Motor control device

Abstract

To provide a motor control device in which a drive circuit can be protected even if a reference current of a protection circuit and an upper limit current allowable for the drive circuit are not matched.SOLUTION: A motor control device 1 according to the present invention includes a three-phase output inverter (drive circuit) 21 that supplies current to a motor M, a protection circuit 4 that determines whether a current is overcurrent on the basis of a detection signal V for detecting a current flowing through the motor M, and an additional circuit 5 that adds a bias voltage Bv to the detection signal V and inputs the resultant signal to the protection circuit 4. The additional circuit 5 adds the bias voltage Bv, which is set to a voltage corresponding to a current equal to or higher than the difference between a current for operating the protection circuit 4 and an upper limit of a current allowable for the three-phase output inverter (drive circuit) 21, to the detection signal V at the time of driving the motor M, offsets the signal, and inputs the final signal VL to the protection circuit 4.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a motor control device.

Background Art

[0002] A motor control device for driving and controlling a motor includes a drive circuit that supplies current to the motor, and a protection circuit that detects an overcurrent flowing through the motor and stops the current supply to the motor.

[0003] More specifically, the protection circuit compares a signal for detecting the current flowing through the motor with a reference voltage, and when the voltage of the signal is higher than the reference voltage, outputs a stop signal, which is a low-level signal, to the drive circuit. The reference voltage is set to the voltage reached by the signal when an overcurrent flowing through the motor flows. The drive circuit includes a plurality of arms connected to the windings of the motor, and switching elements provided on both sides sandwiching the connection points of the arms to the windings. When receiving the stop signal, all the switching elements are turned off to stop the current supply to the motor (see, for example, Patent Document 1).

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] As described above, the conventional motor control device is provided with a protection circuit, which stops the motor when an overcurrent flows through the motor to prevent the motor from burning out. However, the motor, the drive circuit, and the protection circuit may be manufactured by different manufacturers. When the drive circuit and the protection circuit are manufactured by different manufacturers in this way, for example, the current that serves as the criterion for the protection circuit to determine that it is an overcurrent does not match the upper limit value of the current that can flow through the drive circuit. Even if a current greater than the current that the drive circuit can tolerate flows, the protection circuit does not operate, and there is a risk that the switching element in the drive circuit will burn out.

[0006] Therefore, an object of the present invention is to provide a motor control device that can protect the drive circuit even if the reference current of the protection circuit does not match the upper limit current that the drive circuit can tolerate.

Means for Solving the Problems

[0007] To achieve the above object, the motor control device of the present invention includes a drive circuit that supplies current to the motor, a protection circuit that determines whether the current is an overcurrent based on a detection signal for detecting the current flowing through the motor, and an addition circuit that adds a bias voltage to the detection signal and inputs it to the protection circuit. The addition circuit adds a bias voltage set to a voltage corresponding to a current equal to or greater than the difference between the current at which the protection circuit operates and the upper limit of the current that the drive circuit can tolerate to the detection signal during the drive of the motor, and inputs the offset final signal to the protection circuit.

[0008] According to the motor control device configured in this way, even when the current that the drive circuit can tolerate is smaller than the current at which the protection circuit operates, the addition circuit adds a bias voltage corresponding to a current equal to or greater than the difference between the current at which the protection circuit operates and the upper limit of the current that the drive circuit can tolerate to the detection signal and outputs the offset final signal to be input to the protection circuit. Therefore, when a current exceeding the current that the drive circuit can tolerate flows, the protection circuit operates to stop the current supply to the motor and protect the drive circuit.

[0009] In addition, the additional circuit in the motor control device may include a voltage addition line that adds a bias voltage to the detection signal, a switching element provided on the voltage addition line, a comparator that outputs a signal for operating the switching element by comparing a reference voltage and an input voltage, and a voltage dividing circuit that divides the voltage input to the comparator with a resistor and a thermistor and inputs it to the comparator. According to the motor control device configured in this way, since a bias voltage optimal for driving a load whose resistance changes at startup due to temperature changes can be added, the drive circuit can be protected and it becomes optimal for use with a load whose resistance changes at startup due to temperature changes.

