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Claims  |
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What is claimed is:
1. A method of directly driving a three-phase D.C. brushless motor having a
rotor and only one set of three phase stator windings with first, second
and third terminals by means of first, second and third bipolar driving
circuits, said method comprising the steps of:
(A) detecting each time said rotor rotates an electrical rotational angle
of 30 degrees; and
(B) controlling said first, second and third bipolar driving circuits to
successively and cyclically switch the polarities of electric potentials
applied to said first, second and third terminals of said stator windings
by said first, second and third bipolar driving circuits so that the
following conditions occur in order, each time a rotation of said rotor by
an electrical rotational angle of 30 degrees has been detected
(a) said first terminal is subjected to a negative potential, said second
terminal is subjected to a positive potential and said third terminal is
subjected to a negative potential;
(b) said first terminal is subjected to a negative potential, said second
terminal is subjected to a positive potential and said third terminal is
open;
(c) said first terminal is subjected to a negative potential, said second
terminal is subjected to a positive potential and said third terminal is
subjected to a positive potential;
(d) said first terminal is subjected to a negative potential, said second
terminal is open and said third terminal is subjected to a positive
potential;
(e) said first terminal is subjected to a negative potential, said second
terminal is subjected to a negative potential and said third terminal is
subjected to a positive potential;
(f) said first terminal is open, said second terminal is subjected to a
negative potential and said third terminal is subjected to a positive
potential;
(g) said first terminal is subjected to a positive potential, said second
terminal is subjected to a negative potential and said third terminal is
subjected to a positive potential;
(h) said first terminal is subjected to a positive potential, said second
terminal is subjected to a negative potential and said third terminal is
open;
(i) said first terminal is subjected to a positive potential, said second
terminal is subjected to a negative potential and said third terminal is
subjected to a negative potential;
(j) said first terminal is subjected to a positive potential, said second
terminal is open and said third terminal is subjected to a negative
potential;
(k) said first terminal is subjected to a positive potential, said second
terminal is subjected to a positive potential and said third terminal is
subjected to a negative potential; and
(l) said first terminal is open, said second terminal is subjected to a
positive potential and said third terminal is subjected to a negative
potential;
whereby a revolving magnetic field which rotates 30 degrees step by step is
produced.
2. A system for directly driving a three-phase D.C. brushless motor having
a rotor and only one set of three phase stator windings with first, second
and third terminals by means of first, second, and third bipolar driving
circuits, said system comprising:
means for detecting each time said rotor rotates an electrical rotational
angle of 30 degrees to produce a detection signal; and
means for controlling said first, second, and third bipolar driving
circuits to successively and cyclically switch the polarities of electric
potentials applied to said first, second and third terminals of said
stator windings by said first, second and third bipolar driving circuits
in response to said detection signal, so that the following conditions
occur in order:
(a) said first terminal is subjected to a negative potential, said second
terminal is subjected to a positive potential and said third terminal is
subjected to a negative potential;
(b) said first terminal is subjected to a negative potential, said second
terminal is subjected to a positive potential and said third terminal is
open;
(c) said first terminal is subjected to a negative potential, said second
terminal is subjected to a positive potential and said third terminal is
subjected to a positive potential;
(d) said first terminal is subjected to a negative potential, said second
terminal is open and said third terminal is subjected to a positive
potential;
(e) said first terminal is subjected to a negative potential, said second
terminal is subjected to a negative potential and said third terminal is
subjected to a positive potential;
(f) said first terminal is open, said second terminal is subjected to a
negative potential and said third terminal is subjected to a positive
potential;
(g) said first terminal is subjected to a positive potential, said second
terminal is subjected to a negative potential and said third terminal is
subjected to a positive potential;
(h) said first terminal is subjected to a positive potential, said second
terminal is subjected to a negative potential and said third terminal is
open;
(i) said first terminal is subjected to a positive potential, said second
terminal is subjected to a negative potential and said third terminal is
subjected to a negative potential;
(j) said first terminal is subjected to a positive potential, said second
terminal is open and said third terminal is subjected to a negative
potential;
(k) said first terminal is subjected to a positive potential, said second
terminal is subjected to a positive potential and said third terminal is
subjected to a negative potential; and
(l) said first terminal is open, said second terminal is subjected to a
positive potential and said third terminal is subjected to a negative
potential,
whereby a revolving magnetic field which rotates 30 degrees step by step is
produced.
