Drive device for synchronous motor and drive method for synchronous motor

Abstract

This invention provides a highly reliable synchronous motor drive device capable of calculating and limiting DC current with high accuracy without requiring additional structures such as dedicated current detection sensors. It is characterized by comprising: a first DC current calculation unit that calculates a first DC current based on a first voltage command value, the current value of the synchronous motor windings, and the battery voltage; a second DC current calculation unit that calculates a second DC current based on the current value of the synchronous motor windings, the rotational speed of the synchronous motor, and predetermined motor characteristics of the synchronous motor; a model error extraction unit that calculates inverter efficiency based on the first DC current and the second DC current; a DC current correction unit that calculates a calculated DC current based on the first DC current, the second DC current, and the inverter efficiency; and a DC current limiting unit that corrects the current command value of the windings based on the calculated DC current and a predetermined DC current limit value.

Description

Technical Field

[0001] This invention relates to the structure and control of a drive device for driving a synchronous motor, and particularly to a technology for the effective application of electric motors for electric braking in automobiles that require high responsiveness and high reliability. Background Technology

[0002] In recent years, the electrification of auxiliary machine components in automobiles, such as steering and braking systems, has been progressing. Electrification of these components eliminates the need for existing hydraulic systems, leading to improvements in maintainability and vehicle operability / controllability. In such electrified auxiliary machines, power conversion devices such as inverters typically convert the DC power from the auxiliary machine's battery into AC power, which then drives a synchronous motor to operate the actuators for steering and braking.

[0003] With the electrification of critical automotive components such as steering and braking, a redundant structure is required to ensure continued operation even in the event of failures in electrical components such as synchronous motors and inverters. For example, this involves replacing the existing three-phase windings of the motor with multiple three-phase windings and also multiplexing the inverter, thus creating a structure that continues operation even in the event of a failure.

[0004] However, the power that can be supplied from the auxiliary machine battery is limited, and there are situations where the power consumed during steering and braking is determined by the vehicle design. In such cases, countermeasures are sometimes implemented to ensure that the DC current output from the auxiliary machine battery does not exceed the limit.

[0005] As a countermeasure, for example, there is a method as described in Patent Document 1, which involves setting up a sensor to detect DC current and reducing the inverter output to limit the output of DC current when the DC current exceeds a limit value.

[0006] Existing technical documents

[0007] Patent documents

[0008] Patent Document 1: Japanese Patent Application Publication No. 2012-175770 Summary of the Invention

[0009] The problem that the invention aims to solve

[0010] Patent document 1 disclosed a technique for distributing battery power by detecting battery status using sensors or the like and controlling a synchronous motor based on that battery status.

[0011] However, the need for additional structures such as sensors and hardware filters poses a risk of increased costs and larger device size.

[0012] In addition, the aforementioned multi-winding structure requires sensors to be installed in each winding system, which further increases the cost.

[0013] One method to prevent increased costs and limit DC current is, as is generally known, to calculate the effective power based on the inverter's output voltage and output current, and to obtain the DC current by dividing the effective power by the battery voltage.

[0014] However, because the efficiency of the inverter is not taken into account, there is an error in the calculation of the DC current, and there is a possibility that the calculation may not be appropriate and may exceed the limit.

[0015] Therefore, the object of the present invention is to provide a highly reliable synchronous motor drive device that can perform high-precision DC current calculation and limitation without the need for additional structures such as dedicated current detection sensors, and a method for driving the synchronous motor using the same device.

[0016] Technical solutions for solving the problem

[0017] To address the aforementioned issues, the present invention is characterized by comprising: a first DC current calculation unit that calculates a first DC current based on a first voltage command value, the current value of the synchronous motor winding, and the battery voltage; a second DC current calculation unit that calculates a second DC current based on the current value of the synchronous motor winding, the speed of the synchronous motor, and a predetermined motor characteristic of the synchronous motor; a model error extraction unit that calculates inverter efficiency based on the first DC current and the second DC current; a DC current correction unit that calculates a calculated DC current based on the first DC current, the second DC current, and the inverter efficiency; and a DC current limiting unit that corrects the current command value of the winding based on the calculated DC current and a predetermined DC current limit value.