Advantages of the Invention

[0010] According to the motor control device of the present invention, the drive circuit can be protected even if the reference current of the protection circuit and the upper limit current allowed by the drive circuit do not match.

Brief Description of the Drawings

[0011]

Figure 1

Figure 2

Figure 3

Modes for Carrying Out the Invention

[0012] Hereinafter, the present invention will be described based on the embodiments shown in the figures. As shown in FIG. 1, the motor control device 1 includes a drive unit 2 having a three-phase output inverter 21 as a drive circuit that supplies current to the motor M, a control unit 3 that gives commands to the drive unit 2, a protection circuit 4 that determines whether the current is an overcurrent based on a detection signal V for detecting the current flowing through the motor M, and an addition circuit 5 that adds a bias voltage Bv to the detection signal V and inputs it to the protection circuit 4. In addition to the above configuration, the motor control device 1 of the present embodiment includes a position detection unit 6 that detects the induced voltage of a coil (not shown) of the motor M and detects the position of a rotor (not shown) of the motor M in order to drive the sensorless motor M.

[0013] Hereinafter, each part of the motor control device 1 of the present embodiment will be described in detail. The motor control device 1 supplies current to the motor M, which is the control target, to drive the motor M. In the present embodiment, the motor M, which is the control target, drives a hydraulic pump P as a load.

[0014] The motor M is a three-phase brushless motor, although not shown in detail, and includes a rotor (not shown). The drive unit 2 includes three legs each having two sets of switching elements corresponding to the three-phase coils of the motor M. By turning on and off the switching elements of each leg, a three-phase output inverter 21 as a drive circuit that connects the power supply 23 and an arbitrary-phase coil to supply current to the coil, and a PWM modulator 22 that outputs a signal for turning on and off each switching element of the three-phase output inverter 21 are provided. When the PWM modulator 22 receives a signal instructing the voltage to be applied to each phase coil of the motor M from the control unit 3, it generates a PWM signal with a duty ratio corresponding to the signal and applies it to the switching elements. When receiving the PWM signal, the three-phase output inverter 21 turns on and off the switching elements and applies the voltage instructed by the control unit 3 to each phase coil to supply current to the coil, for example, for 120-degree conduction drive. Also, a shunt resistor 7 for detecting the current flowing through each phase coil of the motor M is provided between the drive unit 2 and the ground. Note that the motor M that is the control target of the motor control device 1 is not limited to a sensorless motor and may be a motor with brushes. Also, the drive circuit may be a drive circuit suitable for driving the motor M instead of the three-phase output inverter 21 according to the type of the motor M.

[0015] The position detection unit 6 detects the induced voltage generated in a non-energized coil among the three-phase coils when the rotational speed of the rotor of the motor M becomes a predetermined rotational speed or higher, and detects the zero-crossing point of the induced voltage to detect the position of the rotor. When the motor M makes one electrical rotation, there is a timing when each phase coil is not energized twice. Therefore, the position detection unit 6 performs position detection six times in total at 60-degree intervals for the three-phase coils. Specifically, the position detection unit 6 monitors the terminal voltage of each of the three-phase coils, detects the zero-crossing point of the non-energized coil, and detects the position of the rotor. Since the zero-crossing point of the induced voltage appears at the 30-degree point of the 60-degree non-energized period, the position detection unit 6 can, for example, measure the time from the zero-crossing point of a coil to the zero-crossing point of the next coil to create a time corresponding to 30 degrees and generate a signal to notify the energization timing for each coil.

[0016] The position of the rotor detected by the position detection unit 6 is input to the control unit 3. The position detection unit 6 outputs the position of the rotor to the control unit 3 when the rotational speed of the rotor becomes a predetermined rotational speed or higher and the induced voltage generated in the non-energized coil can be stably detected. However, when the rotational speed of the rotor is less than the predetermined rotational speed, the induced voltage cannot be stably detected, so the position of the rotor is not output to the control unit 3.

