Brushless motor

Details Brushless motor Brushless motor

1999-12-21

2001-04-17

Nguyen, Tran (Department: 2834)

Electrical generator or motor structure

Dynamoelectric

Rotary

C310S049030, C310S256000

Reexamination Certificate

active

06218760

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a brushless motor for driving at least one media-disc such as a magnetic disc, more specifically, it relates to a brushless motor having less torque ripples, cogging and iron loss, and yet having a better torque constant by improving a shape of a stator-core.
BACKGROUND OF THE INVENTION
The brushless motor has been widely used in office automation devices and audio-video devices. Among various brushless motors, a polygon-mirror-scanner-motor employed in laser copying machines, a spindle-motor employed in magnetic-disc-driving-devices are directed to the higher-rotating-speed. The rotating speed of the polygon-mirror-scanner-motor is over 20,000 rpm, and that of the spindle-motor is as high as 12,000 rpm because a memory capacity has been increased and the higher rotating speed has been required.
The higher rotating speed entails the greater vibrations due to powering a motor coil and cogging, and these increased vibrations involve other problems. Regarding noises of the motor, the higher rotating speed induces the motor to rotate in an imbalance manner, which produces additional vibrations. Further, the higher rotating speed increases motor-loss thereby boosting the power consumption. The major loss in power consumption comprises windage loss, axial loss, iron loss and the like. The iron loss among others has two components, i.e. hysteresis loss and eddy-current-loss. In general, the hysteresis loss is proportionate to a number of rotations (more specifically, a current-frequency in the motor coil), and the eddy-current-loss is proportionate to a square of the number of rotations (more specifically, the current-frequency in the motor coil). The eddy-current-loss is thus increased at the greater number of rotations, so that the iron loss takes a greater part of the entire loss.
Smooth rotating is required for the motor to reduce vibration. To achieve this, it is necessary to lower the cogging and eliminate torque ripples. Skew magnetizing to a rotor magnet or laminating a plurality of core pieces forming a stator core is a regular measures to lower the cogging and eliminate the torque ripples.
On the other hand, the following two methods are employed to reduce iron loss:
1. Decreasing the iron loss of the stator core per se by replacing silicon-steel-plates which are laminated to the stator core with the plates having less iron loss, or by annealing the stator core; or
2. Decreasing a number of magnetized polarities thereby lowering the frequency of current running through the motor coil.
Conventional problems due to skewing provided to the rotor magnet and stator core are discussed hereinafter.
When the skewing is provided to the rotor magnet and stator core, torque ripples and cogging decrease; however, the motor efficiency as well as torque constant is lowered. In recent years, the market strongly demands a smaller device with the less vibrations and the lower noise and yet the device should keep the same torque-constant and the motor-efficiency as those of an original device. To achieve this request, the torque ripples and cogging of the motor per se should be reduced.
Japanese Patent Examined Publication No. 2636108 shown in
FIG. 5
already disclosed a method how to reduce cogging by providing skewed magnetization to the rotor magnet.
In this prior art shown in
FIG. 5
, the following formula is established:
(76°


)×0.8 ≦&thgr;2≦(76°


)×1.2
where
&thgr;2=skew angle;
2n=a number of magnet poles of the rotor magnet;
3n=a number of slots of the stator core.
The structure embodied by this formula allows the skew angle at magnetizing the rotor-magnet to be set so that the cogging and torque ripples are reduced without lowering the motor efficiency.
This prior art is effective only when a motor has some dimensional room, thus the prior art is difficult to apply to the motor to be smaller and thinner. In particular, when the motor becomes thinner and uses only a small number of core layers or the height of rotor magnet is too low to measure, this skewed magnetization produces no effects. In this case, neither lowering the cogging nor decreasing the torque ripples is expected, and the motor characteristics Kt (the torque constant) is aggravated.
The problems accompanying the conventional method of reducing the iron loss are described hereinafter.
A plurality of core pieces, i.e. silicon steel plates, are replaced with the plates having the less iron loss, or the plates undergo annealing. These are usual methods for decreasing the iron loss; however, these methods incur cost increase, and the annealed material is vulnerable to corrosion. An appropriate surface treatment is thus required.
Another method is disclosed in Japanese Patent Application Non-examined Publication No. H07-31085 shown in FIG.
6
. The method is to construct the stator core not by laminating a plurality of core pieces but by unitarily forming the stator core, so that the iron loss is reduced.
FIG. 6A
is a cross section of a conventional motor,
FIG. 6B
is a plan view of a stator core of the conventional motor, and
FIG. 6C
is a side view thereof.
In
FIG. 6A
, teeth
253
of stator core
250
are wound by coils
260
, and rotor magnet
280
is disposed around stator core
250
via an annular space. Magnet
280
is fixed to depending
274
of the rotor.
In
FIGS. 6B and 6C
, stator core
250
comprises the following elements:
(a) hole
251
for the stator core to be fixed;
(b) annular base
252
provided around hole
251
; and
(c) teeth
253
protruding from an outer wall of base
252
in a radius direction and being wound by coils
260
. End plates
254
protruding in the axial direction are formed on each tip of respective teeth
253
.
Stator core
250
comprises base
252
and teeth
253
. These elements undergo a press-process and a machining process thereby forming the stator core, and then the stator core undergoes annealing process. This method, i.e. forming the stator core not by laminating a plurality of core pieces but by forming unitarily the elements into the stator core before undergoing the annealing process, can reduce the iron loss.
This method, however, requires a number of processing steps such as press-process, machining-process, and thus produces unstable quality and no advantage of cost. Indeed this method can improve a saturated magnetic flux density; however, it produces only a little effect for reducing the iron loss. This method is difficult to apply to a tall-height motor, and aggravates the motor characteristics Kt if this method is employed.
SUMMARY OF THE INVENTION
The present invention addresses the problems discussed previously, and has the following two objectives:
1. To achieve a method solving the problems applicable with ease to any sizes of motors including a small and a thin sizes.
2. To realize a brushless motor with less torque ripples, cogging and iron loss as well as a better torque constant of motor characteristics.
The brushless motor of the present invention comprises the following elements:
(a) a rotor magnet including a plurality of magnetic poles; and
(b) a stator core facing the rotor magnet via air gap.
The stator core has a plurality of teeth wound by coils, and the respective teeth widths range from 1.7 mm to 2.2 mm inclusive both ends.
A ratio of outer diameter of the stator core vs. tooth width ranges from 8 to 12 inclusive both ends.
The structure discussed above allows the stator core to be produced not by complicated processes but by a press-process with ease and at a high accuracy. This structure also achieves less vibrations and low noise by reducing sources of vibrations such as cogging and torque ripples.


REFERENCES:
patent: 3860843 (1975-01-01), Kawasaki et al.
patent: 4575652 (1986-03-01), Gogue
patent: 5610464 (1997-03-01), Asano et al.
patent: 5798887 (1998-08-01), Yoshida et al.
patent: 5990592 (1999-11-01), Miura et al.
patent: 7-31085 (1995-01-01), None
patent: 2636108 (1997-04-01), None

Affiliated with

Also associated with

Rating

Brushless motor does not yet have a rating. At this time, there are no reviews or comments for this patent.

If you have personal experience with Brushless motor, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Brushless motor will most certainly appreciate the feedback.

Rate now

Comments { 0 }

Profile ID: LFUS-PAI-O-2555547