U.S. patent application number 10/142220 was filed with the patent office on 2002-09-19 for permanent magnet rotating electric machine and electrically driven vehicle employing same.
This patent application is currently assigned to Hitachi, Ltd. and Hitachi Car Engineering Co., Ltd., Hitachi, Ltd. and Hitachi Car Engineering Co., Ltd.. Invention is credited to Kawamata, Shouichi, Koizumi, Osamu, Matsunobe, Yutaka, Oda, Keiji, Shibukawa, Suetaro, Tajima, Fumio.
Application Number | 20020130576 10/142220 |
Document ID | / |
Family ID | 14014701 |
Filed Date | 2002-09-19 |
United States Patent
Application |
20020130576 |
Kind Code |
A1 |
Tajima, Fumio ; et
al. |
September 19, 2002 |
Permanent magnet rotating electric machine and electrically driven
vehicle employing same
Abstract
A rotating electric machine comprises a stator having stator
salient poles, three-phases windings wound around said stator
salient poles, a rotor rotatable held inside the said stator, and
permanent magnets inserted into said rotor and positioned opposite
to said stator salient poles, wherein said three-phase windings are
concentratively wound around each of said stator salient poles,
said windings of each phase are wound around at more than one
stator salient pole, and said windings of each phase have a phase
difference of voltage between at least one of the windings and the
other.
Inventors: |
Tajima, Fumio; (Ibaraki-ken,
JP) ; Matsunobe, Yutaka; (Hitachi-shi, JP) ;
Kawamata, Shouichi; (Hitachi-shi, JP) ; Shibukawa,
Suetaro; (Hitachinaka-shi, JP) ; Koizumi, Osamu;
(Ibaraki-ken, JP) ; Oda, Keiji; (Hitachinaka-shi,
JP) |
Correspondence
Address: |
CROWELL & MORING LLP
INTELLECTUAL PROPERTY GROUP
P.O. BOX 14300
WASHINGTON
DC
20044-4300
US
|
Assignee: |
Hitachi, Ltd. and Hitachi Car
Engineering Co., Ltd.
|
Family ID: |
14014701 |
Appl. No.: |
10/142220 |
Filed: |
May 10, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10142220 |
May 10, 2002 |
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09488637 |
Jan 21, 2000 |
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6396183 |
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09488637 |
Jan 21, 2000 |
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08838745 |
Apr 11, 1997 |
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6034460 |
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Current U.S.
Class: |
310/156.56 |
Current CPC
Class: |
H02K 1/246 20130101;
Y02T 10/64 20130101; H02K 1/276 20130101; H02K 21/16 20130101; H02K
29/03 20130101; Y02T 10/641 20130101; H02K 19/103 20130101; H02K
3/28 20130101; B60L 15/025 20130101; B60L 2200/26 20130101; H02K
2213/03 20130101; Y02T 10/643 20130101 |
Class at
Publication: |
310/156.56 |
International
Class: |
H02K 021/12 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 12, 1996 |
JP |
8-091014 |
Claims
What is claimed is:
1. A permanent magnet rotating electric machine comprising: a
stator having stator salient poles, three-phases windings wound
around said stator salient poles; a rotor rotatable held inside the
said stator; and permanent magnets inserted into said rotor and
positioned opposite to said stator salient poles, wherein said
three-phase windings are concentratively wound around each of said
stator salient poles, said windings of each phase are wound around
at more than one stator salient pole, and said windings of each
phase have a phase difference of voltage between at least one of
the windings and the other.
2. A permanent magnet rotating electric machine according to claim
1, wherein M:P=6n:6n.+-.2 is satisfied where M is the number of
said stator salient poles, P is the number of said permanent
magnets, and n is a positive integer.
3. A permanent magnet rotating electric machine according to claim
1, wherein M:P=3n:3n.+-.1 is satisfied where M is the number of
said stator salient poles, P is the number of said permanent
magnets of said rotor, and n is a positive integer.
4. A permanent magnet rotating electric machine according to claim
1, wherein the number of poles of said permanent magnets is eight
or more.
5. A permanent magnet rotating electric machine according to claim
1, wherein a magnetic pole piece area of said rotor is projected
toward said stator.
6. A permanent magnet rotating electric machine comprising: a
stator having stator salient poles, three-phases windings wound
around said stator salient poles; a rotor rotatable held inside the
said stator; and permanent magnets inserted into said rotor and
positioned opposite to said stator salient poles, wherein said
three-phase windings are concentratively wound around each of said
stator salient poles.
7. A permanent magnet rotating electric machine according to claim
6, further comprising a magnetic material having a higher magnetic
impermeability than said permanent magnets disposed between
adjacent ones of said permanent magnets.
8. An electrically driven vehicle comprising: a permanent magnet
rotating electric machine being coupled to drive wheels comprising:
a stator having stator salient poles, three-phases windings wound
around said stator salient poles; a rotor rotatable held inside the
said stator; and permanent magnets inserted into said rotor and
positioned opposite to said stator salient poles, and control means
for supplying a voltage to said three-phase windings, wherein said
three-phase windings are concentratively wound around each of said
stator salient poles, said windings of each phase are wound around
at more than one stator salient pole, and said control means
supplies voltage which has a phase difference between at least one
of the windings and the other among each phase of three-phase.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates generally to a permanent
magnet rotating electric machine and an electrically driven vehicle
employing same.
[0003] 2. Description of Related Art
[0004] Motors used in electrically driven vehicles, in particular,
driving electric cars must ensure a sufficient running distance
with a limited battery capacity, so that they are desired to be
small, light-weight, and highly efficient.
[0005] For a motor to be small and light weight, it is required to
be suitable for high speed rotation. In this regard, permanent
magnet motors are advantageous over direct-current motors and
induction motor.
[0006] Permanent magnet rotors are classified into a surface magnet
rotor which has permanent magnets positioned along the outer
periphery of the rotor and a so-called internal magnet rotor which
has a permanent magnet holder within a core made of silicon steel
or the like having a higher magnetic permeability than permanent
magnets.
[0007] The surface magnet rotor is advantageous in ease of control,
less influences by reactive magnetic flux of a stator winding, low
noise, and so on. However, the surface magnet rotor also has
several disadvantages such as requirement of reinforced magnets for
high speed rotation, a narrow speed control range due to
difficulties in field weakening control, a low efficiency in high
speed and low load operations, and so on.
