Method for controlling a motor-driven compressor configured for installation in a vehicle

Abstract

A method for controlling the acceleration and deceleration of a motor-driven compressor (10) configured to be installed in a vehicle, wherein the motor-driven compressor (10) comprises an electric motor (11) driven by an electric power supplied from a battery (E), and a control section (40) that controls the drive of the electric motor (11), wherein the control section (40) has an upper limit value of an increase rate of a rotational speed per unit time of the electric motor (11) and an upper limit value of a decrease rate of the rotational speed per unit time of the electric motor (11), the control section (40) sets a target speed and controls the electric motor (11) to be driven so that the speed of the electric motor (11) becomes equal to the target speed, when the control section (40) sets the target speed to a speed of the electric motor (11) requested by another control section (50), the control section (40) changes the speed of the electric motor (11) at an increase rate that is less than or equal to the upper limit of the increase rate or at a decrease rate that is less than or equal to the upper limit of the decrease rate, and under the condition that the control section (40) sets the target rotational speed to a rotational speed limit determined based on a voltage of the battery (E), and the voltage of the battery (E) decreases, as a result of which the control section (40) is able to reduce the rotational speed of the electric motor (11) at a decrease rate exceeding the upper limit of the decrease rate.

Description

Background of the invention

[0001] The present invention relates to a method for controlling a motor-driven compressor configured for installation in a vehicle.

[0002] Conventionally, there is a motor-driven compressor configured for installation in a vehicle and powered by an inverter. The inverter receives electrical energy from a vehicle's battery and converts direct current into alternating current. The motor-driven compressor is driven at a speed specified or commanded by a controller installed in the vehicle. The speed commanded by the controller is represented by discrete values. Therefore, if the speed is frequently changed in response to commands from the controller, a rapid increase or decrease in the speed repeatedly occurs, thereby aggravating noise and vibration (NV). To solve this problem, as in Fig. 4, the motor-driven compressor is driven at a speed having a change per unit time smaller than that of a commanded speed commanded by the controller.

[0003] The motor-driven compressor receives electrical energy from the vehicle's battery and is affected by the battery voltage. For example, if the motor-driven compressor operates at high speed and is stopped when the battery voltage is low, the counter electromotive force voltage (the counter electromotive voltage) of the motor of the motor-driven compressor may exceed the battery voltage. When this happens, an electric current flows from the motor to the battery via the inverter of the motor-driven compressor. At this time, when the counter electromotive voltage of the motor is high, an excessive electric current flows through the inverter, damaging the switching elements of the inverter. To avoid this, the maximum speed at which the motor-driven compressor can be operated must be limited based on the battery voltage.

[0004] Japanese Laid-Open Patent Publication JP H07-120081 A discloses a low-cost and small-sized control drive device for a motor-driven compressor that prevents a motor-driven compressor from being stopped by an overcurrent protection caused by a voltage change of a power source and, without adversely affecting the lubricating function in the compressor, lowers the blown air temperature for a temporary period of air conditioning and increases the rate of increase of the blown air temperature. The control drive device includes a DC voltage generating device, a motor-driven compressor including a motor and compressing refrigerant for air conditioning, and an inverter.The control drive device causes the inverter's frequency increase rate or voltage increase rate to be higher in a low-speed range of a variable-speed range than in a high-speed range. The control drive device also causes the inverter's frequency decrease rate or voltage decrease rate to be higher in the high-speed range of the variable-speed range than in the low-speed range.

[0005] The battery voltage varies depending on the condition of the vehicle. Thus, the power source voltage of the motor-driven compressor may decrease rapidly in some cases. At the time of such a rapid decrease in the power source voltage, the speed decreases only slowly at a rate determined by taking NV (vibration) into account, even if a rapid decrease in the speed of the motor-driven compressor is intended. Therefore, if the motor-driven compressor is operating at a high speed and is stopped when the battery voltage is low, the amount of electric current flowing in the motor of the motor-driven compressor will be greater than before the compressor stopped, as shown in Fig. 5. Excessive electrical current can therefore flow through the battery and damage the inverter.

[0006] To prevent damage to the inverter's switching elements caused by excessive electrical current flowing through the inverter, switching elements with a high withstand voltage can be used. However, this increases the cost and size of the inverter.

