JP2008131684A - Driving system of electric motor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a driving system of an electric motor, which can increase regenerative energy by increasing inductance, and increase a battery discharging time by reducing a current in startup. <P>SOLUTION: The driving system of an electric motor includes a power supply circuit 100, a multi-phase inverter circuit 200 and an electric motor 300. In the system, a two-rotor type axial air gap type brushless DC motor is used in which a rotor is disposed oppositely at a predetermined gap on both sides of the stator along an axial line direction of a rotor output shaft. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an electric motor drive system that recovers and reuses regenerative energy of an electric motor, and more specifically, operation of the system, such as acceleration, constant speed, deceleration, braking, inertia (downhill traveling, etc.), and stopping. The present invention relates to a drive system for an electric motor that can increase the amount of regenerative energy recovered when traveling is repeated and reduce the amount of discharge at the time of starting.

  In the field of transportation equipment, electric motors are used in automobiles and motorcycles. Recently, hybrid motors that are used in combination with gasoline engines, and electric assist functions that assist human power by installing electric motors in part of the drive system of bicycles. Bicycles are often seen.

  For example, as shown in Patent Document 1, this type of electrically assisted bicycle has been mainly used for an electric motor including a DC motor with a brush or a radial air gap brushless DC motor.

  However, when a brushed DC motor or a radial air gap type brushless DC motor is used, there are the following problems. That is, the brushed DC motor has a short life of the brush and the commutator, and has a stator winding that is short-circuited by the brush, so that the energy efficiency is poor, and therefore the discharge duration of the battery is also shortened.

  On the other hand, since the radial air gap type brushless DC motor switches the current flowing through the stator winding by the inverter circuit, it has a long life and high energy conversion efficiency and reliability.

  However, the radial air gap type brushless DC motor cannot be said to have a sufficiently long discharge duration of the battery. Further, in order to increase the output of the radial air gap type brushless DC motor, the dimensions of the stator and the rotor must be increased, which hinders the miniaturization of the bicycle with electric assist.

JP-A-9-117077

  Therefore, in order to solve the above-described problems, the present invention can increase the recovery amount of the regenerative energy of the electric motor, and can reduce the current at the time of starting and extend the discharge duration of the battery. An object of the present invention is to provide an electric motor drive system that can be used.

  In order to solve the above-described problems, the present invention has several features described below. The invention according to claim 1 is a DC power supply circuit in which a battery power supply and a capacitor are connected in parallel, a multiphase inverter circuit comprising switching elements in which flywheel diodes are connected in antiparallel, and the multiphase inverter circuit An electric motor connected to the power supply circuit through the feedback of the position signal of the rotor of the electric motor to the switching element of the multi-phase inverter circuit, by switching the current flowing through the winding of the electric motor, In an electric motor drive system that generates a rotating magnetic field and generates torque in a field rotor, the electric motor includes one stator and two rotors disposed on both sides of the stator along the axial direction of the rotor output shaft. 2-rotor 2-air-gap axial air-gap brush arranged oppositely with a predetermined gap The DC motor is used, and the motor operates intermittently in a torque generation mode for generating torque or a power generation mode for generating electric energy. In the power generation mode, the switching element is turned off to The electric energy generated by the rotation is fed back to the battery power source and the capacitor.

  A second aspect of the present invention is the axial air gap type brushless DC motor according to the first or second aspect, wherein the second stator is further disposed opposite to the other side surface of the rotor with a predetermined gap. 2 rotor 4 air gap type axial air gap type brushless DC motor.

  A third aspect of the present invention is characterized in that, in the first or second aspect, a magnet made of a sintered rare earth magnet or a bonded rare earth magnet is used for the rotor.

  According to a fourth aspect of the present invention, in any one of the first to third aspects, the stator has a fractional slot, and a winding is wound around the fractional slot in concentrated winding. Yes.

  According to a fifth aspect of the present invention, in the first aspect, the stator core of the stator and / or the back yoke of the rotor has a high magnetic flux density lower than 50A400 or 35A360 of JIS-C-2552 and has a high permeability. A low electromagnetic steel plate, a cold-rolled steel plate, or a hot-rolled steel plate is used.

  A sixth aspect of the present invention is characterized in that, in the first aspect, the back yoke of the rotor is formed by laminating one or a plurality of disk-shaped cold-rolled steel plates or hot-rolled steel plates in the axial direction. Yes.