[0008] The internal magnet rotor, in turn, has advantages such as
the capability of high speed rotation by field weakening control
using magnetic pole pieces positioned along the outer periphery of
magnets, the capability of highly efficient rotation in high speed
and low load operations, utilization of reluctance torque, and so
on.
[0009] Prior art internal magnet rotors are described, for example,
in JP-A-5-219669, FIG. 5 of JP-A-7-39091.
[0010] Within large-size permanent magnet motors used in electric
vehicles and so on, those having an internal permanent magnet rotor
employ a distributed winding stator for their stator structure.
[0011] However, permanent magnet motors described in the prior art
have a disadvantage that pulsating torque based on high frequency
components of permanent magnets or auxiliary magnet poles is
produced. Also, cogging torque is produced by influence of
roughness and fineness of magnetic flux of stator salient poles and
roughness and fineness of magnetic flux of permanent magnets, and
smooth rotation of permanent magnet motors cannot be obtained.
Further, since the distributed winding stator has elongated winding
ends, this causes a limitation to reduction in size and weight of
rotating electric machines employing a distributed winding
stator.
SUMMARY OF THE INVENTION
[0012] It is an object of the present invention to provide a
permanent magnet rotating electric machine which has small
pulsating torque and cogging torque, and can be obtained smooth
rotation thereof.
[0013] It is another object of the present invention to provide a
permanent magnet rotating electric machine having shortened winding
ends, and having stator construction being capable to be small,
light-weight.
[0014] To achieve the above object, according to a first aspect,
the present invention provides a permanent magnet rotating electric
machine comprising a stator having stator salient poles,
three-phases windings wound around said stator salient poles, a
rotor rotatably held inside the said stator, and permanent magnets
inserted into said rotor and positioned opposite to said stator
salient poles, wherein said three-phase windings are
concentratively wound around each of said stator salient poles,
said windings of each phase are wound around at more than one
stator salient pole, and said windings of each phase have a phase
difference of voltage between at least one of the windings and the
other.
[0015] Preferably, the permanent magnet rotating electric machine
satisfies M:P=6n:6n.+-.2, where M is the number of the stator
salient poles, P is the number of the permanent magnets, and n is a
positive integer.
[0016] Preferably, the permanent magnet rotating electric machine
satisfies M:P=3n:3n.+-.1, where M is the number of the stator
salient poles, P is the number of the permanent magnets of the
rotor, and n is a positive integer.
[0017] Preferably, in the permanent magnet rotating electric
machine, the number of poles of the permanent magnets is eight or
more.
[0018] Preferably, in the permanent magnet rotating electric
machine, a magnetic pole piece area of the rotor is projected
toward the stator.
[0019] Preferably, in the permanent magnet rotating electric
machine, a magnetic material having a higher magnetic
impermeability than the permanent magnets is disposed between
adjacent ones of the permanent magnets.
[0020] To achieve the above object, according to a second aspect,
the present invention provides a permanent magnet rotating electric
machine comprising a stator having stator salient poles,
three-phases windings wound around said stator salient poles, a
rotor rotatable held inside the said stator, and permanent magnets
inserted into said rotor and positioned opposite to said stator
salient poles, wherein said three-phase windings are
concentratively wound around each of said stator salient poles.
[0021] To achieve the above object, according to an aspect, the
present invention provides an electrically driven vehicle
comprising a permanent magnet rotating electric machine being
coupled to drive wheels comprising a stator having stator salient
poles, three-phases windings wound around said stator salient
poles, a rotor rotatable held inside the said stator, and permanent
magnets inserted into said rotor and positioned opposite to said
stator salient poles, and control means for supplying a voltage to
said three-phase windings, wherein said three-phase windings are
concentratively wound around each of said stator salient poles,
said windings of each phase are wound around at more than one
stator salient pole, and said control means supplies voltage which
has a phase difference between at least one of the windings and the
other among each phase of three-phase.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] FIG. 1 is a partial cross-sectional view of a permanent
magnet rotating electric machine according to a first embodiment of
the present invention, viewed from the front side thereof;
[0023] FIG. 2 is a cross-sectional view taken along the section
line A-A of FIG. 1, illustrating the permanent magnet rotating
electric machine according to the first embodiment of the present
invention;
[0024] FIG. 3 is a circuit diagram illustrating a control circuit
for the permanent magnet rotating electric machine according to the
first embodiment of the present invention;
[0025] FIGS. 4A-4C are explanatory diagrams illustrating torque
generated by the permanent magnet rotating electric machine
according to the first embodiment of the present invention;
[0026] FIGS. 5A-5C are diagrams for explaining the principles of
the permanent magnet rotating electric machine according to the
first embodiment of the present invention;
[0027] FIG. 6 is a cross-sectional view illustrating a permanent
magnet rotating electric machine according to a second embodiment
of the present invention;
[0028] FIG. 7 is a cross-sectional view illustrating a permanent
magnet rotating electric machine according to a third embodiment of
the present invention;
[0029] FIG. 8 is a cross-sectional view illustrating a permanent
magnet rotating electric machine according to a fourth embodiment
of the present invention; and
[0030] FIG. 9 is a block diagram illustrating an electric car
equipped with a permanent magnet rotating electric machine
according to a fifth embodiment of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0031] Permanent magnet rotating electric machines according to a
first embodiment of the present invention will hereinafter be
described with reference to FIGS. 1-5C.
[0032] FIG. 1 is a partial cross-sectional view of a permanent
magnet rotating electric machine according to a first embodiment of
the present invention, viewed from the front side thereof.
[0033] Referring specifically to FIG. 1, a stator 20 of a rotating
electric machine 10 comprises a stator core 22, multi-phase stator
windings 24 wound around the stator core 22, and a housing 26 for
securely holding the stator core 22 on the inner peripheral surface
thereof. A rotor 30 comprises a rotor core 32, permanent magnets 36
inserted into permanent magnet inserting holes 34 formed in the
rotor core 32, and a shaft 38. The shaft 38 is rotatable held by
bearings 42, 44. The bearings 42, 44 are supported by end brackets
46, 48, respectively, which in turn is secured to both ends of the
housing 26.
