JP2003333870A - Inverter system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To make it possible to freely set the voltage ratio of a power source at the input side of an inverter to the other power source connected between coils. <P>SOLUTION: Output terminals of the inverter INV are connected to each terminal at both ends of coils L1, L2. A battery B is arranged between middle points of the coils L1, L2, and a capacitor C1 at the input side of the inverter INV. By adjusting the duty ratio of each transistor of the inverter INV, each input terminal of the coils L1, L2 is controlled independently, and thereby electric potentials at the middle points and phase currents of the coils L1, L2 are controlled. A current flows in such a way as to determine a voltage of the capacitor C1 so that the difference of the electric potentials at the middle points of the coil L1, L2 can become the same with the voltage of the battery B. As a result, the ratio of the voltage of the capacitor C1 to that of the battery B can be set freely. <P>COPYRIGHT: (C)2004,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an inverter system which controls a plurality of inverters to control corresponding coil currents and power conversion between two power sources.

[0002]

2. Description of the Related Art Inverters have been widely used for controlling drive currents of motors. This inverter has an arm made up of an upper switching element and a lower switching element connected in series corresponding to the coil end of the motor, and connects the midpoint of the arm to the coil end. Therefore, the motor drive current can be arbitrarily controlled by controlling the on / off of the upper switching element and the lower switching element of each arm.

In the system disclosed in Japanese Patent Laid-Open No. 11-178114, a DC power supply is connected between the neutral point of the coil of an AC motor driven by an inverter and the negative bus of the inverter. On the other hand, a capacitor is connected between the positive electrode bus and the negative electrode bus of the inverter.

Here, when ON / OFF of each switching element of the inverter is controlled and a drive current is supplied from the inverter to each phase coil of the AC motor, the potential of the connection end (intermediate point of each arm) of the coil of each phase. The average value of should be the neutral point potential of the AC motor. On the other hand, the neutral point potential is
It is fixed to the voltage of the DC power supply connected here. Therefore, if the average value of the potentials of the respective phases in the output of the inverter is not balanced with the anode potential of the DC power source, a zero-phase current that flows in and out of the neutral point flows, and the capacitor is charged or discharged. Then, the zero-phase current always flows so that the average value of the phase potentials output from the inverter and the anode potential of the DC power supply are balanced, and the zero-phase current becomes zero at the balanced stage.

In the case of driving an ordinary AC motor, the neutral point potential is half the inverter input voltage, so that the capacitor voltage is twice the anode potential of the DC power supply. Here, the ratio of the inverter input voltage to the neutral point potential can be changed by controlling the modulation rate, which is the ratio of the ON periods of the upper switching element and the lower switching element in the inverter. That is, by making the ON period of the upper switching element shorter than that of the lower switching element, the neutral point potential becomes lower than the capacitor voltage. Therefore, the DC power supply voltage can be reduced and the inverter input voltage can be increased sufficiently.

According to such a system, the voltage of the DC power supply such as the battery can be kept low and the capacitor voltage, which is the drive voltage of the motor, can be raised. Therefore, when the AC motor has a high output, it can be driven at a high voltage, so that the amount of current for obtaining the same output can be relatively small.

[0007]

However, in the above-mentioned conventional example, by reducing the neutral point potential, the DC power supply voltage and the
The ratio of the capacitor voltage is increased. The drive current of the motor has a voltage waveform that oscillates around the neutral point potential. Therefore, when the maximum amplitude value of the AC voltage of the motor drive current exceeds twice the neutral point voltage (DC power supply voltage), the voltage waveform is distorted. Therefore, there is a problem that the maximum amplitude value of the AC voltage is limited to twice the DC power supply voltage.

It is an object of the present invention to provide an inverter system capable of increasing the voltage ratio between the power source on the input side of the inverter and the power source connected between the coils without suffering from such a limitation.

[0009]

According to the present invention, there is provided an inverter having a plurality of output terminals connected to a plurality of coils and independently controlling potentials at both ends of each coil by PWM control; And a first power storage means provided between any one point of one coil and another coil and capable of storing or discharging power, and power storage or discharging provided at the input side of the inverter. A possible second power storage means, and independently controlling the potentials across the plurality of coils by controlling the inverter,
It is characterized by controlling conversion of electric power between the first and second electric power storage means.

