Motor drive circuit, injection molding machine therewith, and control method of motor drive circuit

Abstract

To reduce a common mode voltage while suppressing a device from being large-sized in a motor drive circuit in which multiple AC motors are connected in parallel to a common power source.SOLUTION: A motor drive circuit 10 comprises a converter 220, servo amplifiers 161 and 162, servo motors 151 and 152, a voltage detection circuit 230, a compensation circuit 240, and a switching circuit 250. The converter 220 converts an AC voltage inputted from a system power supply 20 to a DC voltage. Each inverter generates a three-phase output AC voltage from the converted DC voltage. The servo motors 151 and 152 are driven by using the output AC voltage from the servo amplifiers 161 and 162. The voltage detection circuit 230 detects a common mode voltage of the inputted AC voltage. The compensation circuit 240 generates a compensation signal for offsetting the common mode voltage detected by the voltage detection circuit 230. The switching circuit 250 switches one of the servo motor 151 or 152 to which the compensation signal should be supplied.SELECTED DRAWING: Figure 2

Description

[Technical field]

[0001] The present disclosure relates to a motor drive circuit, an injection molding machine including the same, and a method for controlling the motor drive circuit. [Background technology]

[0002] Japanese Patent Laid-Open Publication No. 10-94244 (Patent Document 1) discloses a configuration in which an active common mode canceller (ACC: Active Common Mode Noise Canceller) is applied to the main circuit of a system that drives an induction motor with a voltage-type PWM (Pulse Width Modulation) inverter. In the system of Japanese Patent Laid-Open Publication No. 10-94244 (Patent Document 1), it is possible to reduce the common mode voltage that occurs when power semiconductor elements in a power conversion device such as an inverter are switched. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Publication No. 10-94244 Summary of the Invention [Problem to be solved by the invention]

[0004] In industrial machines including injection molding machines, a single machine may be equipped with multiple AC motors as driving actuators. In such machines, the drive devices (inverters) for driving the AC motors may be driven using a DC voltage converted by a common converter.

[0005] Here, when the neutral point of the AC motor is grounded, the potential of the neutral point may change due to a common mode voltage caused by switching of the power semiconductors included in the inverter. Then, due to the electrostatic capacitance in the motor, a potential difference occurs between the inner and outer rings of the bearing that supports the main shaft of the motor. Since the oil film thickness of the grease in the bearing is very thin, the potential difference may cause insulation breakdown of the grease and discharge between the inner and outer rings. This discharge current may cause electrolytic corrosion that damages the metal surface of the rolling surface of the bearing, or the discharge current may cause adverse effects on electronic devices due to radiation noise.

[0006] Such common mode voltages can be reduced by using the ACC described in the above Patent Document 1. However, when the configuration of Patent Document 1 is applied to a configuration in which multiple motors are connected in parallel to a common power supply, it becomes necessary to provide an ACC for each motor individually, which can lead to an increase in the size of the motor drive circuit.

[0007] The present disclosure has been made to solve such problems, and has as its objective to reduce the common-mode voltage in a motor drive circuit in which multiple AC motors are connected in parallel to a common power supply, while suppressing an increase in the size of the device. [Means for solving the problem]

[0008] A motor drive circuit according to an aspect of the present disclosure includes a converter, a plurality of inverters, a plurality of motors driven by the inverters, a voltage detection circuit, a compensation circuit, and a switching circuit. Each of the plurality of inverters converts a DC voltage converted from an input AC voltage by the converter into an output AC voltage to drive a corresponding motor. The voltage detection circuit detects a common mode voltage in the input AC voltage. The compensation circuit generates a compensation signal to offset the detected common mode voltage. The switching circuit switches a motor to which the compensation signal is supplied among the plurality of motors.