[0034] A magnetic pole position detector PS for detecting the
position of the permanent magnets 36 of the rotor 30 and an encoder
E for detecting the position of the rotor 30 are disposed on a side
surface of the rotor 30. The operation of the rotating electric
machine 10 is controlled by a control unit, later described with
reference to FIG. 3, in response to a signal of the magnetic pole
position detector PS and an output signal of the encoder E.
[0035] FIG. 2 is a cross-sectional view taken along the section
line A-A of FIG. 1, wherein however, the illustration of the
housing 26 is omitted.
[0036] Referring specifically to FIG. 2, the rotating electric
machine 10 comprises the stator 20 and the rotor 30. The rotor 20
comprises the stator core 22 and the stator windings 24. The stator
core 22 comprises an annular stator yoke 22A and stator salient
poles 22B, and the stator windings 24 are concentratively wound
around the stator salient poles 22B. The respective windings 24 are
configured not to share a magnetic path on gap surfaces. By
employing a stator structure in which the stator windings are
implemented by concentrated windings, the length of end coil
portions can be reduced, and consequently the physical size of the
rotating electric machine can also be reduced. The end coil
portions refer to portions of the stator windings 24 projecting
from the stator core 24 to the left and right directions in FIG. 1.
Since these end coil portions can be reduced, the entire rotating
electric machine can be reduced in length, thus resulting in a
smaller size of the rotating electric machine.
[0037] The U-phase of the stator windings 24 is connected to U1+,
U1-, U2+, U2-, respectively; the V-phase is connected to V1+, V1-,
V2+, V2-, respectively; and W-phase is connected to W1+, W1-, W2+,
W2-, respectively.
[0038] The rotor 30 comprises a rotor core 32 formed of a plurality
of laminated plates made of a highly magnetically permeable
material, for example, silicon steel; four permanent magnets 36
inserted into four permanent magnet inserting holes 34 formed in
the rotor core 32; and a shaft 38. Ten permanent magnets 36 are
positioned in the circumferential direction of the rotor core 32 at
equal intervals such that their polarities are in the opposite
directions from each other.
[0039] The rotor core 32 is formed with the permanent magnet
inserting holes 34 and a hole for passing the shaft 38
therethrough, both formed by punch press. Thus, the rotor 30 is
composed of the rotor core 32 made of laminated silicon steel
plates and formed with the punch-pressed permanent magnet inserting
holes 34 and hole for passing the shaft 38 therethrough, the
permanent magnets 36 inserted into the holes 34, and the shaft 38
extending through the hole.
[0040] The rotor core 32 may be divided in the radial direction
into an inner yoke area 32A and an outer peripheral area 32B. The
outer peripheral area 32B of the rotor core 32 may be further
divided in the circumferential direction into an auxiliary magnetic
pole area 32B1 and a magnetic pole piece area 32B2. The auxiliary
magnetic pole area 32B1, which is an area sandwiched by adjacent
permanent magnet inserting holes 34, functions to prohibit magnetic
circuits of the magnets from passing therethrough and to allow
magnetic flux to be directly generated in the stator by a
magnetomotive force of the stator. The magnetic pole piece area
32B2 is an area positioned outside the permanent magnets 36 within
the outer peripheral area 32B of the rotor core 32, in which
magnetic flux B.phi. from the permanent magnets 36 flows through
gaps between the permanent magnets 36 and the stator 20 into the
stator 20 to form magnetic circuits.
[0041] The permanent magnets 36 can be accommodated in the
permanent magnet inserting holes 34 which are bordered by the
auxiliary magnetic pole area 32B1 in the circumferential direction
and bordered by the magnetic pole piece area 32B2 around the outer
periphery, thus providing a motor suitable for high speed
rotation.
[0042] The concentrated winding stator is generally used in
reluctance motors and small brash-less motors. In this case, the
reluctance motor includes a rotor only having auxiliary magnetic
poles, while the brash-less motor has permanent magnets directly
disposed on the outer surface of a rotor. Thus, the reluctance
motor generate small torque including large pulsating
components.
[0043] With the surface magnetic rotor, on the other hand, it is
relatively difficult to apply a field weakening control thereto.
Accordingly the surface magnetic rotor is likely to cause a loss
due to an eddy current generated in surface magnets to reduce the
efficiency.
[0044] In contrast, a structural combination of a rotor employing
internal permanent magnets and a concentrated winding stator allows
for utilization of torque generated by flux of the permanent
magnets as well as torque generated by reluctance components of the
auxiliary magnetic poles, thereby providing a higher efficiency. In
addition, since the field weakening can be achieved by the effect
of the auxiliary magnetic poles, later described, an operating
region can be significantly expanded, particularly, in a high speed
region.
[0045] Further, since the magnetic pole piece area is made of a
magnetic material, pulsating flux from the stator salient poles can
be mitigated. Also, since the laminated steel core is employed, the
rotating electric machine of the first embodiment is free from eddy
current losses.
[0046] It is assumed in the example illustrated in FIG. 2 that the
rotating electric machine is a three-phase motor which comprises
the permanent magnet rotor 36 with the number of poles being ten,
and the stator with the number of magnetic poles being twelve. When
the number of stator salient poles is represented by M and the
number of the poles of the rotor magnets by P, a structure
satisfying the following relationship:
M:P=6n:6n.+-.2
[0047] (where n is a positive integer) can realize reduced torque
pulsations and an increased utilization ratio of windings (winding
coefficient). It is therefore appreciated that the embodiment
illustrated in FIG. 2 can provide a highly efficient, small and
light-weight rotating electric machine.
[0048] It goes without saying that while the foregoing description
has been made in connection with an example of a motor, the first
embodiment can be similarly applied to a generator.
[0049] Next, a control unit for controlling the permanent magnet
rotating electric machine according to the first embodiment will be
described with reference to FIG. 3.
[0050] FIG. 3 is a circuit diagram of a control circuit for the
permanent magnet rotating electric machine according to the first
embodiment.
[0051] The stator windings 24 of the rotating electric machine 24
are powered from a direct current power source 80 through an
invertor 82. A speed control circuit (ASR) 84 calculates a speed
difference .omega.e from a speed instruction .omega.s and an actual
speed .omega.f derived from positional information .theta. from the
encoder E through an F/V convertor 86, and outputs a torque
instruction in accordance with a PI control scheme (P represents a
proportional term, and I an integral term) or the like, i.e., a
current instruction Is and a rotating angle .theta.1 for the rotor
30.