According to the present invention, since the potentials at both ends of the coils are independently controlled, the average potential of each coil can be arbitrarily determined. Therefore, even if the voltage ratio between the first power storage means and the second power storage means is increased, the average potential of the coil can be increased and the current of the coil can be secured.

Further, according to the present invention, an inverter having a plurality of output terminals connected to a plurality of coils and independently controlling the potentials at both ends of each coil by PWM control, and one of the plurality of coils. First power storage means that is provided between an arbitrary point of the coil and one end of the input side of the inverter and is capable of storing or releasing power, and power storage means that is provided between both ends of the input side of the inverter and capable of storing or releasing power. Second electric power storage means, and independently controlling the potentials across the plurality of coils under the control of the inverter, thereby controlling the conversion of electric power between the first and second electric power storage means. It is characterized by

As described above, when the first power storage means is connected between one coil and the inverter input terminal, the potential difference between the coil and the inverter input terminal is determined by the first power storage means. . Therefore, the average voltage of this coil is determined by the first power storage means, but the average voltage of the other coils can be determined arbitrarily.

The inverter has a plurality of arms each consisting of an upper switching element and a lower switching element connected in series. The midpoint of the plurality of arms is connected to the ends of the plurality of coils. It is preferable to determine the ratio of the voltage of the first power storage means and the voltage of the second power storage means by the modulation rate which is the ratio of the on period of the upper switching element and the on period of the lower switching element. .

As described above, since the average potential of each coil is determined by the modulation rate and the average potential difference between the coils becomes the first power supply voltage, the amplitude of the triangular wave in the PWM control that determines the modulation rate becomes the first. 2 power supply voltage. Therefore,
The voltage value of the second power supply can be set to an arbitrary voltage by controlling the modulation rate.

Further, it is preferable that the potential at both ends of each coil is controlled and the phase current in each coil is controlled by changing the modulation factor in the inverter. As a result, a desired phase current can be supplied to the coil.

It is also preferable that the first power storage means is a battery and the second power storage means is a capacitor. As a result, the inverter input voltage can be set to a high voltage by using the constant voltage battery.

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings.

[System Configuration] FIG. 1 shows an inverter system according to this embodiment. Both ends of the coil L1 are connected to two arm output terminals of the inverter INV. On the other hand, a capacitor C1 as a power source is connected to a pair of input terminals (a positive electrode bus and a negative electrode bus) of this inverter.

One phase of the inverter INV (a pair of arms corresponding to the coil L1) is a switching element 4
Transistors Q1 to Q4 and these transistors Q
It is composed of four diodes D1 to D4 which connect the emitters to the collectors of Q1 to Q4 respectively and flow a current from the emitter side to the collector side. Transistors Q1 and Q2
Are connected in series between the input terminals, and these transistor Q
The connection point of 1 and Q2 is one output terminal. Also,
The transistors Q3 and Q4 are also connected in series between the input terminals,
The connection point of these transistors Q3 and Q4 is the other output terminal. That is, the connection point of the transistors Q1 and Q2 is connected to one end of the coil L1, and the transistor Q1
The connection point of 3 and Q4 is connected to the other end of the coil L2.

Therefore, by turning on the transistors Q1 and Q4, the current from the capacitor C1 flows through the coil L1 in one direction, and by turning on the transistors Q3 and Q2, the current from the capacitor C1 is changed.
Flow 1 in the other direction.

Both ends of the coil L2 are connected to the output ends of one phase (a pair of arms corresponding to the coil L2) of the inverter INV in which a pair of input ends (positive and negative buses) are connected to both ends of the capacitor C1. ing. The portion of the inverter corresponding to the coil L2 has the same structure as the portion corresponding to the coil L1. That is, it is composed of four transistors Q5 to Q8, and four diodes D5 to D8 which respectively connect the emitters and collectors of these transistors Q5 to Q8 and flow a current from the emitter side to the collector side. The transistors Q5 and Q6 are connected in series between the input terminals, the connection point of these transistors Q5 and Q6 is connected to the coil L2 once, and the transistors Q7 and Q8 are also connected in series between the input terminals. The connection point of the transistors Q7 and Q8 is connected to the other end of the coil L2.

Therefore, by turning on the transistors Q5 and Q8, the current from the capacitor C1 flows in one direction through the coil L2, and by turning on the transistors Q7 and Q6, the current from the capacitor C1 is changed.
2 flows in the other direction.