[0007] According to Japanese Patent Laid-Open Publication JP H07-120081 A, the rate of decrease of the inverter's frequency or voltage is higher in the high-speed, variable-speed range than in the low-speed range. However, the publication does not contain any description regarding damage to the inverter's switching elements caused by stopping the motor-driven compressor operating at high speed when the battery voltage is low.

[0008] Patent application JP 2010-48103 A discloses an inverter-powered electric motor for driving an air conditioning compressor in a vehicle, which is controlled to limit a temperature rise of an electromagnetic coil. The electric motor can execute the following two modes: a power output mode for driving the vehicle and a regenerative mode. However, if the motor is not currently executing either of these modes, a first limit value limits the rotational acceleration; if the motor is executing one of these modes, a second limit value limits the rotational acceleration. Whether the motor is executing one of the modes is determined based on the voltage. Disclosure of the invention

[0009] Accordingly, it is an object of the present invention to provide a method for controlling a motor-driven compressor configured to be mounted on a vehicle and capable of preventing damage to switching elements of an inverter of the motor-driven compressor caused by stopping the motor-driven compressor at the time of a rapid voltage change of a vehicle battery, while preventing a rapid change in the rotational speed of the motor-driven compressor.

[0010] To achieve the aforementioned object, according to a first aspect of the present invention, there is provided a method for controlling the acceleration and deceleration of a motor-driven compressor configured to be installed in a vehicle. The motor-driven compressor includes an electric motor driven by electric power supplied from a battery, and a control section that controls the drive of the electric motor. The control section has an upper limit value of an increase rate of a rotational speed per unit time of the electric motor and an upper limit value of a decrease rate of the rotational speed per unit time of the electric motor. The control section sets a target rotational speed and controls the electric motor to be driven so that the rotational speed of the electric motor becomes equal to the target rotational speed.When the control section sets the target speed to a speed of the electric motor requested by another control section, the control section changes the speed of the electric motor at an increase rate less than or equal to the upper limit of the increase rate or a decrease rate less than or equal to the upper limit of the decrease rate. As a result, under the condition that the control section sets the target speed to a speed limit determined based on a voltage of the battery, and the voltage of the battery decreases, the control section is able to decrease the speed of the electric motor at a decrease rate that exceeds the upper limit of the decrease rate.

[0011] According to a second aspect of the present invention, a motor-driven compressor configured to be installed in a vehicle is provided. The motor-driven compressor includes an electric motor driven by electric power supplied from a battery, and a control section that controls the drive of the electric motor. The control section has an upper limit value of an increase rate of a rotational speed per unit time of the electric motor and an upper limit value of a decrease rate of the rotational speed per unit time of the electric motor. The control section sets a target rotational speed and controls the electric motor to be driven so that the rotational speed of the electric motor becomes equal to the target rotational speed.When the control section sets the target speed to a speed of the electric motor requested by another control section, the control section changes the speed of the electric motor at an increase rate less than or equal to the upper limit of the increase rate or a decrease rate less than or equal to the upper limit of the decrease rate. As a result, under the condition that the control section sets the target speed to a speed limit determined based on a voltage of the battery, and the voltage of the battery decreases, the control section is able to decrease the speed of the electric motor at a decrease rate that exceeds the upper limit of the decrease rate.

[0012] Other aspects and advantages of the present invention will become apparent from the following description, taken in conjunction with the accompanying drawings, which illustrate by way of example the principles of the invention. Brief description of the drawings

[0013] The invention, together with objects and advantages thereof, may best be understood by reference to the following description of the presently preferred embodiments together with the accompanying drawings in which: Fig. 1 is a circuit diagram illustrating the arrangement of a motor-driven compressor configured to be installed in a vehicle according to an embodiment of the present invention; Fig. 2 is a flowchart illustrating the drive control of an electric motor; Fig. Figure 3 is a timing diagram illustrating the relationship between vehicle battery voltage, commanded speed, maximum speed limit, and actual speed. Fig. Figure 4 is a timing chart showing the relationship between the actual speed and the commanded speed in a conventional technique. Fig. 5 is a time chart showing a change in motor current amount before and after the motor-driven compressor stops. Detailed description of preferred embodiments

[0014] A method of controlling a motor-driven compressor configured for installation in a vehicle according to an embodiment of the present invention will now be described with reference to the Fig. 1 to 3.

