U.S. patent application number 09/731859 was filed with the patent office on 2001-11-15 for rotary electric machine for vehicle.
Invention is credited to Nakamura, Shigenobu, Umeda, Atsushi.
Application Number | 20010040416 09/731859 |
Document ID | / |
Family ID | 18408062 |
Filed Date | 2001-11-15 |
United States Patent
Application |
20010040416 |
Kind Code |
A1 |
Nakamura, Shigenobu ; et
al. |
November 15, 2001 |
Rotary electric machine for vehicle
Abstract
In a vehicle rotary electric machine, a stator core has a
plurality of slots accommodating a plurality of conductors forming
a plurality of phase windings. The phase winding includes a
plurality of parallel-connected winding sections. Each of the
winding sections is formed of a plurality of series-connected
conductors that is distributed to the plurality of slots neighbor
to each other.
Inventors: |
Nakamura, Shigenobu;
(Anjo-city, JP) ; Umeda, Atsushi; (Okazaki-city,
JP) |
Correspondence
Address: |
PILLSBURY WINTHROP LLP
1600 TYSONS BOULEVARD
MCLEAN
VA
22102
US
|
Family ID: |
18408062 |
Appl. No.: |
09/731859 |
Filed: |
December 8, 2000 |
Current U.S.
Class: |
310/201 ;
310/198; 310/254.1 |
Current CPC
Class: |
H02K 3/28 20130101; H02K
3/12 20130101 |
Class at
Publication: |
310/201 ;
310/254; 310/198 |
International
Class: |
H02K 001/12; H02K
003/00 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 9, 1999 |
JP |
11-350074 |
Claims
What is claimed is:
1. A vehicle rotary electric machine comprising: a rotor having a
plurality of magnetic poles; a stator having a stator core opposed
to said rotor and a multi-phase stator winding, said stator core
having a plurality of slots accommodating a plurality of conductors
connected to form said stator winding; and a frame for supporting
said rotor and stator, wherein said multi-phase stator winding
comprises a plurality of phase windings; each of said phase winding
comprises a plurality of parallel-connected winding sections, and
each winding section comprises a plurality of series-connected
conductors distributed to a plurality of slots that is neighbor to
each other.
2. The vehicle rotary electric machine as claimed in claim 1,
wherein said plurality of winding sections is the same in phase
with one another at opposite ends to be connected in parallel.
3. The vehicle rotary electric machine as claimed in claim 1,
wherein said plurality of phase winding sections has a first
winding section and a second winding section, said first winding
section comprises a series circuit of a portion of a plurality of
conductors disposed in one of said two neighboring slots and a
portion of a plurality of conductors disposed in the other slot,
and said second winding section comprises a series circuit of the
remaining portion of said plurality of conductors disposed in said
one slot and the remaining portion of said conductors disposed in
said the other slot.
4. The vehicle rotary electric machine as claimed in claim 3,
wherein said first winding section comprises a series circuit of a
half of a plurality of conductors disposed in one of said two
neighboring slots and a half of a plurality of conductors disposed
in said the other slot.
5. The vehicle rotary electric machine as claimed in claim 1,
wherein if the number of poles of said rotor is p, the number of
phases of said multi-phase stator winding is n, the number of the
neighboring slots for accommodating conductors of said phase
winding of said stator winding is m, the total number of said slot
is equal to or more than p.times.n.times.m.
6. The vehicle rotary electric machine as claimed in claim 1,
further comprising: a plurality of switch elements connected to
respective phase windings of said multi-phase stator winding.
7. The vehicle rotary electric machine as claimed in claim 1,
wherein said plurality of conductors comprises a plurality of
conductor segments connected at coil ends extended to an end of
said stator core.
8. The vehicle rotary electric machine as claimed in claim 7,
wherein said end of said stator core has a plurality of coil ends
extended to spaced apart from one another.
9. The vehicle rotary electric machine as claimed in claim 1,
wherein said conductors disposed in said slots have a rectangular
cross-section having longer radial sides.
10. The vehicle rotary electric machine as claimed in claim 1,
further comprising a booster for boosting output voltage generated
by said stator winding in a low speed range.