  According to the first aspect of the present invention, by using the axial air gap type brushless DC motor, the synchronous reactance when operating as a generator during regeneration is reduced, the voltage drop due to the reactance is suppressed, and the output voltage is increased. Thus, more regenerative energy can be returned to the power supply side.

  Further, by using a 2-rotor 2-air gap type axial air gap type brushless DC motor in which two rotors are arranged on both sides of one stator, the magnetic resistance is increased and the synchronous reactance can be further reduced.

  According to the second aspect of the present invention, the second stator is further arranged on the other side surface of the rotor, so that a total of four air gaps of two rotors and four air gaps can be obtained, and the magnetic resistance can be further increased. The synchronous reactance can be further reduced.

  According to the third aspect of the present invention, by using a sintered rare earth magnet or a bonded rare earth magnet as the rotor magnet, the torque constant can be further increased, and the starting current can be reduced and the starting time can be shortened. Therefore, it is possible to suppress the battery capacity reduction due to the large current at the start.

  According to the fourth aspect of the present invention, the winding resistance of the stator can be reduced and the energy conversion efficiency can be increased by winding the stator core having fractional slots with concentrated winding. Here, the “fractional slot” referred to in the present invention means a slot where [number of stator slots] / [number of phases × number of poles] = fraction.

  According to the invention described in claim 5, by using an electromagnetic steel sheet, a cold-rolled steel sheet, or a hot-rolled steel sheet having a low magnetic permeability and a high magnetic flux density for the stator core and the back yoke, the synchronous reactance can be reduced and the generator operates. When doing so, the output voltage can be increased.

  According to the invention described in claim 6, by using a disk-shaped cold rolled steel sheet or hot rolled steel sheet laminated in the axial direction as a rotor back yoke, transmission of DC magnetic flux of the permanent magnet is achieved. On the other hand, the AC magnetic flux generated from the stator is difficult to pass due to eddy currents, and the magnetic resistance can be increased and the synchronous reactance can be reduced.

  Next, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a circuit configuration diagram of an electric motor drive system according to an embodiment of the present invention, and FIG. 2 is a schematic diagram of an axial air gap type brushless DC motor used in the electric motor drive system of the present invention.

  As shown in FIG. 1, the electric motor drive system of the present invention includes a power supply circuit 100 formed by connecting a battery power supply 110 and a capacitor 120 charged with regenerative energy of the electric motor in parallel, and a flywheel diode 210 in antiparallel. Are provided with a multiphase inverter circuit 200 composed of a switching element 220 and an electric motor 300 connected to the power supply circuit 10 via the multiphase inverter circuit 200.

  As the battery power source 110, a commercial battery as a general DC power source represented by a lithium ion battery, a nickel metal hydride battery, a nickel cadmium battery, or the like is used. In the present invention, the capacity and voltage of the battery power source 110 can be arbitrarily changed according to the specifications of the drive system.

  As the capacitor 120, an electrolytic capacitor or a large-capacity electric double layer capacitor is used, and a ceramic capacitor or a film capacitor having good high frequency characteristics may be further used in combination.

The capacity of the capacitor 120 is such that the average regenerative energy in one cycle of driving traveling-inertial traveling-braking stop is Pr and the capacitor voltage is Vc.
C ^· Vc 2/2 = Pr
It is preferable that the value is set to be larger than the required C value.

  The multi-phase inverter circuit 200 has a series circuit in which a pair of switching elements 220 such as power transistors and MOSFETs, which are connected in antiparallel with flywheel diodes 210, are provided on the upstream side and the downstream side along the current direction. The series circuit is provided for three phases of U phase, V phase, and W phase.

  A non-connection end of each phase winding of the electric motor 300 is connected to each interconnection point of the switching element 220 of the series circuit for each phase. The rotation control of the electric motor 300 is controlled by PWM (Pulse Width Modulation) or PAM (Pulse Amplitude Modulation), or 120 ° energization or 180 ° energization.

  As shown in FIG. 2, the electric motor 300 includes a stator 310 formed in a disk shape, and a pair of rotors 320 and 320 that are opposed to each other with a predetermined gap (gap) on both side surfaces of the stator 310. A so-called axial air gap type brushless DC motor is used. Each rotor 320, 320 is coaxially fixed to a rotor output shaft 330 that outputs a rotational driving force.

