Motor drive device for controlling motor during coasting

Abstract

This motor drive device comprises: an inverter that drives a motor; a converter that is connected to the inverter via a DC link; a capacitor that is provided to the DC link; a preliminary charging circuit that suppresses an inrush electric current during a period of preliminary charging to the capacitor; a voltage detection unit that detects a DC link voltage; and a control unit that controls the inverter so that the motor during coasting is decelerated when the DC link voltage is equal to or greater than the lower-limit value of the voltage required to drive the motor, and controls the preliminary charging operation performed by the preliminary charging circuit so as to charge the capacitor when the DC link voltage is less than the lower-limit value.

Description

Motor drive device for controlling a motor during coasting

[0001] The present disclosure relates to a motor drive device that controls a motor during coasting.

[0002] If a power outage or power interruption occurs while a motor drive device is driving a motor, the motor drive device will no longer be able to control the motor's rotation, and the motor will coast (free-run state). Methods for stopping a coasting motor include restarting the motor drive device and controlling the motor to decelerate, stopping the motor using a mechanical brake, or stopping the motor using air resistance or friction, etc.

[0003] JP 2014-140272 A JP 2021-072678 A

[0004] There is a demand for a technology that reduces the power loss that occurs when a coasting motor is decelerated and stopped by a motor drive device.

[0005] According to one aspect of the present disclosure, a motor drive device includes: an inverter that drives a motor; a converter that converts AC power supplied from an AC power source into DC power and supplies the DC power to the inverter via a DC link; a pre-charging circuit that has a capacitor provided in the DC link, a pre-charging resistor that suppresses inrush current during a pre-charging period of the capacitor, and a pre-charging switch connected in parallel with the pre-charging resistor, wherein the pre-charging circuit performs a pre-charging operation to charge the capacitor with current supplied via the pre-charging resistor while the pre-charging switch is open, and completes the pre-charging operation of the capacitor when the pre-charging switch is switched from open to closed; a voltage detection unit that detects a DC link voltage, which is a potential difference between a positive power line and a negative power line in the DC link; and a control unit that controls the inverter to decelerate the coasting motor when the DC link voltage is equal to or greater than a lower limit of a voltage required to drive the motor, and controls the pre-charging operation by the pre-charging circuit to charge the capacitor when the DC link voltage is below the lower limit.

[0006] FIG. 1 is a diagram illustrating a motor drive device according to a first embodiment and a second embodiment of the present disclosure; FIG. 2 is a diagram illustrating a motor drive device according to a modification of the first embodiment and the second embodiment of the present disclosure; FIG. 3 is a flowchart illustrating an operation flow of the motor drive device according to the first embodiment and the modification thereof; FIG. 4 is a flowchart illustrating an operation flow of the motor drive device according to the second embodiment and the modification thereof; FIG. 5 is a waveform diagram illustrating an example of operation when stopping a motor that is coasting from a state where the DC link voltage is equal to or higher than a lower limit of the voltage required to drive the motor, in the motor drive device according to the first embodiment and the second embodiment and the modification thereof; FIG. 6 is a waveform diagram illustrating an example of operation when stopping a motor that is coasting from a state where the DC link voltage is equal to or higher than a lower limit of the voltage required to drive the motor, in the motor drive device according to the second embodiment and the modification thereof; FIG. 7 is a waveform diagram illustrating an example of operation when stopping a motor that is coasting from a state where the DC link voltage is lower than a lower limit of the voltage required to drive the motor, in the motor drive device according to the second embodiment and the modification thereof;

[0007] A motor drive device for controlling a motor during coasting according to an embodiment will be described below with reference to the drawings. In the following description, components having the same or similar functions will be assigned the same reference numerals. Duplicate descriptions of those components may be omitted. The scale of the drawings has been appropriately changed to facilitate understanding.

[0008] In the following description, a converter that converts AC power supplied from an AC power source into DC power and outputs it is also referred to as a “rectifier device,” “rectifier circuit,” “rectifier,” or “forward converter.” An inverter that converts DC power into AC power and outputs it is also referred to as an “inverter.” A “DC link” refers to a circuit portion that electrically connects the DC output side of a converter and the DC input side of an inverter, and is also referred to as a “DC link unit,” “DC link,” “DC link unit,” “DC bus,” or “DC intermediate circuit.” A pre-charging circuit is also referred to as an “initial charging circuit” or “inrush current suppression circuit.” A “pre-charging period” refers to a period during which pre-charging of a capacitor is performed. A “DC link voltage” refers to the potential difference between the positive potential on the positive power line of the DC link and the negative potential on the negative power line of the DC link. Furthermore, the “rotational speed” of a motor refers to the “rotational speed of the rotor or rotating shaft” of the motor. A “rotational speed command” of a motor refers to the “rotational speed command for the rotor or rotating shaft” of the motor. The "position" of a motor means the "rotational angle position of the rotor or rotating shaft" of the motor. The "position command" of a motor means the "rotational angle position command for the rotor or rotating shaft" of the motor. Furthermore, the "speed control" of a motor means the "speed control for the rotor or rotating shaft" of the motor. The "position control" of a motor means the "rotational angle position control for the rotor or rotating shaft" of the motor.

[0009] <First embodiment of the present disclosure> Fig. 1 is a diagram showing a motor drive device according to first and second embodiments of the present disclosure. The configuration shown in Fig. 1 is also applicable to a second embodiment described later. However, the operation of the pre-charge control unit 21 differs between the first and second embodiments.

[0010] In the first embodiment of the present disclosure and the second embodiment described below, as an example, a case is shown in which an AC motor 3 is driven by a motor drive device 1 connected to an AC power source 2. The number of phases of the AC power source 2 and the motor 3 is not particularly limited to each embodiment, and may be, for example, three-phase or single-phase. Examples of the AC power source 2 include a three-phase 400V AC power source, a three-phase 200V AC power source, a three-phase 600V AC power source, and a single-phase 100V AC power source. Here, as an example, the AC power source 2 and the motor 3 are each three-phase. Furthermore, the number of motors 3 is not particularly limited to each embodiment, and there may be more than one. Here, as an example, there is one motor 3. Machines in which the motor 3 is provided include, for example, machine tools and robots.

[0011] 1, a motor drive device 1 according to the first embodiment of the present disclosure includes a converter 11, an inverter 12, a capacitor 13, a pre-charging circuit 14, a voltage detection unit 15, a control unit 16, and an electromagnetic contactor 20. The control unit 16 has a converter control unit 101, a motor control unit 102, and a switching control unit 103. An AC reactor, an AC line filter, and the like may be provided on the AC input side of the converter 11, but are not shown here.

[0012] The electromagnetic contactor 20 is provided on the AC input side of the converter 11, and opens and closes the electrical path between the AC power source 2 and the converter 11. The opening and closing operation of the electromagnetic contactor 20 is controlled by a switching control unit 103. A closing operation that forms an electrical path between the AC power source 2 and the converter 11 is achieved by closing the contacts of the electromagnetic contactor 20. An opening operation that interrupts the electrical path between the AC power source 2 and the converter 11 is achieved by opening the contacts of the electromagnetic contactor 20.

[0013] Converter control unit 101 has a voltage detection unit 15, a pre-charging control unit 21, and a regenerative braking control unit 22. Voltage detection unit 15 may be provided outside converter control unit 101. Pre-charging control unit 21 may be provided outside converter control unit 101. Although not shown here, a power line for supplying power to control unit 16 is a separate system from a power line for supplying power from AC power supply 2 to converter 11. Even before converter 11 is powered on, power for driving control unit 16 is supplied.

[0014] The converter 11 and the inverter 12 are connected via a DC link.

[0015] The voltage detection unit 15 detects a DC link voltage, which is a potential difference between the potential of the positive power line 41P and the potential of the negative power line 41N in the DC link. The voltage detection unit 15 may detect the voltage applied between the positive terminal and the negative terminal of the capacitor 13 as the DC link voltage. Data related to the DC link voltage detected by the voltage detection unit 15 is sent to the pre-charge control unit 21 and the regenerative braking control unit 22.