[0009] A control method for a motor drive circuit according to another aspect of the present disclosure relates to a control method for a drive circuit that drives multiple motors using a DC voltage converted by a converter, the control method including the steps of (a) detecting a common-mode voltage in an input AC voltage of the converter by a voltage detection circuit, (b) generating a compensation signal for offsetting the common-mode voltage by a compensation circuit, and (c) switching a motor to which the compensation signal is supplied by a switching circuit based on the common-mode voltage and the operating states of the multiple motors. Effect of the Invention

[0010] In the motor drive circuit according to the present disclosure, in a configuration in which multiple motors are connected in parallel to a common power supply, a switching circuit can be used to switch the motor to which a compensation signal for offsetting the common-mode voltage detected on the power supply side is supplied, thereby making it possible to appropriately reduce the common-mode voltage while preventing the device from becoming larger. [Brief description of the drawings]

[0011] [Figure 1] FIG. 1 is a diagram for illustrating a configuration of an injection molding machine equipped with a motor drive circuit according to a first embodiment. [Diagram 2] FIG. 2 is a diagram for explaining a motor drive circuit according to the first embodiment. [Diagram 3] 10A and 10B are diagrams for explaining common mode noise in a motor drive circuit of a comparative example. [Figure 4] 4 is a flowchart illustrating a control method for the motor drive circuit. [Diagram 5] FIG. 11 is a diagram illustrating a motor drive circuit according to a second embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] Hereinafter, embodiments of the present disclosure will be described in detail with reference to the drawings. In the drawings, the same or corresponding parts are designated by the same reference characters and their description will not be repeated.

[0013] [Embodiment 1] (Configuration of injection molding machine) FIG. 1 is a diagram for explaining the configuration of an injection molding machine 100 equipped with a motor drive circuit according to the first embodiment. For convenience of explanation, the floor surface on which the injection molding machine 100 is placed is defined as an XY plane, and the direction perpendicular to the floor surface is defined as a Z-axis direction. The positive direction of the Z-axis may be referred to as the upper side or upward, and the negative direction as the lower side or downward. Although the injection molding machine 100 in the first embodiment is shown as a horizontal injection molding machine, it is not limited to the horizontal type and may be a vertical injection molding machine. In addition, the features of the present disclosure are also applicable to other industrial machines such as machine tools, press machines, extrusion molding machines, and / or laser processing machines, as long as they have a configuration in which a common AC power source is used to drive multiple motors.

[0014] 1, injection molding machine 100 includes a clamping unit 110 for clamping a mold, an injection unit 120 for melting and injecting an injection material, an operation panel 130, a control unit 140, and a servo amplifier 160. In FIG. 1, clamping unit 110 is disposed on the negative side of the X-axis with respect to injection unit 120.

[0015] The mold clamping device 110 includes a bed 111, a fixed platen 112, a mold clamping housing 113, a movable platen 114, tie bars 115, a mold clamping mechanism 116, molds 117 and 118, and a ball screw 119. The bed 111 is placed on the floor surface, and devices such as the fixed platen 112, the mold clamping housing 113, and the movable platen 114 are mounted on the upper surface of the bed 111.

[0016] The fixed platen 112 is fixed to the end of the bed 111 closer to the injection unit 120 (i.e., in the positive direction of the X-axis). The mold clamping housing 113 is disposed at the end of the bed 111 in the negative direction of the X-axis. The fixed platen 112 and the mold clamping housing 113 are connected by a tie bar 115 including a plurality of bars. The mold clamping housing 113 is movable on the bed 111 in the X-axis direction.

[0017] The movable platen 114 is disposed on the bed 111 between the fixed platen 112 and the clamping housing 113. The movable platen 114 is configured to be movable in the X-axis direction. The clamping housing 113 and the movable platen 114 are connected by a clamping mechanism 116. The clamping mechanism 116 has a toggle mechanism. A ball screw 119 is connected to the toggle mechanism, and the movable platen 114 can be moved in the X-axis direction relative to the clamping housing 113 by driving a servo motor 151 disposed in the clamping housing 113 to rotate the ball screw 119. Note that a direct acting cylinder driven by hydraulic pressure may be used as the clamping mechanism 116.

[0018] Molds 117 and 118 are disposed on the movable platen 114 and the fixed platen 112, respectively. The molds 117 and 118 are disposed between the movable platen 114 and the fixed platen 112, facing each other. By moving the mold 117 in the X-axis direction using the mold clamping mechanism 116, the mold 117 and the mold 118 can be brought into close contact with each other, or the mold 117 can be separated from the mold 118. In the following description, the process of shifting the molds 117 and 118 from a separated state to a close contact state is referred to as "mold clamping". Also, the process of shifting the molds 117 and 118 from a close contact state to a separated state is referred to as "mold opening".

