JPH09205797A - Variable speed driving device for ac motor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To make an interphase reactor unnecessary by making the number of revolutions of a motor to correspond to the sum of the output frequencies of first and second variable frequency converters or the difference between the output frequencies by respectively connecting the frequency converters to stator and rotor windings. SOLUTION: First and second inverters 2 and 3 which are provided as first and second variable frequency converters are respectively connected to the stator winding la and rotor winding 1b of a wound-rotor induction motor 1 so that DC power obtained by converting AC power outputted from a power transformer 7 by means of a converter 6 having a high power factor can be supplied to the motor 1 and the number of revolutions of the motor 1 can correspond to the sum of the output frequencies of the inverters 2 and 3 or the difference between the output frequencies. Therefore, the capacity of the motor can be increased and the motor can be driven at a low speed in an excellent state by increasing the output torque of the motor without using any interphase reactor.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a variable speed drive device for an AC electric motor used in a steel rolling mill, an electric locomotive, a high speed elevator or the like. Especially G
The present invention relates to an AC motor drive device using a variable frequency converter such as an inverter using a self-extinguishing element such as a TO thyristor or IGBT, or a variable frequency converter such as a cycloconverter using a thyristor, and particularly the output of two or more converters. The present invention relates to a drive device capable of increasing the output capacity, reducing the harmonics of the output voltage waveform, and obtaining excellent control performance by combining

[0002]

2. Description of the Related Art A typical conventional multi-inverter for driving an AC motor is a phase-to-phase reactor multi-inverter that combines the outputs of two GTO inverters, an example of which is shown in FIG.
Shown in Inverters (80) and (81) simplified and shown as boxes in FIG.
It is an ordinary three-phase two-level inverter or three-level inverter as shown in (a), (b) and (c).

As a large-capacity inverter suitable for a drive device that requires a large torque in a low-speed rotation region from a stopped state and requires excellent control performance, such as a steel rolling mill drive,
Recently, attention has been paid to this interphase reactor type circuit. This circuit is described, for example, in the literature, “Large-capacity GTO drive system realizing high power factor and harmonic reduction”, Hitachi Review, Vol. 75, (May 1993), pages 31-34, active research and development is being conducted. FIG. 13A is obtained by rewriting the circuit shown in FIG. 1 of the above-mentioned document in accordance with the other drawings of this specification. In this circuit, the outputs of two voltage type three-phase two-level inverters (80) and (81) using GTO are connected to the interphase reactor (8).
2), (83) and (84) are combined and the output is supplied to the induction motor (85). A synchronous machine may be used as the electric motor. GTO switching frequency is several hundred Hz
The phase of the carrier wave is shifted by 180 degrees between the two inverters so that the two inverters are alternately switched, and an output voltage with few harmonics as shown in FIG. 13B is obtained. ing. In this circuit, the two inverters generate the fundamental wave output voltage of the same size and phase,
Since the output current is controlled to be divided in half, the voltage applied to the interphase reactor is only the voltage corresponding to the phase difference of the carrier wave, and the output fundamental wave component is not applied, so even if the output frequency is near 0 Hz. There is no risk of saturation of the reactor magnetic flux, and sufficient output voltage and torque can be obtained.

This method gives an excellent output voltage waveform,
Further, sufficient torque can be secured even in a low frequency low speed region. However, since this method requires three interphase reactors, its price and size, loss, and electromagnetic noise due to the switching voltage waveform applied to the reactor are problems. Moreover, in this method, when the current balance is lost, the iron core of the reactor is saturated, and the current balance is further deteriorated, and operation becomes impossible. Therefore, circuit components such as GTO and PWM control circuit The control becomes complicated because it is necessary to make the characteristics as uniform as possible and to provide a current balance control system on top of that.

[0005]

Since the conventional typical multiple inverter for driving an AC motor is constructed as described above, a large electromagnetic device such as an interphase reactor is required to combine the outputs of the inverters. . As a result, there are problems such as its installation location, reduced efficiency, electromagnetic noise, and economical efficiency. The present invention solves these problems by providing a method that does not require parts such as an interphase reactor.

In addition, 4.5kV, 4kA, 6kV, 6k
Since the large capacity GTO of A class has a large switching loss, it is not easy to use it at a switching frequency higher than about 300 Hz. Therefore, once the required inverter output frequency is determined, it is necessary to generate that output frequency at the lowest possible switching frequency. Therefore, in the above phase-to-phase reactor method, by shifting the carrier wave phase that determines the switching of the two inverters by 180 degrees, the two inverters switch alternately, equivalently doubling the switching frequency, and at a low switching frequency, I try to get a high output frequency. The present invention improves this point and provides a method capable of obtaining a required output frequency, that is, a rotation speed, at a low switching frequency.

In addition, since the frequency is almost 0 during a low speed and large torque operation called stall operation, the current of a specific arm keeps a large value for a long time, so that it is averaged in the operation at a high frequency. In comparison with G
The junction temperature of TO rises remarkably. In the inverter for rolling, the maximum temperature rise at this time should be set within the allowable value.
It is necessary to select the rating of O and design the cooling, and the utilization rate of GTO decreases. The present invention solves this problem and provides a method in which the current does not continue to flow to the GTO of a specific arm during stall operation and the junction temperature can be prevented from rising significantly.

The present invention has been made to solve the above problems, and in a variable speed drive device for an AC motor, a large capacity drive is performed by combining the outputs of two inverters without using an interphase reactor. Configure the device and
Sufficient torque can be secured even in the vicinity of Hz, and the required output frequency, i.e., rotation speed, can be obtained at the lowest switching frequency. In addition, by avoiding current concentration on a specific arm during stall operation, GTO
To provide a new variable speed drive device for AC motors that can avoid temperature rise, and to obtain a highly efficient and high performance variable speed drive device that is compact, economical, and has no reactor electromagnetic noise. To do.

[0009]

According to a first aspect of the present invention, there is provided a variable speed driving device for an AC electric motor, wherein the stator and the rotor have AC multi-phase windings. The first variable frequency converter is connected to the winding and the second variable frequency converter is connected to the rotor winding, and it corresponds to the sum or difference frequency of the output frequencies of these first and second variable frequency converters. It is configured to obtain the rotation speed of the AC motor.

According to a second aspect of the present invention, there is provided a variable speed driving device for an AC electric motor according to the first aspect, wherein one of the first and second variable frequency converters is controlled as a voltage source and the other is controlled as a current source. One of them supplies the excitation current of the AC motor and the other supplies the torque current.

