signal and the ON/OFF operations of the FETs  261 - 264  by second PWM signals to stop the operations of the transforming unit  23  and the inverter circuit  26  (S 113 ). On the other hands, when the overdischarge signal has not been inputted from the battery pack  4  (S 114 : NO), the microcomputer  29  returns to S 107  to continue a voltage control based on the current (load). 
         [0066]    Thus, in the present embodiment, since the occurrence of the overdischarge in the battery pack  4  is detected by both the battery pack  4  and the inverter  2 , it becomes possible to reliably prevent the occurrence of the overdischarge. 
         [0067]    As described above, the mower  1  according to the present embodiment changes the driving power supplied to the AC motor  32  based on the load applied to the AC motor  32 . Specifically, the mower  1  increases the driving power when the load applied to the AC motor  32  is equal to or greater than a predetermined value, and decreases the driving power when the load applied to the AC motor  32  is smaller than a predetermined value. With this construction, it becomes possible to reduce the waste of the electrical power when the mower  1  runs idle. 
         [0068]    Note that the driving power supplied to the AC motor  32  may be changed by changing the duty of first PWM signals without changing the duty of second PWM signal. Further, the driving power supplied to the AC motor  32  may be changed by changing both the duty of first PWM signal and the duty of second PWM signals. 
         [0069]    In this case, as shown in  FIG. 5 , the microcomputer  29  controls the FET  232  with the first PWM signal so that the transformed voltage approaches the first target voltage in S 203 -S 205 , and sets the duty of the second PWM signals to 40% in S 206 . Then, when the duty of the second PWM signals is set to 40% (S 207 : 40%) and the current (load) is greater than the first threshold (S 208 : YES), the microcomputer  29  determines which of the first target voltage and a second target voltage greater than the first target voltage the duty of the first PWM signal is set to a value for (S 208   a ). When the duty of first PWM signal is set to a value for the first target voltage (S 208   a : first target voltage), the microcomputer  29  increases the duty of the first PWM signal to a value for the second target voltage (S 208   b ) and also increases the duty of the second PWM signals to 100% (S 209 ). Thus, the driving power supplied to the AC motor  32  is increased by increasing both the duty of first PWM signal and the duty of second PWM signals. On the other hands, when the duty of first PWM signal is set to a value for the second target voltage (S 208   a : second target voltage), the microcomputer  29  goes to S 209 . 
         [0070]    On the other hands, when the duty of the second PWM signals is set to 100% (S 207 : 100%) and the current (load) is smaller than the second threshold (S 210 : YES), the microcomputer  29  determines which of the first target voltage and a second target voltage greater than the first target voltage the duty of the first PWM signal is set to a value for (S 210   a ). When the duty of the first PWM signal is set to a value for the second target voltage (S 210   a : second target voltage), the microcomputer  29  decreases the duty of the first PWM signal to a value for the first target voltage (S 210   b ) and also decreases the duty of the second PWM signals to 40% (S 211 ). Thus, the driving power supplied to the AC motor  32  is decreased by decreasing both the duty of first PWM signal and the duty of second PWM signals. On the other hands, when the duty of first PWM signal is set to a value for the first target voltage (S 210   a : first target voltage), the microcomputer  29  goes to S 211 . 
         [0071]    With this construction, it becomes possible to not only reduce a waste of an electrical power but also suppresses the heat generated in the FETs  232  and  261 - 264 , when the mower  1  runs idle, or the load applied to the AC motor  32  is small although the mower  1  mows a lawn. 
         [0072]    Next, a mower  1  according to a second embodiment of the present invention will be described with reference to  FIGS. 6 and 7 . 
         [0073]    In the second embodiment, the driving power supplied to the AC motor  32  can be manually changed, although the driving power supplied to the AC motor  32  is automatically changed based on the load applied to the AC motor  32  in the first embodiment. 
         [0074]      FIG. 6  is a circuit diagram of the mower  1  according to the second embodiment. In  FIG. 6 , like parts and components as  FIG. 2  are designated by the same reference numerals, and the description is omitted. 
