Motor control device and air conditioner

Abstract

A motor control device according to one embodiment of the present invention is provided with a DC voltage detector, a rotation speed detector, a limiting torque calculator, and a rotation speed controller. The DC voltage detector detects DC voltage supplied to an inverter that drives a motor. The rotation speed detector detects the rotation speed of the motor. The limiting torque calculator calculates a limiting torque for the motor on the basis of the DC voltage and the rotation speed. On the basis of the limiting torque, the rotation speed controller controls the rotation speed of the motor so as limit the torque of the motor.

Description

Motor control device and air conditioner 【0001】 The present invention relates to a motor control device for controlling a motor and an air conditioner. 【0002】 When driving a motor in the voltage saturation region (weak magnetic flux control region), if an attempt is made to rotate the motor beyond the limit torque, the control of the motor becomes unstable, and for example, there is a possibility that the motor loses synchronization and stops. Therefore, techniques for stably controlling the motor near the limit torque have been developed. Patent Document 1 describes a method of correcting the input (speed command value) of the speed control of the motor near the limit torque. In this method, as a threshold corresponding to the limit torque, a threshold for the phase angle of the voltage command values of the d-axis and q-axis of the vector control is used. This threshold is set in advance by numerical analysis or actual machine tests. For example, when the phase angle of the voltage command value becomes equal to or greater than the threshold, the input of the speed control is corrected by a speed correction value so that the current command value of the q-axis, which is the output of the speed control, does not increase up to the limit value. Thereby, it has become possible to realize highly stable, highly efficient, and highly responsive control characteristics up to the limit torque of the motor. 【0003】 Japanese Unexamined Patent Application Publication No. 2010-142031 【0004】 Generally, the limit torque of a motor varies according to the DC voltage supplied to the inverter that drives the motor and the rotational speed of the motor. For this reason, for example, as in Patent Document 1, when examining the threshold of the voltage phase corresponding to the limit torque in advance by numerical analysis or actual machine tests, it is necessary to change the DC voltage and the rotational speed respectively to set a map of the threshold, etc., and there was a possibility that a great deal of time would be required for setting the threshold. Also, depending on the accuracy of the map of the threshold, a deviation from the actual limit torque may occur, and it is also conceivable that the motor control becomes unstable. 【0005】 In view of the above circumstances, an object of the present invention is to provide a motor control device and an air conditioner capable of easily realizing a stable high torque output near the limit torque. 【0006】To achieve the above objective, a motor control device according to one embodiment of the present invention comprises a DC voltage detector, a rotational speed detector, a limit torque calculator, and a rotational speed controller. The DC voltage detector detects the DC voltage supplied to an inverter that drives the motor. The rotational speed detector detects the rotational speed of the motor. The limit torque calculator calculates the limit torque of the motor based on the DC voltage and the rotational speed. The rotational speed controller controls the rotational speed of the motor to limit the torque of the motor based on the limit torque. 【0007】 In this motor control device, the DC voltage supplied to the inverter and the motor's rotational speed are detected, and the motor's limit torque is calculated from the detected DC voltage and rotational speed. The rotational speed is then controlled to limit the motor's torque using this limit torque. In this way, the limit torque is updated in accordance with changes in DC voltage and rotational speed, allowing the motor's torque output to be stably increased to near the limit torque according to the operating conditions. Furthermore, since the limit torque is calculated, setting the limit torque does not require a significant amount of time. This makes it easy to achieve stable high torque output near the limit torque. 【0008】 The motor control device may further include an induced voltage command value calculator that calculates an induced voltage command value for the motor based on the DC voltage, the rotational speed, and motor parameters relating to the motor. In this case, the limit torque calculator may calculate the limit torque based on the induced voltage command value. 【0009】 By using the induced voltage command value in this way, it becomes possible to calculate the limit torque with high accuracy. 【0010】 The motor control device may further include a motor parameter modifier that modifies the motor parameters based on the operating state of the motor. 【0011】This allows for the correction of motor parameters that change depending on operating conditions, such as the current flowing through the motor and the motor temperature. As a result, it becomes possible to calculate the limit torque with greater accuracy. 【0012】 The motor control device may further include a rotational speed limiter that limits the rotational speed of the motor based on a torque limit ratio, which is the ratio of the motor's torque command value to the limit torque. 【0013】 For example, the limiting torque constantly changes depending on the operating conditions. Therefore, by using a ratio to the limiting torque rather than the torque itself, it becomes possible to appropriately limit the rotational speed according to the operating conditions. 【0014】 The torque command value may be calculated based on the motor speed command value output from a predetermined controller. In this case, the rotation speed limiter may instruct the predetermined controller to maintain or reduce the rotation speed indicated by the speed command value, based on the torque limit ratio. 【0015】 This suppresses the discrepancy between the speed command value and the actual rotational speed, making it possible to stably control the motor near the limit torque. 【0016】 The inverter may be connected to a converter that outputs a variable DC voltage during the operation of the motor. In this case, the DC voltage detector may sequentially detect the DC voltage output from the converter. 【0017】 For example, if the DC voltage changes due to the converter, the limiting torque may also change significantly. Even in such cases, by sequentially detecting the DC voltage output by the converter, it is possible to achieve a stable high torque output. 【0018】An air conditioner according to one embodiment of the present invention may include a motor control device, an inverter for driving the motor, a converter for supplying a DC voltage to the inverter, a compressor having the motor, an outdoor heat exchanger, an expansion valve, an indoor heat exchanger, and a four-way valve, which are sequentially connected by refrigerant piping in a refrigerant circuit. In this case, the limit torque calculator may calculate the limit torque of the motor based on the DC voltage and the rotational speed when the heating operation is performed at or below a predetermined outside air temperature. 【0019】 When the outside temperature is low, electrical components, including the motor, are cooled, increasing the allowable current flowing through each component. Furthermore, during heating operation at low outside temperatures, the density of the refrigerant drawn into the compressor is lower, resulting in lower compression power (power consumption) per rotation and thus lower current flowing to the motor. Therefore, during heating operation at low outside temperatures, the motor's rotational speed can be increased. Additionally, if the actual room temperature is significantly lower than the room temperature requested by the air conditioner user, the air conditioner operates at maximum output. As a result, during heating operation at low outside temperatures, the motor's rotational speed tends to increase, and the limit torque tends to decrease. Thus, under conditions where the limit torque is low and the motor's rotational speed is likely to increase, the limit torque is more easily reached. Even in such cases, calculating the limit torque based on DC voltage and rotational speed allows for appropriate limiting of the output torque, enabling the efficient and stable achievement of high torque output. 【0020】An air conditioner according to one embodiment of the present invention comprises a refrigerant circuit, an inverter, a converter, and a motor control device. The refrigerant circuit is configured by sequentially connecting a compressor having a motor, an outdoor heat exchanger, an expansion valve, an indoor heat exchanger, and a four-way valve via refrigerant piping. The inverter drives the motor. The converter supplies a DC voltage to the inverter. The motor control device includes a DC voltage detector for detecting the DC voltage, a rotational speed detector for detecting the rotational speed of the motor, a limit torque calculator for calculating the limit torque of the motor based on the DC voltage and the rotational speed, and a rotational speed controller for controlling the rotational speed of the motor to limit the torque of the motor based on the limit torque. 【0021】 As described above, the present invention makes it possible to easily achieve stable high torque output near the limit torque. It should be noted that the effects described herein are not necessarily limited, and any of the effects described in this disclosure may be used. 【0022】 This is a schematic diagram showing an example configuration of an air conditioner equipped with a motor control device according to the first embodiment of the present invention. This is a schematic diagram showing an example configuration of the motor control device according to this embodiment. This is a diagram for explaining an example of operation of the current command value calculator according to this embodiment. This is a diagram for explaining an example of operation of the current command value calculator according to this embodiment. This is a diagram for explaining an example of operation of the voltage phase calculator according to this embodiment. This is a schematic diagram showing an example configuration of a torque command value limiter. This is a diagram for explaining limit processing by limit torque T_limit. This is a schematic diagram showing an example configuration of a motor control device according to the second embodiment. 