[0019] With the mold 117 and the mold 118 in close contact with each other in the mold clamping process, the inside of the mold is filled with molten material (resin), which is then cooled and solidified, thereby molding a product of a desired shape. After the product is molded, with the mold 117 separated from the mold 118 in the mold opening process, the molded product can be removed from the mold 117 by operating an ejection mechanism (not shown) disposed on the movable platen 114. The ejection mechanism is driven by a servo motor 152 disposed on the movable platen 114. The process of removing the product using the ejection mechanism is referred to as the "ejection" process.

[0020] The injection device 120 includes a base 121, a heating cylinder 122, a driving device 124, a hopper 125, a nozzle touch device 127, and a temperature sensor 128. The base 121 is placed on the floor surface on the positive side of the X-axis of the bed 111, and the driving device 124 is mounted on the upper surface thereof. Servo motors 153 and 154 are arranged in the driving device 124.

[0021] The driving device 124 is provided with a heating cylinder 122 extending in the X-axis direction. The heating cylinder 122 includes a heater (not shown) for heating the inside, a screw 123, and an injection nozzle 126. The screw 123 is driven by a servo motor 153 in the driving device 124 and configured to be rotatable around the X-axis direction as a rotation axis. The screw 123 is also configured to be movable in the X-axis direction by a servo motor 154. The injection nozzle 126 is provided at the end of the heating cylinder 122 on the clamping device 110 side (i.e., the end in the negative direction of the X-axis). The heating cylinder 122 heats and melts the bead-shaped resin material fed from the hopper 125, and kneads it using the screw 123 to generate a molten material. The process of melting the resin material in this manner is called a "plasticization" process.

[0022] The nozzle touch device 127 is configured by, for example, a mechanism using a hydraulic cylinder or a mechanism using a ball screw, and connects the drive device 124 and the fixed platen 112 of the mold clamping device 110. When the nozzle touch device 127 is configured by a mechanism using a ball screw, the nozzle touch device 127 is driven by the drive device 124 to move the drive device 124 and the heating cylinder 122 in the X-axis direction. The nozzle touch device 127 brings the injection nozzle 126 into contact with the sprue bush of the mold 118 in the mold clamping device 110, and injects the molten material from the injection nozzle 126, thereby filling the cavity of the mold 117, 118 with the molten material. The servo motor 154 applies pressure to the molten material by moving the screw 123 in the heating cylinder 122 in the negative direction of the X-axis, and injects the molten material into the mold 117, 118 and keeps the pressure of the molten material constant after injection.

[0023] The configuration of the nozzle touch mechanism is not limited to the above-mentioned configuration in which the entire injection device is moved by a ball screw arranged between the fixed platen 112 and the driving device 124, and other configurations may be used. For example, a configuration in which a ball screw is used to connect the device frame and a fixed member at the rear of the heating cylinder, and the heating cylinder itself is moved toward the mold, may be used. Alternatively, a configuration in which a ball screw is used to connect a slide base on which the injection device is mounted and the device frame, and the injection device is moved together with the slide base to bring the injection nozzle into contact with the mold.

[0024] The process of injecting the molten material into the molds 117, 118 is referred to as an "injection" process. After the injection process, the process of cooling the molten material filled in the molds 117, 118 while maintaining a constant pressure is referred to as a "pressure holding" process.

[0025] The temperature sensor 128 is disposed near the injection nozzle 126 of the heating cylinder 122. The temperature sensor 128 detects the temperature of the molten material inside the heating cylinder 122 and outputs the temperature to the control device 140. The control device 140 controls the heater based on the detection value of the temperature sensor 128 to adjust the temperature of the molten material to a desired temperature.

[0026] Once the dwell step is completed, the mold opening and ejection steps are carried out to remove the molded product.

[0027] The injection molding machine 100 can continuously form products by cyclically repeating a mold clamping process, an injection process, a pressure holding process, a plasticizing process, a mold opening process, and an ejection process.