11. The vehicle rotary electric machine as claimed in claim 1,
further comprising a switching device which repeats grounding and
opening of the output terminal of said stator winding.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] The present application is based on and claims priority from
Japanese Patent Application Hei 11-350074 filed Dec. 9, 1999, the
contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a rotary electric machine
such as a motor or a generator to be mounted in a ship or a
vehicle.
[0004] 2. Description of the Related Art
[0005] Recently, hybrid vehicles have been manufactured in order to
protect the environment. A hybrid vehicle is driven by a motor
while running at a low speed in a town or the like and is driven by
an engine while running at a high speed in a suburb. There is a
hybrid vehicle, such as a hybrid bus on a regular route, whose
engine is switched off when the vehicle stops temporarily on a
traffic signal. Such a hybrid bus is equipped with an idle-stop
system for automatically stopping the engine. Incidentally, an
inexpensive rotary electric machine that operates both as a motor
and as a generator has been demanded to continuously drive a
compressor and/or a power steering pump while the vehicle engine is
stopped.
[0006] WO98/54823 or U.S. Pat. No. 5,998,903, which corresponds to
WO98/54823, discloses a compact and powerful generator. Such a
generator has a stator winding formed of a plurality of conductor
segments, a stator core having two times as many slots as an
ordinary generator. The conductor segments are connected so that
one of the conductor segments that extended from one of a plurality
of layers of a slot is connected to another that are extended from
another layer of another slot. This arrangement improves the
conductor's space-factor and cooling effect of the stator winding,
resulting in a stator winding of low resistance. In the above
publication, there is an embodiment, in which windings that are
disposed in the neighboring slots are connected in series to be
connected to a rectifier bridge to provide a DC output power. There
is another embodiment, in which windings disposed in the
neighboring slots are connected in parallel to be connected to
respective rectifier bridges to provide a composite DC output
power.
[0007] In a vehicle rotary electric machine, there is a demand for
increasing current capacity of each phase winding. For example, if
the torque of a motor is increased, it is necessary to increase
drive current supplied to the stator winding. It is also necessary
to lower the resistance of phase windings of a generator to
generate a large amount of current.
SUMMARY OF THE INVENTION
[0008] In view of the above subject, a main object of the invention
is to provide a vehicle rotary electric machine that has a stator
winding having a large current capacity.
[0009] Another object of the invention is to accommodate a small
number of conductors in each slot and form a plurality winding
sections connected in parallel, thereby increasing the current
capacity of the stator winding.
[0010] Another object of the invention is to provide new conductor
segments that are suitable for forming a parallel circuit of the
winding sections.
[0011] Another object of the invention is to provide a vehicle
rotary electric machine that can operate as both a motor and a
generator.
[0012] According to a feature of the invention, a phase-winding is
formed of a plurality of parallel winding sections, each of which
is comprised of a plurality of conductors. This plurality of
conductors is distributed to a plurality of neighboring slots.
[0013] As a result, the rotary electric machine can provide a
sufficient torque characteristic, as a motor, and a sufficient
output current characteristic, as a generator.
[0014] It is possible to provide five-phase or six-phase stator
winding. However, parts of rectifying bridges or an inverter
circuit should not be increased too many. From this point of view,
three-phase stator winding is the most suitable. It is also
possible to increase the number of the slots accommodating a
plurality of the conductors forming a phase winding to more than
two. However, it is necessary to take the size of the vehicle
rotary electric machine into account, and two slots are the most
suitable from this point of view. It is possible to increase the
number of parallel winding sections to more than two. However, the
working time for connecting such winding sections in parallel
should be considered. From this point of view, two winding sections
in parallel are the most suitable. Further, the number of
conductors accommodated in each slot can be increased to more than
two. However, current capacity, size, productivity of the rotary
electric machine should be taken into account. For example, four
conductors or less in each slot is the best suitable for a
motor.
[0015] It is possible to provide a structure in which the winding
sections are the same in phase with one another at opposite ends to
be connected in parallel.
[0016] In general, if a winding formed of a plurality of series
connected conductors disposed in a slot and another winding formed
of a plurality of series connected conductors disposed in the next
or neighboring slot are connected in parallel, current may flow
from one to another because of phase difference between two
winding. As a result, the efficiency of the rotary electric machine
is not sufficiently high. However, the rotary electric machine
according to the invention eliminates the phase difference, so that
a sufficiently high efficiency can be maintained.
