Multiphase DC motor and starting method thereof

A starting method for starting a multiphase DC motor having a stator provided with stator coils, a rotor provided with a rotor magnet and detector for detecting revolutions of the rotor, has stepping process and acceleration process. In the stepping process, a plurality of stepping steps is performed. The accelerating process is performed after the stepping process is completed. The plurality of the stepping steps include counter-exciting operation. Each of the plurality of stepping steps is provided by a rotation detection step. The stepping process is completed and the accelerating process is performed when a rotation detector detects that the revolutions of the rotor exceeds a predetermined value.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a multiphase DC motor and starting method 
thereof, and more particularly to an improvement for improving its 
starting characteristics. 
2. Related Art 
A multiphase DC motor has been conventionally used as a motor for 
rotationally driving a magnetic disk unit. This type of motor is also 
referred to as a spindle motor, and it is well known that a spindle motor 
comprises a stator having stator coils supplied with exciting currents, a 
rotor having a rotor magnet generating a rotational force from 
electromagnetic interaction between the rotor magnet and the stator coils, 
and a position detecting sensor for detecting the rotational position 
(rotational angle) of the rotor magnet. 
According to this type of motor, the rotational angle of the rotor magnet 
is detected by a position detecting sensor, and exciting currents to be 
supplied to the stator coils are switched in response to the detection 
signal from the position detecting sensor. A Hall element is used as the 
position detecting sensor. 
In recent years, a so-called sensorless multiphase DC motor has become 
popular in order to minituarize the motor and to prevent characteristics 
of the sensor from being deteriorated. This motor detects the position of 
the rotor magnet based on induced voltages in a coil in which exciting 
currents are not flowing, instead of using the position detecting sensor. 
No counter electromotive force can be obtained when the motor is stopped. 
Accordingly, in the sensorless motor, the rotor is swung when the motor is 
started. For example, in a three-phase spindle motor, a stepping process 
for sequentially supplying the exciting currents is repeated. During the 
stepping process, forward, resting and reverse exciting currents are 
supplied to respective phases. 
However, such a multiphase DC motor has the following technical problem, 
particularly with respect to its starting method. 
According to the multiphase DC motor, the position of the rotor magnet is 
detected due to the induced voltage. However, neither the voltage is 
induced nor the polarity of the magnet is known when the motor is stopped. 
Accordingly, the motor is forcibly started by generating a signal having a 
predetermined pattern. However, the motor may not start due to a low 
torque depending on the position of the rotor, or the rotor may be rotated 
reversely due to a magnetic field generated in a reverse direction when 
powered. 
In order to solve the above problem and improve the starting reliability, 
the inventor of the present invention has proposed a novel starting method 
for a sensorless motor as disclosed in U.S. Pat. No. 5,235,264. This 
starting method includes the step of reversing, in the starting time, an 
exciting current from the positive direction to the negative direction or 
vice versa without including the rest time. When the reversing step is 
performed, the magnetic flux changes drastically to produce a high torque. 
As a result, the starting reliability is improved. 
However, in the above starting method, the reversing step is performed only 
at one phase of the coil at a time so that a sufficient torque cannot be 
obtained. Thus, more improvement has been required. In addition, the time 
required for starting the motor is relatively long, and thus the reduction 
of the starting time has been demanded. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a multiphase DC motor and 
a starting method thereof which can obtain a sufficient starting torque 
when it is powered. 
Another object of the present invention is to provide a multiphase DC motor 
and a starting method thereof which can reduce the starting time. 
Further objects and features of the present invention will be understood 
readily from the following description. 