[0015] A motor-driven compressor is used in an air conditioning system configured for installation in a vehicle. The motor-driven compressor includes a compression section that compresses and discharges refrigerant, an electric motor that serves as the drive source of the compression section, and an inverter that drives the electric motor.

[0016] As in Fig. As shown in Figure 1, a motor-driven compressor 10 includes an electric motor 11 and an inverter 12 serving as a drive circuit that drives the electric motor 11. A coil 25 of the electric motor 11 has a three-phase structure including a U-phase coil 25u, a V-phase coil 25v, and a W-phase coil 25w. The electric motor 11 is a three-phase motor. The U-phase coil 25u, the V-phase coil 25v, and the W-phase coil 25w are connected in a Y-connection.

[0017] The inverter 12 has power switching elements Qu1, Qu2 corresponding to the U-phase coil 25u, power switching elements Qv1, Qv2 corresponding to the V-phase coil 25v and power switching elements Qw1, Qw2 corresponding to the W-phase coil 25w

[0018] The power switching elements Qu1, Qu2, Qv1, Qv2, Qw1, and Qw2 (hereinafter simply referred to as power switching elements Qu1 to Qw2) are implemented as IGBTs. The power switching elements Qu1, Qu2 are connected in series via a lead wire. The lead wire is connected to the U-phase coil 25u. The series-connected body of the power switching elements Qu1, Qu2 receives direct current from a vehicle battery E. The other power switching elements Qv1, Qv2, Qw1, and Qw2 are connected in the same way as the power switching elements Qu1 and Qu2, except that the corresponding coils are different. Therefore, detailed descriptions of the power switching elements Qv1, Qv2, Qw1, and Qw2 are omitted here.

[0019] The inverter 12 has diodes Du1, Dv1, Dw1, Du2, Dv2 and Dw2 connected in parallel to the power switching elements Qu1 to Qw2, and a capacitor C1 connected in parallel to the vehicle battery E.

[0020] The motor-driven compressor 10 has a control device 40 that serves as a control section that controls the inverter 12. Specifically, the control device 40 controls the switching operation of each of the power switching elements Qu1 to Qw2. The control device 40 drives the electric motor 11 by periodically turning the power switching elements Qu1 to Qw2 on and off.

[0021] The control device 40 has an upper limit value for the rate of increase of the rotational speed per unit time of the electric motor 11 and an upper limit value for the rate of decrease of the rotational speed per unit time of the electric motor. Furthermore, the control device 40 sets a target rotational speed and controls the drive of the electric motor 11 such that the rotational speed of the electric motor 11 becomes equal to the target rotational speed.

[0022] The control device 40 is electrically connected to an air conditioning ECU 50, which serves as another control section. When the target speed is set to the speed of the electric motor 11 requested by the air conditioning ECU 50, the control device 40 changes the speed of the electric motor 11 at either an increase rate less than or equal to the upper limit of the increase rate or a decrease rate less than or equal to the upper limit of the decrease rate.

[0023] As in Fig. As shown in Figure 1, the motor-driven compressor 10 includes a voltage sensor 30 serving as a voltage detection section that detects the voltage of the vehicle battery E (the vehicle battery voltage). The voltage sensor 30 transmits a detection result of the vehicle battery voltage to the controller 40.

[0024] The controller 40 includes a CPU and a memory. The memory stores a map or printout representing the relationship between the vehicle battery voltage and a speed limit of the electric motor 11. The speed limit is determined based on the amount of electric current flowing into the inverter 12 due to the difference between the counter electromotive force generated at the time the inverter 12 is stopped and the vehicle battery voltage. That is, the speed limit is a speed of the electric motor 11 at which the inverter 12 is not damaged by stopping the inverter 12. Therefore, the speed limit is set to supply a larger amount of electric current when switching elements with a higher withstand voltage are used. That is, the speed limit varies depending on the specifications of the inverter 12.

[0025] The memory stores the upper limit value of the increase rate of the rotational speed per unit time of the electric motor 11 and the upper limit value of the decrease rate of the rotational speed per unit time of the electric motor 11. Specifically, the control device 40 has the upper limit value of the increase rate of the rotational speed per unit time of the electric motor 11 and the upper limit value of the decrease rate of the rotational speed per unit time of the electric motor 11. Specifically, the rotational speed per unit time of the electric motor 11 is represented by the unit rpm / s.