Therefore, by controlling the switching of the inverter INV, the currents of the coils L1 and L2 can be arbitrarily controlled. For example, the coils L1 and L2 form a two-phase AC motor.

A battery B is arranged between an arbitrary point of the coil L1 (middle point in this example) and an arbitrary point of the coil L2 (middle point). Therefore, the voltage difference between the midpoints of the coils L1 and L2 is fixed to the voltage of the battery B. Therefore, a current flows between the battery B and the capacitor C1 so that the voltage difference between the midpoints of the coils L1 and L2 becomes the voltage of the battery B, and the voltage of the capacitor C1 is determined. The positive electrode of the battery B is connected to the midpoint of the coil L1, and the negative electrode is connected to the midpoint of the coil L2. When the battery B is connected to the midpoint of the two coils L1 and L2, the average potential of the coils and the midpoint potential become the same potential, which facilitates control.

Here, the lengths of the ON periods of the upper transistors Q1, Q3, Q5 and Q7 and the ON periods of the lower transistors Q2, Q4, Q6 and Q8 in the inverter INV are made different from each other, so that the coils L1 and L2 are made different. The average voltage across both ends can be controlled to any value. Therefore, each coil L
It is possible to arbitrarily control the current flowing through the L1 and L2 and the current flowing in and out of the midpoint of both coils.

That is, as shown in FIG. 2, the L1 + output potential command is compared with the triangular wave carrier, and when the triangular wave is larger than the L1 + output potential command, the transistor Q1 is turned on, and when the triangular wave is small, the transistor Q2 is turned on. . When the triangular wave is larger than the L1-output potential command, the transistor Q3 is turned on, when the triangular wave is small, the transistor Q4 is turned on, and when the triangular wave is larger than the L2 + output potential command, the transistor Q5 is turned on and the triangular wave is small. Then, the transistor Q6 is turned on, the transistor Q7 is turned on when the triangular wave is large and the transistor Q8 is turned on when the triangular wave is small according to the L2-output potential command.

At this time, a current flows through the capacitor C1 so that the potential difference between the average potential of the coil L1 and the average potential of the coil L2 becomes the voltage Vb of the battery B. As a result, the voltage of the capacitor C1 (input voltage of the inverter INV) becomes the voltage Vc1 corresponding to the width of the triangular wave in FIG.

That is, the voltage Vb of the battery B, the inverter input voltage (capacitor voltage) Vc1 and the coil L
A circulating current flows between the battery B and the capacitor C1 until the average potentials of 1 and L2 have the magnitude relationship shown in FIG. 2, and the state shown in FIG. 2 becomes a balanced state.

Here, the ratio (modulation rate) of the ON period of the upper transistor and the ON period of the lower transistor in one arm is, as shown in FIG. 2, the magnitude of the output potential command for one cycle of the triangular carrier wave. It is the ratio of
That is, the higher the output potential command, the shorter the period in which the triangular wave exceeds the command value. Then, by setting the period when the triangular wave exceeds the output potential command to be the on period of the upper transistor and the off period of the lower transistor of each phase,
The ratio of the on periods of the upper and lower transistors (that is, the modulation rate) is determined.

Therefore, the potentials (average potentials) across the coils L1 and L2 are determined by the average modulation rate in the pair of arms corresponding to each. Further, the potential difference between the two midpoint potentials is the voltage Vb of the battery B. Therefore, the battery voltage Vb, the modulation rates d1 and d2 of the pair of arms corresponding to the coils L1 and L2, and the capacitor voltage V
The following relationships exist between c1.

Vc1 = Vb / (d1-d2) Since the battery voltage Vb is fixed, the inverter IN
A pair of arms corresponding to the coil L1 of V and the coil L2
The capacitor voltage Vc can be determined by controlling the modulation rates d1 and d2 in the pair of arms corresponding to.

In the above example, the switching transistor is turned on and off without dead time with respect to the carrier wave period Ts of the inverter. That is, when the duty ratio is 50%, the upper and lower transistors are both turned on for a period of 50%. However, in order to completely eliminate the shoot-through current during the switching period, a dead time Td for turning off both the upper and lower transistors is often provided. In this case, the above equation is rewritten and applied as follows.

