JPH09140187A - Power converter - Google Patents

Abstract

PROBLEM TO BE SOLVED: To limits the current while taking account of overcurent protection at low frequency at the time of motor stall or the like by limiting the current command value below a predetermined current limit value depending on the frequency of AC current being fed to a motor. SOLUTION: A current limiter 50 reads out a current limit value I1lim from an angular speed-current limit value table in a memory 51 according to an angular speed corresponding to an AC voltage frequency ω1 being applied to a stator winding from an adder 26. A q-axis stator winding current limit value I1qlim for limiting the current command value Ilq * of q-axis stator winding current variable depending on the load is then calculated based on the current limit value I1lim and a d-axis stator winding current command value I1d * outputted from a speed controller 30. The current command value I1q * of q-axis stator winding current being inputted to an adder 24 is limited below the q-axis stator winding current limit value Ilqlim . Consequently, overcurrent protection at low frequency can be performed inexpensively without derating a power converter 1.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a power converter for converting DC power into AC power of variable voltage and variable frequency and supplying the converted power to an electric motor.

[0002]

2. Description of the Related Art FIG. 5 shows a power converter similar to the conventional example disclosed in Japanese Patent Laid-Open No. 1-103189. In FIG. 5, reference numeral 1 denotes a variable voltage variable frequency power converter composed of switching elements, which converts normal DC power into a desired voltage and frequency and supplies it to the stator winding of the induction motor 2. 3 is a speed detector for detecting the rotor angular velocity of the induction motor 2; 4 is a current detector for detecting the three-phase alternating currents I _1U , I _1V , I _1W flowing into the stator windings of the induction motor 2; This is a three-phase to two-phase converter.
The three-phase AC currents I _1U , I _1V , and I _1W in a two-axis rotational coordinate system (dq coordinate system) that rotates in synchronization with the frequency ω _{1 of the} AC voltage applied to the stator winding. It is converted into a value, that is, the stator winding currents I _1d and I _1q . 6 is a magnetic flux calculator for calculating the magnetic flux Φ _2d that links the rotor from the stator winding currents I _1d and I _1q and the stator winding voltages V _Id and V _{1q in the} dq coordinate system, and 7 is d The voltage command value to be generated by the power converter 1 in the −q coordinate system is set to the actual three-phase instantaneous values V _1U , V _1V , V
_It is a two-phase to three-phase converter that converts to _1W .

Reference numeral 8 denotes a d-axis current controller, which calculates the difference between the d-axis component command value I _1d * of the stator winding current and its actual value I _1d by, for example, proportional-plus-integral calculation to flow a current according to the command value. It is for doing so. _{Reference numeral} 9 is a q-axis current controller for proportionally integrating the difference between the q-axis component command value I _1q * of the stator winding current and its actual value I _1q , in order to flow a current according to the command value. It is a thing. 29 is a magnetic flux controller for controlling the d-axis component Φ _2d of the rotor winding flux linkage to a desired value Φ _2d *, and 30 is the rotor angular velocity ω _r
A speed controller for controlling the desired value ω _r *,
31 is a divider, 32 is a coefficient unit, and the slip frequency command ω _s * is calculated by the divider 31 and the coefficient unit 32. Further, 21 is an integrator, 22, 24, 27 and 28 are subtractors, and 26 is an adder.

Here, the power converter 1 has, for example, the configuration shown in FIG. In FIG. 6, 41 is a DC power source, 4
Reference numerals 2a to 42f are switching elements that are connected to the DC power supply 41 and configure each arm of three phases, 43a to 43f are diodes that are connected in antiparallel to the switching elements, and 45 is a modulation circuit. Further, as the modulation circuit 45, as shown in FIG.
, A carrier wave (triangular wave) generator 46 that outputs a triangular wave as a carrier wave, sine wave modulation control signals (AC voltage command signals V _1U , V _1V ,
A modulation control signal generator 47 for outputting V _1W ) and a comparator 48 for comparing the carrier wave 46a from the carrier wave generator 46 and the modulation control signals 47a to 47c from the modulation control signal generator 47, respectively. From this comparator 48, as shown in FIG.
c is sent.