[0026] The speed limit decreases as the vehicle battery voltage decreases. Even if the command value from the air conditioning ECU 50 is higher than the speed limit, the controller 40 controls the inverter 12 so that the speed of the electric motor 11 becomes equal to the speed limit. Specifically, when the target speed is set to the speed limit, the controller 40 decreases the speed of the electric motor 11 at a decrease rate exceeding the upper limit of the decrease rate when necessary. "Decreasing the speed of the electric motor 11 at a decrease rate exceeding the upper limit of the decrease rate when necessary" indicates the following method.Specifically, when the rotational speed of the electric motor 11 is reduced as the vehicle battery voltage decreases, in other words, the target rotational speed is set to the rotational speed limit, the rotational speed of the electric motor 11 may be reduced at a reduction rate less than or equal to the upper limit of the reduction rate as long as such a reduction does not cause a problem.

[0027] The drive control of the electric motor 11 by the control device 40 after the start of the electric motor 11 will now be described with reference to the Fig. 2 and Fig. 3. The control device 40 receives a command from the air conditioning ECU 50 and then controls the inverter 12 to start the electric motor 11. Thereafter, the control device 40 controls the inverter 12 according to the Fig. 2, which controls the drive of the electric motor 11.

[0028] The procedure performed by the control device 40 at the time of starting the electric motor 11 is the same procedure as from step S9 to step S17 described below after the control device 40 receives a command from the air conditioning ECU 50.

[0029] In step S1, the controller 40 determines whether a predetermined time has elapsed. The predetermined time refers, for example, to a period of approximately 100 ms (milliseconds). If the predetermined time has elapsed, the controller 40 advances the control process to step S2 and calculates the speed limit. The speed limit is calculated using the current vehicle battery voltage provided by the voltage sensor 30 and the map representing the relationship between the vehicle battery voltage and the speed limit.

[0030] After calculating the speed limit, the controller 40 determines in step S3 whether the current target speed is greater than the speed limit. If the current target speed is less than or equal to the speed limit, the controller 40 advances the control process to step S1 without updating the target speed. If the current target speed is greater than the speed limit, the controller 40 advances the control process to step S4 and updates the target speed. Specifically, in step S4, the controller 40 sets the speed limit to the target speed.

[0031] Subsequently, the control device 40 advances the control process to step S5 and determines whether the control device 40 has received a command from the air conditioning ECU 50. If the control device 40 has not received a command from the air conditioning ECU 50, the control device 40 advances the control process to step S6 and calculates the increase or decrease rate using the target speed and the current speed. Next, the control device 40 advances the control process to step S7 and controls the drive of the electric motor 11 based on the calculated increase or decrease rate.

[0032] If the predetermined time has not elapsed in step S1, the controller 40 advances the control process to step S8 and determines whether the controller 40 has received a command from the air conditioning ECU 50. If the controller 40 has not received a command from the air conditioning ECU 50, the controller 40 returns the control process to step S1. If the controller 40 has received a command from the air conditioning ECU 50, the controller 40 advances the control process to step S9 and determines whether the current target rotational speed is not equal to the command value (hereinafter referred to as commanded rotational speed) from the air conditioning ECU 50. If the current target rotational speed is equal to the command value from the air conditioning ECU 50, the controller 40 returns the control process to step S1.

[0033] If the current target rotational speed is not equal to the command value from the air conditioning ECU 50, the control device 40 advances the control process to step S10 and calculates the rotational speed limit. More specifically, if the rotational speed limit calculated in step S2 is stored in the memory separately from the target rotational speed, step S10 can be omitted. Subsequently, in step S11, the control device 40 determines whether the commanded rotational speed is less than or equal to the rotational speed limit. If the commanded rotational speed is less than or equal to the rotational speed limit, the control device 40 advances the control process to step S12 and sets the commanded rotational speed to the target rotational speed. Next, the control device 40 advances the control process to step S13 and calculates the increase and decrease rates using the target rotational speed and the current rotational speed.The control device 40 then advances the control process to step S14 and determines whether the calculated increase and decrease rate is higher than the corresponding upper limit value. If the calculated increase and decrease rate is not higher than the upper limit value, the control device 40 advances the control process to step S15 and controls the drive of the electric motor 11 at the calculated increase or decrease rate. If the calculated increase and decrease rate is higher than the corresponding upper limit value, the control device 40 advances the control process to step S16 and controls the drive of the electric motor 11 at the increase or decrease rate equal to the upper limit value.