[0016] The converter 11 converts AC power supplied from the AC power source 2 via the electromagnetic contactor 20 in a closed state into DC power and outputs the DC power to a DC link, which is the DC output side of the converter 11. In the example shown in FIG. 1 , since the AC power source 2 is a three-phase AC power source, the converter 11 is configured as a three-phase bridge circuit. If the AC power source 2 is a single-phase AC power source, the converter 11 is configured as a single-phase bridge circuit. In the example shown in FIG. 1 , the converter 11 is configured as a rectifier capable of power regeneration. Examples of rectifiers capable of power regeneration include PWM switching control rectifiers and 120-degree conduction control rectifiers. The converter 11 is configured as a bridge circuit of semiconductor switching elements and diodes connected in antiparallel to the semiconductor switching elements. Each semiconductor switching element is controlled on and off in response to a drive command received from the converter control unit 101, thereby performing bidirectional AC / DC power conversion. Examples of semiconductor switching elements include FETs, IGBTs, thyristors, GTOs, and transistors, but other semiconductor switching elements may also be used.

[0017] The regenerative power of the motor 3, generated by the speed control unit 19's deceleration control of the motor 3, is returned to the DC link through the power conversion operation of the inverter 12, resulting in a gradual increase in the DC link voltage. If the DC link voltage becomes too high, it may exceed the withstand voltage of the capacitor 13 and the withstand voltages of the semiconductor switching elements and diodes in the converter 11 and inverter 12, potentially causing damage to these elements. Therefore, a regenerative braking start voltage is set in advance, and when the DC link voltage exceeds the regenerative braking start voltage, an operation is performed to reduce the DC power in the DC link (hereinafter, sometimes referred to as "regenerative braking"). In the example shown in FIG. 1 , a power regeneration operation by the converter 11 is performed as regenerative braking to reduce the DC power in the DC link.

[0018] The regenerative braking control unit 22 controls whether the converter 11 performs a power regeneration operation. The power regeneration operation of the converter 11 is an operation in which semiconductor switching elements in the converter 11 are turned on and off to convert DC power in the DC link into AC power and return this AC power to the AC power source 2. The regenerative braking control unit 22 starts controlling the power regeneration operation of the converter 11 when the DC link voltage becomes equal to or higher than the regenerative braking start voltage while the speed control unit 19 is controlling the inverter 12 to decelerate the coasting motor 3. Furthermore, the regenerative braking control unit 22 stops controlling the power regeneration operation of the converter 11 when the DC link voltage becomes equal to or lower than the regenerative braking end voltage while controlling the converter 11 to return regenerative power generated by the motor 3 to the AC power source 2.

[0019] The regenerative braking start voltage is set to a value lower than, for example, the withstand voltage of the capacitor 13 and the withstand voltage of the semiconductor switching elements and diodes in the converter 11 and inverter 12. The regenerative braking end voltage is set to a value lower than the regenerative braking start voltage. The regenerative braking end voltage is set to a value equal to or higher than the pre-charging completion voltage. Data related to the regenerative braking start voltage and the regenerative braking end voltage are stored in advance in a memory (not shown) in the converter control unit 101 or a memory (not shown) connected to the converter control unit 101.

[0020] The capacitor 13 is provided in a DC link between the DC output side of the converter 11 and the DC input side of the inverter 12. One end of the capacitor 13 is connected to the positive power line 41P of the DC link, and the other end of the capacitor 13 is connected to the negative power line 41N of the DC link. The capacitor 13 is sometimes referred to as a "DC link capacitor," "DC link capacitor," or "smoothing capacitor." The capacitor 13 has the function of suppressing pulsation in the DC output of the converter 11 and the function of storing DC power used by the inverter 12 to generate AC power. Examples of the capacitor 13 include an electrolytic capacitor and a film capacitor.

[0021] 1 , the pre-charging circuit 14 is provided on the positive power line 41P of the DC link. Alternatively, the pre-charging circuit 14 may be provided on the negative power line 41N of the DC link. Alternatively, the pre-charging circuit 14 may be provided on at least two of the three-phase power lines connecting the electromagnetic contactor 20 and the AC input side of the converter 11.

[0022] The pre-charging circuit 14 includes a pre-charging resistor 141 and a pre-charging switch 142 .

[0023] The pre-charging resistor 141 suppresses inrush current during pre-charging of the capacitor 13. The pre-charging switch 142 is connected in parallel to the pre-charging resistor 141. Examples of the pre-charging switch 142 include a relay and a semiconductor switching element, but other switch components may also be used. Examples of the semiconductor switching element include an IGBT, an FET, a thyristor, a GTO, and a transistor, but other semiconductor switching elements may also be used.

[0024] Pre-charging switch 142 opens and closes the electrical path between converter 11 and capacitor 13 under the control of pre-charging control unit 21. Pre-charging control unit 21 commands pre-charging switch 142 to open during the pre-charging period, and commands pre-charging switch 142 to close during the motor driving period, which is a period other than the pre-charging period.

[0025] The pre-charging control unit 21 issues an opening / closing command to the pre-charging switch 142 to selectively switch between an open state in which a current path is formed via the pre-charging resistor 141 and a closed state in which a short circuit is formed without passing through the pre-charging resistor 141. When the electromagnetic contactor 20 is closed under the control of the opening / closing control unit 103, the pre-charging control unit 21 commands the pre-charging switch 142 to open during the pre-charging period, and in response, the pre-charging switch 142 maintains its open state. As a result, during the pre-charging period, the current output from the converter 11 flows into the capacitor 13 via the pre-charging resistor 141, charging the capacitor 13. This pre-charging operation allows the current output from the converter 11 to flow through the pre-charging resistor 141, preventing the occurrence of an inrush current. After the pre-charging operation is completed, the pre-charging control unit 21 commands the pre-charging switch 142 to close, and in response, the pre-charging switch 142 maintains its closed state. During the motor driving period after the pre-charging operation is completed, the current output from the converter 11 flows toward the capacitor 13 and the inverter 12 through the pre-charging switch 142 in the closed state.

[0026] The motor control unit 102 controls the power conversion operation of the inverter 12, thereby controlling the driving of the motor 3. The motor control unit 102 has a speed determination unit 17, a speed detection unit 18, and a speed control unit 19.

[0027] The inverter 12 is connected to the converter 11 via a DC link. The inverter 12 converts DC power from the DC link into AC power by controlling the on / off of semiconductor switching elements within the inverter 12, and supplies this AC power to the motor 3 to drive it. The inverter 12 is composed of semiconductor switching elements and a bridge circuit of diodes connected in antiparallel to the semiconductor switching elements. The inverter 12 is composed of a three-phase bridge circuit if the motor 3 is a three-phase AC motor, and a single-phase bridge circuit if the motor 3 is a single-phase AC motor. In the example shown in FIG. 1 , the motor 3 is a three-phase AC motor, so the inverter 12 is composed of a three-phase bridge circuit. Examples of semiconductor switching elements include FETs, IGBTs, thyristors, GTOs, and transistors, but other semiconductor switching elements may also be used. The power conversion operation of the inverter 12 is controlled, for example, by a PWM switching control method. That is, the inverter 12 receives a drive command (PWM switching command) from the motor control unit 102, converts the DC power in the DC link into AC power and outputs this to the motor 3, or converts the AC power regenerated when the motor 3 decelerates into DC power and returns this to the DC link.

[0028] The speed detection unit 18 detects the rotation speed of the motor 3. Data relating to the rotation speed of the motor 3 detected by the speed detection unit 18 is sent to the speed control unit 19 and the speed determination unit 17.

[0029] The speed control unit 19 generates drive commands for controlling the rotational speed, torque, or position of the rotor of the motor 3 based on the rotational speed of the motor 3 (rotational speed feedback), the current flowing through the windings of the motor 3 (current feedback), the rotational speed command, the torque command, the position command, and the operation program of the motor 3. The drive commands generated by the speed control unit 19 are sent to the semiconductor switching elements in the inverter 12. Note that the configuration of the speed control unit 19 defined here is merely an example, and the configuration of the speed control unit 19 may be defined by including terms such as a position command generation unit, a speed command generation unit, a torque command generation unit, and a switching command generation unit.

[0030] The speed determination unit 17 determines whether the motor 3 is in a coasting state. For example, when the rotational speed of the motor 3 detected by the speed detection unit 18 is equal to or greater than a predetermined value greater than 0 while AC power is not being supplied to the motor 3 from the inverter 12, the speed determination unit 17 determines that the motor 3 is in a coasting state. If the speed determination unit 17 determines that the motor 3 is in a coasting state, the speed determination unit 17 then determines whether the rotational speed of the motor 3 during coasting, detected by the speed detection unit 18, is greater than a threshold value for detecting that the motor 3 is stopped. If the rotational speed of the motor 3 is determined to be equal to or less than the threshold value, this means that the motor 3 is stopped. If the rotational speed of the motor 3 is determined to be greater than the threshold value, this means that the motor 3 is rotating. The threshold value is set to a value (e.g., several rpm to several tens of rpm) that allows detection that the motor 3 is stopped. Note that the numerical values ​​given here are merely examples, and other numerical values ​​may be used. The reason why the determination as to whether the motor 3 has stopped is based on a comparison between the rotation speed of the motor 3 detected by the speed detection unit 18 and a threshold value is to eliminate erroneous determinations by the speed determination unit 17 that are caused by errors contained in the rotation speed detected by the speed detection unit 18.