[0017] Further, a phase winding is formed of a first winding
section and a second winding section. The first winding section
includes a series circuit of a portion of a plurality of conductors
disposed in one of the two neighboring slots and a portion of a
plurality of conductors disposed in the other slot. The second
winding section includes a series circuit of the remaining portion
of the plurality of conductors disposed in the one slot and the
remaining portion of the conductors disposed in the other slot.
[0018] The first winding section may include a series circuit of a
half of a plurality of conductors disposed in one of the two
neighboring slots and a half of a plurality of conductors disposed
in the other slot.
[0019] The conductors of the first winding section and the
conductors of the second winding section are connected in a manner
corresponding to the phase angle, forming a diamond shape, which
equalizes the phase angle of the first and second winding
sections.
[0020] If the number of poles of said rotor is p, the number of
phases of said multi-phase stator winding is n, the number of the
neighboring slots for accommodating conductors of the phase winding
of the stator winding is m, the total number of the slot is equal
to or more than p.times.n.times.m.
[0021] The above structure is important to the vehicle rotary
electric machine. For example, a 16-pole, three-phase and
two-neighboring-slot type rotary electric machine has 96 slots.
This type can provide a suitable size of a vehicle rotary electric
machine. If a rotary electric machine has some empty slots, the
number of slots becomes more than 96.
[0022] The above embodiment may include switch elements connected
to the respective phase windings of the multi-phase stator winding.
The number of the switch elements that are connected to respective
phase windings is proportional to the number of the phases because
each phase winding is formed of the winding sections connected to
each other in parallel. For example, three-phase stator winding is
connected to a three-phase bridge circuit. This structure can make
the number and the size of the switching elements small. It is
desirable to use diodes as the switching elements. A rectifier can
be formed of diodes, and an inverter can be formed of transistors
or MOS transistors.
[0023] The conductor may be a conductor segment connected at a coil
end that is extended to an end of the stator core. The conductor
segment is suitable to a stator having a small number of conductors
in each slot. The conductors that are distributed to neighboring
slots are connected in series to form a winding section, thereby
providing a suitable number of coil-turns.
[0024] In addition, a plurality of coil ends is extended to an end
of the stator core and is spaced apart from one another. It is easy
to line up the coil ends at a prescribed portion so that the coil
ends can be spaced apart from each other. This arrangement is
effective to lessen heat problems. It is also effective to provide
an even arrangement of the coil ends. For example, it is possible
to provide gaps for passing cooling air between the coil ends.
[0025] It is also possible to fill the gaps with resinous material.
Preferably, the conductors disposed in the slots have a rectangular
cross-section having longer radial sides.
[0026] The vehicle rotary electric machine can be provided with a
booster for boosting output voltage generated by the stator winding
in a low speed range. Further, a switching device can be disposed
in the rotary electric machine to repeat grounding and opening of
the output terminal of the stator winding.
BRIEF DESCRIPTION OF THE DRAWINGS
[0027] Other objects, features and characteristics of the present
invention as well as the functions of related parts of the present
invention will become clear from a study of the following detailed
description, the appended claims and the drawings. In the
drawings:
[0028] FIG. 1 is a cross-sectional view illustrating the whole
structure of a rotary electric machine according to a first
embodiment of the invention;
[0029] FIG. 2 is a fragmentary cross-sectional view of a stator
according to the first embodiment;
[0030] FIG. 3 is a circuit diagram of a stator winding according to
the first embodiment;
[0031] FIG. 4 is a winding diagram of the stator winding according
to the first embodiment that is connected to the portions
illustrated in FIG. 3 at line III-III and line IV-IV;
[0032] FIG. 5 is a vector diagram of the stator winding according
to the first embodiment;
[0033] FIG. 6 is a circuit diagram of a control unit according to
the first embodiment;
[0034] FIG. 7 is a perspective view of a conductor segment
according to the first embodiment;
[0035] FIG. 8 is a perspective view of the stator according to the
first embodiment viewed from the inside thereof;
[0036] FIG. 9 is a vector diagram of a stator winding according to
a second embodiment of the invention;
[0037] FIG. 10 is a perspective exploded view of conductor segments
to be inserted into a stator core according to a third embodiment
of the invention;
[0038] FIG. 11 is a circuit diagram of a battery charging system
according to a fourth embodiment of the invention;
[0039] FIG. 12 is a graph showing a relationship between generator
output voltage and generator output power with respect to the
rotation speed of the generator according to the fourth embodiment;
and
[0040] FIG. 13 is a circuit diagram of a battery charging system
according to a fifth embodiment of the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0041] Rotary electric machines according to a first embodiment of
the invention is described with reference to FIGS. 1-8.
