BACKGROUND OF THE INVENTION
1. Field of the Invention
[0001] This invention relates to a DC motor, and more particularly to a DC
motor in which a field generation unit is constituted by a plurality of types of field generating means.
2. Description of the Related Art
[0002] Known types of DC motors include a self-excited series wound motor
and a separately excited motor. A characteristic of the former, i.e. a self-excited series wound motor, is that a magnetic field varies in accordance with a load current. Hence, an increase in the load leads to a large increase in the magnetic field, whereas when the load is small, the magnetic field weakens, leading to a large increase in rotation speed. A characteristic of the latter, i.e. a separately excited motor, meanwhile, is that an amount of field magnetic flux remains constant regardless of the load current. Hence, the torque of a separately excited motor is proportionate to the load current, and therefore increases proportionately in response to an increase in the rotation speed.
[0003] In Japanese Patent Application Publication H5-91705, these
characteristics are applied to form a magnetic field in a DC motor from a magnet field generated by a permanent magnet and a winding field generated by a field core and a field winding. By employing the magnet field and the winding field, a large amount of torque can be generated when the DC motor is driven. Moreover, the magnet field is generated by the permanent magnet at all times, even when a current supplied to the field winding is cut off. When the motor is stopped, therefore, the motor functions as

a generator, or more specifically braking force is applied to a rotary machine by
means of an interaction between inertial rotation of the rotor of the motor and the
magnet field, and as a result, an inertial rotation period of the motor can be shortened.
[0004] Further, Japanese Patent Publication No. 5959583 discloses a motor in
which a field generating unit of the motor is provided with both a permanent magnet and a field coil in order to improve the performance of the motor at high rotation speeds while increasing the torque thereof at low temperatures.
SUMMARY OF THE INVENTION
[0005] In the motors described in Japanese Patent Application Publication
H5-91705 and Japanese Patent Publication No. 5959583, the field generating unit is formed from a field coil and a permanent magnet. Typically, the field coil is held by being sandwiched between the field core and a yoke, while the permanent magnet is held using a holder or the like. The field core is fixed to the yoke by welding or caulking. When different mounting methods are employed, however, the number of production processes increases, leading to increased production costs and equipment costs.
[0006] This invention has been designed to solve the problem described above,
and an object thereof is to provide a DC motor with which production costs and equipment costs can be suppressed.
[0007] A DC motor according to this invention includes an armature, and a field
generation unit disposed on an outer side of the armature, wherein the field generation unit includes a cylindrical yoke, and respective pluralities of permanent magnets and field coil units mounted on an inner periphery of the yoke, and each of the field coil units is formed by molding a field core and a field coil integrally using an

insulating material.
[0008] In the DC motor according to this invention, the field coil unit is formed
by molding the field core and the field coil integrally using an insulating material, and therefore an identical mounting method can be used to mount the permanent magnets and the field coil units on the yoke. As a result, production costs and equipment costs can be suppressed.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] FIG. 1 is a schematic sectional view of a DC motor according to a first
embodiment of this invention;
FIG. 2 is a schematic sectional view of a DC motor according to a second embodiment of this invention;
FIG. 3 is a circuit diagram showing a starter including a DC motor according to a third embodiment of this invention; and
FIG. 4 is a graph showing temporal variation in a battery voltage (a power supply voltage) and a starter current with respect to FIG. 3.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0010] Embodiments of this invention will be described below with reference to
the drawings.
First Embodiment
FIG. 1 is a schematic sectional view showing a DC motor 1 according to a first embodiment of this invention. The DC motor 1 includes an armature 2 and a field generation unit 3 for generating a magnetic field. The armature 2 includes an armature winding, an armature core, and a rectifier, none of which are shown in the

drawing, and is disposed on an inner side of the field generation unit 3 to be free to
rotate.
[0011] The field generation unit 3 is disposed on an outer side of the armature 2,
and includes a yoke 30, a plurality of permanent magnets 31, a plurality of field coil
units 32, and a plurality of holders 33.
[0012] The yoke 30 is used to form a magnetic circuit, and is formed from a
cylindrical magnetic member. The permanent magnets 31 and the field coil units 32
are disposed on an inner periphery of the yoke 30 at equal intervals in a
circumferential direction of the yoke 30.
[0013] The permanent magnets 31 each have an arc-shaped outer form, and
are disposed in contact with an inner peripheral surface of the yoke 30.
[0014] Each of the field coil units 32 is formed by molding a field core 32a and a
field coil 32b integrally using an insulating material 32c such that the field coil unit 32
has a similar arc-shaped outer form to the permanent magnet 31. The field core 32a
is a magnetic member having an outer form with a T-shaped cross-section. The field
coil 32b is a winding wound around the field core 32a. The insulating material 32c is
constituted by resin or the like, and acts to fix the field core 32a and the field coil 32b
to each other integrally.
[0015] The field coil unit 32 can be attached to and detached from the yoke 30
as the field coil unit 32. In other words, the field coil unit 32 is formed in advance,
before being mounted on the yoke 30, and the field core 32a of the field coil unit 32
does not extend directly from the yoke 30. Afield coil is typically formed by winding a
winding around a magnetic pole extending from a yoke. In this typical formation
method, the winding is wound around the magnetic pole in a narrow space, and
therefore outer dimensions of the field coil are likely to vary. The outer dimension