  The axial air gap type brushless DC motor 300 includes a position sensor (not shown) of the rotor 320 for energization control of the brushless DC motor. As the position sensor, a general position sensor such as a Hall element or an optical sensor is used, but a sensorless system may be used.

  The stator 310 includes a plurality of (in this example, nine (9 slots)) stator cores 311 that are annularly arranged with the rotation axis of the rotor output shaft 4 as a central axis. The stator 310 is wound around the outer periphery of each stator core 311. The wire is wound in multiple layers.

  Slots around which the windings are wound are provided between the stator cores 311 of the stator 310. In the present invention, the slots of the stator 310 are fractional slots, and the windings are wound in concentrated windings on the fractional slots. Being turned

  According to this, the winding resistance of the stator can be reduced and the energy conversion efficiency can be increased by winding the stator core having fractional slots with concentrated winding. Here, the “fractional slot” referred to in the present invention refers to the number of slots where [number of stator slots] / [number of phases × number of poles] = fraction.

  The stator core 330 is preferably made of a material having a high magnetic flux density and a low magnetic permeability in order to reduce the synchronous reactance and increase the torque constant. That is, in addition to cold-rolled steel sheets and hot-rolled steel sheets of low-grade electrical steel sheets with low silicon content (preferably electrical steel sheets lower than JIS-C-2552 50A400 or 35A360, such as 50A1300), powder-formed pressure A powdered iron core or the like is used.

  Since the rotors 320 and 320 have the same configuration, only one of them will be described in the following description, and the description of the other will be omitted. The rotor 320 has a disk-shaped rotor back yoke 321, and a rotor magnet 322 is disposed on the rotor back yoke 321 so as to face the stator core 330.

  The rotor back yoke 321 is preferably made of a material having a high magnetic flux density and a low magnetic permeability in order to reduce the synchronous reactance and increase the torque constant in the same manner as the stator core 330 described above. It consists of one or more sheets of rolled or hot rolled steel sheets laminated in the axial direction.

  That is, the rotor back yoke 321 is a low-grade electromagnetic steel sheet with a low silicon content (preferably a material lower than 50A400 or 35A360 of JIS-C-2552, such as 50A1300), a cold-rolled steel sheet or a hot-rolled steel sheet, A powder-molded powder core is used.

  The rotor magnet 322 is made of a sintered rare earth magnet and / or a bonded rare earth magnet. More specifically, rare earth magnets such as neodymium-iron-boron magnets, samarium-cobalt magnets, and samarium-iron-nitrogen magnets are preferably used, but permanent magnets such as ferrite magnets may also be used. Good.

  In the present invention, the rotor 300 has a structure in which the rotor magnet 322 faces the teeth surface of the stator core 330, that is, a so-called SPM (Surface Permanent Magnet) structure, and the difference between the q-axis inductance and the d-axis inductance is within ± 10%. It is suppressed.

  In this example, the axial air gap type brushless DC motor 300 is a so-called 2-rotor 2 air gap type axial air gap type brushless DC motor in which two rotors 320 are arranged on both side surfaces of one stator 310. However, in addition to this, as shown in FIG. 3, an auxiliary stator 340 as a second stator may be further arranged on the other outer surface of the rotor 320 (the surface facing the anti-stator 310).

  According to this, by constructing a so-called two-rotor 4-air gap type axial air gap type brushless DC motor 300A in which air gaps are formed on both surfaces of the rotor 320, the magnetic resistance is increased by the amount of increase in the air gap. The synchronous reactance can be further reduced and the regenerative energy is increased.

  However, when the air gap is increased in this way, the gap length increases and it becomes difficult for the magnetic flux to pass. Therefore, it is preferable to increase the thickness of the rotor magnet 322 to compensate the magnetic flux density.

  When this axial air gap type brushless DC motor 300 is used in a drive system of an electric motor such as a bicycle with electric assist or a hybrid vehicle, for example, during inertia traveling or braking, the electric motor 300 generates regenerative energy as a generator, and the regenerative energy is It is stored in the capacitor 110 and further charged in the battery. FIG. 4 is an equivalent circuit of the present system during the regenerative operation.

FIG. 5A is a voltage vector diagram of a radial air gap type electric motor, and FIG. 5B is a voltage vector diagram of an axial air gap type brushless DC motor. According to this, since the axial air gap type brushless DC motor 300 has _a smaller synchronous reactance Xa than the radial air gap type electric motor, even when the same current i flows, the voltage drop X _a · i Decreases and the output voltage V _ta increases.

