Note: Descriptions are shown in the official language in which they were submitted.
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Permanent ma~net field DC machine
This invention relates to a so-called permanent
magnet field DC machine, i.e. one employing a field
system of permanent magnets.
The amount of magnetic flux relative to the
armature current is substantially constant in conventional
permanent magnet field DC motors having-a field system
constituted by permanent magnets alone. For this reason,
a machine of this ~ind displays shunt-winding output
characteristics, and is incapable of generating sufficient
torque during starting, when a larger current flows
therethrough. Another type of permanent magnet field DC
machine, which is designed in consideration of the magneto-
motive force due to the armature reaction, is disclosed
in Japanese Patent Laid-Open No. 153558/1982 published on
September 22, 1982 with the title "Permanent magnet field
starter with auxiliary poles". This machine has auxiliary
poles made of a magnetic material, such as soft steel,
disposed parallel in the peripheral direction with the
permanent magnets.
However, according to conventional methods, ~his
machine cannot generate a sufficiently large starting
torque, and its rotational speed in the non-loaded state
is small, because the amount of torque generating at the
permanent magnets is large when the machine has no load.
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For this reason, when a permanent magnet field DC machine
is employed as a vehicle starter motor, it acts as a load
on the engine of the vehicle after starting the engine
and returning to the non-loaded state~ For the purpose
of increasing the starting torque, it is necessary to
increase the areas of ~he permanent magnets. When rare-
earth magnets, such as samarium cobalt magnets or neodymium
group magnets, are employ~d as the permanent magnets for
obtaining the same magnetic flux as ferrite magnets and
thin magnets, the cost of the field poles is increased,
since it is necessary to provide these magnets with large
areas.
An object of the present invention is to provide a
permanent magnet field DC machine that can generate a
larger torque under high load, for instance during starting,
and can rotate at a higher rotational speed in the non-
loaded state.
The present invention provides a permanent magnet
field DC machine having magnetic pole pieces made of a
magnetic material whose permeability is greater than the
reversible permeability of the permanent magnets and whose
thickness defined at the magnetizing end is larger than
that defined on the demagnetizing side.
In this arrangement, each of the magnetic pole
pieces of magnetic material is so formed as to reduce the
magnetic gap between the magnetic pole piece and the
armature core on the magnetizing side, thereby efficiently
utilizing the magnetizing effect of the armature reaction
and enabling the field poles to generate great magnetic
flux at the time of starting or under high-load when the
armature current is large, and to reduce the amount of
magnetic flux in the non-loaded state or under small load
when the armature current is small.
The present invention can thus ensure that the
starting torque of the machine can be increased, as well
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as its rotational speed in the non-loaded state.
In the drawings:
FIG. 1 is a radial cross section taken along I-I'
of FIG. 2 of a permanent magnet field motor to which one
embodiment of the present invention is applied;
FIG. 2 is a partial axial cross section of the motor
taken along II-II' of FIG. l;
FIG. 3 is a diagram of the distribution of armature
reaction applied to the magnetic field pole shown in FIG.
10 1,
FIG . 4 is a magnetic flux distribution diagram of
the motor of FIG. 1 during starting;
FIG. 5 is a characteristic diagram showing the
amount of magnetic flux of a field pole corresponding to
the armature current;
FlG. 6 is a characteristic diagram showing the torque
and the rotational speed corresponding to ~he armature
current;
FIGs. 7 to 12 show radial cross sections of other
embodiments relating to FIG. 1 of the present invention;
FIG. 13 is a modification of FIG. 2;
FIG. 14 is a radial cross section taken along XIV-
XIV' of FIG . 15 of a permanent magnet field motor with
auxiliary poles in accordance with an embodiment of the
present invention;
FIG. 15 is a partial axial cross section of the
motor taken along XV-XV' of FIG. 14;
FIG. 16 is a diagram of the distribution of armature
reaction applied to the magnetic pole shown in FIG. 14;
FIG. 17 is a magnetic flux distribution diagram of
the motor of FIG. 14 during starting;
FIG. 18 is a graph of the amount of magnetic flux
of the field pole corresponding to the armature current;
and
FIGs. 19 to 25 are radial cross sections of other
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embodiments rela-ting to FIG. 14 of the present invention.
Referring to FIGs. 1 and 2, a rotor, consisting of
a shaft 1, a commutator 2 and an armature consisting of
an armature core 3 and a coil 4 wound on the core 3, is
supported on end memhers 6a and 6b by bearings 5a and 5b.