[0028] The base 121 houses therein a control device 140 and a servo amplifier 160 for driving the servo motors 151-154. The control device 140 includes a CPU (Central Processing Unit) 141 and a memory 142. The control device 140 acquires detection values ​​of various sensors arranged in the injection molding machine 100 and comprehensively controls the injection molding machine 100. Although not shown in the figure, the servo amplifier 160 includes a plurality of servo amplifiers provided corresponding to the servo motors 151-154, respectively. In the following description, the servo motors 151-154 will also be collectively referred to as "servo motor 150."

[0029] The operation panel 130 is a device for an operator to operate the injection molding machine 100, and includes a display device such as a liquid crystal display and an input device such as a keyboard. The operation panel 130 is connected to the control device 140, and can acquire and display the status of the injection molding machine 100, and output a user operation signal from the input device to the control device 140. The operation panel 130 may be a touch panel in which the display device and the input device are integrated. The operation panel 130 may be attached to the bed 111 or base 121 of the injection molding machine 100, or may be disposed in a position independent of the injection molding machine 100.

[0030] (Motor drive circuit configuration) Fig. 2 is a diagram for explaining the motor drive circuit 10 in the injection molding machine 100 of Fig. 1. Schematically, the motor drive circuit 10 converts AC power from a system power supply 20, which is a three-phase AC power supply, into DC power, and reconverts the converted DC power into AC power by a servo amplifier 160 to drive a corresponding servo motor 150.

[0031] 2, motor drive circuit 10 includes power supply circuit 200, voltage detection circuit 230, compensation circuit 240, switching circuit 250, and servo amplifier 160. Power supply circuit 200 includes filter circuit 211 and converter 220. Motor drive circuit 10 of the first embodiment has a configuration in which a plurality of servo amplifiers are connected to common power supply circuit 200. In FIG. 2, servo amplifiers 161 and 162 that drive servo motors 151 and 152, respectively, are illustrated as the plurality of servo amplifiers. In motor drive circuit 10, inverters for driving servo motors 153 and 154 other than those described above may be further connected to power supply circuit 200.

[0032] The filter circuit 211 includes inductors Lf, Ls connected in series in each phase of the system power supply 20, and a capacitor Cf connected between the inductors Lf and Ls in each phase. The filter circuit 211 removes high-frequency noise components in the AC power supplied from the system power supply 20.

[0033] The converter 220 is controlled by the control device 140, and converts the AC voltage (input AC voltage) that has passed through the filter circuit 211 into a DC voltage and supplies the DC voltage to the DC bus 260. The converter 220 is a three-phase full-bridge type PWM rectifier, and includes switching elements Q1 to Q6 arranged between the power supply lines PL1 and NL1 of the DC bus 260. The a-phase of the filter circuit 211 is connected to a connection node of the switching elements Q1 and Q2 that are connected in series between the power supply lines PL1 and NL1. The b-phase of the filter circuit 211 is connected to a connection node of the switching elements Q3 and Q4 that are connected in series between the power supply lines PL1 and NL1. The c-phase of the filter circuit 211 is connected to a connection node of the switching elements Q5 and Q6 that are connected in series between the power supply lines PL1 and NL1. The power supply line NL1 is ungrounded or capacitively grounded.

[0034] A smoothing capacitor Cdc is disposed between the power supply lines PL1 and NL1 in the DC bus 260. A terminal voltage VDC of the capacitor Cdc is detected by a voltage sensor (not shown) and output to the control device 140. The voltage of the DC bus 260 is, for example, 350 V.

[0035] Servo amplifiers 161 and 162 are connected in parallel to the DC bus 260. Each servo amplifier is configured by a three-phase full-bridge type inverter. Each servo amplifier is controlled by the control device 140, converts the DC voltage from the DC bus 260 into an AC voltage (output AC voltage), and drives the corresponding servo motor. Each of the servo motors 151 and 152 is an AC motor having a star connection, and its neutral point is connected to a ground voltage. The servo amplifiers 161 and 162 output operation signals SP1 and SP2, respectively, indicating that they are in operation to the control device 140.

[0036] The voltage detection circuit 230 is connected between the filter circuit 211 and the converter 220 in the power supply circuit 200. The voltage detection circuit 230 includes a common mode transformer 231 and a voltage sensor 232.

[0037] Common mode transformer 231 has a primary coil provided in each phase of the power path between filter circuit 211 and converter 220, and a secondary coil coupled to each of the primary coils. In common mode transformer 231, a voltage (common mode voltage) corresponding to common mode noise generated in motor drive circuit 10 is generated across the secondary coil. Voltage sensor 232 detects common mode voltage Vcom generated in the secondary coil, and outputs it to control device 140.