According to a third aspect of the present invention, there is provided a variable speed drive device for an AC motor according to the first aspect, wherein one of the first and second variable frequency converters is controlled as a voltage source and the other is used as a current source. The first and second variable frequency converters are controlled and both supply the torque component current and the excitation component current of the AC motor.

According to a fourth aspect of the present invention, there is provided a variable speed drive device for an AC electric motor according to the first aspect, wherein both the first and second variable frequency converters are controlled as current sources, and one of the variable frequencies is controlled. The converter supplies the excitation current and the torque current of the AC motor, and the other variable frequency converter supplies the torque current.

According to a fifth aspect of the present invention, there is provided a variable speed drive device for an AC electric motor according to the first aspect, wherein both the first and second variable frequency converters are controlled as current sources. Both the frequency converters supply the torque component current and the excitation component current of the AC motor.

According to a sixth aspect of the present invention, there is provided a variable speed driving device for an AC electric motor, wherein when the number of turns of the stator winding and the number of turns of the rotor winding are N1 and N2, respectively. The command value of the torque component current supplied to the second variable frequency converter is the inverse ratio of N1 and N2.

According to a seventh aspect of the present invention, there is provided a variable speed drive device for an AC electric motor according to any one of the first to sixth aspects, wherein the output frequency of the first or second variable frequency converter with respect to the rotational speed of the AC electric motor. According to the number of revolutions of the AC motor, the output frequency of the variable frequency converter provided with the pattern is determined by the pattern, and the output frequency of the other variable frequency converter is the AC motor. Is determined according to the rotation coordinates of the winding side to which the other variable frequency converter is connected, which is determined according to the rotation of the.

According to an eighth aspect of the present invention, there is provided a variable speed drive device for an AC electric motor according to any one of the first to seventh aspects, wherein the AC motor has a first or a predetermined rotational speed (hereinafter referred to as a polarity reversal rotational speed). Before the polarity of any of the output frequencies of the second variable frequency converter is reversed from positive to negative or from negative to positive, and the absolute value of the output frequency is not less than a predetermined minimum value. The output frequency is changed to the output frequency after inversion within a predetermined minute time.

According to a ninth aspect of the present invention, in the variable speed drive device for an AC electric motor according to the eighth aspect, the polarity reversal rotation speed is set to be different between when the rotation speed command value of the AC electric motor is rising and when it is falling. Thus, the polarity reversal operation of the output frequency of the variable frequency converter has a hysteresis characteristic.

A variable speed drive device for an AC motor according to a tenth aspect of the present invention is the variable speed drive device according to any one of the first to sixth aspects.
A plurality of AC motors are provided, a common first variable frequency converter is connected to each stator winding of the above AC motor, and an individual second variable frequency converter is connected to each rotor winding. By making the output frequencies of the respective second variable frequency converters different from each other, the respective AC motors can be driven at mutually different rotational speeds.

According to an eleventh aspect of the present invention, there is provided a variable speed drive device for an AC electric motor according to any one of the first to tenth aspects, wherein the first variable frequency converter is a first inverter and the second variable frequency converter is a second variable frequency converter. In the second inverter, the DC input terminals of the first inverter and the second inverter are connected in parallel and connected to a common DC power source.

According to a twelfth aspect of the present invention, there is provided a variable speed drive device for an AC electric motor according to any one of the first to tenth aspects, wherein the first variable frequency converter is a first inverter and the second variable frequency converter is a second variable frequency converter. In the second inverter, DC power supplies for the first inverter and the second inverter are separately provided so that the first and second inverters can operate at DC voltages different from each other.

According to a thirteenth aspect of the present invention, there is provided a variable speed drive device for an AC electric motor according to the twelfth aspect, wherein either one of the first and second inverters has a three-phase two-level inverter.
On the other hand, a 3-phase 3-level inverter is used.

According to a fourteenth aspect of the present invention, there is provided a variable speed drive device for an AC electric motor according to any one of the eleventh to thirteenth aspects, wherein the first and second inverters use a modulation method of self-extinguishing during one cycle of the AC output. A PWM is used in which an arc element performs switching a plurality of times, the frequencies of carrier waves that determine the switching frequencies of the first and second inverters are the same, and the phase difference between the carrier waves is mutually different. It has a.

According to a fifteenth aspect of the present invention, there is provided a variable speed drive device for an AC electric motor according to any one of the first to tenth aspects, wherein one of the first and second variable frequency converters is a cycloconverter and the other is a cycloconverter. It is an inverter.

[0024]

BEST MODE FOR CARRYING OUT THE INVENTION

Embodiment 1. 1 is an overall configuration diagram showing a variable speed drive device for an AC electric motor according to Embodiment 1 of the present invention.
In the figure, (1) is an AC motor to be controlled, and more specifically, it is a winding-type induction motor having a stator winding (1a) and a rotor winding (1b). (2) is a first inverter (INV-A) as a first variable frequency converter connected to the stator winding (1a), and (3) is a first inverter connected to the rotor winding (1b). The second inverter (INV-B), (4) and (5), as the second variable frequency converter, represent the DC capacitors of these inverters, which are assumed to be included in the inverters in the following figures. , Omitted as appropriate. The high power factor converter CONV (6) converts the AC power received through the transformer (7) into DC power and supplies the DC power to the two inverters. Further, during the regenerative operation, the electric power from the electric motor is regenerated to the AC power source.

In FIG. 1, the inverter (2),
Although the converters of (3) and the converter (6) are simply represented by boxes, first, in consideration of application examples in other embodiments described later, first, concrete configuration examples of these converters will be described. The expression method will be described with reference to FIGS. 2 and 3.

As one inverter forming the variable speed drive device of the present invention, a GTO type three-phase two-phase inverter shown in FIG.
Level inverter, IGBT type three-phase two-level inverter shown in (c), or GTO type three-phase three shown in FIG. 2 (b)
Although a level inverter or the like is used, since a 2-level inverter is well known, a 3-level inverter will be described. FIG. 2B shows a circuit using a reverse conducting GTO. Between a positive electrode P and a negative electrode N of a DC power supply having a neutral point terminal or a DC power supply having a neutral point made of a capacitor,
The switching elements S1, S2, S3, S4 are sequentially connected in series, and the connection point of S1 and S2 and the connection point of S3 and S4 are connected to the neutral point terminal via diodes, respectively. The connection point is the output terminal U. Although the two-level inverter can output only two positive and negative voltage levels, this circuit can output three voltage levels as follows. (A) When S1 and S2 are on: Positive potential of DC power supply (b) When S2 and S3 are on: Zero potential of DC power supply (c) When S3 and S4 are on: Negative power of DC power supply As a result, the three-phase three-level inverter provided with three sets of this circuit can reduce the harmonics of the output voltage as compared with the normal two-level inverter.