[0031] If the speed determination unit 17 determines that the rotational speed of the motor 3 during coasting is greater than the threshold value, the control unit 16 determines whether the DC link voltage is equal to or greater than the lower limit of the voltage required to drive the motor 3.

[0032] As described above, the inverter 12 converts DC power in the DC link into AC power and supplies the AC power to the motor 3 to drive the motor 3. If the DC link voltage is too low, the AC voltage output from the inverter 12 as a result of the inverter 12's power conversion operation may not reach the minimum voltage required to normally drive the motor 3. If the motor 3 cannot be normally driven, for example, an overcurrent may be output from the inverter 12, or the difference between the rotation speed detected by the speed detection unit 18 and the speed command value may become extremely large, causing an alarm to be generated in the motor drive device 1. Thus, the DC link voltage has a minimum value required to normally drive the motor 3. In the first and second embodiments and their modified examples of the present disclosure, a value that is somewhat higher (e.g., several percent to several tens of percent) than the minimum DC link voltage required to normally drive the motor 3 is preset as the "lower limit value of the voltage required to drive the motor 3." Generally, the lower limit value of the voltage required to drive the motor 3 is lower than the pre-charge completion voltage. Furthermore, the lower limit of the voltage required to drive the motor 3 depends on the specifications of the motor 3, the power conversion performance of the inverter 12, and the connection state between the motor 3 and the motor drive device 1. Data relating to the lower limit of the voltage required to drive the motor 3 is stored in advance in a memory (not shown) within the motor control unit 102 or a memory (not shown) connected to the motor control unit 102.

[0033] If the speed determination unit 17 determines that the rotational speed of the motor 3 during coasting is greater than the threshold value and the DC link voltage is equal to or greater than the lower limit of the voltage required to drive the motor 3, the speed control unit 19 controls the power conversion operation of the inverter 12 so as to decelerate the motor 3 during coasting.

[0034] Furthermore, if the speed determination unit 17 determines that the rotational speed of the motor 3 during coasting is greater than the threshold value and the DC link voltage is less than the lower limit of the voltage required to drive the motor 3, the pre-charging operation is performed until the DC link voltage reaches or exceeds the pre-charging completion voltage of the capacitor 13. Also, if the speed determination unit 17 determines that the rotational speed of the motor 3 during coasting is equal to or less than the threshold value, the pre-charging operation is performed until the DC link voltage reaches or exceeds the pre-charging completion voltage of the capacitor 13. In the pre-charging operation, the switching control unit 103 commands the electromagnetic contactor 20 to close, and the pre-charging control unit 21 commands the pre-charging switch 142 of the pre-charging circuit 14 to open, thereby charging the capacitor 13 with current flowing from the converter 11 via the pre-charging resistor 141. The pre-charging completion voltage is set to, for example, the potential difference between the positive and negative terminals of the capacitor 13 when the capacitor 13 is fully charged. Data related to the pre-charging completion voltage is stored in advance in a memory (not shown) within the converter control unit 101 or a memory (not shown) connected to the converter control unit 101.

[0035] FIG. 2 is a diagram illustrating a motor drive device according to a modified example of the first embodiment of the present disclosure.

[0036] 1, the converter 11 is configured as a rectifier capable of power regeneration, and therefore, the converter 11 performs a power regeneration operation to reduce the DC power in the DC link. In this modified example in which the converter 11 is configured as a diode rectifier, a discharge operation is performed by the discharge resistor 31 as regenerative braking to reduce the DC power in the DC link.

[0037] As shown in Fig. 2 , the converter 11 in this modification is a diode rectifier that converts AC power supplied from the AC power source 2 via the electromagnetic contactor 20 in a closed state into DC power and outputs the DC power to a DC link, which is the DC output side of the converter 11. The converter 11 is made up of a diode bridge circuit. In the example shown in Fig. 2 , the AC power source 2 is a three-phase AC power source, so the converter 11 is made up of a three-phase bridge circuit. If the AC power source 2 is a single-phase AC power source, the converter 11 is made up of a single-phase bridge circuit.

[0038] A discharge circuit 30 is provided in the DC link. The discharge circuit 30 includes a discharge resistor 31 and a discharge switch 32. The discharge resistor 31 and the discharge switch 32 are connected in series. One end of the discharge circuit 30 is connected to a positive power line 41P in the DC link, and one end of the discharge switch 32 is connected to a negative power line 41N in the DC link. When the discharge switch 32 is in an open state, no current flows through the discharge resistor 31, and no discharging operation is performed. When the discharge switch 32 is in a closed state, current flows from the positive power line 41P in the DC link through the discharge resistor 31 to the negative power line 41N, and a discharging operation is performed. During the discharging operation in which a current flows through the discharge resistor 31, electrical energy is converted into thermal energy in the discharge resistor 31, and power is consumed.

[0039] The regenerative braking control unit 22 controls whether or not the discharge circuit 30 performs a discharge operation. When the DC link voltage is less than the regenerative braking start voltage, the regenerative braking control unit 22 controls the discharge switch 32 to open so as not to perform the discharge operation. When the speed control unit 19 controls the inverter 12 to decelerate the motor 3 during coasting, the regenerative braking control unit 22 controls the discharge switch 32 to close so as to start the discharge operation when the DC link voltage becomes equal to or greater than the regenerative braking start voltage. When the regenerative braking control unit 22 controls the DC power in the DC link to be consumed by the discharge resistor 31, the regenerative braking control unit 22 controls the discharge switch 32 to open so as to end the discharge operation when the DC link voltage becomes equal to or less than the regenerative braking end voltage.

[0040] FIG. 3 is a flowchart showing the operation flow of the motor drive device according to the first embodiment and its modified example of the present disclosure.

[0041] When the motor 3 is being driven by the motor drive device 1, in step S101 the speed determination unit 17 determines whether the motor 3 is in a coasting state. For example, if the rotation speed of the motor 3 detected by the speed detection unit 18 is equal to or greater than a predetermined value greater than 0 when AC power is not being supplied to the motor 3 from the inverter 12, the speed determination unit 17 determines that the motor 3 is in a coasting state.

[0042] If it is determined in step S101 that the motor 3 is in a coasting state, then in step S102, the speed determination unit 17 determines whether the rotational speed of the motor 3 during coasting, detected by the speed detection unit 18, is greater than a threshold value for detecting that the motor 3 is stopped. The result of the determination by the speed determination unit 17 is notified to the speed control unit 19 and the pre-charge control unit 21.

[0043] If it is not determined in step S101 that the motor 3 is in a coasting state, the process proceeds to step S110.

[0044] If it is determined in step S102 that the rotational speed of the motor 3 during coasting is greater than the threshold value, then in step S103, the control unit 16 determines whether the DC link voltage is equal to or greater than the lower limit of the voltage required to drive the motor 3.

[0045] If it is determined in step S103 that the DC link voltage is equal to or greater than the lower limit of the voltage required to drive the motor 3, then in step S106 the speed control unit 19 controls the power conversion operation of the inverter 12 to decelerate the coasting motor 3. The regenerative power of the motor 3, generated by the speed control unit 19's control of deceleration of the motor 3, is returned to the DC link by the power conversion operation of the inverter 12, and as a result, the DC link voltage gradually increases. In this way, if it is determined in step S103 that the DC link voltage is equal to or greater than the lower limit of the voltage required to drive the motor 3, then pre-charging is not performed and deceleration control of the motor 3 is executed in step S106, so that no power loss occurs in the pre-charging circuit 14.

[0046] If it is not determined in step S103 that the DC link voltage is equal to or greater than the lower limit of the voltage required to drive the motor 3, then in step S104, the switching control unit 103 commands the electromagnetic contactor 20 to close, and the pre-charging control unit 21 commands the pre-charging switch 142 of the pre-charging circuit 14 to open, thereby performing a pre-charging operation. At this time, power loss occurs in the pre-charging circuit 14.