[0038] The compensation circuit 240 is an amplifier circuit that amplifies and outputs the common mode voltage Vcom using DC power from the DC bus 260 as a driving power source. The compensation circuit 240 includes complementary transistors S1 and S2 that configure a push-pull type emitter follower circuit, and capacitors C1 and C2 connected in series. The series-connected capacitors C1 and C2 are connected in parallel to the emitter follower circuit.

[0039] A negative terminal of the secondary coil of the common mode transformer 231 is connected to the control terminals of the transistors S1 and S2. A positive terminal of the secondary coil is connected to the connection node of the transistors S1 and S2 and is connected to the input terminal of the switching circuit 250 via the capacitor C3.

[0040] 2, the switching circuit 250 is a one-input two-output type (1P2T type) switch, and is switched by a control signal SEL from the control device 140. The two output terminals of the switching circuit 250 are connected to the neutral points of the servo motors 151 and 152, respectively. When n servo motors are connected in parallel to the converter 220, the switching circuit 250 is a 1PnT type switch.

[0041] When the common mode voltage Vcom is detected by the voltage sensor 232, the control device 140 switches the switching circuit 250 to the servo motor in operation based on the operation signals SP1, SP2 from the servo amplifiers 161, 162. In this manner, the control device 140 supplies the compensation signal generated by the compensation circuit 240 to the motor that is causing the common mode voltage to occur.

[0042] In the motor drive circuit 10 of FIG. 2, the voltage detection circuit 230, the compensation circuit 240 and the switching circuit 250 configure the active common mode canceller (ACC) in the first embodiment.

[0043] (Common mode noise explanation) Here, the occurrence of common mode noise and its effects will be described. Fig. 3 is a diagram for explaining common mode noise in a motor drive circuit 10X of a comparative example. The motor drive circuit 10X in Fig. 3 is configured such that a power supply circuit 200 is provided individually for each servo motor 151.

[0044] In such a motor drive circuit 10X, the potential of the neutral point of the servo motor 151 may fluctuate due to a common mode voltage caused by the switching operation of the converter 220 or the servo amplifier 161. When this fluctuation in the potential of the neutral point occurs, a common mode current flows from the neutral point of the servo motor 151 through the ground potential to the neutral point of the system power supply 20, and returns to the servo motor 151 via the converter 220 and the servo amplifier 161, as shown by the arrow AR1 in FIG.

[0045] At this time, a potential difference occurs between the inner and outer rings of the bearing supporting the main shaft of the servo motor 151, but because the thickness of the grease film in the bearing is very thin, this potential difference can cause insulation breakdown in the grease, resulting in a discharge between the inner and outer rings. This discharge current can cause electrolytic corrosion that damages the metal surface of the rolling surface of the bearing, and the discharge current can cause adverse effects on electronic devices due to radiation noise.

[0046] In order to reduce such common mode noise, an ACC may be used. In the case of a motor drive circuit 10X configured to drive individual motors as in the comparative example of Fig. 3, common mode noise occurs when the target servo motor 151 is driven, so an ACC is provided for each motor as in the above-mentioned Patent Document 1 (JP Patent Publication 10-94244A).

[0047] Here, when a configuration in which an ACC is individually arranged for each motor as in Patent Document 1 is applied to a so-called multi-axis motor drive circuit in which multiple servo motors 151, 152 are connected in parallel to a common power supply circuit 200 as shown in FIG. 2, it is necessary to provide a common mode voltage detection circuit, compensation circuit, and common mode transformer for each motor, which can cause the entire motor drive circuit to become larger.

[0048] Therefore, in the motor drive circuit 10 in the first embodiment, a voltage detection circuit 230 for a common mode voltage and a compensation circuit 240 are arranged for a common power supply circuit 200. Then, the motor being driven is identified from the operating state of each servo motor, and a compensation signal generated by the compensation circuit 240 is supplied to the neutral point of the servo motor identified by the switching circuit 250.