Although the reverse conducting GTO is used in the above circuit diagram, the reverse conducting GTO is a power semiconductor device in which a normal GTO and an anti-parallel diode are integrated on a single silicon wafer, as shown in FIG. It is indicated by the symbol a). It goes without saying that the inverter used in the present invention may use other types of power semiconductor devices, for example, an IGBT and an antiparallel diode. Since the three-level inverter and the two-level inverter in FIG. 2 are both three-phase voltage type inverters, they are simplified and shown in a block diagram as shown in FIG.
A three-level inverter requires a neutral point terminal of the DC power supply, but we think that the capacitor that creates the neutral point is also included in the inverter, so the illustration of the neutral point is omitted. It is represented by one block diagram as shown in FIG. FIG. 1A shows a general voltage source inverter,
(B) shows it in a single diagram, (c) shows a GTO inverter, and (d) shows an IGBT inverter.
(E) and (f) are converters that convert alternating current to direct current. The converter includes the thyristor-type reversible converter shown in FIG. 2 (d) and the inverters shown in FIGS. 2 (a), (b), and (c). There is one that uses as a high power factor converter. The thyristor type reversible converter can be shown in FIG. 3 (g), and the high power factor converter can be shown in FIG. 3 (h). These are exchanges
Bidirectional power conversion between DC is possible. As is well known, the cycloconverter is the reversible converter shown in FIG.
The anti-parallel circuit of the three-phase bridge excluding the filter is used as a constituent element, and three or six thereof are used to configure as shown in FIG. 2 (e). This is simplified to FIG. 3 (i),
Shown as (j).

Returning to the explanation of FIG. 1, if the output frequency of the first inverter INV-A (2) on the stator side is f1,
So-called synchronous speed ns (rpm) of wire wound induction motor
Is n = 120f1 / p, where p is the number of pole pairs. Further, at the frequency f1 on the stator side, the rotation speed is n (rp
m) the second inverter INV-B on the rotor side
If the output frequency of (3) is f2, n = 120 (f1 + f
2) / p. When n is equal to or lower than the synchronous speed, f2 is negative, and when n is equal to or higher than the synchronous speed, f2 is positive. Therefore, f2 of the present invention has an opposite sign to the general slip frequency.
When n is equal to or less than the synchronous speed, as is well known, the primary side of the wound-rotor induction motor (hereinafter, the stator side and the primary side, and the rotor side and the secondary side are synonymous and appropriately used together) Of the electric power input from (1), the slip frequency is extra, and it comes out to the secondary side, so it is necessary to absorb it by the inverter INV-B and return it to the power supply. The electric power that emerges on the secondary side is unfavorable because it leads to a decrease in efficiency as extra circulating electric power. Therefore, only the difference power required for driving is passed through the converter. On the contrary, in order to increase n above the synchronous speed, as is well known, it is necessary to input power corresponding to the negative slip frequency from the inverter INV-B to the rotor. Therefore, the sum of the two electric powers passes through the converter. That is, in this method, only one DC power supply is required, and unnecessary circulating power does not flow to the DC power supply.

In this drive device, the speed of the electric motor is the sum or difference between the frequencies f1 and f2 of the two inverters. If the phase rotation direction shown in FIG. 4 is positive, the speed is f1 + f2.
Becomes Therefore, speed control can be performed by keeping f1 of the inverter INV-A constant and changing f2 of the inverter INV-B, or by changing f1 and f2 at the same time. Since the speed corresponding to f1 + f2 can be obtained, the frequency of the inverter can be 1/2 when designing the drive system of the same rotation speed. Large capacity G
In the case of a TO inverter, its switching loss is large, and the switching frequency has an upper limit of about 300 Hz. However, by adopting the present invention in which two inverters are combined, the application to a drive device becomes easy.

Further, in this drive system, it is not necessary for the converters such as two inverters to have the same ratings of voltage, current, frequency range, etc., and a large capacity drive system having a high degree of freedom in design and a high flexibility can be provided. can get.

Embodiment 2 FIG. 5 shows various form examples examined by paying attention to the control form of both inverters (2) and (3). First, in the drive device of FIG. 5 (a), the winding induction motor (1) The inverter INV-A (2) connected to the primary side stator winding is controlled as a voltage source, that is, a voltage control type, and the inverter INV-B (3) connected to the secondary side rotor winding is It is controlled as a current source, that is, as a current control type. The main circuit itself may be a current source (current source inverter). By using the current control type inverter or the current type inverter, the torque current can be quickly and accurately controlled, and the overcurrent can be avoided. Therefore, it is suitable for driving a rolling mill that requires rapid acceleration / deceleration.

Embodiment 3. In the drive device shown in FIG. 5B, the inverter I connected to the primary side stator winding
Both the NV-A (2) and the inverter INV-B (3) connected to the secondary side rotor winding are controlled as a current control type. The main circuit itself may be a current source (current source inverter). By using a current-controlled inverter or a current-source inverter for both primary and secondary,
Both the secondary and secondary excitation currents and torque currents can be controlled to be non-interfering with each other, and performance superior to that of the system of FIG. 5A can be obtained.

Embodiment 4 FIG. FIG. 5C shows a variable speed drive device for an AC electric motor according to the fourth embodiment. In this example, an inverter INV- connected to the primary side stator winding
Inverter I connected to A (2) and the secondary rotor winding
The control system of NV-B (3) is not limited, but the two inverters are configured so that they may have different DC voltage specifications or operating conditions, apart from their DC power supplies. By doing so, for example, the voltage of the inverter INV-B can be made lower than that of the inverter INV-A to have a small capacity, and the degree of freedom in design can be increased.

Embodiment 5. FIG. 6 shows a variable speed drive device for an AC electric motor according to the fifth embodiment. The inverter INV-A (2) connected to the primary side stator winding is a three-level inverter, and the inverter INV-B (3) connected to the secondary side rotor winding is a two-level inverter.
When manufactured with the same GTO, the capacity ratio between the 3-level inverter and the 2-level inverter is 2: 1, so a system with a drive power of 2 + 1 can be constructed.

