Electric power conversion device, motor control device, and electric power steering device

Abstract

A switching control unit (31, 32) drives a first-phase switching element at a duty ratio of 100% in cases where a target duty ratio for a first phase, which is the phase having the highest target duty ratio from among a plurality of phases of a multi-phase inverter (33), is greater than a prescribed first threshold value set to less than 100%, and drives a second-phase switching element at a corrected duty ratio obtained by increasing and correcting a target duty ratio for a second phase different from the first phase. A voltage-command-value-limiting unit (45) limits a q-axis voltage command value and a d-axis voltage command value so that the corrected duty ratio does not exceed a prescribed second threshold value set to less than 100%.

Description

Power conversion device, motor control device, and electric power steering device

[0001] The present invention relates to a power conversion device, a motor control device, and an electric power steering device.

[0002] The motor control device described in Patent Document 1 below measures phase currents based on the voltage drops across shunt resistors connected in series to switching elements of a three-phase inverter.

[0003] Patent No. 5396948 specification

[0004] When measuring phase currents based on the voltage drops across shunt resistors connected in series to switching elements of a multi-phase inverter, as the duty ratio of the maximum duty phase, which is the phase with the highest duty ratio under PWM control, increases, the timing of switching the switching elements of the maximum duty phase and the timing of sampling the voltage across the shunt resistor become closer together, which can result in switching noise being superimposed on the measured value of the voltage across the shunt resistor, making it impossible to accurately detect the phase currents.

[0005] For this reason, in the motor control device described in Patent Document 1, when the on time of any of the switching elements on the low potential side becomes shorter than the current value detection time, the switching element on the high potential side of the current undetectable phase corresponding to that switching element is turned on, the switching element on the low potential side is kept off, and the duty instruction values ​​of phases other than the current undetectable phase are shifted to the high potential side.

[0006] However, if the duty instruction values ​​of the phases other than the current-detectable phase are shifted to the high potential side, the length of the on-period of the switching element on the low potential side becomes shorter, which may make it impossible to accurately detect the phase current flowing through the shunt resistor. The present invention has been made in consideration of the above circumstances, and aims to improve the measurement accuracy when the duty ratio of the maximum duty phase, which is the phase with the highest duty ratio under PWM control, is high, when measuring phase currents based on the voltage drops of resistor elements connected in series to the switching elements of a multi-phase inverter.

[0007] In order to achieve the above object, a power conversion device according to one aspect of the present invention includes: a multi-phase inverter having upper arm switching elements and lower arm switching elements of a plurality of phases connected in series; a voltage command value generation unit that generates a q-axis voltage command value and a d-axis voltage command value for driving the multi-phase inverter; a voltage command value limiting unit that limits the q-axis voltage command value and the d-axis voltage command value; a voltage command value conversion unit that converts the q-axis voltage command value and the d-axis voltage command value limited by the voltage command value limiting unit into multi-phase voltage command values; a switching control unit that drives a first switching element that is either the upper arm switching element or the lower arm switching element, and a second switching element that is the other switching element of the upper arm switching element and the lower arm switching element other than the first switching element, by PWM control based on the q-axis voltage command value and the d-axis voltage command value limited by the voltage command value limiting unit;

[0008] The switching control unit sets a target duty ratio, which is a target value of the duty ratio of the period during which the first switching element is on in the PWM period, in accordance with a target voltage to be applied to a load of the multi-phase inverter. When the target duty ratio of a first phase, which is a phase having the highest target duty ratio among multiple phases of the multi-phase inverter, is equal to or less than a predetermined first threshold set to be less than 100%, the switching control unit drives the first switching element of the first phase at the target duty ratio. When the target duty ratio of the first phase is greater than the first threshold, the switching control unit drives the first switching element of the first phase at a duty ratio of 100%, and drives the first switching element of the second phase at a corrected duty ratio obtained by increasing and correcting the target duty ratio of a second phase other than the first phase among the multiple phases. The voltage command value limiting unit limits the q-axis voltage command value and the d-axis voltage command value so that the corrected duty ratio does not exceed the predetermined second threshold set to be less than 100%.

[0009] According to the present invention, when measuring phase currents based on the voltage drops across resistor elements connected in series to switching elements of a multi-phase inverter, it is possible to improve measurement accuracy when the duty ratio of the maximum duty phase, which is the phase with the highest duty ratio under PWM control, is high.

[0010] 1 is a configuration diagram showing an overview of an example of an electric power steering device according to an embodiment; FIG. 2 is a configuration diagram showing an overview of an example of an electronic control unit (ECU); FIG. 3 is a block diagram showing an example of the functional configuration of a control calculation unit; FIGS. 1A to 1E are schematic time charts illustrating the switching timing of a switching element and the current measurement timing; FIGS. 1A to 1E are schematic time charts illustrating the switching timing of a switching element and the current measurement timing; FIGS. 1A to 1E are schematic diagrams illustrating a method for setting a duty ratio; FIGS. 1A to 1D are explanatory diagrams of the duty ratio of each phase and the difference in duty ratio between phases; FIGS. 1A and 1B are explanatory diagrams of the effects of the embodiment; FIGS. 1A and 1B are explanatory diagrams of the duty ratio of each phase when the q-axis voltage command value and the d-axis voltage command value are not limited; and FIGS. 1A and 1B are explanatory diagrams of the duty ratio of each phase when the q-axis voltage command value and the d-axis voltage command value are limited. FIG. 2 is a flowchart of an example of a current measurement method according to an embodiment; and FIG. 3 is a configuration diagram showing an overview of a first modified example of an electric power steering device. 10A and 10B are configuration diagrams showing an overview of a second modified example of an electric power steering device, and FIG. 10B are configuration diagrams showing an overview of a third modified example of an electric power steering device.

[0011] Embodiments of the present invention will be described in detail with reference to the drawings. Note that the embodiments of the present invention shown below are merely examples of devices and methods for embodying the technical concept of the present invention, and the technical concept of the present invention does not limit the configuration, arrangement, etc. of component parts to those described below. Various modifications can be made to the technical concept of the present invention within the technical scope defined by the claims.

[0012] 1 is a diagram showing an outline of an example of an electric power steering (EPS) device according to an embodiment. A steering shaft (steering shaft, handle shaft) 2 of a steering wheel (steering handle) 1 is connected to steered wheels 8L, 8R via reduction gears (worm gears) 3, universal joints 4a and 4b, a pinion-rack mechanism 5, tie rods 6a, 6b, and hub units 7a, 7b, which constitute a reduction mechanism.

[0013] The pinion-rack mechanism 5 has a pinion 5a connected to a pinion shaft to which steering force is transmitted from the universal joint 4b, and a rack 5b meshing with the pinion 5a, and converts the rotational motion transmitted to the pinion 5a into linear motion in the vehicle width direction by the rack 5b. The steering shaft 2 is provided with a torque sensor 10 that detects the steering torque Th. The steering shaft 2 is also provided with a steering angle sensor 14 that detects the steering angle θh of the steering wheel 1.

[0014] A motor 20 that assists the steering force of the steering wheel 1 is connected to the steering shaft 2 via a reduction gear 3. In this specification, an example in which the motor 20 is a three-phase motor will be described, but the number of phases of the motor 20 does not have to be three. An electronic control unit (ECU) 30 that controls the electric power steering device is supplied with power from a battery 13 and receives an ignition key signal via an ignition switch 11.