[0018] Furthermore, the present invention is characterized by including: (a) a step of calculating a first DC current based on a first voltage command value, the current value of the synchronous motor winding, and the battery voltage; (b) a step of calculating a second DC current based on the current value of the synchronous motor winding, the speed of the synchronous motor, and a predetermined motor characteristic of the synchronous motor; (c) a step of calculating the inverter efficiency based on the first DC current and the second DC current; (d) a step of calculating a calculated DC current based on the first DC current, the second DC current, and the inverter efficiency; and (e) a step of correcting the current command value of the winding based on the calculated DC current and a predetermined DC current limit value.

[0019] The effects of the invention

[0020] According to the present invention, a highly reliable synchronous motor drive device and a method for driving the synchronous motor using it are realized, which can perform high-precision DC current calculation and limitation without the need for additional structures such as dedicated current detection sensors.

[0021] This can help curb cost increases and improve the reliability of synchronous motors and electric brakes for automobiles equipped with them.

[0022] Other issues, structures, and effects not described above will be explained through the following description of the implementation methods. Attached Figure Description

[0023] Figure 1 This is a block diagram illustrating the main functions of the synchronous motor drive device according to Embodiment 1 of the present invention.

[0024] Figure 2 It means Figure 1 A block diagram showing the main functions of the DC current calculation unit.

[0025] Figure 3 It means Figure 1 A block diagram showing the main functions of the DC current limiting section.

[0026] Figure 4 This is a schematic diagram illustrating the DC current of a conventional synchronous motor drive unit.

[0027] Figure 5 This is a diagram schematically illustrating the DC current of the drive device for the synchronous motor of Embodiment 1 of the present invention.

[0028] Figure 6 This is a diagram showing the schematic structure of the vehicle according to Embodiment 2 of the present invention. Detailed Implementation

[0029] Hereinafter, embodiments of the present invention will be described using the accompanying drawings. Furthermore, the same reference numerals are used for the same structures in each drawing, and detailed descriptions of repeated parts are omitted.

[0030] Furthermore, the following description and accompanying drawings are examples for illustrating the invention, and omissions and simplifications have been appropriately made for clarity. The invention can also be implemented in various other ways. Unless otherwise specified, each constituent element can be single or multiple.

[0031] Example 1

[0032] refer to Figures 1 to 5 The driving device for the synchronous motor according to Embodiment 1 of the present invention and the driving method for the synchronous motor using it will be described. Among them, Figure 4A diagram of the DC current of a conventional synchronous motor drive device is shown to facilitate understanding of the effects of the present invention.

[0033] Figure 1 This is a functional block diagram of the synchronous motor drive device 100 in this embodiment.

[0034] The synchronous motor drive device 100 in this embodiment is as follows: Figure 1 As shown, the main structure includes a current detection unit 300, a power converter 400, a current control unit 500, a current command calculation unit 600, a coordinate transformation unit 700, a DC current calculation unit 800, and a DC current limiting unit 900, which drives a synchronous motor 200.

[0035] The synchronous motor 200 is, for example, a permanent magnet type synchronous motor, which uses permanent magnets (strong magnetic materials) to form the rotor and armature windings to form the stator. Alternatively, in this embodiment, the synchronous motor 200 is a double three-phase winding motor, having a structure with two three-phase windings.

[0036] The synchronous motor drive unit 100 provides AC power to one of the three-phase windings of the dual three-phase winding. Additionally, a synchronous motor drive unit 100', having the same structure as the synchronous motor drive unit 100, provides AC power to the other three-phase winding, thereby driving the dual three-phase winding motor.

[0037] The current detection unit 300 consists of current sensors 300u, 300v, and 300w that detect the three-phase current flowing in the synchronous motor 200. The current sensors 300u, 300v, and 300w are respectively configured for each phase of the synchronous motor 200. The current detection unit 300 detects the currents Iu, Iv, and Iw and outputs them to the coordinate transformation unit 700.

[0038] Furthermore, an example is shown where current sensors 300u, 300v, and 300w are configured for each phase of the synchronous motor 200. However, it is also possible to utilize the fact that the sum of the symmetrical three-phase AC currents is zero, and only two phases (e.g., phase u and phase v) are configured. Alternatively, a structure can be adopted where the three-phase current of the synchronous motor 200 is obtained based on the current flowing in the DC bus (not shown) of the power converter 400. Using these structures reduces the number of current sensors, thus achieving cost reduction.

[0039] The power converter 400 is, for example, an inverter. When driving the synchronous motor 200, it controls the semiconductor switching elements of the power converter 400 to be turned on and off based on the voltage commands Vd* and Vq* from the current control unit 500, and applies voltages Vu, Vv, and Vw to the synchronous motor 200 to drive the synchronous motor 200.

