Method and apparatus for controlling a synchronous motor

An inverter(3) for driving a motor(4) of an electric automobile(20) produces a three-phase PWM current. When the torque reference is zero, alternatively or when the torque reference is zero and motor speed is no higher than a predetermined value, the inverter(3) is controlled to be cut off so that current from a main power battery(9) is suspended or stopped.

FIELD OF THE INVENTION AND RELATED ART STATEMENT 
1. Field of the Invention 
The present invention relates to an improvement in a PWM control method and 
a control apparatus for an inverter which is for controlling a synchronous 
motor. The control method and apparatus are particularly suitable for 
battery-driven electric cars using synchronous motors of permanent magnet 
rotor type. 
2. Description of the Related Art 
In an inverter of pulse width modulation control for driving a motor, 
especially such as a synchronous motor, in general, a triangular waveform 
signal, which is a reference signal is compared with a torque reference 
which instructs or commands an amount of torque of motor. A pulse signal 
of a duty ratio proportional to a level of torque reference is output from 
the comparator. A voltage which is applied to the motor, hence an output 
torque thereof is controlled on the basis of the pulse signal. 
In a conventional pulse width modulation circuit as shown in FIG. 4, a 
reference triangular waveform signal 31 of a triangular waveform signal 
generator 21 is compared with a torque reference 32 by a comparator 22. 
Consequently, as shown in FIG. 6B, a pulse-width modulated output 33 which 
varies gradually in a pulse width as shown in FIG.6 is output from the 
comparator 22. 
A duty ratio of the pulse-width modulation output 33 varies substantially 
in proportion to a level of the torque reference 32. 
With respect to one coil L of the motor, operation of the inverter of the 
pulse width modulation control is described with reference to FIG. 5. 
Switching elements e.g. thyristor T.sub.1 and T.sub.2 are connected in 
series across a direct-current power supply (series connected batteries) 
9. Other similar switching elements T.sub.3 and T.sub.4 are also connected 
in series across the direct-current power supply 9. The coil L is 
connected across a junction J.sub.1 of the switching elements T.sub.1 and 
T.sub.2 and a junction J.sub.2 of the switching elements T.sub.3 and 
T.sub.4. The switching elements T.sub.1 -T.sub.4 are controlled by a 
control circuit (not shown) on the basis of the pulse-width modulation 
output 33. Directions of a current fed from direct-current power supply 9 
and passing through the coil L is controlled as shown by an arrow X or an 
arrow Y, by switching operations of the switching elements T.sub.1 
-T.sub.4. 
When the level as shown in FIG. 7A of the torque reference 32 is zero, or 
neutral between positive and negative level, the switching elements 
T.sub.1 -T.sub.4 are switched so as to output a signal of a waveform as 
shown in FIG. 7B. Then, the duty ratio of the pulse-width modulation 
output 33 is 50 percent. In every one cycle t of the triangular waveform 
signal 31, the current flows through the coil L alternately in the 
directions X and Y as shown in FIG. 7B. A carrier frequency of the 
triangular waveform signal 31 is selected from several kHz to several to 
ten kHz in general. Therefore, the period t of the current passing through 
the coil L is about several ten .mu.s. 
Since the current having a short period t alternately passes through the 
coil L in the direction of the arrow X or the direction of the arrow Y, an 
average current in the coil L is substantially zero and is corresponding 
to the torque reference 32. Hereupon, a current is supplied to the coil L 
from the battery 9, and that the 50% duty ratio current shown in FIG. 7B 
is worthless as a whole. The intensity of this ineffective current is 
about one four hundredth (1/400) of a rated current. 
The worthless or ineffective current is only useful for a case which 
requires a holding torque or regenerative braking. In a conventional 
electric automobile, an electric power from the battery is worthlessly 
dissipated due to the above-mentioned ineffective current. It has been a 
problem to reduce the worthless loss of the electric power of the battery 
9 of an electric car. In the case of the electric automobile having a high 
voltage power source, such loss of the electric power due to the 
ineffective current reaches several hundred watts in total, and therefore 
cannot be disregarded. 
In order to save such loss of the electric power, in a control apparatus of 
the synchronous motor of electric car driven by a series-connected high 
voltage battery, its control system has long been demanded to achieve as 
small worthless current loss as possible. The present invention is to 
solve the above-mentioned subject. 
OBJECT AND SUMMARY OF THE INVENTION 
An object of the present invention is to provide a control method and an 
apparatus of a motor which prevents a loss of an electric energy of a 
battery by interrupting or suspending power supply from the battery when a 
torque reference level is zero, thereby improving an efficiency of the 
battery. 
In the method and apparatus for controlling of an inverter in which a 
synchronous motor is controlled by a pulse width modulation of an output 
of the battery (hereinafter is referred to as PWM control), when the 
torque reference to the synchronous motor is zero, a switching operation 
of the inverter is suspended or stopped. 
