PWM/PAM control mode switching type motor control apparatus, and motor drive and air-conditioner using the same

A motor drive apparatus and an air-conditioner using the motor control apparatus has a feature which allows it to gradually increase a dc voltage value to a predetermined value when starting the switching operation of a chopper circuit by dc voltage control thereof, and when stopping the switching operation of the chopper circuit to be able to gradually decrease the dc voltage value to a predetermined value. Further, the dc voltage command value is adjusted so as to make it possible to maintain a preferred value of dc voltage. Thereby, fluctuation in the number of revolutions of the motor can be prevented even if the converter is started or stopped while the motor is operating, thereby allowing the motor drive apparatus and air-conditioner using the motor control apparatus of the invention to be operated at their maximum capacities.

BACKGROUND OF THE INVENTION 
A conventional motor drive apparatus which combines a power circuit 
including a rectifying circuit for rectifying a.c. to d.c. and a means for 
suppressing harmonics in a power supply current with a motor drive circuit 
is described in JP-A Nos.6-105563 and 7-115788. 
The conventional motor drive apparatus is provided with a converter circuit 
of power factor improvement type using a booster chopper circuit which 
executes suppression of harmonics in the power source current and d.c. 
voltage control simultaneously, and an inverter circuit which drives the 
motor, and the same executes its motor speed control by controlling its 
d.c. voltage and inverter conduction ratio. 
SUMMARY OF THE INVENTION 
According to the prior art described above, when a load coupled to the 
motor is relatively light, the motor speed control is executed by 
controlling such that its d.c. voltage becomes minimum in a range which 
allows suppression of harmonics in the power source current, and at the 
same time by controlling the conduction ratio by means of the inverter. 
When a load coupled to the motor is relatively heavy, the motor speed 
control is executed, while holding the conduction ratio of the inverter at 
100%, by incrementing or decrementing the d.c. voltage by means of the 
converter. In other words, at the time of a light load, PWM control by 
means of the inverter is executed, and at the time of a heavy load, PAM 
control by means of the converter is executed. 
When the converter circuit used is of a type of the power factor 
improvement converter circuit, since it uses a booster chopper circuit, 
its d.c. voltage increases rapidly when a power consumption in the load 
coupled to the converter circuit drops. Therefore, when the power 
consumption in the load coupled to the prior art power factor improvement 
type converter circuit is not large enough, the operation of the converter 
is caused to stop. Namely, the converter is adapted to start its operation 
when the load becomes greater than a predetermined value. 
Therefore, according to the prior art motor speed control, the converter is 
caused to start or stop while the motor is in operation thereby causing 
its d.c. voltage to vary, thus varying the motor speed. 
In the prior art converter control circuit, in order to prevent occurrence 
of a surge in the d.c. voltage at the time of start-up and protect the 
converter, a process to gradually increase the conduction ratio of the 
booster chopper is used. 
However, no consideration has been made in the prior art as to a problem of 
variance in the number of revolutions of the motor resulting from a d.c. 
voltage fluctuation at the time of start or stop of the motor. 
In addition, the prior art motor control apparatus is associated with the 
following problems. 
While in the PAM control mode under a heavy load, when the load of the 
motor is increased further, a d.c. voltage value is increased 
significantly in the converter to a value in the vicinity of its maximum 
limit value specified by a circuit configuration of the converter. 
In the prior art motor drive apparatus, a range of operation is set for the 
motor drive apparatus such that the value of its d.c. voltage will not 
exceed its maximum limit value, or a limiter is set for command values of 
the d.c. voltage such that no command is issued exceeding its maximum 
limit value. Further, a protection circuit is provided for stopping 
operation of the motor drive apparatus in case the value of the d.c. 
voltage happens to exceed the maximum limit value thereof due to 
occurrence of abnormality or malfunctioning. 
In order to operate the motor drive apparatus at the maximum capacity 
thereof, it is necessary to maintain its d.c. voltage at a value in the 
vicinity of the maximum limit value thereof. In particular, for a motor 
drive apparatus which can operate in a wide range from a light load to a 
heavy load by means of the inverter control, it is necessary to provide 
for a high precision d.c. voltage control system which is independent of 
load conditions. Such high precision motor drive apparatus needs to have a 
complicated control system, and thus is costly. 
An object of the present invention is to provide for a motor control 
apparatus which can prevent variance in the number of revolutions of the 
motor at the time of start-up or stop of the converter during operation of 
the motor, and a motor drive apparatus and air-conditioner using the same. 
Another object of the invention is to provide for a motor control apparatus 
which has a d.c. voltage control system which is simple and not costly, 
and can operate at the maximum capacity thereof, and a motor drive 
apparatus and air-conditioner using the same. 
The present invention relates to a power circuit for controlling a d.c. 
voltage, a motor control apparatus for controlling the number of 
revolutions of the motor to a preferred speed, and an air-conditioner 
which uses the same motor control apparatus for driving a compressor motor 
thereof. 