【0023】 Embodiments of the present invention will be described below with reference to the drawings. 【0024】 <First Embodiment> [Air Conditioner] Figure 1 is a schematic diagram showing an example of the configuration of an air conditioner equipped with a motor control device according to the first embodiment of the present invention. As shown in Figure 1, the air conditioner 100 has an indoor unit 1 and an outdoor unit 2. 【0025】The indoor unit 1 is installed and used in an indoor space within a building. The indoor unit 1 includes an indoor heat exchanger 10, an indoor blower 11, and a controller 12. The outdoor unit 2 is installed and used outdoors of the building where the indoor unit 1 is installed, and is connected to the indoor unit 1 via refrigerant piping that circulates the refrigerant. The outdoor unit 2 includes a compressor 20 with a motor M, an outdoor heat exchanger 21, an expansion valve (pressure reducer) 22, a four-way valve (flow path switch) 23, an outdoor blower 24, and a motor control device 30. 【0026】 The air conditioner 100 also has a refrigerant circuit 25 in which a compressor 20 with a motor M, an outdoor heat exchanger 21, an expansion valve 22, an indoor heat exchanger 10, and a four-way valve 23 are sequentially connected by refrigerant piping. Specifically, the outdoor heat exchanger 21, the expansion valve 22, the indoor heat exchanger 10, and the four-way valve 23 connected to the compressor 20 are connected in a ring in this order. The four-way valve 23 switches whether the refrigerant discharged from the compressor 20 flows to the indoor heat exchanger 10 side or to the outdoor heat exchanger 21 side. 【0027】 For example, during heating operation, high-temperature, high-pressure refrigerant (gas refrigerant) discharged from the compressor 20 of the outdoor unit 2 flows into the indoor heat exchanger 10 of the indoor unit 1 via the four-way valve 23. The high-pressure refrigerant that has exchanged heat with air in the indoor heat exchanger 10 (condenser) condenses and liquefies. Subsequently, the high-pressure liquid refrigerant is depressurized by passing through the expansion valve 22 of the outdoor unit 2, becoming a low-temperature, low-pressure gas-liquid two-phase refrigerant that flows into the outdoor heat exchanger 21. The refrigerant that has exchanged heat with outside air in the outdoor heat exchanger 21 (evaporator) vaporizes. Subsequently, the low-pressure refrigerant is drawn into the compressor 20 via the four-way valve 23. 【0028】 For example, during cooling operation, the high-temperature, high-pressure refrigerant discharged from the compressor 20 of the outdoor unit 2 flows into the outdoor heat exchanger 21 via the four-way valve 23. The high-pressure gaseous refrigerant that has exchanged heat with the outside air in the outdoor heat exchanger 21 (condenser) condenses and liquefies. Subsequently, the high-pressure liquid refrigerant is depressurized by passing through the expansion valve 22 of the outdoor unit 2, becoming a low-temperature, low-pressure gaseous two-phase refrigerant, which flows into the indoor heat exchanger 10 of the indoor unit 1. In the indoor heat exchanger 10 (evaporator), the refrigerant that has exchanged heat with the air evaporates and vaporizes. Subsequently, the low-pressure gaseous refrigerant is drawn into the compressor 20 via the four-way valve 23. 【0029】 The indoor fan 11 draws air into the indoor unit 1 and supplies the air, which has undergone heat exchange with the refrigerant in the indoor heat exchanger 10, to the room. The outdoor fan 24 draws air into the outdoor unit 2 and discharges the air, which has undergone heat exchange with the refrigerant in the outdoor heat exchanger 21, to the outside. 【0030】 The controller 12 controls the operation of the entire air conditioner 100. Specifically, the controller 12 outputs control signals that control the operation of each part, such as the indoor fan 11, the outdoor fan 24, and the motor control device 30. This enables actions such as switching between operating modes (heating operation, cooling operation, etc.) and controlling the operation of the compressor 20 (rotation control of the motor M) according to the set temperature. In this embodiment, the controller 12 corresponds to a predetermined controller. 【0031】 Motor M is a compressor motor that drives the compressor 20. As motor M, a permanent magnet synchronous motor (PMSM) is used, which uses permanent magnets in the rotor and windings in the stator. Motor M is provided with three-phase windings (coils) to which three-phase AC voltages output from the IPM 41 (inverter), described later, are applied. Hereinafter, the phases of the three-phase AC will be referred to as U-phase, V-phase, and W-phase. Note that the specific configuration of motor M is not limited, and any type of permanent magnet motor may be used. 【0032】 The motor control device 30 is a device that rotates the motor M based on power supplied from the AC power source 3 and controls the rotational operation of the motor M. Specifically, the motor control device 30 performs vector control of the motor M. In vector control, the current flowing through the windings provided on the stator of the motor M is divided into a current component that generates magnetic flux in the rotor of the motor M (d-axis current) and a current component that generates torque in the rotor (q-axis current), and each current component is controlled independently. In this embodiment, the case in which the motor control device 30 performs vector control of the motor M (compressor motor) that drives the compressor 20 is described, but it is also possible to apply this technology to the vector control of other motors. 【0033】 As shown in Figure 1, the motor control device 30 includes a converter circuit 31, an inverter circuit 32, a calculation circuit 33, a current detector 34, and a DC voltage detector 35. 【0034】 The converter circuit 31 is an AC-DC converter that converts the AC voltage output from the AC power supply 3 into a DC voltage, and supplies the DC voltage to the inverter circuit 32. The converter circuit 31 may be, for example, a circuit that outputs a constant DC voltage, or a circuit that can boost or lower the DC voltage (a circuit with a variable output voltage). The specific configuration of the converter circuit 31 is not limited, and any circuit capable of supplying the DC voltage necessary for the operation of the inverter circuit 32 can be used. In this embodiment, the converter circuit 31 corresponds to a converter. 【0035】 The inverter circuit 32 is a DC-AC converter that converts the DC voltage output from the converter circuit 31 into three-phase AC voltage (three-phase AC voltage of U-phase, V-phase, and W-phase), and supplies the converted three-phase AC voltage to the motor M to drive the motor M. Specifically, the inverter circuit 32 drives the motor M based on the control command value of vector control generated by the calculation circuit 33. Here, the control command value is the command value of the parameters for controlling the motor M. For example, a voltage command value (control command value) that specifies the voltage values ​​to be supplied to each of the U-phase, V-phase, and W-phase of the motor M is input to the inverter circuit 32. The inverter circuit 32 applies the voltage indicated by the voltage command value to each phase. Based on this voltage, current flows to each phase of the motor M, and the motor M is driven. 【0036】 The voltage applied to the motor M by the inverter circuit 32 is controlled using a pulse width modulation (PWM) signal. The voltage command value is a duty cycle command value that specifies the width (duty cycle) of the PWM signal. The specific configuration of the inverter circuit 32 is not limited; for example, any circuit capable of supplying the three-phase AC voltage necessary for the operation of the motor M can be used. In this embodiment, the inverter circuit 32 corresponds to the inverter that drives the motor. 【0037】The arithmetic circuit 33 is a circuit that performs the calculations necessary for controlling the motor M. The arithmetic circuit 33 is constructed using a computer equipped with a CPU (Central Processing Unit) and memory. The arithmetic circuit 33 receives input such as the detected values ​​from the current detector 34 and the DC voltage detector 35, as well as commands and setting values ​​transmitted from the controller 12 installed in the indoor unit 1. Based on these inputs, control command values ​​for performing vector control of the motor M are calculated. Here, duty command values ​​representing the voltage values ​​to be supplied to each phase of the motor M are calculated. 【0038】 The current detector 34 detects the current value of the motor current flowing through the motor M. Here, motor current refers to the drive current that drives the motor M, and is the current flowing through the windings of the motor M. Specifically, the U-phase current, V-phase current, and W-phase current flowing through the windings of the U-phase, V-phase, and W-phase are detected as motor current. Hereafter, the current values ​​of the motor current detected by the current detector 34 will be referred to as the U-phase current value Iu, the V-phase current value Iv, and the W-phase current value Iw. The detection results (Iu, Iv, Iw) of the current detector 34 are output to the calculation circuit 33. 【0039】 The DC voltage detector 35 detects the DC voltage supplied to the inverter circuit 32 (IPM 41, described later) that drives the motor M. In other words, the DC voltage detector 35 also detects the output voltage of the converter circuit 31. Hereafter, the DC voltage supplied to the inverter circuit 32 will be referred to as Vdc. The detection result of the DC voltage detector 35 is output to the calculation circuit 33. As the DC voltage detector 35, any voltage sensor can be used, for example, depending on the magnitude of the DC voltage Vdc. 【0040】 [Motor control device configuration] 【0041】 Figure 2 is a schematic diagram showing an example configuration of the motor control device according to this embodiment. Note that the converter circuit 31 and the DC voltage detector 35 are omitted from the illustration in Figure 2. 