Vc = Vb / {(d1-Td / Ts)-
(D2 + Td / Ts)} Therefore, even when the dead time is provided, the capacitor voltage Vc can be determined by controlling the modulation rates d1 and d2.

Further, in this embodiment, the inverter IN
A pair of output terminals L1 + of each coil L1, L2 of V,
The potentials of L1-, L2 +, and L2- are changed as shown in FIG. As a result, a current according to the difference between the voltages across the coils L1 and L2 flows, and this current flows through the coils L1 and L2.
Drive current (phase current).

As described above, according to this embodiment, the capacitor voltage Vc1 can be controlled by adjusting the voltage applied across the coils L1 and L2 by the inverter INV. Therefore, according to the output voltage of the inverter INV, the input voltage of the inverter (capacitor C1 voltage Vc
By properly controlling 1), the efficiency of the inverter INV can be increased. Further, since the voltage of the battery B can be set arbitrarily, it can be shared with other power sources. For example, in electric vehicles and hybrid vehicles, it is possible to keep the battery voltage low,
It will be advantageous in terms of cost.

Furthermore, the average potential of the coils L1 and L2 does not necessarily have to be close to the ground potential. Therefore, there is not much restriction on the maximum value of the voltage amplitude in each coil current, and the ratio between the battery B voltage Vb and the capacitor voltage Vc1 can be set relatively freely.

Further, the circuit functioning as a DC-DC converter has a reactor with 1/2 phase of the self-inductance of the coil. In the conventional configuration in which the battery is arranged between the neutral point of the motor and the ground, the coil does not function as a reactor for the current flowing in and out of the neutral point,
It was necessary to dispose another reactor between the neutral point and the battery, but the configuration of the present embodiment eliminates the need for such a special reactor. Therefore, it is advantageous in terms of cost, space, and efficiency.

Further, each end potential of each coil L1 and L2 is controlled completely independently by the arm of the corresponding inverter INV. Therefore, the failure of one arm of the inverter INV only affects the potential at one end of one coil. Therefore, it is also possible to separate only that portion and compensate by supplying current to other coils. In particular, if the motor has two or more phases, it is possible to maintain its rotation, and it becomes easy to configure a fail-safe mechanism.

In the above example, both coils L1 and L2
Battery B was connected between the midpoints. However, the connection point may be any point in both coils L1 and L2. For example, the connection point of the battery B to the coil L1 connects the coil L1 to N1.
(L1 + side): A point to be divided into N2 (L1-side),
The connection point of the battery B to the coil L2 connects the coil L2 to N3.
(L2 + side): If it is a point to be divided into N4 (L2- side): (L1 + output potential command amplitude)-(L1- output potential command amplitude) = L1 phase voltage command- (L2 + output) Potential command amplitude)-(L2-output potential command amplitude) = L2 phase voltage command- (L1 + output potential command amplitude): (L1-output potential command amplitude) = N1: N2 (L2 + output potential) Amplitude of command): (Amplitude of L2-output potential command) = N3: N4. The connection point may be any point of the coils L1 and L2.

FIG. 3 shows an example of the configuration when applied to a three-phase motor. The inverter INV is a three-phase coil L
It has a six-arm configuration corresponding to 1, L2, and L3. That is, transistors Q9 to Q1 for two arms
2, diodes D9 to D12 are added. The connection point between the transistors Q9 and Q10 is connected to one end of the coil L3, and the connection point between the transistors Q11 and Q12 is connected to the other end of the coil L3.

On the other hand, the coils L1, L2, L3 are arranged so that their phases are different from each other by 120 degrees, and constitute a motor for rotating one rotor (not shown).

In this embodiment, the battery B is connected between the midpoint of the coil L1 and the midpoint of the coil L3, and the capacitor C2 is placed between the midpoint of the coil L3 and the midpoint of the coil L2. Are arranged. The capacitor C1
Is the input end of the inverter INV (positive bus, negative bus)
It is connected to the.