precision of the field coil unit 32 according to this embodiment is higher than that of the typical field coil described above.
[0016] As described above, each field coil unit 32 is formed by molding the field
core 32a and the field coil 32b integrally using the insulating material 32c, and therefore the same mounting method can be used to mount the permanent magnets
31 and the field coil units 32 on the yoke 30. The method of mounting the permanent magnets 31 and the field coil units 32 on the yoke 30 may be selected as desired, but in this embodiment, the permanent magnets 31 and the field coil units 32 are held on the inner periphery of the yoke 30 by the holders 33. The holders 33 are members having a C-shaped cross-section, and are respectively disposed between the permanent magnets 31 and the field coil units 32. Each holder 33 is compressed between the permanent magnet 31 and the field coil unit 32 in the circumferential direction of the yoke 30 so as to bias the permanent magnet 31 and the field coil unit
32 in the circumferential direction of the yoke 30. The permanent magnets 31 and the field coil units 32 are held on the inner side of the yoke 30 by the biasing force of the respective holders 33.
[0017] An angle a1 formed by respective ends of an arc region in which an
outer peripheral surface of the permanent magnet 31 of one pole contacts the inner peripheral surface of the yoke 30 and a center C of the yoke 30 is preferably set to be identical to an angle a2 formed by respective ends of an arc region in which an outer peripheral surface of the field coil unit 32 of one pole contacts the inner peripheral surface of the yoke 30 and the center C of the yoke 30. In other words, respective widths by which the permanent magnet 31 and the field coil unit 32 extend in the circumferential direction of the yoke 30 are preferably equal. With this configuration, the outer forms of the permanent magnet 31 and the field coil unit 32 can be aligned,

and the layout and mounting method of the permanent magnet 31, the field coil unit 32, and the holder 33 can be standardized more reliably. As a result, a further improvement in productivity and a further reduction in production costs can be expected. As long as equipment for mounting the permanent magnet 31 is provided, new equipment is not required to mount the field coil unit 32, and therefore equipment costs can also be suppressed. With the DC motor 1 according to this embodiment, when considering the feasibility of modifying the DC motor 1 from a configuration in which the field generation unit 3 is constituted by a single type of field generating means to a configuration in which the field generation unit 3 is constituted by a plurality of types of field generating means, the layout need not be modified, and therefore the time for consideration can be shortened.
[0018] In the DC motor 1, each field coil unit 32 is formed by molding the field
core 32a and the field coil 32b integrally using the insulating material 32c, and therefore the same mounting method can be used to mount the permanent magnets 31 and the field coil units 32 on the yoke 30. As a result, production costs and equipment costs can be suppressed.
[0019] Further, the angle a1 formed by the respective ends of the arc region in
which the outer peripheral surface of the permanent magnet 31 of one pole contacts
the inner peripheral surface of the yoke 30 and the center C of the yoke 30 is set to be
identical to the angle a2 formed by the respective ends of the arc region in which the
outer peripheral surface of the field coil unit 32 of one pole contacts the inner
peripheral surface of the yoke 30 and the center C of the yoke 30, and therefore the
layout and mounting method of the permanent magnet 31, the field coil unit 32, and
the holder 33 can be more reliably standardized.
[0020] Second Embodiment

FIG. 2 is a schematic sectional view of a DC motor 1 according to a second embodiment of this invention. As shown in FIG. 2, the configuration of the second embodiment is obtained by adding auxiliary magnetic poles 34 to the configuration of the first embodiment.
[0021] The auxiliary magnetic poles 34 are magnetic members having a
rectangular cross-section, and are disposed adjacent to respective magnetization
increasing sides of the field coil unit 32 and the permanent magnet 31 that generate
magnetic fields. By disposing the auxiliary magnetic poles 34 in this manner, an
increase in torque in a high current region and an increase in rotation speed in a low
current region can be achieved, and as a result, the DC motor 1 can be configured so
as to have characteristics resembling those of a series wound motor.
[0022] The auxiliary magnetic poles 34 together with the holders 33 hold the
permanent magnets 31 and the field coil units 32 on the inner periphery of the yoke 30. Moreover, the auxiliary magnetic poles 34 can be used for positioning so that the permanent magnets 31 and the field coil units 32 are held by the holders 33 while being pressed against the auxiliary magnetic poles 34, and as a result, improvements in mounting precision and mounting efficiency can be expected. All other configurations are identical to the first embodiment.
[0023] Hence, the auxiliary magnetic poles 34 may be added to the field
generation unit 3.
[0024] Third Embodiment
FIG. 3 is a circuit diagram showing a starter 4 that includes a DC motor 1 according to a third embodiment of this invention. The starter 4 shown in FIG. 3 includes the DC motor 1 of either the first or the second embodiment, and is used to start an engine, not shown in the drawing. A mechanical configuration of the DC