[0015] The ECU 30 calculates a current command value of the assist control command based on the steering torque Th detected by the torque sensor 10, the vehicle speed Vh detected by the vehicle speed sensor 12, and the steering angle θh detected by the steering angle sensor 14, and controls the currents (U-phase current Iu, V-phase current Iv, and W-phase current Iw) supplied to the motor 20 based on a voltage command value obtained by performing compensation or the like on the current command value. Note that the steering angle sensor 14 is not essential, and the steering angle θh may be calculated by adding the torsion angle of the torsion bar of the torque sensor 10 to the motor rotation angle θm obtained from a rotation angle sensor 21 that detects the rotation angle of the rotary shaft of the motor 20. Furthermore, the steering angle of the steered wheels 8L, 8R may be used instead of the steering angle θh. For example, the steering angle may be detected by detecting the displacement amount of the rack 5b.

[0016] The ECU 30 includes a computer including, for example, a processor and peripheral components such as a storage device. The processor may be, for example, a CPU (Central Processing Unit) or an MPU (Micro-Processing Unit). The storage device may include any of a semiconductor storage device, a magnetic storage device, and an optical storage device. The storage device may include memories such as a register, a cache memory, and a ROM (Read Only Memory) and a RAM (Random Access Memory) used as a main storage device. The functions of the ECU 30 described below are realized, for example, by the processor of the ECU 30 executing a computer program stored in the storage device.

[0017] The ECU 30 may be formed of dedicated hardware for executing the various information processes described below. For example, the ECU 30 may include a functional logic circuit configured in a general-purpose semiconductor integrated circuit. For example, the ECU 30 may have a programmable logic device (PLD) such as a field-programmable gate array (FPGA).

[0018] 2 is a schematic diagram illustrating an example of an ECU 30 according to an embodiment. The ECU 30 includes a control calculation unit 31, a gate drive circuit 32, a multi-phase inverter 33, a current interruption circuit 34, an interruption drive circuit 35, a voltage drop measurement unit 36, and a motor rotation speed calculation unit 37. The control calculation unit 31 and the gate drive circuit 32 are examples of a "switching control unit" as defined in the claims. A power wiring PW that transmits power from the battery 13 is connected to the ECU 30 via a connector CNT. Signals representing the steering torque Th detected by the torque sensor 10, the vehicle speed Vh detected by the vehicle speed sensor 12, and the steering angle θh detected by the steering angle sensor 14 are also transmitted to the control calculation unit 31 via the connector CNT.

[0019] The control calculation unit 31 calculates a current command value, which is a control target value of the drive current of the motor 20, based on at least the steering torque Th. The control calculation unit 31 calculates a voltage command value obtained by performing compensation or the like on the current command value, and generates gate control signals Sgu, Sgv, and Sgw by PWM-modulating the voltage command value. The gate control signals Sgu, Sgv, and Sgw are PWM signals that control the drive voltages output from the multi-phase inverter 33 to the U-phase coil 20u, V-phase coil 20v, and W-phase coil 20w of the motor 20.

[0020] Based on the gate control signals Sgu, Sgv, and Sgw, the gate drive circuit 32 performs on / off control of the switching elements Qu1, Qv1, Qw1, Qu2, Qv2, and Qw2 of the multi-phase inverter 33 (described later). For example, if the switching elements Qu1, Qv1, Qw1, Qu2, Qv2, and Qw2 are field effect transistors (FETs), the gate drive circuit 32 generates gate signals for the field effect transistors based on the gate control signals Sgu, Sgv, and Sgw.

[0021] The multi-phase inverter 33 includes a three-phase bridge connected between a positive line that is connected to a DC power supply Vdc and receives DC power and a ground line. The three-phase bridge includes switching element pairs in which switching elements Qu1, Qv1, and Qw1 are connected in series to upper arms of the U-phase, V-phase, and W-phase, and switching elements Qu2, Qv2, and Qw2 are connected in series to lower arms of the U-phase, V-phase, and W-phase, respectively.

[0022] The U-phase current Iu supplied to the U-phase coil 20u of the motor 20 is supplied from the connection point of the switching elements Qu1 and Qu2, the V-phase current Iv supplied to the V-phase coil 20v is supplied from the connection point of the switching elements Qv1 and Qv2, and the W-phase current Iw supplied to the W-phase coil 20w is supplied from the connection point of the switching elements Qw1 and Qw2. The U-phase current Iu, V-phase current Iv, and W-phase current Iw are passed through the U-phase coil 20u, V-phase coil 20v, and W-phase coil 20w of the motor 20 via a current interruption circuit 34.

[0023] The current interruption circuit 34 has three phase interruption FETs QAu, QAv, and QAw for interrupting the phase currents of the motor. The control calculation unit 31 outputs a control signal Sm that controls the conduction and interruption of the current interruption circuit 34 to the interruption drive circuit 35. The interruption drive circuit 35 outputs gate signals to the phase interruption FETs QAu to QAw in response to the control signal Sm, and passes or interrupts the U-phase current Iu, V-phase current Iv, and W-phase current Iw from the multi-phase inverter 33 to the motor 20.

[0024] A smoothing capacitor Cs is connected in parallel to the multi-phase inverter 33. The smoothing capacitor Cs may be, for example, an electrolytic capacitor. The ECU 30 may include a plurality of smoothing capacitors connected in parallel as the smoothing capacitor Cs. Shunt resistors ru, rv, and rw are connected in series between the ground line and the switching elements Qu2, Qv2, and Qw2 of the lower arms of the U-phase, V-phase, and W-phase.

[0025] The voltage drop measurement unit 36 ​​measures the voltage drop across the shunt resistors ru, rv, and rw due to the current flowing through the lower arm switching elements Qu2, Qv2, and Qw2. The voltage drop measurement unit 36 ​​outputs measured values ​​Vud, Vvd, and Vwd of the voltages across the shunt resistors ru, rv, and rw to the control calculation unit 31. The motor rotation speed calculation unit 37 calculates the motor rotation angle θm (e.g., motor electrical angle) of the motor 20 based on the detection signal of the rotation angle sensor 21 and outputs the calculated value to the control calculation unit 31.

[0026] 3 is a block diagram showing an example of the functional configuration of the control calculation unit 31. The control calculation unit 31 includes a current command value calculation unit 40, subtractors 41 and 42, a current limiting unit 43, a proportional-integral (PI) control unit 44, a voltage limiting unit 45, a two-phase / three-phase conversion unit 46, a dead time compensating unit 47, a third-order harmonic compensating unit 48, adders 49 to 51, a duty shift unit 52, a PWM control unit 53, a current calculation unit 54, a three-phase / two-phase conversion unit 55, and an angular velocity conversion unit 56, and drives the motor 20 by vector control.

[0027] The current command value calculation unit 40 calculates a q-axis current command value Iq0 and a d-axis current command value Id0 to be passed through the motor 20 based on the steering torque Th, the vehicle speed Vh, the motor rotation angle θm of the motor 20, and the rotation angular velocity ω of the motor 20. Meanwhile, the current calculation unit 54 calculates measured values ​​Iud, Ivd, and Iwd of the U-phase current, V-phase current, and W-phase current of the motor 20 based on measured values ​​Vud, Vvd, and Vwd of the voltages across the shunt resistors ru, rv, and rw measured by the voltage drop measurement unit 36 ​​of Fig. 2. The voltage drop measurement unit 36 ​​and the current calculation unit 54 are an example of a "current measurement unit" recited in the claims.