Sixth Embodiment The system shown in FIG. 7 is a system capable of operating a plurality of electric motors at slightly different speeds. The primary side stator winding of the two electric motors (1-1) and (1-2) has a common inverter INV-A (2).
And a separate inverter (3
-1) and (3-2) are connected. Inverter (3
By making the frequencies of -1) and (3-2) have a difference of Δf, it is possible to cope with the difference in speed of the electric motor. This is a system suitable for electric locomotives and the like where there is a slight difference in the rotational speed of each wheel.

Embodiment 7 FIG. 8 is a block diagram including a control system of a variable speed drive device for an AC electric motor according to a seventh embodiment of the present invention. The inverter INV-A (2) is connected to the stator side of the wound-rotor induction motor (1), and the inverter INV-B (3) is connected to the rotor side of the inverter INV-A.
Is controlled as a voltage source, the frequency of which is determined by the VF oscillator (115). The inverter INV-B is controlled as a current source as described later.

In this example, a two-pole machine is used in an easy-to-understand manner, and a pulse generator (10) for one rotation of 6000 pulses is used, so that one cycle is 6000 pulses. Therefore, the output of the VF oscillator is converted into phase information by the up / down counter (111) of 6000 counts, the sine wave and cosine wave waveform memory (106) is read according to the count number, and this is read by the voltage command circuit (117). ) Give to. The up / down counter is used here because the primary frequency not only rotates in the forward direction but also rotates in the reverse direction. It switches between up and down depending on whether the primary frequency is positive or negative.

The FV pattern generation circuit (116) determines the output voltage according to the frequency. The voltage command circuit (117) creates a three-phase voltage command having an amplitude proportional to the output of the output, and gives it to the voltage control circuit (118). The voltage control circuit creates a command for the output voltage of the inverter INV-A, and the PWM circuit (10
Give to 1). This PWM circuit normally uses a triangular wave comparison method to determine the switching of the inverter. This PWM
Although not shown in the figure, the triangular wave signal of the circuit has a phase difference of 180 degrees at the same frequency as the triangular wave of the PWM circuit (102) described later, thereby reducing harmonics of the switching frequency. ing. In this figure, the voltage feedback control is not performed, but if it is performed, better characteristics can be obtained.

On the other hand, an inverter INV-B (3) is connected to the rotor side of the wound-rotor induction motor (1), which is controlled as a current source. The current control system is configured on a synchronous rotation coordinate axis that rotates in agreement with the secondary induced electromotive force of the wound-rotor induction motor. It is the up / down counter (110) that determines the synchronous rotation coordinates. The up / down counter is driven up by the output of the pulse generator (10) corresponding to the rotation speed n of the electric motor, and is driven down by the pulse of the VF oscillator (115) corresponding to the frequency f1 on the stator side. ing. Because n
Is the rotation speed (rps) in the case of a two-pole machine, the relationship between frequency and n is n = f1 + f2, so the up / down counter corresponds to f2, so the signal given to the up / down counter is f2 = n. This is because it becomes −f1. With this configuration, at the moment when the U phases of the stator and the rotor coincide with each other at the time of starting, the 0 phase is adjusted by 1 pulse / pulse of rotation generation. In this way, the up / down counter shows the phase information that matches the induced electromotive force on the rotor side. Note that, in the above description and FIG. 8, for simplicity, the case where the electric motor is in the forward rotation and the output frequency of the inverter INV-A (2) is in the forward rotation is shown. However, the circuit shown in FIG. 9 is required to cope with the case where the electric motor rotates in the reverse direction and the case where the frequency of the inverter INV-A (2) rotates in the reverse direction. In FIG. 9, (123) and (124) are
It is a pulse addition circuit that obtains an output pulse that is the sum of the frequencies of two pulses. These adder circuits are required for n
This is because both of them may be input to the up side or the down side depending on the signs of and f1. In the figure, the rotation speed n and the pulse f1 of the VF oscillator (115)
Is configured to be switched by a switch circuit between connecting to the up side or the down side of the up-down counter according to a sign indicating whether it is forward rotation or reverse rotation.
That is, the switch circuit (125) is configured to input to the up side when n is positive and to the down side when n is negative. Further, in the switch circuit (126), when f1 is in forward rotation, it is input to the down side, and in reverse rotation, it is input to the up side so that f2 = n-f1 is always maintained.

Returning to FIG. 8, a sine wave
The cosine wave generation circuit (107) outputs sin and cos waves that are the reference of synchronous rotation coordinates, and based on this, the secondary current signal i2 (U, V, W) obtained from the current detector (9) is set to 3 phase/
The γδ coordinate conversion circuit (105) converts the signals into iγ2 and iδ2 signals. The γ-axis is the direct axis and corresponds to the exciting current, and the δ-axis is the horizontal axis and corresponds to the torque current.

In the above, the conversion from the three-phase coordinate UVW to the synchronous rotation coordinate γδ is as follows for the three-phase current, for example. Iδ = IU sinωt + IV sin (ωt-2π / 3) + IW sin (ωt + 2π / 3) Iγ = IU cosωt + IV cos (ωt-2π / 3) + IW cos (ωt + 2π / 3) When the voltage command given to the PWM circuit is VXδ, VXγ and it is converted from γδ coordinates to UVW coordinates,
It is as follows. VXU = VXδ sin ωt + VXγ cos ωt VXV = VXδ sin (ωt -2π / 3) + VXγ cos (ωt -2π / 3) VXW = VXδ sin (ωt + 2π / 3) + VXγ cos (ωt + 2π / 3)

The speed control circuit (109) generates a torque current command iδ2 ^* based on the command ωre ^{* of the} speed command circuit (108) and the speed signal ωre obtained from the pulse generator. In order to independently control the γ-axis exciting current and the δ-axis torque current, the torque current command iδ2 ^* and the exciting current command iγ2 ^* = 0 are given to the decoupling current control circuit (112).
The signals of ω1, ωre, iγ1 and iδ1 necessary for performing the decoupling control of the γ-axis and δ-axis currents of the secondary current are also given to the decoupling current control circuit (112).