Features of the invention for accomplishing the above-mentioned objects of 
the invention will be described in the following. A motor control 
apparatus according to the invention is comprised of: a converter circuit 
having a rectifying circuit, a smoothing circuit, and a chopper circuit 
which increments or decrements its d.c. voltage by means of switching 
operation of switching elements and an energy storage effect by 
inductance; an inverter circuit which is connected to an output side of 
the converter circuit; a d.c. voltage detection circuit which detects a 
d.c. voltage value at the output side of the converter circuit; a d.c. 
voltage control means which controls switching operation of the chopper 
circuit on the basis of an output value from the d.c. voltage detection 
circuit and a d.c. voltage command value such that the d.c. voltage value 
having been detected becomes equal to the d.c. voltage command value; an 
inverter control circuit which controls the switching operation of the 
inverter circuit which drives the motor; a speed control means which 
produces a d.c. voltage command and a conduction ratio signal to the d.c. 
voltage control means and the inverter control circuit to execute the 
speed control of the motor; and the motor. This motor control apparatus, 
when starting the switching operation of the chopper circuit, by means of 
the d.c. voltage control means is characterized by executing the steps of: 
enabling the switching operation of the chopper circuit; controlling a 
d.c. voltage value to be increased gradually to a predetermined value. 
When stopping the switching operation of the chopper circuit, the same is 
characterized by executing the steps of: controlling a d.c. voltage value 
to be decreased gradually to a predetermined value; then stopping the 
switching operation of the chopper circuit. 
According to these features of the invention described above, fluctuation 
of the d.c. voltage occurring at the time of start-up or stoppage of the 
converter can be suppressed, and a smooth d.c. voltage control can be 
realized. Further, when a motor drive circuit is coupled to the converter 
circuit as a load thereof, fluctuation of the motor speed can be 
suppressed thereby realizing a stable speed control thereof. 
Further, an improved d.c. voltage control corresponding to changes in the 
power supply voltage can be provided by the steps of: detecting a d.c. 
voltage of the converter during its stoppage; defining a value of this 
detected d.c. voltage as a d.c. voltage minimum value; and defining an 
initial value of a d.c. voltage command value to be applied at the time of 
start-up of the converter by adding an appropriate value to the d.c. 
voltage minimum value. 
Still further, an improved criterion for determining stoppage of the 
converter can be provided by the steps of: comparing a d.c. voltage and a 
command value corresponding thereto; judging whether or not the d.c. 
voltage is responsive to the command value; and stopping the converter 
when the d.c. voltage is no more compliant with the command value. 
The another feature of the motor control apparatus which is comprised of: a 
converter circuit which has the rectifying circuit for rectifying the a.c. 
power supply to d.c., the smoothing circuit, and the chopper circuit which 
increments or decrements the d.c. voltage using the switching operation of 
switching elements and the energy storage effect by inductance; the 
inverter circuit coupled to the output side of the converter circuit; the 
d.c. voltage detection circuit for detecting the d.c. voltage value at the 
output side of the converter circuit; the d.c. voltage control means for 
controlling the switching operation of the chopper circuit on the basis of 
the output value from the d.c. voltage detection circuit and the d.c. 
voltage command value such that the d.c. voltage value becomes equal to 
the d.c. voltage command value; the inverter control circuit for 
controlling the switching operation of the inverter circuit for driving 
the motor; the speed control means for controlling the motor speed by 
outputting the d.c. voltage command value and the conduction ratio signal 
to the d.c. voltage control means and the inverter control circuit; and 
the motor, resides in that by means of the d.c. voltage control means, the 
d.c. voltage command value is no more increased upon detection of the d.c. 
voltage detection value which exceeds a predetermined value. 
According to this another feature of the invention described above, without 
need of provision of a high precision d.c. voltage control system, it 
becomes possible to control the d.c. voltage at the maximum value thereof, 
thereby enabling the motor coupled to the motor control apparatus of the 
invention to be operated at the maximum output capacity. Further, 
independent of the load conditions, a constant d.c. voltage maximum value 
can be obtained. 
Still further, the d.c. voltage value can be controlled at a predetermined 
value by decrementing the d.c. voltage command value sequentially when the 
d.c. voltage detection value exceeds a predetermined value. 
Furthermore, a problem of an overshoot of the d.c. voltage value can be 
prevented by decreasing a value of increase to be applied to the d.c. 
voltage command value when the d.c. voltage detection value approaches to 
the vicinity of a predetermined value.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
EMBODIMENT 1 
A motor drive apparatus according to a first embodiment of the invention 
will be described in detail in the following. 