[0052] A phase shift circuit 88 shifts the phase of pulses from the
encoder E, i.e., the positional information .theta. from the
encoder E in accordance with the rotating angle .theta.1 instructed
from the speed control circuit (ASR) 84. A sine wave/cosine wave
generator 90 generates a sine wave output by shifting the phase of
an induced voltage of each of the stator windings 24 (three phases
in this embodiment) based on the position detector PS for detecting
the positions of the magnetic poles of the permanent magnets of the
rotor 30 and the positional information .theta. on the rotor 30
having its phase shifted by the phase shift circuit 88. The amount
of phase shift may be zero.
[0053] A two-phase/three-phase convertor circuit 92 outputs current
instructions Isa, Isb, Isb to the respective phases in accordance
with the current instruction Is from the speed control circuit
(ASR) 84 and an output of the sin wave/cosine wave generator 90.
The respective phases individually have current control systems
(ACR) 94A, 94B, 94C which control respective phase currents by
providing the invertor 82 with signals in accordance with the
current instructions Isa, Isb, Isc and current detecting signals
Ifa, Ifb, Ifc. In this event, a combined current of the respective
phase currents is always formed at a position perpendicular to the
field flux or at a phase shifted position, so that characteristics
equivalent to those of a direct current motor can be achieved
without commutator.
[0054] When the rotating electric machine of the first embodiment
is applied to an electric car, the control unit has a torque
control system for directly controlling the torque instead of the
speed control circuit 84. In other words, the speed control circuit
84 is replaced with a torque control circuit. The torque control
circuit receives torque Ts as an input signal, calculates torque Te
from the torque Ts and actual torque Tf detected by a torque
detector, and outputs a torque instruction in accordance with a PI
control scheme (P represents a proportional term, and I an integral
term) or the like, i.e., a current instruction Is and a rotating
angle .theta.1 for the rotor 30.
[0055] In a permanent magnet rotating electric machine, since
torque is directly proportional to a current, a current control
system may be provided instead of the speed control circuit 84.
[0056] The connection of the stator windings 24 is made in
accordance with a three-phase stator winding scheme. More
specifically, U1+, U1-, U2+, U2- are connected in the illustrated
order in the U-phase; V1+, V1-, V2+, V2- are connected in the
illustrated order in the V-phase; and W1+, W1-, W2+, W2- are
connected in the illustrated order in the W-phase. Here, between
the windings constituting the respective phases, for example,
between U1+and U2-, and between U1- and U2+ in the U-phase; between
V1+and V2-, and between V1- and V2+ in the V-phase; and between W1+
and W2-, and between W1- and W2+ in the W-phase, there is a phase
difference of 30 degrees in electrical angle. Specifically
explaining with reference to FIG. 2, for example, an angle .theta.1
between the stator salient poles U1+ and U2- is 30 degrees, while
adjacent permanent magnets 36 of the rotor 30 are angularly spaced
by angles .theta.2. In this way, within the stator salient poles
which are wound by the stator windings connected to the same phase,
at least one stator salient pole has a phase shifted with respect
to the associated permanent magnet. Take, as an example, a stator
salient pole around which the winding U1- is wound and a stator
salient pole around which the winding U2+ is wound. Assuming that
U1- is in phase with the permanent magnet 36A, U1- is shifted from
the permanent magnet 36B by an angular distance of 30 degrees. This
contributes to a reduction in pulsating pulse which may cause a
problem in the concentrated winding stator. The reason for this
reduction will be described later with reference to FIG. 4.
[0057] A concentrated winding should be constructed such that
respective windings do not overlap on the gap surface as
illustrated in FIG. 1. This eliminates interference between the
respective windings, and a small, light-weight and simple rotating
electric machine can be realized.
[0058] Also, by selecting adjacent windings to be connected to the
same phase as illustrated, the connection is facilitated.
Specifically, in the U-phase, U1+ and U2- are adjacent, and U1- and
U2+ are adjacent. In the V-phase, V1+ and V2- are adjacent, and V1-
and V2+ are adjacent. Similarly, in the W-phase, W1+ and W2- are
adjacent, and W1- and W2+ are adjacent, thus facilitating the
connection of these windings.
[0059] Next, the reason for the reduction in torque pulsation will
be explained with reference to FIGS. 4A-4C.
[0060] FIGS. 4A-4C show the torque generated by the permanent
magnet rotating electric machine according to the first embodiment
of the present invention.
[0061] FIG. 4A represents torque which is generated when the
respective stator windings of U1+, U1-, V1+, V1-, W1+, W1- are
applied with a sine wave current based on a signal from the sine
wave/cosine wave generator circuit 90 illustrated in FIG. 3. While
uniform torque would be generated if no harmonics were included,
the inclusion of harmonic components caused by the permanent
magnets, harmonic components due to the auxiliary magnetic poles,
and so on cause torque pulsation at a period of 60 degrees in
electrical angle, as illustrated.
[0062] FIG. 4B represents torque which is generated when the
respective stator windings of U2+, U2-, V2+, V2-, W2+, W2- are
applied with a sine wave current. Since the represented torque
includes harmonic components caused by the permanent magnets,
harmonic components due to the auxiliary magnetic poles, and so on,
as is the case of the torque represented in FIG. 4A, torque
pulsations are generated at a period of 60 degrees in electrical
angle.
[0063] It should be noted herein that since there is a phase
difference of 30 degrees in electrical angle between the stator
salient poles around which U1+, U1-, V1+, V1-, W1+, W1- of the
stator windings 24 are wound and the stator salient poles around
which U2+, U2-, V2+, V2-, W2+, W2- of the stator windings 24 are
wound, the torque pulsations generated thereby are in opposite
phase to each other.
[0064] Thus, a combination of torque of FIGS. 4A and 4B exhibits
reduced pulsations as shown in FIG. 4C.
[0065] Referring back to FIG. 2, in the example in which the ratio
of the number of permanent magnet M to the number of stator salient
poles P is determined to be 10:12, the cogging torque of the
permanent magnet rotating electric machine exhibits a number of
pulsations per rotation equal to the least common multiple of the
number of permanent magnets and the number of stator salient poles
, i.e., 60 per rotation in this example. Generally, the cogging
torque is smaller as the number of pulsations per rotation is
larger.