[0047] In step S105 , the pre-charging control unit 21 determines whether the DC link voltage has reached or exceeded the pre-charging completion voltage of the capacitor 13 .

[0048] If it is not determined in step S105 that the DC link voltage has reached or exceeded the pre-charging completion voltage of the capacitor 13, the process returns to step S104.

[0049] If it is determined in step S105 that the DC link voltage has become equal to or greater than the pre-charging completion voltage of the capacitor 13, the switching control unit 103 commands the electromagnetic contactor 20 to close, and the pre-charging control unit 21 commands the pre-charging switch 142 of the pre-charging circuit 14 to close, thereby terminating the pre-charging operation, and the process proceeds to step S106. Steps S104 and S105 are repeatedly executed, thereby performing the pre-charging operation until the DC link voltage becomes equal to or greater than the pre-charging completion voltage of the capacitor 13.

[0050] In step S106, the speed control unit 19 controls the power conversion operation of the inverter 12 to decelerate the coasting motor 3. The regenerative power of the motor 3 generated by the speed control unit 19's control of deceleration of the motor 3 is returned to the DC link by the power conversion operation of the inverter 12, and as a result, the DC link voltage gradually increases.

[0051] In step S107, the regenerative braking control unit 22 determines whether the DC link voltage has reached or exceeded the regenerative braking start voltage.

[0052] If it is not determined in step S107 that the DC link voltage has reached or exceeded the regenerative braking start voltage, the process returns to step S102.

[0053] If it is determined in step S107 that the DC link voltage has reached or exceeded the regenerative braking start voltage, in step S108, the switching control unit 103 commands the electromagnetic contactor 20 to close, and the regenerative braking control unit 22 controls the converter 11 to perform a power regeneration operation. At this time, a power loss occurs due to the power conversion operation of the converter 11. Note that in the modified example shown in FIG. 2 , the regenerative braking control unit 22 controls the discharge switch 32 to close, thereby causing the discharge resistor 31 to consume the DC power in the DC link. At this time, a power loss occurs in the discharge resistor 31.

[0054] In step S109, the regenerative braking control unit 22 determines whether the DC link voltage has become equal to or lower than the regenerative braking end voltage.

[0055] If it is not determined in step S109 that the DC link voltage has become equal to or lower than the regenerative braking end voltage, the process returns to step S108.

[0056] If it is determined in step S109 that the DC link voltage has become equal to or lower than the regenerative braking end voltage, the regenerative braking control unit 22 ends control of the converter 11 to perform the power regeneration operation, and the process returns to step S102. In the modified example shown in Fig. 2, the regenerative braking control unit 22 controls the discharge switch 32 to open, thereby ending control of dissipating DC power in the DC link through the discharge resistor 31, and the process returns to step S102.

[0057] The above-described processes of steps S103 to S109 are repeatedly executed as long as it is determined in step S102 that the rotational speed of the coasting motor 3 is greater than the threshold value. However, if it is determined in step S103 that the DC link voltage is equal to or greater than the lower limit of the voltage required to drive the motor 3, pre-charging is not performed and deceleration control of the motor 3 is executed in step S106, so that no power loss occurs in the pre-charging circuit 14. This makes it possible to reduce the power loss that occurs when the motor drive device 1 decelerates and stops the coasting motor 3.

[0058] If it is determined in step S102 that the rotation speed of the motor 3 during coasting is equal to or less than the threshold value, the process proceeds to step S110.

[0059] In step S110, the pre-charge control unit 21 determines whether the DC link voltage has reached or exceeded the pre-charge completion voltage of the capacitor 13. If, after steps S103 to S109, it is determined in step S102 that the rotational speed of the motor 3 during coasting is equal to or lower than the threshold value, the speed control unit 19 stops deceleration control of the motor 3, and then the process proceeds to step S110.

[0060] If it is not determined in step S110 that the DC link voltage has reached or exceeded the pre-charging completion voltage of the capacitor 13, then in step S111 the switching control unit 103 commands the electromagnetic contactor 20 to close, and the pre-charging control unit 21 commands the pre-charging switch 142 of the pre-charging circuit 14 to open, thereby performing the pre-charging operation. At this time, power loss occurs in the pre-charging circuit 14.

[0061] If it is determined in step S110 that the DC link voltage has reached or exceeded the pre-charging completion voltage of the capacitor 13, the pre-charging control unit 21 instructs the pre-charging switch 142 of the pre-charging circuit 14 to close, thereby terminating the pre-charging operation. At this point, the DC link voltage has been charged to the pre-charging completion voltage of the capacitor 13, allowing the motor drive device 1 to drive the motor 3 as usual.

[0062] 1 , a motor drive device 1 according to a second embodiment of the present disclosure includes a converter 11, an inverter 12, a capacitor 13, a pre-charging circuit 14, a voltage detection unit 15, a control unit 16, and an electromagnetic contactor 20. The control unit 16 includes a converter control unit 101, a motor control unit 102, and an opening / closing control unit 103. The converter control unit 101 includes a voltage detection unit 15, a pre-charging control unit 21, and a regenerative braking control unit 22. The motor control unit 102 includes a speed determination unit 17, a speed detection unit 18, and a speed control unit 19.

[0063] In the second embodiment of the present disclosure, the operation of the pre-charge control unit 21 when the speed determination unit 17 determines that the rotational speed of the motor 3 during coasting is greater than the threshold value and the DC link voltage is less than the lower limit of the voltage required to drive the motor 3 is different from that in the first embodiment.

[0064] In the second embodiment of the present disclosure, when the speed determination unit 17 determines that the rotational speed of the motor 3 during coasting is greater than the threshold value and the DC link voltage is less than the lower limit of the voltage required to drive the motor 3, the pre-charging operation is performed until the DC link voltage becomes equal to or greater than the lower limit of the voltage required to drive the motor 3. When the speed determination unit 17 determines that the rotational speed of the motor 3 during coasting is equal to or less than the threshold value, the pre-charging operation is performed, as in the first embodiment, until the DC link voltage becomes equal to or greater than the pre-charging completion voltage of the capacitor 13. In the pre-charging operation, the switching control unit 103 commands the electromagnetic contactor 20 to close, and the pre-charging control unit 21 commands the pre-charging switch 142 of the pre-charging circuit 14 to open, thereby charging the capacitor 13 with a current flowing from the converter 11 via the pre-charging resistor 141.

[0065] The configurations and operations of the converter 11, inverter 12, capacitor 13, pre-charging circuit 14, voltage detection unit 15, electromagnetic contactor 20, motor control unit 102, and switching control unit 103 are as described in relation to the first embodiment shown in Fig. 1. The configurations and operations of the voltage detection unit 15 and regenerative braking control unit 22 in the converter control unit 101 are as described in relation to the first embodiment shown in Fig. 1. The modified example using power consumption by the discharge resistor 31 described with reference to Fig. 2 can also be applied to the second embodiment.

[0066] FIG. 4 is a flowchart showing the operation flow of the motor drive device according to the second embodiment of the present disclosure and its modified example.

[0067] When the motor 3 is being driven by the motor drive device 1, in step S201 the speed determination unit 17 determines whether the motor 3 is in a coasting state. For example, if the rotation speed of the motor 3 detected by the speed detection unit 18 is equal to or greater than a predetermined value greater than 0 when AC power is not being supplied to the motor 3 from the inverter 12, the speed determination unit 17 determines that the motor 3 is in a coasting state.

[0068] If it is determined in step S201 that the motor 3 is in a coasting state, then in step S202, the speed determination unit 17 determines whether the rotation speed of the motor 3 during coasting, detected by the speed detection unit 18, is greater than a threshold value for detecting that the motor 3 is stopped. The result of the determination by the speed determination unit 17 is notified to the speed control unit 19 and the pre-charge control unit 21.

[0069] If it is not determined in step S201 that the motor 3 is in a coasting state, the process proceeds to step S210.

[0070] If it is determined in step S202 that the rotation speed of the motor 3 during coasting is greater than the threshold value, then in step S203, the control unit 16 determines whether the DC link voltage is equal to or greater than the lower limit of the voltage required to drive the motor 3.