[0028] In the phase with the highest duty ratio among the U-phase, V-phase, and W-phase, the period during which the switching element of the lower arm is on is short, making it impossible to accurately detect the voltage across the shunt resistor. Therefore, the current calculation unit 54 may measure the phase current of the phase with the highest duty ratio based on the voltages across the shunt resistors of the other two phases other than the phase with the highest duty ratio and Kirchhoff's law (the sum of the U-phase current Iu, V-phase current Iv, and W-phase current Iw is 0).

[0029] For example, when the duty ratio of the phase with the highest duty ratio is equal to or less than a predetermined value, the phase current of the phase with the highest duty ratio may be measured based on the voltage across the shunt resistor of the phase with the highest duty ratio, and when the duty ratio of the phase with the highest duty ratio is greater than the predetermined value, the phase current of the phase with the highest duty ratio may be measured based on the voltage across the shunt resistors of the other two phases and Kirchhoff's law.

[0030] A three-phase / two-phase conversion unit 55 converts the measured values ​​Iud, Ivd, and Iwd of the U-phase current, V-phase current, and W-phase current into a q-axis current iq and a d-axis current id. Subtractors 41 and 42 calculate a q-axis deviation current Δq0 and a d-axis deviation current Δd0 by subtracting the fed-back q-axis current iq and d-axis current id from a q-axis current command value Iq0 and a d-axis current command value Id0, respectively. A current limiting unit 43 limits the upper limits of the q-axis deviation current Δq0 and the d-axis deviation current Δd0. The limited q-axis deviation current Δq and d-axis deviation current Δd are input to a PI control unit 44.

[0031] The PI control unit 44 calculates a basic q-axis voltage command value vq0 and a basic d-axis voltage command value vd0 such that the q-axis deviation current Δq and the d-axis deviation current Δd are zero, respectively. For example, the basic q-axis voltage command value vq0 and the basic d-axis voltage command value vd0 may be duty ratios that are ratios of the voltage command values ​​to the power supply voltage. For example, the basic q-axis voltage command value vq0 and the basic d-axis voltage command value vd0 may be zero-midpoint duty ratios that vary within a range from −50% to +50% with 0% as the center value.

[0032] The voltage limiting unit 45 sets the q-axis voltage command value vq and the d-axis voltage command value vd by limiting the basic q-axis voltage command value vq0 and the basic d-axis voltage command value vd0. The function of the voltage limiting unit 45 will be described in detail later. The two-phase / three-phase conversion unit 46 converts the q-axis voltage command value vq and the d-axis voltage command value vd into a first U-phase voltage command value vu1, a first V-phase voltage command value vv1, and a first W-phase voltage command value vw1, respectively.

[0033] Dead time compensator 47 outputs a dead time compensation value that compensates for the dead time of multi-phase inverter 33 to adders 49 to 51. Third harmonic compensator 48 generates third harmonic components for improving the voltage utilization rate and outputs them to adders 49 to 51. The third harmonic components are harmonic components having a frequency three times the fundamental frequency, which is the frequency of the fundamental U-phase voltage command value vu0, the fundamental V-phase voltage command value vv0, and the fundamental W-phase voltage command value vw0.

[0034] Adders 49 to 51 add the dead-time compensation value and the third-order harmonic component to the first U-phase voltage command value vu1, the first V-phase voltage command value vv1, and the first W-phase voltage command value vw1, respectively, to calculate a second U-phase voltage command value vu2, a second V-phase voltage command value vv2, and a second W-phase voltage command value vw2. When the largest voltage command value among the second U-phase voltage command value vu2, the second V-phase voltage command value vv2, and the second W-phase voltage command value vw2 is equal to or smaller than a threshold, duty shifter 52 sets U-phase voltage command value vu, V-phase voltage command value vv, and W-phase voltage command value vw based on the second U-phase voltage command value vu2, the second V-phase voltage command value vv2, and the second W-phase voltage command value vw2, respectively, and outputs the set values ​​to PWM control unit 53. On the other hand, if the maximum voltage command value among the voltage command values ​​vu2, vv2, and vw2 is greater than the threshold value, the duty shift unit 52 increases and corrects the U-phase voltage command value vu, the V-phase voltage command value vv, and the W-phase voltage command value vw, and outputs them to the PWM control unit 53.

[0035] In addition, when the basic q-axis voltage command value vq0, the basic d-axis voltage command value vd0, the first U-phase voltage command value vu1, the first V-phase voltage command value vv1, the first W-phase voltage command value vw1, and the second U-phase voltage command value vu2, the second V-phase voltage command value vv2, and the second W-phase voltage command value vw2 are set as zero midpoint duty ratios that vary within a range from −50% to +50% around 0% as a center value, the duty shift unit 52 corrects the midpoint of the duty ratios to output the U-phase voltage command value vu, the V-phase voltage command value vv, and the W-phase voltage command value vw as duty ratios that vary within a range from 0% to 100% around 50% as a center value. The function of the duty shift unit 52 will be described in detail later.

[0036] The PWM control unit 53 generates gate control signals Sgu, Sgv, and Sgw by PWM-modulating the U-phase voltage command value vu, the V-phase voltage command value vv, and the W-phase voltage command value vw, and outputs them to the gate drive circuit 32 shown in Fig. 2. The angular velocity conversion unit 56 calculates the rotational angular velocity ω of the motor 20 based on the change over time in the motor rotational angle θm. The motor rotational angle θm and the rotational angular velocity ω are input to the current command value calculation unit 40 and used for vector control.

[0037] (Details of Duty Shift Unit 52) ​​The details of the duty shift unit 52 will be described. First, the relationship between the switching timing of the switching elements Qu1, Qv1, Qw1, Qu2, Qv2, and Qw2 of the multi-phase inverter 33 and the current measurement timing will be described. In the following description, it is assumed that the period during which the switching elements Qu1, Qv1, and Qw1 of the upper arms of the U-phase, V-phase, and W-phase of the multi-phase inverter 33 are on is equal to the PWM period T ＰＷＭ The duty ratios of the pulses are denoted as "Du," "Dv," and "Dw," respectively.

[0038] Furthermore, the target values ​​of the duty ratios Du, Dv, and Dw specified by the second U-phase voltage command value vu2, the second V-phase voltage command value vv2, and the second W-phase voltage command value vw2 are expressed as "target duty ratios Dut, Dvt, and Dwt." When the second U-phase voltage command value vu2, the second V-phase voltage command value vv2, and the second W-phase voltage command value vw2 are set as zero midpoint duty ratios, the midpoint of the duty ratio is corrected to 50%, thereby shortening the on-period of the upper arm switching element to the PWM period T ＰＷＭ is converted into a duty ratio value.

[0039] Of the U, V, and W phases, the phase with the highest target duty ratio is referred to as the “maximum duty phase,” the phase with the smallest target duty ratio is referred to as the “minimum duty phase,” and phases other than the maximum and minimum duty phases are referred to as “intermediate duty phases.” The maximum duty phase is an example of a “first phase” as defined in the claims, and the intermediate and minimum duty phases are examples of a “second phase” as defined in the claims.