[0040] The current control unit 500 receives the d-axis current command Id* from the current command calculation unit 600, the q-axis current command Iq* output from the DC current limiting unit 900, and the d-axis current Id and q-axis current Iq output from the coordinate transformation unit 700, and controls them in a manner that makes Id and Iq consistent with the command values ​​Id* and Iq*.

[0041] For example, a commonly known structure combines PI control with non-interference control. These controls output voltage commands Vd* and Vq* for the dq axes, which are then sent to the power converter 400.

[0042] The current command calculation unit 600 determines the d-axis current command Id* and the first q-axis current command Iq1* for driving the synchronous motor 200 based on the output of the upper-level control (not shown), namely the torque command T*, and the speed ω of the synchronous motor 200.

[0043] For example, a lookup table prepared based on the pre-calculated characteristics of the synchronous motor 200, or a mathematical model of the synchronous motor 200, is used to determine this. The output Id* of the current command calculation unit 600 is sent to the current control unit 500, and another output Iq1* is input to the DC current limiting unit 900, which will be described later.

[0044] The coordinate transformation unit 700 performs coordinate transformation on the three-phase currents Iu, Iv, and Iw of the synchronous motor 200, transforming them into d-axis current Id and q-axis current Iq. In this transformation, the electrical phase θ of the synchronous motor 200 is used for the rotational coordinate transformation. The outputs Id and Iq of the coordinate transformation unit 700 are sent to the DC current calculation unit 800 and the current control unit 500. In the DC current calculation unit 800, the DC current is calculated, as described later, and in the current control unit 500, the dq-axis voltage commands Vd* and Vq* are calculated.

[0045] The DC current calculation unit 800 takes the d-axis current Id and q-axis current Iq from the coordinate transformation unit 700, the d-axis voltage command Vd* and q-axis voltage command Vq* from the current control unit 500, the battery voltage Vbat, and the rotational speed ω of the synchronous motor 200 as inputs to calculate the calculated DC current value Idc^. Furthermore, the calculated Idc^ is sent to the DC current limiting unit 900.

[0046] The DC current limiting unit 900 takes the calculated DC current value Idc^ as input and calculates the q-axis current compensation value Iqcmp. Furthermore, it compensates for the q-axis current by adding it to the first q-axis current command Iq1* calculated in the current command calculation unit 600, thereby limiting the DC current or increasing the output.

[0047] Figure 2 yes Figure 1 Functional block diagram of the DC current calculation unit 800.

[0048] The DC current calculation unit 800 includes a first DC current calculation unit 810, a second DC current calculation unit 820, a model error extraction unit 830, and a DC current correction unit 840.

[0049] The first DC current calculation unit 810 takes the d-axis voltage command Vd*, the q-axis voltage command Vq*, the d-axis current Id, the q-axis current Iq, and the battery voltage Vbat as inputs to calculate the first DC current Idc1^.

[0050] The first DC current Idc1^ is calculated, for example, using the generally known formula (1).

[0051] Idc1^=(Vd*×Id+Vq*×Iq) / Vbat…(1)

[0052] Equation (1) is based on the balance between the inverter's input power (i.e., the product of DC current and DC voltage) and the inverter's output voltage (i.e., the inner product of voltage and current) to obtain the first DC current Idc1^.

[0053] However, in general, in power converters such as inverters, energy loss occurs during power conversion, so the output power of the inverter is less than the input power. Therefore, the actual DC current, when considering the inverter efficiency, becomes Equation (2).

[0054] Idc=(Vd*×Id+Vq*×Iq) / Vbat / η…(2)

[0055] The commonly known equation (1) does not take into account the inverter efficiency, so it will inevitably produce errors. If the inverter efficiency η is measured in advance, the DC current can be calculated with good accuracy, but the inverter efficiency varies due to its operating voltage and current, or temperature, so it is difficult to measure in advance.

[0056] Therefore, in this embodiment, a structure having a second DC current calculation unit 820, a model error extraction unit 830, and a DC current correction unit 840 is adopted.

[0057] The second DC current calculation unit 820 takes the d-axis current Id, the q-axis current Iq, the motor speed ω, and the battery voltage Vbat as inputs to calculate the second DC current Idc2^. The second DC current calculation unit 820 is, for example, a module that calculates DC current based on motor characteristics. In this embodiment, equations (3), (4), and (5) are used.

