Brushless DC motor and a method of generation power therewith

An improved 10-pole 9-slot brushless DC motor used as a spindle motor used in devices, such as a computer hard disk driver or a laser scanner, is provided which reduces cogging torque and improves driving torque. The motor includes a field magnet having an annular permanent magnet which is formed of 10 magnetic poles attached to each other in a radial orientation, for generating a flux field and an armature formed by winding an armature core formed of nine slots and nine salient-poles with windings connected in three phases. Also, the field magnet of the motor is rotated by the rotating magnetic field generated by the armature windings. Since the motor has a short cycle and low cogging torque, the motor is very stable. Also, torque ripple is low and the torque characteristics as a whole are improved, thus achieving high efficiency.

BACKGROUND OF THE INVENTION 
The present invention relates to a brushless direct current (DC) motor used 
as a spindle motor in devices, such as a hard disk drive for a computer or 
a laser scanner, and more particularly, to a brushless DC motor having a 
low cogging torque and, at the same time, improved torque characteristics. 
The spindle motor used in hard disk drives or laser scanners is required to 
operate at high speeds, be small in size, and very precise. In particular, 
in the case of the hard disk drive, the recording data spacing on a disk 
becomes narrow according to the recent high-density trend, and thus 
repeatable/non-repeatable run-out become a serious problem due to the 
vibration of the spindle motor for driving a hard disk. In order to meet 
these requirements, the spindle motor is required to have a low cogging 
torque and, simultaneously, be highly efficient. However, the prior art 
does not satisfy both requirements. 
For example, a 12-pole 9-slot brushless DC motor usually used as a spindle 
motor comprises, as shown in FIG. 1, a field magnet 1 for generating a 
flux field, having twelve N and S magnetic poles alternately bonded in a 
radial orientation, an armature core 2 formed by alternately arranging 
nine salient-poles 3 and slots 4, nine armature windings 5 wound around 
each salient-pole 3, and a field yoke 6 surrounding the outer perimeter of 
the field magnet 1. 
The nine armature windings 5 are divided into three groups, and connected 
to each other to apply differing phase voltages to the respective groups. 
Referring to the connection diagram of FIG. 2, windings 5.sub.A1, 5.sub.A2 
and 5.sub.A3 to which phase voltage A is applied are sequentially wound 
around first, fourth and seventh salient-poles 3a, 3d and 3g, windings 
5.sub.B1, 5.sub.B2, and 5.sub.B3 to which phase voltage B is applied are 
sequentially wound around second, fifth and eighth salient-poles 3b, 3e 
and 3h, and the remaining windings 5.sub.C1, 5.sub.C2 and 5.sub.C3 to 
which phase voltage C is applied are sequentially wound around third, 
sixth and ninth salient-poles 3c, 3f and 3i. That is, as these armature 
windings sequentially generate rotating magnetic fields, torque is created 
between the flux passing between the rotating magnetic fields and the 
poles of the field magnet 1, and the field magnet 1 is rotated with 
respect to the armature core 2 together with the field yoke 6 by means of 
the generated torque. 
In this conventional brushless DC motor, the torque is generally dependent 
on the magnetic forces of the permanent magnet for generating the flux 
field. The problem is that cogging torque increases with an increase in 
torque, thereby enlarging torque ripple. 
The cycle of a cogging torque in degrees (.degree.) is expressed as: 
##EQU1## 
wherein C denotes the cycle, T.sub.c denotes cogging torque, L.C.M. 
denotes the least common multiple and MP.sub.n and S.sub.n denote the 
number of magnetic poles and slots, respectively. Under identical magnetic 
conditions of the field magnet, when the cogging torque cycle is long, 
considerable energy is required during one cycle, thus increasing the 
cogging torque; and, if the cycle is short, the cogging torque decreases. 