To achieve the objects, according to the first aspect of the present 
invention, the starting method for a multi-phase direct current (DC) motor 
includes: 
a plurality of stepping steps for supplying the exciting currents to stator 
coils to start the multi-phase direct current motor, each of the stepping 
steps including a step for performing a counter-exciting operation in 
which the exciting currents are reversed without a substantial rest time; 
rotation detection steps of detecting a rotation of the motor, each of the 
rotation detection steps being provided following a respective one of the 
plurality of stepping steps; and 
step of, when the rotation detection step detects that the number of 
revolutions (rotation speed) of the rotor does not reach a predetermined 
value, performing next stepping step, and of, when the rotation detection 
step detects that the number of revolutions of the rotor exceeds the 
predetermined value, completing the stepping process and performing an 
accelerating step of accelerating the rotor. 
According to the second aspect of the present invention, the multiphase 
direct current (DC) motor, includes: 
a stator provided with stator coils to which exciting currents are 
supplied; 
a rotor provided with a rotor magnet acquiring a rotation torque by 
electromagnetic interaction with the stator; 
bearing means inserted between the stator and the rotor; 
revolution detecting means for detecting number of revolutions of the 
motor; and 
control means, connected to the stator coils and the revolution detecting 
means, for controlling the rotation of the rotor by supplying the exciting 
currents to the stator coils to perform a stepping process and an 
accelerating process in a starting operation such that the exciting 
currents are supplied to each phase of the stator coils so as to perform, 
in the stepping process, a counter-exciting operation in which the 
exciting currents are reversed without including a rest time 
substantially, and the stepping process is completed when the rotation 
detecting means detects that the number of revolutions of the rotor 
exceeds a predetermined value. 
According to the present invention, in the counter-exciting operation, the 
exciting currents are reversed without substantial rest time. More 
specifically, the flowing directions of the exciting currents are reversed 
from the negative direction to positive direction or positive direction to 
negative direction without substantial rest time. Due to this, the 
starting probability of the motor is greatly improved.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The present invention will now be described in more detail with reference 
to the accompanying drawings. 
FIGS. 1 through 7C show a first embodiment of a multiphase direct current 
(DC) motor according to the present invention. 
As shown in FIG. 1, a multiphase motor has a bracket 10 which is mounted on 
a lower housing 12 of a magnetic recording apparatus. An upwardly 
protruding portion 16 is formed on the inner peripheral portion of an 
upward opened annular concave portion 14 of the bracket 10. A through hole 
18 extends vertically through the central portion of the upwardly 
protruding portion 16. The bracket 10 can be formed integrally with the 
lower housing 12. The lower end portion of an approximately cylindrical 
shaft 20 is fixedly inserted in the through hole 18 of the bracket 10 so 
that the shaft 20 is erected on the central portion of the bracket 10. 
The lower portion of the inner peripheral portion of a stator core 22 is 
fixedly mounted on the outer peripheral surface of the upwardly protruding 
portion 16. An annular spacer 24 for positioning the stator core 22 in an 
axial direction is disposed between the surface of the lower end portion 
of the inner peripheral surface of the stator core 22 and the bottom 
surface of the annular concave portion 14. 
The stator core 22 has a plurality of detents arranged circumferentially at 
intervals. A predetermined number of windings of a stator coil 23 is wound 
around each detent. In the embodiment, the stator coils 23 comprise 
three-phase coils u, v and w to which currents are supplied as described 
later. 
The lead 26 of the three-phase stator coils u, v and w is pulled downward 
through a lead hole 28 penetrating a bottom plate of the annular concave 
portion 14 of the bracket 10, and electrically connected to a flexible 
circuit board 30 which is connected to a control system 32 shown in FIG. 
2. Exciting currents from the control system 32 are supplied to the stator 
coils u, v and w. An annular thrust plate 34 protruding radially and 
outwardly is integrally formed on the upper portion of the shaft 20. The 
upper and bottom surfaces of the thrust plate 34 are formed perpendicular 
to the outer peripheral portion having approximately cylindrical surface 
of the shaft 20. 
A sleeve member 36 is approximately cylindrical and an outer diameter of 
the upper end portion of which is widened. The inner peripheral portion of 
the sleeve member 36 comprises an approximately cylindrical radial sliding 
portion 38 having a small diameter, an intermediate inner diameter portion 
40 which is widened at the upper portion of the radial sliding portion, 
and a large inner diameter portion 42 which is further widened at the 
upper portion of the intermediate inner portion 40. 