[0071] If it is determined in step S203 that the DC link voltage is equal to or greater than the lower limit of the voltage required to drive the motor 3, then in step S206 the speed control unit 19 controls the power conversion operation of the inverter 12 to decelerate the coasting motor 3. The regenerative power of the motor 3, generated by the speed control unit 19's control of deceleration of the motor 3, is returned to the DC link by the power conversion operation of the inverter 12, and as a result, the DC link voltage gradually increases. In this way, if it is determined in step S203 that the DC link voltage is equal to or greater than the lower limit of the voltage required to drive the motor 3, then pre-charging is not performed and deceleration control of the motor 3 is executed in step S206, so that no power loss occurs in the pre-charging circuit 14.

[0072] If it is not determined in step S203 that the DC link voltage is equal to or greater than the lower limit of the voltage required to drive the motor 3, then in step S204, the switching control unit 103 commands the electromagnetic contactor 20 to close, and the pre-charging control unit 21 commands the pre-charging switch 142 of the pre-charging circuit 14 to open, thereby performing a pre-charging operation. At this time, power loss occurs in the pre-charging circuit 14.

[0073] In step S205, the pre-charge control unit 21 determines whether the DC link voltage has reached or exceeded the lower limit of the voltage required to drive the motor 3.

[0074] If it is not determined in step S205 that the DC link voltage is equal to or greater than the lower limit of the voltage required to drive the motor 3, the process returns to step S204.

[0075] If it is determined in step S205 that the DC link voltage has reached or exceeded the lower limit of the voltage required to drive the motor 3, the switching control unit 103 commands the electromagnetic contactor 20 to open, and the pre-charging control unit 21 commands the pre-charging switch 142 of the pre-charging circuit 14 to close, thereby terminating the pre-charging operation, and the process proceeds to step S206. Steps S204 and S205 are repeatedly executed, thereby performing the pre-charging operation until the DC link voltage reaches or exceeds the lower limit of the voltage required to drive the motor 3. The lower limit of the voltage required to drive the motor 3 is lower than the pre-charging completion voltage. Therefore, according to the second embodiment of the present disclosure, the pre-charging operation, which is executed when it is not determined that the DC link voltage is reached or exceeded the lower limit of the voltage required to drive the motor 3, is performed until the DC link voltage reaches or exceeds the lower limit of the voltage required to drive the motor 3. This further reduces power loss in the pre-charging circuit 14 compared to the first embodiment, in which the pre-charging operation is performed until the DC link voltage reaches or exceeds the pre-charging completion voltage.

[0076] In step S206, the speed control unit 19 controls the power conversion operation of the inverter 12 to decelerate the coasting motor 3. The regenerative power of the motor 3 generated by the speed control unit 19's control of deceleration of the motor 3 is returned to the DC link by the power conversion operation of the inverter 12, and as a result, the DC link voltage gradually increases.

[0077] In step S207, the regenerative braking control unit 22 determines whether the DC link voltage has reached or exceeded the regenerative braking start voltage.

[0078] If it is not determined in step S207 that the DC link voltage has reached or exceeded the regenerative braking start voltage, the process returns to step S202.

[0079] If it is determined in step S207 that the DC link voltage has reached or exceeded the regenerative braking start voltage, in step S208, the switching control unit 103 commands the electromagnetic contactor 20 to close, and the regenerative braking control unit 22 controls the converter 11 to perform a power regeneration operation. At this time, a power loss occurs due to the power conversion operation of the converter 11. Note that in the modified example shown in FIG. 2 , the regenerative braking control unit 22 controls the discharge switch 32 to close, thereby causing the discharge resistor 31 to consume the DC power in the DC link. At this time, a power loss occurs in the discharge resistor 31.

[0080] In step S209, the regenerative braking control unit 22 determines whether the DC link voltage has become equal to or lower than the regenerative braking end voltage.

[0081] If it is not determined in step S209 that the DC link voltage has become equal to or lower than the regenerative braking end voltage, the process returns to step S208.

[0082] If it is determined in step S209 that the DC link voltage has become equal to or lower than the regenerative braking end voltage, the regenerative braking control unit 22 ends control of the converter 11 to perform the power regeneration operation, and the process returns to step S202. In the modification shown in Fig. 2, the regenerative braking control unit 22 controls the discharge switch 32 to open, thereby ending control of dissipating DC power in the DC link through the discharge resistor 31, and the process returns to step S202.

[0083] The above-described processes of steps S203 to S209 are repeatedly executed as long as it is determined in step S202 that the rotational speed of the motor 3 during coasting is greater than the threshold. However, if it is determined in step S203 that the DC link voltage is equal to or greater than the lower limit of the voltage required to drive the motor 3, the pre-charging operation is not executed and the deceleration control of the motor 3 is executed in step S206. This prevents power loss in the pre-charging circuit 14. Furthermore, if it is determined that the DC link voltage is not equal to or greater than the lower limit of the voltage required to drive the motor 3, the pre-charging operation is executed until the DC link voltage reaches or exceeds the lower limit of the voltage required to drive the motor 3. This further reduces power loss in the pre-charging circuit 14 compared to the first embodiment, in which the pre-charging operation is executed until the DC link voltage reaches or exceeds the pre-charging completion voltage. This reduces power loss that occurs when the motor drive device 1 decelerates and stops the coasting motor 3.

[0084] If it is determined in step S202 that the rotation speed of the motor 3 during coasting is equal to or less than the threshold value, the process proceeds to step S210.

[0085] In step S210, the pre-charge control unit 21 determines whether the DC link voltage has reached or exceeded the pre-charge completion voltage of the capacitor 13. If, after steps S203 to S209, it is determined in step S202 that the rotational speed of the motor 3 during coasting is equal to or lower than the threshold value, the speed control unit 19 stops deceleration control of the motor 3, and then the process proceeds to step S210.

[0086] If it is not determined in step S210 that the DC link voltage has reached or exceeded the pre-charging completion voltage of the capacitor 13, then in step S211 the switching control unit 103 commands the electromagnetic contactor 20 to close, and the pre-charging control unit 21 commands the pre-charging switch 142 of the pre-charging circuit 14 to open, thereby performing the pre-charging operation. At this time, power loss occurs in the pre-charging circuit 14.

[0087] If it is determined in step S210 that the DC link voltage has reached or exceeded the pre-charging completion voltage of the capacitor 13, the pre-charging control unit 21 commands the pre-charging switch 142 of the pre-charging circuit 14 to close, thereby terminating the pre-charging operation. The process then ends. At this point, the DC link voltage has been charged to the pre-charging completion voltage of the capacitor 13, allowing the motor drive device 1 to drive the motor 3 as usual.

[0088] <Operation Example> Figure 5 is a waveform diagram showing an operation example when stopping a coasting motor from a state in which the DC link voltage is equal to or higher than the lower limit of the voltage required to drive the motor, in the motor drive device according to the first and second embodiments and their modifications of the present disclosure. From top to bottom, Figure 5 shows the DC link voltage, the rotational speed of the motor 3, and the accumulated loss in the motor drive device 1. In the example shown in Figure 5, the pre-charging completion voltage and the regenerative braking end voltage are set to the same value.

[0089] At time t0, motor 3 starts coasting. At time t0, the DC link voltage is lower than the pre-charge completion voltage and higher than the lower limit of the voltage required to drive motor 3. From time t0 to time t1, the DC link voltage gradually decreases due to the natural discharge of capacitor 13. Furthermore, the accumulated loss gradually increases due to the natural discharge of capacitor 13. Because motor 3 is coasting, the rotational speed of motor 3 gradually decreases due to air resistance and friction.

[0090] At time t1, the speed control unit 19 starts controlling the power conversion operation of the inverter 12 so as to decelerate the coasting motor 3. The speed control unit 19 continues deceleration control of the motor 3 until time t3, when the rotational speed of the motor 3 falls below a threshold for detecting that the motor 3 is stopped. From time t1 to time t3, the regenerative power of the motor 3, generated by the speed control unit 19's deceleration control of the motor 3, is converted into DC power by the inverter 12 and returned to the DC link, causing the DC link voltage to rise. Furthermore, power loss occurs due to the power conversion operation of the inverter 12, causing the cumulative loss to rise.

[0091] When the DC link voltage exceeds the regenerative braking start voltage, the regenerative braking control unit 22 controls the converter 11 to perform power regeneration. However, in the example shown in FIG. 5 , at time t3 before the DC link voltage exceeds the regenerative braking start voltage, the rotational speed of the motor 3 falls below the threshold for detecting that the motor 3 is stopped, and the speed control unit 19 stops deceleration control of the motor 3. Because deceleration control of the motor 3 is stopped before the DC link voltage exceeds the regenerative braking start voltage, the regenerative braking control unit 22 does not control the power regeneration operation of the converter 11. In the motor drive devices 1 according to the first and second embodiments and their modified examples of the present disclosure, the capacitor 13 is not pre-charged between time t1 and time t3, and therefore no loss due to the pre-charging operation occurs. Furthermore, because the pre-charging operation is not performed, the DC link voltage is less likely to exceed the regenerative braking start voltage, and therefore the regenerative braking control unit 22 is less likely to control the power regeneration operation of the converter 11.

