Inverter system with motor insulation checking function

Abstract

An inverter system includes a converter (3), an inverter (6), a first switch SW1 connected between the converter (3) and an AC power source (1), a capacitor (5) smoothing DC power in a DC bus (4), a resistor Rr connected from a positive voltage side of the DC bus (4) to ground, a second switch SW2 connected between the resistor Rr and ground, and a controller (10) controlling drive of the inverter system. The controller (10) is configured to: turn on the second switch SW2 and obtain a first end voltage E R1 of a resistor Rx when the first switch SW1 is in an off state after charging the capacitor (5); turn on an element connected to a negative voltage side of the DC bus (4) among semiconductor elements of the inverter (6), then obtain a second end voltage E R2 of the resistor Rx; and check an insulation resistance of a motor (7) based on the end voltages E R1 , E R2 .

Description

[0001] Cross-references to related applications

[0002] This application claims priority to Japanese Patent Application No. 2020-183819, filed on November 2, 2020, the entire contents of which (including the description, claims, drawings and abstract) are incorporated herein by reference. Technical Field

[0003] This invention discloses an inverter system that has the function of checking the insulation resistance of a motor to be driven. Background Technology

[0004] In manufacturing facilities, sudden failures can impact productivity, making it desirable to proactively detect any malfunctions and perform preventative maintenance. Particularly in machine tools performing metalworking, the following malfunctions frequently occur: water-based cutting fluids used for machining generate moisture, which adheres to and penetrates the motor, causing insulation degradation. Therefore, it is desirable to inspect and diagnose the motor's insulation resistance.

[0005] In the past, to diagnose motor insulation resistance, the motor was disconnected from the inverter during non-operational periods of the machine so that the motor insulation resistance could be measured using an insulation resistance tester, and this process was performed periodically. However, in recent years, in complex machine structures, performing checks by disconnecting the motor requires a significant amount of work and time, inevitably reducing productivity.

[0006] To reduce the workload of motor insulation testing, a technique was designed to measure insulation resistance using the functions of an inverter. For example, in Patent Document 1, a voltage divider circuit (such as one configured with resistors R1 and R2 and connected from the negative voltage side of the inverter's DC bus) is described. Figure 4 (As shown) the inverter is coupled to ground via switch SW2, and at this time, by simultaneously turning on the semiconductor elements constituting the inverter, a closed circuit including the motor insulation resistance Rx is formed, and the voltage appearing in the voltage divider circuit is detected. At this time, by opening switch SW1, the inverter circuit is disconnected from the AC power supply.

[0007] exist Figure 4 In this circuit, when switch SW2 and any semiconductor element located on the upper side of the inverter are connected, current flows through resistor R1 via the following path: positive voltage side of the DC bus → inverter element → motor circuit → motor insulation resistance Rx → ground → resistor R2 → switch SW2 → resistor R1 → negative voltage side of the DC bus. By detecting the voltage appearing in resistor R1, the value of motor insulation resistance Rx, which is the target for measurement, can be derived.

[0008] However, in practical inverter systems, the motor insulation resistance Rx cannot be accurately calculated due to leakage currents from the semiconductor elements constituting the inverter and converter, as well as leakage currents caused by other components within the inverter system. In particular, measurement accuracy tends to deteriorate in systems where multiple inverters are connected to a single DC bus to control multiple motors. This problem is addressed by referring to... Figure 5 Let me explain. Motor A is the target motor being measured. One of the upper semiconductor elements of inverter A is turned on to apply the voltage from the positive side of the DC bus to motor A. Regarding motor B, although all semiconductor elements of inverter B are off, the leakage current I... L1 I L2 I L3 The current flows through these semiconductor elements, and through the insulation resistance Rx2 of motor B to resistor R1. Therefore, the voltage appearing in resistor R1 reflects not only the insulation resistance Rx1 of motor A, but also the error component caused by the insulation resistance Rx2 of motor B.

[0009] To solve this problem, for example, in Patent Document 2, the negative side voltage of the DC bus is applied to the motor B by turning on all the lower semiconductor elements of the inverter B. When the switch SW2 is turned on, the voltage applied to the motor is set to a potential close to ground, thereby preventing the flow of leakage current.