[0049] With this configuration, the common-mode voltage detection circuit and compensation circuit can be standardized to reduce the number of detection circuits and compensation circuits, and the supply path of the compensation signal to each motor can be simplified. Therefore, in a multi-axis motor drive circuit, the common-mode voltage can be reduced while preventing the device from becoming large.

[0050] (Control Method) Next, a method for controlling the ACC in the motor drive circuit 10 of the first embodiment will be described with reference to Fig. 4. Fig. 4 is a flowchart for explaining a method for controlling the motor drive circuit 10. The flowchart shown in Fig. 4 is called from the main routine when a predetermined condition is met, and is executed by the CPU 141 in the control device 140.

[0051] 4, in step (hereinafter, step is abbreviated as S) 100, control device 140 acquires common mode voltage Vcom detected by voltage sensor 232 of voltage detection circuit 230. Then, in S110, control device 140 determines whether or not the detected common mode voltage Vcom is zero.

[0052] If the common mode voltage Vcom is zero (Vcom=0: YES in S110), the control device 140 determines that common mode noise is not occurring, skips the subsequent processing, and returns to the processing of the main routine. On the other hand, if the common mode voltage Vcom is other than zero (Vcom≠0: NO in S110), the control device 140 determines that common mode noise is occurring, and proceeds to the processing of S120.

[0053] In S120, the control device 140 acquires operation information of each servo motor by the operation signals SP1 and SP2 from the servo amplifiers 161 and 162. Then, in S130, the control device 140 determines a compensation target motor to which a compensation signal generated by the compensation circuit 240 should be supplied based on the acquired operation signals SP1 and SP2. Specifically, the control device 140 identifies a currently operating servo motor from information included in the operation information, such as the current value of each servo motor, the amount of change in the encoder, and / or the operation flag, and determines the currently operating servo motor as a compensation target motor. Then, in S140, the control device 140 switches the switching circuit 250 to the compensation target servo motor and supplies a compensation signal to the servo motor. This reduces common mode noise caused by the compensation target motor.

[0054] Here, when multiple servo motors are driven simultaneously, the phase of the common mode voltage detected in the power supply circuit 200 may change depending on the operating state of each servo motor during operation. In other words, the phase of the common mode voltage does not necessarily match the phase of the drive voltage supplied to each servo motor. Therefore, even if the compensation signal generated by the compensation circuit 240 is supplied to multiple servo motors during operation, the common mode voltage may not be properly cancelled out. Therefore, in the first embodiment, the compensation process by the ACC is executed only when any one of the servo motors is operating independently.

[0055] As described above, by performing control according to the flowchart in Fig. 4, it is possible to reduce the common mode voltage in a motor drive circuit having a configuration in which multiple motors are connected to a common power supply circuit, thereby preventing electrolytic corrosion of the motor bearings and reducing the impact on other devices.

[0056] The "servo amplifier 161" and the "servo amplifier 162" in the first embodiment correspond to the "first inverter" and the "second inverter" in this disclosure, respectively. The "servo motor 151" and the "servo motor 152" in the first embodiment correspond to the "first motor" and the "second motor" in this disclosure, respectively.

[0057] [Embodiment 2] In the second embodiment, another configuration example of the voltage detection circuit in FIG. 2 and a method of supplying compensation signals to each servo motor will be described.

[0058] Fig. 5 is a diagram for explaining a motor drive circuit 10A according to a second embodiment. In Fig. 5, voltage detection circuit 230 in Fig. 2 is replaced with a voltage detection circuit 230A, and a common mode transformer for supplying a compensation signal is added to the supply path of the drive voltage to each servo motor. In Fig. 5, the description of elements that overlap with Fig. 2 will not be repeated.

[0059] 5, the voltage detection circuit 230A includes a voltage detection unit 231A and a voltage sensor 232A. The voltage detection unit 231A includes capacitors C11, C12, and C13, one end of which is connected to the a-phase, b-phase, and c-phase of the filter circuit 211, respectively. The other ends of the capacitors C11, C12, and C13 are connected to each other. In other words, the capacitors C11, C12, and C13 are star-connected to the a-phase, b-phase, and c-phase of the filter circuit 211. A connection node of the capacitors C11, C12, and C13 (i.e., the neutral point of the power supply circuit 200) is connected to the control terminals of the transistors S1 and S2 of the compensation circuit 240. The voltage sensor 232A detects a voltage Vcom at the connection node of the capacitors C11, C12, and C13, and outputs the detection signal to the control device 140. When no common-mode noise is generated, the voltage of the connection node becomes zero.