Accordingly, in the prior art, the cogging torque cycle was shortened by 
forming auxiliary slots on the salient-poles of the armature core or 
auxiliary salient-poles on the slots thereof, which resulted in a 
reduction in the cogging torque. However, the formation of auxiliary slots 
or auxiliary salient-poles involves a difficult process, and the coil 
winding becomes very complicated. 
In another example disclosed in U.S. Pat. No. 4,847,712, shown in FIG. 3, a 
spindle motor having low cogging torque is proposed in which the spindle 
motor is an eight-pole nine-slot brushless DC motor including a field 
magnet 11 having eight poles, an armature core 12 having nine 
salient-poles 13 and slots 14, nine armature windings 15 divided into A, B 
and C phases and wound around each salient-pole 13, which are connected to 
each other as shown in FIG. 4. Thus, the cogging torque cycle is shortened 
by as much as half of the 10.degree. of that of the 12-pole 9-slot motor, 
and the value of the cogging torque is also greatly reduced. However, as 
shown in FIG. 9, with the reduction of cogging torque, the torque 
performance degrades so that its efficiency is reduced by 10 to 20% as 
compared to that of the 12-pole 9-slot motor. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an improved brushless 
DC motor having low-cogging torque and high efficiency which improves 
torque performance without transforming the shape of the motor by forming 
additional auxiliary slots or salient-poles on an armature core. 
To accomplish the above object, there is provided a brushless DC motor 
which generates power from the torque generated between a flux field and a 
rotating magnetic field, comprising: means for generating the flux field; 
and means for forming the rotating magnetic field with respect to the flux 
field, wherein the means for generating the flux field includes an annular 
field magnet having ten magnetic poles alternately arranged in a 
ring-shaped form along a circumferential orientation and magnetically 
bonded to each other in a radial orientation, each having an opposing 
polarity and the means for forming the rotating magnetic field includes an 
armature core having nine salient-poles at regular intervals from the 
circumference opposing the surface of the magnetic pole of the field 
magnet, and nine slots each formed between the adjacent salient-poles, and 
a plurality of armature windings wound around each salient pole which is 
wound with at least one armature winding. 
Also, it is preferable that the plurality of armature windings are divided 
into three groups of windings wound around three consecutive salient-poles 
of the armature core, and connected in three phases to permit voltages of 
different phases to be applied to the respective groups.

DETAILED DESCRIPTION OF THE INVENTION 
A brushless DC motor according to an embodiment of the present invention 
which is shown in FIG. 5 is constituted by 10 poles and 9 slots, in which 
the number of windings around each salient pole is 57 turns, the width of 
a tooth is 3.6 mm, a torque constant is 139.6 cm/A, a line resistance 
between A to B, B to C or C to A shown in FIG. 7 is 5.50.OMEGA., the lines 
are connected as shown in FIG. 6, and each pole is made by magnetizing the 
material of NEOM10 with the capacitance of 200 .mu.F and the voltage of 
1200V. 
Referring to FIG. 5, a 10-pole 9-slot brushless DC motor according to the 
present invention includes an annular field magnet 21 having ten magnetic 
poles, as a means for generating a flux field, an armature core 22 formed 
of nine salient-poles 23 and nine slots 24, which function as a means for 
forming rotating magnetic fields, nine armature windings 25 wound around 
the nine salient-poles 23, and a field yoke 26 surrounding the outer 
perimeter of the field magnet 21. Here, the field magnet 21 is a permanent 
annular magnet which is formed by arranging ten magnets of the same size 
in a circumferential orientation. The ten magnets are attached to each 
other so that the N and S poles are alternating. The field magnet 21 is 
attached to the field yoke 26 and rotates with the field yoke. 