         [0075]    The mower  1  according to the second embodiment is provided with an energy-saving switch  201  and a resistor  202 , while being not provided with the current detecting resistor  27 . The main body  3  is driven at an energy-saving mode when the energy-saving switch  201  is turned ON. 
         [0076]    The energy-saving switch  201  and the resistor  202  are connected in series between the three-terminal regulator  222   a  and GND so that the resistor  202  is directly connected to the three-terminal regulator  222   a . The connecting point between the resistor  202  and the energy-saving switch  201  is connected to the microcomputer  29 . With this construction, when the energy-saving switch  201  is turned ON (energy-saving mode), 0V (Low) is inputted to an input port B of the microcomputer  29 . On the other hands, when the energy-saving switch  201  is turned OFF, a predetermined DC voltage outputted from the three-terminal regulator  222   a  is inputted to the input port B of the microcomputer  29 . 
         [0077]    When the energy-saving switch  201  is turned OFF, the microcomputer  29  alternately turns ON the first set and the second set at 100% of duty by second PWM signals. On the other hands, the energy-saving switch  201  is turned ON, the microcomputer  29  alternately turns ON the first set and the second set at 70% of duty by second PWM signals. With this construction, a user can change the driving power supplied to the AC motor  32  in accordance with the user&#39;s wish. Therefore, for example, if the user turns ON the energy-saving switch  201  when mowing a little lawn, it becomes possible to reduce the waste of the electrical power. 
         [0078]    Next, a voltage control performed by the microcomputer  29  will be described with reference to  FIG. 7 . The descriptions of S 301 -S 305  and S 309 -S 311  are omitted, since the operations in S 301 -S 305  and S 309 -S 311  are identical with the operations in S 101 -S 105  and S 112 -S 114  in  FIG. 3 , respectively. 
         [0079]    In the second embodiment, in S 306 , the microcomputer  29  determines whether or not the energy-saving switch  201  has been turned ON (S 306 ). When the energy-saving switch  201  has been turned ON (S 306 : YES), the microcomputer  29  sets the duty of the second PWM signals to 70% (S 307 ). On the other hands, when the energy-saving switch  201  has not been turned ON (S 306 : NO), the microcomputer  29  sets the duty of the second PWM signals to 100% (S 308 ). 
         [0080]    As described above, since the mower  1  according to the second embodiment is provided with the energy-saving switch  201 , a user can change the driving power supplied to the AC motor  32  in accordance with the user&#39;s wish. Therefore, for example, if the user turns ON the energy-saving switch  201  when mowing a little lawn, it becomes possible to reduce the waste of the electrical power. 
         [0081]    Note that a variable resistor having a dial may be disposed instead of the energy-saving switch  201 . In this case, as show in  FIG. 8 , the driving power can be changed at non-step form by changing the resistance value of the variable resistor with the dial. 
         [0082]    Further, in the second embodiment, the microcomputer  29  decreases the driving power supplied to the AC motor  32  by decreasing the duty of the FETs  261 - 264 , when the energy-saving switch  201  is turned ON. However, the microcomputer  29  may decrease the driving power supplied to the AC motor  32  by decreasing the duty of the FET  232  when the energy-saving switch  201  is turned ON. 
         [0083]    In this case, as shown in  FIG. 9 , the microcomputer  29  controls the FET  232  with the first PWM signal so that the transformed voltage approaches the first target voltage in S 403 -S 405 . Then, when the energy-saving switch  201  is turned ON (S 406 : YES), the microcomputer  29  decreases the duty of the first PWM signal so that a third target voltage smaller than the first target voltage is outputted from the transforming unit  23  (S 406   a ), and sets the duty of the second PWM signals to 70% (S 407 ). On the other hand, When the energy-saving switch  201  is turned OFF (S 406 : NO), the microcomputer  29  increases the duty of the first PWM signal so that the second target voltage greater than the first voltage is outputted from the transforming unit  23  (S 406   b ), and sets the duty of the second PWM signals to 100% (S 408 ). 
         [0084]    With this construction, it becomes possible to not only reduce a waste of an electrical power but also suppresses the heat generated in the FETs  232  and  261 - 264 . 