In this configuration, as shown in FIG. 4, three output potential commands L1 +, L1-, of the inverter INV are provided.
L2 +, L2-, L3 +, L3- are compared with the triangular wave carrier, and when the output potential command is larger than the triangular wave carrier, the respective coils L1, L2, L3 of the inverter INV.
The upper transistors of the pair of arms corresponding to are turned on, and the lower transistors are turned on when smaller than the triangular wave carrier. At this time, the potential difference between the average potential of the coil L1 and the average potential of the coil L3 is the voltage Vb of the battery B.
A current flows through the capacitor C so that As a result, the voltage Vc of the capacitor C becomes a voltage corresponding to the width of the triangular wave in FIG. Also, the middle point of the coil L3 and the coil L
The voltage of the capacitor C2 connected between the two midpoints is
It is determined as the voltage difference between the voltage command value of the coil L3 and the voltage command value of the coil L2.

That is, the voltage Vb of the battery B, the inverter input voltage Vc1 and the capacitor voltage Vc2 are shown in FIG.
Battery B and capacitor C until the relationship shown in
A circulating current flows between C1 and C2, and the state shown in FIG. 4 becomes a balanced state.

Even in such a system for a three-phase motor coil, as in the case described above, the coil L
The inverter input voltage Vc1 can be arbitrarily set with respect to the voltage of the battery B by freely setting the average potentials at 1, L2 and L3. In particular, in the case of a failure of the arm of the inverter for one coil, it is possible to control the current to the other coil and maintain the rotation of the motor.

FIG. 5 shows an example of connection to a three-phase PM (permanent magnet) motor. In this example, the negative electrode of the battery B is connected to the midpoint of the coil L2, and the positive electrode of the battery B is connected to the midpoint of the coils L1 and L3. Therefore, the midpoint potentials of the coil L1 and the coil L3 are the same potential. The middle point of the coil L1 and the coil L3
If a capacitor is provided between the midpoints, the circuit becomes equivalent to the circuit of FIG.

In the figure, the magnetic poles shown on the stator are magnetic poles formed by a circulating current acting as a DC-DC converter. As can be seen from this figure, according to the present embodiment, the inductance using the motor magnetic circuit is formed in the circuit through which the circulating current flows, and this is used for power conversion. Therefore, it is not necessary to add a reactor.

Further, an attractive repulsive force is generated between the magnetic pole of the circulating current and the magnetic pole of the rotor. However, since the rotational direction components of the motor cancel each other out, no torque ripple occurs and the rotation is not adversely affected. In FIG. 5, the force in the radial direction remains without being cancelled, but the rigidity of the shaft is sufficiently high so that it does not matter. It is also possible to cancel the radial force by making the magnetic poles of the motor multi-pole.

FIG. 6 is a diagram showing another structural example of a system having a two-phase coil. In the system of FIG. 6, the positive electrode of the battery B is connected to an arbitrary one point (in this example, an intermediate point divided by N1: N2) of the coil L1, and the negative electrode is connected to the negative electrode bus of the inverter INV. .

As a result, as shown in FIG. 7, the average potential of the coil L1 is the voltage V2 which is the voltage of the battery B.
Is set to. Therefore, by setting the amplitude of the phase potential command for the coil L1 to be equal to or less than twice the voltage V2 in the peak width,
Distortion of the phase current in the coil L1 can be prevented. On the other hand, the phase potential command for the coil L2 can be set arbitrarily.

In this example, the connection point of the battery B to the coil L1 is such that the coil L1 is N1 (L1 + side): N2 (L
1-side), (L1 + amplitude of output potential command)-(L1-amplitude of output potential command) = phase potential command of L1 phase ((amplitude of L1 + output potential command) :( If the condition of (L1-amplitude of output potential command) = N1: N2 is satisfied, the connection point may be any point of the coil L1.

With such a configuration, the same operational effect as that of the above-described embodiment can be obtained. Further, although the configuration of the two-phase coil is shown in FIGS. 6 and 7, the same configuration can be applied even when the number of phases is three or more. For one or more batteries,
It can be connected to one end of the input side of the inverter. It should be noted that the voltage of the battery may be any voltage as long as the voltage at the connection point of the coil can be set to an intermediate voltage of the voltage on the input side of the inverter, or may be connected to the positive bus.

The present invention can be used, for example, for vehicles (electric vehicles, hybrid vehicles, etc.) that generate a driving force with a multiphase motor, but is not limited to this, and a multiphase motor that can be driven with an inverter is used. It is applicable to all inverter systems.