3. A system as claimed in claim 2 wherein said controlling means includes a
current control circuit for controlling values of the currents through the
stator windings in response to said switching.
4. A system as claimed in claim 2 wherein said controlling means includes a
voltage control circuit for controlling values of voltages applied across
terminals of the stator windings in response to said switching.
5. A system as claimed in claim 2 wherein said controlling means includes a
regulated voltage circuit which continually applies a constant voltage
across terminals of the stator windings.
6. A three-phase D.C. brushless motor having a rotor and only one set of
three phase stator windings with first, second and third bipolar driving
circuits, said motor comprising:
means for detecting each time said rotor rotates an electrical rotational
angle of 30 degrees to produce a detection signal; and
means for controlling said first, second and third bipolar driving circuits
to successively and cyclically switch the polarities of electric
potentials applied to said first, second and third bipolar driving
circuits in response to said detection signal, so that the following
conditions occur in order:
(a) said first terminal is subjected to a negative potential, said second
terminal is subjected to a positive potential and said third terminal is
subjected to a negative potential;
(b) said first terminal is subjected to a negative potential, said second
terminal is subjected to a positive potential and said third terminal is
open;
(c) said first terminal is subjected to a negative potential, said second
terminal is subjected to a positive potential and said third terminal is
subjected to a positive potential;
(d) said first terminal is subjected to a negative potential, said second
terminal is open and said third terminal is subjected to a positive
potential;
(e) said first terminal is subjected to a negative potential, said second
terminal is subjected to a negative potential and said third terminal is
subjected to a positive potential;
(f) said first terminal is open, said second terminal is subjected to a
negative potential and said third terminal is subjected to a positive
potential;
(g) said first terminal is subjected to a positive potential, said second
terminal is subjected to a negative potential and said third terminal is
subjected to a positive potential;
(h) said first terminal is subjected to a positive potential, said second
terminal is subjected to a negative potential and said third terminal is
open;
(i) said first terminal is subjected to a positive potential, said second
terminal is subjected to a negative potential and said third terminal is
subjected to a negative potential;
(j) said first terminal is subjected to a positive potential, said second
terminal is open and said third terminal is subjected to a negative
potential;
(k) said first terminal is subjected to a positive potential, said second
terminal is subjected to a positive potential and said third terminal is
subjected to a negative potential; and
(l) said first terminal is open, said second terminal is subjected to a
positive potential and said third terminal is subjected to a negative
potential,
whereby a revolving magnetic field which rotates 30 degrees step by step is
produced. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of and apparatus for driving a
three-phase D.C. brushless motor.
2. Description of the Prior Art
Before describing a prior-art system for driving a three-phase D.C.
brushless motor, the relationship among the vector of the magnetic field
of a stator, the relative angle .theta. of the magnetic field of a rotor
and a torque T developing in the rotor will be explained. As shown in a
vector diagram of FIG. 7 A of the accompanying drawings, an angle 103
which the vector 102 of a rotor field defines relative to a stator field
101 is denoted by .theta., and a torque 104 which a rotor generates
counterclockwise on this occasion is denoted by T. Then, the relationship
between the relative angle .theta. and the torque T ordinarily becomes a
waveform whose period is 360 degrees as shown in FIG. 7 B. In addition, it
is common that the waveform approximates a sine wave as regards a D.C.
brushless motor fabricated so as not to degrade the efficiency thereof. It
is also well-known that, in a case where the rotor is multipolar, the
relative angle .theta. differs from the mechanical rotational angle of the
rotor. In the ensuing description, accordingly, angles concerning the
rotor shall indicate electrical rotational angles, and cases where
mechanical rotational angles are meant will be clearly stated as such.