Referring to the connection diagram in FIG. 6, the nine armature windings 
are divided into three groups: the first group is comprised of windings 
25.sub.A1, 25.sub.A2 and 25.sub.A3 wound around the first three 
salient-poles 23a, 23b and 23c, the second group is comprised of windings 
25.sub.B1, 25.sub.B2 and 25.sub.B3 wound around the next three 
salient-poles 23d, 23e and 23f, and the last group is comprised of 
windings 25.sub.C1, 25.sub.C2 and 25.sub.C3 wound around the last three 
salient-poles 23g, 23h and 23i. Here, differing-phase voltages A, B and C 
are applied to the respective groups. Also, referring to FIG. 7, windings 
25.sub.A1, 25.sub.A2 and 25.sub.A3, 25.sub.B1, 25.sub.B2 and 25.sub.B3 and 
25.sub.C1, 25.sub.C2 and 25.sub.C3 to which phases A, B and C are 
respectively applied are formed in a three-phase Y-connection. 
The above 10 pole 9 slot brushless DC motor according to the present 
invention is driven by a driving circuit (not shown) constituted by a 
diode and a thyristor, as in the prior art. Also, in order for the motor 
to rotate smoothly, the present invention detects the position of a rotor 
(field magnet) by means of a counter electromotive force waveform induced 
from each winding upon rotation of the motor, and commutates the direction 
of current flowing in the coils of each phase whenever the rotor rotates a 
12.degree. interval. 
The phase to which current should initially be provided for starting the 
rotation of the motor is different depending on the position of the rotor. 
That is, if the current is initially provided from phase A to phase B, the 
next current is provided from phase A to phase C, then from phase B to 
phase C, then from phase B to phase A, then from phase C to phase A, then 
from phase C to phase B, and then from phase A to phase B. 
The results of employing the 10-pole 9-slot brushless DC motor as a spindle 
motor used in a hard disk drive, the cogging torque and the overall torque 
characteristics are described in FIGS. 8 and 9, respectively, with the 
characteristics of the conventional 12-pole 9-slot and 8-pole 9-slot 
motors. The values of the graphs can be expressed numerically by Tables 1 
and 2. 
TABLE 1 
______________________________________ 
Cogging Torque Characteristics 
10-pole 12-pole 8-pole 
Cogging Torque 
9-slot Motor 
9-slot Motor 
9-slot Motor 
______________________________________ 
Cycle 4.degree. 10.degree. 5.degree. 
Maximum Nm/m! 
0.0031 0.0605 0.0043 
______________________________________ 
TABLE 2 
______________________________________ 
Driving Torque Characteristics 
10-pole 12-pole 8-pole 
Driving Torque 
9-slot Motor 
9-slot Motor 
9-slot Motor 
______________________________________ 
Average Nm/m! 
0.6293 0.6133 0.5507 
______________________________________ 
Referring to TABLE. 1, in the 10-pole 9-slot brushless DC motor according 
to the present invention, the cycle of the cogging torque is shortened to 
40.degree.. Thus, the frequency of the cogging torque increases such that 
it has no influence on the system having a relatively low-frequency 
working environment. Thus, stability is enhanced due to reduced vibration 
upon driving. Also, it is not significant that the average value of the 
cogging torque becomes 0 during one rotation of the rotor. From comparing 
the values in Table 1, the maximum cogging torque of the 10-pole 9-slot 
brushless DC motor according to the present invention is reduced 
significantly over those of the conventional 12-pole 9-slot and 8-pole 
9-slot motors. 
Also, in view of the overall torque characteristics, the 10-pole 9-slot 
brushless DC motor according to the present invention has a torque ripple 
characteristic similar to that of the conventional 8-pole 9-slot motor. 
However, as shown in TABLE. 2, the average torque of the present invention 
is larger than that of the conventional 8-pole 9-slot motor, which proves 
the present invention to be highly efficient. 
As described above, the present invention provides a highly-efficient 
brushless DC motor having improved overall torque characteristics in spite 
of having low cogging torque. Also, this motor used as a spindle motor, in 
devices such as a hard disk drive or laser scanner, sufficiently satisfies 
the requirements of a spindle motor and also contributes to the enhanced 
stability of the system. In addition, the present invention makes the 
brushless DC motor very practical. 
The present invention is not limited to the example described above and 
shown in the drawings, and it is apparent that changes and transformation 
may be effected within the following scope of the claims.