[0066] In a conventionally used motor having a general surface
magnet rotor and a concentrated winding stator, the ratio of the
number of permanent magnets M to the number of stator salient poles
P is typically 2:3. This ratio corresponds to 10:15 when the number
of permanent magnets M is changed from two to ten which is the
number of permanent magnets M in the example illustrated in FIG. 2.
In this case, the number of pulsations per rotation of the cogging
torque is calculated to be 30 which is the least common multiple of
10 and 15. It will be understood from this discussion that the
structure of the first embodiment can reduce the cogging torque
more than conventional motor of the same type.
[0067] In addition, pulsating torque possibly occurring when a
current is conducted can be reduced by the principles shown in FIG.
4.
[0068] Next, the operation principles of the field weakening
control for the permanent magnet rotating electric machine
according to the first embodiment will be explained with reference
to FIGS. 5A-5C.
[0069] Torque T generated by a permanent magnet rotating electric
machine is generally expressed by the following equation:
T={E0.multidot.Iq+(Xq-Xd).multidot.Id.multidot.Iq}/w
[0070] where E0 is an induced voltage; Xq is reactance on q-axis;
Xd is reactance on d-axis; Id is a current on d-axis; Iq is a
current on q-axis; and w is an angular rotational speed.
[0071] Referring first to FIG. 5A, a permanent magnet 36 is
positioned on d-axis, and an auxiliary magnetic pole area 32B1
having a higher magnetic permeability than the permanent magnet 36
is positioned on q-axis. In this arrangement, respective vectors
are represented in FIG. 5A. A current Im, which is a combination of
the d-axis current Id and the q-axis current Iq, is controlled in
the illustrated direction by the current instructions Isa, Isb, Isc
generated by the control circuit illustrated in FIG. 3,
calculations of output positions of the magnetic pole position
detector PS and the encoder E of the rotating electric machine, and
so on.
[0072] In the foregoing equation, the first term expresses a
component of torque generated by the permanent magnet, and the
second term expresses a reluctance component generated by the
auxiliary magnetic pole area 32B1.
[0073] A rotating electric machine for electric car must be
controlled so as to maximize the torque/current particularly during
a low speed operation. FIG. 5A shows a vector diagram when the
rotating electric machine is controlled to generate a maximum
torque current. In this event, the rotating electric machine is
controlled to apply an increased magnetomotive force to the
auxiliary magnetic pole 32B1, thus taking advantage of the torque
generated by the permanent magnet, expressed by the first term, as
well as the reluctance torque generated by the auxiliary magnetic
pole 32B1, expressed by the second term.
[0074] In a high speed region, on the other hand, the torque may be
small. Rather, the Id component is increased to cancel the induced
voltage E0 of the permanent magnet by Xd.multidot.Id in order to
weaken the flux of the permanent magnet 36, whereby the rotating
electric machine can be rotated up to a high speed region. FIG. 5B
shows a vector diagram during a high speed operation.
[0075] The currents Id, Iq are controlled by the phase shift
circuit 88 of the control circuit illustrated in FIG. 3.
[0076] Referring next to FIG. 5C, a broken line T2 represents
torque generated by a conventional surface magnet rotating electric
machine. It can be seen from the broken line T2 that the torque is
decreased in a high speed region. A solid line T1, in turn,
represents the relationship between the torque and the speed of the
permanent magnet rotating electric machine according to the first
embodiment, provided by the control described above. Since the
current can more easily pass through as compared with the
conventional surface magnet rotating electric machine, the
permanent magnet rotating electric machine of the first embodiment
can be operated in a higher speed region.
[0077] According to the first embodiment, since a concentrated
winding stator is employed, the end coil portions of the stator can
be reduced, so that a smaller rotating electric machine can be
provided.
[0078] Also, since the stator salient poles, having wound
therearound the stator windings connected to the same phase,
include at least one salient pole which has a different phase with
respect to the associated permanent magnet, this configuration
reduces the pulsating torque which may cause a problem in the
concentrated winding stator.
[0079] Further, since the permanent magnet rotor is provided with
auxiliary magnetic poles, a structure suitable for field weakening
control is realized, thereby providing a rotating electric machine
appropriate to high speed rotation.
[0080] Furthermore, since an auxiliary magnetic pole area made of a
magnetic material having a higher magnetic permeability than the
permanent magnets is positioned between the permanent magnets,
increased torque can be generated.
[0081] Moreover, the permanent magnets are surrounded by silicon
steel plates, so that a structure suitable for high speed rotation
can be provided.
[0082] Next, a permanent magnet rotating electric machine according
to another embodiment of the present invention will be described
with reference to FIG. 6.
[0083] FIG. 6 is a cross-sectional view illustrating the permanent
magnet rotating electric machine according to a second embodiment
of the present invention.
[0084] The second embodiment is characterized by a three-phase
motor structure which comprises a permanent magnet rotor 36 having
ten poles (P=10) and a stator having nine magnetic poles (M=9).
Thus, when the ratio of the number of stator salient poles M to the
number of magnetic poles of the stator magnet P (M:P) is
3n:3n.+-.1, reduced torque pulsations and an increased utilization
ratio of windings (winding coefficient) can be realized, so that a
highly efficient, small, and light-weight rotating electric machine
can be provided.
[0085] Referring specifically to FIG. 6, the rotating electric
machine 10 comprises a stator 20 and a rotor 30. The rotor 20
comprises a stator core 22 and a stator windings 24. The stator
core 22 comprises an annular stator yoke 22A and stator salient
poles 22B, and the stator windings 24 are concentratively wound
around the stator salient poles 22B. The respective windings 24 are
configured not to share a magnetic path on gap surfaces. By
employing a stator structure in which the stator windings are
implemented by concentrated windings, the length of end coil
portions can be reduced, and consequently the physical size of the
rotating electric machine can also be reduced.
[0086] The U-phase of the stator windings 24 is connected to U1+,
U1-, U2+, U2-, respectively; the V-phase is connected to V1+, V1-,
V2+, V2-, respectively; and W-phase is connected to W1+, W1-, W2+,
W2-, respectively.
[0087] The rotor 30 comprises a rotor core 34 formed of a plurality
of laminated plates made of a highly magnetic permeable material,
for example, silicon steel; four permanent magnets 36 inserted into
four permanent magnet inserting holes 34 formed in the rotor cores
32; and a shaft 38. Ten permanent magnets 36 are positioned in the
circumferential direction of the rotor core 32 at equal intervals
such that their polarities are in the opposite directions from each
other.