FIG. 8 schematically shows the connection of a conventional thee-phase D.C.
brushless motor and a prior-art bipolar drive circuit which drives the
three-phase D.C. brushless motor while controlling current. As illustrated
in FIG. 8, this thee-phase D.C. brushless motor includes a rotor 217 and
three stator windings 201, 202 and 203 having phase differences of 120
degrees between the respectively adjacent ones. In addition, three
magnetic sensors 220 are arranged at intervals of 60 degrees so as to
detect the rotational angles of the rotor 217 at steps of 60 degrees. A
group of electronic switches 240 are disposed in association with the
stator windings 201, 202 and 203 and the magnetic sensors 220. Each of the
electronic switches is turned "on" (is closed) when a logic signal applied
thereto is "1", and it is turned "off" (is opened) when a logic signal
applied thereto is "0". Usually, these electronic switches are constructed
of power transistors etc. The prior-art bipolar drive system turns "on"
and "off" the electronic switches, thereby to select two of the three
stator windings in succession and to cause currents to flow through only
the two stator windings. Such ways of causing currents to flow number six
in total because there are three combinations of the two stator windings
to be selected, the direction of the current being reversible for each of
the combinations, and these six ways generate six sorts of magnetic fields
defining angles of 60 degrees between the adjacent ones as shown in FIG.
9(A), respectively. Accordingly, a revolving magnetic field at steps of 60
degrees can be formed by turning"on" and "off" the group of electronic
switches to change-over the currents of the stator windings in succession.
The motor is rotated by forming the revolving magnetic field in accordance
with the rotation of the rotor. That is, a logic circuit 218 decides the
positions of the rotor at the steps of 60 degrees on the basis of the
output signals of the three magnetic sensors 220 and turns "on" and "off"
in accordance with the decided positions, whereby the revolving magnetic
field is formed at the steps of 60 degrees. This signifies that the
currents are caused to flow through the stator windings in successive
change-over fashion so as to generate magnetic fields with which the most
effective torques are obtained at any angular positions of the rotor. To
provide a better understanding of the logical operation of FIG. 8,
reference is made to Table 2 which shows a truth table of logic circuit
218. The prior-art drive circuit is furnished with a current control
circuit or voltage control circuit 219, which produces a current or
voltage proportional to an instructed voltage 250. It is common to employ
the current control circuit 219 in a case where the torque is to be
controlled irrespective of the rotational frequency of the motor, and to
employ the voltage control circuit 219 in a case where the rotational
frequency is to be controlled irrespective of the load of the motor.
FIG. 10 shows the relationship of the torque T to the rotational angle
.theta. of the rotor in the case of adopting the prior-art drive method,
as stated above. The waveform of the torque T is indicated by a solid
line, and the fluctuation width of the torque is indicated by dT.sub.0.
Assuming that the torque waveform in FIG. 10 is a sine wave, the torque
fluctuation width dT.sub.0 becomes:
dT.sub.0 =1-sin 60.degree.=approximately 0.134
and the torque T drops about 13.4% with respect to the maximum value
thereof. Moreover, this corresponds to an assumed case where no error is
involved in the detection of the angular positions of the rotor, and in
actuality, an angular error a.degree. develops in the detection of the
angular positions of the rotor due to, e.g., the mounting errors of the
magnetic sensors or the like rotor position detector elements. FIG. 11
illustrates this situation, and a torque fluctuation width dT.sub.a in
this case becomes still greater as compared with dT.sub.0. That is,
assuming that the torque waveform in FIG. 11 is a sine wave, the torque
fluctuation width dT.sub.a becomes:
dT.sub.a =1-sin (60-a).degree.
By way of example, at a =3.degree., dT.sub.a =approximately, 0.161 and the
torque T drops about 16.1% with respect to the maximum value thereof.
The above description has concerned the static torque fluctuations, but
when the motor is being rotated, the transient characteristics of the
values of the currents flowing through the inductances of the windings
also need to be considered. With the prior-art drive system described
alone, current I having flowed through the inductance of a certain winding
is rendered null, while at the same time current I is suddenly caused to
flow through the inductance through which no current has flowed. In
actuality, however, the total value of the currents which flow into the
motor undergoes a great change on account of, e.g., the generation of a
counter electromotive force which is determined by the time constant of
the inductance and the rotational frequency of the motor. Due to this
change, a great dynamic torque fluctuation arises besides the
aforementioned static torque fluctuations at the time of the switching of
phases. In this manner, with the foregoing prior-art system for driving
the three-phase D.C. brushless motor, the overall torque fluctuation
becomes considerably great. Moreover, on account of the great torque
fluctuation, considerably loud noise has sometimes occurred during the
rotation of the motor.