In other words, even when the voltages induced in the motor are the same, the magnitudes of the voltage drops X _r · i and X _a · i due to the synchronous reactances X _r and X _{a become} high and low in the output voltage V _tr · V _ta . since a direct impact, axial airgap brushless DC motor, because synchronous reactance X _a is small, the voltage drop X _a · i decreases. As a result, the output voltage increases and the regenerative energy increases. Further, since the inductance of the stator winding is small, the transient response characteristic is also improved, so that the regenerative energy can be quickly returned to the power supply side.

  This is also clear from the following formula. Referring to FIGS. 6A and 6B, the axial air gap type brushless DC motor used in the drive system of the present invention is expressed by the following equation (1) from the law of ampere-round integration. .

ここで、２ＮａＩは〔起磁力〕、２ｌｔ／μ_１Ｓ_１は〔ステータコア部分の磁気抵抗〕、２ｌｙ_１／μ_２Ｓ_２は〔ロータコアの磁気抵抗〕、４（ｇ＋ｍ）／μ_０Ｓ_３は〔磁石とギャップ部分の磁気抵抗〕である。
また、Ｎ_ｒは〔巻数／ティース〕、Ｉは〔電流〕、ｌ_ｔは〔ティース磁路長〕、ｌ_ｙ１は〔ロータヨーク磁路長〕、ｌ_ｙ２は〔ステータヨーク磁路長〕、μ_０は〔エアギャップとマグネットの透磁率〕、μ_１は〔ティースの透磁率〕、μ_２は〔バックヨークの透磁率〕、Ｓ_０は〔ロータマグネットに対するステータティース先端部の面積〕、Ｓ_１は〔ティースの断面積〕、Ｓ_２は〔バックヨークの磁路断面積〕、ｇは〔ギャップ長さ〕、ｍは〔マグネット厚さ〕、Φは〔磁束〕である。

Here, 2NaI is [magnetomotive force], 2lt / μ ₁ S ₁ is [magnetic resistance of stator core part], 2ly ₁ / μ ₂ S ₂ is [magnetic resistance of rotor core], 4 (g + m) / μ ₀ S ₃ is [Magnetic resistance of magnet and gap portion].
Further, the _{N r} [turns / tooth], I is [current], _{l t} is [teeth magnetic path _{length], l y1} is [yoke magnetic path _{length], l y2} is [stator yoke magnetic path length], mu ₀ the [permeability of the air gap and the magnet], the mu ₁ [permeability of the tooth], the mu ₂ [permeability of the back yoke], S ₀ is [area of stator teeth tip with respect to the rotor magnet], S ₁ is [cross-sectional area of the teeth], S ₂ is [path cross-sectional area of the back yoke], g is [a gap length], m is [magnet thickness], [Phi is [flux].

When μ ₁ >> μ ₀ , μ ₂ >> μ ₀ , μ ₀ = 1, S ₁ ≈S ₂ ≈S ₃ ≈S _a , the expression (1) is expressed as the following expression (2): Is done.

Furthermore, the case of [flux linkage] [psi, the [inductance] was L _a, equation (3) and (4).

ここで、Ｐは〔極数〕，ｎは〔回転速度（ｒｐｍ）〕である。 As a result, synchronous reactance _{X a} is a formula (5) shown below.

Here, P is [number of poles] and n is [rotational speed (rpm)].

ここで、２ＮｒＩは〔起磁力〕、〈２ｌｙ／μ_１Ｓ_１＋ｌｙ_１／μ_１Ｓ_２〉は、〔ステータコア部分の磁気抵抗〕、ｌｙ_２／μ_２Ｓ_３は〔ロータコアの磁気抵抗〕、４（ｇ＋ｍ）／μ_０Ｓ_４は〔磁石とギャップ部分の磁気抵抗〕であり、Ｎ_ｒは〔巻数／ティース〕、Ｉは〔電流〕、ｌ_ｔは〔ティース磁路長〕、ｌ_ｙ１は〔ロータヨーク磁路長〕、ｌ_ｙ２は〔ステータヨーク磁路長〕、μ_０は〔エアギャップとマグネットの透磁率〕、μ_１は〔ティースの透磁率〕、μ_２は〔バックヨークの透磁率〕、Ｓ_０は〔ロータマグネットに対するステータティース先端部の面積〕、Ｓ_１は〔ティースの断面積〕、Ｓ_２は〔バックヨークの磁路断面積〕、ｇは〔ギャップ長さ〕、ｍは〔マグネット厚さ〕、Φは〔磁束〕である。 Similarly, in the case of a radial air gap type electric motor, it is expressed as the following formula (6).

