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Claims  |
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What is claimed is:
1. A method for controlling a motor, comprising: a step of determining
speed-torque characteristics of a motor which is to be controlled, in a
state in which no field-weakening current is flowing and in a state in
which a field-weakening current is flowing at one level, respectively;
a step of determining a first torque command current by calculation from a
deviation between a speed command to the motor and a present speed of the
motor;
a step of determining a torque that develops at the present speed in the
state in which no field-weakening current is flowing on the speed-torque
characteristic and a torque that develops at the present speed in the
state in which the field-weakening current is flowed at one level on the
speed-torque characteristic, respectively;
a step of calculating torque currents corresponding to the torques
determined in the preceding step from the torques and torque constants of
the speed-torque characteristics; and
a step of comparing the first torque command current determined from the
deviation between the speed command to the motor and the present speed of
the motor, with the torque currents corresponding to the torques,
respectively, and determining and outputting a second torque command
current and a field-weakening current depending on whether or not the
first torque command current falls upon a field-weakening range of torque
currents corresponding to the torques.
2. The method as claimed in claim 1, wherein the motor is reduced in output
by heat generation of the motor and returned back to a normal state of the
motor by a system that utilizes field-weakening control to control the
motor, the method further comprising:
a step of gradually decreasing the torque current with a value of the
field-weakening current fixed;
a step of correcting the torque current while the field-weakening current
is reduced;
a step of holding a final state for a specific time period;
a step of gradually increasing the torque current level with the
field-weakening current value fixed; and
a step of correcting the torque current while the field-weakening current
level is increased.
3. The method as claimed in claim 1, wherein the motor is controlled by
controlling regenerative braking force, the method further comprising:
a step of calculating a required field-weakening current from the speed of
the motor and a speed-torque characteristic of the motor; and
a step of calculating a torque current from the calculated field-weakening
current value.
4. A method for controlling a motor, comprising:
a step of determining a torque command current by calculation from a
deviation between a speed control to a motor which is to be controlled and
a present speed of the motor;
a step of determining whether or not a sum of voltage vectors of the motor
is within a restrictive circle that depends on an applied voltage to the
motor, when the torque command current is set with no field-weakening
current flowing and at the present speed; and
a step of determining a field-weakening current and a torque current with a
field-weakening factor varied, when the sum of the voltage vectors of the
motor is out of the restrictive circle that depends on the applied voltage
to the motor, on a condition that the sum of the voltage vectors of the
motor is within the restrictive circle that depends on the applied voltage
to the motor, and that the torque current is made equal to a value
resulting from multiplying the torque command current determined from the
speed deviation, with the field-weakening factor.
5. The method as claimed in claim 4, wherein the motor is controlled by
controlling regenerative braking force, the method further comprising:
a step of calculating a required field-weakening current from the speed of
the motor and a speed-torque characteristic of the motor; and
a step of calculating a torque current from the calculated field-weakening
current value.
6. An apparatus for controlling a motor, comprising:
a means for determining a torque command current by calculation from a
deviation between a speed control to a motor which is to be controlled and
a present speed of the motor;
a means for determining whether or not a sum of voltage vectors of the
motor is within a restrictive circle that depends on an applied voltage to
the motor, when the torque command current is set with no field-weakening
current flowing and at the present speed; and
a means for determining a field-weakening current and a torque current with
a field-weakening factor varied, when the sum of the voltage vectors of
the motor is out of the restrictive circle that depends on the applied
voltage to the motor, on a condition that the sum of the voltage vectors
of the motor is within the restrictive circle that depends on the applied
voltage to the motor, and that the torque current is made equal to a value
resulting from multiplying the torque command current determined from the
speed deviation, with the field-weakening factor.
7. The apparatus as claimed in claim 6, wherein the motor is controlled by
controlling regenerative braking force, the apparatus further comprising:
a means for calculating a required field-weakening current from the speed
of the motor and a speed-torque characteristic of the motor; and
a means for calculating a torque current from the calculated
field-weakening current value.