[0010] Reference List

[0011] Patent documents

[0012] Patent Document 1: JP2009-204600A

[0013] Patent Document 2: JP2015-169479A

[0014] Patent Document 3: WO2013 / 018411A

[0015] While patent documents 1 and 2 disclose techniques for eliminating the influence of insulation resistance in any motor other than the target motor being measured, in actual inverter systems, there are additional factors that degrade the accuracy of insulation resistance measurements. These factors are addressed by referring to... Figure 3 Let me explain. Figure 3 The inverter system shown is configured such that converter 3 can perform reversible conversion using semiconductor elements (e.g., IGBTs) and is used to recover regenerated power from the load motor 7. Power regeneration is indispensable for applications that perform acceleration and deceleration operations very frequently (e.g., machine tool spindle drive applications), therefore, this type of inverter system is widely used.

[0016] In the converter 3 that performs power regeneration, since current flows through the AC power line via switch control, a line filter 2 is typically inserted between the converter 3 and the AC power supply 1 to smooth the current. For example... Figure 3 As shown, the line filter 2 is typically configured with a reactor and a capacitor, and as a capacitor, it is usually connected in parallel with a discharge resistor of about several hundred kΩ.

[0017] When the insulation resistance measurement methods disclosed in Patent Documents 1 and 2 are applied to, for example... Figure 3 When measuring the inverter system shown, not only is the insulation resistance of the target motor reflected, but the leakage current from the semiconductor elements of converter 3 is also included in the current flowing to the detection resistor Rr via the discharge resistor located in line filter 2, thus introducing an error in the detected voltage. Furthermore, although line filter 2 can be configured without a discharge resistor, in this case, leakage current flows due to the voltage accumulated in the capacitor, which also becomes an error factor. Additionally, the insulation resistance R of the reactor in line filter 2... F The presence of these factors also constitutes another error factor. Therefore, it is evident that the inability to accurately measure the insulation resistance of a motor exists due to these multiple error factors.

[0018] Furthermore, Patent Document 3 discloses that when leakage current from the semiconductor elements constituting the inverter or from the DC bus circuit of the inverter is present, the motor insulation resistance cannot be accurately measured. As a solution to this problem, Patent Document 3 discloses that when measuring the insulation resistance, the measurement is first performed while the semiconductor elements of the inverter are in an off state, and the measurement of the motor insulation resistance is stopped when a predetermined value or higher insulation resistance is detected at that moment. In other words, there is a problem that the motor insulation resistance cannot be measured when the aforementioned error factors are present. Summary of the Invention

[0019] The present invention discloses an inverter system with motor insulation checking function, comprising: a converter that converts AC power supplied from an AC power source into DC power and outputs the DC power to a DC bus; a first switch that connects and disconnects the converter from the AC power source; an inverter that includes multiple semiconductor elements and converts DC power charged on the DC bus into AC power and applies the AC power to the motor; a capacitor that smooths the DC power in the DC bus; a resistor that is connected to ground from either the positive voltage side or the negative voltage side of the DC bus; a second switch that connects and disconnects the connection path between the resistor and ground; a first voltage detection circuit that detects the terminal voltage across the resistor; and a controller that controls the drive of the inverter system. The controller is configured to perform: a charging process, which includes turning on a first switch and charging a capacitor; a first detection process, which includes turning on a second switch after the charging process, while the inverter, converter, and the first switch are all in the off state, and then obtaining the output from the first voltage detection circuit as a first terminal voltage; a second detection process, which includes turning on the second switch after the charging process, while both the converter and the first switch are in the off state, and turning on any element of the inverter's semiconductor components connected to either the positive voltage side or the negative voltage side of the DC bus, and then obtaining the output from the first voltage detection circuit as a second terminal voltage; and a checking process, which includes checking the quality of the motor's insulation resistance based at least on the first terminal voltage and the second terminal voltage, after the first and second detection processes.