A schematic block diagram of the motor drive apparatus of the first 
embodiment of the invention is shown in FIG. 1. The motor drive apparatus 
of this embodiment of the invention is comprised of: a converter circuit 2 
which controls an amplitude of a d.c. voltage using a rectifying circuit 
and a booster chopper circuit; an inverter circuit 3 which inverts the 
d.c. voltage to an a.c. voltage with a predetermined value; a motor 
control means 8 which executes speed control of a brushless d.c. motor 4 
in response to a speed command; a position sense circuit 9 which senses 
positions of magnetic poles of brushless d.c. motor 4; a converter control 
circuit 6 which controls the converter circuit 2 on the basis of a 
correction d.c. voltage signal from the motor control means 8 and a 
converter ON/OFF signal; a driver 5 which drives inverter circuit 3 on the 
basis of a PWM signal and a drive signal from motor control means 8; and a 
current detection circuit 7 which detects a current from a.c. power supply 
1 and transmits the current detected to motor control means 8. 
FIG. 2 shows an internal configuration of motor control means 8 of the 
invention. Motor control means 8 executes speed control of brushless d.c. 
motor 4 on the basis of a speed signal which was computed from a position 
sense signal output from position sense circuit 9 and a speed command 
signal provided externally. Since the motor control means 8 of the 
invention uses a microcomputer, all of the arithmetic operations within 
the motor control means 8 are executed by software processing thereof. 
A position sense signal detected by position sense circuit 9 is input to a 
drive signal generator 83 and a speed calculation unit 84. Drive signal 
generator 83 produces a drive signal on the basis of the position sense 
signal. Speed calculation unit 84 computes a speed of the brushless d.c. 
motor 4 from the position sense signal, and also issues a motor stop 
signal when the motor is determined at stoppage. 
Converter operation determination unit 82 in response to an input current 
value from current detection circuit 7 outputs an operation enable signal 
only when the input current value exceeds a preset value. 
The d.c. voltage correction arithmetic unit 81 calculates a correction d.c. 
voltage Ed' on the basis of d.c. voltage value Ed output from converter 
circuit 2 and d.c. voltage command value Ed* output from d.c. voltage 
control arithmetic means 801. Here, correction d.c. voltage signal Ed' is 
calculated such that a certain value (d.c. voltage fixed command value 
Vr)is output when d.c. voltage value Ed coincides with d.c. voltage 
command value Ed*. More specifically, Ed' is calculated according to 
equation 1 as follows. 
EQU Ed'=(Ed/Ed*).times.Vr (1) 
where, Ed': d.c. voltage correction value, Ed: d.c. voltage detection 
value, Ed*: d.c. voltage command value, and Vr: d.c. voltage fixed command 
value. 
Necessity of calculation of the d.c. voltage correction according to 
equation 1 in d.c. voltage correction arithmetic unit 81 will be described 
in the following. 
Converter control circuit 6 is provided with a d.c. voltage control 
circuit, which is not shown. A d.c. voltage command value to this d.c. 
voltage control circuit is a fixed value (d.c. voltage fixed command 
value), thereby the d.c. voltage control of the d.c. voltage control 
circuit is enabled by changing the detection gain of a d.c. voltage 
detection circuit thereof (not shown). Thereby, the d.c. voltage to be 
input to converter control circuit 6 needs to be corrected. 
If the d.c. voltage control circuit has a configuration to allow input of 
an arbitrary d.c. voltage command value, there is no need to execute the 
d.c. voltage correction calculation according to equation 1. Further, if 
the converter control circuit 6 is not provided with the d.c. voltage 
control circuit described above, a d.c. voltage control unit instead of 
the d.c. voltage correction arithmetic unit 81 may be provided. 
Speed control means 80 obtains a speed variance between the speed command 
signal and the speed signal, and calculates, on the basis of the speed 
variance obtained above, a PWM signal to inverter circuit 3 and a d.c. 
voltage command value Ed* to d.c. voltage correction arithmetic unit 81. 
PWM/PAM control determination unit 802 determines whether the speed control 
of brushless d.c. motor 4 should be executed by the PWM control using the 
inverter or by the PAM control using the converter on the basis of a motor 
stop signal from speed calculation unit 84, a PWM signal from PWM duty 
calculation unit 803, and d.c. voltage command value Ed* from d.c. voltage 
control arithmetic means 801. 
The d.c. voltage control arithmetic means 801 calculates d.c. voltage 
command value Ed* on the basis of the speed variance, a control status 
signal from PWM/PAM control determination unit 802, a motor stop signal 
from speed calculation unit 84 and an operation enabling signal from 
converter operation determination unit 82, and also outputs a converter 
operation flag and converter ON/OFF signal. The d.c. voltage command value 
Ed* and converter ON/OFF signal are output to converter control circuit 6, 
and the converter operate flag is output to PWM/PAM control determination 
unit 802. 