The end members 6a and 6b are fixed to a cvlindrical yoke
7. Permanent magnets 8, consisting e.g. of ferrite magnets,
and magnetic pole pieces 9 are disposed around the inner
periphery of the yoke 7. The thickness of each of these
magnets 8 in the radial direction is greatest over a
portion thereof defined by an angle ~A on a demagnetizing
side 11, and gradually decreases over a portion defined
by an angle ~B from the demagnetizing side toward a
magnetizing end 10 thereof. The magnets ~ face the armature
core 3 with a gap therebetween. A magnetic pole piece 9,
made of a magnetic material such as soft sceel which has
a high permeability, is disposed between the yoke 7 and the
portion of each permanent magnet 8 defined by the angle
~B. The thickness of each of the magnetic pole pieces 9
in the radial direction is greatest at each magnetizing
end 10 and gradually decreases towards the corresponding
demagnetizing side 11. Therefore, the magnetic gap
between each magnetic pole piece 9 and the armature core
3 is smallest at the magnetizing end 10 and increases
towards the demagnetizing side.
In the embodiment thus arranged, the magnetomotive
force of the armature reaction acts on the field poles
when the armature coil is energized. As shown in FIG. 3,
the magnetomotive force of the armature reaction acts as
a magnetizing force on the left hand side of the center
of magnetism 0-0' and as a demagnetizing force on the
right hand side, when a current flows through the armature
coil in the direction from behind the plane of the figure
to the front.
Generally, the demagnetizing force Ha is expressed
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by the following formula (1). lt is in proportion to the
angLe ~ from the cen-ter of magnetism and in reverse
proportion to the thickness t of the magnet in the
direction of the magnetization thereof.
H oC ~/t ............................ (1)
The demagnetizing force acting on the permanent magnet 8
has a maximum value of Ha. In this embodiment the
radial thickness of t is made large so that the magnet
can resist this demagnetizing force Ha. The intensity of
the field system of the magnet is thereby maintained. A
demagnetizing force Hb also acts Oll a boundary portion 12
between each permanent magnet 8 and the corresponding
magnetic pole piece 9, but the magnitude of the demagnet-
izing force Hb is approximately 3a, as shown in FIG. 3.
The magnetic pole piece 9 disposed between each
permanent magnet 8 and the yoke 7 is made from a magnetic
material having a high permeability, as described above,
so that it generates a large amount of magnetic flux,
because of the magnetiæing effect of the armature reaction
when the armature current flows at a higher rate during
starting or when under load. This state is shown in the
distribution chart of FIG. 4. The magnetic pole piece 9
is formed so that its radial thickness increases from the
boundary portion 12 of the demagnetizing side to the
magnetizing end 10 thereof, so that the magnetic flux
generated by the armature reaction is led to the magnet-
izing end 10. In addition, the magnetizing end 10 faces
the armature core 3 via a gap, and the magnetic gap is
narrow in the vicinity of the magnetizing end 10, so
that the amount of the flux 14 is large at the magnet-
izing end 10 of the pole piece 9. This arrangement
ensures a larger amount 18 of magnetic flux during
starting or under load, when the armature current flows
at a high level, as expressed by a solid-line curve
~A in FIG. 5. This arrangement is free from short-
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circuited magnetic flux, since the magnetic gap betweeneach pole piece 9 and the armature core 3 is wide at
about the center of magne-tism thereof.
The radial thickness of the permanent magnet 8 is
largest at the angle ~A portion and gradually decreases
toward the magnetizing end 10 over the angle ~B portion,
as described above. Accordingly, the cross sectional
area of the permanent magnet 8 is small and the amount
of magnetic flux 16 generated from the permanent magnet
8 is reduced, as expressed by the solid-line curve ~M
in FIG. 5, while the amount of magnetic flux 15 generated
by a magnetic pole constituted by the permanent magnet
alone is as expressed by the broken-line curve ~M'.
However, as expressed by the curve ~ in FIG. 5,
the amount o~ magnetic flux 17 of the field pole, which
is the sum of ~M and ~A, is small in the substantially
non-loaded state when the armature current is small,
and is large when the load and hence the armature current
is large. When this embodiment is used as an electric
motor, it exhibits a larger rotational speed 21 when it is
not loaded and the armature current is small, and can
output its greatest torque 19 when it is started or is
loaded when the armature current is large, as shown in
FIG. 6. In FIG. 6, numerals 22 and 20 respectively
represent characteristic curves of rotational speed and
torque versus armature current in the conventional
apparatus.