The sleeve member 36 is mounted on the shaft 20 from the bottom portion of 
the member 36 before the shaft 20 is fixedly inserted in the through hole 
18. An annular thrust holding plate 44 is inserted in the large inner 
diameter portion 42 of the sleeve member 36 with the inner peripheral 
portion of the thrust holding plate 44 being slightly apart radially from 
the shaft 20. The upper portion of the large inner diameter portion 42 is 
calked inwardly at four points with 90 degrees apart each other, for 
example so as to fix the thrust holding plate 44. The thrust holding plate 
44 is fixedly inserted in radially inwardly opened annular concave portion 
46 formed in the intermediate inner diameter portion 40 by the thrust 
holding plate 44 and the sleeve member 36. The lower portion of the shaft 
20 protrudes downwardly from the sleeve member 36. The shaft 20 is mostly 
engaged with the through hole 18 of the bracket 10. A portion 
corresponding to the radial sliding portion 38 of the outer peripheral 
portion having approximately cylindrical surface of the shaft 20 serves as 
a radial receiving portion. 
A rotor hub 48 made of a ferromagnetic material is approximately 
cylindrical. The rotor hub 48 is fixedly and coaxially mounted on the 
upper outer peripheral portion of the sleeve member 36. The lower end 
portion of the rotor hub 48 is inserted in the annular concave portion 14. 
An annular extending portion 50 for supporting the bottom surface of the 
inner peripheral portion of a hard disk is formed on the outer peripheral 
portion of the rotor hub 48. Note that the rotor hub 48 and the sleeve 
member 36 may be integrally formed. 
A cylindrical rotor magnet 52 facing to the stator core 22 with a gap in 
the radial direction is fixedly inserted in the inner peripheral portion 
of the rotor hub 48. A top surface of the rotor magnet 52 is in contact 
with the bottom surface of the upper outer peripheral portion of the 
sleeve member 36 so that the rotor magnet 52 is positioned in an axial 
direction. 
A herringbone groove 54 is formed in approximately upper half annular 
portion of the radial sliding portion 38 of the sleeve member 36. The 
herringbone groove 54 and the radial receiving portion of the shaft 20 
constitute a radial dynamic pressure bearing portion 56. The bearing 
portion 56 permits the filled liquid lubricant to produce a radial load 
bearing pressure by the rotation of the sleeve member 36 in a 
predetermined direction. The herringbone groove 54 may be formed in the 
radial receiving portion 38 of the shaft 20. Further, a groove other than 
the herringbone groove may be adopted. 
The upper and lower annular surfaces (axial receiving portion) of the 
thrust plate 34 and the upper and lower annular surfaces (axial sliding 
portion) of the annular concave portion 46 constitute the axial dynamic 
pressure bearing portions 58 anal 60. The upper and lower annular surfaces 
of the thrust plate 34 face parallelly to the upper and lower annular 
surfaces of the annular concave portion 46, and liquid lubricant is filled 
therebetween to form a slight gap in an axial direction. The herringbone 
groove (not shown) is formed entirely and circumferentially on the upper 
and lower annular surfaces of the thrust plate 34. The herringbone groove 
permits the lubricant filled between the upper and lower annular surfaces 
of the annular concave portion 46 to produce a high pressure by the 
rotation of the sleeve member 36 and the thrust holding plate 44 in a 
predetermined direction. The herringbone groove may be formed on the upper 
and lower annular surfaces of the annular concave portion 46 facing to the 
upper and lower annular surfaces of the thrust plate 36. Further, the 
groove other than the herringbone groove may be adopted. 
