Electrical generator or motor structure – Dynamoelectric – Rotary
Reexamination Certificate
2001-06-19
2002-10-15
Ramirez, Nestor (Department: 2834)
Electrical generator or motor structure
Dynamoelectric
Rotary
C310S179000, C310S184000, C310S198000
Reexamination Certificate
active
06465918
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a direct-current (DC) brushless motor, and a polygon scanner and an image forming apparatus having the same, and more particularly to a DC brushless motor which rotates a rotor by generating a rotating magnetic field while switching conduction to windings fixed on a stator core securely disposed in correspondence to the rotatable rotor having permanent magnets fixed thereon, a polygon scanner which has the DC brushless motor for rotating a rotating body having a polygon mirror fixed thereon to scan a laser beam for writing data, and an image forming apparatus for writing an image carrier or a photosensitive drum through a laser beam to form an image on the image carrier in an electrophotographic system. The present invention has particular applications in electrophotographic image forming apparatus suitable for a copying machine, a printer, a facsimile machine, a combination of these machines, or the like.
2. Discussion of the Background
Electrophotographic image forming apparatus employing a laser writing system for use in a digital copying machine, a laser printer, a facsimile apparatus, a combination of these machine, or the like have become rapidly more pervasive because of their high performance including a high printing quality, high speed printing capability, low noise and so on, as well as because of a reduction in price. A polygon scanner, which is a component of the laser writing system for these machines, is required to have the capability of rotating at an appropriate rotational speed corresponding to an image forming speed and a pixel density of the higher performance image forming apparatus.
Particularly, with an increasingly higher image forming speed and pixel density, the polygon scanner in the laser writing system is required to provide a high rotational speed exceeding 20,000 revolutions per minute, so that some conventional polygon scanners of a ball bearing type are not sufficient to satisfy a required quality in regard to an effective life of bearings, noise caused by the bearings, or the like.
For this reason, a polygon scanner employing a dynamic pressure air bearing has been proposed for higher rotational speeds by the same inventors as the present application (see for example Laid-open Japanese Patent Application No. 11-38346).
With such a polygon scanner having the capability of rotating at a higher rotational speed, power consumption is increased as the rotational speed is higher. Since a difference in efficiency between employed motors as driving power sources for polygon scanners noticeably appears in the difference in power consumption between the motors, a reduction in power consumption through an improvement in motor efficiency has been a critical issue.
For example, a DC brushless motor of a so-called radial gap inner rotor type is known (see Laid-open Japanese Patent Application No. 8-149775). Specifically, this type of DC brushless motor is composed of a rotor having permanent magnets fixed thereon, a stator core disposed outside the rotor with a predetermined spacing therebetween, a plurality of windings wound around the stator cores, and so on. A rotating magnetic field is generated by switching conduction to the windings to rotate the rotor.
Conventionally known stator cores used in DC brushless motors of this type may be classified into an open slot type, a half-open slot type, and a closed slot type. The open slot type stator rotor is described, for example, in Kokichi Ohkawa “Permanent Magnet Motor,” p185, published by Sogo Denshi Shuppan, 1975.
Since the closed slot type involves difficulties in a winding method and hence a higher manufacturing cost, the open slot type and the half-open slot type are generally considered more convenient due to their relatively easy winding operations.
In recent years, however, there is a tendency of preferentially manufacturing DC brushless motors of a so-called radial gap outer rotor type, which provide a higher production efficiency in a winding operation step than the radial gap inner rotors that employ the open slot type or half-open slot type stator rotor.
This is because the stator rotor used in the radial gap outer rotor type DC brushless motor has an open slot formed in an outer peripheral portion so that winding can be made more easily than the radial gap inner rotor type, which has an open slot in an inner peripheral portion.
In recent years, DC brushless motors of the radial gap outer rotor type have been widespread, and accordingly, a manufacturing cost thereof has been also reduced.
U.S. Pat. No. 5,382,853 discloses such a DC brushless motor of the radial gap outer rotor type, which includes a permanent magnet having four magnetized poles, six pole shoes and six windings.
Referring now to
FIG. 1
, which illustrates a conventional DC brushless motor
200
, a permanent magnet
201
has four poles formed of two pairs of two polarities, and is rotatably supported by a rotor
202
. A stator core
203
is disposed inside the permanent magnet
201
concentrically therewith.
The stator core
203
, made of a ferromagnetic material, is formed with six T-shaped pole shoes
203
a
, each of which is wound with a winding
204
. That is, six windings
204
are wound around the six pole shoes
203
a.
The windings
204
include three phases designated as a U-phase, a V-phase and a W-phase in
FIG. 1
, where a set of two windings U
1
, U
2
form the U-phase; a set of two windings V
1
, V
2
form the V-phase; and a set of two windings W
1
, W
2
form the W-phase.
A rotating position detecting mechanism
206
includes three rotating position detector elements
206
a
,
206
b
,
206
c
disposed at intervals of 60°, which generate rotating position detecting signals that are used by a driver circuit
205
(see
FIG. 3
) to switch conduction such that two phases are selected for conduction.
When the three rotating position detector elements
206
a
,
206
b
,
206
c
of the rotating position detecting mechanism
206
detect N, S, N poles, respectively, two phases, U-phase and V-phase, are selected and energized.
A current flows into the windings from the U
1
-phase and out of the V
1
-phase, causing the T-shaped pole shoe
203
a
wound with the U
1
-phase and U
2
-phase to have the S-polarity;
and the T-shaped pole shoe
203
a
wound with the V
1
-phase and V
2
-phase to have the N-polarity. Consequently, a magnetic repellent force or a magnetic attractive force acts between the permanent magnet
201
and the stator core
203
to rotate the permanent magnet
201
in the counter-clockwise direction as indicated by an arrow A in FIG.
1
.
Referring now to
FIG. 2
to explain how the windings
204
are wound around the respective pole shoes
203
a
. Viewed from the permanent magnet
201
, U
1
and U
2
in the U-phase of the winding
204
are wound in the same direction and connected to each other such that a current conducted therethrough causes the T-shaped shoe poles
203
a
, wound with windings U
1
, U
2
, to have the same magnetic polarity (see again FIG.
1
).
Similarly, V
1
and V
2
in the V-phase of the winding
204
and W
1
and W
2
in the W-phase of the winding
204
are wound in the same direction and connected to each other.
Referring next to
FIG. 3
, three winding groups
207
including the three sets of U-phase, V-phase, and W-phase windings
204
are connected in a Y-shaped connection configuration as generally indicated by reference numeral
208
. Each of the three U-phase, V-phase, and W-phase windings
204
in the groups
207
has one end connected to an associated driver circuit
205
, which switches the phases of the conducted windings
204
in accordance with rotating position detecting signals of the rotating position detecting mechanism
206
, not shown in FIG.
3
.
Referring next to
FIGS. 4A and 4B
, a description will be made on how the conduction is switched on the basis of a rotating position a detection made by the three rotating position detector elements
206
a
,
206
b
,
206
c
of
Itami Yukio
Suzuki Mitsuo
Dinh Le Dang
Ramirez Nestor
Ricoh & Company, Ltd.
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