As described above, the cross sectional area of each
permanent magnet 8 is small in this embodiment so that
the weight of the permanent magnets can be reduced to a
significant extent. ~he magnetic pole piece 9 face
the armature core at the magnetizing end thereof, but
its radial thickness decreases toward the demagnetizing
side so that the magnetic gap is increased, thereby
preventing the occurrence of eddy-current losses due
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to slot ripples. Therefore, there is no delay of magnetic
flux relative to the rise in armature current.
The present invention has been described above with
respect to a quadrapole permanent magnet field DC machine,
but it is possible for the principle of -the invention to
be applied to other multi-pole machines, such as dipole
or six-pole machines. The present invention is effective
for generators as well as electric motors. Each permanent
magnet 8 is formed of a ferrite magnet in the above
embodiment, but the material of the permanent magnet 8 is
not limited; other kinds of materials, such as samarium
cobalt and neodymium magnets, whi~h are rare-earth magnets,
iron and boron magnets can be used. The material of each
magnetic pole piece 9 can be laminated silicon steel plates
or a ferrite core.
FIG. 7 shows an arrangement in which each magnetic
pole piece 9 is disposed on the magnetizing side only,
relative to the center of magnetism 0-0', whereby the
same magnetizing effect of the present invention can be
realized. FI~s. 8, 9, and 10 show other arrangements
in which each line of the boundary between the magnetic
pole piece 9 and the permanent magnet 8 is in the form of
a circular arc. These arrangements also exhibit the
same effects as that of FIG. 1. FIG. 11 shows still
another arrangement in which portions of the magnet 8 and
the pole piece 9 having a small thickness are cut off so
as to form gaps 13 and 23, thereby preventing damage to
the edges of the magnet ~ and the pole piece 9, and
facilitating their manufacture. FIG. 12 shows still
another arrangement in which edge portions of the magnet
8 and the pole piece 9 that are oppositely located in the
peripheral direction are cut off so that a magnetic density
distribution in the form of a sine wave is provided in
the gap between the field pole and the armature core,
thereby reducing noise and vibration of the motor.
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In the arrangement shown in FIGs. 1 and 2, the pole
piece 9 and the magnet 8 have the same axial length, but
they can be different in length. Tha-t is, when an axial
length ~ of the armature core is assumed, the axial length
of the magne~ic pole piece 9 is set to be about 1.2 ~ ,
as shown in FIG. 13, over which the magnetizing force of
the armature reaction is distributed, while the axial
length of the permanent magnet 8 is set to be 1.3 ~ to2.~ ~.
It is therby possible to introduce a large amount of
magnetic flu~ from the axial end of the permanent magnet
8, which is out of the effective range of the armature
reaction, into the armature core, in the loaded state or
during starting when the armature current flows at a
high level. A larger amount of magnetic flux is thus
obtained.
In the above described embodiment of the present
invention, the magnetic pole piece whose thic~ness is
decreased toward the demagnetizing side is formed at a
portion of the permanent magnet so that the magnetizing
effect of the armature reaction can be efficiently
utilized, thereby obtaining a larger amount of magnetic
flux when the current flows at a high level. In addition,
the amount of magnetic flux generating from the permanent
magnet is reduced, as the area of the permanent magnet
is small. Accordingly, a motor having a field pole in
accordan~e with the present invention exhibits a direct-
winding characteristic whereby the motor can output a
larger torque when the current flows at a high level,
and can rotate at a high rotational speed in the non-
loaded state when the current flows at a low level. Themotor can thereby be reduced in size and manufacturing
cost. In addition, it is possible to greatly reduce the
cost of the magnets when rare-earth magnets are used
whose cost per weight is high, since the weight of the
permanent magnet is reduce.
g
Another embodiment of permanent magnet field DC
machine according to the present invention will now be
described. FIG. 1~ is a radial cross section through
a quadrapole permanent magnet field DC machine with
auxiliary poles, and FIG. 15 is an axial cross section
of the same. As shown in FIGs. 14 and 15, the rotor,
consisting of a shaft 1, a commutator 2 and an armature
consisting of an armature core 3 and a coil 4 wound on
the armature core 3, is supported on end m~mbers 6a and
6b by bearings 5a and 5b. The end members 6a and 6b are
fixed to a cylindrical yoke 7. Auxiliary poles 80 fixed
to the yoke 7 and having a peripheral angle of 01 are
made of a magnetic material, e.g., soft steel, and act
to intensify the magnetomotive force of the armature
reaction. They face the armature core 3 via a gap.