【0042】The motor control device 30 calculates the limit torque of the motor M in the voltage saturation region and performs motor control using the calculation result of the limit torque. Here, the voltage saturation region is a high rotation region of the motor M where the output voltage amplitude Va that becomes the amplitude of the voltage command value is saturated and field weakening control is performed. Further, the motor control device 30 performs maximum torque / current control or the like that controls the motor M by varying the output voltage amplitude Va in a region other than the voltage saturation region (hereinafter referred to as the normal control region). 【0043】 As shown in FIG. 2, the motor control device 30 includes a PWM modulation processor 40 and an IPM (Intelligent Power Module) 41. Further, the motor control device 30 includes a shunt resistor 42, current sensors 43a and 43b, and a 3φ current calculator 44. Note that the motor control device 30 may have either the shunt resistor 42 or either one of the current sensors 43a and 43b. 【0044】 Further, the motor control device 30 includes, as functional blocks realized by the arithmetic circuit 33 shown in FIG. 1, a u, v, w / d-q converter 45, an axis error calculator 46, a PLL (Phase Locked Loop) controller 47, a position estimator 48, and a 1 / Pn processor 49. Further, the motor control device 30 includes, as functional blocks realized by the arithmetic circuit 33, a subtracter 50, a speed controller 51, a voltage amplitude adjuster 52, an induced voltage command value calculator 53, a torque command value limiter 54, a current command value calculator 55, a voltage phase calculator 56, a voltage command value calculator 57, and a d-q / u, v, w converter 58. 【0045】 In the present embodiment, the inverter circuit 32 shown in FIG. 1 is configured by the PWM modulation processor 40 and the IPM 41. Note that the PWM modulation processor 40 may be realized by the arithmetic circuit 33. 【0046】 The PWM modulation processor 40 generates a six-phase PWM signal based on the U-phase output voltage command value Vu*, V-phase output voltage command value Vv*, W-phase output voltage command value Vw* output from the d-q / u, v, w converter 58 described later and the PWM carrier signal, and outputs the generated six-phase PWM signal to the IPM. 【0047】 Based on the six-phase PWM signal output from the PWM modulator 40, the IPM41 generates an AC voltage to be applied to each of the U-phase, V-phase, and W-phase of the motor M by converting the DC voltage Vdc supplied from the converter circuit 31, and applies each AC voltage to the U-phase, V-phase, and W-phase of the motor M. 【0048】 The shunt resistor 42 is a resistor element for detecting the motor current in a single shunt method. The current sensors 43a and 43b are current sensors provided on two of the three wirings corresponding to each phase of the motor M (here, the wirings of the U-phase and V-phase), and are configured using, for example, a CT (Current Transformer) or the like. Here, the current detector 34 shown in FIG. 1 is constituted by the motor current sensors (the shunt resistor 42 or the current sensors 43a and 43b) and the three-phase current calculator 44. 【0049】 When the bus current is detected by the single shunt method using the shunt resistor 42, the three-phase current calculator 44 calculates the U-phase current value Iu, V-phase current value Iv, and W-phase current value Iw of the motor M from the six-phase PWM switching information output from the PWM modulator 40 and the detected bus current. Also, when the U-phase current and V-phase current are detected by the current sensors 43a and 43b, the three-phase current calculator 44 calculates the remaining W-phase current value Iw based on Kirchhoff's law of "Iu + Iv + Iw = 0". The three-phase current calculator 44 outputs the calculated current values Iu, Iv, Iw of each phase to the u, v, w / d-q converter 45. Note that the three-phase current calculator 44 may be realized by the arithmetic circuit 33. 【0050】 Based on the electrical angular phase θdq indicating the current rotor position output from the position estimator 48, the u, v, w / d-q converter 45 converts the three-phase U-phase current value Iu, V-phase current value Iv, and W-phase current value Iw output from the three-phase current calculator 44 into two-phase d-axis current Id and q-axis current Iq. Then, the u, v, w / d-q converter 45 outputs the d-axis current Id and q-axis current Iq to the axis error calculator 46, voltage amplitude adjuster 52, and induced voltage command value calculator 53. 【0051】The U-phase current value Iu, V-phase current value Iv, and W-phase current value Iw of motor M are detected values ​​obtained by detecting the current values ​​of each phase of motor M. Therefore, the d-axis current Id and q-axis current Iq, which are obtained by converting the U-phase current value Iu, V-phase current value Iv, and W-phase current value Iw, can also be said to be detected values. In the vector control of motor M, the d-axis current Id and q-axis current Iq are used as detected values ​​of the motor current flowing through motor M. 【0052】 The axis error calculator 46 uses the d-axis voltage command value Vd* and q-axis voltage command value Vq* output from the voltage command value calculator 57, the d-axis current Id and q-axis current Iq output from the u,v,w / d-q converter 45, and the estimated electrical angular velocity ωe output from the PLL controller 47 to calculate the axis error Δθ (the difference between the estimated rotation axis and the actual rotation axis). The axis error calculator 46 then outputs the calculated axis error Δθ to the PLL controller 47. 【0053】 The PLL controller 47 calculates the current estimated angular velocity, which is the electrical angle estimated angular velocity ωe, based on the axis error Δθ output from the axis error calculator 46, and outputs the calculated electrical angle estimated angular velocity ωe to the position estimater 48, the 1 / Pn processor 49, the induced voltage command value calculator 53, the torque command value limiter 54, the current command value calculator 55, the voltage phase calculator 56, and the axis error calculator 46. 【0054】 The position estimator 48 estimates the electrical angular phase θdq based on the estimated electrical angular velocity ωe output from the PLL controller 47. The position estimator 48 then outputs the estimated electrical angular phase θdq to the d-q / u,v,w converter 58 and the u,v,w / d-q converter 45. 【0055】 The 1 / Pn processor 49 calculates the estimated mechanical angular velocity ωm by dividing the estimated electrical angular velocity ωe output from the PLL controller 47 by the number of pole pairs Pn of the motor M, and outputs the calculated estimated mechanical angular velocity ωm to the subtractor 50. 【0056】Thus, the motor control device 30 estimates the electrical angle estimated angular velocity ωe and the mechanical angle estimated angular velocity ωm, which represent the rotational speed of the motor M, based on the motor current detection result. In this disclosure, estimating the rotational speed of the motor M from the motor current detection result is equivalent to detecting the rotational speed of the motor M. In this embodiment, a rotational speed detector for detecting the rotational speed of the motor is configured by a motor current sensor (shunt resistor 42 or current sensors 43a and 43b), a 3φ current calculator 44, a u, v, w / d-q converter 45, an axis error calculator 46, a PLL controller 47, and a 1 / Pn processor 49. 【0057】 The method for detecting the rotational speed of motor M is not limited; for example, motor M may be equipped with a rotational angle sensor (position sensor) and a rotational speed sensor, and these sensors may be used to detect the rotational speed of motor M. 【0058】 The subtractor 50 calculates the angular velocity error Δω by subtracting the current estimated angular velocity, which is the mechanical angular velocity ωm output from the 1 / Pn processor, from the mechanical angular velocity command value ωm* input to the motor control device 30 from an external controller (in this case, the controller 12 shown in Figure 1), and outputs the calculated angular velocity error Δω to the speed controller 51. Here, the mechanical angular velocity command value ωm* is the speed command value of the motor M output from the controller 12. 【0059】 The speed controller 51 generates a torque command value for the motor M (hereinafter referred to as the pre-limit torque command value T*') such that the angular velocity error Δω input from the subtractor 50 approaches zero, and outputs the generated pre-limit torque command value T*' to the torque command value limiter 54. The speed controller 51 is configured, for example, using an integrator and a proportionalizer, and generates the pre-limit torque command value T*' by PI control. 【0060】In this embodiment, the subtractor 50 and the speed controller 51 calculate the torque command value (pre-limit torque command value T*') of the motor M based on the speed command value (mechanical angular velocity command value ωm*) of the motor M output from the controller 12. Furthermore, in the block downstream of the speed controller 51, the rotational speed of the motor M is controlled by controlling the torque command value of the motor M. In other words, the motor control device 30 controls the rotation of the motor M in the dimension of the torque of the motor M. 【0061】 The voltage amplitude regulator 52 calculates a voltage amplitude command value Va* based on the d-axis current Id and q-axis current Iq output from the u,v,w / d-q converter 45. Here, the voltage amplitude command value Va* is the command value of the output voltage amplitude Va. Specifically, it outputs a voltage amplitude command value Va* adjusted so that the d-axis current Id and q-axis current Iq trace the MTPA curve (maximum torque / current control curve). For example, the voltage amplitude regulator 52 calculates the d-axis current Id_mtpa on the MTPA curve from the current q-axis current Iq, and adjusts the voltage amplitude command value Va* by PI control or the like so that there is no error between the calculated d-axis current Id_mtpa and the current d-axis current Id. The voltage amplitude regulator 52 calculates the voltage amplitude command value Va* according to equations (1.1) and (1.2) shown below, for example. 