Power conversion device and method therefor

Abstract

To inhibit occurrence of overcurrent protection during startup of an electric motor thereby preventing an operation delay when the electric motor is driven.SOLUTION: A power conversion device 1 includes a DC conversion unit 101, a smoothing capacitor 103, a DC voltage boosting unit 102, an AC conversion unit 104, an output control unit 110 that controls output of the DC voltage boosting unit 102 and output of the AC conversion unit 104, and a frequency command unit 111. In accordance with a command sent from the frequency command unit 111, the output control unit 110 monitors the level of the DC voltage charged in an output-side capacitor 107, and a switching element 108 turns off a supply of a DC voltage outputted from the output-side capacitor 107 when the level of the DC voltage is equal to or higher than a prescribed voltage level, and the switching element 108 turns on a supply of the DC voltage outputted from the output-side capacitor 107 at a timing when the level of the DC voltage becomes below the prescribed voltage level. Accordingly, output control is performed on an AC electric motor 300.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to a power conversion device and a power conversion method. [Background technology]

[0002] Background art includes JP 2013-46539 A (Patent Document 1). This publication discloses an inverter control circuit that includes a full-wave rectifying diode bridge that rectifies AC voltage, a boost circuit that is connected to the full-wave rectifying diode bridge and boosts the rectified voltage, a storage battery on the input side of the boost circuit, switching means for using either a commercial power source or the storage battery as a power source, an inverter that converts DC power to AC power, and control means that controls the inverter to drive a motor with PWM (Pulse Width Modulation) control or PAM (Pulse Amplitude Modulation) control.

[0003] Furthermore, Patent Document 1 discloses a technology in which the control means drives the motor with PWM control when a commercial power source is used as the power source, and drives the motor with PAM control when a storage battery is used as the power source. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] JP 2013-46539 A Summary of the Invention [Problem to be solved by the invention]

[0005] Patent Document 1 describes a method in which a capacitor placed between a boost circuit and an inverter circuit can be driven by a storage battery even in a low voltage state. In this case, if the capacitor is in a higher voltage state than necessary due to the influence of the motor side (output side), the high voltage at the time of PAM control output may have some effect on the drive of the motor.

[0006] As such, Patent Document 1 does not disclose a method for suppressing the output voltage when starting output, and therefore if a high voltage has already been charged to the capacitor, starting output when the capacitor is heavily charged will result in an excessive current flowing through the motor, activating the overcurrent protection.

[0007] In particular, in systems where the DC voltage charged in the smoothing capacitor is passed through a boost circuit to charge the output capacitor and output to the motor, leakage current, induced voltage, the system installation environment, and emergency shutdown after charging are likely to occur. Therefore, when the output capacitor is charged with voltage from the motor side, the charge will remain unless the inverter control circuit actively performs output operation. This poses the problem that, depending on the installation environment of the system that supplies power to the motor, the voltage does not drop at the stage of starting the motor, and overcurrent protection occurs frequently.

[0008] When considering this from the user's perspective, a problem arises in that the power supply may be suddenly cut off after startup, resulting in unintended overcurrent protection for the user, which may interfere with the operation of the motor.

[0009] Therefore, the present invention has been made in consideration of the above-mentioned problems, and aims to provide a power conversion device and a method thereof that can prevent delays in operation when driving an electric motor by not generating overcurrent protection when starting the electric motor. [Means for solving the problem]