As expedients for reducing such a torque fluctuation, it has heretofore
been proposed to improve the magnetic circuit of a motor and to improve a
driving method. Among the prior-art methods proposed, the method improving
the magnetic circuit of the motor includes a method which improves the
magnetization pattern of a rotor, a method which improves the
configuration of stator windings or the configuration of the cores of the
stator windings, and so forth. Any of these methods, however, crushes the
vicinity of the maximum value of a torque curve to thereby flatten the
torque fluctuation. Since the vicinity of the maximum value of the torque
curve is originally a part of raising the efficiency of the motor,
crushing this part incurs the disadvantage that the efficiency of the
motor is lowered.
On the other hand, the prior-art method of reducing the torque fluctuation
through the improvement of the driving method includes a method according
to which the correction values of torque fluctuation components
corresponding to the rotational angles of a rotor are recorded in a
read-only memory beforehand, and corrections are made so as to flatten the
torque fluctuation by referring to the recorded values, and so forth. Any
of these methods, however, premises the joint use thereof with means for
detecting the rotational angles at a high resolution and involves the
disadvantage of high cost. Moreover, with the methods, in a region where a
torque curve falls, current which enters a motor is relatively increased,
thereby to lift up a torque. The region where the torque curve falls is
originally a part in which the efficiency of the motor is low. Therefore,
when the current is increased here, a copper loss increases relatively, to
incur the disadvantage that the efficiency of the motor lowers.
An object of the present invention is to provide a method of and apparatus
for driving a three-phase D.C. brushless motor which can eliminate the
problems of the prior art as stated above.
Another object of the present invention is to provide a three-phase D.C.
brushless motor which can eliminate the problems of the prior art as
stated above.
SUMMARY OF THE INVENTION
According to an aspect of the present invention, there is provided a method
of driving a three-phase D.C. brushless motor having rotor and stator
windings of three-phases, the method including the steps of: (a) causing
currents to flow through stator windings of only two phases among the
stator windings of three phases; (b) causing currents to flow through the
stator windings of all the three phases; and (c) alternately switching to
the above step (a) or to the above step (b) each time the rotor rotates 30
degrees in terms of an electrical rotational angle, whereby torque
fluctuation can be reduced.
According to another aspect of the present invention, there is provided an
apparatus for driving a three-phase D.C. brushless motor having a rotor
and stator windings of three phases, the apparatus comprising detection
means for detecting an electrical angle of the rotor every 30 degrees, and
control means for alternately switching to the step of causing currents to
flow through stator windings of only two phases among the stator windings
of three phases or to the step of causing currents to flow through the
stator windings of all the three phases in response to the detection by
the detection means, whereby torque fluctuation can be reduced.
For a motor having only one set of star-connected stator windings with
three terminals, according to a feature of the present invention, at a
step of driving the motor, one of the three terminals is subjected to a
positive potential, another of the three terminals is subjected to a
negative potential, and the remaining one of the three terminals is open.
In this step, an electric current flows in series through the stator
winding connected between the positive terminal and the neutral point and
the stator winding connected between the neutral point and the negative
terminal, while no electric current flows through the stator winding
connected between the neutral point and the open terminal. Therefore, in
such step, an electric current flows through only two stator windings. At
another step of driving the motor, the terminal that was open at the
previous step is subjected to a positive or negative potential. This
results in two of the three terminals being subjected to the same
potential. Therefore, substantially the same electric currents flow in
parallel through the two stator windings connected between the neutral
point and the two same potential terminals, and an electric current equal
to the sum of electric currents flowing through the two stator windings
flows through the remaining stator winding. Namely, in this step, electric
currents flow through all the three stator windings. A stator magnetic
field, as produced when electric current flows through only two stator
windings, and a stator magnetic field, as produced when electric current
flows through all three stator windings, are out of phase by an electrical
rotational angle of 30 degrees.
For a motor having only one set of delta-connected stator windings with
three terminals, according to a feature of the present invention, at a
step of driving the motor, one of the three terminals is subjected to a
positive potential, another of the three terminals is subjected to a
negative potential, and the remaining one of the three terminals is open.