As described above, the sleeve member 36, the rotor hub 48, etc. are 
arranged to be freely rotated through the lubricant with respect to the 
shaft 20, the stator core 22 and the like. The displacement of the 
rotating sleeve member 36 with respect to the shaft 20 in the radial 
direction can be sufficiently constrained to a small value by the radial 
dynamic pressure bearing portion 56. Furthermore, the displacement of the 
rotating sleeve member 36 with respect to the shaft 20 in an axial 
direction can be sufficiently constrained to a small value by the axial 
dynamic pressure bearing portions 58 and 60. 
When the sleeve member 36 is rotated with respect to the shaft 20, the 
lubricant filled in the gaps between the sleeve member 36 and the shaft 20 
and the thrust holding plate 44 and the shaft 20 is introduced into the 
herringbone groove portion of the radial dynamic pressure bearing portion 
56 and the herringbone portion of the axial dynamic pressure bearing 
portions 58 and 60. The radial dynamic pressure bearing portion 56 permits 
the introduced lubricant to produce mainly the radial load bearing 
pressure, and the axial dynamic pressure bearing portions 58 and 60 permit 
the introduced lubricant to produce mainly the axial load bearing 
pressure. 
FIG. 2 shows a rotation control system 32 of the motor shown in FIG. 1. The 
control system 32 can obtain a remarkable advantage to be described later 
when used for a motor using the dynamic pressure bearing. However, a 
similar advantage can be achieved even when the control system 32 used for 
a motor using a normal ball bearing. 
The rotation control system 32 in FIG. 2 comprises a counterelectromotive 
force detecting circuit 70, a control circuit 72 (which constitutes 
control means), a driver circuit 74 (which constitutes driver means), a 
power circuit 76, a sequencer 78, and a stepping timer 80. The 
counterelectromotive force detecting circuit 70 detects the 
counterelectromotive force induced in the three-phase coils u, v, w 
(stator coil 23) in the rest time in which the exciting currents are not 
supplied. A detection signal from the counterelectromotive force detecting 
circuit 70 is input to the control circuit 72. The driver circuit 74 is 
connected to the output stage of the control circuit 72. The sequencer 78 
includes an exciting counter 82. 
The power circuit 76 receives an output signal from the driver circuit 74 
operating in accordance with command signal from the control circuit 72, 
and supplies the exciting currents to the coils u, v, and w in the form of 
a pattern based on an exciting pattern signal. The control circuit 72 
controls the steady operation of the motor after being started in 
accordance with the signal from the counterelectromotive force detecting 
circuit 70. 
The sequencer 78 generates preliminarily set internal stepping pattern 
signals in response to a control signal from the control circuit 72. In 
this embodiment, the internal stepping pattern signals in which 6 stepping 
steps of (i) u.fwdarw.v, (ii) w.fwdarw.v, (iii) w.fwdarw.u, (iv) 
v.fwdarw.u, (v) v.fwdarw.w, and (iv) u.fwdarw.w are repeated are generated 
with respect to the stator coils u, v, and w, as shown in FIG. 3. 
The exciting counter 82 changes the internal stepping pattern signals in 
accordance with the control signal from the control circuit 72. For 
example, when the exciting counter 82 is set to +1, the exciting pattern 
signals in which the stepping steps of (i) through (vi) shown in FIG. 3 
are repeated are supplied to the driver circuit 74. When the exciting 
counter 82 is set to +2, the exciting pattern signals in which the steps 
of (i), (iii), and (v) shown in FIG. 3 skipped every other step in a 
forward direction are repeated are supplied to the driver circuit 74. Note 
that a counter exciting operation to be described later can also be 
achieved by skipping the internal stepping pattern signals shown in FIG. 3 
every other step in the backward direction or skipping the internal 
stepping pattern signals every two steps in a forward or backward 
direction. 
In the control flow shown in FIG. 4, when the control circuit 72 starts 
upon receiving a start signal, the initialize operation is performed in 
step S1. Then, in step S2, the exciting counter 82 is set to 2, the 
stepping timer 80 is set to T1, and a repetition number n for supplying 
the exciting currents when powered is set. In this embodiment, n is set to 
6 but can be set to an arbitrary value. The supplement time period of the 
exciting currents is set and changed while taking account of the resonant 
frequency of the head system (such as a magnetic head for writing into 
and/or reading data from a recording medium and an arm supporting the 
magnetic head) or a drive system (a recording medium and a rotor hub 48 of 
the spindle motor) of the magnetic disk drive in which the motor is used. 