In the example of FIG. 8, the secondary excitation current command iγ2 ^* = 0
However, if this is done, the inverter INV-
Since the primary voltage is determined by A, all the exciting current required for it is supplied from the primary. As another method, since the exciting current required to generate the primary voltage determined by the inverter INV-A can be known from the characteristics of the motor, all necessary exciting currents can be given as the secondary exciting current command iγ2 ^*. It is possible. In this case, there may be some excess or deficiency in the exciting current given from the secondary, but the difference is the primary inverter IN
Supplied naturally from VA. Further, it is possible to arbitrarily share the exciting current between the inverters on the primary side and the secondary side. As described above, the exciting current of the T-type equivalent circuit is
It can be understood that it can be supplied from any of the following.

The decoupling current control circuit (112) converts the output current signal of the inverter INV-B from the current detector (9) into the feedback signal iγ2, i.
Based on δ2 and the above command value, command values Vγ2 ^* and Vδ2 ^* of the voltage to be output from the inverter are output. This γδ axis signal is converted into a three-phase signal by the γδ / 3-phase conversion circuit (104),
It is given to the PWM circuit (102). In this way, the exciting current component and the torque current component of the output current of the inverter INV-B are controlled.

With the above construction, the electric motor (1) 1
The secondary voltage, that is, the magnetic flux and the frequency are determined by the inverter INV-A, and the torque is controlled by the inverter INV-B. The frequency f2 of the inverter INV-B is automatically determined by the up / down counter according to the rotation of the electric motor, and an arbitrary number of revolutions can be obtained from zero speed to the synchronous speed or higher within the range that the output frequency of the inverter INV-B can permit.

FIG. 10 shows a frequency control pattern for four-quadrant operation. When the frequency rotation direction is determined as shown in FIG. 4, the speed of the electric motor is determined by the sum of the frequency f1 of the inverter INV-A and the frequency f2 of the inverter INV-B. The horizontal axis is f1 + f2, which is tentatively named the effective frequency, which corresponds to the number of revolutions. F1 on the vertical axis,
f2 and rotation speed n (rps in the case of a two-pole machine) are taken. In this example, in the case of normal rotation, the pattern of f1 determined in advance with respect to the rotational speed is fixed at 20 Hz, and the rotational speed of the electric motor is accelerated to a speed corresponding to 0 to 40 Hz (2 ns with a synchronous speed for 20 Hz of n s), The rotation coordinate on the secondary side naturally changes, the frequency f2 of the secondary side converter naturally changes from -20 Hz to +20 Hz, and the speed is controlled. In the case of reverse rotation, f1 depends on the speed change of the motor
Is changed from +20 Hz to -20 Hz. As a result, f2 does not change at -20Hz,
The rotation speed is controlled from 0 to a speed corresponding to -40 Hz (-2 ns).

As described above, in this system, the frequency f1
Is required to be changed according to the motor speed.
Then, a pattern generating circuit (113) of f1 is provided, and its output is used as a frequency command of the inverter INV-A through a first-order delay circuit having a time constant of about 0.1 to 1 second. This time constant is provided in order to avoid a drastic change in frequency, and is necessary when using a pattern as shown in FIG. 12 described later.

Embodiment 8. Next, another example shown in FIG. 11 will be described. In this case, both the inverter INV-A connected to the stator winding and the inverter INV-B connected to the rotor winding are controlled as current sources. Hereinafter, the description of the same parts as the case of FIG. 8 including the control system of the secondary-side inverter INV-B will be omitted, and the different parts of the control will be mainly described.

In this device, the inverter INV-A also has γ.
In order to control the current on the δ coordinate, the primary current signal i1 of the current detector (8) is set to 1 by the three-phase / γδ conversion circuit (103).
It is converted into the synchronous rotation coordinate on the next side, and the signal is fed back to the decoupling current control circuit (121). For this non-interference control, although not shown, ω1, ωre, i
Since γ2 and iδ2 are required, these signals are also given.

When two current control type converters are connected to the primary and secondary of one winding type induction motor, it is necessary to give a current command so that both current controls are consistent. Considering this problem from the T-type equivalent circuit of the induction motor, when the number of turns of the primary and secondary windings of the wound-type induction motor is N1 and N2, the torque current flowing to the primary side can be seen from the T-type equivalent circuit. iδ1 flows on the secondary side as a value iδ2 of the inverse ratio of the number of turns. Therefore, in FIG. 11, first, the torque current command on the secondary side is set to iδ2 ^*, and based on that, the primary torque current command is set to iδ1 ^* = (N2 / N1) iδ2 ^* and given from the torque current command circuit (119). There is.

In the example of this figure, the exciting current is set to iγ2 ^* = 0 and is supplied from the primary side. In a simple system, constant excitation is used, so the required excitation current command is i
It is given as γ1 ^{* from the} excitation current command circuit (120) to the decoupling current control circuit (121). The outputs of the decoupling current control circuit are the voltage commands Vγ1 ^* and Vδ1 ^* to be output by the inverter INV-A. This signal is converted into three phases by the γδ / 3-phase conversion circuit (122), and then the PWM circuit (1
In 01), it is used as the inverter switching signal. Of course,
As described with reference to FIG. 8, the exciting current may be supplied from the secondary side without setting iγ2 ^* = 0.
It can also be shared by the secondary supply. In this system, not only the secondary current but also the primary current is made non-interfering with the γ-axis current and the δ-axis current, so that better control characteristics than the system of FIG. 8 can be obtained.

Ninth Embodiment Next, a control method capable of avoiding the current concentration on the specific arm described above, here, as an example, a frequency f1 when controlling the speed from forward rotation to reverse rotation without using a frequency of 5 Hz or less, A method of controlling f2 will be described with reference to FIG. In this figure, the vertical axis is the frequency of each inverter. The horizontal axis is f1
+ F2. Winding-type induction motor speed n (rpm)
Is n = 120 (f1 + f2) /, where p is the number of pole pairs
Since it is p, the horizontal axis corresponds to the rotation speed. f1 + f2
Since the number of rotations is determined by, this is named the effective frequency for convenience. Therefore, this figure shows how to control f1 and f2 as the rotation speed changes. In this example, the upper limit of the frequency of the inverter is 20 Hz for both f1 and f2, so a speed corresponding to the maximum effective frequency f1 + f2 = 40 Hz is obtained. As described in the explanation of the control system of FIG. 8 or FIG. 11, f1 is first determined based on the rotation speed of the electric motor, and as a result, the rotational coordinate of the secondary side is determined, and as a result, f2 is naturally controlled. The system is composed of f1 and f
2 is going to be controlled.