In this step, an electric current flows through the stator winding
connected directly between the positive and negative stator windings, and
an electric current equal to substantially one half of the above-mentioned
electric current flows through the remaining two stator windings connected
between the positive and negative terminals through the open terminal.
Therefore, in such step, electric currents flow through all three stator
windings. At another step of driving the motor, the terminal that was open
at the previous step is subjected to a positive or negative potential.
This results in two of the three terminals being subjected to the same
potential. While no electric current flows through the stator winding
connected between the same potential terminals, substantially the same
electric current flows in parallel through the remaining two stator
windings. Namely, in this step, electric current flows through only two
stator windings. The stator magnetic field, as produced when electric
current flows through all three stator windings, is out of phase by an
electrical rotational angle of 30 degrees.
As seen from the above, according to the present invention, a revolving
magnetic field which rotates 30 degrees step by step can be produced for
both a motor having star-connected stator windings and a motor having
delta-connected stator windings, by alternately switching from the step in
which electric currents flow through only two stator windings to the step
in which electric currents flow through all three stator windings.
Although the present invention can be applied to both a motor having
star-connected stator windings and a motor having delta-connected stator
windings as mentioned above only an embodiment of the present invention
applied to a motor having star-connected stator windings will be
described.
The control means may include a current control circuit which can change
values of the currents to flow through the stator windings, in response to
the switching, or a voltage control circuit which can change values of
voltages to be applied across terminals of the stator windings, in
response to the switching, or a regulated voltage circuit which
continually applies a constant voltage across terminals of the stator
windings, in spite of the switching.
According to still another aspect of the present invention, there is
provided a three-phase D.C. brushless motor having rotor and stator
windings of three phases, the motor containing detection means for
detecting an electrical angle of the rotor every 30 degrees so that the
motor can be driven by alternately switching to the step of causing
currents to flow through stator windings of only two phases among the
stator windings of three phases or to the step of causing currents to flow
through the stator windings of all the three phases each time the rotor
rotates 30 degrees in terms of an electrical rotational angle.
The present invention will now be described in further detail with regard
to preferred embodiments as illustrated in the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagram schematically showing the construction of an apparatus
for driving a three-phase D.C. brushless motor as an embodiment of the
present invention;
FIG. 2 is a diagram showing timings indicative of the relationships between
the torque fluctuation and control signals of the motor in the driving
apparatus of FIG. 1;
FIG. 3 is a diagram for explaining a torque fluctuation which would be
caused in the driving system of the present invention in a case where an
error is involved in the mounting position of a magnetic sensor element;
FIG. 4A, 4B, 5A and 5B are diagrams for explaining the vectors of revolving
magnetic fields which are generated by the driving system of the present
invention in FIG. 1;
FIG. 6 is a diagram for explaining a case where a torque curve is
distorted;
FIG. 7A and 7B are diagrams for explaining the relationship among the
magnetic field of a rotor, the vector of a revolving magnetic field and a
torque produced in the rotor, in a conventional three-phase D.C. brushless
motor;
FIG. 8 is a schematic diagram showing an example of a prior-art drive
circuit for a three-phase D.C. brushless motor;
FIG. 9 is a diagram for explaining the vectors of revolving magnetic fields
which are generated in a prior-art driving method and the driving method
of the present invention;
FIG. 10 is a diagram showing the variation of a torque which is produced in
a rotor in the prior-art driving method; and
FIG. 11 is a diagram for explaining the fluctuation of a torque which would
be caused in the rotor in the prior-art driving method in a case where an
error is involved in the mounting position of a magnetic sensor element.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Before describing the preferred embodiments of the present invention, the
principle of the driving system of the present invention, will be first
elucidated in comparison with the prior-art driving system described
above.
According to the principle of the present invention, even the prior-art
drive circuit in FIG. 8 can cause currents to flow through all the stator
windings by the use of the group of electronic switches 240 if it is
improved. More specifically, one of the terminals of the three stator
windings is connected to a power source side, another terminal is
connected to a ground side, and the remaining terminal is connected to the
power source side or the ground side, whereby the currents can be caused
to flow through all the three stator windings. On this occasion, two of
the three stator windings form parallel branches. Assuming that the
resistance values of the respective stator windings be substantially
equal, a current equal to a half of a current to flow through the
remaining stator winding flows through each of the two parallel branches.