[0010] In order to solve the above-mentioned problems and achieve the above-mentioned object, one embodiment of the present invention is a power conversion device that is connected to an external power source and an electric motor, inputs an AC voltage from the external power source, and outputs a DC voltage to the electric motor, the power conversion device including a DC conversion unit that converts the input AC voltage into a DC voltage, a DC voltage smoothing unit that smooths and charges the DC voltage, a DC voltage boost unit that has a switching element and boosts the DC voltage charged in the DC voltage smoothing unit, an AC conversion unit that converts the DC voltage boosted by the DC voltage boost unit into an AC voltage, and an output control unit that controls outputs of the DC voltage boost unit and the AC conversion unit. and a frequency command unit that outputs a command to the output control unit, wherein the output control unit monitors the level of the DC voltage charged to the DC voltage smoothing unit in response to a command from the frequency command unit, turns off the supply of the DC voltage output from the DC voltage smoothing unit to the switching element while the level of the DC voltage is equal to or higher than a predetermined voltage level, and turns on the supply of the DC voltage output from the DC voltage smoothing unit to the switching element when the level of the DC voltage falls below the predetermined voltage level, thereby performing power control for the electric motor.

[0011] Another embodiment of the present invention is a power conversion method for controlling power supply to an electric motor based on a DC voltage smoothed by switching a switching element on and off, comprising the steps of: monitoring a level of the smoothed DC voltage in response to a frequency command supplied from an external source; determining whether the monitored DC voltage level is equal to or higher than a predetermined voltage level that activates overcurrent protection; controlling the switching element to an off state while a determination is made that the DC voltage level is equal to or higher than the predetermined voltage level; controlling the switching element to an on state at the timing when a determination is made that the voltage level is below the predetermined voltage level; and controlling the power supply to the electric motor in response to the on state of the switching element. Effect of the Invention

[0012] According to the present invention, it is possible to provide a power conversion device and a power conversion method that can prevent delays in operation when driving a motor by not generating overcurrent protection when starting the motor. [Brief description of the drawings]

[0013] [Figure 1] 1 is a configuration diagram illustrating an example of a power conversion device according to an embodiment of the present invention. [Diagram 2] 5 is a flowchart showing an example of the operation of an output control unit in the embodiment of the present invention. [Diagram 3] FIG. 4 is an explanatory diagram showing waveforms when an output control unit performs output control in one embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. Note that the following description and drawings are merely examples for explaining the present invention, and appropriate omissions and simplifications have been made in order to clarify the explanation. The present invention can also be implemented in various other forms. Furthermore, unless otherwise specified, each component may be singular or plural.

[0015] An embodiment of the present invention will be described below with reference to the drawings. First, a power conversion device according to an embodiment of the present invention will be described with reference to FIG. 1. FIG. 1 is a configuration diagram showing an example of a power conversion device according to this embodiment. As shown in FIG. 1, the power conversion device 1 of this embodiment is connected to a three-phase AC power source 200 as a power source and an AC motor 300 for driving a motor, and includes a DC conversion unit 101, a DC voltage boost unit 102, a smoothing capacitor 103, an AC conversion unit 104, an output control unit 110, a frequency command unit 111, an operation and display unit 120, and a storage unit 121. The operation and display unit 120 may be a part of the power conversion device 1 or an external unit, and if it is external, it is connected by a bus for communication. Furthermore, the operation and display unit 120 itself may be connected to an external device or network through a bus to acquire data.

[0016] DC voltage boost unit 102 is composed of output side capacitor 107, boost circuit switching element 108, reactor 109 and free wheel diode 112 which constitute a DC voltage smoothing unit that smoothes the DC voltage, and is the main part which supplies power to AC conversion unit 104 after the voltage value remaining in output side capacitor 107 falls below a predetermined value when AC motor 300 is started.

[0017] Next, each component will be described. The three-phase AC power supply 200 is a power supply of three-phase AC voltage supplied from, for example, a power company or AC voltage supplied from a generator, and outputs the AC voltage to the DC conversion unit 101. The DC conversion unit 101 is configured, for example, by a DC conversion circuit configured with diodes or a DC conversion circuit using an IGBT (Insulated Gate Bipolar Transistor) and a flywheel diode, and converts the AC voltage input from the three-phase AC power supply 200 into a DC voltage and outputs it to the smoothing capacitor 103. The example in FIG. 1 shows an example in which the DC conversion unit 101 is configured with diodes.