[0042] As shown in FIG. 1, vehicle generator-motor 1 as a vehicle
rotary electric machine includes front frame 11 and rear frame 12.
Front frame 11 and rear frame 12 rotatably support rotor 2 and also
fixedly support stator 3. Rear frame 12 has output terminal 13.
[0043] Shaft 21 of rotor 2 extends through front frame 11 and
projects from frame 11 to have pulley 22 fixed at the left of front
end thereof. Pulley 22 is linked to a vehicle drive engine via a
belt (not shown). The right or rear end of shaft 21 projects
backward from rear frame 12 to a portion where brush unit 14 is
disposed. Brush unit 14 has a pair of brushes 15 disposed in
contact with a pair of slip rings 23 that is carried by shaft 21.
Shaft 21 carries Lundell-type magnetic core 24. Magnetic core 24
has a plurality of pole pieces 24a on the outer periphery thereof.
Magnetic core 24 is comprised of front pole core 24b and rear pole
core 24c. Magnetic field coil 25 is disposed between both pole
cores 24band 24c to polarize the plurality of pole pieces 24a.
Field coil 25 is connected to the pair of slip rings 23 to be
energized via brush unit 14. Magnetic core 23 has front fan 26 and
rear fan 27, which are respectively fixed on the opposite ends
thereof. Front and rear fans 26 and 27 take air from air-intake
windows 16 and 17 formed at frames 11 and 12 and discharge the air
from air discharge windows 18 and 19.
[0044] Stator 3 is disposed opposite the plurality of pole pieces
24a of rotor 2. Stator 3 is a cylindrical member disposed around
rotor 2. Stator 3 has stator core 31 that is formed of a plurality
of laminated steel sheets and stator winding 32 mounted on stator
core 31. One end of stator winding 32 is connected to output
terminal 13 as output lead wire 33.
[0045] As shown in FIG. 2, stator core 31 has a plurality of
axially extending slots 310. The plurality of slots 310 is formed
to open at the inner periphery of stator core 31 opposite pole
pieces 24a, where teeth 312 are also formed. There are 96 slots in
this embodiment. Two conductors are disposed in each slot 311.
Conductor 34 disposed in a radially inner layer and conductor 35
disposed in a radially outer layer are radially piled or stratified
in each slot. Conductors 34 and 35 have a rectangular cross-section
whose radial sides are longer than the circumferential sides so
that wider teeth can be provided. The wide teeth provide excellent
heat conduction from conductors to the stator and a suitable
distance between respective coil ends outside the slots. Insulators
36 of a thin insulation sheet are respectively disposed between
conductors 34, 35 and the inner surfaces of slots 311.
[0046] Stator winding 32 is formed of conductors 34 and 35
respectively disposed in the inner and outer layers of a plurality
of slots 311. As shown in FIGS. 3 and 4, the plurality of slots 311
forms six groups that are different in electric phase from one
another. For example, the first slot group includes slots #4, #10,
#16-#88 and #94 (slot numbers indicated in the middle of the
winding diagrams), the second slot group includes #5, #11, #17-#89
and #95, as shown in FIGS. 3 and 4. The third, fourth, fifth and
sixth slot groups are disposed in the stator core in the same
manner. The first and second slot groups are disposed neighbor to
each other and accommodate an X-phase winding. The third and fourth
slot groups are disposed next or neighbor to each other and
accommodate a Y-phase winding, and the fifth and sixth slot groups
are disposed neighbor to each other and accommodate a Z-phase
winding.
[0047] Basically, conductors 34 and 35 are alternately connected in
series. In other words, one of conductors 34 and 35 accommodated in
a slot is series-connected to the other of conductors 34 and 35
accommodated in another slot that is six-slot-pitch spaced apart
from the aforestated slot. Thus, twelve wave-wound winding units
that round stator core 32 are formed. Two conductors accommodated
in a slot are respectively connected to other conductors to form
two wave-wound winding units. In other words, the first slot group
accommodates two wave-wound winding units 37a and 38a.