[0020] In the above configuration, the inverter system further includes a second voltage detection circuit that detects the voltage of the DC bus. The controller can: in the first detection process, additionally obtain the output from the second voltage detection circuit as the first DC bus voltage; in the second detection process, additionally obtain the output from the second voltage detection circuit as the second DC bus voltage; and in the check process, calculate the insulation resistance of the motor based on the first DC bus voltage, the second DC bus voltage, the first terminal voltage, the second terminal voltage, and the resistance value of the resistor.

[0021] In the above configuration, the controller can calculate the insulation resistance Rx of the motor according to Equation 1 below, where E DC1 E represents the voltage of the first DC bus. DC2 E represents the voltage of the second DC bus. R1 E represents the voltage at the first terminal. R2 Rr represents the voltage at the second terminal, and Rr represents the resistance value of the resistor.

[0022]

[0023] The inverter system with motor insulation testing function disclosed in this invention can accurately measure the insulation resistance of the target motor by eliminating the influence of error factors such as the discharge resistor included in the line filter, the insulation resistance of any motor other than the target motor, and the insulation resistance of the reactor in the line filter. Therefore, the need for periodic motor disconnection and insulation resistance checks for preventative maintenance can be eliminated. Attached Figure Description

[0024] One or more embodiments of the present invention will be described based on the following figures, in which:

[0025] Figure 1 This is a configuration diagram of the inverter system;

[0026] Figure 2 This is a flowchart illustrating the operation for measuring the insulation resistance of a motor;

[0027] Figure 3 This is a diagram explaining the problems in insulation resistance measurement in an inverter system;

[0028] Figure 4 It is a configuration diagram of an inverter system based on the background technology; and

[0029] Figure 5 This is a configuration diagram of another inverter system based on the background technology.

[0030] List of reference numerals

[0031] 1: AC power supply, 2: line filter, 3: converter, 4: DC bus, 5: capacitor, 6: inverter, 7: motor, 8: second voltage detection circuit, 9: first voltage detection circuit, 10: controller. Detailed Implementation

[0032] Figure 1 This is a configuration diagram of the inverter system. AC power supply 1 is connected to line filter 2 via first switch SW1. The three-phase AC power passing through line filter 2 is input to converter 3 to convert the three-phase AC power into DC power, which is output to DC bus 4. Capacitor 5, used to smooth the DC power, is connected to DC bus 4.

[0033] DC bus 4 is connected to the DC input terminal of inverter 6. Inverter 6 converts DC power into AC power by executing the switching control of semiconductor elements, and applies the AC power to motor 7.

[0034] The motor 7 has the following structure: the coil is housed within a metal (i.e., conductive) housing, and insulation between the housing and the coil is provided by insulating material. However, when moisture seeps into the housing due to prolonged use, the insulation resistance decreases, and leakage current flows between the coil and the housing. Figure 1 In this context, the insulation resistance is represented by Rx. Since the housing is fixed to the machine body, the leakage current flowing through the insulation resistance Rx flows accordingly out to the machine body. The controller 10 controls the switching on and off of the first switch SW1 and the second switch SW2, as well as the semiconductor elements constituting the converter 3 and the inverter 6. The controller 10 also performs steps 0 to 4, further explained below, to check the insulation resistance Rx of the motor 7 based on the detection values ​​of the first voltage detection circuit 9 and the second voltage detection circuit 8, described below. The controller 10 is configured, for example, with a computer including a processor and memory.

[0035] In the inverter system described above, to measure the insulation resistance Rx of motor 7, resistor Rr is connected from the negative voltage side of DC bus 4 and coupled to ground (i.e., the machine body) via the second switch SW2. Although resistor Rr is connected to the negative voltage side of DC bus 4 in this example, alternatively, resistor Rr can also be connected to the positive voltage side. When connected to the positive voltage side, equivalent functionality can be achieved by swapping the positive voltage side (or upper side) and the negative voltage side (or lower side) in the operation of the semiconductor elements described below. In the inverter system, a first voltage detection circuit 9 is provided for detecting the voltage across resistor Rr and a second voltage detection circuit 8 is provided for detecting the DC voltage of DC bus 4.

[0036] Now refer to Figure 2 The flowchart describes the operation sequence used to check the insulation resistance Rx.