[0017] The control unit 3 obtains the speed of the motor M from the electrical angle of the rotor obtained from the position detection unit 6, obtains a current command from the deviation between the obtained speed and the speed command, obtains a voltage command from the deviation between the current of each phase coil detected by the potential difference of the shunt resistor 7 and the current command, and converts the voltage command into three-phase voltage commands using the electrical angle of the rotor. Then, the control unit 3 inputs the obtained voltage command for each phase as a command to the drive unit 2. Note that the configuration of the control unit 3 described above is an example, and design changes can be made as long as the motor M can be driven. The control unit 3 may receive an input of a speed command from a higher-level control device and generate a voltage command to be applied to the motor M when obtaining the voltage command from the speed command, or may obtain the voltage command from the torque command when speed control is not required.

[0018] The protection circuit 4 receives the input of the final signal VL, which is a voltage signal obtained by summing a detection signal V for detecting the current flowing through the motor M and a bias voltage Bv added to the detection signal V, and determines whether the current flowing through the motor M is an overcurrent. Thus, the final signal VL input to the protection circuit 4 is a voltage signal offset by the bias voltage Bv from the detection signal V. When the protection circuit 4 determines that the current flowing through the motor M is an overcurrent, it outputs a stop signal that is a high-level signal to the drive unit 2, and when the current is not an overcurrent, it outputs a drive signal that is a low-level signal to the drive unit 2. The drive unit 2 includes, for example, a switch (not shown) between the power supply 23 and the three-phase output inverter 21, and when a stop signal is input from the protection circuit 4, the switch opens to stop the current supply to the motor M. Note that the drive unit 2 may turn off the switching elements of each arm to stop the current supply to the motor M when it receives the stop signal.

[0019] Specifically, the protection circuit 4 is a comparator in which the final signal VL, which is a voltage obtained by adding the detection signal V, which is the potential difference across both ends of the shunt resistor 7 provided between the drive unit 2 and the ground, and the bias voltage Bv loaded by an addition circuit 5 described later, is input to the positive terminal, and a threshold voltage to be compared with the final signal VL is input to the negative terminal.

[0020] The potential difference across both ends of the shunt resistor 7 is proportional to the current flowing through the motor M, and the threshold voltage is set to the potential difference across both ends of the shunt resistor 7 when an overcurrent flows through the motor M. When the potential difference across the shunt resistor 7 exceeds the threshold voltage, the protection circuit 4 determines that an overcurrent has flowed through the motor M and operates to output a stop signal to the drive unit 2. The current flowing through the motor M when the protection circuit 4 operates in this way is used as a reference current for detecting an overcurrent.

[0021] However, the protection circuit 4 receives as an input the final signal VL, which is a voltage signal obtained by adding the bias voltage Bv to the detection signal V, which is the potential difference across both ends of the shunt resistor 7. Therefore, when a current smaller than the current corresponding to the bias voltage Bv by the current serving as the operation reference flows through the motor M, the protection circuit 4 erroneously determines that an overcurrent is flowing through the motor M and operates.

[0022] Specifically, when the final signal VL exceeds the threshold voltage, the protection circuit 4 outputs a stop signal, which is a high-level signal, to the drive unit 2, and when the input detection signal V is equal to or lower than the threshold voltage, the protection circuit 4 outputs a drive signal, which is a low-level signal, to the drive unit 2. When the stop signal is input, the drive unit 2 stops the current supply to the motor M as described above, and when the drive signal is input, the drive unit 2 supplies current to the motor M.

[0023] The addition circuit 5 includes two resistors 53 and 54 installed in series on a voltage addition line 52 that connects the power supply 51 to the drive circuit side of the shunt resistor 7, and a signal line 55 that connects between the resistors 53 and 54 to the positive terminal of the protection circuit 4. With respect to the detection signal V, which is the potential difference of the shunt resistor 7, the voltage created by dividing the voltage of the power supply 51 by the resistors 53 and 54 is added as the bias voltage Bv and input to the positive terminal of the protection circuit 4.