Furthermore, also when the torque reference is zero and the rotating speed 
of the synchronous motor is no higher than a predetermined reference 
value, the switching operation of the inverter is suspended or stopped. 
When the torque reference is zero and the state that the rotating speed of 
the synchronous motor is no higher than the predetermined reference value 
continues for a predetermined time length, the switching operation of the 
inverter is suspended. 
According to the present invention, when the torque reference level to the 
motor is zero, the switching operation of the inverter is suspended and 
current supply from a power source e.g. a battery is stopped, and thereby, 
the electric energy of the battery can be saved, and efficiency of use of 
the battery is improved. 
When the torque reference to the motor is zero and the rotating speed of 
the motor is no higher than the predetermined reference value, or when the 
torque reference level to the motor is zero and the state that the 
rotating speed of the motor is no higher than the predetermined reference 
value continues for the predetermined time length, the switching operation 
of the inverter is suspended. Therefore, when the rotating speed of the 
motor is larger than the reference value in deceleration step, a charging 
current produced by regenerative braking is stored in the battery. When 
the rotating speed of the motor is no higher than the reference value, the 
current supply to the motor is suspended, and the energy of the battery 
can be saved. Since a loss of the energy in the battery is prevented, the 
mileage of automobile covered by one charging of the battery greatly 
increases for example. 
According to the present invention, a service life of the battery also 
increases by preventing the loss of the electric energy in the battery and 
improves the efficiency in use of the electric power.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
An embodiment of the present invention will be described in detail with 
reference to FIG. 1 through FIG. 6B. 
The embodiment shown in FIG. 1 and FIG. 2 is an example which is applied to 
an electric automobile. FIG.1 is a plan view of an electric automobile 20 
and shows an arrangement of relevant equipments. Referring to FIG. 1, a 
permanent magnet rotor type synchronous motor 4 (hereinafter is simply 
referred to as motor) is controlled by a control apparatus 7, and rotates 
driving wheels 10. A transmission signal 1A representing a transmission 
position of a shift lever system 1 is applied to the control apparatus 7. 
An acceleration-pedal pushing-down degree signal 2A representing 
pushing-down degree of an accelerator pedal 2 and a brake signal 6A 
representing operation of a brake pedal 6 are applied to the control 
apparatus 7. A speed sensor 5 (which is a tachometer) detects rotation 
direction and rotation speed of the motor 4, and a speed feedback signal 
5A is output from the speed sensor 5. The speed feedback signal 5A is 
applied to the control apparatus 7. A current sensor 11 detects a current 
of the motor 4 and output a current feedback signal 11A. The current 
feedback signal 11A is applied to the control apparatus 7. The control 
apparatus 7 is fed from a battery 8 for the control apparatus 7. A current 
from a battery 9 is supplied to the motor 4 by control of the control 
apparatus 7 through an inverter 3. 
The control apparatus 7 generates an torque reference 17A by processing the 
acceleration-pedal pushing-down degree signal 2A, the brake pedal pushing 
degree signal 6A, the speed feedback signal 5A and the current feedback 
signal 11A. The torque reference 17A is applied to the inverter 3 so as to 
control an output current and operation frequency of the inverter 3. 
The inverter 3 carries out a switching operation by receiving the torque 
reference 17A, and the current from the battery 9 is supplied to the motor 
4. The electric automobile 20 is driven by rotation of driving wheels 10 
through rotation of the motor 4. 
FIG. 2 is a block diagram of the motor control apparatus of the present 
embodiment. Referring to FIG. 2, a CPU 7A receives the acceleration-pedal 
pushing-down degree signal 2A from the accelerator 2, the brake pedal 
pushing degree signal 6A from the brake 6 and the speed feedback signal 5A 
from the speed sensor 5. An output of the CPU 7A is applied to a current 
control unit 7B. The current control unit 7B supplies the torque reference 
17A to a servo control unit 3B so as to control the motor 4 in compliance 
with the current feedback signal 11A of the current sensor 11. The CPU 
generates an inverter ON/OFF signal 18 which activates or deactivates the 
inverter 3. The inverter ON/OFF signal 18 is applied to the servo control 
unit 3B. When the inverter ON/OFF signal 18 is in ON state, the inverter 3 
is activated. On the contrary, when the inverter ON/OFF signal 18 is in 
OFF state, the inverter 3 is deactivated thereby to stop or suspend its 
current, hence current from the battery 8. 
In the CPU 7A of the control apparatus 7, the acceleration-pedal 
pushing-down degree signal 2A is processed to produce a speed deviation 
signal 19A. The speed deviation signal 19A is applied to the current 
control unit 7B as a current signal which is converted by a known PI 
control operation. 
In the current control unit 7B, the speed deviation signal 19A is compared 
with the current feedback signal 11A from the current sensor 11, which 
detects the output current of the inverter 3, and a current deviation 
signal is obtained. 