【0062】 In equations (1.1) and (1.2), "Ld", "Lq", and "Ψa" represent the d-axis inductance, q-axis inductance, and armature flux linkage of the motor M, respectively. "Kp" and "Ki" represent the proportional gain and integral gain, respectively. 【0063】Furthermore, if the voltage amplitude command value Va* exceeds the output voltage limit value Vdq_limit, the voltage amplitude regulator 52 limits the voltage amplitude command value Va* to the output voltage limit value Vdq_limit as shown in equation (2) below. Here, the output voltage limit value Vdq_limit is obtained by converting the DC voltage Vdc (detection result of the DC voltage detector 35) supplied from the converter circuit 31 to the IPM 41 into a voltage value in the dq rotating coordinate axis system, which is the control system. The conversion process to the output voltage limit value Vdq_limit may be performed in the DC voltage detector 35 or by a converter (not shown). By limiting the voltage amplitude command value Va* to the output voltage limit value Vdq_limit, flux weakening control is achieved. 【0064】 The induced voltage command value calculator 53 calculates the induced voltage command value Vo* based on the voltage amplitude command value Va* according to the motor model equations shown in equations (3.1) and (3.2), based on the current d-axis current Id and q-axis current Iq and the estimated electrical angle angular velocity ωe. The induced voltage command value Vo* is the command value of the induced voltage Vo of the motor M. The process for calculating the induced voltage command value Vo* is described below. 【0065】 First, the basic equations representing the output voltage amplitude Va and induced voltage Vo are shown. Equations (3.1) and (3.2) are the voltage equations of the PMSM representing the d-axis voltage Vd and q-axis voltage Vq. Equation (4) is the theoretical formula for the output voltage amplitude Va, and equation (5) is the theoretical formula for the induced voltage Vo of the motor M. 【0066】 In equations (3.1) and (3.2), "R" is the resistance value (winding resistance) of the motor M. "p" is a differential operator representing the time derivative, and "p・Ld・Id" and "p・Lq・Iq" (p-term voltage) are terms representing the voltage drop across the inductance due to the change in current. Note that if the current changes of the d-axis current Id and the q-axis current Iq are sufficiently small, the p-term voltage may be ignored. 【0067】From equations (3.1) to (5), the equation relating the voltage amplitude command value Va* and the induced voltage command value Vo* is given by equation (6). The induced voltage command value calculator 53 calculates the induced voltage command value Vo* according to equation (6) and outputs the calculated induced voltage command value Vo* to the torque command value limiter 54 and the current command value calculator 55. 【0068】 For example, near the limit torque where flux weakening control is in operation, the voltage amplitude command value Va* in equation (6) is limited by the voltage amplitude regulator 52 and becomes a voltage equivalent to the DC voltage Vdc (output voltage limit value Vdq_limit) (see equation (2)). That is, the induced voltage command value calculator 53 calculates the induced voltage command value Vo* for the motor M based on the DC voltage Vdc (output voltage limit value Vdq_limit), the rotational speed (electrical angle estimated angular velocity ωe), and the motor parameters related to the motor M. In equation (6), the motor parameters are the resistance value R, d-axis inductance Ld, q-axis inductance Lq, and armature flux linkage Ψa of the motor M. By calculating the induced voltage command value Vo* in this way, the accuracy of calculating the limit torque, which will be described later, can be improved. 【0069】 The torque command value limiter 54 calculates the limit torque of the motor M, limits the pre-limit torque command value T*' using the limit torque, and outputs the torque command value T* to the current command value calculator 55. In this disclosure, the limit torque is the limit value of the torque of the motor M. The limit torque is typically defined by the DC voltage Vdc supplied to the IPM 41 that drives the motor M and the rotational speed of the motor M. 【0070】In this embodiment, the limit torque on the constant induced voltage ellipse is calculated using the induced voltage command value Vo*, the estimated electrical angle angular velocity ωe, and motor parameters. The constant induced voltage ellipse is, for example, the trajectory of the current (the trajectory of the current vector (Id, Iq)) where the induced voltage command value Vo* is equal when the estimated electrical angle angular velocity ωe is kept constant. The trajectory of the current tracing the limit torque that can be output on the constant induced voltage ellipse is generally called the MTPV curve (maximum torque / voltage control curve) or the MTPF curve (maximum torque / magnetic flux control curve). Therefore, the operation of the torque command value limiter 54 is equivalent to limiting the torque of the motor M by the maximum torque on the MTPV curve (MTPF curve). 【0071】 Furthermore, the torque command value limiter 54 sets the rotational speed limit state SPEED_LIMIT_STATE according to the ratio of the torque command value T* to the limit torque, and notifies the higher-level controller 12 of this set value. The specific operation of the torque command value limiter 54 will be explained in detail later with reference to Figure 5, etc. 【0072】 The current command value calculator 55 calculates the q-axis current command value Iq* and the d-axis current command value Id* based on the intersection of the constant torque curve, which is the trajectory of the current where the torque command value T* is constant, and the constant induced voltage ellipse, which is the trajectory of the current where the induced voltage command value Vo* and the estimated electrical angle angular velocity ωe are constant. The current command value calculator 55 outputs the calculated q-axis current command value Iq* and d-axis current command value Id* to the voltage phase calculator 56. 【0073】 The intersection point of the constant torque curve and the constant induced voltage ellipse can be calculated, for example, using the motor torque equation shown in equation (7) and the induced voltage equation shown in equation (8). In equation (7), T is the torque of the motor M, and in equation (8), Vo is the induced voltage of the motor M. 【0074】 By eliminating the d-axis current Id from equations (7) and (8), we can obtain a quartic equation for the q-axis current Iq, as shown in equation (9). In equation (9), "ΔL = Ld - Lq". 【0075】As a solution to the quartic equation shown in equation (9), by applying methods such as Newton's method to the quartic equation shown in equation (9), it is possible to derive a solution corresponding to the q-axis current command value Iq* at the point where the constant torque curve, which is the trajectory of the current where the torque command value T* is constant, and the constant induced voltage ellipse, which is the trajectory of the current where the induced voltage Vo and the estimated angular velocity ωe of the electrical angle intersect. 【0076】 Furthermore, after calculating the q-axis current command value Iq*, the current command value calculator 55 calculates the d-axis current command value Id* based on the q-axis current command value Iq* according to equation (10), which is obtained by transforming the induced voltage equation shown in equation (8) into a d-axis current equation. 【0077】 Here, in equation (10), the choice of whether to select a positive or negative sign for the square root (√) can be determined by calculating the torque at the point where a straight line (hereinafter sometimes referred to as the "M-point boundary line") that is parallel to the Iq axis and passes through point M (-Ψa / Ld, 0), which is the center of the constant induced voltage ellipse, intersects with the constant induced voltage ellipse (hereinafter sometimes referred to as the "M-point boundary torque T_M"), and comparing the M-point boundary torque T_M with the torque command value T*. 【0078】 Figures 3A and 3B illustrate an example of the operation of the current command value calculator according to this embodiment. The procedure for calculating the d-axis current command value Id* and the q-axis current command value Iq* is shown below. 【0079】 The current command value calculator 55 first calculates the d-axis current Id_M at point M according to equation (11). 【0080】 Next, the current command value calculator 55 calculates the q-axis current Iq_M at the point where the M-point boundary line intersects with the constant induced voltage ellipse. The q-axis current Iq_M can be calculated by substituting the d-axis current Id_M at point M into equation (8), and is therefore calculated according to equation (12). 【0081】 Therefore, the current command value calculator 55 calculates the torque T_M on the M point boundary according to equation (13). 【0082】The current command value calculator 55 then determines the d-axis current command value Id* according to equations (14.1) and (14.2) based on the relationship between the torque command value T* and the torque T_M on the boundary of point M. Equation (14.1) shows the d-axis current command value Id* when "torque command value T* ≤ torque T_M on the boundary of point M" (see Figure 3A), and equation (14.2) shows the d-axis current command value Id* when "torque command value T* > torque T_M on the boundary of point M" (see Figure 3B). 【0083】 The current command value calculator 55 outputs the d-axis current command value Id* and the q-axis current command value Iq* calculated as described above to the voltage phase calculator 56. 【0084】 The voltage phase calculator 56 calculates the provisional d-axis voltage command value Vd_m and provisional q-axis voltage command value Vq_m in feedforward according to the motor model equations shown in equations (15.1) and (15.2), based on the estimated electrical angle angular velocity ωe, the d-axis current command value Id*, and the q-axis current command value Iq*, and then calculates the voltage phase command value δ* according to equation (16) based on the provisional d-axis voltage command value Vd_m and provisional q-axis voltage command value Vq_m. 