[0092] From time t3 to time t4, the DC link voltage gradually decreases due to the natural discharge of capacitor 13. Furthermore, the cumulative loss gradually increases due to the natural discharge of capacitor 13. Since the deceleration control of motor 3 by speed control unit 19 is stopped at time t3, the DC link voltage gradually decreases due to the natural discharge of capacitor 13, but the degree of increase in cumulative loss is small.

[0093] After time t4, the DC link voltage continues to be maintained at the pre-charging completion voltage by compensating for the energy consumed by the natural discharge of the capacitor 13 from the AC power supply 2 via the converter 11. Due to the natural discharge of the capacitor 13, the cumulative loss gradually increases.

[0094] Fig. 6 is a waveform diagram showing an example of the operation of a conventional motor drive device when stopping a coasting motor from a state in which the DC link voltage is equal to or greater than the lower limit of the voltage required to drive the motor. From top to bottom, Fig. 6 shows the DC link voltage, the motor rotation speed, and the accumulated loss in the motor drive device. In the example shown in Fig. 6, the pre-charging completion voltage and the regenerative braking end voltage are set to the same value.

[0095] At time t0, the motor begins to coast. At time t0, the DC link voltage is lower than the pre-charge completion voltage and higher than the lower limit of the voltage required to drive the motor. From time t0 to time t1, the DC link voltage gradually decreases due to the natural discharge of the capacitor. Furthermore, the accumulated loss gradually increases due to the natural discharge of the capacitor. Because the motor is coasting, the rotational speed of the motor gradually decreases due to air resistance and friction.

[0096] In conventional motor drive devices, the capacitor is pre-charged until the DC link voltage reaches the pre-charge completion voltage before decelerating the coasting motor. In the example shown in FIG. 6 , the pre-charge operation for the capacitor begins at time t1. During the pre-charge period from time t1 to time t2, the DC link voltage rises, and the cumulative loss also increases due to the power loss caused by the pre-charge operation. The motor continues coasting during this period. As described above, in conventional motor drive devices, the capacitor is pre-charged until the DC link voltage reaches the pre-charge completion voltage before decelerating the coasting motor. In contrast, according to the first and second embodiments and their modifications of the present disclosure, as described with reference to FIG. 5 , the deceleration control for the coasting motor 3 is performed without pre-charge operation as long as the DC link voltage is equal to or higher than the lower limit of the voltage required to drive the motor 3, thereby reducing power loss.

[0097] At time t2, the DC link voltage exceeds the pre-charge completion voltage, completing the pre-charge operation for the capacitor, and starting to control the inverter's power conversion operation so that the coasting motor decelerates. The regenerative power of the motor generated by the motor deceleration control is converted into DC power by the inverter and returned to the DC link, causing the DC link voltage to rise. Furthermore, the power conversion operation of the inverter causes power loss, resulting in an increase in the cumulative loss.

[0098] When the regenerative power from the motor is large, the DC link voltage continues to rise and exceeds the regenerative braking start voltage, causing the converter to start regenerative power generation or the discharge resistor to start discharging at time t3. The converter continues regenerative power generation or the discharge resistor continues discharging until time t4, when the DC link voltage drops to the regenerative braking end voltage. The cumulative loss increases due to the power losses incurred during the inverter's power conversion operation and the converter's regenerative power generation or the discharge resistor's discharge. Conventional motor drive devices always pre-charge the capacitor until the DC link voltage reaches the pre-charge completion voltage before decelerating a coasting motor. This increases the likelihood that the regenerative power generated during motor deceleration will cause the DC link voltage to exceed the regenerative braking start voltage, thereby increasing the likelihood of regenerative braking. In contrast, according to the first and second embodiments and their modifications of the present disclosure, as described with reference to FIG. 5 , pre-charging is not performed if the DC link voltage is equal to or greater than the lower limit of the voltage required to drive the motor 3. This reduces the likelihood that the DC link voltage will exceed the regenerative braking start voltage and therefore reduces the likelihood of regenerative braking.

[0099] The deceleration control of the motor is performed until time t4 when the rotation speed of the motor falls below the threshold for detecting that the motor has stopped. At time t4, the rotation speed of the motor falls below the threshold for detecting that the motor has stopped, so the deceleration control of the motor is stopped.

[0100] After time t4, the power consumed by the natural discharge of the capacitor is compensated for by the AC power supply via the converter, thereby maintaining the DC link voltage at the pre-charge completion voltage. The natural discharge of the capacitor gradually increases the cumulative loss.

[0101] 7 is a waveform diagram showing an example of operation of the motor drive device according to the second embodiment and its modifications when stopping a coasting motor from a state in which the DC link voltage is below the lower limit of the voltage required to drive the motor. From top to bottom, Fig. 7 shows the DC link voltage, the rotational speed of the motor 3, and the accumulated loss in the motor drive device 1. In the example shown in Fig. 7, the pre-charging completion voltage and the regenerative braking end voltage are set to the same value.

[0102] At time t0, motor 3 starts coasting. At time t0, the DC link voltage is lower than the pre-charge completion voltage and also lower than the lower limit of the voltage required to drive motor 3. From time t0 to time t1, the DC link voltage gradually decreases due to the natural discharge of capacitor 13. Furthermore, the accumulated loss gradually increases due to the natural discharge of capacitor 13. Because motor 3 is coasting, the rotational speed of motor 3 gradually decreases due to air resistance and friction.

[0103] At time t1, the pre-charging operation starts, which gradually increases the DC link voltage and the cumulative loss.

[0104] When the DC link voltage reaches or exceeds the lower limit of the voltage required to drive the motor 3 at time t2, the pre-charging operation is terminated, and the speed control unit 19 starts controlling the power conversion operation of the inverter 12 to decelerate the coasting motor 3. The speed control unit 19 continues deceleration control of the motor 3 until time t3, when the rotational speed of the motor 3 falls below a threshold for detecting that the motor 3 is stopped. From time t2 to time t3, the regenerative power of the motor 3, generated by the speed control unit 19's deceleration control of the motor 3, is converted into DC power by the inverter 12 and returned to the DC link, causing the DC link voltage to rise. Furthermore, power loss occurs due to the power conversion operation of the inverter 12, causing the cumulative loss to increase.

[0105] 7 , the rotational speed of the motor 3 falls below the threshold for detecting that the motor 3 is stopped at time t3 before the DC link voltage reaches or exceeds the regenerative braking start voltage, and therefore the speed control unit 19 stops deceleration control of the motor 3. Because the deceleration control of the motor 3 is stopped before the DC link voltage reaches or exceeds the regenerative braking start voltage, the regenerative braking control unit 22 does not control the power regeneration operation of the converter 11. In the motor drive device 1 according to the second embodiment and its modification example of the present disclosure, the pre-charging operation is performed only until the DC link voltage reaches the lower limit of the voltage required to drive the motor 3. Therefore, the period during which the pre-charging operation is performed is shorter than in conventional motor drive devices that perform the pre-charging operation until the DC link voltage reaches the pre-charging completion voltage, thereby reducing power loss. Furthermore, the motor drive device 1 according to the second embodiment and its modified example of the present disclosure performs pre-charging for a shorter period of time than conventional motor drive devices, and therefore the DC link voltage is less likely to exceed the regenerative braking start voltage, and therefore the regenerative braking control unit 22 is less likely to control the power regeneration operation of the converter 11.

[0106] From time t3 to time t4, the DC link voltage gradually decreases due to the natural discharge of capacitor 13. Furthermore, the cumulative loss gradually increases due to the natural discharge of capacitor 13. Since the deceleration control of motor 3 by speed control unit 19 is stopped at time t3, the DC link voltage gradually decreases due to the natural discharge of capacitor 13, but the degree of increase in cumulative loss is small.

[0107] After time t4, the DC link voltage continues to be maintained at the pre-charging completion voltage by compensating for the energy consumed by the natural discharge of the capacitor 13 from the AC power supply 2 via the converter 11. Due to the natural discharge of the capacitor 13, the cumulative loss gradually increases.