Next, the operation will be described. First, the current control system will be described. The three-phase AC currents I _1U , I _1V , I _1W flowing into the stator windings of the induction motor 2 are detected by the current detector 4. Three-phase - two-phase converter 5, the three-phase alternating current I _1U, I _1V, the I _1W, three-phase AC voltage V _1U applied to the stator winding, V _1V, the frequency omega ₁ of the V _1W It is converted into stator winding currents I _1d and I _1q viewed from a two-axis orthogonal coordinate system (dq coordinate system) that rotates in synchronization. Three-phase alternating current I _1U ,
Conversion from I _1V , I _1W to stator winding currents I _1d , I _1q is performed by the following equation.

[0006]

(Equation 1)

[0007] d-axis current controller 8, d-axis current command value I _1d of the stator winding * the formula (1), the difference was a proportional integral operation with the stator winding current I _1d sought (2) , And outputs the d-axis voltage command value V _1d of the stator winding voltage. Similarly,
Also for the q-axis component, the q-axis current controller 9 calculates a proportional integral between the difference between the q-axis current command value I _1q * of the stator winding and the stator winding current I _1q obtained by the equations (1) and (2). Then, the q-axis voltage command value V _1q of the stator winding voltage is output.

D-axis voltage command value V _1d and q-axis voltage command value V _1q
Is converted into actual three-phase instantaneous voltages V _1U , V _1V and V _1W by the two-phase to three-phase converter 7. Three-phase instantaneous voltage V _1U , V _1V , V _1W from the d-axis voltage command value V _1d and the q-axis voltage command value V _1q
Is converted by the following equation. As a result, the obtained three-phase instantaneous voltages V _1U , V _1V , and V _1W are actually generated from the power converter 1, and a desired current can flow.

[0009]

(Equation 2)

Next, the slip frequency calculation will be described.
If the above current control system operates at a sufficiently high speed, the stator winding currents I _1d and I _1q will be I _1d * = I _1d and I _1q , respectively.
It can be regarded as * = I _1q . At this time, the stator winding currents I _1d , I
_The equation of state of the system of the induction motor 2 when _1q is regarded as an input is as follows.

[0011]

(Equation 3)

Here, α, β and γ are constants determined by the induction motor 2. Φ _2d is a d-axis component rotor interlinkage magnetic flux (hereinafter referred to as d-axis component magnetic flux), ω _r is a rotor angular velocity, ω _s is a slip frequency, and ω _s = ω ₁ −ω _r … ( 7) Now, if the slip frequency ω _s is the equation (8), the equation (5) becomes the equation (9).

[0013]

(Equation 4)

Here, since α <0, the q-axis component magnetic flux Φ _2q
Approaches zero over time. Thus, after a certain time, it can be considered that Φ _2q = 0.

A command value ω _s * of the slip frequency ω _s is calculated by the divider 31 and the coefficient multiplier 32 based on the equation (8). The slip frequency command value ω _s * and the rotor angular velocity ω _r are added by the adder 26, the AC voltage frequency ω ₁ applied to the stator winding is calculated, and the two-phase / three-phase converter 7 and the power converter are calculated. AC voltage of a frequency omega ₁ is applied to the actual induction motor 2 by one.

Next, the magnetic flux control will be described. If the q-axis component magnetic flux is Φ _2q = 0 by the slip frequency control described above, controlling the magnetic flux means controlling the d-axis component magnetic flux Φ _2d . If Φ _2q = 0, the equation (4) becomes the following equation, and the d-axis stator winding current I _1d
By operating, the d-axis component magnetic flux Φ _2d can be controlled to a desired value.

[0017]

(Equation 5)

In the magnetic flux controller 29, the difference between the d-axis component magnetic flux command value Φ _2d * and the d-axis component magnetic flux Φ _2d is proportional-integrally calculated to output the stator winding current command value I _1d *. The value of the d-axis component magnetic flux Φ _2d is obtained by the magnetic flux calculator 6.

Next, speed control will be described. Ф _2q = 0 by slip frequency control, Φ _2d = Φ by magnetic flux control
If it can be controlled to _2d * (constant), the equation (6) becomes the equation (11), and the rotor angular velocity ω _r can be controlled to a desired value by operating the q-axis stator winding current I _1q. become.

[0020]

(Equation 6)

The speed controller 30 proportionally calculates the difference between the rotor angular speed command value ω _r * and the actual measurement value ω _r, and outputs the command value I _1q * of the q-axis stator winding current I _1q . The power converter 1 includes a modulation signal 45a created by the modulation circuit 45.
To 45c and their respective inverted signals 45d to 45f, the switching elements 43a to 43 forming each arm.
f is turned on and off to control the AC voltage supplied to the induction motor 2.