【0085】 In equations (15.1) and (15.2), the voltage drops across the inductance "p・Ld・Id" and "p・Lq・Iq" (p-term voltages) associated with current changes when the d-axis current Id and q-axis current Iq fluctuate due to torque control, for example, for the purpose of vibration suppression, are taken into consideration. However, if the current changes in the d-axis current Id and q-axis current Iq are sufficiently small, the p-term voltages may be ignored. 【0086】Figure 4 is a diagram illustrating an example of the operation of the voltage phase calculator according to this embodiment. Figure 4 shows an output voltage vector 15 having a voltage amplitude command value Va* calculated by the voltage amplitude adjuster 52 according to equations (1.2) and (2). The voltage phase command value δ* calculated by the voltage phase calculator 56 is used as the angle that the output voltage vector 15 makes with the q-axis, i.e., as the phase of the output voltage vector 15. In this way, even in the voltage saturation region where the output voltage amplitude Va is limited to or less than the DC voltage Vdc that the IPM 41 can output (actually, the output voltage limit value Vdq_limit equivalent to the DC voltage Vdc), it is possible to generate a voltage phase command value δ* corresponding to the torque command value T* through calculation processing. The voltage phase calculator 56 outputs the calculated voltage phase command value δ* to the voltage command value calculator 57. 【0087】 The voltage command value calculator 57 calculates the d-axis voltage command value Vd* and the q-axis voltage command value Vq* by performing a coordinate transformation from polar coordinates to Cartesian coordinates according to equations (17.1) and (17.2) based on the voltage amplitude command value Va* and the voltage phase command value δ*, and outputs them to the d-q / u,v,w converter 58 and the axis error calculator 46. 【0088】 The d-q / u,v,w converter 58 converts the two-phase d-axis voltage command value Vd* and q-axis voltage command value Vq* output from the voltage command value calculator 57 into three-phase U-phase output voltage command value Vu*, V-phase output voltage command value Vv*, and W-phase output voltage command value Vw* based on the current rotor position, which is the electrical angular phase (dq-axis phase) θdq, output from the position estimater 48. The d-q / u,v,w converter 58 then outputs the U-phase output voltage command value Vu*, V-phase output voltage command value Vv*, and W-phase output voltage command value Vw* to the PWM modulation processor 40. 【0089】 [Torque Command Value Limiter] Figure 5 is a schematic diagram showing an example of the configuration of a torque command value limiter. As shown in Figure 5, the torque command value limiter 54 includes a limit torque calculator 60, a torque command limit processor 61, a torque limit ratio calculator 62, and a rotational speed limit state determiner 63. 【0090】The torque command value limiter 54 performs processing to limit the torque command value T* in the voltage saturation region where the torque of the motor M takes a value close to the limit torque T_limit. The operation of each part of the torque command value limiter 54 in the voltage saturation region will be described below. 【0091】 The limit torque calculator 60 calculates the limit torque T_limit of the motor M based on the DC voltage Vdc and the rotational speed. The DC voltage Vdc is the DC voltage supplied to the IPM 41 of the inverter circuit 32 and defines the maximum voltage that can be supplied to the motor M. The rotational speed is the current rotational speed of the motor M, and is typically the estimated electrical angular velocity ωe calculated by the PLL controller 47. By using the DC voltage Vdc and the estimated electrical angular velocity ωe, it is possible to calculate the limit torque T_limit according to the operating state of the motor M. 【0092】 In this embodiment, the limit torque calculator 60 calculates the limit torque T_limit based on the induced voltage command value Vo*. Specifically, the limit torque calculator 60 calculates the limit torque T_limit on the constant induced voltage ellipse obtained from the motor parameters, the electrically estimated angular velocity ωe, and the induced voltage command value Vo*. By using the induced voltage command value Vo* in this way, it becomes possible to set the constant induced voltage ellipse with high precision, and thus to calculate the limit torque T_limit with high precision. 【0093】 In the voltage saturation region, the voltage amplitude regulator 52 limits the voltage amplitude command value Va* to the DC voltage Vdc (output voltage limit value Vdq_limit), and the induced voltage command value calculator 53 calculates the induced voltage command value Vo* based on the DC voltage Vdc. By using the induced voltage command value Vo* based on the DC voltage Vdc, the limit torque T_limit corresponding to the DC voltage Vdc can be calculated. The method for calculating the limit torque T_limit will be described below. 【0094】The d-axis current Id and q-axis current Iq on the constant induced voltage ellipse satisfy the induced voltage equation for motor M shown in equation (8). Furthermore, the torque T of motor M is a function of the d-axis current Id and q-axis current Iq, as shown in the torque equation for motor M in equation (7). The limit torque calculator 60 calculates the d-axis current Id and q-axis current Iq that satisfy the relationship in equation (8) and maximize the torque T in equation (7), and calculates the limit torque T_limit from the calculation result. 【0095】 As described in the explanation of the torque command value limiter 54, the trajectories of the currents (d-axis current Id and q-axis current Iq) tracing the limit torque T_limit on the constant induced voltage ellipse are MTPV curves (MTPF curves). Hereinafter, the d-axis current Id and q-axis current Iq that become the limit torque T_limit on the constant induced voltage ellipse will be referred to as Id_mtpv and Iq_mtpv, respectively. 【0096】 In the limit torque calculator 60, the amplitude Ψo of the constant induced voltage ellipse based on the induced voltage command value Vo* is calculated according to equation (18), and the parameter ΔΨd, which is described by the amplitude Ψo of the constant induced voltage ellipse and the motor parameters, is calculated according to equation (19). 【0097】 Using the parameter ΔΨd, the d-axis current Id_mtpv, which gives the limiting torque T_limit on the constant induced voltage ellipse, is calculated according to equation (20). Substituting equation (20) into equation (8), the q-axis current Iq_mtpv, which gives the limiting torque T_limit on the constant induced voltage ellipse, is calculated according to equation (21). 【0098】 By substituting the d-axis current Id_mtpv and q-axis current Iq_mtpv, calculated according to equations (20) and (21), into the torque equation shown in equation (7), the limit torque T_limit is calculated as shown in equation (22). 【0099】The torque command limit processor 61 limits the pre-limit torque command value T*' output from the speed controller 51 by the limit torque T_limit, according to the limit torque T_limit output from the limit torque calculator 60, and outputs it as the final torque command value T*. 【0100】 For example, if the pre-limit torque command value T*' is greater than the limit torque T_limit, the value of the limit torque T_limit is used as the torque command value T*, as shown in equation (23.1). On the other hand, if the pre-limit torque command value T*' is less than or equal to the limit torque T_limit, the value of the pre-limit torque command value T*' is used as the torque command value T*, as shown in equation (23.2). 【0101】 Figure 6 is a diagram illustrating the limiting process using the limiting torque T_limit. The constant induced voltage ellipse 16 shown in Figure 6 is the trajectory of the current vector where the induced voltage command value Vo* based on the DC voltage Vdc is equal, when the estimated electrical angle velocity ωe is kept constant. The point where the constant induced voltage ellipse 16 intersects the maximum torque / magnetic flux control curve is the point where the limiting torque T_limit is obtained. 【0102】 Here, a constant torque curve 17 representing a pre-limit torque command value T*' that is greater than the limit torque T_limit is schematically shown by a dotted curve. In this case, the constant torque curve 17 of the pre-limit torque command value T*' is a curve that does not intersect with the constant induced voltage ellipse 16. 【0103】 The limiting process using the limiting torque T_limit is a process that limits the final torque command value T* by setting T* = T_limit when T*' > T_limit, as shown in equation (23.1). As a result of this process, the constant torque curve 17 (solid line curve) representing the torque command value T* will be tangent to the constant induced voltage ellipse 16 at the point where the constant induced voltage ellipse 16 intersects with the maximum torque / magnetic flux control curve. Thus, the limiting process using the limiting torque T_limit can be said to be a process that reduces the pre-limiting torque command value T*' to the limiting torque T_limit so that the constant torque curve 17 is tangent to the constant induced voltage ellipse 16. 【0104】 Furthermore, as explained with reference to Figure 2, the current command value calculator 55, voltage phase calculator 56, and voltage command value calculator 57, which are located downstream of the torque command value limiter 54, calculate the d-axis voltage command value Vd* and the q-axis voltage command value Vq* that specify the rotational speed of the motor M, based on the final torque command value T* calculated after the limit processing using the limit torque T_limit described above. 【0105】 In this manner, the motor control device 30 controls the rotational speed of the motor M to limit the torque of the motor M based on the limiting torque T_limit. This makes it possible to fully extract the maximum torque that the motor M can output, even in the voltage saturation region where the motor M operates near the limiting torque T_limit. In this embodiment, the rotational speed controller is configured by the torque command limit processor 61 shown in Figure 5, and the speed controller 51, current command value calculator 55, voltage phase calculator 56, and voltage command value calculator 57 shown in Figure 2. 