[0088] The rotor core 32 is formed with the permanent magnet
inserting holes 34 and a hole for passing the shaft 38
therethrough, both formed by punch press. Thus, the rotor 30 is
composed of the rotor core 32 made of laminated silicon steel
plates and formed with the punch-pressed permanent magnet inserting
holes 34 and hole for passing the shaft 38 therethrough, the
permanent magnets 36 inserted into the holes 34, and the shaft 38
extending through the hole.
[0089] The rotor core 32 may be divided in the radial direction
into an inner yoke area 32A and an outer peripheral area 32B. The
outer peripheral area 32B of the rotor core 32 may be further
divided in the circumferential direction into an auxiliary magnetic
pole area 32B1 and a magnetic pole piece area 32B2. The auxiliary
magnetic pole area 32B1, which is an area sandwiched by adjacent
permanent, magnet inserting holes 34, functions to prohibit
magnetic circuits of the magnets from passing therethrough and to
allow magnetic flux to be directly generated in the stator by a
magnetomotive force of the stator. The magnetic pole piece area
32B2 is an area positioned outside the permanent magnets 36 within
the outer peripheral area 32B of the rotor core 32, in which
magnetic flux B.phi. from the permanent magnets 36 flows through
gaps between the permanent magnets 36 and the stator 20 into the
stator 20 to form a magnetic circuit.
[0090] The permanent magnets 36 can be accommodated in the
permanent magnet inserting holes 34 which are bordered by the
auxiliary magnetic pole area 32B1 in the circumferential direction
and bordered by the magnetic pole piece area 32B2 around the outer
periphery, thus providing a rotating electric machine suitable for
high speed rotation.
[0091] Further, since the magnetic pole piece area is made of a
magnetic material, pulsating flux from the stator salient poles can
be mitigated. Also, since the laminated steel core is employed, the
rotating electric machine of the second embodiment is free from
eddy current losses.
[0092] It is assumed in the example illustrated in FIG. 6 that the
rotating electric machine is a three-phase motor which comprises
the permanent magnet rotor 36 with the number of poles P being ten,
and the stator with the number of magnetic poles being nine. When
the number of stator salient poles is represented by M and the
number of the poles of the rotor magnets by P, a structure
satisfying the following relationship:
M:P=3n:3n.+-.1
[0093] (where n is a positive integer) can realize reduced torque
pulsations and an increased utilization ratio of windings (winding
coefficient), so that a highly efficient, a small and light-weight
rotating electric machine can be provided.
[0094] The connection of the stator windings 24 is made in
accordance with a three-phase stator winding scheme. More
specifically, U1+, U1-, U2+ are connected in the illustrated order
in the U-phase; V1+, V1-, V2+ are connected in the illustrated
order in the V-phase; and W1+, W1-, W2+ are connected in the
illustrated order in the W-phase. Here, the windings constituting
the respective phases, for example, U1+ and U1-, and U1- and U2+ in
the U-phase; V1+ and V1-, V1- and V2+ in the V-phase; and W1+ and
W1-, W1- and W2+ in the W-phase, have a phase difference of 20
degrees in electrical angle. In this way, the stator salient poles
having wound therearound the stator windings connected to the same
phase, increase at least one stator salient pole which has a
different phase with respect to the associated permanent magnet.
Take, as an example, a stator salient pole having wound therearound
the winding U1- and a stator salient pole having wound therearound
the winding U2+. Assuming that U1- is in phase with the permanent
magnet 36A, U1- is shifted from the permanent magnet 36B by an
angular distance of 30 degrees. This contributes to a reduction in
pulsating torque which may cause a problem in the concentrated
winding stator.
[0095] An electrical angle between adjacent stator salient poles
22B is calculated to be 200 degrees (180.times.(10/9)=200), and 20
degrees when taking into account the phase difference. The cogging
torque of the permanent magnet rotating electric machine exhibits a
number of pulsations per rotation equal to the least common
multiple of the number of permanent magnets and the number of
stator salient poles , i.e., 90 per rotation in this example.
[0096] In the example illustrated in FIG. 2 in which the ratio of
the number of permanent magnets M to the number of stator salient
poles P is 10:12, the cogging torque of the permanent magnet
rotating electric machine exhibits pulsations of 60 per rotation.
It is understood from this discussion that the second embodiment
can further reduce the cogging torque.
[0097] It goes without saying that while the foregoing description
has been made in connection with an example of a motor, the second
embodiment can be similarly applied to a generator.
[0098] According to the second embodiment, since a concentrated
winding stator is employed, the end coil portions of the stator can
be reduced in length, so that a smaller rotating electric machine
can be provided.
[0099] Also, since the stator salient poles, having wound
therearound stator windings connected to the same phase, include at
least one salient pole which has a different phase with respect to
the associated permanent magnet, this configuration reduces the
pulsating torque which may cause a problem in the concentrated
winding stator.
[0100] In addition, the cogging torque can be further reduced.
[0101] Further, since the permanent magnet rotor is provided with
auxiliary magnetic poles, a structure suitable for field weakening
control is realized, thereby providing a rotating electric machine
appropriate to high speed rotations.
[0102] Furthermore, since an auxiliary magnetic pole area made of a
magnetic material having a higher magnetic permeability than the
permanent magnets is positioned between the permanent magnets,
increased torque can be generated.
[0103] Moreover, the permanent magnets are surrounded by silicon
steel plates, so that a rotating electric machine suitable for high
speed rotations can be provided.
[0104] Next, a permanent magnet rotating electric machine according
to a third embodiment of the present invention will be described
with reference to FIG. 7.
[0105] FIG. 7 is a cross-sectional view illustrating the permanent
magnet rotating electric machine according to the third embodiment
of the present invention.
[0106] The third embodiment is characterized by a three-phase motor
structure which comprises a permanent magnet rotor 36 having twelve
poles (P=12) and a stator having eight magnetic poles (M=8). Since
this structure can increase the utilization ratio of windings
(winding coefficient), a highly efficient, small, and light-weight
rotating electric machine can be provided.
[0107] Referring specifically to FIG. 7, the rotating electric
machine 10 comprises a stator 20 and a rotor 30. The rotor 20
comprises a stator core 22 and a stator windings 24. The stator
core 22 comprises an annular stator yoke 22A and stator salient
poles 22B, and the stator windings 24 are concentratively wound
around the stator salient poles 22B. The respective windings 24 are
configured not to share a magnetic path on gap surfaces. By
employing a stator structure in which the stator windings are
implemented by concentrated windings, end coil portions can be
reduced in length, and consequently the physical size of the
rotating electric machine can also be reduced.