Magnetic pole pieces 9 with a peripheral angle of ~2
and made of a magnetic material are fixed to the yoke 7
to abut the auxiliary poles 80 in the peripheral direction,
and are disposed partially on the demagnetizing side.
Permanent magnets 8 are disposed around the inner per-
iphery of the cylindrical yoke 7. Each magnet 8 consists
of a magnet 101 ~Fig. 16) having a smaller thickness and
disposed under the magnetic pole piece 9 on the side of
the gap, and a magnet 102 having a greater thickness
disposed on the demagnetizing side toward the end 11
thereof. The directions of magnetization of the magnets
101 and 102 are the same, when these magnets are disposed
in the same pole. The radial thickness of the magnet 101
is half that of the magnet 102.
In this arrangement, the magnetomotive force of the
armature reaction acts on the field poles when the arm-
ature coil is energized. As shown in FIG. 16, the
magnetomotive force of the armature reaction acts as a
magnetizing force on the left hand side of the center of
magnetism 0-0' and as a demagnetizing force on the right
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hand side, when a current flows through the armature coil
in the direction from behind the plane of the figure to
the front. The demagnetizing force acting on the permanent
magnet 8 has a maximum value of lla at the demagnetizing
end ll of the magnet 102. The radial thickness t is made
large so that the magnet 102 can resist this demagnetizing
force Ha.
A demagnetizing force Hb acts on the magnet 101
which is laminated and laid on the magnetic pole piece 9
at the portion 12 on the demagnetizing side. Since the
angle 3B from the center of the magnetism of the magnet
lOl is about 1/3 of the angle ~A of the magnet 102 and the
thickness tl of the former is aboùt a half of that of the
latter, the demagnetizing force HB becomes approximately
3 times Ha, as apparent from the formula (l).
FIG. 17 shows the magnetic flux distribution of this
embodiment. A large amount of magnetic flux is generated
by the magnetizing effect of the armature reaction on
each auxiliary pole 80 which is formed of a material of
high permeability. Each magnetic pole piece 9 formed of
a magnetic ma~erial having high permeability, as that of
the auxiliary pole acts to reduce the magnetomotive force
consumption of the yoke, because it forms a part of the
magnetic flux flow path. The pole piece 9 leads the
reaction magnetic flux of the armature reaction to the
magnetizing side, thus acting in the same manner as in
the case of the auxiliary pole. There is substantially
no occurrence of short-circuited magnetic flux due to
the armature reaction which does not contribute to the
torque generation, since the magnetic gap between the
armature core 3 and each pole piece 9 is large. Accordingly,
as expressed by the solid line 26 in FIG. 18, a larger
amount of magnetic flux is generated at a larger armature
current ia2. In this embodiment, the permeance coefficient
of the permanent magnet 8 is reduced, because of the
reduced ~adial ~hickness of the permanent magnet 101. For
this reason, the amount of magnetic flux generated by the
permanent magnet 101 is small, compared with the conven-
tional arrangements. Therefore, as expressed by the
solid line 26 in FIG. 18, the amount of magnetic flux of
the field pole is small at a small armature current ial
substantially in the non-loaded state, compared with that
of the prior art expressed by the broken line 27.
In this embodiment of the present invention, the
amount of flux of the magnetic pole is small in the non-
loaded state, but it is large in a loaded condition or
during starting when the armature current is large,
compared with the conventional arrangements. For this
reason, the rotational speed in the non-loaded stated is
high and a large torque can be obtained in the loaded
state or during starting of the machine. When this
embodiment is used as a starter motor, it does not act as
a load on the engine.
In addition, the volume of each permanent magnet 8
is reduced, since the radial thickness of the magnet 101
is reduced while maintaining the resistance to the
demagneti7ing field of the armature reaction, as described
above. It is thus possible to realize a permanent magnet
that is reduced in weight and cost.
The description has been made with respect to a
quadrapole permanent magnet field DC machine, but it is
possible for the principle of the invention to be applied
to other multipole machines such as dipole or six-pole
machines. The present invention is effective for
generators as well as electric motors. Each permanent
magnet may be integrally formed or composed of two parts.