[0018] Smoothing capacitor 103 smoothes the DC voltage input from DC conversion unit 101, and charges the DC voltage, and supplies the DC voltage to output side capacitor 107 through boost switching element 108 and reactor 109 in DC voltage boost unit 102. For example, when the output of AC generator 300 itself is a DC voltage, smoothing capacitor 103 may be configured to input the DC voltage directly from the generator without passing through DC conversion unit 101. The voltage value of smoothing capacitor 103 is output to output control unit 110, for example, through a voltage divider circuit and an AD converter.

[0019] The boost switching element 108 controls the charging voltage of the output side capacitor 107 according to instructions from the output control unit 110. The reactor 109 is arranged to prevent excessive current from flowing when the boost switching element 108 switches and charges the output side capacitor 107. The freewheel diode 112 is arranged for protecting the boost switching element 108, the reactor 109, and the output side capacitor 107.

[0020] The output-side capacitor 107 smoothes the DC voltage input through the step-up switching element 108 and the reactor 109, and supplies the smoothed DC voltage to the AC conversion unit 104. The voltage value of the output-side capacitor 107 is output to the output control unit 110 through, for example, a voltage divider circuit and an AD converter. The current detection unit 106 detects the current flowing through the AC motor 300, and outputs the current to the output control unit 110.

[0021] The AC conversion unit 104 is composed of an AC conversion circuit using, for example, an IGBT and a flywheel diode, and receives the DC voltage of the output side capacitor 107 as an input, converts the DC voltage to an AC voltage using a pulse output waveform input from a frequency command unit 111, and outputs the AC voltage to the AC motor 300. The pulse output waveform may be either a PWM waveform or a PAM waveform.

[0022] The output control unit 110 is configured as a computer including a CPU, a ROM storing a program for controlling the operation of this embodiment, and receives as input a command frequency commanded by a frequency command unit 111 and the output voltages of the smoothing capacitor 103 and the output side capacitor 107. The output control unit 110 outputs a pulse output waveform calculated using the calculated command frequency to the power conversion unit 104. The output control unit 110 outputs a pulse waveform calculated using the calculated command voltage to the boost switching element 108.

[0023] The frequency command unit 111 receives as input a target voltage (speed command) obtained from the outside via the operation and display unit 120, which is a user interface, calculates a frequency and voltage command value to be output for gradually increasing the speed of the AC motor 300, and outputs the calculated frequency and voltage command value to the output control unit 110. The operation and display unit 120 is configured, for example, with a control panel on a control panel or a user interface such as an operator connected to the power conversion device 1 itself, and outputs setting values ​​such as the target voltage selected by the operator to the memory unit 111. Then, the operation and display unit 120 outputs, for example, a hardwired analog command value or a digital command value via communication to the frequency command unit 111 as a frequency command.

[0024] The memory unit 121 is composed of, for example, an EEPROM or a RAM, and stores various data input from the operation / display unit 120 as input. When the power supply of the power conversion device 1 is turned on, the data previously stored in the memory unit 121 is output to the output control unit 110 or the frequency command unit 111.

[0025] Next, the output control operation will be described with reference to Fig. 2 and Fig. 3. Here, an example of frequency control for driving the motor of the AC motor 300 after optimizing the capacitor voltage so that the output side capacitor voltage value falls below a predetermined value when starting PAM output will be described. Fig. 2 is a flow chart showing the output control unit 110 according to this embodiment outputting a pulse output waveform to the power conversion unit 104 and the boost switching element 108, and Fig. 3 is an explanatory diagram showing the waveform when the output control unit 110 according to this embodiment performs output control.

[0026] First, in the output control unit 110, a frequency command is input from the frequency command unit 111 (step S201). When the frequency command becomes a finite value (zero or more) (step S202), the voltage value charged in the output side capacitor 107 is first obtained in order to start PWM control (step S203).