8. An apparatus for controlling a motor, comprising:
a means for determining speed-torque characteristics of a motor which is to
be controlled, in a state in which no field-weakening current is flowing
and in a state in which a field-weakening current is flowing at one level,
respectively;
a means for determining a first torque command current by calculation from
a deviation between a speed command to the motor and a present speed of
the motor;
a means for determining a torque that develops at the present speed in the
state in which no field-weakening current is flowing on the speed-torque
characteristic and a torque that develops at the present speed in the
state in which the field-weakening current is flowing at one level on the
speed-torque characteristic, respectively;
a means for calculating torque currents corresponding to the torques
determined in the torque determining means from the torques and torque
constants of the speed-torque characteristics; and
a means for comparing the first torque command current determined from the
deviation between the speed command to the motor and the present speed of
the motor, with the torque currents corresponding to the torques,
respectively, and determining and outputting a second torque command
current and a field-weakening current depending on whether or not the
first torque command current falls upon a field-weakening range of torque
currents corresponding to the torques.
9. The apparatus as claimed in claim 8, wherein the motor is reduced in
output by heat generation of the motor and returned back to a normal state
of the motor by a system that utilizes field-weakening control to control
the motor, the apparatus further comprising:
a means for gradually decreasing the torque current with a value of the
field-weakening current fixed;
a means for correcting the torque current while the field-weakening current
is reduced;
a means for holding a final state for a specific time period;
a means for gradually increasing the torque current level with the
field-weakening current value fixed; and
a means for correcting the torque current while the field-weakening current
level is increased.
10. The apparatus as claimed in claim 8, wherein the motor is controlled by
controlling regenerative braking force, the apparatus further comprising:
a means for calculating a required field-weakening current from the speed
of the motor and a speed-torque characteristic of the motor; and
a means for calculating a torque current from the calculated
field-weakening current value.
11. The apparatus as claimed in claim 8, comprising:
an encoder provided to the motor;
a speed detection means for detecting a rotational speed of the motor from
an output signal of the encoder;
a position detection means for detecting a position of a rotor of the motor
from an output signal of the encoder;
an address generation means for generating a digital address signal
corresponding to the position of the rotor of the motor in response to an
output of the position detection means;
a speed control means for outputting a speed control command signal of a
torque current axis which is hereinafter, referred to as a q-axis and a
speed control command signal of a field-weakening current axis which is
hereinafter, referred to as a d-axis, which orthogonally cross each other,
in correspondence with a difference between a speed command input and an
output signal of the speed detection means;
a q-axis waveform storage means and a d-axis waveform storage means for
storing each one cycle of q-axis waveform data and d-axis waveform data
for use of driving the motor, and for receiving inputs of digital address
signals outputted from the address generation means as address inputs to
read the q-axis waveform data and the d-axis waveform data for driving the
motor, in correspondence with the rotor position of the motor;
a q-axis integrating D/A conversion means and a d-axis integrating D/A
conversion means for adding up a q-axis speed control command signal and a
d-axis speed control command signal outputted from the speed control means
to the q-axis waveform data and the d-axis waveform data for driving the
motor outputted from the q-axis waveform storage means and the d-axis
waveform storage means, respectively, and for performing D/A conversion
upon addition results;
a d-axis and q-axis addition means for adding and synthesizing output
signals of the q-axis integrating D/A conversion means and the d-axis
integrating D/A conversion means;
a current control circuit for outputting an error signal between an output
signal of the d-axis and q-axis addition means and a detection signal of a
load current flowing in the motor;
a pulse-width modulation (hereinafter, referred to as PWM) control circuit
for generating a pulse-width modulation signals correspondingly to an
output signal of the current control circuit; and
a PWM inverter for driving the motor with an output signal of the PWM
control circuit.
12. The apparatus as claimed in claim 11, further comprising:
a phase control circuit for storing phase correction data for correcting a
change in phase lag due to a change in the rotational speed of the motor
and for receiving an input of an output signal of the speed detection
means and for outputting an output of phase correction data corresponding
to the rotational speed of the motor; and
a q-axis addition means and a d-axis addition means for adding the phase
correction data outputted from the phase control circuit to the digital
address signal outputted from the address generation means and for feeding
addition results to the q-axis waveform storage means and the d-axis
waveform storage means.