[0040] Figures 4(a) and 4(b) are schematic time charts of PWM signals that drive the upper and lower arm switching elements of the maximum duty phase, respectively; Figures 4(c) and 4(d) are schematic time charts of PWM signals that drive the upper and lower arm switching elements of the medium duty phase or minimum duty phase, respectively; and Figure 4(e) is a schematic time chart of the output voltage of an AD converter that samples the voltage across a shunt resistor of the medium duty phase or minimum duty phase.

[0041] code T ＰＷＭ indicates a PWM period (i.e., one control period of PWM control), and symbol T ＨＯＦＦ indicates an off period in which the upper arm switching element is in an off state, and symbol T ＬＯＮ indicates an on-period in which the lower arm switching element is in an on-state. When shunt resistors ru, rv, and rw are connected in series to the lower arm switching elements Qu2, Qv2, and Qw2 (in the case of a so-called "downstream shunt system"), the voltage drop measurement unit 36 ​​measures the on-period T ＬＯＮ The central time tc (or the off period T ＨＯＦＦ The voltage across the shunt resistor is sampled at the center time tc of the PWM period T ＰＷＭ Multiple of (n × T ＰＷＭ ) (n is a natural number).

[0042] As a result, as the duty ratio of the maximum duty phase increases, the switching timings thd, thr, tlr, and tld of the switching elements of the maximum duty phase and the sampling timing tc of the voltage across the shunt resistors of the intermediate duty phase and minimum duty phase become closer, which may result in switching noise being superimposed on the measured value of the voltage across the shunt resistor, making it impossible to detect the phase current correctly.

[0043] Therefore, when the target duty ratio of the maximum duty phase is greater than a predetermined first threshold Dth1 set to less than 100%, the duty shift unit 52 sets (shifts) the target duty ratio of the maximum duty phase to 100%. That is, the target duty ratio of the maximum duty phase is increased to 100%. As a result, the switching element of the maximum duty phase is driven at a duty ratio of 100%. That is, the period during which the upper arm switching element of the maximum duty phase is turned on is set to the PWM period T ＰＷＭ The ratio of the maximum duty ratio to the maximum duty ratio is 100%, and the period during which the lower arm switching element of the maximum duty phase is turned on is equal to the PWM period T ＰＷＭ The ratio of the maximum duty phase to the maximum duty phase is set to 0%, and the upper arm switching element and the lower arm switching element are driven.

[0044] 5(a) to 5(e) are schematic time charts illustrating the switching timing of the switching elements and the current measurement timing when the target duty ratio of the maximum duty phase is shifted to 100%. Figures 5(a) and 5(b) are schematic time charts of PWM signals that drive the upper and lower arm switching elements of the maximum duty phase, respectively. Figures 5(c) and 5(d) are schematic time charts of PWM signals that drive the upper and lower arm switching elements of the intermediate duty phase or the minimum duty phase, respectively. Figure 5(e) is a schematic time chart of the output voltage of an AD converter that samples the voltage across a shunt resistor of the intermediate duty phase or the minimum duty phase.

[0045] 5(a) and 5(b), by shifting the target duty ratio of the maximum duty phase to 100%, switching of the switching element of the maximum duty phase does not occur, and therefore switching noise is not superimposed on the measured values ​​of the voltages across the shunt resistors of the intermediate duty phase and the minimum duty phase. As a result, the phase currents of the intermediate duty phase and the minimum duty phase can be correctly detected.

[0046] 6 is an explanatory diagram of a method for setting the duty ratio by the duty shift unit 52. The dashed line D1t indicates the target duty ratio of the maximum duty phase among the target duty ratios specified by the second U-phase voltage command value vu2, the second V-phase voltage command value vv2, and the second W-phase voltage command value vw2 input to the duty shift unit 52. The dashed-dotted line D2t indicates the target duty ratio of the intermediate duty phase or the minimum duty phase. For ease of explanation, the example of FIG. 6 shows a case in which the target duty ratio D1t of the maximum duty phase is gradually increased to 100% over time, and the target duty ratio D2t of the intermediate duty phase or the minimum duty phase is fixed.

[0047] On the other hand, a solid line D1 indicates the duty ratio of the maximum duty phase among the duty ratios specified by the U-phase voltage command value vu, the V-phase voltage command value vv, and the W-phase voltage command value vw output by the duty shift unit 52. A two-dot chain line D2 indicates the duty ratio of the intermediate duty phase or the minimum duty phase.

[0048] The duty shift unit 52 determines whether the target duty ratio D1t of the maximum duty phase is equal to or less than the first threshold value Dth1. If the target duty ratio D1t of the maximum duty phase is equal to or less than the first threshold value Dth1, the duty shift unit 52 does not increase the target duty ratios D1t and D2t. That is, the target duty ratios D1t and D2t are set as the duty ratios D1 and D2 without any change. As a result, the switching element of the maximum duty phase is driven at the target duty ratio D1t. Furthermore, the switching elements of the intermediate duty phase and the minimum duty phase are driven at the target duty ratio D2t.

[0049] On the other hand, when the target duty ratio D1t of the maximum duty phase is greater than the first threshold Dth1, the duty shift unit 52 corrects the target duty ratio D1t to 100%. In other words, the target duty ratio D1t is corrected to 100% and the corrected duty ratio D1=100% is output as the voltage command value for the maximum duty phase. As a result, the switching element of the maximum duty phase is driven at the corrected duty ratio D1=100%.

[0050] Furthermore, when the target duty ratio D1t of the maximum duty phase is greater than the first threshold Dth1, the duty shift unit 52 outputs the corrected duty ratio D2 obtained by increasing the target duty ratio D2t of the intermediate duty phase and the minimum duty phase as the voltage command value for the intermediate duty phase and the minimum duty phase. As a result, the switching elements of the intermediate duty phase and the minimum duty phase are driven by the corrected duty ratio D2.

[0051] For example, the duty shift unit 52 calculates a duty difference ΔD = (100% - D1t) by subtracting the target duty ratio D1t of the maximum duty phase from 100%, sets the sum of the target duty ratio D2t of the intermediate duty phase and the minimum duty phase plus the duty difference ΔD as a corrected duty ratio D2 = (D2t + ΔD), and outputs the corrected duty ratio D2 as a voltage command value for the intermediate duty phase and the minimum duty phase.

[0052] As a result, when the target duty ratio D1t is greater than the first threshold Dth1, the correction amount ΔD for correcting the duty ratios of the intermediate duty phase and the minimum duty phase becomes smaller as the difference between 100% and the target duty ratio D1t of the maximum duty phase is smaller (in other words, the greater the amount by which the target duty ratio D1t exceeds the first threshold Dth1). In this way, by increasing the duty ratios of the intermediate duty phase and the minimum duty phase by the duty difference ΔD as the duty ratio of the maximum duty phase is shifted to 100%, fluctuations in the U-phase current Iu, V-phase current Iv, and W-phase current Iw caused by the duty ratio shift can be suppressed or prevented. The reason for this is explained below.

[0053] The solid line, dashed line, and dashed dotted line in Fig. 7(a) respectively show the waveforms of the duty ratios Du, Dv, and Dw of the U-phase, V-phase, and W-phase when the duty ratios are not shifted. The solid line, dashed line, and dashed dotted line in Fig. 7(b) respectively show the correlation duty between the U-phase and V-phase (i.e., the difference Du-Dv between the duty ratios of the U-phase and V-phase), the correlation duty between the V-phase and W-phase (Dv-Dw), and the correlation duty between the W-phase and U-phase (Dw-Du).