[0034] If the commanded speed is greater than the number of speed thresholds in step S11, the controller 40 advances the control process to step S17 and sets the speed threshold to the target speed. The controller 40 then advances the control process to step S6.

[0035] If the controller 40 receives a command from the air conditioning ECU 50 in step S5, the controller 40 advances the control process to step S18 and determines whether the commanded speed is greater than or equal to the target speed (the speed limit). If the commanded speed is greater than or equal to the target speed, the controller 40 advances the control process to step S6 without updating the current target speed. If the commanded speed is less than the target speed, the controller 40 advances the control process to step S12.

[0036] Next, with reference to Fig. 3 describes the manner in which the speed of the electric motor 11 controlled by the controller 40 changes due to changes in the commanded speed and the speed limit.

[0037] As in Fig. As shown in Figure 3, before time t1, the electric motor 11 is in a state driven at a speed lower than the speed limit. When, at time t1, the controller 40 receives a command corresponding to a speed higher than the current speed and higher than the speed limit from the air conditioning ECU 50, the electric motor 11 is controlled so that the speed of the electric motor 11 reaches the speed limit at an increase rate less than or equal to the corresponding upper limit. As a result, the speed of the electric motor 11 reaches the speed corresponding to the speed limit at time t2.

[0038] Although the commanded speed is greater than the current speed after time t2, the speed limit does not change from time t2 to time t3. Therefore, the electric motor 11 is not accelerated. Specifically, when the commanded speed is greater than the current speed, the electric motor 11 is not accelerated if it is in a state where the speed of the electric motor 11 is equal to the speed limit.

[0039] When the vehicle battery voltage increases at time t3, the speed limit also begins to increase at a constant rate based on the vehicle battery voltage. Therefore, the electric motor 11 is controlled so that the current speed reaches the speed limit at an increase rate less than or equal to the corresponding upper limit. At this time, the increase rate of the speed limit is higher than the upper limit of the increase rate. As a result, the speed of the electric motor 11 reaches the speed equal to the speed limit with a delay relative to the time at which the speed limit reaches the speed corresponding to the vehicle battery voltage, in other words, at time t4. The speed of the electric motor 11 is not changed until time t5, when the vehicle battery voltage begins to decrease.

[0040] The vehicle battery voltage starts decreasing at time t5 and stops decreasing from time t6. The speed limit decreases along with the vehicle battery voltage in the period from time t5 to time t6. In the present embodiment, the decreasing rate of the speed limit is higher than the upper limit of the decreasing rate of the speed of the electric motor 11. Therefore, the electric motor 11 is controlled to be driven so that the speed of the electric motor 11 reaches the speed equal to the speed limit at a decreasing rate that exceeds the upper limit of the decreasing rate.

[0041] The present embodiment achieves the following advantages.

[0042] The controller 40 controls the drive of the electric motor 11 arranged in a motor-driven compressor configured to be installed in a vehicle, with the vehicle battery E serving as the power source for the compressor. The controller 40 has the upper limit of the increase rate of the rotational speed per unit time of the electric motor 11 and the upper limit of the decrease rate of the rotational speed per unit time of the electric motor 11. The controller 40 sets a target rotational speed and controls the electric motor 11 so that the rotational speed of the electric motor 11 becomes equal to the target rotational speed.When the target speed is set to the speed of the electric motor 11 requested by the air conditioning ECU 50, the controller 40 changes the speed of the electric motor 11 either at an increase rate less than or equal to the upper limit of the increase rate or at a decrease rate less than or equal to the upper limit of the decrease rate. When the target speed is set to the speed limit determined based on the battery voltage, the controller 40 reduces the speed of the electric motor at a decrease rate that exceeds the upper limit of the decrease rate if necessary.

[0043] In this case, even if the voltage of the vehicle battery E decreases rapidly, the rotational speed of the electric motor 11 is rapidly reduced due to such a decrease in the voltage of the vehicle battery E. This prevents damage to the switching elements of the inverter 12 of the motor-driven compressor 10 due to an increase in electric current caused by the stopping of the motor-driven compressor 10.

[0044] The embodiment described above can be modified as follows.

[0045] The increase rate at which the speed of the electric motor 11 is increased to the target speed and the decrease rate at which the speed of the electric motor 11 is decreased to the target speed both do not necessarily have to be constant until the speed of the electric motor 11 reaches the target speed, but can be varied.