[0024] When a bias voltage Bv is added to the detection signal V by the additional circuit 5, the final signal VL input to the protection circuit 4 becomes a high voltage offset by the bias voltage Bv from the voltage corresponding to the current flowing through the motor M. Therefore, when a current smaller than the current corresponding to the bias voltage Bv by the current corresponding to the threshold voltage flows through the motor M, the protection circuit 4 detects an overcurrent and outputs a stop signal to the drive unit 2. The bias voltage Bv added to the detection signal V is set so that when a current exceeding the upper limit current that the three-phase output inverter 21 as a drive circuit can tolerate flows through the final signal VL input to the protection circuit 4, the overcurrent can be detected and a high-level signal can be output. Therefore, for example, the voltage obtained by subtracting the threshold voltage corresponding to the current at which the protection circuit 4 operates from the detection signal V when the upper limit current that the three-phase output inverter 21 can tolerate flows is used as the bias voltage Bv, and the resistance values of the resistors 53 and 54 are set so that the additional circuit 5 can output the bias voltage Bv. That is, the bias voltage Bv may be set to a voltage corresponding to a current equal to or greater than the difference between the current at which the protection circuit 4 operates and the upper limit of the current that the three-phase output inverter 21 as a drive circuit can tolerate. By setting the bias voltage Bv in this way, it is possible to suppress a current greater than the current that the three-phase output inverter 21 as a drive circuit can tolerate from flowing through itself. In addition, the setting of the bias voltage Bv is performed at the time of design or shipment of the motor control device 1 according to the specifications of the protection circuit 4 and the three-phase output inverter 21 as a drive circuit of different manufacturers.

[0025] Note that in this embodiment, the additional circuit 5 may be grounded to the ground between the resistors 53 and 54 of the voltage addition line 52 via a capacitor in order to remove noise and mitigate fluctuations in the final signal VL input to the protection circuit 4.

[0026] The motor control device 1 configured as described above adds the bias voltage Bv to the detection signal V, which is the potential difference across both ends of the shunt resistor 7, during the driving of the motor M, and outputs the final signal VL offset from the detection signal V and inputs it to the protection circuit 4.

[0027] Then, depending on the height of the bias voltage Bv added to the detection signal V, when a current exceeding the current that the three-phase output inverter 21 can tolerate flows through the motor M even though no current serving as a reference for overcurrent corresponding to the threshold voltage is flowing, the protection circuit 4 outputs a stop signal assuming that an overcurrent is flowing through the motor M. That is, when the bias voltage Bv is added to the detection signal V, although actually only a current corresponding to the potential difference across the shunt resistor 7 flows through the motor M, the protection circuit 4 compares the voltage obtained by adding the bias voltage Bv to the voltage corresponding to the actually flowing current with the threshold voltage, and thus outputs a stop signal when the voltage exceeds the threshold voltage.

[0028] When the final signal VL input to the protection circuit 4 becomes equal to or higher than the threshold voltage, it is regarded as an overcurrent and a stop signal is input to the drive unit 2. Therefore, the current supplied from the drive unit 2 to the motor M is limited to the upper limit value, which is the value obtained by subtracting the current corresponding to the bias voltage Bv from the current corresponding to the voltage threshold.

[0029] As shown by line A in FIG. 2, the upper limit value is a current value corresponding to the threshold voltage and is equal to or lower than the upper limit of the current that the three-phase output inverter 21 can tolerate (line B in FIG. 2). Thus, by limiting the current supplied to the motor M in this way, the motor control device 1 supplies a current (line D in FIG. 2) that is limited to be smaller than the current (line C in FIG. 2) supplied to the motor M when the addition circuit 5 is not provided, as shown in FIG. 2. FIG. 2 is a diagram showing the transition of the current in one phase of the coil during energization when limiting the current when energizing the three-phase coils of the three-phase output inverter 21. In FIG. 2, the current flowing through the coil reaches the upper limit value and the supply of the current is stopped, the current flowing through the coil is limited to the upper limit value and decreases, after the current decreases, the next PWM signal is turned on and the current is supplied, and again, the situation where the supply of the current is stopped when the current reaches the upper limit value continues.

[0030] Since the motor control device 1 restricts the current flowing through the motor M in this way, it is possible to prevent an overcurrent from flowing through the switching elements of the three-phase output inverter 21, and it is possible to prevent the switching elements from burning out.