Here, d.c. voltage command value Ed* assumes a minimum value when the 
control status signal indicates the PWM control mode, and has a d.c. 
voltage command value corresponding to a speed variance when the control 
status signal indicates the PAM control mode. In other words, the d.c. 
voltage is incremented or decremented corresponding to the speed variance. 
The converter ON/OFF signal becomes a converter ON when the motor stop 
signal indicates that the motor is in operation and the operate enable 
signal indicates that the converter operation is allowed. By the converter 
ON signal of the converter ON/OFF signal, converter circuit 2 starts its 
operation so as to coincide d.c. voltage value Ed with d.c. voltage 
command value Ed*. 
PWM duty calculation unit 803 calculates and outputs a PWM signal on the 
basis of the speed variance and the control status signal from PWM/PAM 
control determination unit 802. Here, the PWM signal takes a value 
representing a conduction ratio corresponding to a speed variance when the 
control status signal indicates the PWM control mode. And, when the 
control status signal indicates the PAM control mode, the PWM signal 
designates 100% of conduction ratio. By way of example, when the motor 
stop signal indicates a motor stoppage status, the PWM signal designates 
0% of conduction ratio. Namely, conduction of brushless d.c. motor 4 is 
prohibited. 
With reference to FIG. 3, operations in the d.c. voltage control arithmetic 
means 801 will be described. FIG. 3 shows process steps of the d.c. 
voltage control arithmetic means 801 in a flow chart. 
In step (811), whether the brushless d.c. motor 4 is in operation or 
stoppage is determined according to the motor stop signal from speed 
calculation unit 84. 
When the motor is at stoppage, in step (812), the converter operation flag 
is cleared to be ready for the converter to stop, and the converter ON/OFF 
signal is set at converter OFF. At this instant, d.c. voltage command 
value Ed* coincides with d.c. voltage reference value Vd2 (a full-wave 
rectification voltage value of the power supply voltage). 
When the motor is in operation, in step (813), it is determined whether or 
not to operate the converter on the basis of the operation enabling signal 
from converter operation determination unit 82. When the converter 
operation enabling signal designates operation enable, the step moves to 
(819), and when the same designates converter stoppage, the step moves to 
(815), respectively. 
When the step moves to (815), processes to decrement d.c. voltage Ed 
gradually then to stop the converter are executed (815-818). 
In step (815), the converter operation flag is cleared. In step (816), d.c. 
voltage command value Ed* and d.c. voltage reference value Vd2 are 
compared. When d.c. voltage command value Ed* becomes smaller than d.c. 
voltage reference value Vd2, the step goes to (817) where a converter OFF 
signal is produced to stop the converter. 
To the contrary, when d.c. voltage command value Ed* is greater than d.c. 
voltage reference value Vd2, the step advances to (818) where 3 V is 
subtracted from d.c. voltage command value Ed*. The reason why d.c. 
voltage command value Ed* is compared with d.c. voltage reference value 
Vd2 in (816) is because that there is a possibility that an actual d.c. 
voltage value Ed does not coincide with d.c. voltage command value Ed*. 
Alternatively, when the step moves from (813) to (819), processes to start 
the converter and increase the d.c. voltage gradually are executed 
(819-822). In addition, when the PAM control is to be executed, PAM 
control processes are executed in steps (823-825). 
In step (819), for readying to operate the converter, a converter ON signal 
as the converter ON/OFF signal is output to converter control circuit 6. 
In step (820), d.c. voltage command value Ed* and d.c. voltage reference 
value Vd1 (a full-wave rectification voltage of a.c. power supply 1 plus 
10 V) are compared, then in step (822), d.c. voltage command value Ed* is 
incremented by adding 3 V until d.c. voltage command value Ed* equals d.c. 
voltage reference value Vd1. 
When d.c. voltage command value Ed* becomes greater than d.c. voltage 
reference value Vd1, the step moves to (821) where for readying to operate 
the converter, a converter operation flag is set, and which is output to 
PWM/PAM control determination unit 802. 
In step (823), the control mode is determined whether it is in the PWM 
control or the PAM control on the basis of the control status signal from 
PWM/PAM control determination unit 802. When it is determined to be in the 
PAM control, the step moves to (824) where d.c. voltage command value Ed* 
is computed from the speed variance, and this d.c. voltage command value 
Ed* computed is output to d.c. voltage correction arithmetic unit 81 in 
step (825). 
This sequence of processing described above is repeated cyclicly at a 
control cycle which is sufficient to ensure adequate speed control of the 
motor. 
Now, operations in PWM/PAM control determination unit 802 will be described 
in detail with reference to FIG. 4, which depicts processing of PWM/PAM 
control determination in a flowchart. 