         [0085]    Next, a mower  1  according to a third embodiment of the present invention will be described with reference to  FIGS. 10 and 11 . 
         [0086]      FIG. 10  is a circuit diagram of the mower  1  according to the third embodiment. In  FIG. 10 , like parts and components as  FIG. 2  are designated by the same reference numerals, and the description is omitted. 
         [0087]    In the first embodiment, when the power switch  221  is turned ON, the battery voltage of the battery pack  4  is supplied to the microcomputer  29  via the power source  22  even if the trigger switch  31  is turned OFF. As the result, an electrical power is wasted. In the third embodiment, the mower  1  is provided with a power switch detecting diode  10  and a trigger detecting unit  11  in order to reduce a waste of an electrical power when the trigger switch is turned OFF. 
         [0088]    An anode of the power switch detecting diode  10  is connected to a low-voltage side of the power switch  221 , and a cathode of the power switch detecting diode  10  is connected to the first terminal  32   a  of the AC motor  32  via the trigger switch  31 . With this construction, when the power switch  221  is turned ON, the battery voltage of the battery pack  4  is applied to the AC motor  32 . 
         [0089]    The cathode of the power switch detecting diode  10  is also connected to the source of the FET  261 . Therefore, when the FET  261  is turned ON, the DC voltage outputted from the rectifying/smoothing circuit  24  is applied to the AC motor  32 . 
         [0090]    The trigger detecting unit  11  includes resistors  111  and  112  connected in series between the second terminal  32   b  of the AC motor  23  and the GND, in other words, between the drain and the source of the FET  264 . When both the power switch  221  and the trigger switch  31  are turned ON, the battery voltage of the battery pack  4  is applied to the trigger detecting unit  11  through the power switch  221 , the power switch detecting diode  10 , the trigger switch  31 , and the AC motor  32 . The battery voltage of the battery pack  4  is divided by the resistors  111  and  112 , and outputted to the microcomputer  29  as a trigger detecting signal. 
         [0091]    Note that the cathode of the power switch detecting diode  10  may be connected to the source of the FET  263 , and the trigger detecting unit  11  may be connected between the drain and the source of the FET  262 . 
         [0092]    In the present embodiment, when the trigger switch  31  is turned OFF, that is, the trigger detecting signal is not inputted from the trigger detecting unit  11  into the microcomputer  29 , the microcomputer  29  stops the ON/OFF operations of the FETs  232  and  261 - 264  by a first PWM signal and second PWM signals. With this construction, it becomes possible to reduce a waste of an electrical power when the trigger switch is turned OFF. 
         [0093]    Next, a voltage control performed by the microcomputer  29  will be described with reference to  FIG. 11 . 
         [0094]    A flowchart shown in  FIG. 11  starts when the power switch  221  is turned ON in a state where the battery pack  4  has been connected to the inverter  2 , or when the battery pack  4  is connected to the inverter  2  in a state where the power switch  221  has been turned ON. When the power switch  221  is turned ON and the battery pack  4  is connected to the inverter  2 , a driving power is generated by the voltage regulator circuit  222 , and the drive of the microcomputer  29  is started with the driving power. 
         [0095]    First, the microcomputer  29  determines whether or not the trigger detecting signal is inputted from the trigger detecting unit  11 , that is, the battery voltage of the battery pack  4  is applied to the trigger detecting unit  11  through the power switch  221 , the power switch detecting diode  10 , the trigger switch  31 , and the AC motor  32  (S 501 ). When the trigger detecting signal is inputted from the trigger detecting unit  11  (S 501 : YES), the microcomputer  29  determines that the trigger switch  31  is turned ON and starts the ON/OFF operation of the FET  232 , that is, the transforming operation of the transformer  231  by a first PWM signal (S 502 ). 
         [0096]    Next, the microcomputer  29  determines, based on the transformed voltage detected by the transformed voltage detecting unit  25 , whether or not the transformed voltage is greater than a target voltage (for example, 141V) (S 503 ). When the transformed voltage is greater than the target voltage (S 503 : YES), the microcomputer  29  reduces the duty of the first PWM signal (S 504 ). On the other hands, when the transformed voltage is smaller than the target voltage (S 503 : NO), the microcomputer  29  increases the duty of the first PWM signal (S 505 ). Thus, the supply of the AC voltage to the AC motor  32  starts. 