[0108] The U-phase of the stator windings 24 is connected to U1,
U2, U3, U4, respectively; the V-phase is connected to V1, V2, V3,
V4, respectively; and W-phase is connected to W1, W2, W3, W4,
respectively.
[0109] The rotor 30 comprises a rotor core 34 formed of a plurality
of laminated plates made of a highly magnetic permeable material,
for example, silicon steel; four permanent magnets 36 inserted into
four permanent magnet inserting holes 34 formed in the rotor cores
32; and a shaft 38. Ten permanent magnets 36 are positioned in the
circumferential direction of the rotor core 32 at equal intervals
such that their polarities are in the opposite directions from each
other.
[0110] The rotor core 32 is formed with the permanent magnet
inserting holes 34 and a hole for passing the shaft 38
therethrough, both formed by punch press. Thus, the rotor 30 is
composed of the rotor core 32 made of laminated silicon steel
plates and formed with the punch-pressed permanent magnet inserting
holes 34 and hole for passing the shaft 38 therethrough, the
permanent magnets 36 inserted into the holes 34, and the shaft 38
extending through the hole.
[0111] The rotor core 32 may be divided in the radial direction
into an inner yoke area 32A and an outer peripheral area 32B. The
outer peripheral area 32B of the rotor core 32 may be further
divided in the circumferential direction into an auxiliary magnetic
pole area 32B1 and a magnetic pole piece area 32B2. The auxiliary
magnetic pole area 32B1, which is an area sandwiched by adjacent
permanent magnet inserting holes 34, functions to prohibit magnetic
circuits of the magnets from passing therethrough and to allow
magnetic flux to be directly generated in the stator by a
magnetomotive force of the stator. The magnetic pole piece area
32B2 is an area positioned outside the permanent magnets 36 within
the outer peripheral area 32B of the rotor core 32, in which
magnetic flux B.phi. from the permanent magnets 36 flows through
gaps between the permanent magnets 36 and the stator 20 into the
stator 20 to form a magnetic circuit.
[0112] The permanent magnets 36 can be accommodated in the
permanent magnet inserting holes 34 which are bordered by the
auxiliary magnetic pole area 32B1 in the circumferential direction
and bordered by the magnetic pole piece area 32B2 around the outer
periphery, thus providing a rotating electric machine suitable for
high speed rotation.
[0113] Further, since the magnetic pole piece area is made of a
magnetic material, pulsating flux from the stator salient poles can
be mitigated. Also, since the laminated steel core is employed, the
rotating electric machine of the second embodiment is free from
eddy current losses.
[0114] It is assumed in the example illustrated in FIG. 7 that the
rotating electric machine is a three-phase motor which comprises
the permanent magnet rotor 36 with the number of poles P being
twelve, and the stator with the number of magnetic poles being
eight. Since such a structure achieves an increased utilization
ratio of the windings (winding coefficient), a highly efficient,
small and light-weight rotating electric machine can be
provided.
[0115] The connection of the stator windings 24 is made in
accordance with a three-phase stator winding scheme. More
specifically, U1, U2, U3, U4 are connected in the illustrated order
in the U-phase; V1, V2, V3, V4 are connected in the illustrated
order in the V-phase; and W1, W2, W3, W4 are connected in the
illustrated order in the W-phase. The windings forming parts of the
U-phase, V-phase, W-phase have a phase difference of 60 degrees
between each other.
[0116] In the third embodiment, the stator salient poles having
wound therearound the stator windings connected to the same phase
are in phase with the associated permanent magnets, so that a
reduction in torque pulsation is not expected. However, since the
salient poles in phase with the permanent magnets are positioned in
a symmetric configuration, a well balanced structure can be
provided. More specifically explaining with reference to the
U-phase, the respective salient poles U1, U2, U3, U4 are positioned
symmetrically about the shaft 38.
[0117] It goes without saying that while the foregoing description
has been made in connection with an example of a motor, the third
embodiment can be similarly applied to a generator.
[0118] According to the third embodiment, since a concentrated
winding stator is employed, the end coil portions of the stator can
be reduced in length, so that a smaller rotating electric machine
can be provided.
[0119] Also, since the permanent magnet rotor is provided with
auxiliary magnetic poles, a structure suitable for field weakening
control is realized, thereby providing a rotating electric machine
appropriate to high speed rotations.
[0120] Further, since an auxiliary magnetic pole area made of a
magnetic material having a higher magnetic permeability than the
permanent magnets is positioned between the permanent magnets,
increased torque can be generated.
[0121] Moreover, the permanent magnets are surrounded by silicon
steel plates, so that a rotating electric machine suitable for high
speed rotations can be provided.
[0122] Next, a permanent magnet rotating electric machine according
to a fourth embodiment of the present invention will be described
with reference to FIG. 8.
[0123] FIG. 8 is a cross-sectional view illustrating the permanent
magnet rotating electric machine according to the fourth embodiment
of the present invention.
[0124] The fourth embodiment is characterized by a three-phase
motor structure which comprises a permanent magnet rotor 36 having
twelve poles (P=12) and a stator having eight magnetic poles (M=8).
Since this structure can increase the utilization ratio of windings
(winding coefficient), a highly efficient, small, and light-weight
rotating electric machine can be provided.
[0125] In addition, a magnetic pole piece area of the rotor is
projected toward the magnetic poles of the stator, such that a
sinusoidal magnetic flux distribution is produced.
[0126] Referring specifically to FIG. 8, the rotating electric
machine 10 comprises a stator 20 and a rotor 30. The rotor 20
comprises a stator core 22 and a stator windings 24. The stator
core 22 comprises an annular stator yoke 22A and stator salient
poles 22B, and the stator windings 24 are concentratively wound
around the stator salient poles 22B. The respective windings 24 are
configured not to share a magnetic path on gap surfaces. By
employing a stator structure in which the stator windings are
implemented by concentrated windings, end coil portions can be
reduced in length, and consequently the physical size of the
rotating electric machine can also be reduced.
[0127] The U-phase of the stator windings 24 is connected to U1,
U2, U3, U4, respectively; the V-phase is connected to V1, V2, V3,
V4, respectively; and W-phase is connected to W1, W2, W3, W4,
respectively.