In step (831), a converter operation flag from d.c. voltage control 
arithmetic unit 801 is detected to determine whether the converter is 
operating or at stoppage. When the converter is determined to be at 
stoppage, since the PAM control is not allowed, the step jumps to (832) 
where the PWM control is forcibly set, and the d.c. voltage command value 
is fixed at d.c. voltage reference value Vd1(the full-wave rectification 
voltage of a.c. power supply 1 plus 10 V) from d.c. voltage control 
arithmetic means 801. 
When the converter is determined to be in operation, the step moves to 
(833) where a present control status is confirmed. When the control status 
is in the PWM control mode, the step moves to (834) where it is determined 
whether the PWM signal conduction ratio to the inverter is 100% or not, 
and also whether the motor requires further acceleration (positive sign 
for the speed variance) or not. Only when the conduction ratio is 100% and 
further acceleration is required, the step moves to (835) where the 
control status signal is set in the PAM control mode, and is output to 
d.c. voltage control arithmetic means 801 and PWM duty calculation unit 
803. Then, PWM duty calculation unit 803 fixes the conduction ratio of the 
PWM signal at 100% to be output to driver 5. 
When the conduction ratio is less than 100%, or no further acceleration is 
required, the step goes to (832). When the PAM control status is verified 
in (833), the step moves to (836) where it is determined whether d.c. 
voltage command value Ed* is smaller than a d.c. voltage reference value 
Vd3 (d.c. voltage reference value Vd1 minus 5 V) or not, and also whether 
the motor requires further deceleration (negative sign of the speed 
variance) or not. Only when d.c. voltage command value Ed* is smaller than 
d.c. voltage reference value Vd3, and further deceleration is required, 
the step moves to (832) where the PWM control is set and d.c. voltage 
command value Ed* is changed. 
When d.c. voltage command value Ed* is greater than d.c. voltage reference 
value Vd3, or the further deceleration is not required, the step moves to 
(837) where the PAM control is maintained. The processing in (837) is the 
same as in (835). 
D.C voltage reference values Vd1-Vd3 used in the processing hereinabove are 
determined from d.c. voltage values Ed obtained during the motor in 
operation and the converter at stoppage. However, when the voltage of a.c. 
power supply 1 is fixed, predetermined fixed values may be used instead of 
these reference values. 
With reference to FIG. 5, changes of d.c. voltage Ed, d.c. voltage command 
value Ed*, PWM signal conduction ratio D, and the number of revolutions N 
of the motor are indicated which were obtained when the speed control of 
brushless d.c. motor 4 was executed by means of the motor drive apparatus 
of the invention. Voltage values, conduction ratios and the number of 
revolutions are indicated on the ordinate, and time is indicated on the 
abscissa. Vd1 corresponds to d.c. voltage reference value Vd1, Vd2 
corresponds to d.c. voltage reference value Vd2, and Vd3 corresponds to 
d.c. voltage reference value Vd3, respectively. 
When the motor is started at time t0, conduction ratio D and the number of 
revolutions N are caused to increase. When the motor starts driving, and 
an input current (not shown) is increased to exceed a preset value at time 
t1, the converter is started so as to gradually increase d.c. voltage Ed 
from Vd2 to Vd1. At this time, a rate of increase of conduction ratio D 
becomes smaller since d.c. voltage Ed increases. 
When dc voltage command value Ed* reaches Vd1 at time t2, dc voltage Ed 
stops its increase. When a further increase of the number of revolutions N 
of the motor is required, conduction ratio D is increased up to 100% so as 
to increase the number of revolutions of the motor. 
When a still further acceleration is required after conduction ratio D 
reached 100% at time t3, the control status is changed from the PWM 
control to the PAM control, then dc voltage Ed (dc voltage command value 
Ed*) is increased so as to increase the number of revolutions N of the 
motor while maintaining the conduction ratio D fixed at 100%. 
Now, at time t4, when the number of revolutions N of the motor is to be 
decreased, dc voltage Ed (dc voltage command value Ed*) is caused to 
decrease at first contrary to the steps of acceleration. At time t5 at 
which dc voltage command value Ed* coincides with Vd3, when a further 
deceleration of the motor speed N is required, the control status is 
changed from the PAM control to the PWM control, and conduction ratio D is 
caused to decrease so as to decelerate the motor while maintaining dc 
voltage Ed (dc voltage command value Ed*) fixed at Vd1. 
At time t6 at which the number of revolutions N of the motor is decreased 
substantially, the load becomes lighter, and the input current becomes 
less than the preset value, then the dc voltage Ed(dc voltage command 
value) is caused to decrease gradually from Vd1 to Vd2. When the dc 
voltage value reaches Vd2 at t7, the converter is stopped its operation. 
To further decrease the number of revolutions N, conduction ratio D is 
caused to decrease further. At time t8, the motor stops its operation. 
According to the features of the motor drive apparatus of the invention, 
the speed control of brushless dc motor 4 is executed by means of the PWM 
control using the inverter when the load coupled to the motor is light, 
and by means of the PAM control using the converter when the load of the 
motor becomes higher. 