[0027] Then, in the output control section 110, the voltage of the output side capacitor 107 is taken in and the voltage value is monitored, and it is determined whether the voltage value is equal to or higher than a voltage level that does not cause an overcurrent (hereinafter referred to as a predetermined voltage level) (step S204). For example, in a situation where the voltage value of the output side capacitor 107 is charged to a high voltage due to leakage current, induced voltage, the installation environment of the system, emergency cutoff after charging, or the like even though the power supply to the AC motor 300 is stopped (natural charging, etc.), the step-up switching element 108 is turned off (stopped), and control is executed not to supply DC voltage to the output side capacitor 107 (step S205).

[0028] In this case, as shown in Fig. 3, the output side capacitor 107 is in a naturally charged state, and during the naturally charging period, the voltage value 400 of the output side capacitor 107 exceeds a predetermined voltage level 500, and the boost switching element 108 is maintained in an off state. During this naturally charging period, control (PWM) by the output control unit 110 is not yet performed. The voltage level exceeding the predetermined voltage level 500 here refers to a voltage level that causes an overcurrent.

[0029] Furthermore, a current conversion value of the current detection unit 106, for example, a value obtained by converting a three-phase AC current into a primary current effective value, is input, and the voltage application value to the AC motor 300 by the PWM output waveform is controlled by setting, for example, the rated current of the AC motor 300 or the rated current of a power conversion device, whichever is lower, as a designated current value. As a result, the output control unit 110 controls the current output while controlling the current value flowing through the AC motor 300 to a state equal to or lower than the designated current value previously set by the user via the operation / display unit 120 and stored in the storage unit 121 (step S206).

[0030] As a result, as shown in Fig. 3, the voltage of the output side capacitor 107 is consumed, and processing is performed for early start of the AC motor 300. That is, by performing this voltage consumption, the waiting time of the system operation is significantly shortened. In this case, as shown in Fig. 3, during the charging consumption period, the PWM output by the output control unit 110 is controlled in accordance with the output of the effective value current shown in Fig. 3, and the voltage of the output side capacitor 107 (voltage value 400) is gradually reduced.

[0031] In the above description, the voltage determination in step S204 is performed once, but the process may be repeated several times to increase the accuracy.

[0032] Regarding the above-mentioned specified current value, if there are other constraints, such as thermal current value limits or current value limits due to wiring, the specified current value may be set according to various limit values. Also, the output voltage phase in PWM control may be fixed or variable.

[0033] Furthermore, the voltage value 400 of the output side capacitor 107 continues to be monitored, and when the voltage value 400 of the output side capacitor 107 becomes less than a predetermined voltage level 500 (step S204), the boost switching element 108 is switched to the on state (operating state), and a process is executed to supply a DC voltage to the output side capacitor 107 based on the command voltage and at the same time perform PAM output control (step S207).

[0034] 3, when the voltage value 400 of the output side capacitor 107 falls below a predetermined voltage level 500, it becomes time to start the AC motor 300 (start-up period of the AC motor 300), and the boost switching element 108 is controlled to be in the ON state. At the same time, a voltage is supplied to the output side capacitor 107 at the rear stage, and the AC motor 300 gradually accelerates.

[0035] In this way, even in a situation where the voltage value of the output-side capacitor 107 is high and the voltage is being charged, the AC motor 300 (motor) can be started without a large current growth and an overcurrent. That is, even in a situation where voltage is supplied from the AC motor 300 (motor) side, it is possible to control the output current and start the motor of the AC motor 300 by suppressing the current at the start of the AC motor 300 and consuming the voltage.

[0036] As described above, according to this embodiment, the AC motor 300 is prevented from being in an overcurrent state when started, so that the overcurrent protection is activated every time the motor is started, and the motor is not stopped each time. In this way, there is no waiting time for start-up due to overcurrent protection, so it is possible to realize control that does not delay the operation when driving the AC motor 300. From the user's point of view, this has the effect of preventing a phenomenon in which the power supply is suddenly cut off after start-up, causing overcurrent protection that is not intended by the user.