In step S3, the exciting currents are supplied to the coils u, v, and w in 
accordance with the contents set in step S2. According to the conditions 
set in step S2, the exciting counter 82 is set to +2, and the stepping 
timer 80 is set to T1. Therefore, the exciting currents are supplied to 
the coils in the order of u.fwdarw.w, w.fwdarw.v, and v.fwdarw.u during 
3T1 period, as shown in FIGS. 5A through 5C. 
When such exciting currents are supplied to the coils u, v, and w, the 
counter exciting operation is sequentially performed once in each phase in 
which a direction of the exciting current is reversed from the negative 
direction to the positive direction, as indicated by thick arrows shown in 
FIGS. 5A through 5C. According to the counter exciting operation of this 
embodiment, the directions of the exciting currents are reversed from the 
negative direction to the positive direction. However, the counter 
exciting operation may be achieved by reversing the exciting currents from 
the positive direction to the negative direction. Then, in step S4, it is 
determined whether or not the motor is rotated. This determination can be 
implemented by using a known zero-crossing method. 
When it is determined that the motor is rotated (the number of revolutions 
(rotation frequency or rotation speed) of the motor 20 reaches a 
predetermined value) in step S4, the stepping process is completed, the 
counter exciting operation is switched to a normal bipolar driving in step 
S5. In the bipolar driving, the exciting currents are sequentially and 
repetitively supplied to the stator coil 23 in the order of (i) 
u.fwdarw.w, (ii) u.fwdarw.v, (iii) w.fwdarw.v, (iv) w.fwdarw.u, (v) 
v.fwdarw.u, and (vi) v.fwdarw.w (step S6). In the accelerating step, the 
control circuit 72 controls the driver circuit 74 in accordance with the 
detection signal from the counterelectromotive force detecting circuit 70. 
Note that a normal unipolar driving may be adopted instead of the bipolar 
driving. 
When it is determined that the motor 20 is not rotated (the number of 
revolutions of the motor 23 does not reach a predetermined value) in step 
S4, the repetition number n is incremented by one in step S7 and it is 
determined whether or not the repetition number n is larger than 6 in step 
S8. If it is determined that the repetition number n is equal to or 
smaller than 6 in step S8, the flow returns to step S3 in which the 
stepping is advanced to the next step and the counter exciting operation 
is performed. 
If it is determined that the repetition number n is larger than 6, the 
repetition number n is set to 4 in step S9, and then the flow returns to 
step S3 in which the counter exciting operation is performed again from 
the fourth stepping step. Note that the value of the repetition number n 
set in step S9 is not limited to 4 and an arbitrary integer value within 
the range set in step S2 can be set. 
When the motor is started in accordance with the above described procedure, 
the counter exciting operation is sequentially performed in the coils u, 
v, and w in the stepping process as shown in the time charts of FIGS. 5A 
through 5C, whereby the stepping steps are repetitively performed 6 times 
at maximum. When the counter exciting operation is performed in the 
multi-phases, the variation of the magnetic flux density becomes large and 
thus the large and continuously increased torque is achieved. The increase 
of the torque is repetitively performed and thus the starting probability 
is improved. 
As a result, the starting current can be reduced and the power dissipation 
for starting the motor can be lowered compared to those of a conventional 
motor. Furthermore, the presence or absence of the rotation can be 
detected during the sequence, further reduction of the power dissipation 
for the start operation can be achieved. 
FIGS. 6 and 7 show a modification of the control procedure by the control 
system 32 and only the features thereof will be described below. 