[0022]

By the way, in the conventional power converter, in order to suppress the junction temperature of the switching elements 42a to 42f constituting each arm of the power converter 1 to be equal to or lower than an allowable value, it is common. Is provided with current limiting means (not shown) for limiting the magnitudes of the current command values, for example, the d-axis component command value I _1d * and the q-axis current command value I _1q * to arbitrary fixed values. However, since the thermal time constants of the junctions of the switching elements 42a to 42f are as short as several to several tens of seconds, the current must be limited to a considerably low value in consideration of the stall state of the induction motor 2. That is, since the switching elements 42a to 42f are energized with the instantaneous value of the alternating current, at a low frequency of the alternating current with respect to the thermal time constant of several to several tens of seconds, a sine wave of the alternating current is present in a certain phase. The maximum value is energized for several to several tens of seconds or more, and the junction temperature may be saturated.

Therefore, it is necessary to set the limit value of the current limiting means so that the junction temperature does not exceed the allowable value even when the maximum value of the alternating current exceeds the thermal time constant. Therefore, the current is limited even in the normal acceleration / deceleration operation and the like, so that it is necessary to considerably derate and use the power converter 1, and there is a problem that the device becomes expensive and large.

The present invention has been made to solve the above problems, and provides a power converter having a current limiting function in consideration of overcurrent protection at a low frequency such as when the electric motor is stalled. With the goal.

[0025]

SUMMARY OF THE INVENTION A power converter according to the present invention converts a DC power into an AC power having a variable voltage and a variable frequency and supplies the converted power to an electric motor, and controls an angular velocity of the electric motor. To control the stator winding current based on the speed control means, the current command value output from the speed control means, and the detected value of the current flowing in the stator winding of the electric motor to generate the power converter. In a power converter including a current control unit that outputs a stator winding voltage, a current limiting unit that limits the current command value to a predetermined current limit value or less in accordance with the frequency of the alternating current supplied to the electric motor. It is characterized by having.

Further, the current limiting means includes a memory having an angular velocity-current limit value table calculated according to a thermal time constant of a junction of a switching element constituting the power converter and an allowable junction temperature value. The present invention is characterized in that the current limit value according to the frequency of the alternating current supplied to the electric motor is read out and the current command value is limited to the current limit value or less.

Further, the current limiting means is characterized in that the current limiting value is read from the memory in accordance with the frequency of the AC voltage applied to the stator winding.

Further, the current limiting means is characterized in that the current limiting value is read from the memory according to the rotor angular velocity.

The current control means includes a d-axis current control means and a q-axis current control means in a two-axis rotational coordinate system, and the current limiting means controls the current limit value and the d-axis current control. Calculating a q-axis current limit value based on the d-axis current command value input to the means, and limiting the q-axis current command value input to the q-axis current control means to the q-axis current limit value or less. It is characterized by.

The electric motor is an induction motor, and the current control means is a d-axis current control means in a biaxial rotating coordinate system that rotates in synchronization with the frequency of the AC voltage applied to the stator winding. And q-axis current control means.

Further, the electric motor is a permanent magnet synchronous electric motor, and the current control means includes a d-axis current control means and a q-axis current control means in a biaxial rotation coordinate system that rotates in synchronization with the rotor angle. It is characterized by being prepared.

[0032]

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiment 1 FIG. Hereinafter, embodiments of the present invention will be described with reference to the drawings. 1 is a configuration diagram showing a power conversion device according to a first embodiment. 1, the same parts as those of the conventional example shown in FIG. 5 are denoted by the same reference numerals, and the description thereof will be omitted. As a new code, 50
Is a current limit value read from a memory 51 described later according to the AC voltage frequency ω ₁ applied to the stator winding of the induction motor 2, and based on the read current limit value and the d-axis stator winding current command value. A current limiter that calculates a q-axis stator winding current limit value and limits the current command value of the q-axis stator winding current to the calculated q-axis stator winding current limit value or less.
Is calculated based on the thermal time constants of the junctions of the switching elements 42a to 42f forming the arms of the power converter 1 and the allowable junction temperature values. For example, the angular velocity as a table showing the relationship between the angular velocity and the current limit value shown in FIG. It is a memory that stores a current limit value table.