Here, 2NrI is [magnetomotive force], <2ly / μ ₁ S ₁ + ly ₁ / μ ₁ S ₂ > is [magnetic resistance of stator core portion], and ly ₂ / μ ₂ S ₃ is [magnetic resistance of rotor core], 4 (g + m) / μ 0 S 4 are [magnetoresistive the magnet and the gap portion], the _{N r} [turns / tooth], I is [current], the _{l t} [teeth magnetic path _{length], l y1} is [Rotor yoke magnetic path length], _ly2 is [stator yoke magnetic path length], μ ₀ is [air permeability of the air gap and magnet], μ ₁ is [magnetic permeability of the teeth], and μ ₂ is [magnetic permeability of the back yoke]. ], S ₀ is [area of stator teeth tip with respect to the rotor magnet], S ₁ is [cross-sectional area of the teeth], S ₂ is [path cross-sectional area of the back yoke], g is [a gap length], m is [Magnet thickness] and Φ are [magnetic flux].

Here, when μ ₁ >> μ ₀ , μ ₂ >> μ ₀ , μ ₀ = 1, S ₁ ≈S ₂ ≈S ₃ ≈S _a and Expression (6) are expressed by the following Expression (7) It is expressed as

Further, when [number of magnetic flux linkages] is ψ and [inductance] is L _r , Equations (8) and (9) are obtained.

ここで、Ｐは〔極数〕，ｎは〔回転速度（ｒｐｍ）〕である。 As a result, the synchronous reactance _Xr is _expressed by the following equation (10).

Here, P is [number of poles] and n is [rotational speed (rpm)].

ここで、Ｋは〔定数〕、Ｄｒは〔ステータコアの外径〕、Ｄｇは〔ロータの外径〕、ｌは〔ステータコア長さ〕、Ａｃは〔アンペア導体数〕、Ｂは〔磁束密度〕である。 Referring to FIGS. 8A and 8B, the torque Tr of the radial air gap type motor is expressed by the following equation (11).

Here, K is [constant], Dr is [outer diameter of stator core], Dg is [outer diameter of rotor], l is [stator core length], Ac is [number of ampere conductors], and B is [magnetic flux density]. is there.

ここで、Ｋは〔定数〕、Ｄｒは〔ステータコアの外径〕、Ｄｇは〔ロータの外径〕、ｌは〔ステータコア長さ〕、Ａｃは〔アンペア導体数〕、Ｂは〔磁束密度〕である。 On the other hand, referring to FIGS. 9A and 9B, the torque Ta of the axial air gap type brushless DC motor is expressed by the following equation (12).

Here, K is [constant], Dr is [outer diameter of stator core], Dg is [outer diameter of rotor], l is [stator core length], Ac is [number of ampere conductors], and B is [magnetic flux density]. is there.

There is an empirical correlation between Do and Dg and Di and Dg, respectively.
Dg = γ · Dr (γ = 0.6 to 0.8≈0.7)
Di = δ · Do (δ = 0.5 to 0.7≈0.6)
However, Dr = Do

Here, the radial air gap type brushless DC motor shown in FIG. 8 and the axial air gap type brushless DC motor shown in FIG. 9 both have 6 slots and 4 poles, and the same torque (Tr = Ta), the currents are equal, and the number of ampere conductors (Ac) and magnetic flux density (B) are also equal. Further, when the external dimensions are Do = Dr = 100 (mm), the core lamination dimension l = 50 (mm) in the axial direction of the radial motor, and the reactances Xa and Xr are compared from the equations (5) and (10),
Xa / Xr = N _a ² · S _a / 2N _r ² S _r ≈1 / 4
Next, synchronous reactance X _a of the axial air gap type brushless DC motor, as compared with the synchronous reactance X _r of the radial air gap type electric motor, it can be seen to be approximately a fraction of. Thereby, the axial air gap type brushless DC motor can supply a higher output voltage to the power supply side.

  Next, as shown in FIG. 7, the battery capacity (Ah) is usually not constant with respect to the discharge current, and the capacity is reduced as the discharge current increases. In general, an electric motor has the largest current flowing at the time of starting, and reducing the starting current has a great effect in preventing a reduction in battery capacity.