According to the modification, the control steps executed by the control 
circuit 72 are different. More specifically, when the control circuit 72 
operates upon receiving the start signal, the initialize operation is 
performed in step S20, and the exciting counter 82 and the stepping timer 
80 are reset. In step S21, the exciting counter 82 is set to +2 and the 
stepping timer 80 is set to T2. Furthermore, the repetition number n for 
supplying the exciting current for the start operation is set. In this 
modification, n is set to 6 but may be set to an arbitrary integer value. 
Then, in step S22, the first stepping step is performed in accordance with 
the contents set in the step S21, and thus the exciting currents are 
supplied to the coils u, v, and w. According to the conditions set in the 
step S21, the exciting counter is set to +2, and the stepping timer 80 is 
set to T2. Therefore, the exciting currents flowing in the order of 
u.fwdarw.w, w.fwdarw.v, and v.fwdarw.u for the time of 3T2 are supplied to 
the coils in accordance with the exciting stepping pattern signals 
supplied from the sequencer 78 to the driver circuit 74, as shown in FIGS. 
7A to 7C. 
When such exciting currents are supplied to the coils u, v, and w, the 
exciting operation is sequentially performed once in each phase in which 
the direction of the exciting current is reversed from the negative 
direction to the positive direction without including the rest time, as 
shown in FIG. 7. In this case, the cycle of the exciting current to be 
supplied to the coil 23 first is preferably set to be approximately 
identical with, or divided frequency of (1/2, 1/3 . . . ), or n (n=2, 3 . 
. . ) times the resonant frequency of the head system or the drive system 
of the magnetic disk drive. Due to this, the head can be effectively 
floated over the recording medium when the motor is powered. Then, in step 
S23, it is determined whether or not the motor is rotated. This 
determination uses the known zero-crossing method as in the previous 
embodiment. 
If it is determined in step S23 that the motor is rotated, the stepping 
process is completed and then the accelerating step is executed. More 
specifically, the conventional bipolar driving is executed in step S23 and 
then the control is advanced to the accelerative driving (step S25). On 
the other hand, if it is determined in step S23 that the motor is not 
rotated, it is determined in step S26 whether or not the repetition number 
n is larger than 6. If it is determined in step S26 that the repetitive 
number n is equal to or smaller than 6, a predetermined time t is added to 
the set time T2 of the stepping timer 80 in step S27. Furthermore, the 
repetition number n is incremented by one in step S28, and the flow 
returns to step S22 in which the counter exciting operation is performed 
again. Although being different depending on a type of the motor, the set 
time T2 and the predetermined time t may be set approximately 18 msec and 
2 msec, respectively. 
If it is determined in step S23 that the motor is not rotated, the stepping 
steps are repeated 6 times at maximum. Each time the flow passes the step 
S27, the set time of the timer 80 is incremented by t, resulting in that 
the time for supplying the exciting currents in FIG. 8, the control system 
102 includes: a power circuit 104 to which one terminals of the stator 
coils u, v, and w are connected; a driver circuit 106 the output stage of 
which is connected to the power circuit 104; a control circuit 108 an 
output stage of which is connected to the driver circuit 106; a sequencer 
110 connected to the output stage of the control circuit 108; and a 
stepping timer 112. The sequencer 110 includes an exciting counter 114. 
Upon receiving the output signal from the driver circuit 106 operating in 
accordance with the command from the control circuit 108, the power 
circuit 104 supplies the exciting currents in the form of the pattern 
signals set by the sequencer 110 to the coils u, v, and w. The control 
circuit 108 controls the starting operation of the motor and the rotation 
of the motor after being started. Upon receiving the control signal from 
the control circuit 108, the sequencer 110 supplies the preliminary set 
pattern signals to the driver circuit 106. In the stepping process, the 
sequencer 110 generates the internal stepping pattern signals, changes the 
exciting stepping pattern signals in response to the signal from the 
control circuit 108, and supplies them to the driver circuit 106. Further, 
in the rotational driving (accelerating process and constant speed 
process), the sequencer 110 generates the internal drive pattern signals, 
changes the the counter exciting operation is sequentially increased by 
the predetermined time t (see FIG. 7). If it is determined in step S26 
that the repetition number n is larger than 6, the flow returns to step 
S22 in which the counter exciting operation is performed again from the 
fourth stepping step. Before return to step S22, the set time of the 
stepping timer 80 is decreased by 9t in step S29 (when n=6, the set time 
will be (3T2+15t) and thus 9t is subtracted from (3T2+15t)). Furthermore, 
the repetition number n is decreased by 3 in step S30. Accordingly, the 
repetition number n starts from 4 when the flow returns to step S22. 
