Power conversion device and method thereof

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 Art

[0002] As background art, there is Japanese Patent Application Laid-Open No. 2013-46539 (Patent Document 1). This publication discloses a full-wave rectifier diode bridge for rectifying an AC voltage, a booster circuit connected to the full-wave rectifier diode bridge for boosting the rectified voltage, a storage battery on the input side of the booster circuit, switching means for using either a commercial power supply or the storage battery as a power source, an inverter for converting DC power into AC power, and control means for controlling the inverter to drive a motor by PWM (Pulse Width Modulation) control or PAM (Pulse Amplitude Modulation) control.

[0003] Further, in this Patent Document 1, there is disclosed a technique in the control means for driving the motor by PWM control when using the commercial power supply as a power source and driving the motor by PAM control when using the storage battery as a power source.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] Patent Document 1 describes a method in which a capacitor arranged between a booster circuit and an inverter circuit can be driven even in a low voltage state by a storage battery. In this case, in a situation where the capacitor is in a voltage state higher than necessary due to the influence from the output side, i.e., the motor side, there is a possibility that the high voltage at the time of PAM control output may have some influence on the driving of the motor.

[0006] Thus, since Patent Document 1 does not disclose a method for starting output by suppressing the output voltage, if a high voltage has already been charged to the capacitor as charge, starting output with a large charge presents the problem of causing an excessive current to flow through the motor and activating the overcurrent protection.

[0007] In particular, in systems where a DC voltage charged to a smoothing capacitor is passed through a boost circuit to charge an output capacitor and then output to an electric motor, leakage current, induced voltage, system installation environment, and emergency shutdown after charging are prone to occur. Therefore, if voltage is charged to the output capacitor from the motor side, the charge will remain unless the inverter control circuit performs an active output operation. This creates a problem where, depending on the installation environment of the system supplying power to the motor, the voltage may not drop during the motor startup phase, leading to frequent overcurrent protection.

[0008] From a user's perspective, this point reveals a problem: the power supply is suddenly cut off after startup, and the overcurrent protection, which the user did not intend, interferes with the operation of the motor.

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

[0010] To solve the above-mentioned problems and achieve the above objective, one embodiment of the present invention is a power conversion device connected to an external power source and an electric motor, which receives an AC voltage from the external power source and outputs a DC voltage to the electric motor, comprising: a DC conversion unit that converts the input AC voltage to DC; a DC voltage smoothing unit that smooths and charges the DC voltage; a DC voltage boosting unit having a switching element that 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 boosting unit to AC; and an output control unit that controls the outputs of the DC voltage boosting unit and the AC conversion unit. The system includes a frequency command unit that outputs a command to the output control unit, wherein the output control unit monitors the level of DC voltage charged to the DC voltage smoothing unit in response to a command from the frequency command unit, controls the supply of DC voltage output from the DC voltage smoothing unit to the switching element to the off setting while the DC voltage level is above a predetermined voltage level, and controls the supply of DC voltage output from the DC voltage smoothing unit to the switching element to the on setting when the DC voltage level falls below the predetermined voltage level, thereby controlling the power to the motor.

[0011] Another embodiment of the present invention is a power conversion method for controlling the supply of power to an electric motor based on a DC voltage smoothed by switching a switching element on and off, comprising the steps of: monitoring the 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 greater than a predetermined voltage level that activates overcurrent protection; controlling the switching element to an off state while it is determined to be equal to or greater than the predetermined voltage level; controlling the switching element to an on state when it is determined to be less than the predetermined voltage level; and controlling the supply of power to the electric motor according to the on state of the switching element. [Effects 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 an electric motor by not generating overcurrent protection when the electric motor is started. [Brief explanation of the drawing]

[0013] [Figure 1] This is a configuration diagram showing an example of a power conversion device in one embodiment of the present invention. [Figure 2] This flowchart shows an example of the operation of the output control unit in one embodiment of the present invention. [Figure 3] This is an explanatory diagram showing the waveform when the output control unit performs output control in one embodiment of the present invention. [Modes for carrying out the invention]

[0014] Embodiments of the present invention will be described below with reference to the drawings. Note that the following description and drawings are merely illustrative examples for explaining the present invention, and have been omitted or simplified as appropriate for clarity of explanation. Furthermore, the present invention can be implemented in various other forms. Also, unless otherwise specified, each component may be singular or plural.