[0031] As described above, the motor control device 1 includes a three-phase output inverter (drive circuit) 21 that supplies current to the motor M, a protection circuit 4 that determines whether the current is an overcurrent based on a detection signal V for detecting the current flowing through the motor M, and an addition circuit 5 that adds a bias voltage Bv to the detection signal V and inputs it to the protection circuit 4.

[0032] According to the motor control device 1 configured as described above, even when the current that can be tolerated by the three-phase output inverter 21 as a drive circuit is smaller than the current that serves as a reference for the overcurrent corresponding to the threshold voltage of the protection circuit 4, the addition circuit 5 adds the bias voltage Bv to the detection signal V for the potential difference across both ends of the shunt resistor 7 and outputs the final signal VL that is offset and inputs it to the protection circuit 4. Therefore, when a current exceeding the current that can be tolerated by the three-phase output inverter (drive circuit) 21 flows, the protection circuit 4 operates and outputs a high-level signal to the drive unit 2 to stop the current supply to the motor M and protect the three-phase output inverter (drive circuit) 21. Thus, according to the motor control device 1, even when the current serving as the reference for the protection circuit 4 to operate does not match the upper limit current that the three-phase output inverter (drive circuit) 21 can tolerate, and the upper limit current of the three-phase output inverter (drive circuit) 21 is smaller than the current serving as the reference for the protection circuit 4 to operate, the three-phase output inverter (drive circuit) 21 can be protected.

[0033] Also, the addition circuit 5 in the motor control device 1 of the present embodiment includes a power supply 51 for adding the bias voltage Bv to the detection signal V, and resistors 53 and 54 that divide the voltage of the power supply 51 to create the bias voltage Bv. According to the motor control device 1 configured as described above, it is possible to protect the three-phase output inverter (drive circuit) 21 with an inexpensive circuit configuration without using an expensive amplifier circuit that uses an operational amplifier to input the final signal VL to the protection circuit 4, and since it is provided with a dedicated power supply 51, the addition circuit 5 can be easily added and installed.

[0034] Note that, in the motor control device 1 described above, a predetermined bias voltage is added to the signal and the final signal VL is input to the protection circuit 4. However, the bias voltage Bv may be adjusted according to the temperature. When adjusting the bias voltage according to the temperature, one or both of the resistors 53 and 54 of the addition circuit 5 in the motor control device 1 of one embodiment may be a resistor such as a thermistor whose resistance value changes due to temperature change. If one or both of the resistors 53 and 54 are resistors such as thermistors, the resistance value of one or both of the resistors 53 and 54 changes according to the temperature at the location where the motor control device 1 is installed. Since the motor control device 1 is often installed near the load driven by the motor M, when the motor M starts, the temperature of the resistors 53 and 54 can be regarded as being approximately the same as the temperature of the hydraulic pump P as the load. Therefore, the height of the bias voltage Bv can be adjusted depending on the temperature of the hydraulic pump P.

[0035] When the motor control device 1 configured as described above has the resistor 53 as a thermistor whose resistance decreases as the temperature rises and the resistor 54 as a resistor whose resistance value changes little due to temperature change, the resistance value of the resistor 53 decreases as the temperature rises. Therefore, the bias voltage Bv is increased as the temperature rises. When the temperature of the hydraulic pump P increases, the motor control device 1 can reduce the resistance value of the resistor 53, increase the bias voltage Bv, and drive the motor M while increasing the degree of reduction of the current flowing through the motor M. Therefore, according to the motor control device 1, when the temperature is high, the viscosity of the hydraulic oil in the hydraulic pump P decreases and the resistance of the load decreases. Therefore, the degree of current reduction can be increased to reduce the torque of the motor M and drive the hydraulic pump P as the load with power saving. When the temperature is low, the viscosity of the hydraulic oil in the hydraulic pump P increases and the resistance of the load increases. Therefore, the degree of current reduction can be reduced to increase the torque of the motor M and ensure sufficient torque to drive the hydraulic pump P as the load.