[0054] On the other hand, the solid line, dashed line, and dashed-dotted line in Figure 7(c) show the waveforms of the duty ratios Du, Dv, and Dw of the U, V, and W phases, respectively, when the duty ratios are shifted. The solid line, dashed line, and dashed-dotted line in Figure 7(d) show the correlation duty (Du-Dv) between the U and V phases, the correlation duty (Dv-Dw) between the V and W phases, and the correlation duty (Dw-Du) between the W and U phases, respectively, when the duty ratios are shifted. For ease of explanation, Figures 7(a) and 7(b) show waveforms when the dead time compensation value and third-order harmonic component are omitted, while Figures 7(c) and 7(d) show waveforms when the third-order harmonic component is omitted.

[0055] As can be seen from Figures 7(a) and 7(c), shifting the duty ratio changes the waveforms of the duty ratios Du, Dv, and Dw of the U, V, and W phases. However, by increasing and correcting the duty ratios of the intermediate duty phase or minimum duty phase by the duty difference ΔD as the duty ratio of the maximum duty phase is shifted to 100%, the difference in duty ratio between the phases does not change even when the duty ratio is shifted, as shown in Figures 7(b) and 7(d).

[0056] In this way, by increasing the duty ratios of the intermediate duty phase and the minimum duty phase as the duty ratio of the maximum duty phase is shifted to 100%, changes in the potential difference between the terminals of the motor 20 are suppressed or prevented even when the duty ratio is shifted, and fluctuations in the U-phase current Iu, V-phase current Iv, and W-phase current Iw can be suppressed or prevented.

[0057] Next, the effects of the embodiment will be described. Figures 8(a) and 8(b) show the measured values ​​of the U-phase current Iud (solid line), the V-phase current Ivd (dashed line), and the W-phase current Iwd (dashed line) when the duty ratio Du of the U-phase, which is the maximum duty phase, gradually increases from 80% to 100%. Figure 8(a) shows the measured values ​​when the duty ratio is not shifted, and Figure 8(b) shows the measured values ​​when the duty ratio is shifted. For ease of explanation, the duty ratios Dv and Dw of the V-phase and W-phase are fixed at 80%.

[0058] When the duty ratio is not shifted ( FIG. 8A ), as the duty ratio Du of the U-phase, which has the maximum duty ratio, approaches 100%, switching noise caused by the switching of the U-phase upper arm switching element Qu1 makes it impossible to accurately detect the V-phase current Ivd and the W-phase current Iwd, as shown by the ellipse P1. Because the U-phase current Iud is calculated from the V-phase current Ivd and the W-phase current Iwd according to Kirchhoff's law, if the V-phase current Ivd and the W-phase current Iwd cannot be accurately detected, as shown by the ellipse P2. On the other hand, when the duty ratio is shifted ( FIG. 8B ), the V-phase current Ivd and the W-phase current Iwd can be accurately detected, as shown by the ellipse P3, even when the U-phase duty ratio Du approaches 100%. As a result, the U-phase current Iud can also be accurately detected.

[0059] (Details of Voltage Limiting Unit 45) The voltage limiting unit 45 in FIG. 3 will be described in detail. When the target duty ratio of the maximum duty phase is greater than the first threshold Dth1, the duty shift unit 52 increases the target duty ratio D2t of the duty intermediate phase. If the corrected duty ratio D2 becomes large, the period during which the switching element of the lower arm is on during the duty intermediate phase may become too short. As a result, it may become impossible to accurately detect the voltage across the shunt resistor during the duty intermediate phase. The duty intermediate phase is an example of the "phase with the second highest target duty ratio" set forth in the claims.

[0060] FIG. 9( a) shows the duty ratios Du, Dv, and Dw of the U-phase, V-phase, and W-phase (i.e., the U-phase voltage command value vu, the V-phase voltage command value vv, and the W-phase voltage command value vw) when the target duty ratio of the maximum duty phase is greater than the first threshold Dth1. FIG. 9( b) shows an enlarged view of the portion indicated by the arrow A1 in FIG. 9( a). The second threshold Dth2 in FIG. 9( b) is a predetermined threshold value (current detection limit threshold) of the duty ratio at which the voltage across the shunt resistor can be accurately detected. The second threshold Dth2 is set to a value less than 100%. If the duty ratio exceeds the second threshold Dth2, the period during which the switching element of the lower arm is on becomes too short, which may result in the voltage across the shunt resistor not being accurately detected.

[0061] For example, the second threshold Dth2 may be set according to the settling time required for the voltage across the shunt resistor measured by the voltage drop measurement unit 36 ​​to stabilize after the switching of the lower-arm switching element. For example, the second threshold Dth2 may be set so that the length of the on-period of the lower-arm switching element in the duty intermediate phase does not become shorter than the settling time. For example, the second threshold Dth2 may be set to a value smaller than the first threshold Dth1.

[0062] 9B, in the portion indicated by the arrow B1, the duty ratio Dw of the W-phase, which is the duty intermediate phase (i.e., the W-phase voltage command value vw), exceeds the second threshold value Dth2. For example, adding a dead time compensation value to the voltage command value makes it easier for the duty ratio of the duty intermediate phase to exceed the second threshold value Dth2. Therefore, when the target duty ratio of the maximum duty phase is greater than the first threshold value Dth1, the voltage limiting unit 45 limits the basic q-axis voltage command value vq0 and the basic d-axis voltage command value vd0 to set the q-axis voltage command value vq and the d-axis voltage command value vd so that the duty ratio D2 of the duty intermediate phase does not exceed the second threshold value Dth2.

[0063] For example, the voltage limiting unit 45 determines the magnitude of the voltage vector of the basic q-axis voltage command value vq0 and the basic d-axis voltage command value vd0.

[0064]

[0065] Based on this, the limit gain G ｄｕｔｙ may be calculated.

[0066]

[0067] The constant D in the above equation (1) Ｌｔｄ is a duty limit value that is set to a value less than 100%. Ｌｔｄ is appropriately set so that the duty ratio D2 of the intermediate duty phase does not exceed the second threshold value Dth2 when the target duty ratio of the maximum duty phase is greater than the first threshold value Dth1. DT is a dead time compensation value added to the voltage command value by the dead time compensator 47. DT For example, the PWM period T ＰＷＭ The division result (C DT =T ＰＷＭ Since there is often a discrepancy between the design value and the actual value of the dead time, an appropriately set value may be used as the dead time compensation value C. DT The constant (2 / √3) is a coefficient corresponding to the reciprocal of the rate of decrease in the amplitude of the three-phase voltage command values ​​due to the superposition of the third-order harmonic component (amplitude before compensation / amplitude after compensation). When the third-order harmonic component is not superimposed on the three-phase voltage command values, the coefficient (2 / √3) may be omitted.

[0068] The voltage limiting unit 45 limits the gain G ｄｕｔｙ is greater than or equal to "1" (i.e., greater than or equal to 100%). ｄｕｔｙ is equal to or greater than "1", the voltage limiting unit 45 outputs the basic q-axis voltage command value vq0 and the basic d-axis voltage command value vd0 as the q-axis voltage command value vq and the d-axis voltage command value vd without limiting them. On the other hand, the limiting gain G ｄｕｔｙ is less than "1", the voltage limiting unit 45 ｄｕｔｙ / 2) to the basic q-axis voltage command value vq0 and the basic d-axis voltage command value vd0, and the product is calculated as the q-axis voltage command value vq=(G ｄｕｔｙ / 2) × vq0 and d-axis voltage command value vd = (G ｄｕｔｙ / 2) × vd0.