First, in the case of the stall operation in which the speed is near zero, if the primary frequency is set to f1 = 5 Hz, the secondary frequency naturally becomes f2 = -5 Hz, and the operation is performed at zero speed. As shown in the rotation direction of the frequency in FIG. 4, if the secondary frequency f2 is rotated in the same direction as the primary frequency f1,
The motor speed is zero. This has a primary frequency of 5 Hz and a slip frequency of 5 Hz. Therefore, even in stall operation, both inverters are operating at 5Hz,
It is possible to avoid that a current with a large peak value continues to flow to a specific arm.

Next, when f1 is increased to 20 Hz, the rotation speed n is accelerated to a value corresponding to 20-5 = 15 Hz.
In this case, the secondary frequency is naturally determined from the rotating coordinate system, but remains unchanged at -5 Hz. This is because all changes in speed are covered by changes in f1,
This is because the pattern of f1 is determined. f1 + f2 = 1
When the pattern is determined such that f1 is changed from 20 Hz to 10 Hz when it becomes 5 Hz, f2 changes from -5 Hz to +5 Hz as a result of f1 changing. That is, the polarity of the output frequency f2 is reversed. At this time, f1 is 2
The speed of changing from 0 Hz to 10 Hz is a time constant circuit of about 0.1 second so as to avoid a sudden change. Also, 15
Hysteresis is provided so that two frequencies, that is, f1 between 20 Hz and 10 Hz and f2 between -5 Hz and 5 Hz do not come and go when stopped at a speed command (polarity reversal rotation speed) equivalent to Hz.

Next, when f1 is kept at 10 Hz and the speed is accelerated to 30 Hz, f2 naturally changes from 5 Hz to 20 Hz. Since f2 is the upper limit at 20 Hz, when f1 is accelerated while changing from 10 Hz to 20 Hz, f2 does not change as a result.
Stop at Hz. As described above, if f1 is determined by the pattern together with the speed, the zero frequency is swept by f2, so that it is possible to control from zero to the maximum speed without using a frequency of 5 Hz or less.

Although the reverse rotation side is also shown in the same figure, in the reverse rotation region, only the movements of f1 and f2 are reversed, and the other operation is the same as the above, so the explanation is omitted. Even in the reverse rotation region, f1 is mainly controlled, and as a result, f2 is determined.

In the above description of each embodiment, the inverter INV-A is used as the primary side and the inverter INV-B is used as the secondary side for the sake of convenience, but the primary and secondary of the wound-rotor induction motor are basically the same. Since it is a function, it goes without saying that the same function can be obtained by exchanging both.

Further, the converter serving as the DC power source may be a thyristor type reversible converter, or as shown in FIG.
A diode converter may be used when there is no regeneration in the circuit of (b). As the inverter, various GTO and IGBT inverters as shown in FIG. 2 can be used.

Although each functional block of the control system has been described as a hardware circuit, it goes without saying that these can also be realized by software in a microprocessor.

The advantages envisioned from the various embodiments to which the present invention is applied are listed below.

It is possible to realize a large-capacity drive device by directly combining the outputs of two inverters inside the motor without the need for an interphase reactor. As a result, problems such as electromagnetic noise of the interphase reactor, loss, and installation location are eliminated. Moreover, sufficient torque can be secured even at low speed operation.

Since the number of revolutions corresponding to the sum of the output frequencies of the two inverters can be obtained, the output frequency is set to 1
It can be / 2. That is, for example, a 20 Hz inverter
A speed equivalent to 40 Hz can be obtained by using the table. This means
It leads to the ability to design the switching frequency low,
Excellent efficiency and economic design are obtained.

It is not necessary to reduce the frequency of the inverter to near zero during the stall operation of the electric motor, and both can be set to several Hz (for example, 5 Hz) or more, so that current concentration on a specific arm can be avoided and the junction of the element can be avoided. It is possible to avoid low-frequency pulsation with a large temperature and improve the utilization factor of the element.

The capacity and rating of the two inverters may be the same capacity, same voltage, and same current rating with the winding ratio of the electric motor set to 1: 1, but they do not have to be the same and can be different ratings. In extreme cases, one can be a cycloconverter. Since there are various selections of the winding ratio of the electric motor,
Greater freedom of design. Therefore, a wide range of specifications can be covered by combining several types of developed main circuits.

Since electric power is supplied from both the armature and the rotor, it is possible to avoid the current from becoming too large compared to the armature voltage as in the interphase reactor method, which is advantageous in designing the electric motor.

Unlike the vector control of the cage induction motor, the secondary current can be directly controlled, so that a high-performance control system can be easily configured. Also, even if a low resistance secondary winding is used, there is no control difficulty, so the efficiency can be significantly improved compared to the normal vector control of a basket-type induction motor in combination with the ability to lower the switching frequency. Therefore, it is possible to realize a large energy saving of the large capacity drive device.

In a conventional winding type induction motor driving device such as variable speed pumping, the primary source is a commercial power source and the secondary side is 20
%, An inverter having a capacity of about 10% was provided, and drive control was performed before and after the synchronous speed. The conventional idea is to design one inverter for one driving device, and it seems that the use of two inverters for driving a wound-rotor induction motor has not been considered as uneconomical and meaningless. Although the method of the present invention is uneconomical for a drive device of small and medium capacity,
This is advantageous when designing a large-capacity drive device that is difficult to manufacture with one GTO inverter. Since it has a slip ring, it has some disadvantages compared to a basket-type induction motor, but its drawbacks are eliminated due to the significant energy saving effect that cannot be realized by other methods. Note that the drive device according to the present invention is typically used for steel rolling mills, but in addition to that, it can be considered to be an electric locomotive, an electric motor drive of an electric propulsion ship, and the like. It is also suitable for driving pumps and blowers, and for several hundred kW IGBT inverters in high speed elevators.

[0068]

As described above, the variable speed drive device for an AC electric motor according to claim 1 is a fixed speed drive device for an AC electric motor having AC multiphase windings on both the stator and the rotor. Connect the first variable frequency converter to the child winding and the second variable frequency converter to the rotor winding, and adjust the output frequency of these first and second variable frequency converters to the sum or difference frequency. Since it is configured to obtain the corresponding rotation speed of the AC motor, it is possible to combine the outputs of the two variable frequency converters without using an interphase reactor, and it is possible to secure large capacity and low speed operation. .

The variable speed drive device according to claim 2 is
One of the first and second variable frequency converters is controlled as a voltage source and the other is controlled as a current source, and either one supplies the excitation current of the AC motor and both supply the torque current. As a result, it is possible to combine converters of different voltage type and current type, which increases the degree of freedom in designing, the torque current can be quickly and accurately controlled, and an overcurrent can be avoided.