         [0097]    Next, the microcomputer  29  determines whether or not the battery voltage detected by the battery voltage detecting unit  21  is smaller than the first overdischarge voltage (S 506 ). When the battery voltage detected by the battery voltage detecting unit  21  is smaller than the overdischarge voltage (S 506 : YES), the microcomputer  29  stops the ON/OFF operation of the FET  232  by a first PWM signal and the ON/OFF operations of the FETs  261 - 264  by second PWM signals to stop the operations of the transforming unit  23  and the inverter circuit  26  (S 507 ). As the result, the power supply to the AC motor  32  is stopped. 
         [0098]    When the battery voltage detected by the battery voltage detecting unit  21  is equal to or greater than the overdischarge voltage (S 506 : NO), the microcomputer  29  determines whether or not the overdischarge signal has been inputted from the battery pack  4  (S 508 ). When the overdischarge signal has been inputted from the battery pack  4  (S 508 : YES), the microcomputer  29  stops the ON/OFF operation of the FET  232  by a first PWM signal and the ON/OFF operations of the FETs  261 - 264  by second PWM signals (S 507 ). 
         [0099]    On the other hands, when the overdischarge signal has not been inputted from the battery pack  4  (S 508 : NO), the microcomputer  29  determines whether or not the trigger detecting signal is inputted from the trigger detecting unit  11  again (S 509 ). When the trigger signal is inputted from the trigger detecting unit  11  (S 509 : YES), the microcomputer  29  returns to S 502 . On the other hands, when the trigger signal is not inputted from the trigger detecting unit  11  (S 509 : NO), the microcomputer  29  stops the ON/OFF operation of the FET  232  by a first PWM signal and the ON/OFF operations of the FETs  261 - 264  by second PWM signals (S 510 ), and returns to S 501 . 
         [0100]    As described above, in the present embodiment, when the trigger switch  31  is turned OFF, the microcomputer  29  stops the ON/OFF operations of the FETs  232  and  261 - 264 . Thus, it becomes possible to reduce a waste of an electrical power. Further, since the ON/OFF operations of the FETs  232  and  261 - 264  is stopped when the trigger switch  31  is turned OFF, the heat is prevented from being generated in the FETs  232  and  261 - 264 , thereby the break of the FETs  232  and  261 - 264  being prevented. 
         [0101]    Next, a mower  1  according to a fourth embodiment of the present invention will be described with reference to  FIG. 12 . 
         [0102]      FIG. 12  is a circuit diagram of the mower  1  according to the fourth embodiment. In  FIG. 12 , like parts and components as  FIG. 10  are designated by the same reference numerals, and the description is omitted. 
         [0103]    The mower  1  according to the fourth embodiment is not provided with the power switch detecting diode  10 . In the present embodiment, when the power switch  221  is turned ON in a state where the trigger switch  31  is turned OFF, the microcomputer  29  starts the ON/OFF operation of the FET  232  by a first PWM signal. However, with respect to the FETs  261 - 264 , the microcomputer  29  turns ON only the FET  261  by second PWM signals. With this construction, when the trigger switch  31  is turned ON, the DC voltage outputted from the rectifying/smoothing unit  24  is applied to the trigger detecting unit  11  through the FET  261 , the trigger switch  31 , and the AC motor  32 , and divided by the resistors  111  and  112 , and outputted to the microcomputer  29  as the trigger detecting signal. Further, in the present embodiment, when the trigger detecting signal is inputted from the trigger detecting unit  11  into the microcomputer  29 , the microcomputer  29  starts the ON/OFF operations of all of the FETs  261 - 264 . 
         [0104]    As described above, in the present embodiment, when the trigger switch  31  is not turned ON, the microcomputer  29  stops the ON/OFF operations of the FETs  261 - 264 . Thus, it becomes possible to reduce a waste of an electrical power. Further, since the ON/OFF operations of the FETs  261 - 264  is stopped when the trigger switch  31  is not turned ON, the heat is prevented from being generated in the FETs  261 - 264 , thereby the break of the FETs  261 - 264  being prevented. 