[0060] Common mode transformer 171 is provided in a power path between servo amplifier 161 and servo motor 151. A secondary coil of common mode transformer 171 is provided in each phase of the power path. A positive terminal of a primary coil of common mode transformer 171 is connected to one output terminal of switching circuit 250. A negative terminal of the primary coil of common mode transformer 171 is connected to a connection node of capacitor C1 and capacitor C2 in compensation circuit 240.

[0061] Common mode transformer 172 is provided in a power path between servo amplifier 162 and servo motor 152. A secondary coil of common mode transformer 172 is provided in each phase of the power path. A positive terminal of a primary coil of common mode transformer 172 is connected to the other output terminal of switching circuit 250. A negative terminal of the primary coil of common mode transformer 172 is connected to a connection node of capacitor C1 and capacitor C2 in compensation circuit 240.

[0062] In the second embodiment, similarly to the first embodiment, the common mode voltage generated in the motor drive circuit 10A can be reduced by switching the switching circuit 250 based on the common mode voltage Vcom detected by the voltage detection circuit 230A and the operation signals SP1, SP2 from the servo amplifiers 161, 162 to supply a compensation signal to the common mode transformer corresponding to the servo motor in operation. This makes it possible to prevent electrolytic corrosion of the motor bearings and reduce the impact on other devices.

[0063] The "common mode transformer 171" and the "common mode transformer 172" in the second embodiment correspond to the "first common mode transformer" and the "second common mode transformer" in this disclosure, respectively.

[0064] [Aspects] It will be appreciated by those skilled in the art that the exemplary embodiments described above are examples of the following aspects.

[0065] (Item 1) A motor drive circuit according to one aspect includes a converter, a first inverter, a second inverter, a first motor, a second motor, a voltage detection circuit, a compensation circuit, and a switching circuit. The converter converts an input AC voltage from a three-phase AC power supply into a DC voltage. The first inverter and the second inverter convert the DC voltage to generate a three-phase output AC voltage. The first motor is driven using the output AC voltage from the first inverter. The second motor is driven using the output AC voltage from the second inverter. The voltage detection circuit detects a common mode voltage in the input AC voltage. The compensation circuit generates a compensation signal to offset the common mode voltage detected by the voltage detection circuit. The switching circuit switches between the first motor and the second motor to which the compensation signal is supplied.

[0066] (2) The motor drive circuit according to the first aspect further includes a control device for controlling the switching circuit. The control device switches the switching circuit based on the common mode voltage and the operating states of the first motor and the second motor.

[0067] (Item 3) In the motor drive circuit described in item 2, when a common mode voltage occurs, the control device controls the switching circuit to supply a compensation signal to the motor in operation of the first motor or the second motor.

[0068] (4) In the motor drive circuit according to the third aspect, the first motor and the second motor each have a neutral point connected to a ground voltage. The compensation signal is supplied to the neutral point of the motor selected by the switching circuit.

[0069] (5) The motor drive circuit according to the third aspect further includes a first common mode transformer arranged on a power supply line from the first inverter to the first motor, and a second common mode transformer arranged on a power supply line from the second inverter to the second motor. The compensation signal is supplied to the common mode transformer of the motor selected by the switching circuit.

[0070] (Item 6) In the motor drive circuit according to any one of items 1 to 5, the voltage detection circuit is a common mode transformer.

[0071] (Item 7) In the motor drive circuit according to any one of items 1 to 5, the voltage detection circuit includes first to third capacitors that are star-connected to the three-phase AC power supply.

[0072] (Item 8) An injection molding machine according to one aspect is equipped with the motor drive circuit according to any one of items 1 to 7.