The variable speed drive device according to claim 3 is
Either one of the first or second variable frequency converters is controlled as a voltage source and the other is controlled as a current source, and the first and second variable frequency converters are both the torque component current of the AC motor. Since the exciting current is supplied, it is easier to balance the capacities of both converters as compared with the one in the previous section.

A variable speed drive device according to a fourth aspect is
Both the first and second variable frequency converters are controlled as current sources, and one of the variable frequency converters supplies the excitation current and the torque current of the AC motor, and the other variable frequency converter. Supplies a current corresponding to the torque, so that decoupling control can be performed on both the stator side and the rotor side, and control characteristics are improved.

A variable speed drive device according to a fifth aspect of the present invention is
Both the first and second variable frequency converters are controlled as current sources, and the first and second variable frequency converters both supply the torque component current and the excitation component current of the AC motor. Compared to, it is easier to balance the capacity of both converters.

A variable speed drive device according to a sixth aspect of the present invention is
When the number of turns of the stator winding and the number of turns of the rotor winding are N1 and N2, respectively, the command value of the torque component current supplied to the first and second variable frequency converters is the inverse ratio of N1 and N2. Therefore, the current control on both the stator side and the rotor side is surely coordinated to obtain smooth control characteristics.

A variable speed drive device according to a seventh aspect of the present invention is
The output frequency of the variable frequency converter provided with the pattern according to the rotation speed of the AC motor is provided by providing a pattern in which the output frequency of the first or second variable frequency converter is predetermined with respect to the rotation speed of the AC motor. The frequency is determined by the above pattern, and the output frequency of the other variable frequency converter is determined by the rotational coordinate of the winding side connected to the other variable frequency converter, which is determined according to the rotation of the AC motor. Therefore, the frequency control of both converters is smoothly performed.

The variable speed drive device according to claim 8 is
The polarity of the output frequency of either the first or second variable frequency converter is inverted from positive to negative or from negative to positive at a predetermined rotation speed of the AC motor (hereinafter, referred to as polarity reversal rotation speed). At the same time, the absolute value of the output frequency is changed to the output frequency before reversal and the output frequency after reversal within a predetermined minute time when the absolute values are all above a predetermined minimum value. At this time, it is possible to quickly pass through the zero frequency, and to prevent a current having a large peak value from continuously flowing through a specific arm of the converter.

The variable speed drive device according to a ninth aspect is
The polarity reversal operation of the variable frequency converter has a hysteresis characteristic by setting the polarity reversal rotation speed while shifting the rotation speed command value of the AC motor when the rotation speed command value rises and when it falls. Even if the rotation speed command value of is stopped at the polarity reversal rotation speed, stable operation characteristics are secured.

According to a tenth aspect of the present invention, there is provided a variable speed drive device comprising a plurality of AC motors, wherein each stator winding of the AC motor is provided with a common first variable frequency converter and each rotor winding. Separate second variable frequency converters are connected to the lines, and the output frequencies of the second variable frequency converters are made different from each other, so that the AC motors have different rotation speeds. Since it is drivable, the number of converters on the stator side remains the same, and by simply increasing the number of converters on the rotor side, it is possible to drive multiple AC motors at mutually different rotational speeds. .

In the variable speed drive device according to an eleventh aspect, the first variable frequency converter is the first inverter, the second variable frequency converter is the second inverter, and the first inverter and the first inverter are the same. Since the DC input terminals of the two inverters are connected in parallel and connected to the common DC power supply, only one DC power supply is required and no unnecessary circulating power flows to the DC power supply.

According to a twelfth aspect of the present invention, there is provided a variable speed drive device in which the first variable frequency converter is a first inverter and the second variable frequency converter is a second inverter. Since the DC power sources for the two inverters are separately provided so that the first and second inverters can operate at DC voltages different from each other, the degree of freedom in selecting the ratings of both inverters increases.

According to the variable speed drive device of the thirteenth aspect, one of the first and second inverters has a three-phase two-phase type.
Since the level inverter and the three-phase three-level inverter are used on the other side, it is possible to realize a system that takes advantage of the features of both the two-level inverter and the three-level inverter.

According to a fourteenth aspect of the present invention, the variable speed drive device uses PWM as a modulation method for the first and second inverters, in which the self-extinguishing element switches a plurality of times during one cycle of the AC output. Moreover, since the frequencies of the carrier waves that determine the switching frequencies of the first and second inverters are made the same and the phases of the carrier waves are made to have a mutual phase difference, the combined current flowing out from both inverters is Harmonic components of are suppressed.

Further, in the variable speed drive device according to the fifteenth aspect, since either the first or second variable frequency converter is the cycloconverter and the other is the inverter, the degree of freedom in selecting the converter is further increased. Will increase.

[Brief description of drawings]

FIG. 1 is a configuration diagram showing a variable speed drive device according to a first embodiment of the present invention.

FIG. 2 is a circuit diagram of various inverters and converters used for explaining the present invention.

FIG. 3 is a block diagram for simplifying and illustrating various inverters and converters.

FIG. 4 is a diagram showing a positive direction of phase rotation of primary and secondary frequencies in the variable speed drive device of the present invention.

FIG. 5 is a configuration diagram showing a variable speed drive device for an AC electric motor according to embodiments 2 to 4 of the present invention.

FIG. 6 is a configuration diagram showing a variable speed drive device for an AC electric motor according to a fifth embodiment of the present invention.

FIG. 7 is a configuration diagram showing a variable speed drive device for an AC electric motor according to a sixth embodiment of the present invention.

FIG. 8 is a block diagram showing a variable speed drive device for an AC electric motor according to a seventh embodiment of the present invention including a control system thereof.

FIG. 9: Forward / reverse rotation of electric motor and inverter INV
It is a block diagram which shows the circuit which switches the input of an up-down counter according to the forward / reverse rotation of the output frequency of -A (2).

FIG. 10 is a diagram showing an example of a control method for explaining how to change the frequencies of the two inverters according to the rotation speed command of the electric motor in the present invention.

FIG. 11 is a block diagram showing a variable speed drive device for an AC electric motor according to an eighth embodiment of the present invention including a control system thereof.

FIG. 12 is a diagram showing an example of a control method for explaining how to change the frequencies of the two inverters according to the rotation speed command of the electric motor without using the frequency of 5 Hz or less in the present invention.