[0108] Fig. 8 is a waveform diagram showing an example of operation of a conventional motor drive device when stopping a coasting motor when the DC link voltage is below the lower limit of the voltage required to drive the motor. From top to bottom, Fig. 8 shows the DC link voltage, the motor rotation speed, and the accumulated loss in the motor drive device. In the example shown in Fig. 8, the pre-charging completion voltage and the regenerative braking end voltage are set to the same value.

[0109] At time t0, the motor begins to coast. At time t0, the DC link voltage is lower than the pre-charge completion voltage and also lower than the lower limit of the voltage required to drive the motor. As the motor is coasting, the rotational speed of the motor gradually decreases due to air resistance and friction. The cumulative loss hardly increases.

[0110] In a conventional motor drive device, the capacitor is pre-charged until the DC link voltage reaches the pre-charge completion voltage before decelerating a coasting motor. In the example shown in FIG. 8 , the pre-charge operation for the capacitor is initiated at time t1. During the pre-charge period from time t1 to time t2, the DC link voltage rises, and the cumulative loss also increases due to the power loss caused by the pre-charge operation. The motor continues coasting during this period. As described above, in a conventional motor drive device, the capacitor is pre-charged until the DC link voltage reaches the pre-charge completion voltage before decelerating a coasting motor. In contrast, according to the second embodiment and its modified example of the present disclosure, the pre-charge operation is only performed until the DC link voltage reaches the lower limit of the voltage required to drive the motor 3, as described with reference to FIG. 7 . Therefore, the pre-charge operation period is shorter than in a conventional motor drive device that performs the pre-charge operation until the DC link voltage reaches the pre-charge completion voltage, thereby reducing power loss. Furthermore, the motor drive device 1 according to the second embodiment and its modified example of the present disclosure performs pre-charging for a shorter period of time than conventional motor drive devices, and therefore the DC link voltage is less likely to exceed the regenerative braking start voltage, and therefore the regenerative braking control unit 22 is less likely to control the power regeneration operation of the converter 11.

[0111] At time t2, the DC link voltage exceeds the pre-charge completion voltage, completing the pre-charge operation for the capacitor, and starting to control the inverter's power conversion operation so that the coasting motor decelerates. The regenerative power of the motor generated by the motor deceleration control is converted into DC power by the inverter and returned to the DC link, causing the DC link voltage to rise. Furthermore, the power conversion operation of the inverter causes power loss, resulting in an increase in the cumulative loss.

[0112] When the regenerative power from the motor is large, the DC link voltage continues to rise and exceeds the regenerative braking start voltage, so at time t3 the converter starts regenerative power operation or the discharge resistor starts discharging. The converter continues regenerative power operation or the discharge resistor continues discharging until time t4, when the DC link voltage drops to the regenerative braking end voltage. The cumulative loss increases due to the power loss caused by the inverter's power conversion operation and the converter's regenerative power operation or the discharge resistor's discharging operation.

[0113] The deceleration control of the motor is performed until time t4 when the rotation speed of the motor falls below the threshold for detecting that the motor has stopped. At time t3, the rotation speed of the motor falls below the threshold for detecting that the motor has stopped, so the deceleration control of the motor is stopped.

[0114] After time t4, the power consumed by the natural discharge of the capacitor is compensated for by the AC power supply via the converter, thereby maintaining the DC link voltage at the pre-charge completion voltage. The natural discharge of the capacitor gradually increases the cumulative loss.

[0115] <Common to the embodiments and their modifications of the present disclosure> The motor drive device 1 includes at least one processor, which is an arithmetic processing device. Examples of the arithmetic processing device include an IC, an LSI, a CPU, an MPU, and a DSP. The arithmetic processing device includes a control unit 16 and other processing units. Each of these units in the arithmetic processing device is a functional module implemented by a program executed on the processor. For example, if the control unit 16 and other processing units are implemented in a program format, the functions of each unit can be realized by operating the arithmetic processing device in accordance with the program. The program for executing each process in the control unit 16 and other processing units may be provided in a form recorded on a computer-readable recording medium such as a semiconductor memory, a magnetic recording medium, or an optical recording medium. Alternatively, the control unit 16 and other processing units may be implemented as a semiconductor integrated circuit into which a program for implementing the function of each unit is written.

[0116] The motor drive device 1 also includes at least one memory serving as a storage device. The memory includes various storage units within the control unit 16 and other processing units. Examples of the memory include electrically erasable and recordable nonvolatile memory such as EEPROM (registered trademark), or high-speed read / write random access memory such as DRAM or SRAM. The storage device may also include a hard disk drive (HDD) or solid state drive (SSD). The memory stores programs for operating the control unit 16 and other processing units. The memory also stores data related to the pre-charge completion voltage, the regenerative braking start voltage, the regenerative braking end voltage, a threshold for detecting whether the motor 3 is stopped, and a minimum voltage required to drive the motor 3. The memory also stores various programs and data related to the motor drive device 1.

[0117] As described above, according to the first and second embodiments and their modified examples of the present disclosure, it is possible to reduce the power loss that occurs when a coasting motor is decelerated and stopped by a motor drive device.

[0118] In conventional motor drive devices, when a coasting motor is decelerated to a stop, a pre-charging operation is always performed after the motor drive device is restarted. Therefore, if the coasting motor has a large kinetic energy, regenerative power from the decelerating motor is returned to the DC link, potentially causing the DC link voltage to become too high. To prevent the DC link voltage from becoming too high, the DC link voltage is controlled by actively performing regenerative braking, such as by using a converter to regenerate power or by discharging a discharge resistor. Therefore, conventional motor drive devices suffer from significant power loss in the pre-charging circuit and power loss due to regenerative braking. In contrast, according to the first and second embodiments and their modifications of the present disclosure, when a coasting motor is decelerated to a stop after the motor drive device is restarted, if the DC link voltage is equal to or higher than the lower limit of the voltage required to drive the motor, the coasting motor is decelerated without performing a pre-charging operation, thereby eliminating power loss in the pre-charging circuit. Furthermore, since pre-charging is not performed if the DC link voltage is equal to or higher than the lower limit of the voltage required to drive the motor, the DC link voltage is less likely to exceed the regenerative braking start voltage, and therefore the possibility of power loss due to regenerative braking is reduced.

[0119] Furthermore, in conventional motor drive devices, a pre-charging operation is always performed until the DC link voltage reaches the pre-charging completion voltage before deceleration control of a coasting motor, resulting in power loss in the pre-charging circuit. In contrast, according to the first and second embodiments and their modifications of the present disclosure, if the DC link voltage is equal to or higher than the lower limit of the voltage required to drive the motor before deceleration control of the coasting motor, the coasting motor is decelerated without performing a pre-charging operation, so no power loss occurs in the pre-charging circuit.

[0120] Furthermore, in conventional motor drive devices, the capacitor is always pre-charged until the DC link voltage reaches the pre-charge completion voltage before decelerating the coasting motor, which increases the possibility that the DC link voltage will exceed the regenerative braking start voltage due to regenerative power generated during deceleration of the motor, thereby increasing the possibility of power loss due to regenerative braking.In contrast, according to the first and second embodiments and their modifications of the present disclosure, pre-charging is not performed if the DC link voltage is equal to or higher than the lower limit of the voltage required to drive the motor, which reduces the possibility that the DC link voltage will exceed the regenerative braking start voltage and therefore reduces the possibility of power loss due to regenerative braking.

[0121] Furthermore, according to the second embodiment and its modification of the present disclosure, the pre-charging operation is performed only until the DC link voltage reaches a lower limit of the voltage required to drive the motor 3, which is lower than the pre-charging completion voltage. The lower limit of the voltage required to drive the motor 3 is lower than the pre-charging completion voltage. Therefore, according to the second embodiment and its modification of the present disclosure, the period during which the pre-charging operation is performed is shorter than in a conventional motor drive device that performs the pre-charging operation until the DC link voltage reaches the pre-charging completion voltage, thereby reducing power loss. Furthermore, according to the second embodiment and its modification of the present disclosure, the period during which the pre-charging operation is performed is shorter than in a conventional motor drive device, making it less likely that the DC link voltage will reach or exceed the regenerative braking start voltage, and therefore less likely to cause power loss due to regenerative braking.