[0037] <Step 0>

[0038] During the measurement process, firstly, the state of switch SW1 is checked. If the first switch SW1 is already turned on for some reason (e.g., motor 7 is already in operation before the measurement process begins), capacitor 5 is in a state of being charged with DC voltage. In this case, the process is simplified to step 1. On the other hand, if the first switch SW1 is in the off state at the start of the measurement process and capacitor 5 is not being charged, the first switch SW1 is temporarily turned on to charge capacitor 5.

[0039] <Step 1>

[0040] When capacitor 5 is charging, a disconnect signal is then applied to the semiconductor elements of inverter 6 and converter 3, and the first switch SW1 is also disconnected. At this time, capacitor 5 remains charged using DC voltage, which will be used to perform measurement operations.

[0041] <Step 2>

[0042] Next, the second switch SW2 is turned on. At this time, inverter 6 and converter 3 are in the off state, and due to leakage current from the semiconductor elements, the power supply is interrupted. Figure 3 The dashed line indicates that the current flows through resistor Rr. At this time, the second voltage detection circuit 8 detects the DC voltage of DC bus 4 as the first DC bus voltage E. DC1 Furthermore, the first voltage detection circuit 9 detects the voltage across the resistor Rr as the first terminal voltage E. R1 .

[0043] <Step 3>

[0044] Next, an activation signal is applied to any of the upper semiconductor elements of inverter 6, which drives motor 7, the target of measurement. Since the coils in this motor are typically internally connected, simply activating any one of the three phases is sufficient. Subsequently, the voltage across resistor Rr is detected as the second voltage E. R2 And the DC bus voltage is detected as the second DC bus voltage E. DC2 .

[0045] At this time, since the DC voltage of DC bus 4 is applied to motor 7, any current flowing through the insulation resistance Rx flows to resistor Rr. If the insulation resistance Rx is sufficiently high and no current flows through it, the voltage E at the second terminal will be... R2 It becomes equal to the first terminal voltage E detected in step 2 above. R1 On the other hand, as Rx decreases, the current increases, causing E to... R1 <E R2 The condition remains true, and the current flowing through Rx is detected as an increase in current.

[0046] The second DC bus voltage E detected in step 3 DC2 Basically equal to the first DC bus voltage E DC1 However, when the transition from step 2 to step 3 takes a relatively long time, capacitor 5 discharges, causing its DC voltage to decrease, resulting in E... DC1 >E DC2 Keep it true.

[0047] The time difference between steps 2 and 3 is explained below. As mentioned above, the first terminal voltage E detected in step 2... R1This is caused by leakage current from the semiconductor device. It is generally known that leakage current from a semiconductor device varies with device temperature. Therefore, by performing step 2 immediately before step 3, the first terminal voltage E can be detected at approximately the same temperature. R1 Second terminal voltage E R2 Therefore, highly accurate insulation resistance measurements can be performed.

[0048] <Step 4>

[0049] Use the E obtained in the above steps R1 E R2 E DC1 and E DC2 The value of the motor insulation resistance Rx is calculated according to the following equation.

[0050]

[0051] While the above description illustrates an example (where, in step 2, the measurement is performed while all semiconductor elements are in the off state), it is also possible to alternatively apply an on signal to the semiconductor elements located on the negative voltage side of the DC bus. In this case, leakage current from the semiconductor elements can be reduced, thus further improving the measurement accuracy of the insulation resistance Rx. Furthermore, steps 2 and 3 can be performed by swapping their order. In other words, if step 3 is performed before step 2, E is detected first. R2 and E DC2 E was then detected R1 and E DC1 However, the insulation resistance Rx can be calculated in the same way according to the above equation.

[0052] Although the above description mentions examples including either an inverter or a motor, the present invention can be similarly applied to, for example... Figure 5 This is implemented in an inverter system that drives two or more motors. In this case, after performing step 2 once, steps 3 and 4 are repeated a number of times corresponding to the number of motors to calculate the insulation resistance Rx value of each motor. When the number of motors is large, the DC voltage of DC bus 4 gradually decreases during the repeated execution of steps 3 and 4, which leads to a deterioration in measurement accuracy. For example, this decrease in measurement accuracy can be prevented by adding an additional step 2 operation each time a measurement is performed for one motor.