【0106】 If the above limit processing is not performed, the final torque command value T* may exceed the limit torque T_limit, and it is conceivable that there will be no intersection point between the constant torque curve 17 determined by the torque command value T* and the constant induced voltage ellipse 16 determined by the electrically estimated angular velocity ωe and the induced voltage command value Vo*. In this case, there is a possibility that the current command value calculated by the subsequent current command value calculator 55 will not be able to be calculated. In contrast, by performing the above limit processing, an intersection point between the constant torque curve 17 and the constant induced voltage ellipse 16 will always be obtained, thus avoiding a situation where the current command value cannot be calculated. This makes it possible to achieve stable control. 【0107】Furthermore, the limit processing by the torque command limit processor 61 can also be applied to the calculation of the integral term of the speed controller 51. For example, it is expected that the deviation (angular velocity error Δω) input to the speed controller 51 will increase when the torque command value T* is limited. In this case, the speed controller 51, which calculates the pre-limit torque command value T*' by PI control or the like, may experience a wind-up phenomenon where the integral term increases. To avoid such an increase in the integral term, the integral term of the speed controller 51 may be limited using the limit torque T_limit. This makes it possible to prevent the wind-up phenomenon of the speed controller 51. 【0108】 Returning to Figure 5, the torque limit ratio calculator 62 and the rotational speed limit state determination unit 63 limit the rotational speed of the motor M based on the torque limit ratio R_trq_lim, which is the ratio of the torque command value T* of the motor M to the limit torque T_limit. In this embodiment, the torque limit ratio calculator 62 and the rotational speed limit state determination unit 63 constitute a rotational speed limiter. 【0109】 As described above, the torque command value T* is obtained by limiting the pre-limit torque command value T*' by the limit torque T_limit. For example, if T*' ≤ T_limit, the torque command value T* will be the same as the pre-limit torque command value T*'. Therefore, the torque command value T*, like the pre-limit torque command value T*', is a command value calculated based on the motor M speed command value (mechanical angular velocity command value ωm*) output from the controller 12. In this embodiment, as will be explained below, the rotational speed of the motor M is limited by limiting the mechanical angular velocity command value ωm*, which is the basis of the torque command value T*, based on the torque limit ratio R_trq_lim. 【0110】 The torque limit ratio calculator 62 calculates the torque limit ratio R_trq_lim and outputs it to the rotational speed limit state determiner 63. The torque limit ratio R_trq_lim is calculated according to equation (24). 【0111】For example, the limiting torque T_limit constantly changes depending on the operating conditions of the motor M (DC voltage Vdc and rotational speed). Therefore, if the current torque command value T* is used directly as the threshold for determining the limiting torque, it is difficult to limit the rotational speed according to the limiting torque T_limit. In contrast, the torque limiting ratio R_trq_lim shown in equation (24) is a parameter that indicates how large the current torque command value T* is compared to the limiting torque T_limit. In this way, by using a ratio to the limiting torque T_limit rather than the torque command value T* itself, it becomes possible to appropriately limit the rotational speed according to the operating conditions of the motor M. 【0112】 Furthermore, if the torque command value T* is near the torque limit T_limit, a hunting phenomenon may occur in the subsequent rotational speed limit state determination process 63, where the determination result frequently changes. For this reason, the torque limit ratio R_trq_lim may be filtered using an LPF (low-pass filter). This makes it possible to prevent the hunting phenomenon described above. 【0113】 Furthermore, while the torque limit ratio R_trq_lim was calculated here using the torque command value T*, it is also possible to calculate the torque limit ratio R_trq_lim using, for example, the pre-limit torque command value T*'. In either case, the rotational speed limiting operation described below is possible. 【0114】 The rotational speed limit state determination unit 63 compares the torque limit ratio R_trq_lim with the torque limit ratio setting value and determines the rotational speed limit state SPEED_LIMIT_STATE near the torque limit. The determined rotational speed limit state SPEED_LIMIT_STATE is output to the higher-level controller (in this case, the controller 12 of the air conditioner 100). The controller 12 generates the mechanical angular velocity command value ωm* of the motor M based on the rotational speed limit operation described below. 【0115】Here, the torque limit ratio setting value is the threshold value for performing the rotational speed limit operation. From equation (24), R_trq_lim ≤ 1, so the torque limit ratio setting value is set to 1 or less. In this embodiment, two threshold values ​​are set as the torque limit ratio setting value: the upward resistance ratio: R_keep and the reduction ratio: R_down. Note that R_keep < R_down < 1. The table below shows the determination process for rotational speed limit operation. 【0116】 For example, if R_trq_lim ≤ R_keep, the rotational speed limit state is set to "rotational speed limit off" and a control signal (SPEED_LIMIT_STATE) indicating that the rotational speed limit is being released is output. In this case, the controller 12 does not impose any restrictions on the mechanical angular velocity command value ωm*, and normal control is performed to arbitrarily increase or decrease ωm*. 【0117】 Furthermore, if R_keep < R_trq_lim ≤ R_down, the rotational speed limit state is set to "rotational speed limit ON," which limits the rotational speed, and a control signal (SPEED_LIMIT_STATE) is generated indicating that an increase in rotational speed is prohibited. In this case, the controller 12 prohibits control to increase the mechanical angular velocity command value ωm*, and ωm* is maintained. Control to decrease the mechanical angular velocity command value ωm* is still possible. 【0118】 Furthermore, if R_down < R_trq_lim, the rotational speed limit state is set to "rotational speed limit ON," which limits the rotational speed, and a control signal (SPEED_LIMIT_STATE) is generated indicating that the rotational speed should be reduced (decreased). In this case, the controller 12 prohibits control to increase or maintain the mechanical angular velocity command value ωm*, and instead executes control to forcibly decrease the mechanical angular velocity command value ωm*. 【0119】The upward resistance ratio R_keep and the reduction ratio R_down are set as follows, for example: R_keep = 0.95 R_down = 0.99 In this example setting, the interval between R_down and R_keep is larger than the interval between the maximum value of R_trq_lim (=1) and R_down. This makes it possible to avoid the phenomenon of speed command hunting caused by the speed limit state determined by the torque limit ratio R_trq_lim hunting. Note that the specific values ​​of the torque limit ratio settings (R_keep and R_down) are not limited to the above example. For example, R_keep and R_down may be adjusted by tuning, etc. 【0120】 In this way, the rotational speed limiting state determination unit 63 instructs the controller 12 to maintain or decelerate the rotational speed indicated by the mechanical angular velocity command value ωm* based on the torque limit ratio R_trq_lim. This suppresses the discrepancy between the mechanical angular velocity command value ωm* and the actual rotational speed, making it possible to stably control the motor near the limit torque T_limit. 【0121】 Up to this point, the operation of the motor control device 100 in the voltage saturation region has been mainly described. On the other hand, in the normal control region, the voltage amplitude adjuster 52 calculates a voltage amplitude command value Va* adjusted so that the d-axis current Id and q-axis current Iq trace the MTPA curve (maximum torque / current control curve), and the voltage amplitude command value Va* is output as is without being limited by the DC voltage Vdc (output voltage limit value Vdq_limit). As a result, in the normal control region, maximum torque / current control is performed using a voltage amplitude command value Va* that is less than or equal to the output voltage limit value Vdq_limit. 【0122】 Furthermore, in the normal control domain, it is not always necessary to perform limiting processing for the torque command value T*. Therefore, in the normal control domain, for example, the torque command value limiter 54 may not be operated, and no limiting processing for the torque command value T* may be performed. In this case, the pre-limiting torque command value T*' is output to the current command value calculator 55 as the torque command value T*, regardless of its actual value. 【0123】In addition, the operation of the motor control device 30 in the normal control region is not limited. For example, in the normal control region, a control method other than the voltage command value generation described above (for example, a method of independently controlling the d-axis current and q-axis current) may be used as a method for controlling the rotational speed of the motor M. Regardless of the control method in the normal control region, in the voltage saturation region, the limit torque T_limit is calculated and the torque command value T* is limited, so that the maximum torque that the motor M can output can be fully extracted. 【0124】 In the motor control device 30 according to this embodiment, the DC voltage Vdc supplied to the inverter circuit 32 and the estimated electrical angular velocity ωe, which is the rotational speed of the motor M, are detected, and the limit torque T_limit of the motor M is calculated from the detected DC voltage Vdc and estimated electrical angular velocity ωe. The rotational speed is controlled to limit the torque of the motor M using this limit torque T_limit. In this way, the limit torque T_limit is updated in accordance with the changes in the DC voltage Vdc and estimated electrical angular velocity ωe, so the torque output of the motor M can be stably increased to near the limit torque T_limit according to the operating state. Furthermore, since the limit torque T_limit is calculated, it does not take a lot of time to set the limit torque T_limit. As a result, it is possible to easily achieve a stable high torque output near the limit torque T_limit. 