13. The apparatus as claimed in claim 11, further comprising:
an induced voltage detector for detecting an induced voltage of the motor;
an induced-voltage decision means for comparing the induced voltage of the
motor detected by the induced voltage detector with a target induced
voltage and for making a decision in response to the comparison;
a phase control circuit for outputting phase correction data corresponding
to an output signal of the induced-voltage decision means;
a q-axis addition means and a d-axis addition means for adding phase
correction data outputted from the phase control circuit to the digital
address signal outputted from the address generation means and for feeding
an addition result to the q-axis waveform storage means and the d-axis
waveform storage means, whereby the induced voltage of the motor is
approximated to the target induced voltage.
14. The apparatus as claimed in claim 11, wherein the q-axis waveform data
and the d-axis waveform data for driving the motor, which have been stored
in the q-axis waveform storage means and the d-axis waveform storage
means, respectively, are waveform data of approximately a trapezoidal wave
in which a specified harmonic component has been added to a
fundamental-wave component.
15. The apparatus as claimed in claim 8, with which an electromobile having
a protection apparatus is provided, the protection apparatus protecting an
electromobile having a controller which is put into operation in response
to a turn-on of a key switch and which controls a speed of a motor by
controlling a flow of a torque current and a field-weakening current in
the motor from a primary battery;
the protection apparatus comprising a protective switch provided in
parallel to the key switch; and
the controller having a decision means which monitors the speed of the
motor, presence or absence of the field-weakening current, and its value,
and which decides whether or not an induced voltage of the motor has come
to such a state that an overcharging current is flowing in the primary
battery, when the key switch is turned off with the field-weakening
current interrupted; and a protective-switch drive means for turning on
the protective switch in response to an AND-condition of an overcharge
decision signal of the decision means and a turn-off of the key switch.
16. The apparatus as claimed in claim 15, wherein the protection apparatus
further comprises an alarm means which is activated by the
protective-switch drive means. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
The present invention relates to a method and an apparatus for controlling
a motor, by which, for example, a synchronous motor that serves as a power
source of an electromobile is controlled.
Based on the establishment of the vector control theory, it has been common
practice that d-axis current (hereinafter, referred to as I.sub.d current)
is controlled generally for more efficient control (for example, see
"Flux-Weakening Regime Operation of an Interior Permanent-Magnet
Synchronous Motor Drive" by Thomas M. Jahns of IEEE Transactions on
Industry Applications, Vol. IA-23, No. 4, July/August 1987, and European
Patent Publication No. 0 503 879 A2).
Recently, by making aggressive use of the I.sub.d current for the purpose
of the control of high-speed range rotation of a motor, the
field-weakening control is beginning to be introduced, in which the motor
is weakened in effective magnetic flux by the flow of the I.sub.d current
so as to be enabled to perform high-speed rotation.
FIG. 21 is a schematic arrangement view of an electromobile. In FIG. 21,
there are shown a car body 51, front wheels 52, rear wheels 53, a motor
54, a transmission 55, and a battery 56. A controller 57 receives inputs
of an acceleration signal and a brake signal. In this electromobile, the
motor 54 is operated with the battery 56 used as an energy source, the
drive force of the motor 54 being transferred to the rear wheels 53 via
the transmission 55. The motor 54 is controlled by the controller 57.
For electromobiles, the following advantages are obtained by controlling
the d-axis current.
Assume that the motor cannot increase its rotational speed beyond a certain
value (for example, 5000 rpm). As the electromobile speed becomes higher,
the rotational speed of the motor must be made higher in linkage with the
wheels. However, since the motor, having reached 5000 rpm, could not
increase its rotational speed any more, its transmission is exploited to
increase the speed.