When the motor is started as described above, the counter exciting 
operation is performed in each coil u, v, and w as shown in the time 
charts of FIGS. 7A through 7C, and the stepping steps are repeated 6 
times. Each time the stepping steps are repeated, the supplement time of 
the exciting current is gradually increased so that the starting 
probability can further be improved. When the number of revolutions of the 
rotor exceeds a predetermined value during the stepping process, the 
stepping process is completed and the control is advanced to the 
accelerating process. Accordingly, the time of the stepping process can be 
shortened resulting in reduced starting time. 
The motor shown in FIG. 1 can also be driven by the control system shown in 
FIGS. 8 through 11. In exciting drive pattern signals in response to the 
signal from the control circuit 108, and supplies them to the driver 
circuit 106. In the embodiment, the internal stepping pattern signals and 
the internal drive pattern signals are comprised of substantially 
identical pattern signals. These pattern signals are arranged such that 6 
steps of (i) u.fwdarw.v, (ii) w.fwdarw.v. (iii) w.fwdarw.u, (iv) 
v.fwdarw.u, (v) v.fwdarw.w, and (vi) u.fwdarw.w are repeated with respect 
to the stator coils u, v, and w. 
Upon receiving the signal from the control circuit 108, the exciting 
counter 114 changes the internal stepping pattern signal (or the internal 
drive pattern signal) in the sequencer 110 in the stepping process (or in 
the accelerating process and the constant speed process). For example, if 
the value of the exciting counter 114 is set to 11, the internal stepping 
pattern signals (or the internal drive pattern signals) are converted into 
the exciting stepping pattern signals (or the exciting drive pattern 
signals) such that the exciting currents in which the steps (i) through 
(vi) shown in FIG. 9 are repeated are supplied to the stator coils u, v, 
and w. Further, if the value of the exciting counter 114 is set to 2, the 
internal stepping pattern signals (or the internal drive pattern signals) 
are converted into the exciting stepping pattern signals (or the exciting 
drive pattern signals) such that the exciting currents in which the steps 
of (i), timer 112 is set to T3, T4, and T5. Accordingly, the internal 
pattern signals are skipped every two steps in a backward direction, and 
thus the exciting currents are supplied in the order of u.fwdarw.v, 
v.fwdarw.w, and w.fwdarw.u each for the time of T3, T4 and T5. 
When the exciting currents are supplied to each stator coils u, v and w, 
the counter-exciting operation in which the exciting currents are reversed 
from the negative to positive direction is sequentially performed in the 
coils u, v and w in the order of the stator coils u, v and w without 
including the rest time, as shown in FIG. 11. Due to this, the change of 
the magnetic flux density becomes large in the stator core. Accordingly, a 
higher torque is generated even in the rotation after started, and thus 
the starting probability of the motor is greatly improved. 
Further, in the previous process (stepping process), the set time of the 
stepping timer 42 is preferably set to T1&lt;T2&lt;T3&gt;T4&gt;T5 in which the set 
time is longest in the intermediate step and gradually shortened as the 
step advances toward the first step or the final step. Furthermore, the 
repetition frequency (1/set time T1-T5 of the stepping timer 112) of the 
exciting current in each step is preferably set to such a value that the 
frequency is gradually decreased from the natural oscillation frequency as 
the step advances toward the first step or (iii) and (v) are repeated 
while skipping every other step in a forward direction are supplied to the 
stator coils u, v, and w. 