[0046] To reduce the speed of the electric motor 11 at the time when the vehicle battery voltage decreases, the speed reduction rate does not always need to be greater than the corresponding upper limit. The reduction rate may be less than or equal to the upper limit, for example, if the difference between the speed limit, which has decreased due to the decrease in the vehicle battery voltage, and the current speed of the electric motor 11 is small.

[0047] In the embodiment shown above, the rotational speed of the electric motor 11 can be increased at an increase rate higher than or equal to the corresponding upper limit value when the rotational speed limit value is the target rotational speed. However, any other suitable configuration is allowed as long as at least the rotational speed of the electric motor 11 can be reduced at a decrease rate higher than or equal to the corresponding upper limit value. For example, the target rotational speed and the current rotational speed at a time between step S6 and step S7 of Fig. 2 are compared with each other. If the target speed is greater than the current speed, the control process may proceed to step S7. If the current speed is greater than the target speed, the control process may proceed to step S14.

[0048] The upper limit of the rate of increase can be either equal to or different from the upper limit of the rate of decrease.

[0049] Power MOSFETs can be used instead of IGBTs as the power switching elements Qu1 to Qw2 of the inverter 12. Each power MOSFET has a parasitic diode. The parasitic diodes function as diodes Du1 to Dw2, which are connected to the power switching elements Qu1 to Qw2. This eliminates the need to connect the diodes Du1 to Dw2, which is cumbersome, and simplifies the assembly.

[0050] As long as the motor-driven compressor 10 is used in a vehicle, the motor-driven compressor 10 may be used in any suitable device, such as a forced induction device other than an air conditioner, configured to be installed in a vehicle.

[0051] The control device is not limited to the air conditioning ECU 50.

[0052] As long as the motor-driven compressor 10 has a drive section that is rotated and driven by a motor, the motor-driven compressor 10 may be any suitable compressor, such as a turbo compressor such as a centrifugal compressor and an axial compressor, or a positive displacement compressor such as a swash plate compressor, a scroll compressor and a reciprocating compressor.

Claims

[1] A method for controlling the acceleration and deceleration of a motor-driven compressor (10) configured to be installed in a vehicle, wherein the motor-driven compressor (10) comprises an electric motor (11) driven by electric power supplied from a battery (E), and a control section (40) that controls the drive of the electric motor (11), wherein the control section (40) has an upper limit value of an increase rate of a rotational speed per unit time of the electric motor (11) and an upper limit value of a decrease rate of the rotational speed per unit time of the electric motor (11), the control section (40) sets a target speed and controls the electric motor (11) to be driven so that the speed of the electric motor (11) becomes equal to the target speed, when the control section (40) sets the target speed to a speed of the electric motor (11) requested by another control section (50), the control section (40) changes the speed of the electric motor (11) at an increase rate that is less than or equal to the upper limit of the increase rate or at a decrease rate that is less than or equal to the upper limit of the decrease rate, and under the condition that the control section (40) sets the target rotational speed to a rotational speed limit determined based on a voltage of the battery (E), and the voltage of the battery (E) decreases, as a result of which the control section (40) is able to reduce the rotational speed of the electric motor (11) at a decrease rate exceeding the upper limit of the decrease rate. [2] Motor-driven compressor (10) configured for installation in a vehicle, comprising an electric motor (11) driven by an electric power supplied by a battery (E); and a control section (40) which controls the drive of the electric motor (11), wherein the control section (40) has an upper limit value of an increase rate of a rotational speed per unit time of the electric motor (11) and an upper limit value of a decrease rate of the rotational speed per unit time of the electric motor (11), the control section (40) sets a target speed and controls the electric motor (11) to be driven so that the speed of the electric motor (11) is equal to the target speed, when the control section (40) sets the target speed to a speed of the electric motor (11) requested by another control section (50), the control section (40) changes the speed of the electric motor (11) at an increase rate that is less than or equal to the upper limit of the increase rate, or at a decrease rate that is less than or equal to the upper limit of the decrease rate, and under the condition that the control section (40) sets the target rotational speed to a rotational speed limit determined based on a voltage of the battery (E), and the voltage of the battery (E) decreases, as a result of which the control section (40) is able to reduce the rotational speed of the electric motor (11) at a decrease rate exceeding the upper limit of the decrease rate.

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