That is, in the structure shown in FIG. 1, in the speed controller 30 as speed control means for controlling the rotor angular speed ω _r to a desired value ω _r *, the magnetic flux controller 29 is a stator winding of the induction motor 2. The three-phase alternating currents I _1U , I _1V , and I _1W flowing into the wires are rotated in synchronization with the frequency ω _{1 of the} alternating voltage applied to the stator winding to form a two-axis rotating coordinate system (dq coordinate system). While the converted d-axis component command value I _1d * of the stator winding current is transmitted, q of the stator winding current is transmitted.
The axial component command value I _1q * is sent out, and the difference between the d-axis component command value I _1d * of the stator winding current and its actual value I _1d is calculated by, for example, proportional integration, and the current according to the command value is obtained. And a d-axis current controller 8 for _supplying a current, and for supplying a current according to the command value by, for example, proportionally integrating the difference between the q-axis component command value I _1q * of the stator winding current and its actual value I _1q . q
The stator winding voltage V to be generated by the power converter 1 by controlling the stator winding current with the shaft current controller 9
The current limiter 50 and the memory 51 constitute the current control means for outputting _1d and _V1q. The frequency of the alternating current supplied to the electric motor, that is, the stator winding output from the current control means. A current limiting unit is configured to limit the q-axis component command value I _1q * of the stator winding current to a predetermined current limit value or less according to the output frequency of the voltage.

Next, the current limiter 50 according to the first embodiment.
Will be described. The current limiter 50 is the adder 2
The current limit value I _1lim is read from the angular velocity-current limit value table in the memory 51 in accordance with the angular velocity according to the AC voltage frequency ω ₁ applied to the stator winding output from _{No. 6,} and the read current limit value is read. A q-axis that limits the current command value I _1q * of the q-axis stator winding current that fluctuates according to the load by I _1lim and the d-axis stator winding current command value I _1d * output from the speed controller 30. Stator winding current limit value I
_1qlim is calculated according to the equation (12). I _1qlim = √ (I _1lim ² −I _1d * ² ) (12) Then, the current command value I _1q * of the q-axis stator winding current input to the adder 24 is set to the q-axis stator winding current. Limit value I
Limit to _1qlim or less.

As described above, when the junctions of the switching elements 42a to 42f constituting each arm of the power converter 1 are heated according to the frequency of the alternating current that is applied, that is, the output frequency ω ₁ of the stator winding voltage. Since the current command value I _1q * of the q-axis stator winding current that varies depending on the load is limited based on the relationship between the frequency and the current limit value calculated from the constant value and the allowable value of the junction temperature,
A low limit value is set for overcurrent protection at low frequencies such as during stalls, and it is limited by a high current limit value during normal acceleration / deceleration operation.Therefore, it is necessary to derate the power converter according to an abnormal condition. In addition, a power conversion device that is inexpensive and improves the prevention of device destruction can be obtained.

Embodiment 2 Next, FIG. 3 shows a second embodiment.
It is a block diagram which shows the power converter device which concerns on. The second embodiment shown in FIG. 3 is different from the first embodiment shown in FIG. 1 in that the current limiter 50 corresponds to the frequency of the alternating current supplied to the induction motor 2 in the first embodiment. AC voltage frequency ω ₁ applied to the stator winding
And the current limit value is read from the memory 51 according to the AC voltage frequency applied to the stator winding, whereas in the second embodiment, the rotor angular velocity ω which is the detection output of the rotor angular velocity detector 3 is read. _The current limit value is read directly from the memory 51 in accordance with _r , and other configurations are similar to those of the first embodiment shown in FIG.

Next, the current limiter 50 according to the second embodiment.
Will be described. The AC voltage frequency ω ₁ applied to the stator winding is the sum of the rotor angular velocity ω _r which is the output of the rotor angular velocity detector 3 and the slip frequency command value ω _s *, and the slip frequency command value ω _s * Is sufficiently small with respect to the rotor angular velocity ω _r . In the second embodiment, the current limiter 50 uses the AC voltage frequency ω ₁ applied to the stator winding.
Instead of, the rotor angular velocity ω _r that is the output of the rotor angular velocity detector 3 is used, and according to the rotor angular velocity ω _r ,
The current limit value I _1lim is read from the memory 51. And
From the current limit value I _1lim which is the output of the memory 51 and the d-axis stator winding current command value I _1d *, the current command value I of the q-axis stator winding current that fluctuates according to the load according to the above equation (12). _The q-axis stator winding current limit value I _{1qli m} that limits _1q * is calculated. Further, the current limiter 50 uses the q-axis stator winding current limit value I _{1ql im} to set the q-axis stator winding current current command value I _1q * to the q-axis stator winding current limit value I _1qlim.
Restrict to the following.