[0058] Vd^=R×Id-ωLqIq…(3)

[0059] Vq^=R×Iq+ωLdId+ωΦ…(4)

[0060] Idc2^=(Vd^×Id+Vq^×Iq) / Vbat…(5)

[0061] Here, R is the winding resistance of the synchronous motor 200, Ld and Lq are the d-axis inductance and q-axis inductance, respectively, and Φ is the linkage flux of the permanent magnet of the synchronous motor 200.

[0062] In equations (3) and (4), the input voltage of the synchronous motor 200 is calculated using the motor model. In equation (5), the second DC current Idc2^ is calculated using the input voltage, d-axis current Id, and q-axis current Iq obtained in equations (3) and (4).

[0063] Equations (3) to (5) do not take into account the ideal DC current of the inverter.

[0064] The model error extraction unit 830 has an error extraction unit 831 and an error correction unit 832. It takes the first DC current Idc1^ and the second DC current Idc2^ as inputs, and corrects and calculates the inverter efficiency information η^ based on these values.

[0065] The error correction unit 832 takes the second DC current Idc2^ and the inverter efficiency information η^ as input, calculates equation (6), and sends it to the error extraction unit 831.

[0066] Idc2^×η^…(6)

[0067] The error extraction unit 831 takes the first DC current Idc1^ and the output of the error correction unit 832 as inputs and outputs the inverter efficiency information η^.

[0068] In the error extraction unit 831 of this embodiment, the inverter efficiency information η^ is calculated and output using equation (7).

[0069] η^=(η^×Idc2^-Idc1^) / s...(7)

[0070] Here, s is the Laplace operator.

[0071] According to the relationship between equations (1) and (2), the first DC current Idc1^ is obtained by multiplying the actual DC current Idc by the inverter efficiency η. That is, it is a value that includes information about the inverter efficiency η.

[0072] On the other hand, the second DC current Idc2^ is an ideal DC current obtained based on a motor model representing the characteristics of the synchronous motor 200, without considering the inverter efficiency.

[0073] By extracting only the information of inverter efficiency η from the first DC current Idc1^ which includes information about inverter efficiency η and the second DC current Idc2^ which does not include information about inverter efficiency η, the DC current can be calculated with good accuracy.

[0074] The output of the model error extraction unit 830, namely the inverter efficiency information η^, is sent to the DC current correction unit 840.

[0075] The DC current correction unit 840 takes the first DC current Idc1^ and the inverter efficiency information η^ as inputs to calculate the estimated value of the DC current Idc^. In this embodiment, the calculation is performed according to equation (8).

[0076] Idc^=Idc1^ / η^…(8)

[0077] Figure 3 yes Figure 1 Functional block diagram of DC current limiting unit 900.

[0078] The DC current limiting unit 900 includes a q-axis current correction unit 910, which takes the calculated DC current value Idc^ and the predetermined DC current limit value Idclmt as inputs, and calculates the q-axis current correction amount Iqcmp using the q-axis current correction unit 910. The calculated Iqcmp is added to the first q-axis current command Iq1*, and output as the q-axis current command Iq*, which is then sent to the current control unit 500.

[0079] In this embodiment, the DC current limiting unit 900 and the q-axis current correction unit 910 calculate Iqcmp using equation (9), for example.

[0080] Iqcmp=(Idclmt-Idc^)×Kdc…(9)

[0081] Here, Kdc is the amplification gain.

[0082] By adopting the form of Equation (9), when the calculated DC current Idc^ exceeds the limit value Idclmt, the output is reduced by decreasing the q-axis current, thereby keeping it within the DC current limit value.

[0083] In addition, when the calculated DC current Idc^ is less than the limit value Idclmt, operating by increasing the q-axis current can improve responsiveness.

[0084] Next, use Figure 4 and Figure 5 This explains the effects of applying the present invention when using a dual three-phase motor as a synchronous motor 200.

[0085] Figure 4This is a schematic diagram illustrating the DC current of a conventional synchronous motor drive without the application of this invention.

[0086] When using a dual three-phase motor as a synchronous motor 200, DC current flows in each of the two three-phase winding systems. In this case, if errors exist in the winding system and inverter components, such as... Figure 4 The imbalance occurs in the direct current, as shown.

[0087] When the DC current of the first system or the DC current of the second system exceeds the DC current limit, the output of the dual three-phase winding motor is limited in order to limit the DC current. The DC current of the first system is suppressed within the DC current limit, but the DC current of the second system, which does not need to be limited, is also limited. Therefore, there is a risk of deterioration in responsiveness.