[0054]

As described above, according to the present invention,
Since the potentials at both ends of the plurality of coils are independently controlled, the average potential of each coil can be arbitrarily determined. Therefore, even if the voltage ratio between the first power supply and the second power supply is increased, the average potential of the coil can be increased and the current of the coil can be secured.

[Brief description of drawings]

FIG. 1 is a diagram showing a system configuration having a two-phase coil.

FIG. 2 is a diagram showing a relationship between an inverter input voltage and a DC power supply voltage in a system having a two-phase coil.

FIG. 3 is a diagram showing a system configuration having a three-phase coil.

FIG. 4 is a diagram showing a relationship between an inverter input voltage and a DC power supply voltage in a system having a three-phase coil.

FIG. 5 is a diagram showing a relationship between a connection example and a magnetic pole formed by a circulating current when applied to a three-phase PM motor.

FIG. 6 is a diagram showing another configuration of a system having a two-phase coil.

FIG. 7 is a diagram showing a relationship between an inverter input voltage and a DC power supply voltage in another configuration of a system having a two-phase coil.

[Explanation of symbols]

B battery, C1 and C2 capacitors, D1 to D12
Diodes, INV inverters, L1 to L3 coils, Q1 to Q12 transistors (switching elements).

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Moriya Kazunari             Aichi Prefecture Nagachite Town Aichi District             Ground 1 Toyota Central Research Institute Co., Ltd. (72) Inventor Hiroki Otani             Aichi Prefecture Nagachite Town Aichi District             Ground 1 Toyota Central Research Institute Co., Ltd. (72) Inventor Hideo Nakai             Aichi Prefecture Nagachite Town Aichi District             Ground 1 Toyota Central Research Institute Co., Ltd. (72) Inventor Yukio Inuma             Aichi Prefecture Nagachite Town Aichi District             Ground 1 Toyota Central Research Institute Co., Ltd. (72) Inventor Shoichi Sasaki             1 Toyota Town, Toyota City, Aichi Prefecture Toyota Auto             Car Co., Ltd. (72) Inventor Junwamoto             1 Toyota Town, Toyota City, Aichi Prefecture Toyota Auto             Car Co., Ltd. (72) Inventor Eiji Sato             1 Toyota Town, Toyota City, Aichi Prefecture Toyota Auto             Car Co., Ltd. (72) Inventor Masayuki Komatsu             1 Toyota Town, Toyota City, Aichi Prefecture Toyota Auto             Car Co., Ltd. F-term (reference) 5H007 AA04 AA07 BB01 BB06 CA02                       CB04 CB05 CC05 CC07 CC12                       CC23 DA06 DB01 EA02 GA01                 5H576 AA15 BB10 CC04 DD05 DD07                       EE11 EE21 HA02 HB02 HB05                       JJ26

Claims (5)

[Claims] 1. An inverter having a plurality of output terminals connected to a plurality of coils and independently controlling the potentials at both ends of each coil by PWM control; and an arbitrary one of the plurality of coils. A first power storage unit that is provided between a point and an arbitrary point of another coil and that can store or release power, and a second power that is provided between both ends of the input side of the inverter and that can store or release power. And a storage unit, wherein the inverter system controls the conversion of electric power between the first and second electric power storage units by independently controlling the potentials at both ends of the plurality of coils under the control of the inverter. 2. An inverter having a plurality of output terminals connected to a plurality of coils and independently controlling the potentials at both ends of each coil by PWM control; and an arbitrary one of the plurality of coils. A first power storage unit provided between the point and one end of the inverter on the input side and capable of storing or releasing power, and a second power storage unit provided between both ends of the input side of the inverter for storing or releasing power. An inverter system for controlling the conversion of electric power between the first and second electric power accumulating means by independently controlling the electric potentials across the plurality of coils under the control of the inverter. 3. The system according to claim 1, wherein the inverter has a plurality of arms each including an upper switching element and a lower switching element connected in series, and a plurality of midpoints of the plurality of arms are provided. Of the first power storage means and the second power storage means according to the modulation rate which is the ratio of the ON period of the upper switching element and the ON period of the lower switching element. An inverter system that determines the voltage ratio. 4. The inverter system according to claim 3, wherein the modulation factor in the inverter is changed to control the potential across each coil and to control the phase current in each coil. 5. The inverter system according to claim 1, wherein the first power storage means is a battery and the second power storage means is a capacitor.