This being the case, the d-axis current control (field-weakening control)
herein proposed, if effected, allows the motor, which only could rotate up
to 5000 rpm without field-weakening control (solid line A.sub.1) to be
rotatable up to, for example, 10,000 rpm by virtue of effecting the
field-weakening control (solid line A.sub.2), as shown in FIG. 22. As a
result, it can be expected to provide an electromobile without any
transmission. In FIG. 22, reference characters TT.sub.1 and TT.sub.1 /2
denote torques while the motor is at rest.
Also, further advantages can be expected, such as reduction in the cost due
to the omission of the transmission, and improvement in efficiency due to
the effect of the reduction in weight.
Moreover, if proper control is performed, more efficient control is
effected, which means that limited energy (battery) serves for more
efficiency, such advantages as extended running distances can be expected.
It is noted that if a motor that is rotatable up to 10000 rpm as the motor
characteristic was created, the motor would be too large in size, with a
greatly increased weight, to be adopted for the electromobile.
However, almost no means has been published for actually realizing the
field-weakening control.
SUMMARY OF THE INVENTION
Accordingly, the object of the present invention is to provide a method and
an apparatus for controlling a motor, by which field-weakening control is
performed so that the motor can be controlled with high efficiency.
In accomplishing these and other objects, according to a first aspect of
the present invention, there is provided a method for controlling a motor,
comprising:
a step of determining speed-torque characteristics of a motor which is to
be controlled, in a state in which no field-weakening current is flowing
and in a state in which a field-weakening current is flowing at one level,
respectively;
a step of determining a first torque command current by calculation from a
deviation between a speed command to the motor and a present speed of the
motor;
a step of determining a torque that develops at the present speed in the
state in which no field-weakening current is flowing on the speed-torque
characteristic and a torque that develops at the present speed in the
state in which the field-weakening current is flowing at one level on the
speed-torque characteristic, respectively;
a step of calculating torque currents corresponding to the torques
determined in the preceding step from the torques and torque constants of
the speed-torque characteristics; and
a step for comparing the first torque command current determined from the
deviation between the speed command to the motor and the present speed of
the motor, with the torque currents corresponding to the torques,
respectively, and determining and outputting a second torque command
current and a field-weakening current depending on whether or not the
first torque command current falls upon a field-weakening range of torque
currents corresponding to the torques.
According to a second aspect of the present invention, there is provided a
method for controlling a motor, comprising:
a step of determining a torque command current by calculation from a
deviation between a speed control to a motor which is to be controlled and
a present speed of the motor;
a step of determining whether or not a sum of voltage vectors of the motor
is within a restrictive circle that depends on an applied voltage to the
motor, when the torque command current is set with no field-weakening
current flowing and at the present speed; and
a step of determining a field-weakening current and a torque current with a
field-weakening factor varied, when the sum of the voltage vectors of the
motor is out of the restrictive circle that depends on the applied voltage
to the motor, on a condition that the sum of the voltage vectors of the
motor is within the restrictive circle that depends on the applied voltage
to the motor, and that the torque current is made equal to a value
resulting from multiplying the torque command current determined from the
speed deviation, with the field-weakening factor.
According to a third aspect of the present invention, there is provided an
apparatus for controlling a motor, comprising:
a means for determining speed-torque characteristics of a motor which is to
be controlled, in a state in which no field-weakening current is flowing
and in a state in which a field-weakening current is flowing at one level,
respectively;
a means for determining a first torque command current by calculation from
a deviation between a speed command to the motor and a present speed of
the motor;
a means for determining a torque that develops at the present speed in the
state in which no field-weakening current is flowing on the speed-torque
characteristic and a torque that develops at the present speed in the
state in which the field-weakening current is flowing at one level on the
speed-torque characteristic, respectively;
a means for calculating torque currents corresponding to the torques
determined in the torque determining means from the torques and torque
constants of the speed-torque characteristics; and
a means for comparing the first torque command current determined from the
deviation between the speed command to the motor and the present speed of
the motor, with the torque currents corresponding to the torques,
respectively, and determining and outputting a second torque command
current and a field-weakening current depending on whether or not the
first torque command current falls upon a field-weakening range of torque
currents corresponding to the torques.