[0069] 10A shows the duty ratios Du, Dv, and Dw of the U-phase, V-phase, and W-phase (i.e., the U-phase voltage command value vu, the V-phase voltage command value vv, and the W-phase voltage command value vw) when the basic q-axis voltage command value vq0 and the basic d-axis voltage command value vd0 are limited. FIG. 10B shows an enlarged view of the portion indicated by the arrow A2 in FIG. 10A. In the portion indicated by the arrow B2, the duty ratio Dw of the W-phase, which is the duty intermediate phase (i.e., the W-phase voltage command value vw), is limited to be equal to or less than the second threshold value Dth2. This prevents the period during which the switching element of the lower arm of the duty intermediate phase is on from becoming too short, which would make it impossible to accurately detect the voltage across the shunt resistor rw.

[0070] 9A, attention is now focused on the portion indicated by the arrow C1. In the portion indicated by the arrow C1, the duty ratio Dw of the W-phase, which is the minimum duty phase (i.e., the W-phase voltage command value vw), approaches "0" due to the superposition of the third-order harmonic component. When the duty ratio approaches "0," the period during which the switching elements of the upper arm are on becomes too short, and there is a risk of a problem in which current does not flow through the switching elements of the upper arm even if the duty ratio is slightly greater than "0."

[0071] 10A, by limiting the basic q-axis voltage command value vq0 and the basic d-axis voltage command value vd0, the duty ratio of the minimum duty phase can be prevented from being set to a value close to "0." This makes it possible to avoid the problem of current not flowing through the switching element of the upper arm even if the duty ratio is slightly larger than "0."

[0072] (Operation) Figure 11 is a flowchart of an example of a current measurement method according to an embodiment. In step S1, the torque sensor 10 and the vehicle speed sensor 12 detect the steering torque Th and the vehicle speed Vh of the steering axle 2. In step S2, the current command value calculation unit 40 calculates a q-axis current command value Iq0 and a d-axis current command value Id0 based on the steering torque Th and the vehicle speed Vh. The PI control unit 44 calculates a basic q-axis voltage command value vq0 and a basic d-axis voltage command value vd0 based on the current deviations of the q-axis current iq and the d-axis current id from the q-axis current command value Iq0 and the d-axis current command value Id0. The PI control unit 44 calculates a zero-midpoint duty ratio, which varies within a range from -50% to +50% with 0% as the center value, as the basic q-axis voltage command value vq0 and the basic d-axis voltage command value vd0.

[0073] In step S3, the voltage limiting unit 45 sets the q-axis voltage command value vq and the d-axis voltage command value vd by limiting the basic q-axis voltage command value vq0 and the basic d-axis voltage command value vd0. Specifically, the voltage limiting unit 45 sets the q-axis voltage command value vq and the d-axis voltage command value vd by limiting the limit gain G ｄｕｔｙ is calculated, and the limit gain G ｄｕｔｙ is equal to or greater than "1", the voltage limiting unit 45 outputs the basic q-axis voltage command value vq0 and the basic d-axis voltage command value vd0 as they are as the q-axis voltage command value vq and the d-axis voltage command value vd without limiting them.

[0074] On the other hand, the limit gain G ｄｕｔｙ is less than "1", the voltage limiting unit 45 ｄｕｔｙ / 2) to the basic q-axis voltage command value vq0 and the basic d-axis voltage command value vd0, and the product is calculated as the q-axis voltage command value vq=(G ｄｕｔｙ / 2) × vq0 and d-axis voltage command value vd = (G ｄｕｔｙ / 2) × vd0.

[0075] In step S4, two-phase / three-phase converter 46 converts q-axis voltage command value vq and d-axis voltage command value vd into a first U-phase voltage command value vu1, a first V-phase voltage command value vv1, and a first W-phase voltage command value vw1, respectively. In step S5, adders 49 to 51 add the dead-time compensation value and the third-order harmonic component to the first U-phase voltage command value vu1, the first V-phase voltage command value vv1, and the first W-phase voltage command value vw1, respectively, to calculate a second U-phase voltage command value vu2, a second V-phase voltage command value vv2, and a second W-phase voltage command value vw2.

[0076] In step S6, the duty shift unit 52 corrects the midpoints of the duty ratios of the second U-phase voltage command value vu2, the second V-phase voltage command value vv2, and the second W-phase voltage command value vw2 to convert them into duty ratios that vary within a range from 0% to 100% with 50% as the center value. In step S7, the duty shift unit 52 selects the maximum duty phase.

[0077] In step S8, the duty shift unit 52 determines whether the target duty ratio D1t of the maximum duty phase is equal to or less than the first threshold value Dth1. If the target duty ratio D1t is equal to or less than the first threshold value Dth1 (step S8: Y), the process proceeds to step S9. If the target duty ratio D1t is not equal to or less than the first threshold value Dth1 (step S8: N), the process proceeds to step S10.

[0078] In step S9, the duty shift unit 52 outputs the second U-phase voltage command value vu2, the second V-phase voltage command value vv2, and the second W-phase voltage command value vw2, the midpoints of which have been corrected in step S6, as the U-phase voltage command value vu, the V-phase voltage command value vv, and the W-phase voltage command value vw. As a result, the switching element of the maximum duty phase is driven at the target duty ratio D1t. Furthermore, the switching elements of the intermediate duty phase and the minimum duty phase are driven at the target duty ratio D2t. Thereafter, the process proceeds to step S12.

[0079] In step S10, the duty shift unit 52 corrects the duty ratio of the maximum duty phase to 100% by shifting the target duty ratio D1t to a duty ratio of 100%. Also, the duty ratios of the intermediate duty phase and the minimum duty phase are corrected to the sum (D2t + ΔD) obtained by adding the duty difference ΔD = (100% - D1t) to the target duty ratio D2t.

[0080] In step S11, the duty shift unit 52 outputs the duty ratios corrected in step S10 as a U-phase voltage command value vu, a V-phase voltage command value vv, and a W-phase voltage command value vw. As a result, the switching elements of the maximum duty phase, the intermediate duty phase, and the minimum duty phase are driven with the duty ratios corrected in step S10. Thereafter, the process proceeds to step S12.

[0081] In step S12, the voltage drop measurement unit 36 ​​and the current calculation unit 54 measure the U-phase current Iu, the V-phase current Iv, and the W-phase current Iw. For example, the current calculation unit 54 may measure the phase currents of the intermediate duty phase and the minimum duty phase based on the voltages across the shunt resistors of the intermediate duty phase and the minimum duty phase, and then measure the phase current of the maximum duty phase based on these phase currents of the intermediate duty phase and the minimum duty phase and Kirchhoff's law. The process then ends.

[0082] (Modifications) (1) In the above embodiment, the shunt resistors ru, rv, and rw are connected in series to the lower arm switching elements Qu2, Qv2, and Qw2 (the so-called “downstream shunt system”). In this case, the upper arm switching elements Qu1, Qv1, and Qw1 are examples of the “first switching elements” recited in the claims, and the lower arm switching elements Qu2, Qv2, and Qw2 are examples of the “second switching elements” recited in the claims.