FIG. 13 is a diagram showing an example of a configuration of a conventional interphase reactor type multiple inverter and its output waveform.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 winding type induction motor, 2 first inverter, 3 second inverter, 4, 5 DC filter capacitor, 6
C for current detection such as DC power supply or converter which becomes DC power supply, 7 power supply transformer, 8 and 9 hall sensor
T, 10 1 pulse / revolution and 6000 pulse / revolution pulse generator, 101, 102 PWM circuit, 10
3,105 3 phase to γδ axis coordinate conversion circuit, 104
Coordinate conversion circuit from γδ axis to three phases, 106, 107
Sine wave and cosine wave generation circuit, 108 speed command circuit, 1
09 speed control circuit, 110 UP-DOWN counter, 111 counter or UP-DOWN counter,
112 Decoupling current control circuit for secondary current, 113 Circuit for deciding frequency of primary side inverter according to rotation speed,
114 a time constant circuit for preventing a sudden change in frequency, 115
A circuit that generates a pulse with a frequency corresponding to an input signal, 1
16 A circuit that determines the voltage amplitude according to the frequency of the primary side inverter, 117 A circuit that determines the instantaneous value of the voltage command of the primary side inverter, 118 A voltage control circuit of the primary side inverter, 119 1 according to the winding ratio of the winding type induction motor Command circuit for determining secondary side torque current, 120 Circuit for determining primary side exciting current command, 121 Primary current decoupling current control circuit, 122 γδ axis to three-phase coordinate conversion circuit, 123,124 Two pulses Pulse adder circuit for obtaining output pulse of sum of frequencies, 125, 126 Switch circuit for switching input pulse output to a side when positive, to b side when negative depending on a signal indicating positive or negative of frequency ,
f1 Output frequency of the first inverter, f2 Output frequency of the second inverter.

 ─────────────────────────────────────────────────── ─── Continued Front Page (72) Inventor Masato Koyama 2-3-3 Marunouchi, Chiyoda-ku, Tokyo Sanryo Electric Co., Ltd.

Claims (15)

[Claims] 1. A variable speed drive device for an AC electric motor having AC multiphase windings for both a stator and a rotor, wherein a first variable frequency converter is provided for the stator winding and a first variable frequency converter is provided for the rotor winding. An alternating current characterized by connecting two variable frequency converters to obtain the rotation speed of the AC motor corresponding to the sum or difference frequency of the output frequencies of these first and second variable frequency converters. Variable speed drive of electric motor. 2. The first or second variable frequency converter,
2. An alternating current according to claim 1, wherein one of them controls as a voltage source and the other controls as a current source, and one supplies an exciting current of the AC motor and both supply a torque current. Variable speed drive of electric motor. 3. The first or second variable frequency converter,
One of them is controlled as a voltage source and the other is controlled as a current source, and both the first and second variable frequency converters supply a torque component current and an excitation component current of the AC motor. The variable speed drive device for an AC electric motor according to claim 1. 4. The first and second variable frequency converters are both controlled as current sources, and one of the variable frequency converters supplies the excitation current and the torque current of the AC motor, and the other. 2. The variable speed drive device for an AC motor according to claim 1, wherein said variable frequency converter supplies a current corresponding to a torque. 5. The first and second variable frequency converters are both controlled as current sources, and the first and second variable frequency converters both supply the torque component current and the excitation component current of the AC motor. The variable speed drive device for an AC electric motor according to claim 1, wherein 6. When the number of turns of the stator winding and the number of turns of the rotor winding are N1 and N2, respectively, the command value of the torque component current supplied to the first and second variable frequency converters is N1 and N2. The variable speed drive device for an AC motor according to claim 4 or 5, characterized in that 7. A variable pattern in which the output frequency of the first or second variable frequency converter is predetermined with respect to the rotational speed of the AC electric motor, and the pattern is provided according to the rotational speed of the AC electric motor. The output frequency of the frequency converter is determined by the above pattern, and the output frequency of the other variable frequency converter is determined by the rotation coordinate of the winding side connected to the other variable frequency converter, which is determined according to the rotation of the AC motor. 7. The variable speed drive device for an AC electric motor according to claim 1, wherein the variable speed drive device is configured to determine. 8. The polarity of the output frequency of either the first or the second variable frequency converter is positive to negative or negative to positive at a predetermined rotational speed of the AC motor (hereinafter referred to as polarity reversal rotational speed). In addition, the output frequency is changed from the pre-reversal output frequency to the post-reversal output frequency within a predetermined minute time. A variable speed drive device for an AC electric motor according to any one of claims 1 to 7. 9. A polarity reversal operation of the output frequency of the variable frequency converter is provided with a hysteresis characteristic by setting the polarity reversal rotation speed while shifting the rotation speed command value of the AC motor when the rotation speed command value rises and when it falls. 9. The variable speed drive device for an AC electric motor according to claim 8, wherein 10. A plurality of alternating current motors, wherein each stator winding of the alternating current motor has a common first variable frequency converter, and each rotor winding has an individual second variable frequency converter. A converter is connected to each of the second variable frequency converters so that the output frequencies of the second variable frequency converters are different from each other so that the AC motors can be driven at mutually different rotational speeds. 7. A variable speed drive device for an AC motor according to any one of 1 to 6. 11. The first variable frequency converter is a first inverter, the second variable frequency converter is a second inverter, and the DC input terminals of the first inverter and the second inverter are connected in parallel. 11. The variable speed drive device for an AC motor according to claim 1, wherein the variable speed drive device is connected to a common DC power source. 12. The first variable frequency converter is a first inverter, the second variable frequency converter is a second inverter, and DC power supplies for the first inverter and the second inverter are separately provided. The variable speed drive device for an AC motor according to any one of claims 1 to 10, wherein the first and second inverters are allowed to operate at mutually different DC voltages. 13. A variable speed drive for an AC electric motor according to claim 12, wherein one of the first and second inverters uses a three-phase two-level inverter and the other uses a three-phase three-level inverter. apparatus. 14. A PWM system in which a self-extinguishing element switches a plurality of times during one cycle of an AC output is used as a modulation system for the first and second inverters, and the first and second inverters are used. 14. The AC motor according to any one of claims 11 to 13, characterized in that the frequencies of carrier waves that determine the switching frequencies of the above are the same, and further the phases of these carrier waves have a phase difference from each other. Variable speed drive. 15. The variable speed AC motor according to claim 1, wherein one of the first and second variable frequency converters is a cycloconverter and the other is an inverter. Drive.