According to a fourth aspect of the present invention, there is provided an
apparatus for controlling a motor, comprising:
a means for determining a torque command current by calculation from a
deviation between a speed control to a motor which is to be controlled and
a present speed of the motor;
a means for determining whether or not a sum of voltage vectors of the
motor is within a restrictive circle that depends on an applied voltage to
the motor, when the torque command current is set with no field-weakening
current flowing and at the present speed; and
a means for determining a field-weakening current and a torque current with
a field-weakening factor varied, when the sum of the voltage vectors of
the motor is out of the restrictive circle that depends on the applied
voltage to the motor, on a condition that the sum of the voltage vectors
of the motor is within the restrictive circle that depends on the applied
voltage to the motor, and that the torque current is made equal to a value
resulting from multiplying the torque command current determined from the
speed deviation, with the field-weakening factor.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other objects and features of the present invention will become
clear from the following description taken in conjunction with the
preferred embodiments thereof with reference to the accompanying drawings,
in which:
FIG. 1 is a speed-torque characteristic chart showing an algorithm of a
method for controlling a motor according to a first embodiment of the
present invention;
FIG. 2 is a speed-torque characteristic chart similarly showing the
algorithm of the method for controlling a motor according to the first
embodiment of the present invention;
FIG. 3 is a voltage vector diagram of motor sections showing an algorithm
of a method for controlling a motor according to a second embodiment of
the present invention;
FIG. 4 is a voltage vector diagram of motor sections similarly showing the
algorithm of the method for controlling a motor according to the second
embodiment of the present invention;
FIG. 5 is a schematic view showing the method of determining a weakening
factor;
FIG. 6 is a speed-torque characteristic chart showing the algorithm of the
method for controlling a motor according to a third embodiment of the
present invention;
FIG. 7 is a torque current-time characteristic chart similarly showing the
algorithm of the method for controlling a motor according to the third
embodiment of the present invention;
FIG. 8 is a torque-speed and time characteristic chart similarly showing
the algorithm of the method for controlling a motor according to the third
embodiment of the present invention;
FIG. 9 is a torque-speed characteristic chart showing the algorithm of a
method for controlling a motor according to a fourth embodiment of the
present invention;
FIG. 10 is a flowchart of the method of the first embodiment;
FIG. 11 is a flowchart of the method of the second embodiment;
FIG. 12 is a block diagram showing the arrangement of a motor controller
according to a fifth embodiment of the present invention;
FIG. 13 is a characteristic chart showing the relationship between the
number of revolutions, i.e.--rotational speed, and phase delay of a motor;
FIG. 14 is a block diagram showing the arrangement of a motor controller
according to a sixth embodiment of the present invention;
FIG. 15 is a waveform diagram of U phase and W phase terminal voltages
applied to the motor in the motor controller according to a seventh
embodiment of the present invention;
FIG. 16 is a waveform diagram of U-W phase terminal-to-terminal voltage
applied to the motor also of the seventh embodiment;
FIG. 17 is a waveform diagram showing terminal voltages of U phase and W
phase applied to the motor in the motor controller according to the
seventh embodiment of the present invention;
FIG. 18 is a waveform diagram of U-W phase terminal-to-terminal voltage
applied to the motor also of the seventh embodiment;
FIG. 19 is a block diagram showing the arrangement of an electromobile
having a protection apparatus according to a ninth embodiment of the
present invention;
FIG. 20 is a speed-torque characteristic chart of a motor for explaining
the disadvantage of overcharge of the primary battery;
FIG. 21 is a schematic arrangement view of an electromobile;
FIG. 22 is a torque-speed characteristic chart showing an aspect of the
field-weakening control of a motor;
FIG. 23 is a view showing the construction of an apparatus for controlling
a motor according to the first embodiment of the present invention; and
FIG. 24 is a view showing the construction of an apparatus for controlling
a motor according to the second embodiment of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Before the description of the present invention proceeds, it is to be noted
that like parts are designated by like reference numerals throughout the
accompanying drawings.
Hereinbelow, embodiments of the present invention are described with
reference to the accompanying drawings.