[0036] Furthermore, when the motor control device 1 has the resistor 53 as a resistor with a small change in resistance value due to temperature change and the resistor 54 as a thermistor whose resistance increases as the temperature rises, the resistance value of the resistor 54 increases as the temperature rises, so the bias voltage Bv increases as the temperature rises. When the temperature of the hydraulic pump P rises, the motor control device 1 increases the resistance value of the resistor 54, increases the bias voltage Bv, and can start the motor M while increasing the degree of reduction of the current flowing through the motor M. According to the motor control device 1 configured in this way, when the temperature is high, the viscosity of the hydraulic oil in the hydraulic pump P decreases and the resistance of the load decreases. Therefore, the degree of reduction of the current is increased to reduce the torque of the motor M, and the hydraulic pump P as the load can be driven with power saving. When the temperature is low, the viscosity of the hydraulic oil in the hydraulic pump P increases and the resistance of the load increases. Therefore, the degree of reduction of the current is decreased to increase the torque of the motor M, and sufficient torque can be ensured to drive the hydraulic pump P as the load.

[0037] Also, when the motor control device 1 has the resistor 53 as a thermistor whose resistance decreases as the temperature rises and the resistor 54 as a thermistor whose resistance increases as the temperature rises, the resistance value of the resistor 53 decreases and the resistance value of the resistor 54 increases as the temperature rises, so the bias voltage Bv increases as the temperature rises. When the temperature of the hydraulic pump P rises, the motor control device 1 decreases the resistance value of the resistor 53 and increases the resistance value of the resistor 54, increases the bias voltage Bv, and can start the motor M while increasing the degree of reduction of the current flowing through the motor M. According to the motor control device 1 configured in this way, when the temperature is high, the viscosity of the hydraulic oil in the hydraulic pump P decreases and the resistance of the load decreases. Therefore, the degree of reduction of the current is increased to reduce the torque of the motor M, and the hydraulic pump P as the load can be driven with power saving. When the temperature is low, the viscosity of the hydraulic oil in the hydraulic pump P increases and the resistance of the load increases. Therefore, the degree of reduction of the current is decreased to increase the torque of the motor M, and sufficient torque can be ensured to drive the hydraulic pump P as the load. Thus, when the motor control device 1 is configured to adjust the height of the bias voltage Bv according to the temperature, it is optimal for use with a load whose starting resistance changes due to temperature changes.

[0038] Further, when adjusting the bias voltage according to the outside air temperature and the temperature of the hydraulic pump P as a load, as in the first modified motor control device 1A of the embodiment shown in FIG. 3, a switching element 60 is provided between the power supply 51 and the resistor 53 of the voltage addition line 52 of the addition circuit 5, and the switching element 60 is turned on and off to adjust the height of the bias voltage Bv. A voltage dividing circuit C that changes the duty ratio of the on signal input to the gate of the switching element 60 depending on the temperature may be provided. In the motor control device 1A, a CR circuit composed of a resistor and a capacitor may be provided between the drive circuit side of the shunt resistor 7 and the voltage addition line 52 to reduce the noise of the detection signal V.

[0039] As shown in FIG. 3, the voltage dividing circuit C that inputs a voltage signal that becomes an on signal to the gate of the switching element 60 includes a comparator 61 that inputs a signal to the gate of the switching element 60, and resistors 63 and a thermistor 64 that divide the voltage of the power supply 62. Further, a reference voltage is applied to the minus terminal of the comparator 61, and a voltage obtained by dividing the voltage of the power supply June 2023 10 62 between the resistor 63 and the thermistor 64 is input as a signal to the plus terminal. Note that two resistors for setting the positive feedback gain are provided between the plus terminal of the operational amplifier (not shown) and the plus terminal and the output terminal of the comparator 61, respectively.

[0040] When the voltage input to the plus terminal of the comparator 61 is higher than the reference voltage input to the minus terminal, the comparator 61 outputs a voltage signal that is a high-level signal to the switching element 60 to turn off the switching element 60. When the voltage input to the plus terminal is lower than the reference voltage input to the minus terminal, a voltage signal that is a low-level signal is output to turn on the switching element 60.