[0083] However, the present invention is not limited to the downstream shunt type, and can also be applied to a configuration in which shunt resistors are connected in series to the upper arm switching elements Qu1, Qv1, Qw1 (a so-called "upstream shunt type" configuration). In this case, the upper arm switching elements Qu1, Qv1, Qw1 are examples of the "second switching elements" set forth in the claims, and the lower arm switching elements Qu2, Qv2, Qw2 are examples of the "first switching elements" set forth in the claims.

[0084] In this case, the minimum duty phase (i.e., the phase with the smallest target duty ratio) turns on the lower arm switching element (first switching element) for a period equal to the PWM period T ＰＷＭ Hereinafter, the period during which the lower arm switching element is turned on is set to the PWM period T ＰＷＭ The ratio of this to the lower duty ratio is sometimes referred to as the "lower duty ratio."

[0085] The duty shift unit 52 shifts the duty ratio of the minimum duty phase to 0% when the target duty ratio of the minimum duty phase is less than a predetermined threshold set to a value greater than 0%. As a result, when the target value of the lower duty ratio in the minimum duty phase is greater than a first threshold set to a value less than 100%, the duty shift unit 52 shifts the target value of the lower duty ratio to 100%. In addition, the gate drive circuit 32 corrects and decreases the duty ratios of the intermediate duty phase and the maximum duty phase in conjunction with shifting the duty ratio of the minimum duty phase to 0%.

[0086] (2) In the above explanation, an example was described in which the power conversion device of the present invention is applied to a column assist type electric power steering device, also known as an upstream assist type. However, the power conversion device of the present invention may also be applied to a downstream assist type electric power steering device. Below, configuration examples in which the power conversion device of the present invention is applied to single pinion assist type, rack assist type, and dual pinion assist type electric power steering devices as examples of downstream assist type electric power steering devices will be explained. In the case of a downstream assist type, the motor 20, rotation angle sensor 21, and ECU 30 may not be separate units, but may be an integrated MCU (Motor Control Unit) for waterproofing purposes, as shown by the dashed lines in Figures 12 to 14.

[0087] FIG. 12 shows a configuration example in which the power conversion device of the present invention is applied to a single-pinion assist electric power steering device. A steering wheel 1 is connected to one universal joint 4a of an intermediate shaft via a steering shaft 2. An input shaft 4c of a torsion bar (not shown) is connected to the other universal joint 4b. A pinion-rack mechanism 5 includes a pinion gear (pinion) 5a, a rack bar (rack) 5b, and a pinion shaft 5c. The input shaft 4c and the pinion-rack mechanism 5 are connected by a torsion bar (not shown) that twists due to a misalignment in the rotational angle between the input shaft 4c and the pinion-rack mechanism 5. A torque sensor 10 electromagnetically measures the torsion angle of the torsion bar as the steering torque Th of the steering wheel 1. A motor 20 that assists the steering force of the steering wheel 1 is connected to the pinion shaft 5c via a reduction gear 3, and a rotation angle sensor 21 calculates rotation angle information of the motor rotation shaft of the motor 20.

[0088] (3) Figure 13 shows a configuration example in which the power conversion device of the present invention is applied to a rack-assist type electric power steering device. A spiral groove (not shown) is formed on the outer peripheral surface of the rack bar 5b, and a similar lead spiral groove (not shown) is also formed on the inner peripheral surface of the nut 81. A ball screw is formed by arranging multiple rolling elements in the rolling path formed by these spiral grooves. A belt 84 is wound around a drive pulley 82 connected to a rotating shaft 20a of a motor 20 that assists the steering force of the steering wheel 1, and a driven pulley 83 connected to the nut 81, and the rotational motion of the rotating shaft 20a is converted into linear motion of the rack bar 5b. A rotation angle sensor 21 calculates rotation angle information of the motor rotating shaft of the motor 20.

[0089] (4) Figure 14 shows a configuration example in which the power conversion device of the present invention is applied to a dual pinion assist type electric power steering device. The dual pinion assist type electric power steering device has a second pinion shaft 85 and a second pinion gear 86 in addition to a pinion shaft 5c and a pinion gear 5a, and a rack bar 5b has first rack teeth (not shown) that mesh with the pinion gear 5a and second rack teeth (not shown) that mesh with the second pinion gear 86. A motor 20 that assists the steering force of the steering wheel 1 is connected to the second pinion shaft 85 via a reduction gear 3, and a rotation angle sensor 21 similarly calculates rotation angle information of the motor rotation shaft of the motor 20.

[0090] (Effects of the Embodiments) (1) A power conversion device according to an embodiment includes: a multi-phase inverter having upper arm switching elements and lower arm switching elements of a plurality of phases connected in series; a voltage command value generation unit that generates a q-axis voltage command value and a d-axis voltage command value for driving the multi-phase inverter; a voltage command value limiting unit that limits the q-axis voltage command value and the d-axis voltage command value; a voltage command value conversion unit that converts the q-axis voltage command value and the d-axis voltage command value limited by the voltage command value limiting unit into a multi-phase voltage command value; a switching control unit that drives a first switching element that is one of the upper arm switching elements and the lower arm switching elements and a second switching element that is the other of the upper arm switching elements and the lower arm switching elements other than the first switching element, by PWM control, based on the q-axis voltage command value and the d-axis voltage command value limited by the voltage command value limiting unit; and a current measurement unit that measures a current flowing through the second switching element, based on a voltage drop across a resistor connected in series to the second switching element.

[0091] The switching control unit sets a target duty ratio, which is a target value of the duty ratio for which the period during which the first switching element is on occupies a PWM period, in accordance with a target voltage to be applied to a load of the multi-phase inverter, and drives the first switching element of the first phase at the target duty ratio when the target duty ratio of a first phase, which is the phase with the highest target duty ratio among the multiple phases of the multi-phase inverter, is equal to or less than a first threshold value set to less than 100%, and drives the first switching element of the first phase at a duty ratio of 100% when the target duty ratio of the first phase is greater than the first threshold value, and drives the first switching element of the second phase at a corrected duty ratio obtained by increasing and correcting the target duty ratio of a second phase other than the first phase among the multiple phases.

[0092] The voltage command value limiting unit limits the q-axis voltage command value and the d-axis voltage command value so that the corrected duty ratio does not exceed a predetermined second threshold value set to less than 100%. This prevents superimposition of switching noise from the switching elements of the first phase when measuring the phase current of a second phase other than the first phase having the highest target duty ratio. This also prevents changes in the phase current caused by shifting the duty ratio of the first phase having the highest target duty ratio to 100%. This also prevents the inability to accurately detect the phase current of the second phase due to an excessively short on-period of the second switching element of the second phase. As a result, the measurement accuracy of the phase current can be improved when the duty ratio of the first phase, which is the phase having the highest duty ratio under PWM control, is high.

[0093] (2) For example, the second threshold may be set to a value smaller than the first threshold. Alternatively, for example, the second threshold may be set according to the settling time required for the measured value of the voltage drop across the resistor element detected by the current measuring unit to stabilize after switching of the second switching element. Alternatively, for example, the second threshold may be set so that the length of the on-period of the second-phase second switching element is not shorter than the settling time. This allows the second threshold to be set so that the on-period of the second-phase second switching element is not too short even if the target duty ratio of the second phase is increased when the duty ratio of the first phase is high.

