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 203a, each of which is wound with a winding 204. That is, six windings 204 are wound around the six pole shoes 203a.
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 U1, U2 form the U-phase; a set of two windings V1, V2 form the V-phase; and a set of two windings W1, W2 form the W-phase.
A rotating position detecting mechanism 206 includes three rotating position detector elements 206a, 206b, 206c disposed at intervals of 60.degree., 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 206a, 206b, 206c 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 U1-phase and out of the V1-phase, causing the T-shaped pole shoe 203a wound with the U1-phase and U2-phase to have the S-polarity; and the T-shaped pole shoe 203a wound with the V1-phase and V2-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 203a. Viewed from the permanent magnet 201, U1 and U2 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 203a, wound with windings U1, U2, to have the same magnetic polarity (see again FIG. 1).
Similarly, V1 and V2 in the V-phase of the winding 204 and W1 and W2 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 FIG. 4, 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 206a, 206b, 206c of the rotating position detecting mechanism 206 and the driver circuits 205 (FIG. 3) in response to the rotating position detection, as well as on a rotating magnetic field generated corresponding to switched conduction and the rotation induced by the rotating magnetic field of the rotor 202 having the permanent magnet 201 fixed thereon.
Specifically, FIG. 4 shows that the phases subjected to conduction are switched every 300 to generate a rotating magnetic field which causes the rotor 202 having the permanent magnet 201 fixed thereon to rotate in the counter-clockwise direction as indicated by an arrow A in FIG. 1.
As the rotor 202 is rotated over an angular distance of 180.degree., the conduction is switched six times by the driver circuits 205, shown in FIG. 3, so that the conduction is switched twelve times during a full rotation of the rotor 202.
In the conventional DC brushless motor, as well as a polygon scanner and an image forming apparatus employing this DC brushless motor, however, magnetic circuits passing through the stator core are formed between the U1-phase and V1-phase windings and between the U2-phase and V2-phase windings of the DC brushless motor, respectively. Therefore, magnetic flux concentrates on two regions around the magnetic circuits, so that driving torques acting on the permanent magnets fixed on the rotor also concentrate on the two regions, thereby limiting effective utilization of magnetic forces generated by the permanent magnet over the entire periphery of the rotor. Particularly, the DC brushless motor suffers from a low rotation driving efficiency at a higher rotational speed due to an air flow loss, a switching loss, and so on, so that the DC brushless motor disadvantageously requires a high power consumption and a large size to compensate for the low efficiency.