The ways of causing the currents to flow number six, and form six
vectorial magnetic fields whose magnitudes are equal and whose directions
differ 60 degrees between the adjacent ones. The group of six vectorial
magnetic fields are shown in FIG. 9(B). As illustrated in FIG. 9(B), this
group of vectors shifts 30 degrees in direction with respect to the group
of vectors (refer to FIG. 9(A)) formed in the case of causing the currents
to flow through only two of the stator windings as stated in the
description of the prior art. When the two groups of vectors are
superposed after the magnitudes of the vectors thereof are substantially
equalized by adjusting current values in the case of causing the currents
to flow through all the three stator windings, twelve vectors whose
magnitudes are equal and whose directions differ 30 degrees between the
adjacent ones are formed as illustrated in FIG. 9(C). This indicates that
the group of switches 240 are successively changed-over to select the two
groups of vectors with the phase shifts of 30 degrees therebetween
alternately and in order, whereby a revolving magnetic field of steps of
30 degrees is established. That is, the group of switches 240 are
successively changed-over at intervals of 30 degrees in accordance with
the rotational positions of the rotor, and the rotor establishes the
revolving magnetic field with which the most effective and uniform torques
are always obtained, whereupon the motor can be rotated.
A torque fluctuation on that occasion becomes as indicated by a curve 601
of solid line in FIG. 2. Besides, a torque fluctuation width in that case
is indicated by dt.sub.0, which is seen to be considerably less than the
torque fluctuation width dT.sub.0 in the prior-art driving system.
More specifically, assuming that the torque curve 601 in FIG. 2 is a sine
wave,
dt.sub.0 =1-sin 75.degree.=0.034
which is sharply improved as compared with the torque fluctuation dT.sub.0
=0.134 in the prior-art system.
Next, the influence on the torque fluctuation will be described as to a
case where an angular error a exists in the detection of the angular
position of the rotor. As illustrated in FIG. 3, assuming the torque
waveform to be a sine wave, a torque fluctuation width is:
dt.sub.a =1-sin (75-a).degree.
By comparing this fluctuation width with the fluctuation width dT.sub.a in
FIG. 11, it is understood that the influence of the error of the angular
position detection of the rotor is also relieved. For example, at a=3,
dt.sub.a =1-sin (72.degree.)=0.049
which is sharply reduced in comparison with the torque fluctuation dT.sub.1
=0.161 for the same detection angle error in the prior-art system.
It is the basic principle ,of the present invention that, in this manner,
the step of causing currents to flow through only two of the stator
windings of three phases and the step of causing currents to flow through
all the three stator windings are alternately switched each time the rotor
rotates 30 degrees, whereby the torque fluctuation is decreased.
In FIG. 2, positions of marks o indicated by reference numeral 602 are the
angular positions of the rotor at which the winding currents are to be
switched in the driving system of the present invention, whereas positions
of marks x indicated by reference numeral 603 are the angular positions of
the rotor at which the winding currents are to be switched in the
prior-art driving system. As understood from the illustration, in the
prior-art driving system, the angular position of the rotor to change-over
the windings appears each time the rotor rotates 60 degrees, whereas in
the present invention, the angular position to change-over the windings
appears each time the rotor rotates 30 degrees, and it shifts 15 degrees
relative to the case of the prior-art driving system. In order to perform
the driving system of the present invention, accordingly, means capable of
directly detecting or estimating such angular positions of the rotor needs
to be comprised.