First Embodiment
A method for controlling a motor according to a first embodiment of the
present invention is described with reference to FIGS. 1, 2, and 10.
FIG. 1 is a speed-torque characteristic chart showing the algorithm for
determining the weakening current (field-weakening current) of a
synchronous motor in the method for controlling a motor according to the
first embodiment of the present invention. FIG. 10 is a flowchart of the
method. In FIG. 1, numeral 1 denotes a characteristic curve that shows
when no weakening current is flowing, i.e., that depends on the battery
voltage and the motor constant, where the resultant torque constant and
no-load rotational speed of the motor are assumed to be K.sub.T1 and
.omega..sub.1, respectively. Numeral 2 denotes a characteristic curve that
shows when a weakening current I.sub.d1 is flowing, where the resultant
torque constant and no-load rotational speed of the motor are assumed to
be K.sub.T2 and .omega..sub.2, respectively. Numeral 3 denotes a
characteristic curve that shows, like the characteristic curve 2, when a
weakening current I.sub.d2 is flowing, where the resultant torque constant
and no-load rotational speed of the motor are assumed to be K.sub.T3 and
.omega..sub.3, respectively.
An apparatus for controlling a motor according to the first embodiment of
the present invention which can carry out the method is suitably used for
an electromobile. As shown in FIG. 23, the apparatus includes a
speed-torque characteristics determining means 100 for carrying out step
#1 in FIG. 10, a first torque command current determining means 101 for
carrying out step #2, a torque determining means 102 for carrying out step
#3, a torque current calculating means 103 for carrying out step #4, and a
first torque command current comparing means 104 for carrying out steps
#5-#9.
Now the algorithm for determining the weakening current of the motor is
described below.
First, from a deviation between a speed command .omega.* and the present
speed .omega., a torque command current I*.sub.q0 is determined by
calculation.
The level of this torque command current I*.sub.q0 is I*.sub.q0 in FIG. 1,
and the broken line in the figure has a slope that depends on the
magnitude of the speed gain. It is noted that the figure is approximated
by a straight line for an easier understanding of the explanation.
Next, the torque generated at the present speed .omega. is determined from
the characteristic curves 1, 2, and 3 at step #1 in FIG. 10. From FIG. 1,
the torque for characteristic curve 1 is T.sub.1, the torque for
characteristic curve 2 is T.sub.2, and the torque for characteristic curve
3 is T.sub.3.
The relationship among torque constant K.sub.t, torque T, and torque
current i is given by Eq. 1. Therefore, from the torque constants K.sub.T1
to K.sub.T3 of the characteristic curves 1, 2, and 3 and the then
resulting torques T.sub.1 to T.sub.3, and necessary torque currents
I.sub.q1 to I.sub.q3 are calculated by using Eq. 2.
T=K.sub.t .times.i (Eq. 1)
I.sub.q1 =T.sub.1 /K.sub.T1, I.sub.q2 =T.sub.2 /K.sub.T2, I.sub.q3 =T.sub.3
/ K.sub.T3 (Eq. 2)
Then, by comparing the torque command current I*.sub.q0 determined from the
speed deviation at step #2 and the levels of the torque currents I.sub.q1
to I.sub.q3 that can be generated at the present speed at step #3 with
each other, the weakening current I*.sub.d and the torque command current
I*.sub.q are determined according to the following conditions at steps
#4-9 and outputted at step #11.
(1) I*.sub.q0 .ltoreq.I.sub.q1
torque command current: I*.sub.q =I*.sub.q0
weakening current: I*.sub.d =0
(2) I.sub.q1 <I*.sub.q0 .ltoreq.I.sub.q2
torque command current: I*.sub.q =I*.sub.q0 .times.(.omega..sub.2
/.omega..sub.1)
weakening current: I*.sub.d =I.sub.d1
(3) I.sub.q2 <I*.sub.q0
torque command current: I*.sub.q =I*.sub.q0 .times.(.omega..sub.3
/.omega..sub.1)
weakening current: I*.sub.d =I.sub.d2
It is noted that the weakening currents I.sub.d1 and I.sub.d2 are
calculated previously by using Eq. 3 from the motor constant and weakening
factor.