[0073] (Item 9) A control method for a motor drive circuit according to one embodiment includes the following steps. The motor drive circuit includes a converter, a first inverter and a second inverter, a first motor and a second motor, a voltage detection circuit connected to a three-phase AC power supply, a compensation circuit, and a switching circuit. The converter converts an input AC voltage from the three-phase AC power supply into a DC voltage. The first inverter and the second inverter convert the DC voltage to generate a three-phase output AC voltage. The first motor is driven using the output AC voltage from the first inverter. The second motor is driven using the output AC voltage from the second inverter. The above control method includes the steps of (a) detecting a common mode voltage in the input AC voltage by the voltage detection circuit, (b) generating a compensation signal for canceling the common mode voltage by the compensation circuit, and (c) switching the switching circuit based on the common mode voltage and the operating states of the first motor and the second motor.

[0074] The embodiments disclosed herein should be considered to be illustrative and not restrictive in all respects. The scope of the present invention is defined by the claims, not by the description of the embodiments described above, and is intended to include all modifications within the meaning and scope of the claims. [Explanation of symbols]

[0075] 10,10A,10X motor drive circuit, 20 system power supply, 100 injection molding machine, 110 mold clamping device, 111 bed, 112 fixed platen, 113 mold clamping housing, 114 movable platen, 115 tie bar, 116 mold clamping mechanism, 117,118 mold, 120 injection device, 121 base, 122 heating cylinder, 123 screw, 124 drive device, 125 hopper, 126 injection nozzle, 127 nozzle touch device, 128 temperature sensor, 130 operation panel, 140 control device, 141 CPU, 142 memory, 151 to 154 servo motor, 160,161,162 servo amplifier, 171,172 common mode transformer, 200 power circuit, 211 filter circuit, 220 converter, 230,230A Voltage detection circuit, 231 common mode transformer, 231A voltage detection section, 232, 232A voltage sensor, 240 compensation circuit, 250 switching circuit, 260 DC bus, C1 to C3, C11 to C13, Cdc, Cf capacitors, Lf, Ls inductors, PL1, NL1 power supply lines, Q1 to Q6 switching elements, S1, S2 transistors.

Claims

1. A converter that converts an input AC voltage from a three-phase AC power source into a DC voltage; a first inverter and a second inverter for converting the DC voltage to generate a three-phase output AC voltage; a first motor driven by an output AC voltage from the first inverter; a second motor driven by an output AC voltage from the second inverter; a voltage detection circuit for detecting a common mode voltage in the input AC voltage; a compensation circuit that generates a compensation signal for offsetting the common mode voltage detected by the voltage detection circuit; a switching circuit that switches between the first motor and the second motor to which the compensation signal is supplied, the motor drive circuit comprising:

2. A control device for controlling the switching circuit is further provided.

2. The motor drive circuit according to claim 1, wherein the control device switches the switching circuit based on the common mode voltage and operating states of the first motor and the second motor.

3. 3. The motor drive circuit according to claim 2, wherein the control device controls the switching circuit to supply the compensation signal to the operating motor of the first motor and the second motor when the common mode voltage occurs.

4. each of the first motor and the second motor has a neutral point connected to a ground voltage; 4. The motor drive circuit of claim 3, wherein the compensation signal is supplied to a neutral point of the motor selected by the switching circuit.

5. a first common mode transformer disposed in a power supply line from the first inverter to the first motor; a second common mode transformer disposed in a power supply line from the second inverter to the second motor, 4. The motor drive circuit of claim 3, wherein the compensation signal is supplied to a common mode transformer of a motor selected by the switching circuit.

6. 6. The motor drive circuit according to claim 1, wherein the voltage detection circuit is a common mode transformer.

7. 6. The motor drive circuit according to claim 1, wherein the voltage detection circuit includes first to third capacitors that are star-connected to a three-phase AC power supply.

8. An injection molding machine equipped with the motor drive circuit according to any one of claims 1 to 5.

9. A method for controlling a motor drive circuit, comprising the steps of: The motor drive circuit includes: A converter that converts an input AC voltage from a three-phase AC power source into a DC voltage; a first inverter and a second inverter for converting the DC voltage to generate a three-phase output AC voltage; a first motor driven by an output AC voltage from the first inverter; a second motor driven by an output AC voltage from the second inverter; a voltage detection circuit connected to the three-phase AC power supply; A compensation circuit; A switching circuit, (a) detecting a common mode voltage in the input AC voltage by the voltage detection circuit; (b) generating a compensation signal by the compensation circuit to cancel the common mode voltage; (c) switching the switching circuit based on the common mode voltage and operating conditions of the first motor and the second motor.

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