With reference to FIG. 6, a change of the number of revolutions of the 
motor drive apparatus according to the embodiment of the invention which 
was applied for controlling a motor for driving an air-conditioner's 
compressor is depicted in comparison with that of the prior art. A thick 
solid line represents the present invention and a thin solid line 
represents the prior art. The number of revolutions of the compressor is 
depicted on ordinate, and the time is depicted on abscissa. 
When the compressor is started at time t0 toward a command revolution 
target value 1, the converter is caused to start at time t1. At this 
instant, according to the prior art, since its dc voltage rises abruptly 
simultaneous with the start-up of the converter, its revolution control 
system cannot follow such abrupt changes thereby allowing an abrupt 
increase and an overshoot in the number of revolutions of the compressor 
to occur. This phenomenon takes place at the start-up of the converter in 
an initial stage of operation of the air-conditioner during which the 
pressure of air-conditioner cycle is low, and in particular, its 
occurrence is remarkable when a motor drive apparatus having a revolution 
control system having a slow response is used. In contrast to this prior 
art, according to the motor drive apparatus of the embodiment of the 
invention, since dc voltage Ed is increased gradually upon start-up of the 
converter, there occurs no abrupt change in the number of revolutions. 
Further, when a command revolution number (not shown) is changed to target 
value 2 at time t2, the load is reduced at time t3 to satisfy the 
condition for the converter to stop. At this instant, according to the 
prior art, since the converter is stopped at t3, its dc voltage Ed drops 
abruptly, thereby accordingly dropping the number of revolutions of the 
compressor. At this instant, when a quantity of variance in the number of 
revolutions becomes great, pole position sensing for the motor becomes 
difficult, thereby sometimes causing the compressor to halt its operation. 
To the contrary, according to the invention, since its dc voltage Ed is 
gradually decreased from Vd1 to Vd2, and the converter is stopped when the 
dc voltage reaches the full-wave rectification voltage value of the power 
supply voltage (Vd2), there occurs no abrupt change in the number of 
revolutions of the compressor, thereby ensuring the compressor motor of 
the air-conditioner to be controlled stably in the number of revolutions 
thereof. 
According to the features of the motor drive apparatus of the embodiment of 
the invention, even if the converter is started or stopped while the motor 
is operating, rapid changes in the dc voltage can be eliminated, and the 
number of revolutions of the motor can be controlled stably. 
The description of the invention hereinabove is made on condition that the 
dc voltage control system of the converter is ensured to operate 
adequately, and that the dc voltage is controlled precisely in response to 
the dc voltage command value. However, it is difficult to ensure that the 
dc voltage control in practical circuits can be operated under every 
conditions. In particular, in the vicinity of the minimum value of the 
chopper conduction ratio in the booster chopper circuit, its conduction 
ratio does not change linearly. Thereby, dc voltage Ed cannot be 
controlled linearly to the limit of the full-wave rectification value of 
the power supply voltage. In other words, there is a difference in 
resultant dc voltage values between cases obtained with the chopper 
operation of the converter being stopped and obtained with the chopper 
conduction ratio at the minimum value thereof. 
Therefore, there arises a particular point from which dc voltage Ed will 
not fall compliant with a dc voltage command value Ed* which is 
decreasing. According to the motor drive apparatus of the first embodiment 
of the invention, the dc voltage command value is caused to decrease 
gradually, and when dc voltage command value Ed* arrives at dc voltage 
reference value Vd2, the converter is caused to stop. However, when dc 
voltage Ed comes to a point of value Vd' which is slightly larger than dc 
voltage reference value Vd2 and from which the dc voltage Ed does not 
drop, a range of values lower than this dc voltage value Vd' is considered 
to be the range in which the converter circuit can no more control the dc 
voltage Ed, thereby, it is preferable to stop the operation of the 
converter at this dc voltage value Vd'. 
EMBODIMENT 2 
Now, a second embodiment of the invention will be described. This second 
embodiment of the invention has a modified type of dc voltage control 
arithmetic means 801 different from that of the first embodiment. The dc 
voltage control arithmetic means 801 has a feature that it can execute a 
smooth dc voltage control even if the power supply voltage fluctuates. 
In the case of the commercial power supply, its power supply voltage 
fluctuates .+-.15%. Thereby, if dc voltage command value Ed* (initial 
value) and dc voltage reference values Vd are used as a fixed value, there 
arises a problem that the dc voltage control as initially conceived cannot 
be executed when the power supply voltage fluctuates. The dc voltage 
control arithmetic means 801 according to the second embodiment of the 
invention is contemplated to solve the above-mentioned problem 
Processing in dc voltage control arithmetic means 801 of the embodiment of 
the invention is depicted in a flowchart of FIG. 7. Differences from the 
dc voltage control arithmetic means 801 (in FIG. 3) of the first 
embodiment of the invention reside in providing three steps including 
steps (841), (842) and (843). The other steps are the same as those of the 
first embodiment of the invention. 