As described above, the power converter 1 is operated according to the frequency of the alternating current, that is, the rotor angular velocity ω _r.
Of the switching elements 42a to 42f forming each arm of
To limit the current command value I _1q * of the q-axis stator winding current that fluctuates according to the load based on the relationship between the frequency-current limit value calculated from the junction thermal time constant and the allowable junction temperature value. Therefore, as in the first embodiment, a low limit value is set for overcurrent protection at a low frequency such as during stall, and a high current limit value is set during normal acceleration / deceleration operation, etc. There is no need to derate the device according to an abnormal condition, and an inexpensive power conversion device that improves prevention of damage to the device can be obtained.

Embodiment 3 Next, FIG. 4 shows a third embodiment.
It is a block diagram which shows the power converter device which concerns on. In the third embodiment shown in FIG. 4, the first embodiment shown in FIGS.
2 is different from the induction motor 2 in Embodiments 1 and 2 in that the permanent magnet synchronous motor 52 is adopted in Embodiment 3 as shown in FIGS. The same parts as those in the first and second embodiments shown are designated by the same reference numerals and the description thereof will be omitted. 52 as a new code
Is a permanent magnet synchronous motor, 53 is a permanent magnet synchronous motor 52
Is a rotary encoder that is a detector of the rotor angle θ and the angular velocity ω _r , and 55 is a three-phase alternating current I _1U , I _1V , I _1W flowing into the stator winding detected by the current detector 4, In a two-axis rotational coordinate system (dq coordinate system) that rotates in synchronization with the three-phase to two-phase converter for converting the stator winding currents I _1d and I _1q , and 57 is dq It is a two-phase to three-phase converter that converts the voltage command value to be generated by the power converter 1 in the coordinate system into the actual three-phase instantaneous values _V1U , _V1V , and _V1W .

Further, reference numeral 58 denotes d for attempting to flow a current according to the command value by, for example, performing proportional integral calculation of the difference between the d-axis component command value I _1d * of the stator winding current and its actual value I _1d. Although it is an axis current controller, generally, the value of the d-axis component command value I _1d * is zero. _{Reference numeral} 59 is a q-axis current controller for attempting to flow a current according to the command value by, for example, proportionally integrating the difference between the q-axis component command value I _1q * of the stator winding current and its actual value I _1q . Furthermore, 62, 64, 66-
Reference numeral 69 is an adder or subtractor, 61 and 63 are interference compensators for respectively compensating the voltage drop due to the inductance of the stator winding of the permanent magnet synchronous motor 52 with respect to the d-axis component and the q-axis component respectively, and 65 is a rotation. It is a compensator for the velocity electromotive voltage generated in proportion to the child angular velocity ω _r .

Next, the current limiter 50 according to the third embodiment
Will be described. Since the AC voltage frequency applied to the rotor winding is equal to the rotor angular velocity ω _r which is the output of the rotor angular velocity detector 53, the current limiter 50
The current limit value I _1lim is read from the angular velocity-current limit value table in the memory 51 according to the rotor angular velocity ω _r . The current limiter 50 then _{reads the} read current limit value I _1lim.
The q-axis stator winding current limit value I _1qlim, which limits the current command value I _1q * of the q-axis stator winding current that fluctuates according to the load, is calculated according to the equation (12). for children winding current command value I _1d * is zero, q for limiting the current command value of q-axis stator winding current that varies according to the current limit value I _1Lim which is the output of memory 51 to load I _1q * It is output as the axial stator winding current limit value I _1qlim , and the current command value I _1q * of the q-axis stator winding current is output as the q-axis stator winding current limit value I.
Limit to _1qlim or less.