Here, the condition that the torque Ta of the axial air gap type brushless DC motor becomes larger than the torque Tr of the radial air gap type electric motor is expressed by Expression (13).

Therefore, in order to satisfy the condition of Ta / Tr> 1, the equation (14) is obtained.

  According to this, the axial air gap type brushless DC motor is larger for the same current when the dimensional condition such that the motor core outer diameter (Do) / axial core stacking dimension (l)> 2 is satisfied. Torque can be output. As a result, the torque constant (= torque / current) can be increased, the starting current required to obtain the same torque can be reduced, and the starting time can be shortened, thus extending the battery discharge duration. be able to.

As described above, according to the present invention, by using a 2-rotor type axial air gap type brushless DC motor for a drive system of an electric motor,
(1) Since more regenerative energy of the electric motor can be generated, an electric motor drive system that efficiently uses the regenerative energy can be obtained.
(2) Since the starting current of the electric motor can be kept low, the discharge time of the battery can be lengthened, and the driving time of the electric motor by one charge can be extended.

  The electric motor drive system of the present invention is preferably applied to electric assist type bicycles, hybrid cars, trains, etc., but may be used by being incorporated in a part of the system in which the electric motor is incorporated. .

The circuit block diagram of the drive system of the electric motor which concerns on one Embodiment of this invention. The schematic diagram of the axial air gap type brushless DC motor of the drive system of this invention. The schematic diagram which shows the modification of an axial air gap type brushless DC motor. FIG. 4 is an equivalent circuit diagram of the drive system during the regenerative operation of the electric motor. (A) Voltage vector diagram of radial air gap type motor, (b) Voltage vector diagram of axial air gap type brushless DC motor. (A) Explanatory drawing for calculating the synchronous reactance of an axial air gap type brushless DC motor, (b) Explanatory drawing for calculating the synchronous reactance of a radial air gap type motor. The schematic diagram which shows the correlation of the capacity | capacitance of a battery, and discharge current. (A), (b) Explanatory drawing for calculating the torque of a radial air gap type motor. (A), (b) Explanatory drawing for calculating the torque of an axial air gap type brushless DC motor.

Explanation of symbols

DESCRIPTION OF SYMBOLS 100 Power supply circuit 110 Battery 120 Capacitor 200 Multiphase inverter circuit 210 Flywheel diode 220 Switching element 300 Electric motor 310 Stator 320, 320 Rotor 340 Auxiliary stator

Claims (6)

A DC power supply circuit formed by connecting a battery power supply and a capacitor in parallel, a multiphase inverter circuit composed of switching elements in which flywheel diodes are connected in antiparallel, and the power supply circuit connected via the multiphase inverter circuit. A position signal of the rotor of the motor is fed back to the switching element of the multi-phase inverter circuit, and a current flowing through the winding of the motor is switched, thereby creating a rotating magnetic field to the field rotor. In an electric motor drive system that generates torque,
As the electric motor, a two-rotor, two-air-gap axial arrangement in which one stator and two rotors arranged on both sides of the stator are arranged opposite each other with a predetermined gap along the axial direction of the rotor output shaft. Using an air gap type brushless DC motor,
The electric motor operates intermittently in a torque generation mode for generating torque or in a power generation mode for generating electric energy. In the power generation mode, the electric energy generated by the rotation of the rotor is turned off by turning off the switching element. Is fed back to the battery power source and the capacitor.   The axial air gap type brushless DC motor is a two-rotor 4-air gap type axial air gap brushless DC motor in which a second stator is further arranged oppositely with a predetermined gap on the other side surface of each rotor. The motor drive system according to claim 1, wherein:   The motor drive system according to claim 1, wherein the rotor is a magnet made of a sintered rare earth magnet or a bonded rare earth magnet.   4. The electric motor drive system according to claim 1, wherein the stator has fractional slots, and windings are wound around the fractional slots in a concentrated manner.   For the stator core of the stator and / or the back yoke of the rotor, an electromagnetic steel sheet, a cold-rolled steel sheet, or a hot-rolled steel sheet having a low magnetic permeability and a high magnetic flux density lower than 50A400 or 35A60 of JIS-C2552. The motor drive system according to claim 1, wherein:   2. The electric motor drive system according to claim 1, wherein the back yoke of the rotor is formed by laminating one or more disk-shaped cold-rolled steel plates or hot-rolled steel plates in the axial direction.

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