In step (841), it is determined whether the converter is operating or not. 
Only when the converter is off, the step goes to (842) where dc voltage Ed 
is detected, and its detected value Ed is set as dc voltage command value 
Ed*. Processing in steps (841) and (842) is for setting a value of a 
full-wave rectification voltage of the power supply voltage as an initial 
value of dc voltage command value Ed* at the time of start-up of the 
converter. By this processing above, the power supply voltage can be 
estimated. Thereby, dc voltage command value Ed* (initial value) and dc 
voltage reference value Vd can be determined from the power supply voltage 
estimated above. In this embodiment of the invention, dc voltage command 
value Ed* (initial value) is determined from the power supply voltage. 
Step (843) is a process to determine whether or not the dc voltage control 
is operating normally and accurately, in other words, whether dc voltage 
value Ed is decreasing in response to dc voltage command value Ed*. More 
specifically, in this step, dc voltage detection value Ed and dc voltage 
command value Ed* are compared. When a difference between dc voltage 
command value Ed* and dc voltage detection value Ed becomes greater than 
10 V, the step goes to (817) to stop the operation of the converter. 
By way of example, the dc voltage reference value Vd1 and dc voltage 
reference value Vd2 may be determined from the dc voltage value Ed having 
been detected in step (842). However, in this second embodiment of the 
invention, by taking into account the fluctuation of the power supply 
voltage, they are determined respectively to be a full-wave rectification 
voltage value at a maximum fluctuation and a minimum fluctuation of the 
power supply voltage. 
By provision of a motor drive apparatus using the dc voltage control 
arithmetic means 801 according to the second embodiment of the invention, 
a smooth dc voltage control can be attained even if there occurs 
fluctuation in the power supply during the operation of the converter. 
By way of example, the converter circuit 2 and dc voltage control 
arithmetic means 801 having been described both in the first and the 
second embodiments of the invention may be applied as a power circuit. 
However, when estimating a power supply voltage from the dc voltage Ed, a 
process once to normalize the dc voltage Ed is required. More 
particularly, a process to conduct dc voltage Ed through the load is 
required prior to its detection. 
EMBODIMENT 3 
Now, a third embodiment of the invention will be described in the 
following. This third embodiment of the invention is comprised of still 
another type of dc voltage control arithmetic means 801 different from 
that of the first embodiment of the invention. This dc voltage control 
arithmetic means 801 of the third embodiment is contemplated to provide 
for a motor drive apparatus operable at a maximum capacity thereof. 
Steps of operation in this dc voltage control arithmetic means 801 of the 
third embodiment of the invention are depicted in a flowchart of FIG. 8. 
In step (851), it is determined whether the brushless dc motor 4 is 
operating or not from the motor stop signal. When the motor is at 
stoppage, the step moves to (852) where the converter operate flag is 
cleared, and the converter ON/OFF signal is set to converter OFF. At this 
instant, its dc voltage command value becomes dc voltage reference value 
Vd2 (a full-wave rectification voltage value of the power supply voltage). 
When the motor is operating, the step goes to (853) where it is determined 
whether or not the converter may be started on the basis of the operation 
enabling signal. 
When the converter is determined to be off, the step goes to (852) to 
execute the process thereof. When the converter is determined operation 
enable, the step goes to (854) where it is determined whether the current 
motor control status is in the PWM control mode or in the PAM control 
mode. When in the PWM control status, the step goes to (855) where a 
converter operation flag is set, and the converter is driven. At this 
instant, dc voltage command value Ed* becomes dc voltage reference value 
Vd1 (a full-wave rectification voltage value of the power supply voltage 
plus 10 V). 
In the case of the PAM control status, in step (856), an 
increment/decrement value .DELTA.Ed* of dc voltage command value Ed* is 
calculated from a speed variance. Here, .DELTA.Ed* is preset to have a 
positive sign when its speed variance is positive, namely, when 
acceleration is required, and a negative sign when its speed variance is 
negative, namely, when deceleration is required. 
In step (857), it is determined whether increment/decrement value 
.DELTA.Ed* of the dc voltage command value Ed* is positive or negative, 
and when negative, the step moves to (858) where dc voltage command value 
Ed* is reduced. It is preset such that when dc voltage command value Ed* 
is reduced, dc voltage Ed is reduced also. 
When dc voltage command increment/decrement value .DELTA.Ed* is positive, 
the present dc voltage Ed is detected in step (859). Then, in step (860), 
dc voltage Ed is compared with a preset value which is obtained by 
subtracting 10 V from the dc voltage maximum reference value. If dc 
voltage value Ed is smaller than the preset value of dc voltage maximum 
reference value minus 10 V, the step jumps to (858) to execute the process 
thereof. 