The stepping timer 112 sets the supplement time of the exciting currents 
set by the exciting counter 114 in accordance with the signal from the 
control circuit 108. FIG. 10 shows an example of the control steps 
executed by the control circuit 108, and FIGS. 11A through 11C show time 
charts of the exciting currents supplied to the stator coils u, v, and w 
in the start operation and the constant speed operation of the motor. 
In the control steps shown in FIG. 10, the control circuit 108 operates 
upon receiving the start signal. First, the exciting counter 114 is set to 
+1 in step S41, and then the exciting counter 114 is set to +4 and the 
stepping timer 112 is set to T1, in step S42. Due to this, the exciting 
currents are supplied to the stator coils u, v, and w in the order of 
u.fwdarw.v, v.fwdarw.w and w.fwdarw.u, each for the time of T1 as shown in 
FIGS. 11A through 11C. 
In step S43, the exciting counter 114 is set to +4, and the stepping timer 
112 is set to T2. In this step too, the exciting currents are supplied for 
the time of T2 in the order of u.fwdarw.v, v.fwdarw.w, and w.fwdarw.u as 
in step S42, as shown in FIG. 10. In steps S4, S5, S6, the exciting 
counter 114 is set to +4 and the stepping the final step from the step 
having the natural oscillation frequency. 
By setting the frequency to the values described above, the starting of the 
motor can be performed more smoothly by the resonant operation. 
Note that the conduction time in each step of the stepping timer 112 may be 
set to be constant (T1=T2=T3=T4=T5) as a particular case. When the above 
conduction process is executed each for the time set by the stepping timer 
112, it is determined whether or not the motor is rotated in step S47. If 
it is determined in step S47 that the motor is not rotated, the flow 
returns to step S41, and the stepping process is repetitively performed by 
the same procedure. 
On the other hand, if it is determined that the motor is rotated in step 
S47, the flow advances to the next steps S8 and S9 in which the 
counter-exciting operation similar to that described above is performed to 
rotate the motor (accelerating process, constant-speed process). 
According to the control method of the multi-phase DC motor performed by 
the above-described procedure, the counter-exciting operation in which the 
conduction direction is reversed without including the rest time is 
sequentially performed in each phase in the start operation, resulting in 
greatly-increased torque due to the counter-exciting operation. Even after 
the motor is started and the rotation begins, the counter-exciting 
operation is sequentially performed in a plurality of phases. Accordingly, 
the magnetic flux density is largely varied, resulting in higher torque 
even in the rotation, similar to the start operation. 
FIGS. 12A to 12C show a modification of the above-described control method, 
and the feature thereof will be described below. According to the 
modification, the exciting counter 114 is set to +2 in steps S2 through 
S6, S8 and S9 in the flowchart of FIG. 10 executed by the control circuit 
108. 
As is apparent from the pattern signals shown in FIG. 9, when the exciting 
counter 114 is set to +2, the exciting currents are supplied in each step 
in the order of coils u.fwdarw.v, w.fwdarw.u, and v.fwdarw.w as shown in 
FIG. 12. In this case, the counter-exciting operation in which the 
exciting currents are reversed from the positive direction to the negative 
direction without including the rest time is sequentially performed in the 
order of the stator coils u, w, and v, as shown by the thick arrows in 
FIGS. 12A to 12C. Due to this, the starting probability of the motor is 
greatly improved as well as the torque at the constant-speed operation is 
increased. Thus, the same advantage as in the above embodiment can be 
achieved. 
In the above embodiments and modifications, the counter-exciting operation 
is performed by skipping every other internal pattern signals in the 
forward direction or every two internal pattern signals in the backward 
direction. However, the counter-exciting operation can also be performed 
by skipping every other internal pattern signals in the backward direction 
or every two internal pattern signals in the backward direction. 
In the above description, a case in which the present invention is applied 
to a three-phase DC motor has been exemplified. However, the present 
invention is not limited to the above embodiments and can be applied to 
other multi-phase motors.