Now, the means for detecting the angular postions of the rotor will be
described. In FIG. 2, reference numeral 604 indicates signal outputs from
individual magnetic sensor elements in the case where the six magnetic
sensors each of which generates a binary signal in accordance with the
polarity of the magnetic field of the rotor are arranged at intervals of
30 degrees. Owing to these signals, it is possible to detect the twelve
possible angular positions of the rotor indicated by reference numeral 602
in FIG. 2, at 0 degrees, 30 degrees, 60 degrees, 90 degrees, 120 degrees,
150 degrees, 180 degrees, 210 degrees, 240 degrees, 270 degrees, 300
degrees and 330 degrees. In the prior-art driving system, the angular
position of the rotor may be detected every 60 degrees, and hence, the
three magnetic sensors at the intervals of 60 degrees suffice. On the
other hand, in the present invention, as understood from FIG. 2, the
magnetic sensors are arranged at the positions shifting 15 degrees on both
the right and left sides of the positions in the prior-art driving system
and therefore six magnetic sensors are required. As these magnetic
sensors, any of Hall elements, magneto resistance elements, etc. can be
used. As the means for detecting the angular position of the rotor every
30 degrees in this manner, there are also a method employing elements
other than the magnetic sensors, a method jointly using different
detection means, a method estimating the positions in relation to time,
etc., which will be explained later.
Next, there will be described methods by which, in the driving system of
the present invention, in alternately changing-over the case of causing
currents to flow through only two of the three stator windings and the
case of causing currents to flow through all the three stator windings,
the magnitudes of the vectors of the vector groups developed in the
respective cases are substantially equalized.
Considered as the first method is an expedient in which the values of the
currents to flow through the stator windings are switched by a single
current control circuit. As stated before, in order to equalize the
magnitudes of the vectors on the respective occasions of the group of
stator magnetic-field vectors (refer to FIG. 9(A)) formed in the case of
causing currents to flow through only two of the three stator windings and
the group of stator magnetic-field vectors (refer to FIG. 9(B)) formed in
tee case of causing currents to flow through all the three stator
windings, the values of the currents to flow through the stator windings
need to be changed in accordance with the respective cases. The extent of
a ratio at which the currents may be changed, will be elucidated.
As to the case of causing currents to flow through only two of the three
stator windings, the current value is denoted by I.sub.tw, and the
magnitude of the magnetic field vector generated is denoted by H.sub.tw.
Since these two quantities are proportional, the following holds with a
proportion constant denoted by K.sub.tw :
Ht.sub.w =K.sub.tw .times.I.sub.tw
On the other hand, as to the case of causing currents to flow through all
the three windings, the current value is denoted by I.sub.th, and the
magnitude of the magnetic field vector generated is denoted by H.sub.th.
Since these two quantities are proportional, the following holds with a
proportion constant denoted by K.sub.th :
H.sub.th =K.sub.th .times.I.sub.th
In order to establish uniform revolving magnetic fields, the current value
I.sub.tw and I.sub.th need to be selected so as to hold:
H.sub.tw =H.sub.th
and they need to be switched for the respective cases.
Next, there will be described a driving apparatus as the preferred
embodiment of the present invention for driving the three-phase D.C.
brushless motor on the basis of such a principle of the present invention.
FIG. 1 schematically shows the circuit connection of the driving
apparatus. As illustrated in FIG. 1, in this driving apparatus, magnetic
sensors 220 are arranged in a number of six, and the output signals
thereof are input to a logic circuit 230. The signals indicated by
reference numeral 604 in FIG. 2 are the output signals from the respective
magnetic sensors.
In accordance with the angular positions of the rotor detected from these
signals, the logic circuit 230 generates signals 210 for turning "on" and
"off" a group of electronic switches 240 so as to establish the revolving
magnetic field as steps of 30 degrees as explained before and
simultaneously generates a signal (indicated by reference numeral 606 in
FIG. 2 and FIG. 1) instructive of either the case of causing currents to
flow through all the stator windings or the case of causing currents to
flow through only two of the stator windings. Owing to this signal, an
electronic switch 204 changes-over instructive voltages V.sub.tw and
V.sub.th for a current control circuit 219. Table 1 is a truth table of
logic circuit 230. In FIG. 2, a section indicated by reference numeral 607
expresses the ways of causing currents to flow through the stator
windings. Regarding the terminals A, B and C of the stator windings,
letter H denotes the connection of the terminal to a power source side,
letter L denotes the connection thereof to a ground side, and letter F
denotes the floating or disconnection thereof. The current control circuit
219 is the same as in FIG. 8, and produces a current proportional to the
instructive voltage. Therefore, if the voltages V.sub.tw and V.sub.th are
set so as to produce the current values I.sub.tw and I.sub.th adapted to
hold H.sub.tw =H.sub.th as stated before, the revolving magnetic field of
uniform magnitude can be formed.