[0015] Hereinafter, one embodiment of the present invention will be described with reference to the drawings. First, a power converter, which is one embodiment of the present invention, will be described with reference to Figure 1. Figure 1 is a configuration diagram showing an example of a power converter according to this embodiment. The power converter 1 of this embodiment is connected to a three-phase AC power supply 200 that serves as the power source and an AC motor 300 that drives the motor, as shown in Figure 1, for example, and includes a DC conversion unit 101, a DC voltage boosting unit 102, a smoothing capacitor 103, an AC conversion unit 104, an output control unit 110, a frequency command unit 111, an operation / display unit 120, and a storage unit 121. The operation / display unit 120 may be part of the power converter 1 or an external component, and in the case of an external component, it shall be connected by a communication bus. Furthermore, the operation / display unit 120 itself may be connected to an external device or network via the bus to acquire data.

[0016] The DC voltage boosting unit 102 consists of an output capacitor 107, a switching element 108 for the boost circuit, a reactor 109, and a freewheeling diode 112, which constitute a DC voltage smoothing unit that smooths the DC voltage. It is the main part that supplies power to the AC conversion unit 104 after the voltage value remaining in the output capacitor 107 falls below a predetermined value when the AC motor 300 is started.

[0017] Next, we will explain the individual components. The three-phase AC power supply 200 is a power source for three-phase AC voltage supplied by, for example, a power company or an AC voltage supplied by a generator, and outputs the AC voltage to the DC conversion unit 101. The DC conversion unit 101 is composed of, for example, a DC conversion circuit made up of diodes or a DC conversion circuit using IGBTs (Insulated Gate Bipolar Transistors) and flywheel diodes, and converts the AC voltage input from the three-phase AC power supply 200 into DC voltage and outputs it to the smoothing capacitor 103. Figure 1 shows an example in which the DC conversion unit 101 is made up of diodes.

[0018] The smoothing capacitor 103 smooths the DC voltage input from the DC conversion unit 101 and charges it, and in the DC voltage boosting unit 102, it supplies the DC voltage to the output capacitor 107 through the boost switching element 108 and the reactor 109. For example, if the output of the AC generator 300 itself is a DC voltage, the smoothing capacitor 103 may be configured to receive the DC voltage directly from the generator without going through the DC conversion unit 101. The voltage value of the smoothing capacitor 103 is output to the 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 capacitor 107 in accordance with the instruction of the output control unit 110. The reactor 109 is arranged so that the current does not flow excessively when the boost switching element 108 switches and charges the output capacitor 107. The freewheeling diode 112 is arranged for the protection of the boost switching element 108, the reactor 109, and the output capacitor 107.

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

[0021] The AC conversion unit 104 is composed of, for example, an AC conversion circuit using an IGBT and a freewheeling diode. Taking the DC voltage input from the output capacitor 107 as an input, it converts the DC voltage into an AC voltage according to the pulse output waveform input from the frequency command unit 111 and outputs it 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 has a computer configuration including a CPU and a ROM storing a program for controlling the operation of the present embodiment. It takes as inputs the command frequency commanded by the frequency command unit 111 and the output voltages of the smoothing capacitor 103 and the output 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 takes as input the target voltage (speed command) obtained from the outside through the operation and display unit 120, which is a user interface, calculates the frequency and voltage command values to be output for gradually increasing the speed of the AC motor 300, and outputs them to the output control unit 110. The operation and display unit 120 is composed of a user interface such as an operation panel of an operation console or an operator connected to the power conversion device 1 itself, and outputs the set values such as the target voltage selected by the operator to the storage unit 111. Then, the operation and display unit 120 uses, for example, an analog command value by hardwiring or a digital command value via communication as a frequency command and outputs it to the frequency command unit 111.