[0088] Therefore, by providing a synchronous motor drive device 100 and a synchronous motor drive device 100' that have a DC current calculation unit 800 and a DC current limiting unit 900 as in this embodiment, the problem can be solved.

[0089] Figure 5 This is a diagram schematically illustrating the direct current when the invention is applied to a dual three-phase motor.

[0090] The DC current calculation unit 800 can accurately calculate the DC current by taking into account the inverter efficiency information η^ that has not been considered before. Therefore, even if DC current imbalance occurs in the first system and the second system, it can suppress them within the DC current limit value.

[0091] Furthermore, because it allows for an increase in the DC current of the second system that does not require limitation, it improves responsiveness and enables high-response control of the actuators of electrically assisted machine components.

[0092] As explained above, in this embodiment, the synchronous motor drive device 100 has a structure including a DC current calculation unit 800 and a DC current limiting unit 900. In the DC current calculation unit 800, the inverter efficiency information η^ is extracted by calculating a first DC current Idc1^ that includes inverter efficiency information η^ and a second DC current Idc2^ that does not include inverter efficiency information η^, thereby improving the accuracy of DC current calculation.

[0093] In addition, a structure is adopted in which the DC current Idc^ calculated by the DC current calculation unit 800 is input to the DC current limiting unit 900, and compared with the predetermined DC current limit value Idclmt to correct the q-axis current command Iq*, thereby suppressing the DC current to the limit value, increasing the output and improving responsiveness while having a margin.

[0094] Furthermore, a structure with the same structure as the synchronous motor drive device 100 is adopted. When the synchronous motor drive device 100' is used as the synchronous motor 200, in the event of DC current imbalance, the q-axis current command Iq* is corrected in a limiting manner for the DC current exceeding the limit value, and the q-axis current command Iq* is corrected in an increasing manner for the DC current with a margin in the limit value, thereby improving the responsiveness.

[0095] This embodiment allows for appropriate limitation of the DC current in the electric assist device, contributing to improved reliability of redundant electric assist devices. Furthermore, by applying it to responsive steering and braking systems, it enables the provision of more responsive electric assist devices, thereby enhancing drivability.

[0096] Example 2

[0097] refer to Figure 6 The vehicle of Embodiment 2 of the present invention will be described. Figure 6 This is a diagram showing the schematic structure of vehicle 1 in this embodiment.

[0098] In this embodiment, an example will be described where the vehicle body 2 is equipped with drive units 100 and 100' of the synchronous motor described in Embodiment 1, and four electric brakes that comprehensively drive and control the braking of the four wheels respectively.

[0099] In this embodiment, vehicle 1 is as follows: Figure 6 As shown, the vehicle body 2 has wheels 3 and an electric brake 4. In addition, the vehicle 1 is equipped with a drive unit 100, 100' of a synchronous electric motor as described in Embodiment 1.

[0100] In addition, the other components are mounted in the same way as in a normal vehicle, but are not necessary for the description of the embodiments of the present invention, so the description is omitted.

[0101] Figure 6 In this system, a coordinate system is adopted with the front-to-back direction of vehicle 1 as the x-axis (front direction is positive), the left-to-right direction as the y-axis (left direction is positive), and the up-to-down direction as the z-axis (up direction is positive).

[0102] in addition, Figure 6 In the diagram, FL represents the structure corresponding to the left front, Fr represents the right front, RL represents the left rear, and Rr represents the right rear. Taking wheel 3 as an example, 3FL, 3Fr, 3RL, and 3Rr represent the left front wheel, right front wheel, left rear wheel, and right rear wheel, respectively.

[0103] The electric brake 4 is a device used to brake the rotation of the wheel 3. In this embodiment, there are four electric brakes: the electric brake 4FL for the left front wheel 3FL, the electric brake 4Fr for the right front wheel 3Fr, the electric brake 4RL for the left rear wheel 3RL, and the electric brake 4Rr for the right rear wheel 3Rr.

[0104] In the electric brake 4, a double three-phase winding motor (six-phase motor) with two three-phase windings is used as a synchronous motor for driving. The drive unit 100 of the synchronous motor provides AC power to one of the three-phase windings, and the drive unit 100' of another synchronous motor with the same structure as the drive unit 100 provides AC power to the other three-phase winding. Thus, the double three-phase winding motor is driven, and the electric brake 4 (4FL, 4Fr, 4RL, 4Rr) brakes the wheels 3 (3FL, 3Fr, 3RL, 3Rr).