         [0105]    Note that the trigger detecting unit  11  may be disposed between the drain and the source of the FET  262 . In this case, when the power switch  221  is turned ON in a state where the trigger switch  31  is turned OFF, the microcomputer  29  turns ON only the FET  263  instead of the FET  261 . 
         [0106]    Further, the microcomputer  29  reduces the duty of the first PWM signal when the trigger switch  31  is not turned ON than when the trigger switch  31  is turned ON. With this construction, it becomes possible to more effectively reduce a waste of an electrical power when the trigger switch  31  is not turned ON. However, by the first PWM signal whose duty is reduced, a voltage such the microcomputer  29  can determine that the trigger switch  31  has been turned ON must applied to the trigger detecting unit  11 . 
         [0107]    While the invention has been described in detail with reference to the embodiments thereof, it would be apparent to those skilled in the art that various changes and modifications may be made therein without departing from the spirit of the invention. 
         [0108]    For example, the inverter  2  may be incorporated into the main body  3 , although the inverter  2  is detachable from the main body  3  in the above embodiments. In this case, the circuits provided in the inverter  2  in the above embodiments are provided in the main body  3 . Therefore, the manufacturing cost is greatly reduced by using the AC motor in a similar as the conventional AC mower. 
         [0109]    Further, the microcomputer  29  may stop one of the FET  232  and FETs  261 - 264  in order to stop the power supply to the AC motor  32 . 
         [0110]    Further, a DC motor may be used instead of the AC motor  32 . In this case, the voltage is adjusted before supplied to the DC motor. 
         [0111]    Further, the mower  1  may be provided with another FET connected to the power switch  221  in series, and the battery pack  4  may be outputs the overdischarge signal to the gate of the FET when detecting the occurrence of the overdischarge. Thus, the life of the battery pack  4  is reliably prevented from being shorten, since the power supply to the microcomputer  29  is also stopped when the occurrence of the overdischarge is detected. 
         [0112]    Further, at least one of the inverter  2  and the battery pack  4  may be provided with an alarm unit, such as, a display or a buzzer, that informing a user of the occurrence of the overdischarge, and stop the power supply to the microcomputer  29  after informing the user of the occurrence of the overdischarge. With this construction, the life of the battery pack  4  is prevented from being shorten without giving the user a feeling of strangeness. 
         [0113]    Further, the second overdischarge threshold in the battery pack  4  may be set to a value smaller than the first overdischarge threshold in the inverter  2 , although the first overdischarge threshold is set to a value smaller than the second overdischarge threshold in the above embodiments. In this case, S 112  and S 114  of  FIG. 3 , S 212  and S 214  of  FIG. 5 , S 309  and S 311  of  FIG. 7 , S 409  and S 411  of  FIG. 9 , and S 506  and S 508  of  FIG. 11  are performed in a reverse order. Further, the occurrence of the overcurrent may be also detected by both the battery pack  4  and the inverter  2 . 
         [0114]    Further, the electrical power tool of the present invention is not limited to the mower. The present invention can be applied to an electrical power tool including a trigger switch and driven with an AC electrical power such as a hedge trimmer, a circular saw, a jigsaw, a grinder, and a driver. 
         [0115]    Further, a plurality of battery pack  4  may be mounted on the main body  4 , and be used sequentially. With this construction, it becomes possible to use the mower  1  for a long time. 
         [0116]    Further, the control of the transformed voltage performed in S 102 -S 105  of  FIG. 3 , S 202 -S 205  of  FIG. 5 , S 302 -S 305  of  FIG. 7 , S 402 -S 405  of  FIG. 9 , and S 502 -S 505  of  FIG. 11  and the detection of the occurrence of the overdischarge performed in S 112 -S 114  of  FIG. 3 , S 212 -S 214  of  FIG. 5 , S 309 -S 311  of  FIG. 7 , S 409 -S 411  of  FIG. 9 , and S 506 -S 508  of  FIG. 11  can be performed in any step in the flowcharts and can be performed at a same time. 
         [0117]    Further, the duty is not limited to a value described in the above embodiments.