I.sub.d1 =(.phi./L).multidot.(1-1/n.sub.0)
I.sub.d2 =(.phi./L).multidot.(1-1/n.sub.1) (Eq. 3)
where n.sub.0 =weakening factor (=.omega..sub.2 /.omega..sub.1)
n.sub.1 =weakening factor (=.omega..sub.3 /.omega..sub.1)
L=inductance of motor
.phi.=effective magnetic flux of motor
For making a decision as to whether the torque command current I*.sub.q0
determined from the speed deviation is smaller or larger than the torque
currents I.sub.q1, I.sub.q2, and I.sub.q3 needed to produce a torque that
is generated at the present speed, it is preferable to provide a
hysteresis to the currents I.sub.q1, I.sub.q2, and I.sub.q3 so that the
torque command current I*.sub.q and the weakening current I*.sub.d can be
prevented from chattering. Further, it is also preferable that the width
of the hysteresis can be set independently of the currents I.sub.q1,
I.sub.q2, and I.sub.q3.
In the above description, it has been assumed that the weakening current is
flowing in two patterns. However, it is needless to say that even if the
weakening current is further increased in value for smoother rotation of
the motor, the above algorithm can be used to accomplish similar control.
Moreover, in the embodiment, when the weakening current I*.sub.d and the
torque command current I*.sub.q are determined, the above three conditions
are used. However, the following two conditions can be used: a first
condition where no weakening current flows as shown in the above condition
(1); and a second condition where a weakening current flows as shown in
the above condition (2) when I.sub.q2 is not considered therein.
The weakening current I.sub.d (as well as the torque current I.sub.q) is
determined by the above-described algorithm.
Nextly, since the maximum current value I.sub.max of the motor is
restricted by the switching device, the upper limit of the torque current
I.sub.q is restricted by the following equation at step #10. That is,
since
I.sub.max.sup.2 =I.sub.d.sup.2 +I.sub.q.sup.2 (Eq. 4)
the result is that
##EQU1##
Therefore, if the restriction of the torque current I.sub.q is added, the
speed-torque characteristic of the motor results in a stepwise
characteristic as shown in FIG. 2. In FIG. 2, a point B.sub.1 is the
position of a torque corresponding to that I.sub.max =I.sub.q, a point
B.sub.2 is the position of a torque corresponding to I.sub.q satisfying
that I.sub.max.sup.2 =I.sub.d1.sup.2 +I.sub.q.sup.2, and a point B.sub.3
is the position of a torque corresponding to I.sub.q satisfying that
I.sub.max.sup.2 =I.sub.d2.sup.2 +I.sub.q.sup.2.
In this connection, it is apparent from FIG. 2 that the larger the number
of patterns of weakening current, the smoother the rotation of the motor
can be obtained.
According to the method for controlling a motor of the first embodiment,
the weakening current can be determined by a simple method, and the motor
can be controlled with a high efficiency.
Second Embodiment
A method for controlling a motor according to a second embodiment of the
present invention is now described with reference to FIGS. 3, 4, and 11.
FIG. 3 is a vector diagram for explaining the algorithm for determining the
weakening current (field-weakening current) of a synchronous motor by the
method for controlling a motor according to the second embodiment of the
present invention. The diagram shows vectors of voltages of each part of
the motor when the torque current I.sub.q and the weakening current
I.sub.d flow to drive the motor. FIG. 11 is the flowchart of the method.
In FIG. 3, K.sub.t is the torque constant of the motor, L.sub.q is the
torque (current axis which is hereinafter referred to as a q-axis)
inductance of the motor, L.sub.d is the field-weakening (current axis
which is hereinafter referred to as a d-axis) inductance of the motor, R
is the resistance of the motor, P is the polar logarithm, .omega..sub.m is
the present speed, and V is the applied voltage to the motor.
Further in FIG. 3, (K.sub.t..omega..sub.m) is an induced voltage that the
motor develops when the motor is rotating at the speed of .omega..sub.m.