[0122] Although the present disclosure has been described in detail above, the present disclosure is not limited to the individual embodiments and individual variations described above. Various additions, substitutions, modifications, partial deletions, etc. are possible for these embodiments and variations within the scope of the gist of the present disclosure, or within the scope of the gist of the present disclosure derived from the content of the claims and their equivalents. These embodiments and variations can also be implemented in combination. For example, in the above-described embodiments and variations, the order of each operation and the order of each process are shown as examples and are not limited to these. The same applies when numerical values ​​or mathematical formulas are used in the description of the above-described embodiments and variations.

[0123] <Supplementary Notes> The following supplementary notes are further disclosed regarding the above-described embodiment and modifications.

[0124] (Note 1) An inverter 12 that drives a motor 3; a converter 11 that converts AC power supplied from an AC power source 2 into DC power and supplies the DC power to the inverter 12 via a DC link; a capacitor 13 provided on the DC link; a pre-charging circuit 14 that has a pre-charging resistor 141 that suppresses inrush current during a pre-charging period for the capacitor 13, and a pre-charging switch 142 connected in parallel to the pre-charging resistor 141, the pre-charging circuit 14 performing a pre-charging operation to charge the capacitor 13 with current supplied via the pre-charging resistor 141 while the pre-charging switch 142 is open, and completing the pre-charging operation of the capacitor 13 when the pre-charging switch 142 is switched from open to closed; a voltage detection unit 15 that detects a DC link voltage that is a potential difference between the potential of a positive side power line 41P and the potential of a negative side power line 41N on the DC link; a control unit that controls the inverter to decelerate the motor during coasting when the DC link voltage is equal to or greater than a lower limit of a voltage required to drive the motor, and controls a pre-charging operation by the pre-charging circuit to charge the capacitor when the DC link voltage is less than the lower limit. (Supplementary Note 2) The motor drive device according to Supplementary Note 1, further comprising a speed determination unit that determines whether the rotational speed of the motor during coasting is greater than a threshold for detecting that the motor is stopped, wherein the determination by the control unit whether the DC link voltage is greater than or equal to the lower limit or less than the lower limit is made when the speed determination unit determines that the rotational speed of the motor during coasting is greater than the threshold. (Supplementary Note 3) The motor drive device according to Supplementary Note 1, wherein, when the DC link voltage is less than the lower limit, the control unit controls the pre-charging operation by the pre-charging circuit to charge the capacitor until the DC link voltage becomes equal to or greater than a pre-charging completion voltage of the capacitor. (Supplementary Note 4) The motor drive device 1 according to Supplementary Note 1, wherein, when the DC link voltage is less than a lower limit value, the control unit 16 controls the pre-charging operation by the pre-charging circuit 14 to charge the capacitor 13 until the DC link voltage becomes equal to or greater than the lower limit value.(Supplementary Note 5) The motor drive device 1 according to Supplementary Note 1, wherein, when the controller 16 is controlling the inverter 12 to decelerate the coasting motor 3, if the DC link voltage becomes equal to or higher than a regenerative braking start voltage, the controller 16 starts controlling the converter 11 to convert DC power in the DC link into AC power and return it to the AC power source 2. (Supplementary Note 6) The motor drive device 1 according to Supplementary Note 5, wherein, when the controller 16 is controlling the converter to return regenerative power generated by the motor 3 to the AC power source 2, if the DC link voltage becomes equal to or lower than a regenerative braking end voltage, the controller 16 stops controlling the converter 11 to convert DC power in the DC link into AC power and return it to the AC power source 2. (Supplementary Note 7) The motor drive device 1 according to Supplementary Note 1, further comprising a discharge resistor 31 provided in the DC link, and wherein, when the controller 16 is controlling the inverter 12 to decelerate the coasting motor 3, if the DC link voltage becomes equal to or higher than a regenerative braking start voltage, the controller 16 starts controlling the discharge resistor 31 to consume the DC power in the DC link. (Supplementary Note 8) The motor drive device 1 according to Supplementary Note 7, wherein the control unit 16 terminates control of discharging DC power in the DC link through the discharge resistor 31 when the DC link voltage becomes equal to or lower than a regenerative braking end voltage, in a case where the control unit 16 is controlling regenerative power generated by the motor 3 to be consumed through the discharge resistor 31. (Supplementary Note 9) The motor drive device 1 according to Supplementary Note 2, wherein when the speed determination unit 17 determines that the rotational speed of the motor 3 during coasting is equal to or lower than a threshold, the control unit 16 controls a pre-charging operation by the pre-charging circuit 14 to charge the capacitor 13 until the DC link voltage becomes equal to or higher than a pre-charging completion voltage of the capacitor 13.

[0125] REFERENCE SIGNS LIST 1 Motor drive device 2 AC power supply 3 Motor 11 Converter 12 Inverter 13 Capacitor 14 Pre-charging circuit 15 Voltage detection unit 16 Control unit 17 Speed ​​determination unit 18 Speed ​​detection unit 19 Speed ​​control unit 20 Electromagnetic contactor 21 Pre-charging control unit 22 Regenerative braking control unit 30 Discharge circuit 31 Discharge resistor 32 Discharge switch 41N Negative side power line 41P Positive side power line 101 Converter control unit 102 Motor control unit 103 Open / close control unit 141 Pre-charging resistor 142 Pre-charging switch

Claims

1. A motor drive device comprising: an inverter for driving a motor; a converter for converting AC power supplied from an AC power source into DC power and supplying the DC power to the inverter via a DC link; a capacitor provided on the DC link; a pre-charging circuit having a pre-charging resistor for suppressing inrush current during a period in which the capacitor is pre-charged, and a pre-charging switch connected in parallel to the pre-charging resistor, the pre-charging circuit performing a pre-charging operation to charge the capacitor with current supplied via the pre-charging resistor while the pre-charging switch is open, and completing the pre-charging operation of the capacitor when the pre-charging switch is switched from open to closed; a voltage detection unit for detecting a DC link voltage, which is the potential difference between a positive power line and a negative power line in the DC link; and a control unit for controlling the inverter so that the motor decelerates during coasting when the DC link voltage is equal to or greater than a lower limit of a voltage required to drive the motor, and for controlling the pre-charging operation by the pre-charging circuit to charge the capacitor when the DC link voltage is less than the lower limit.

2. A motor drive device as described in claim 1, further comprising a speed determination unit which determines whether the rotational speed of the motor while coasting is greater than a threshold value for detecting that the motor is stopped, and the determination by the control unit of whether the DC link voltage is greater than or equal to the lower limit value or less than the lower limit value is executed when the speed determination unit determines that the rotational speed of the motor while coasting is greater than the threshold value.

3. The motor drive device according to claim 1, wherein, when the DC link voltage is less than the lower limit, the control unit controls the pre-charging operation by the pre-charging circuit to charge the capacitor until the DC link voltage becomes equal to or greater than a pre-charging completion voltage of the capacitor.

4. The motor drive device according to claim 1, wherein, when the DC link voltage is less than the lower limit, the control unit controls the pre-charging operation by the pre-charging circuit to charge the capacitor until the DC link voltage becomes equal to or greater than the lower limit.

5. The motor drive device according to claim 1, wherein, when the control unit is controlling the inverter so as to decelerate the motor during coasting and the DC link voltage reaches or exceeds a regenerative braking start voltage, the control unit starts to control the converter so as to convert the DC power in the DC link into AC power and return it to the AC power source.

6. The motor drive device according to claim 5, wherein, in a state in which the control unit is controlling the converter to return regenerative power generated by the motor to the AC power supply, when the DC link voltage becomes equal to or lower than a regenerative braking end voltage, the control unit terminates control of the converter to convert the DC power in the DC link into AC power and return it to the AC power supply.

7. The motor drive device according to claim 1, further comprising a discharge resistor provided in the DC link, wherein the control unit, while controlling the inverter to decelerate the motor during coasting, starts control so that the DC power in the DC link is consumed by the discharge resistor when the DC link voltage reaches or exceeds a regenerative braking start voltage.

8. The motor drive device according to claim 7, wherein, in a case where the control unit is controlling the regenerative power generated by the motor to be consumed by the discharge resistor provided in the DC link, when the DC link voltage becomes equal to or lower than a regenerative braking end voltage, the control unit terminates controlling the DC power in the DC link to be consumed by the discharge resistor.

9. The motor drive device according to claim 2, wherein, when the speed determination unit determines that the rotational speed of the motor during coasting is equal to or lower than the threshold value, the control unit controls the pre-charging operation by the pre-charging circuit to charge the capacitor until the DC link voltage becomes equal to or higher than a pre-charging completion voltage of the capacitor.

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