[0037] The embodiment described above is expected to be particularly effective when applied to a VAH inverter.

[0038] The present invention is not limited to the above-described embodiment, but includes various modified examples. For example, the above-described embodiment has been described in detail to clearly explain the present invention, and is not necessarily limited to an embodiment having all of the configurations described. In addition, it is possible to replace a part of the configuration of this embodiment with another configuration, and it is also possible to add another configuration to a certain configuration. In addition, it is possible to add, delete, or replace a part of the configuration of this embodiment with another configuration. [Explanation of symbols]

[0039] 1 Power conversion device 101 DC converter 102 DC voltage booster 103 Smoothing capacitor 104 AC conversion unit 106 Current detection section 107 Output Capacitor 108 Boost switching element 109 Reactor 110 Output control section 111 Frequency command unit 112 Freewheeling Diode 120 Operation / display section 121 Storage section

Claims

1. A power conversion device connected to an external power source and an electric motor, receiving an AC voltage from the external power source and outputting a DC voltage to the electric motor, a DC converter that converts the input AC voltage into DC; A DC voltage smoothing unit that smoothes and charges the DC voltage; a DC voltage boosting unit having a switching element and boosting the DC voltage charged in the DC voltage smoothing unit; an AC converter that converts the DC voltage boosted by the DC voltage booster into an AC voltage; an output control unit that controls outputs of the DC voltage boost unit and the AC conversion unit; A frequency command unit that outputs a command to the output control unit; Equipped with the output control unit monitors the level of the DC voltage charged in the DC voltage smoothing unit in response to a command from the frequency command unit, turns off the supply of the DC voltage output from the DC voltage smoothing unit to the switching element while the level of the DC voltage is equal to or higher than a predetermined voltage level, and turns on the supply of the DC voltage output from the DC voltage smoothing unit to the switching element when the level of the DC voltage falls below the predetermined voltage level, thereby controlling power to the electric motor, the power conversion device characterized by the above.

2. 2. The power conversion device according to claim 1, wherein the output control unit outputs the charged voltage under PWM (Pulse Width Modulation) control while the level of the DC voltage is equal to or higher than the predetermined voltage level, and switches from the PWM control to PAM (Pulse Amplitude Modulation) control at a timing when the level of the DC voltage falls below the predetermined voltage level, and performs output control to output the charged voltage under the PAM control.

3. 2. The power conversion device according to claim 1, wherein the frequency command unit gradually increases the output of the output control unit toward a predetermined target voltage using the command under the PAM control.

4. 2. The power conversion device according to claim 1, wherein the predetermined voltage level is set to a level that does not cause an overcurrent.

5. A power conversion method for controlling power supply to an electric motor based on a DC voltage smoothed by switching on and off a switching element, comprising the steps of: monitoring a level of the smoothed DC voltage in response to an externally provided frequency command; determining whether the monitored DC voltage level is greater than or equal to a predetermined voltage level that activates overcurrent protection; controlling the switching element to an off state while it is determined that the voltage is equal to or higher than the predetermined voltage level; controlling the switching element to an on state at a timing when it is determined that the voltage level is less than the predetermined voltage level; controlling power supply to the electric motor in response to an on state of the switching element; A power supply method comprising:

6. 6. The power conversion method according to claim 5, wherein the charged voltage is output under PWM (Pulse Width Modulation) control while it is determined that the voltage is equal to or higher than the predetermined voltage level, and when it is determined that the voltage is less than the predetermined voltage level, the PWM control is switched to PAM (Pulse Amplitude Modulation) control, and power supply to the electric motor is controlled under the PAM control.

7. 6. The power conversion method according to claim 5, wherein an output voltage for supplying power to the motor is gradually increased toward a preset target voltage in accordance with a frequency command under the PAM control.

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