motor 1 is as shown in either FIG. 1 or FIG. 2.
[0025] As shown in FIG. 3, the starter 4 includes, in addition to the DC motor 1,
a battery 5, a start switch 6, a starter switch 7, and a short-circuit switch 8. The
battery 5 serves as a power supply for supplying electric power to the DC motor 1.
[0026] The start switch 6 is connected between the battery 5 and a connection
point between a drive coil 71 and a holding coil 72 of the starter switch 7, to be described below.
[0027] The starter switch 7 includes a normally open contact 70, the drive coil
71, and the holding coil 72. The normally open contact 70 includes first and second fixed contacts 70a, 70b, and a movable contact 70c. The first fixed contact 70a is connected to the battery 5, and the second fixed contact 70b is connected to the short-circuit switch 8 and the field coil 32b of the DC motor 1. The movable contact 70c connects the first and second fixed contacts 70a, 70b to each other when a current is passed through the drive coil 71 and the holding coil 72. The drive coil 71 and the holding coil 72 are connected in series between the second fixed contact 70b and the ground.
[0028] The short-circuit switch 8 includes a normally open contact 80 and a
drive coil 81. The normally open contact 80 includes first and second fixed contacts 80a, 80b, and a movable contact 80c. The first fixed contact 80a is connected to the second fixed contact 70b of the starter switch 7, and the second fixed contact 80b is connected to the armature 2 of the DC motor 1. The field coil 32b of the DC motor 1 is connected in parallel to the first and second fixed contacts 80a, 80b. The movable contact 80c connects the first and second fixed contacts 80a, 80b to each other when a voltage applied to one end of the drive coil 81 reaches a predetermined value. The drive coil 81 is connected between the second fixed contact 80b and the ground.

[0029] Next, an operation of the starter 4 shown in FIG. 3 will be described.
First, in a first step, the start switch 6 is closed in response to an engine
startup request, whereby a current is supplied from the battery 5 to the drive coil 71
and the holding coil 72 of the starter switch 7 through the start switch 6. At this time,
a current is also supplied to the armature 2 and the field coil 32b of the DC motor 1.
[0030] In a second step, the normally open contact 70 of the starter switch 7 is
closed (the first and second fixed contacts 70a, 70b are connected by the movable
contact 70c) by energizing the drive coil 71 and the holding coil 72. Here, the holding
coil 72 is connected to the battery 5 via the start switch 6, and therefore, when the
start switch 6 is closed, the holding coil 72 remains energized. As a result, the
normally open contact 70 of the starter switch 7 is kept closed.
[0031] Accordingly, rotary torque is generated in the DC motor 1, causing a
meshed pinion gear to start moving toward the side of a ring gear provided on a
crankshaft of the engine. When the normally open contact 70 of the starter switch 7
is closed, a current la flowing through the drive coil 71 substantially disappears, while
a current lb is supplied from the battery 5 to the field coil 32b and the armature 2
through the starter switch 7. As a result, rotary force from the DC motor 1 is
transmitted to the crankshaft of the engine, whereby startup of the engine begins.
[0032] In a third step, following the elapse of a predetermined time, a voltage
applied to one end of the drive coil 81 of the short-circuit switch 8 starts to increase in response to an increase in the rotation speed of the DC motor 1 due to the action of a reverse voltage generated after the DC motor 1 starts to rotate, and when this voltage reaches a predetermined value, a plunger of the short-circuit switch 8 is driven such that the normally open contact 80 of the short-circuit switch 8 is closed (the first and second fixed contacts 80a, 80b are connected by the movable contact 80c). As a