[0041] The voltage input to the positive terminal of the comparator 61 changes depending on the temperature by the thermistor 64 whose resistance value increases as the temperature rises. When the temperature rises, the resistance value of the thermistor 64 decreases and the voltage input to the positive terminal becomes lower. Therefore, the higher the temperature, the easier it is for the comparator 61 to output a low-level signal and to turn on the switching element 60. Conversely, the higher the temperature, the easier it is for the comparator 61 to output a high-level signal and to turn off the switching element 60. Thus, also by the motor control device 1A configured in this way, the height of the bias voltage Bv can be adjusted depending on the temperature. When the temperature is high, the viscosity of the hydraulic oil in the hydraulic pump P decreases and the resistance of the load decreases, so the degree of current reduction is increased to reduce the torque of the motor M and drive the hydraulic pump P, which is the load, with power saving. When the temperature is low, the viscosity of the hydraulic oil in the hydraulic pump P increases and the resistance of the load increases, so the degree of current reduction is decreased to increase the torque of the motor M and ensure sufficient torque to drive the hydraulic pump P, which is the load.

[0042] As described above, the additional circuit 5 in the motor control device 1A includes a voltage addition line 52 that adds the bias voltage Bv to the detection signal V, a switching element 60 provided on the voltage addition line 62, a comparator 61 that outputs a signal for operating the switching element 60 by comparing the reference voltage and the input voltage, and a voltage dividing circuit C that divides the voltage input to the comparator 61 with the resistor 63 and the thermistor 64 and inputs it to the comparator 61. According to the motor control device 1A configured in this way, since the additional circuit 5 can adjust the height of the bias voltage Bv according to the temperature around the motor control device 1, it is possible to add an optimal bias voltage Bv for driving a load whose resistance to rotation at startup changes due to temperature changes, so that the three-phase output inverter (drive circuit) 21 can be protected and it becomes optimal for use in a load whose resistance at startup changes due to temperature changes.

[0043] In addition, in the voltage dividing circuit C, the thermistor 64 may be arranged on the power supply 62 side of the resistor 63, and the thermistor 64 may be a thermistor whose resistance increases due to a temperature rise. Also, when the switching element 60 is a switching element that turns on when a high-level signal is input and turns off when a low-level signal is input, the voltage dividing circuit C may increase the voltage input to the comparator 61 due to a temperature rise and decrease the voltage input to the comparator 61 due to a temperature drop. To achieve this, the resistance characteristic of the thermistor 64 with respect to temperature and the connection order of the thermistor 64 and the resistor 63 with respect to the power supply 62 may be determined.

[0044] As described above, the preferred embodiments of the present invention have been described in detail. However, modifications, deformations, and changes are possible without departing from the scope of the claims.

Description of Reference Numerals

[0045] 1, 1A ··· motor control device, 4 ··· protection circuit, 5 ··· additional circuit, 21 ··· three-phase output inverter (drive circuit), 51 ··· power supply, 52 ··· voltage addition line, 53, 54, 63 ··· resistors, 60 ··· switching element, 61 ··· comparator, 64 ··· thermistor, Bv ··· bias voltage, C ··· voltage dividing circuit, M ··· motor, V ··· detection signal, VL ··· final signal

Claims

1. A drive circuit that supplies current to a motor, a protection circuit that determines whether the current is an overcurrent based on a detection signal for detecting the current flowing through the motor, and an addition circuit that adds a bias voltage to the detection signal and inputs it to the protection circuit, wherein the addition circuit adds a bias voltage set to a voltage corresponding to a current equal to or greater than the difference between the current at which the protection circuit operates and the upper limit of the current that the drive circuit can tolerate to the detection signal during driving of the motor, and inputs the offset final signal to the protection circuit A motor control device characterized by the above.

2. The addition circuit includes a voltage addition line that adds a bias voltage to the detection signal, a switching element provided on the voltage addition line, a comparator that outputs a signal for operating the switching element by comparing a reference voltage and an input voltage, and a voltage dividing circuit that divides the voltage input to the comparator with a resistor and a thermistor and inputs it to the comparator The motor control device according to claim 1, characterized by the above.

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