(I.sub.q.R) is a voltage that develops when the torque current I.sub.q has
flowed through the resistance component of the motor.
(.omega.L.sub.q.I.sub.q) is a voltage that develops due to the q-axis
inductance when the motor is rotating at the speed of .mu..sub.m.
(I.sub.d.R) is a voltage that develops when the weakening current I.sub.d
has flowed through the resistance component of the motor.
(.omega.L.sub.d.L.sub.d) is a voltage that develops due to the d-axis
inductance when the motor is rotating at the speed of .omega..sub.m.
In this case, the d-axis current is advanced in phase by 90.degree.
relative to the q-axis current.
An apparatus for controlling a motor according to the second embodiment of
the present invention which can carry out the method is suitably used for
an electromobile. As shown in FIG. 24, the apparatus includes a torque
command current determining means 110 for carrying out steps #41 in FIG.
11, a deciding means 111 for carrying out steps #52 and #53, and a
field-weakening factor and torque determining means 112 for carrying out
steps #42-51.
Now the algorithm for determining the d-axis current is described.
(A) Now, for the motor to rotate with the torque current I.sub.q, the
weakening current I.sub.d, and the rotational speed .omega..sub.m, the sum
of the vectors of the aforementioned developed voltages should be within a
restrictive circle for an applied voltage V of the motor in FIG. 3.
(B) The torque command current I*.sub.q0 is calculated from the deviation
between the command speed .omega.*.sub.e and the present speed
.omega..sub.e at step #41 in FIG. 11.
(C) Based on the calculated torque command current I*.sub.q0, a
determination is made as to whether or not the weakening current should be
flowed. First, at step #42, if the following expression is satisfied
(K.sub.t..omega..sub.e +I*.sub.q0.R).sup.2
+(.omega.L.sub.q.I*.sub.q0).sup.2 .ltoreq.V.sup.2 (Eq. 6)
then the weakening current I.sub.d =0 and the torque current I.sub.q
=I*.sub.q0 at step #43. At this time, the limitation process which is
similar to that at step #10 in the first embodiment is carried out at step
#44 and then the processed data is outputted at step #45. Also, if the
following expression is satisfied
(K.sub.t..omega..sub.e +I*.sub.q0.R).sup.2
+(.omega.L.sub.q.I*.sub.q0).sup.2 >V.sup.2 (Eq. 7)
then the weakening current flows. That is, first, a weakening factor n is
set at step #46 and then processes at step #47-#51 are carried out. That
is, at step #47, a determination is made as to whether or not the
weakening factor n is within a limited range. If yes, the process at step
#48 is carried out and if no, the process at step #50 is carried out.
(D) The weakening current I*.sub.d and the torque current I*.sub.q are
determined in the following way:
(D-1) First, at steps #48 and #50, the weakening current I*.sub.d is
determined by
I*.sub.d =(.phi./L.sub.d).(1-1/n) (Eq. 8)
where .phi. is the effective magnetic flux (=K.sub.t /P) of the motor and n
is the weakening factor (n>1).
(D-2) Next, the torque current I*.sub.q is determined in the following way.
Although the motor current is determined by
I.sup.2 =I*.sub.q.sup.2 +I*.sub.d.sup.2 (Eq. 9)
yet the motor current is limited to I.sub.max due to the upper limit of the
switching device For this purpose, I.sub.max.sup.2 .gtoreq.I*.sub.q0.sup.2
+I*.sub.d.sup.2 should be satisfied at step #51 after the process at step
#50 is carried out. If it is determined at step #51 that this equation is
not satisfied, then the weakening factor n is re-set at step #46. Then,
the torque current I*.sub.q0 is varied to satisfy the above equation, and
then at step #48, the torque current I*.sub.q to be outputted to the motor
is determined by
##EQU2##
When the process at step #48 is carried out, the torque current I*.sub.q
and the weakening current I*.sub.d are outputted at step #49.
(D-3) Next, if it is determined at step #51 that this equation is
satisfied, it is determined at step #52 whether or not the voltage V is
within the restrictive circle for the voltage V (described in (A)), by Eq.
11:
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