Then, setting such current values I.sub.tw and I.sub.th will be described.
It is assumed that all the stator windings have equal D.C resistances R.
It is also assumed that current I flowing through each stator winding and
the magnitude of a magnetic field generated be proportional, and that the
magnetic fields be vectorially combinable. That is,
H=K.times.I
where K denotes a proportion constant, which is assumed to have the same
value for all the windings.
FIG. 4 A illustrates a case where, by turning "on" and "off" the group of
electronic switches, the terminal of one of the Y-connected windings is
connected to the power source side, the terminal of another is connected
to the ground side, and the terminal of the remaining one is floated,
thereby causing currents to flow through two of the three windings.
Letting I.sub.tw denote the value of a power source current on this
occasion, equal currents of I.sub.tw flow through the stator winding 201
and the stator winding 202, respectively. Therefore, the magnitude
H.sub.tw of the combined magnetic field in this case becomes:
##EQU1##
as shown in FIG. 4 B. In addition, since the combined resistance of the
stator windings in this case is 2R, the copper loss of the motor becomes:
P.sub.tw =(I.sub.tw).sup.2 .times.2R (2)
FIG. 5 A illustrates a case where, by turning "on" and "off" the group of
electronic switches, the terminal of one of the Y-connected windings is
connected to the power source side, and the terminals of the remaining two
windings are grounded, thereby causing currents to flow through all the
three windings. Assuming that all the resistances of the stator windings
be equal, and letting I.sub.th denote the value of a power source current,
a current of I.sub.th flows through the stator winding 201, and currents
each being of I.sub.th /2 flow through the stator windings 202 and 203,
respectively. Therefore, the magnitude H.sub.th of the combined magnetic
field in this case becomes:
H.sub.th =3/2.times.K.times.I.sub.th (3)
as shown in FIG. 5 B. In addition, since the combined resistance of the
stator windings in this case is (3/2)R, the copper loss becomes:
P.sub.th =(I.sub.th).sup.2 .times.(3/2)R
For the purpose of establishing H.sub.tw =H.sub.th, accordingly, it is
understood from Eq. (1) and Eq. (3) that I.sub.tw and I.sub.th may be set
so as to hold the following relation:
##EQU2##
Besides, in the case where I.sub.tw and I.sub.th are set in this manner,
the copper loss becomes as follows in view of Eqs. (4) and (5):
##EQU3##
In view of this equation and Eq. (2), the following is held:
P.sub.th =P.sub.tw (7)
This signifies that the copper losses become equal in both the step of
causing the currents to flow through all the three windings and the step
of causing the currents to flow through only the two windings. Thus, it is
understood that, according to the driving system of the present invention,
the efficiency is equal to the efficiency in the prior-art driving method,
so the torque fluctuation can be reduced without sacrificing the
efficiency.
Next, there will be elucidated dynamic torque fluctuations in the case of
considering the transient characteristics of the values of currents
flowing through the inductances of the windings. With note taken of one
stator winding, in the prior-art driving system, the current value changes
between 0 and I.sub.tw. In contrast, in the driving system of the present
invention, the current value changes in the sequence of 0, (1/.sqroot.3)
I.sub.tw, I.sub.tw, (2/.sqroot.3) I.sub.tw, I.sub.tw, (1/.sqroot.3)
I.sub.tw, and 0, so that the transient fluctuation of the current value is
less than in the prior-art driving system. That is, in the prior-art
driving system, the current value changes from 0% to 100%, whereas in the
driving system of the present invention, the current value changes in the
sequence of 0%, 58%, 100%, 115%, 100%, 58% and 0%, and hence, the change
width of the current value is 58% at the maximum. Accordingly, the dynamic
torque fluctuations which arise due to the transient fluctuations of the
current values become less than in the prior-art driving system.
Under such a simplified assumption, the ration of I.sub.th to I.sub.tw may
be set at 2/.sqroot.3 times. In actuality, however, this value is not
always valid on account of the distortion of the torque, non-uniformities
in the windings and the magnetic circuits, and so on. Accordingly, the
ratio may be properly set so as to minimize the torque fluctuations,
depending upon the character | | |