[0048] Two winding units 37a and 37b, which are accommodated in the
neighboring first and second slot groups, are connected in series
by crossing wire 37c to form winding section 37. Accordingly, a
plurality of conductors of winding section 37 is distributed to two
neighboring slot groups. Opposite ends 37d and 37e of winding
section 37 are extended in the axial direction from coil ends.
Remaining two winding units 38a and 38b of all the four wave-wound
winding units accommodated in the first and second slot groups are
series-connected by crossing wire 38c to form winding section 38.
Opposite ends 38d and 38e of winding section 38 are extended in the
axial direction from coil ends. Ends 37d and 38d are connected to
each other and to X-phase output terminal 33a. On the other hand,
end 37e and end 38e are connected to each other to form a neutral
point N. As a result, two winding sections 36 and 37 accommodated
in the two neighboring slot groups are connected in parallel. Thus,
the X-phase winding having two conductors each slot is formed.
Y-phase winding and Z-phase winding are also formed in the same
manner as described above.
[0049] Winding section 37 and winding section 38 generate voltage
in the same phase at the opposite ends. Therefore, the voltages in
vector can be illustrated in a diamond shape, as shown in FIG. 5,
which illustrates voltages in vector of the star connected X, Y and
Z-phase windings. Thus, a three-phase winding can be formed of six
different winding sections.
[0050] Three output terminals 33 (33a, 33b, 33c) of three-phase
winding 32 are connected to control unit 4 via respective terminals
13 (13a, 13b, 13c). Control unit 4 is connected to a vehicle
battery. Control unit 4 includes a three-phase bridge circuit
comprised of MOS-FETS and a control circuit for controlling the
gate voltage of the MOS-FETS and field current, as shown in FIG. 6.
Control unit 4 operates rotary electric machine 1 as a three-phase
AC generator or a three-phase synchronous motor according to the
vehicle running conditions or electric load conditions. In other
words, control unit 4 has a function to convert alternating current
to direct current and a function to switch on or off current that
is supplied to the armature winding.
[0051] Inner conductors 34 and outer conductors 35 are formed of
conductor segments, and a plurality of conductor segments is
connected in series to form a series-connected winding. As shown in
FIGS. 7 and 8, most portion of the stator winding is comprised of
U-shaped conductor segments 30, and the ends of the stator winding
37d, 37e, 38d and 38e are comprised of I-shaped conductor segments.
Crossing wires 37c and 38c are comprised of U-shaped conductor
segments the span of which is one slot-pitch shorter than a pole
pitch. All the conductor segments are made of film-coated copper
wires as shown in FIG. 2.
[0052] In FIG. 7, conductor segment 30 is formed of a copper wire,
which is bent to form hairpin-shaped segment 30a, thereafter,
twisted at portions A near turn portion 30b, and twisted at
portions B near the ends thereof. Portions A are twisted before the
segments are mounted in stator core 31, and portions B are twisted
after the segments are mounted in stator core 31.
[0053] Turn portion 30b and inclined portions 30c and 30d of
conductor segments 30 form a coil end 30m on an end of stator
winding 32. Inclined portions 30g and 30h and segment ends 30i and
30j form another coil end 30n on the other end of stator winding
32. Segment ends 30i and 30j of one conductor segment 30 are
respectively connected to the segment ends of other conductor
segments by welding, forming joint portions 39. Straight portion
30e is disposed in one slot 311 as inner conductor34, and straight
portion 30f is disposed in another slot 311 as outer conductor 35.
The one slot 311 and the another slot 311 are a predetermined
number of pole-pitches spaced from each other.
[0054] As shown in FIG. 8, a plurality of coil ends 30m forms a
ring-shaped coil-end group. Each of the plurality of coil ends 30m
is spaced apart from another to form gaps, which cooling air is
taken in.
[0055] When the rotary electric machine operates as a generator,
engine torque is transmitted to pulley 22, which rotates rotor 2.
When field coil 25 is energized, magnetic core 24 is polarized to
form sixteen magnetic poles. Consequently, magnetic flux is
supplied to stator winding 32 of stator 3, and AC voltage is
generated therein. The AC voltage is rectified by control unit 4,
and DC output voltage is provided.
[0056] When the rotary electric machine operates as a motor, direct
current is supplied from a battery to control unit 4, which
provides a three-phase AC voltage through its switching operation.