【0125】 One method for controlling the motor's rotational speed near its torque limit is to pre-set a voltage phase limit value equivalent to the torque limit through numerical analysis or actual machine verification, and then use that set value to limit the input for speed control. However, this method has the problem that it takes a considerable amount of time to set the voltage phase limit value. This is because the torque limit changes depending on the DC voltage and rotational speed. 【0126】Furthermore, while the rotational speed command value, which is the basis for the input value of speed control, is set by a higher-level controller, if the higher-level controller is not notified of input restrictions for speed control, the actual rotational speed may deviate from the rotational speed command value. For example, if calculations of a voltage model using the rotational speed command value are performed in the process of generating the voltage command value, model errors may occur due to the deviation of the actual rotational speed from the rotational speed command value, which may result in unstable motor control. 【0127】 In this embodiment, as described above, the torque command value T* of the motor M is limited using the limit torque T_limit calculated from the DC voltage Vdc and the estimated electrical angular velocity ωe. In other words, the limit torque T_limit does not need to be set in advance by numerical analysis or actual machine verification. As a result, even if the limit torque T_limit changes according to the operating state of the motor M (DC voltage Vdc and rotational speed), it is possible to appropriately limit the torque command value T* without tuning. 【0128】 Furthermore, in this embodiment, the rotational speed limit state SPEED_LIMIT_STATE, determined based on the torque limit ratio R_trq_lim shown in equation (24), is notified to the higher-level controller 12. The controller 12 performs processing to maintain or decelerate the rotational speed command value (mechanical angular velocity command value ωm*) input to the motor control device 30 according to the rotational speed limit state SPEED_LIMIT_STATE. This makes it possible to avoid situations in which the actual rotational speed of the motor M deviates from the mechanical angular velocity command value ωm* without following it. As a result, it is possible to sufficiently improve the control stability near the limit torque T_limit. 【0129】 <Second Embodiment> A motor control device according to a second embodiment of the present invention will now be described. In the following description, parts that are similar to the configuration and operation of the motor control device 30 described in the above embodiment will be omitted or simplified. 【0130】Figure 7 is a schematic diagram showing an example of the configuration of a motor control device according to the second embodiment. The motor control device 130 shown in Figure 7 has a configuration that adds a motor parameter modifier 70 to the motor control device 30 shown in Figure 2. The other configurations of the motor control device 130 are the same as those of the motor control device 30 shown in Figure 2. 【0131】 The motor parameter corrector 70 corrects motor parameters related to motor M based on the operating state of motor M. This makes it possible to correct motor parameters that change depending on the operating state, such as the current flowing through motor M and motor temperature. As a result, it becomes possible to calculate the limit torque T_limit and other parameters with higher accuracy. 【0132】 As shown in Figure 7, the motor parameter modifier 70 includes an inductance setter 71 and an armature flux linkage calculator 72. In this embodiment, an example of modifying the motor parameters, namely the inductance of the motor M (d-axis inductance Ld and q-axis inductance Lq) and the armature flux linkage Ψa, will be described. The motor parameters modified by the motor parameter modifier 70 are output to various parts of the motor control device 130 as appropriate and used for calculation processing. Hereinafter, the values ​​of the motor parameters modified according to the operating state will be referred to as set values. 【0133】 The inductance setter 71 variably sets the d-axis inductance setting value Ld_s and the q-axis inductance setting value Lq_s according to the operating state of the motor M. Generally, the inductance of a motor M has magnetic saturation characteristics that change according to the motor current flowing through the motor M. Therefore, when the motor current changes according to the operating state such as the load on the motor M, the actual inductance also changes. By appropriately setting the inductance setting values ​​(Ld_s and Lq_s) that take into account the characteristics of the actual inductance, the accuracy of calculating the limit torque T_limit can be improved. 【0134】As a method for setting the inductance, the magnetic saturation characteristics of the inductance may be stored in ROM as a map in advance, and the inductance value corresponding to the current (detected values ​​of the d-axis current Id and q-axis current Iq) which changes according to the operating state may be referenced from the map. Below are examples of maps for the d-axis inductance Ld and the q-axis inductance Lq. 【0135】 For example, when setting the d-axis inductance setting value Ld_s, the d-axis inductance Ld corresponding to the d-axis current Id is read out. Similarly, when setting the q-axis inductance setting value Lq_s, the q-axis inductance Lq corresponding to the q-axis current Iq is read out. A map may also be used in which the d-axis inductance Ld (or q-axis inductance Lq) is specified by two parameters: the d-axis current Id and the q-axis current Iq. Furthermore, the values ​​of each inductance may be set by linear interpolation or the like. 【0136】 The armature flux linkage calculator 72 sets the armature flux linkage setting value Ψa_s variably according to the operating conditions. It is generally known that the armature flux linkage Ψa has a temperature characteristic. For example, in the case of a neodymium magnet, the armature flux linkage Ψa decreases as the temperature increases and increases as the temperature decreases, having a negative temperature coefficient. One example of a method for setting the armature flux linkage setting value Ψa is to store the temperature characteristics of the armature flux linkage Ψa as a map in ROM in advance, similar to the case of the inductance setter 71, and read the corresponding value according to the motor temperature. 【0137】 If the motor temperature cannot be detected, the armature flux linkage Ψa may be estimated using, for example, the motor's voltage equation. Equation (25) shows the q-axis voltage equation including the armature flux linkage Ψa. 【0138】 Substituting the armature flux linkage setting value Ψa_s into equation (25), the model value Vq_mdl for the q-axis voltage Vq is given by equation (26). 【0139】Furthermore, the armature flux linkage setting value Ψa_s is modified by integral control as shown in equation (27) so that there is no deviation between the q-axis voltage command value Vq* actually applied to the motor M and the q-axis voltage model value Vq_mdl. Here, "G" is the gain for determining the integral response. 【0140】 The q-axis voltage equation described above includes the d-axis inductance Ld and the winding resistance R. In general, the magnetic saturation characteristics of the d-axis inductance Ld change less with respect to current compared to the q-axis inductance Lq. Also, the error in the winding resistance R has a smaller impact on the overall voltage than the error in the induced voltage, i.e., the error in the armature flux linkage Ψa. For these reasons, changes in the d-axis inductance Ld and the winding resistance R can be ignored without causing significant problems. Furthermore, by setting the correct inductance (d-axis inductance setting value Ld_s and q-axis inductance setting value Lq_s) using the inductance setter 71 described above, the estimation accuracy of the armature flux linkage setting value Ψa_s can also be improved. 【0141】 In this way, by appropriately varying the motor parameters used in motor control according to the operating conditions, the accuracy of calculating the limit torque T_limit is improved, and the maximum torque of the motor M can be further increased. 【0142】 <Other Embodiments> The present invention is not limited to the embodiments described above, and various other embodiments can be realized. 【0143】 The present invention can be applied to a configuration equipped with a converter circuit 31 that outputs a variable DC voltage Vdc. That is, the inverter (IPM 41) that drives the motor M may be connected to the converter circuit 31 that outputs a variable DC voltage Vdc during the operation of the motor M. In this case, the DC voltage detector 35 sequentially detects the DC voltage Vdc output from the converter circuit 31. 【0144】As the converter circuit 31 that outputs a variable DC voltage Vdc, for example, a circuit capable of outputting DC voltages Vdc of different voltage levels by boosting or stepping down the DC voltage obtained by rectifying the output of the AC power supply 3 is used. The converter circuit 31 outputs a variable DC voltage Vdc during the operation of the motor M in response to a control signal from a higher-level controller 12 or the like. 【0145】 For example, when the load on the motor M is large or when it is rotating at high speed, a relatively high DC voltage Vdc is output, and when the load on the motor M is small or when it is rotating at low speed, a relatively low DC voltage Vdc is output. Even when the DC voltage Vdc changes in this way, the DC voltage detector 35 monitors the DC voltage Vdc, making it possible to calculate the limit torque T_limit according to the current DC voltage Vdc. This makes it possible to appropriately limit the output torque (torque command value T*) of the motor M to the limit torque T_limit, thereby realizing highly efficient and high-output motor control. 