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

[0025] Next, the output control operation will be described using FIGS. 2 and 3. Here, when starting the PAM output, 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 is below a predetermined value will be described. FIG. 2 is a flowchart in which the output control unit 110 according to the present embodiment outputs commands for the pulse output waveform to the power conversion unit 104 and the boost switching element 108, and FIG. 3 is an explanatory diagram showing waveforms when the output control unit 110 according to the present embodiment performs output control.

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

[0027] Then, the output control unit 110 receives the voltage of the output capacitor 107 and monitors its voltage value to determine whether it is above a voltage level that does not cause overcurrent (hereinafter referred to as a predetermined voltage level) (step S204). For example, if the voltage of the output capacitor 107 is high due to leakage current, induced voltage, the system's installation environment, emergency shutdown after charging, etc., even though the power supply to the AC motor 300 has been stopped (such as natural charging), the boost switching element 108 is turned off (stopped), and control is performed to not supply DC voltage to the output capacitor 107 (step S205).

[0028] In this case, as shown in Figure 3, the output capacitor 107 is in a state of natural charging, and during this natural charging period, the voltage value 400 of the output capacitor 107 is at a voltage level exceeding a predetermined voltage level 500, and the boost switching element 108 remains in the off state. During this natural 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 the voltage level at which overcurrent occurs.

[0029] Furthermore, the current detection unit 106 takes the current conversion value, for example, the value obtained by converting the three-phase AC current to the primary current RMS value, as input, and the lower of, for example, the rated current of the AC motor 300 or the rated current of the power converter is set as the specified current value, and the voltage applied to the AC motor 300 by the PWM output waveform is controlled. As a result, the output control unit 110 controls the output of the current while controlling the current value flowing through the AC motor 300 to a state that is less than or equal to the specified current value that has been set in advance by the user through the operation / display unit 120 and stored in the memory unit 121 (step S206).

[0030] As a result, as shown in Figure 3, the voltage of the output capacitor 107 is consumed, and processing is performed to enable early startup of the AC motor 300. In other words, this voltage consumption significantly reduces the waiting time for system operation. In this case, as shown in Figure 3, during the charge 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 Figure 3, and the voltage of the output capacitor 107 (voltage value 400) is gradually reduced.

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

[0032] Regarding the specified current value mentioned above, if there are other constraints, such as thermal current limits or current limits due to wiring, the specified current value may be set to match 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 capacitor 107 is continuously monitored, and if the voltage value 400 of the output capacitor 107 falls below a predetermined voltage level of 500 (step S204), the boost switching element 108 is switched to the ON state (operating state), and a process is executed in which a DC voltage is supplied to the output capacitor 107 based on the command voltage, and at the same time, PAM output control is performed (step S207).

[0034] In this case, as shown in Figure 3, the voltage value 400 across the output capacitor 107 falls below a predetermined voltage level 500, which is the starting timing for the AC motor 300 (starting period of the AC motor 300), and the boost switching element 108 is controlled to the ON state. At the same time, voltage is supplied to the output capacitor 107 to the subsequent stage, and the AC motor 300 is gradually accelerated.

[0035] In this way, even when the output capacitor 107 is charged with a high voltage value, the AC motor 300 can be started without excessive current growth or overcurrent. In other words, even when voltage is supplied from the AC motor 300, the output current can be controlled and the motor of the AC motor 300 can be started by suppressing the current during startup and consuming the voltage.

[0036] As explained above, according to this embodiment, since the AC motor 300 is kept in a state where it will not experience overcurrent during startup, the overcurrent protection does not activate each time it starts up, eliminating the need to stop the motor each time. In this way, the waiting time for startup due to overcurrent protection is eliminated, making it possible to achieve control that does not delay the operation when driving the AC motor 300. From the user's perspective, this also has the effect of preventing the phenomenon in which the power supply is suddenly cut off after startup, causing unintended overcurrent protection.