The three-phase AC voltage is applied to stator winding 32 to form
a rotating magnetic field, which rotates rotor 2 as a magnetic
rotor member.
[0057] The rotary electric machine has six phase-windings
(wave-wound winding sections). However, a three-phase bridge
circuit of the control unit, which is more compact and inexpensive,
can be used.
[0058] Two conductors, which are accommodated in two layers in each
slot, make comparatively small portions of coil ends thereof
overlap each other. Therefore, the heat dissipation is improved. In
addition, cooling air passes through between coil ends so that
energy loss caused by heat can be reduced.
[0059] The rotary electric machine has a stator core in which
winding sections are disposed 30.degree. in electric angle
different from each other. Therefore, magnetic pulsation can be
moderated so that noises can be reduced.
[0060] It is possible to use a multi-phase ring-connected stator
winding according to a second embodiment of the invention, as shown
in FIG. 9, instead of the star-connected stator winding shown in
FIG. 5.
[0061] Furthermore, it is possible to add a structure of cooling
the stator core and stator winding, thereby improving the
efficiency.
[0062] Instead of the conductor segments shown in FIG. 7, it is
possible to use the conductor segments according to a third
embodiment of the invention, as shown in FIG. 10. In this case, the
number of conductors in each slot is four. The number of the
conductors can be selected according to the driving torque and the
output electric power. Conductor segments 300 are mainly formed of
large segments 301 and small segments 302. Therefore, four
conductors are lined up along a radial direction in each slot as
illustrated by broken lines in FIG. 10. Here, one slot-group has
two two-turn lap-wound winding units. It is also possible to form
four wave-wound winding units. Thus, the same-phase winding
sections can be formed of either wave-wound winding units or
lap-wound winding units. A half of the conductor segments
accommodated in one of neighboring two slot groups and a half of
the conductor segments accommodated in the other group are
connected in series to form a winding section. Thus, two winding
sections are connected in parallel to each other to form a phase
winding. Insulators 360 are preferably curled into columnar members
as shown in FIG. 10.
[0063] It is also possible to provide a stator having four or more
electric phases.
[0064] A rotary electric machine according to a fourth embodiment
has a battery charging system shown in FIG. 11. The battery
charging system has booster 41 for boosting output voltage of
control unit 401 up to a voltage higher than a battery voltage.
Booster 41 is controlled by control unit 401. Control unit 401
controls the generator output voltage to provide a maximum electric
power while the engine rotation speed is in a low speed range. When
the generator output voltage is lower than a battery voltage,
control unit 401 controls booster 41 to boost the output voltage
thereof, thereby charging the battery. For example, when the rotary
electric machine rotates at a low speed, such as speed N1 or N2 in
FIG. 12, the output voltage is lowered to voltage V1 or V2 that is
lower than the battery voltage VB. Subsequently, the voltage V1 or
V2 is boosted to a voltage higher than the battery voltage VB. FIG.
12 shows generator output power in Y-axis and generator output
voltage in X-axis when the engine speed is N1 and N2 (N1<N2).
The above described structure is effective to supplement the
generator output power in case the number of conductors in each
slot is so small that the generator output power is not high enough
in a low speed range. This structure is well fit for an idle-stop
system that stops a vehicle engine temporarily when the engine runs
at an idling speed.
[0065] A battery charging system of a rotary electric machine
according to a fifth embodiment of the invention is shown in FIG.
13. The battery charging system is equipped with switch element 42
for grounding the output voltage of control unit 402. This battery
charging system controls switch element 42 to repeat the switching
operation, thereby generating voltage Vo that is higher than
battery voltage Vb so as to charge a battery. This also can
supplement the generation power in a low speed range.
[0066] It is also possible to combine the systems shown in FIGS. 11
and 13. That is, induced voltage by repeated switching operation of
grounding the output terminal is set to a voltage that corresponds
to a maximum output power. Thus, the output power in a low speed
range can be increased.
[0067] In the foregoing description of the present invention, the
invention has been disclosed with reference to specific embodiments
thereof. It will, however, be evident that various modifications
and changes may be made to the specific embodiments of the present
invention without departing from the broader spirit and scope of
the invention as set forth in the appended claims. Accordingly, the
description of the present invention is to be regarded in an
illustrative, rather than a restrictive, sense.
* * * * *