【0146】 Furthermore, when setting a threshold for limiting torque through numerical analysis or actual machine testing, it becomes necessary to set a threshold for each DC voltage Vdc, which can significantly increase the workload. In contrast, by applying the present invention, it is possible to sequentially calculate an appropriate limit torque T_limit, thus avoiding the aforementioned increase in workload. As a result, even in configurations where the DC voltage Vdc changes, it becomes easy to achieve stable high torque output near the limit torque. 【0147】 Furthermore, in the above embodiment, a method was described in which the DC voltage Vdc output from the converter circuit 31 is directly detected by a DC voltage detector 35 configured with a voltage sensor. This makes it possible to accurately calculate the limit torque T_limit by referring to the actual DC voltage Vdc, even when, for example, the output of the converter circuit 31 is variable or the DC voltage Vdc fluctuates according to the operation of the motor M. 【0148】Furthermore, as a method for detecting the DC voltage Vdc, for example, a method of reading a set value of the DC voltage Vdc may be used. For example, when using a converter circuit 31 with a variable output, a control signal specifying the DC voltage Vdc is output to the converter circuit 31 from a higher-level controller 12 or the like. The set value of the DC voltage Vdc (or output voltage limit value Vdq_limit) corresponding to this control signal may be read from memory or the like. Alternatively, when using a converter circuit 31 that outputs a constant DC voltage Vdc, the set value of the DC voltage Vdc (or output voltage limit value Vdq_limit) that has been set in advance may be read. This eliminates the need to provide a voltage sensor or the like, and makes it possible to reduce the cost of the device. 【0149】 In the above embodiment, a method of motor control using the calculation result of the limit torque T_limit, regardless of the operating conditions of the air conditioner 100, was described. However, the invention is not limited to this, and for example, motor control using the limit torque T_limit may be performed according to the operating conditions of the air conditioner 100. 【0150】 For example, the limit torque calculator 60 may calculate the limit torque T_limit of the motor M based on the DC voltage and rotational speed when the heating operation is performed at a predetermined ambient temperature T0 or below. The predetermined ambient temperature T0 is a threshold for determining a low ambient temperature, and is set to, for example, T0 = 0°, but may be set to other temperatures. 【0151】 For example, the air conditioner 100 is equipped with an outside air temperature sensor that detects the temperature outside. When the temperature T detected by the outside air temperature sensor is less than or equal to a predetermined outside air temperature T0, and heating operation is performed, the limit torque T_limit is calculated by the method described above, and motor control (torque limiting, etc.) using the limit torque T_limit is performed. In other cases, motor control that does not use the limit torque T_limit (for example, maximum torque / current control, etc.) may be performed. 【0152】Generally, when the outside temperature is low, electrical components, including the motor M, are cooled, and the allowable current flowing through each component increases. Also, during heating operation when the outside temperature is low, the density of the refrigerant drawn into the compressor 20 in the refrigerant circuit 25 shown in Figure 1 decreases, resulting in lower compression power (power consumption) per rotation and lower current flowing through the motor M. For this reason, the rotational speed of the motor M can be increased during heating operation when the outside temperature is low. Furthermore, if the actual room temperature is lower than the room temperature requested by the user of the air conditioner 100 by a predetermined temperature or more, the air conditioner 100 operates at maximum output. As a result, during heating operation when the outside temperature is low, the rotational speed of the motor M tends to increase and the limiting torque tends to decrease. 【0153】 Thus, under conditions where the limit torque is low and the rotational speed of motor M tends to be high, the torque of motor M is more likely to reach its limit torque. Even in such cases, by calculating the limit torque T_limit based on the DC voltage and rotational speed, it becomes possible to appropriately limit the output torque of motor M, thereby efficiently achieving stable high torque output. 【0154】 In the above embodiment, a method for calculating the limit torque T_limit based on the amplitude Ψo (=Vo* / ωe) of the constant induced voltage ellipse shown in equation (18) was described. This method involves calculating the induced voltage command value Vo* based on the DC voltage Vdc according to equation (6), and then calculating the limit torque T_limit from the calculation result. The method for calculating the limit torque T_limit is not limited, and for example, the limit torque T_limit may be calculated without using the induced voltage command value Vo*. 【0155】 For example, the amplitude Ψo of the constant induced voltage ellipse may be calculated using the value obtained by multiplying the DC voltage Vdc by a coefficient α (Vdc × α) according to equation (28) shown below. The coefficient α is a value of 1 or less, and is set in advance so that, for example, Vdc × α is close to the induced voltage command value Vo* based on the DC voltage Vdc. 【0156】Alternatively, for example, the amplitude Ψo of the constant induced voltage ellipse may be calculated using the value obtained by subtracting the voltage β generated by the motor current from the DC voltage Vdc (Vdc-β), according to equation (29) shown below. The voltage β due to current generation is calculated in advance, for example, as the value obtained by multiplying the maximum current Imax flowing through the motor by the winding resistance R (Imax × R). 【0157】 For example, the limiting torque T_limit can be calculated using the amplitude Ψo of the constant induced voltage ellipse shown in equation (28) or equation (29) and the above-mentioned equations (19) to (22). In this way, it is also possible to calculate the limiting torque T_limit directly from the DC voltage Vdc without calculating the induced voltage command value Vo*. 【0158】 It is also possible to combine at least two of the feature features of the present invention described above. In other words, the various feature features described in each embodiment may be combined arbitrarily without distinction between embodiments. Furthermore, the various effects described above are merely examples and are not limiting, and other effects may also be exhibited. 【0159】 M...Motor 12...Controller 25...Refrigerant circuit 30, 130...Motor control device 31...Converter circuit 32...Inverter circuit 34...Current detector 35...DC voltage detector 52...Voltage amplitude adjuster 53...Induced voltage command value calculator 54...Torque command value limiter 60...Maximum torque calculator 61...Torque command limit processor 62...Torque limit ratio calculator 63...Rotation speed limit state determination device 70...Motor parameter modifier 100...Air conditioner

Claims

1. A motor control device comprising: a DC voltage detector for detecting the DC voltage supplied to an inverter that drives a motor; a rotational speed detector for detecting the rotational speed of the motor; a limit torque calculator for calculating the limit torque of the motor based on the DC voltage and the rotational speed; and a rotational speed controller for controlling the rotational speed of the motor to limit the torque of the motor based on the limit torque.

2. A motor control device according to claim 1, further comprising an induced voltage command value calculator that calculates an induced voltage command value for the motor based on the DC voltage, the rotational speed, and motor parameters relating to the motor, wherein the limit torque calculator calculates the limit torque based on the induced voltage command value.

3. A motor control device according to claim 2, further comprising a motor parameter modifier for modifying motor parameters related to the motor based on the operating state of the motor.

4. A motor control device according to claim 1, further comprising a rotational speed limiter that limits the rotational speed of the motor based on a torque limit ratio, which is the ratio of the torque command value of the motor to the limit torque.

5. A motor control device according to claim 4, wherein the torque command value is calculated based on a motor speed command value output from a predetermined controller, and the rotation speed limiter instructs the predetermined controller to maintain or reduce the rotation speed indicated by the speed command value based on the torque limit ratio.

6. A motor control device according to any one of claims 1 to 5, wherein the inverter is connected to a converter that outputs a variable DC voltage during the operation of the motor, and the DC voltage detector sequentially detects the DC voltage output from the converter.

7. An air conditioner comprising a motor control device according to any one of claims 1 to 5, the air conditioner comprising: an inverter for driving the motor; a converter for supplying a DC voltage to the inverter; a compressor having the motor; an outdoor heat exchanger; an expansion valve; an indoor heat exchanger; and a four-way valve, which are sequentially connected by refrigerant piping; wherein the limit torque calculator calculates the limit torque of the motor based on the DC voltage and the rotational speed when the heating operation is performed at or below a predetermined outside air temperature.

8. An air conditioner comprising: a refrigerant circuit in which a compressor having a motor, an outdoor heat exchanger, an expansion valve, an indoor heat exchanger, and a four-way valve are sequentially connected by refrigerant piping; an inverter for driving the motor; a converter for supplying a DC voltage to the inverter; a DC voltage detector for detecting the DC voltage; a rotational speed detector for detecting the rotational speed of the motor; a limit torque calculator for calculating the limit torque of the motor based on the DC voltage and the rotational speed; and a motor control device having a rotational speed controller for controlling the rotational speed of the motor to limit the torque of the motor based on the limit torque.

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