As described above, even when the electric motor is a permanent magnet synchronous electric motor, the switching element 42a forming each arm of the power converter 1 is in accordance with the frequency of the supplied alternating current, that is, the rotor angular velocity ω _r. Limit the current command value I _1q * of the q-axis stator winding current that fluctuates according to the load based on the relationship between the frequency-current limit value calculated from the junction thermal time constant and the allowable junction temperature of ~ 42f. Therefore, as in the first embodiment, a low limit value is set for overcurrent protection at a low frequency such as during stall, and a high current limit value is set for normal acceleration / deceleration operation. There is no need to derate the power conversion device in accordance with an abnormal condition, and it is possible to obtain a power conversion device that is inexpensive and improves the prevention of device damage.

As described above, according to the present invention, the current limiting means for limiting the current command of the current control means to a predetermined value or less causes the junction of the switching element to be switched in accordance with the frequency of the alternating current supplied to the electric motor. Since the limit value of the current limiting means is changed according to the relationship between the frequency-current limit value calculated from the thermal time constant and the allowable value of the junction temperature, a low limit value is set for overcurrent protection at low frequencies such as during stalls. Since it is set and is limited by a high current limit value during normal acceleration / deceleration operation, it is not necessary to derate the power conversion device at the time of an abnormality, and an inexpensive power conversion device that improves prevention of device damage can be obtained. effective.

[Brief description of the drawings]

FIG. 1 is a block configuration diagram showing a power conversion device according to a first embodiment of the present invention.

2 is a characteristic diagram illustrating an angular velocity-current limit value table provided in a memory 51 in FIG.

FIG. 3 is a block configuration diagram showing a power conversion device according to a second embodiment of the present invention.

FIG. 4 is a block configuration diagram showing a power conversion device according to a third embodiment of the present invention.

FIG. 5 is a block diagram showing a conventional power converter.

FIG. 6 is a block configuration diagram showing details of the power converter 1 in FIG.

7 is a block configuration diagram showing details of a modulation circuit in FIG.

FIG. 8 is a signal waveform diagram of each part in FIG.

[Explanation of symbols]

1 power converter, 2 induction motor, 3 rotor angular velocity detector, 4 current detector, 5 three-phase to two-phase converter, 6 magnetic flux converter, 7 two-phase to three-phase converter, 8 d-axis current controller, 9 q-axis current controller, 29 magnetic flux controller, 30 speed controller, 50 current limiter,
51 memory, 52 permanent magnet synchronous motor, 58 d-axis current controller, 59 q-axis current controller.

Claims (7)

[Claims] 1. A power converter for converting DC power into AC power of variable voltage and variable frequency and supplying the converted power to an electric motor, speed control means for controlling an angular speed of the electric motor, and output from the speed control means. And a current control means for controlling the stator winding current based on the detected current value flowing in the stator winding of the motor and outputting the stator winding voltage to be generated by the power converter. A power conversion device comprising: a power conversion device, comprising current limiting means for limiting the current command value to a predetermined current limit value or less according to a frequency of an alternating current supplied to the electric motor. 2. An angular velocity calculated according to a thermal time constant of a junction of a switching element forming the power converter and an allowable junction temperature value,
A memory having a current limit value table is provided, and a current limit value corresponding to the frequency of the alternating current supplied to the electric motor is read from the memory, and the current command value is limited to a current limit value or less. Item 1. The power converter according to item 1. 3. The power converter according to claim 2, wherein the current limiting means reads the current limiting value from the memory according to the frequency of the AC voltage applied to the stator winding. 4. The power converter according to claim 2, wherein the current limiting means reads the current limiting value from the memory according to the rotor angular velocity. 5. The current control means includes a d-axis current control means and a q-axis current control means in a biaxial rotation coordinate system, and the current limiting means includes the current limit value and the d-axis current control. Calculating a q-axis current limit value based on the d-axis current command value input to the means, and limiting the q-axis current command value input to the q-axis current control means to the q-axis current limit value or less. The power conversion device according to any one of claims 1 to 4, characterized in that. 6. The electric motor is an induction motor, and the current control means is a d-axis current control means in a biaxial rotating coordinate system that rotates in synchronization with a frequency of an AC voltage applied to a stator winding. 6. The power converter according to claim 5, further comprising a q-axis current control means. 7. The electric motor is a permanent magnet synchronous electric motor, and the current control means includes a d-axis current control means and a q-axis current control means in a biaxial rotation coordinate system that rotates in synchronization with a rotor angle. The power conversion device according to claim 5, wherein the power conversion device is provided.