If dc voltage value Ed is larger than the preset value of dc voltage 
maximum reference value Edmax-10 V, the step goes to (861) where dc 
voltage value Ed and dc voltage maximum reference value Edmax are 
compared. 
If dc voltage value Ed is smaller than dc voltage maximum reference value 
Edmax, the step goes to (862) where dc voltage command increment/decrement 
value .DELTA.Ed* is divided by 4 to produce such that 
.DELTA.Ed*=.DELTA.Ed*/4, which is added on to dc voltage command value Ed* 
in step (858). 
According to the third embodiment of the invention, it is arranged such 
that if dc voltage command increment/decrement value .DELTA.Ed* is 
negative, the step jumps to (858) unconditionally to decrement dc voltage 
command value Ed*. However, it may be arranged such that additional 
processes subsequent to step (859) are included to compare dc voltage 
value Ed with dc voltage maximum reference value Edmax, and to increase a 
decremental width of dc voltage command value Ed if dc voltage value Ed 
exceeds dc voltage maximum reference value Edmax. 
If dc voltage value Ed is equal to dc voltage maximum reference value 
Edmax, the step goes to (864) where dc voltage command value Ed is not 
altered (Ed*=Ed*), and is output as dc voltage command value Ed* in step 
(864). 
If dc voltage value Ed is larger than dc voltage maximum reference value 
Edmax, the step goes to (865) where dc voltage command value Ed* is 
reduced by a decrement preset value .DELTA.V. Then, in step (864), its 
decremented dc voltage command value Ed* is output. 
The decrement preset value .DELTA.V and other values used here are 
determined on the basis of experiments conducted to obtain a stable dc 
voltage control. Further, the processes described above are repeated 
cyclicly at a control cycle which is sufficient to ensure adequate and 
effective speed control of the motor. 
With reference to FIG. 9, dc voltage control characteristics are indicated 
which were obtained with brushless dc motor 4 when its speed control was 
executed using the motor drive apparatus using dc voltage control 
arithmetic means 801 according to the third embodiment of the invention. 
FIG. 9 shows a relationship between dc voltage command value Ed* versus dc 
voltage value Ed. D.C voltage value Ed is shown on ordinate, and dc 
voltage command value Ed* is shown on abscissa. 
Under a normal load condition in which the characteristics of its dc 
voltage control system are maintained advantageously, since its dc voltage 
value Ed coincides with its dc voltage command value Ed*, Edmax is output 
at Ed*1. However, under a heavy load or a light load, there arises an 
error in the dc voltage control characteristics, thereby causing a problem 
that Edmax is not attained at Ed*1 of the dc voltage command value, or 
contrarily the dc voltage value Ed exceeds Edmax. 
According to the dc voltage control arithmetic means 801 of this embodiment 
of the invention, under a heavy load condition, since dc voltage command 
value Ed* is increased to Ed*2, its dc voltage value can be controlled up 
to Edmax. To the contrary, under a light load condition, since dc voltage 
command value Ed* will not go up more than Ed*0, its dc voltage value will 
not rise more than Edmax. Thereby, irrespective of load conditions, the dc 
voltage can be always controlled up to maximum value Edmax. 
Now, with reference to FIG. 10, changes of dc voltage command value Ed* and 
dc voltage value Ed versus time elapsed are depicted, which were obtained 
with a motor drive apparatus for driving an air-conditioner compressor 
motor using the dc voltage control arithmetic means 801 according to this 
embodiment of the invention. 
Assume that at time t0 the motor is started and the converter is in 
operation. Until time t1, the speed control of the motor is executed by 
the PWM control, thereby causing no changes in dc voltage command value 
Ed* and dc voltage value Ed. After t1, the control status is switched over 
to the PAM control, thereby causing dc voltage value Ed to rise. 
When the number of revolutions of the motor increases, the load increases 
accordingly, thereby causing a variance to occur between dc voltage 
command value Ed* and dc voltage value Ed corresponding thereto. When dc 
voltage value Ed reaches a preset value of (Edmax-10 V), an incremental 
rate .DELTA.Ed* of dc voltage command value Ed* is suppressed such that dc 
voltage value Ed is adjusted not to exceed Edmax. 
When dc voltage value Ed reaches Edmax at t3, dc voltage command value Ed* 
is adjusted not to increase, and dc voltage value Ed is maintained at 
Edmax. 
If, however, dc voltage value Ed is caused to overshoot at t3, dc voltage 
command value Ed* is caused to decrease such that dc voltage value Ed 
coincides with Edmax. 
By way of example, FIG. 10 depicts such a case in which dc voltage Ed is 
caused to overshoot. Although they are drawn in linear lines to simplify 
the explanation, actually, they are represented by smooth curves, and 
variance between dc voltage command value Ed* and dc voltage value Ed is 
not so large as indicated in the drawing.