[0094] (3) The voltage command value limiting unit may set a limiting gain based on a ratio obtained by subtracting the dead-time compensation value from a predetermined upper limit value set to less than 100%, and dividing the difference by the magnitude of the voltage vector of the q-axis voltage command value and the d-axis voltage command value, and may limit the q-axis voltage command value and the d-axis voltage command value by multiplying the q-axis voltage command value and the d-axis voltage command value by the limiting gain. This makes it possible to limit the q-axis voltage command value and the d-axis voltage command value so that the corrected duty ratio of the second phase does not exceed the second threshold value.

[0095] (4) The power conversion device may include a third-order harmonic compensator that superimposes a third-order harmonic component on a multiphase voltage command value. The voltage command value limiter may set a limiting gain based on a product of a ratio obtained by dividing a difference obtained by subtracting a dead-time compensation value from a predetermined upper limit value set to less than 100% by the magnitude of the voltage vector of the q-axis voltage command value and the d-axis voltage command value, and multiplying the result by a coefficient corresponding to a rate of decrease in amplitude of the multiphase voltage command value due to the superimposition of the third-order harmonic component. This allows the q-axis voltage command value and the d-axis voltage command value to be limited so that the corrected duty ratio of the second phase does not exceed a second threshold. (5) The voltage command value limiter may limit the q-axis voltage command value and the d-axis voltage command value so that the corrected duty ratio of a phase having the second-highest target duty ratio among the multiple phases of the multiphase inverter does not exceed a second threshold. This allows the q-axis voltage command value and the d-axis voltage command value to be limited so that the period during which the second switching element in the phase having the second-highest target duty ratio is on is not too short.

[0096] (6) The switching control unit may set the corrected duty ratio to be the sum of the target duty ratio of the second phase and the difference obtained by subtracting the target duty ratio of the first phase from 100%. This prevents a change in the phase current caused by shifting the duty ratio of the first phase, which has the highest target duty ratio, to 100%.

[0097] 1...Steering wheel, 2...Steering shaft, 3...Reduction gear, 4a, 4b...Universal joint, 4c...Input side shaft, 5...Pinion rack mechanism, 5a...Pinion gear (pinion), 5b...Rack bar (rack), 5c...Pinion shaft, 6a, 6b...Tie rod, 7a, 7b...Hub unit, 8L, 8R...Steering wheels, 10...Torque sensor, 11...Ignition switch, 12...Vehicle speed sensor, 13...Battery, 14...Steering angle sensor, 20...Motor, 20a...Rotating shaft, 20u...U-phase coil, 20v...V-phase coil, 20w...W-phase coil, 21...Rotation angle sensor, 30...Electronic control unit, 31...Control calculation unit, 32... Gate drive circuit, 33... multi-phase inverter, 34... current interruption circuit, 35... interruption drive circuit, 36... voltage drop measurement unit, 37... motor rotation speed calculation unit, 40... current command value calculation unit, 41, 42... subtractor, 43... current limiting unit, 44... proportional integral control unit, 45... voltage limiting unit, 46... two-phase / three-phase conversion unit, 47... dead time compensation unit, 48... third-order harmonic compensation unit, 49, 50, 51... adder, 52... duty shift unit, 53... PWM control unit, 54... current calculation unit, 55... three-phase / two-phase conversion unit, 56... angular velocity conversion unit, 81... nut, 82... drive pulley, 83... driven pulley, 84... belt, 85... second pinion shaft, 86... second pinion gear

Claims

1. a multi-phase inverter having a series connection of upper arm switching elements and lower arm switching elements of a plurality of phases; a voltage command value generating unit that generates a q-axis voltage command value and a d-axis voltage command value for driving the multi-phase inverter; a voltage command value limiting unit that limits the q-axis voltage command value and the d-axis voltage command value; a voltage command value conversion unit that converts the q-axis voltage command value and the d-axis voltage command value limited by the voltage command value limiting unit into a multi-phase voltage command value; a switching control unit that drives a first switching element which is either one of the upper arm switching element and the lower arm switching element, and a second switching element which is the other switching element other than the first switching element among the upper arm switching element and the lower arm switching element, based on the q-axis voltage command value and the d-axis voltage command value limited by the voltage command value limiting unit, by PWM control; a current measuring unit that measures a current flowing through the second switching element based on a voltage drop across a resistor connected in series to the second switching element; Equipped with The switching control unit is a target duty ratio, which is a target value of a duty ratio of a period during which the first switching element is turned on in a PWM cycle, is set in accordance with a target voltage to be applied to a load of the multi-phase inverter; When the target duty ratio of a first phase, which is a phase having the highest target duty ratio among a plurality of phases of the multi-phase inverter, is equal to or less than a predetermined first threshold value set to less than 100%, the first switching element of the first phase is driven at the target duty ratio; When the target duty ratio of the first phase is greater than the first threshold value, the first switching element of the first phase is driven at the duty ratio of 100%, and the first switching element of the second phase is driven at a corrected duty ratio obtained by increasing and correcting the target duty ratio of a second phase other than the first phase among the multiple phases, The voltage command value limiting unit is a limit gain is set based on a ratio obtained by dividing a difference obtained by subtracting a dead time compensation value from a predetermined upper limit value set to less than 100% by a magnitude of a voltage vector of the q-axis voltage command value and the d-axis voltage command value; multiplying the q-axis voltage command value and the d-axis voltage command value by the limit gain to limit the q-axis voltage command value and the d-axis voltage command value so that the corrected duty ratio does not exceed a predetermined second threshold value that is set to be less than 100%. A power conversion device comprising:

2. The power conversion device according to claim 1 , wherein the second threshold value is set to a value smaller than the first threshold value.

3. 2. The power conversion device according to claim 1, wherein the second threshold is set according to a settling time until a measured value of the voltage drop across the resistance element detected by the current measuring unit becomes stable after switching of the second switching element.

4. 4. The power conversion device according to claim 3, wherein the second threshold value is set so that the length of an on-period of the second switching element of the second phase does not become less than the settling time.

5. (delete)

6. a third harmonic compensator that superimposes a third harmonic component on the multiphase voltage command value, the voltage command value limiting unit sets a limiting gain based on a product obtained by multiplying a ratio obtained by dividing a difference obtained by subtracting a dead-time compensation value from a predetermined upper limit value set to less than 100% by a magnitude of a voltage vector of the q-axis voltage command value and the d-axis voltage command value by a coefficient corresponding to a reduction rate of an amplitude of the multiphase voltage command value due to superposition of the third harmonic component.

2. The power conversion device according to claim 1 .

7. 2. The power conversion device according to claim 1, wherein the voltage command value limiting unit limits the q-axis voltage command value and the d-axis voltage command value such that the corrected duty ratio in a phase having the second highest target duty ratio among the multiple phases of the multi-phase inverter does not exceed the second threshold value.

8. 2. The power conversion device according to claim 1, wherein the switching control unit sets, as the corrected duty ratio, a sum obtained by adding a difference obtained by subtracting the target duty ratio of the first phase from 100% to the target duty ratio of the second phase.

9. A power conversion device according to any one of claims 1 to 4 and 6 to 8; a controller that controls the multi-phase inverter that drives an electric motor based on a measurement result of the current measurement unit; A motor control device comprising:

10. The motor control device according to claim 9 ; a polyphase motor controlled by the motor control device; An electric power steering device, characterized in that the polyphase motor applies a steering assist force to a steering system of a vehicle.