[0105] The vehicle 1 in this embodiment is configured as described above, and the output of the synchronous motor drive units 100, 100' is distributed to four electric brakes 4 (4FL, 4Fr, 4RL, 4Rr), thereby comprehensively driving and controlling the four electric brakes that brake the four wheels respectively.

[0106] Furthermore, the present invention is not limited to the above embodiments, but includes various modifications. For example, the above embodiments are described in detail for ease of understanding of the present invention and are not limited to having all the structures described. Additionally, a portion of the structure of one embodiment can be replaced with the structure of another embodiment, and structures of other embodiments can be added to the structure of one embodiment. Furthermore, for a portion of the structure of each embodiment, other structures can be added, deleted, or replaced.

[0107] Explanation of reference numerals in the attached figures

[0108] 1…vehicle, 2…car body, 3 (3FL, 3Fr, 3RL, 3Rr)…wheel, 4 (4FL, 4Fr, 4RL, 4Rr)…electric brake, 100, 100'…drive unit for synchronous motor, 200…synchronous motor, 300…current detection unit, 300u, 300v, 300w…current sensor, 400…power converter, 500…current control unit, 600…current command calculation unit, 700…coordinate transformation unit, 800…DC current calculation unit, 810…first DC current calculation unit, 820…second DC current calculation unit, 830…model error extraction unit, 831…error extraction unit, 832…error correction unit, 840…DC current correction unit, 900…DC current limiting unit, 910…q-axis current correction unit.

Claims

1. A drive device for a synchronous motor, characterized in that, include: The first DC current calculation unit calculates the first DC current based on the d-axis voltage command, q-axis voltage command, d-axis current value, q-axis current value and battery voltage of the synchronous motor; The second DC current calculation unit calculates the second DC current based on the d-axis current value and q-axis current value of the winding of the synchronous motor, the speed of the synchronous motor, and the specified motor characteristics of the synchronous motor. The model error extraction unit calculates the inverter efficiency based on the first DC current and the second DC current; The DC current correction unit calculates and estimates the DC current based on the first DC current, the second DC current, and the inverter efficiency. and The DC current limiting unit corrects the q-axis current command value of the winding based on the calculated DC current and the predetermined DC current limit value.

2. The synchronous motor drive device as described in claim 1, characterized in that: The model error extraction unit includes an error extraction unit and an error correction unit. The error correction unit calculates the product of the second DC current and the inverter efficiency. The error extraction unit calculates the inverter efficiency based on the first DC current and the output of the error correction unit.

3. The drive apparatus of a synchronous motor as set forth in claim 1, characterized by have: A current detection unit for detecting the three-phase current flowing in the synchronous motor; and The coordinate transformation unit transforms the three-phase current into d-axis current and q-axis current.

4. The synchronous motor drive device as described in claim 1, characterized in that: The synchronous motor is a double three-phase winding motor with two three-phase windings. The synchronous motor's drive unit provides AC power to one of the two three-phase windings, and the drive unit of the other synchronous motor, which has the same structure as the synchronous motor's drive unit, provides AC power to the other three-phase winding, thereby driving the synchronous motor.

5. The synchronous motor drive device as described in claim 4, characterized in that: The synchronous motor is a six-phase motor that drives the electric brake of the vehicle. The system integrates and controls four electric brakes that brake the four wheels of the vehicle individually.

6. A drive method of a synchronous motor characterized by comprising: Includes the following steps: (a) The steps of calculating the first DC current based on the d-axis voltage command, q-axis voltage command, d-axis current value, q-axis current value and battery voltage of the synchronous motor; (b) The step of calculating the second DC current based on the d-axis current value and q-axis current value of the winding of the synchronous motor, the speed of the synchronous motor and the specified motor characteristics of the synchronous motor; (c) The step of calculating the inverter efficiency based on the first DC current and the second DC current; (d) The step of calculating the DC current based on the first DC current, the second DC current, and the inverter efficiency; (e) The step of correcting the q-axis current command value of the winding based on the calculated DC current and the predetermined DC current limit value.

7. The driving method for a synchronous motor as described in claim 6, characterized in that: Step (c) includes: (c1) The step of calculating the product of the second DC current and the inverter efficiency; (c2) a step of calculating the inverter efficiency based on the first direct current and the operation result of the (cl) step. (c2) a step of calculating the inverter efficiency based on the first direct current and the operation result of the (cl) step.

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