[0128] The rotor 30 comprises a rotor core 34 formed of a plurality
of laminated plates made of a highly magnetic permeable material,
for example, silicon steel; four permanent magnets 36 inserted into
four permanent magnet inserting holes 34 formed in the rotor cores
32; and a shaft 38. Ten permanent magnets 36 are positioned in the
circumferential direction of the rotor core 32 at equal intervals
such that their polarities are in the opposite directions from each
other.
[0129] The rotor core 32 is formed with the permanent magnet
inserting holes 34 and a hole for passing the shaft 38
therethrough, both formed by punch press. Thus, the rotor 30 is
composed of the rotor core 32 made of laminated silicon steel
plates and formed with the punch-pressed permanent magnet inserting
holes 34 and hole for passing the shaft 38 therethrough, the
permanent magnets 36 inserted into the holes 34, and the shaft 38
extending through the hole.
[0130] The rotor core 32 may be divided in the radial direction
into an inner yoke area 32A and an outer peripheral area 32B. The
outer peripheral area 32B of the rotor core 32 may be further
divided in the circumferential direction into an auxiliary magnetic
pole area 32B1 and a magnetic pole piece area 32B2. The auxiliary
magnetic pole area 32B1, which is an area sandwiched by adjacent
permanent magnet inserting holes 34, functions to prohibit magnetic
circuits of the magnets from passing therethrough and to allow
magnetic flux to be directly generated in the stator by a
magnetomotive force of the stator. In the fourth embodiment, the
magnetic pole piece area of the rotor is projected toward the
stator magnetic poles 22B to shape a sinusoidal magnetic flux
distribution.
[0131] The permanent magnets 36 can be accommodated in the
permanent magnet inserting holes 34 which are bordered by the
auxiliary magnetic pole area 32B1 in the circumferential direction
and bordered by the magnetic pole piece area 32B2 around the outer
periphery, thus providing a rotating electric machine suitable for
high speed rotation.
[0132] Further, since the magnetic pole piece area is made of a
magnetic material, pulsating flux from the stator salient poles can
be mitigated. Also, since the laminated steel core is employed, the
rotating electric machine of the second embodiment is free from
eddy current losses.
[0133] It is assumed in the example illustrated in FIG. 8 that the
rotating electric machine is a three-phase motor which comprises
the permanent magnet rotor 36 with the number of poles P being
twelve, and the stator with the number of magnetic poles being
eight. Since such a structure achieves an increased utilization
ratio of the windings (winding coefficient), a highly efficient,
small and light-weight rotating electric machine can be
provided.
[0134] The connection of the stator windings 24 is made in
accordance with a three-phase stator winding scheme. More
specifically, U1, U2, U3, U4 are connected in the illustrated order
in the U-phase; V1, V2, V3, V4 are connected in the illustrated
order in the V-phase; and W1, W2, W3, W4 are connected in the
illustrated order in the W-phase. The windings forming parts of the
U-phase, V-phase, W-phase have a phase difference of 60 degrees
between each other.
[0135] In the fourth embodiment, the stator salient poles, having
wound therearound the stator windings connected to the same phase,
are in phase with the associated permanent magnets, so that a
reduction in torque pulsation is not expected. However, since the
salient poles in phase with the permanent magnets are positioned in
a symmetric configuration, a well balanced structure can be
provided. More specifically explaining with reference to the
U-phase, the respective salient poles U1, U2, U3, U4 are positioned
symmetrically about the shaft 38.
[0136] It goes without saying that while the foregoing description
has been made in connection with an example of a motor, the fourth
embodiment can be similarly applied to a generator.
[0137] According to the fourth embodiment, since a concentrated
winding stator is employed, the end coil portions of the stator can
be reduced in length, so that a smaller rotating electric machine
can be provided.
[0138] Also, since the permanent magnet rotor is employed, a
structure suitable for field weakening control is realized, thereby
providing a rotating electric machine appropriate to high speed
rotations.
[0139] Moreover, the permanent magnets are surrounded by silicon
steel plates, so that a rotating electric machine suitable for high
speed rotations can be provided.
[0140] While the foregoing respective embodiments have been
described in connection with a control system which controls a
sinusoidal current with respect to the position of the rotor, it
goes without saying that the present invention may also be applied
to a 120 degree conductive brash-less motor scheme which does not
perform a current control.
[0141] Also, while the foregoing description has been made with
reference to an internal rotation type motor, the present invention
may also be applied to external rotation type motors, generators,
and linear motors.
[0142] Next, an electric car employing a permanent magnet rotating
electric machine according to a fifth embodiment will be described
with reference to FIG. 9.
[0143] FIG. 9 is a block diagram illustrating the configuration of
an electric car which is equipped with a permanent magnet rotating
electric machine according to the fifth embodiment of the present
invention.
[0144] A body 100 of the electric car is supported by four wheels
110, 112, 114, 116. Since this electric car is a front-wheel driven
type, a permanent magnet rotating electric machine 120 is directly
coupled to a front wheel shaft 154. The permanent magnet rotating
electric machine 120 has a structure as illustrated in FIG. 2, 6, 7
or 8. A control unit 130 is provided for controlling driving torque
of the permanent magnet rotating electric machine 120. A battery
140 is provided as a power source for the control unit 130.
Electric power from the battery 140 is supplied to the permanent
magnet rotating electric machine 120 through the control unit 130,
thereby driving the permanent magnet rotating electric machine 120
to rotate the wheels 110, 114. The rotation of a steering wheel 150
is transmitted to the two wheels 110, 114 through a transmission
mechanism including a steering ring gear 152, a tie rod, a knuckle
arm, and so on to change the angle of the wheels 110, 114.
[0145] It should be noted that while in the foregoing embodiment,
the permanent magnet rotating electric machine has been described
to be used for driving wheels of an electric car, the permanent
magnet rotating electric machine may also be used for driving
wheels of an electric locomotive or the like.
[0146] According to the fifth embodiment, when the permanent magnet
rotating electric machine is applied to an electrically driven
vehicle, particular to an electric car, a small, light-weight, and
highly efficient permanent magnet rotating electric machine can be
equipped in the vehicle, thus making it possible to provide an
electric car which can run a longer distance with the amount of
electric power accumulated in one recharging operation.
* * * * *