The material of the permanent magnet is not limited
specifically, and it is possible to use magnetic materials
such as ferrite magnets, rare-earth magnets involving
samarium cobalt, cerium, cobalt, neodymium, iron and
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boron magnets, and plastic magnets. The auxiliary pole
and magnetic pole piece may be integrally formed. With
reference to FIG. 16, the values of the angle ~B and the
thickness tl of the magnet 101 can be respectively set
to be about ~A/3 and t/2 in relation to those of the
magnets 102. It is possible to freely select the dimensions
of the magnet 101. Theoretically, they should satisfy the
relationship: ~A x tl = ~B x t. For instance, 9B may be
aA/2 when the thickness tl of the ma~net 101 is assumed
to be about t/2. However, in practice, the dimension
eA/3 = 9B and tl = t/2, as given above, can be selected
(instead of tl - t/3) to achieve an even stronger demagnet-
ization force on the magnet.
It is possi~le to arrange that the radial thickness
of an auxiliary pole 9' decreases toward the demagnetizing
side, as shown in a radial cross section of FIG. 19 This
arrangement facilitates the integral formation of the
permanent magnets, so as to save cost and labor when
assembling the machine. In the arrangement of FIG. 19,
the thickness of the magnet is decreased at an end 12
compared with that shown in FIG. 14. The resistance to
the demagnetizing force is thereby further improved as
well as the magnetizing effect.
FIG. 20 shows in radial cross section of another
modified example in which the edge of the demagnetizing
end 12 of the auxiliary pole g which abuts on the magnet
102 is cut to form a gap 23. A short circuit of the
reaction magnetic flux is thereby more positively prevented,
and the leakage flux of the permanent magnet can be
reduced.
The same effect in accordance with the present
invention can be realized by the arrangement shown in
FIG. 21 in which the cross sectional shape of the pole
piece 9 is triangular. The same effect as in FIG. 14 is
also possible in the arrangement shown in FIG. 22 in
- 13 -
~hich a laminar auxiliary pole 80' is fixed by welding
to the underside of a magnetic pole piece 9" which is
integrally formed from the magnetizing end portion to
the demagnetizing side. The process of fixing the
auxiliary pole 80' to the yoke is simplified by this
arrangement. It is a matter of course that, as shown in
FIG. 23, edge portions of the magnet portions 101 and
102 constituting the permanent magnet 8, which respectively
abut on the auxiliary pole 80 and the magnetic pole piece
9 in the peripheral direction, are cut so as to form a
gap 130. In the arrangement shown in FIG. 24, the aux-
iliary pole 80 and the magnetic pole piece 9 are disposed
in the manner shown in FIG. 14, and a permanent magnet
8 is disposed alone at the demagnetiziny end so as to
form a substantial space 23 under the magnetic pole
piece 9, thereby greatly reducing the weight of the magnet.
This permanent magnet 8 may be in the form of an L or a
trapezoid L-shape for the same effect. FIG. 25 shows
another arrangement in which the boundary line between
the pole piece 9 and the magnet 8 is in the form of a
circular arc.
In the embodiment shown in FIGs.14 and 15, the
auxiliary pole 80 and the magnetic pole piece 9 have the
same axial length as that of the magnetic portions 101
and 102 of the permanent magnet 8, but the former length
can be different from the latter. That is, when an axial
length ~ of the armature core is assumed, the axial
length of each of the auxiliary pole and magnetlc pole
piece is set about 1.2 ~ while the axial length of the
permanent magnet 9 is set from 1.3 ~ to 2.0 ~ It is
thereby possible, during starting, to introduce a large
amount of magnetic flux from the axial end of the
permanent magnet 8 into the armature core, thus obtaining
a larger amount of magnetic flux.
In the above described embodiment of the present
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invention, -the first auxiliary pole and the magnetic pole
piece extend from the magnetizing side to a part of the
demagne-tizing side, thereby conducting the magnetizing
effect of the armature reaction. A larger amount of
magnetic flux can be generated during starting or under
load when the current flows at a higher level, thus
realizing a motor having a larger torque. The thickness
of the permanent magnet portion disposed under the magnetic
pole piece is reduced, so that the permeance coefficient
of the permanent magnet becomes small, and the amount of
magnetic flux in the non-loaded state of the motor is
reduced. For this reason, the rotational speed of the
motor in the non-loaded state can be increased and the
reliability of the motor can be improved. In addition,
it is possible to reduce the weight of the permanent
magnet and hence the cost of the motor, since the volume
of the permanent magnet can be reduced.
As is apparent from the above description, the
amount of magnetic flux can be increased under high load,
for instance at the time of starting, and can be reduced
under low load, thus providing a permanent magnet field
DC machine capable of outputting a higher torque under
high load and capable of rotating at a higher rational
speed under low load, namely,a device having preferable
output characteristics for vehicle starters.