[0053] Furthermore, when it is desirable to simplify the calculation of insulation resistance Rx, only the E obtained in steps 2 and 3 can be used. R1 and E R2 Based on the difference between these values ​​(i.e., E)R2 -E R1 To determine an approximate value for the insulation resistance Rx, that is, due to the difference (E... R2 -E R1 The difference (E) increases as the insulation resistance Rx deteriorates, so the difference (E) can be increased. R2 -E R1 The difference is compared with a predetermined threshold, and when the difference exceeds the threshold, it can be determined that the insulation resistance Rx is insufficient. When using this process, steps 2 and 3 must be performed consecutively within a short period of time. Since any change in the DC bus voltage is sufficiently small during this time, E DC1 ≈E DC2 It can be assumed to remain true.

Claims

1. An inverter system with motor insulation testing function, comprising: A converter that converts AC power supplied from an AC power source into DC power and outputs the DC power to a DC bus; A first switch connects and disconnects the converter from the AC power source; An inverter includes multiple semiconductor elements and converts the DC power charged on the DC bus into AC power and applies the AC power to a motor; A capacitor that smooths the DC power in the DC bus; A resistor that is connected to ground from either the positive or negative voltage side of the DC bus; The second switch connects and disconnects the connection path between the resistor and the ground; A first voltage detection circuit detects the terminal voltage across the resistor. The second voltage detection circuit detects the voltage of the DC bus. as well as The controller controls the drive of the inverter system, wherein The controller is configured to perform: The charging process includes turning on the first switch and charging the capacitor; The first detection process includes: after the charging process, when the inverter, the converter and the first switch are all in the off state, turning on the second switch, and then obtaining the output from the first voltage detection circuit as the first terminal voltage, and additionally obtaining the output from the second voltage detection circuit as the first DC bus voltage. The second detection process includes: after the charging process, when both the converter and the first switch are in an open state, turning on the second switch, and turning on any element of the semiconductor element of the inverter connected to either the positive voltage side or the negative voltage side of the DC bus; then obtaining the output from the first voltage detection circuit as the second terminal voltage, and additionally obtaining the output from the second voltage detection circuit as the second DC bus voltage; and The inspection process includes: checking the quality of the insulation resistance Rx of the motor after the first detection process and the second detection process; The controller calculates the insulation resistance Rx of the motor according to the following equation 1. Among them, E DC1 E represents the voltage of the first DC bus. DC2 E represents the voltage of the second DC bus. R1 E represents the voltage at the first terminal. R2 Rr represents the voltage at the second terminal, and Rr represents the resistance value of the resistor.

2. An inverter system with motor insulation testing function, comprising: A converter that converts AC power supplied from an AC power source into DC power and outputs the DC power to a DC bus; A first switch connects and disconnects the converter from the AC power source; An inverter includes multiple semiconductor elements and converts the DC power charged on the DC bus into AC power and applies the AC power to a motor; A capacitor that smooths the DC power in the DC bus; A resistor that is connected to ground from either the positive or negative voltage side of the DC bus; The second switch connects and disconnects the connection path between the resistor and the ground; A first voltage detection circuit detects the terminal voltage across the resistor. as well as The controller controls the drive of the inverter system, wherein The controller is configured to perform: The charging process includes turning on the first switch and charging the capacitor; The first detection process includes: after the charging process, when the inverter, the converter and the first switch are all in the off state, turning on the second switch, and then obtaining the output from the first voltage detection circuit as the first terminal voltage; The second detection process includes: after the charging process, when both the converter and the first switch are in an open state, turning on the second switch, and turning on any element of the semiconductor element of the inverter connected to either the positive voltage side or the negative voltage side of the DC bus, and then obtaining the output from the first voltage detection circuit as the second terminal voltage; and The inspection process includes: after the first detection process and the second detection process, if the value of the second terminal voltage minus the first terminal voltage exceeds a predetermined threshold, then it is determined that the insulation resistance of the motor is insufficient.

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