[0037] The embodiments described above are expected to be particularly effective when applied to VAH inverters.

[0038] It should be noted that the present invention is not limited to the embodiment described above, and various modifications are included. For example, the embodiment described above is explained in detail for the purpose of clearly illustrating the present invention, and is not necessarily limited to having all the configurations described. Furthermore, it is possible to replace some of the configurations of this embodiment with other configurations, and it is also possible to add other configurations to a certain configuration. In addition, it is possible to add, delete, or replace some of the configurations of this embodiment with other configurations. [Explanation of Symbols]

[0039] 1. Power converter 101 DC conversion section 102 DC voltage boosting section 103 Smoothing Capacitor 104 AC conversion unit 106 Current detection unit 107 Output capacitor 108 Boost Switching Element 109 Reactor 110 Output control unit 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, which receives an AC voltage from the external power source and outputs a DC voltage to the electric motor, A DC conversion unit that converts the input AC voltage into DC, A DC voltage smoothing unit that smooths the DC voltage and charges the DC voltage, A DC voltage boosting unit having a switching element that boosts the DC voltage charged in the DC voltage smoothing unit, An AC conversion unit converts the DC voltage boosted by the DC voltage boosting unit into AC voltage, An output control unit that controls the output of the DC voltage boosting unit and the AC conversion unit, A frequency command unit that outputs commands to the output control unit, Equipped with, The DC voltage smoothing section includes an output capacitor. The power conversion device is characterized in that the output control unit monitors the level of DC voltage charged in the DC voltage smoothing unit when the power supply to the motor is stopped in response to a command from the frequency command unit, turns off the supply of DC voltage output from the DC voltage smoothing unit to the switching element while the DC voltage level is above a predetermined voltage level, and turns on the supply of DC voltage output from the DC voltage smoothing unit to the switching element when the DC voltage level falls below the predetermined voltage level, thereby controlling the power to the motor.

2. A power conversion device according to claim 1, characterized in that the output control unit outputs the DC voltage charged to the DC voltage smoothing unit using PWM (Pulse Width Modulation) control while the DC voltage level is above a predetermined voltage level, and switches the PWM control to PAM (Pulse Amplitude Modulation) control and outputs the DC voltage charged to the DC voltage smoothing unit using PAM control when the DC voltage level falls below the predetermined voltage level.

3. A power conversion device according to claim 1, wherein the frequency command unit, under PAM control, uses the command to gradually increase the output of the output control unit toward a pre-prepared target voltage.

4. A power conversion device according to claim 1, characterized in that the predetermined voltage level is set to a level that does not cause overcurrent.

5. A power conversion method for controlling the power supply to an electric motor based on a DC voltage smoothed by switching a switching element on and off, The steps include monitoring the level of the smoothed DC voltage in response to a frequency command supplied from an external source while the power supply to the electric motor is stopped, The steps include determining whether the monitored DC voltage level is equal to or greater than a predetermined voltage level that triggers overcurrent protection, The steps include controlling the switching element to the off state while it is determined that the voltage level is above the predetermined level, The steps include controlling the switching element to the ON state at the timing when it is determined that the voltage level is below the predetermined level, The steps include controlling the power supply to the motor according to the ON state of the switching element, A power supply method characterized by including the following.

6. A power conversion method according to claim 5, characterized in that, while it is determined that the voltage is above a predetermined voltage level, the DC voltage charged to the DC voltage smoothing unit is output by PWM (Pulse Width Modulation) control, and when it is determined that the voltage is below the predetermined voltage level, the PWM control is switched to PAM (Pulse Amplitude Modulation) control, and the power supply to the motor is controlled by PAM control.

7. A power conversion method according to claim 5, characterized in that the output voltage for supplying power to the motor is gradually increased toward a predetermined target voltage in accordance with a frequency command under PAM control.

Images