result, the field coil 32b is short-circuited such that only the permanent magnet 31
continues to generate a magnetic field. The short-circuit switch 8 is activated
automatically by the action of the reverse voltage, and therefore complicated control is
not required. Hence, the system can be simplified. Moreover, by setting the
voltage value at which the short-circuit switch 8 closes optimally, a timing of the short
circuit and a voltage reduction occurring during the short circuit can be adjusted.
[0033] FIG. 4 is a graph showing temporal variation in a battery voltage (a
power supply voltage) and a starter current with respect to FIG. 3. An inrush current that flows through the DC motor 1 when the starter switch 7 is closed in the second step (T2) described above is set as 11. When the voltage of the battery 5 is set as V0, a unique internal resistance of the battery 5 is set as RB, a resistance value of the field coil 32b is set as RF, an internal resistance of the DC motor 1 is set as RM, and respective wiring resistances are set as RW, the inrush current 11 is expressed by the following formula.
11 = Vo/(Rb +RW +RF + RM) Formula (1)
[0034] In the third step (T3) described above, the unique internal resistance RB
of the battery 5, the internal resistance RM of the DC motor 1, and the respective wiring resistances RW exist. Note that prior to the third step, a current flows through the DC motor 1, and therefore a reverse voltage E is generated by the DC motor 1 when the DC motor 1 starts to rotate.
When the current flowing through the DC motor 1 at this time is set as 12, 12 and a voltage drop of the battery 5 are expressed by Formula (2), shown below.
12 = (V0 - E)/(RB + RW + RM) Formula (2)
[0035] When a motor constant is set as k, a magnetic flux amount is set as O,
and the motor rotation speed is set as n, the reverse voltage E is expressed by

Formula (3), shown below.
E = kxOxn Formula (3)
In other words, the current 12 flowing through the DC motor 1 can be adjusting by varying the specifications.
[0036] When the voltage value at which the short-circuit switch 8 closes is set to
be high, a short circuit occurs only when the reverse voltage E generated by the DC motor 1 increases. In other words, when the reverse voltage E increases, a relationship of 11 > 12 is established in Formula (2), and in Formula (3), the short-circuit switch 8 is switched ON in a region where the rotation speed of the DC motor 1 is high. Hence, the short circuit is delayed, and as a result, engine startup (a period extending from T2 to T3) takes a longer time.
[0037] Therefore, by setting the voltage value at which the short-circuit switch 8
closes such that 11 and 12 are equal, as shown in FIG. 4, the amount of time required
for the short-circuit switch 8 to close can be shortened. As a result, power flickers
can be suppressed, and engine startup can be achieved more quickly.
[0038] According to this embodiment, the drive coil 71 of the starter switch 7,
the field coil 32b, and the respective wiring resistances exist on a circuit in the first step, the field coil 32b and the respective wiring resistances exist in the second step, and the respective wiring resistances exist in the third step. In other words, the electrical resistance decreases in three stages. Furthermore, the DC motor 1 starts to rotate prior to the third step in response to the current and the current lb, whereby the reverse voltage of the starter increases such that the currents are suppressed. Since the field coil 32b is used as a resistor, there is no need to provide a separate resistor, and therefore reductions in structural complexity and size can be achieved. Moreover, the current flowing through the field coil 32b can be reduced by the short

circuit, enabling a reduction in field magnetic flux such that a changeover from a
torque type characteristic to a rotary type characteristic is achieved, and as a result,
the engine can be started quickly. Hence, an operating period of the starter can be
shortened, and the quietness thereof can be improved. Furthermore, copper loss
caused by the field coil 32b is reduced, leading to an improvement in efficiency.
[0039] With the starter 4 according to this embodiment, therefore, first, when
the starter is started, the field coil 32b disposed between the power supply and the DC motor 1 is energized, and following the elapse of a predetermined time, the current flowing through the field coil 32b is reduced by the short-circuit switch 8. In so doing, the initial inrush current can be reduced, thereby suppressing a voltage reduction, and as a result, the engine can be started efficiently, without causing power flickers in in-vehicle electrical devices. Moreover, the short-circuit switch 8 is activated automatically by the reverse voltage, and therefore complicated control using a sensor for detecting rotation of the DC motor 1, a timer circuit, and so on is not required. As a result, the system can be simplified.
[0040] Furthermore, since both the permanent magnet 31 and the field coil 32b
are provided, the strength of the magnetic field can be increased, leading to an increase in motor torque, by energizing the field coil 32b, and therefore the engine can be started reliably even when a large amount of friction torque is generated thereby, such as at a low temperature, for example.

What is claimed is:
1. A DC motor comprising:
an armature (2); and
a field generation unit (3) disposed on an outer side of the armature (2),
wherein the field generation unit (3) includes a cylindrical yoke (30), and respective
pluralities of permanent magnets (31) and field coil units (32) mounted on an inner
periphery of the yoke (30), and
each of the field coil units (32) is formed by molding a field core (32a) and a field coil
(32b) integrally using an insulating material (32c).
2. The DC motor according to claim 1, wherein
an angle (a1) formed by respective ends of an arc region in which an outer peripheral surface of the permanent magnet (31) of one pole contacts an inner peripheral surface of the yoke (30) and a center (C) of the yoke (30) is set to be identical to an angle (a2) formed by respective ends of an arc region in which an outer peripheral surface of the field coil unit (32) of one pole contacts the inner peripheral surface of the yoke (30) and the center (C) of the yoke (30).