The current deviation signal is processed into the torque reference signal 
17A by known PI controlling, and the torque reference signal 17A is 
applied to the servo control unit 3B, so that a current corresponding to 
the acceleration-pedal pushing-down degree signal 2A is output from the 
inverter 3. A torque reference 32 in FIG. 4 for the PWM control is 
generated on the basis of the torque reference signal 17A. 
When a level of the torque reference signal 17A is high, an amplitude of a 
pulse-width modulation output 33 in the PWM control increases, and an 
output current also increases. On the contrary, when the level of the 
torque reference signal 17A is low, the pulse width of the PWM output 33 
decreases and the output current also decreases. 
In the servo control unit 3B, the pulse-width modulation output 33 
(hereinafter is referred to as PWM output 33) which is the output of the 
comparator 22 is generated as shown in FIG. 4. The PWM output 33 obtained 
by comparison of torque reference 32 and the triangular waveform signal 31 
is shown in FIG. 6B. The control unit 3B controls output of the PWM output 
33 to a gate unit 3C in compliance with the inverter ON/OFF signal 18 
which is inputted from the control apparatus 7. 
In operation, the inverter ON/OFF signal 18 is not output when the level of 
the torque reference signal 17A is zero (FIG. 3, Steps 41, 46, 47). In 
another operation method, the inverter ON/OFF signal 18 is not output (OFF 
state) when a level of the torque reference signal 17A is zero and the 
rotating speed of the motor 4 is no higher than a predetermined reference 
value. When the inverter ON/OFF signal 18 is not applied to the servo 
control unit 3B, a switching signal is not applied to the gate unit 3C 
from the servo control unit 3B. 
In the gate unit 3C, a switching signal 22A is generated in compliance with 
the PWM output 33 output from the servo control unit 3B. The switching 
circuit 23 of FIG. 5 is disposed in an output unit 3D and is controlled by 
the switching signal 22A. The output unit 3D comprises three switching 
circuits 23 for the motor 4 of three phases. FIG. 5 shows one example of 
switching circuit 23 for one phase. Output transistors T.sub.1 -T.sub.4 of 
each switching circuit 23 are controlled by the switching signal 22A, and 
the current of the battery 9 is supplied to the motor 4. When the inverter 
ON/OFF signal 18 to the servo control unit 3B is in OFF state, operation 
of the output unit 3D is suspended and the current of the battery 9 is not 
supplied to the motor 4. 
The output unit 3D is controlled by the abovementioned switching signal 
22A, and makes switching operation of the output current of the battery 9. 
A three-phase driving output based on the PWM output 33 (FIGS. 2,4) 
obtained by the switching operation is utilized for controlling the motor 
4. 
In deceleration step of the electric automobile, a charging current from 
the motor 4 produced by regenerative braking is supplied to the battery 9 
when speed is higher than a predetermined value. However, when the speed 
is lowered below a predetermined value, for example 250 r.p.m., whereat 
the regeneration voltage is too small for supplying to the battery, and on 
the other hand the motor rotation loses smoothness, operation of the 
inverter 3 is suspended and the circuit of the regenerative braking is cut 
off from the battery 9, thereby preventing useless discharging of the 
battery 9 to the circuit and improving smoothness of running. A current 
supply to the motor 4 is also suspended and useless consumption of the 
battery 9 due to the current to the motor 4 is prevented. Consequently, 
the energy of the battery 9 can be used very efficiently. Since the 
electric power of the battery 9 is saved, maximum travel distance of the 
electric automobile covered by one charge of the battery 9 can be greatly 
increased. 
In the embodiment shown in FIG. 2 the current control unit 7B is disposed 
in the control apparatus 7, but it can be disposed in the inverter 3. 
Though the control apparatus 7 and the inverter 3 are separately 
configured in the embodiment, these two can be configured as one apparatus 
carrying out functions of both. 
Furthermore, the embodiment has been configured that the inverter ON/OFF 
signal 18 is not output when the level of the torque reference 17A is zero 
and the rotating speed of the motor 4 is no higher than the reference 
value. 
Another configuration may be made such that the inverter ON/OFF signal 18 
is suspended, when the level of the torque reference 17A is zero, or 
alternatively, when the level of the torque reference 17A is zero and that 
the state of the motor speed being no higher than a second reference 
value, for example 300 r.p.m., continues for a predetermined time length, 
the operation of switching timing output of the servo control unit 3B is 
suspended (FIG. 3, Steps 41, 43, 46, 47), thereby to stop the operation of 
the inverter 3. In such motor rotation speed, although a very small 
regeneration current would charge the battery, the amount of current is 
too small to be meaningful, while low frequency vibration of the motor 
occurs and causes uncomfortable noise to the vehicle body, and also gives 
adverse effect to the motor and power transmission mechanism. Therefore, 
continuation for long time of motor driving (electrifying) at the motor 
speed no higher than the second reference value e.g. said 300 r.p.m. for a 
predetermined time length, for example one minute, should better be 
suspended. Accordingly, this configuration has a technical advantage in 
the low speed driving for a certain time length. 
Although the present invention has been described in terms of the presently 
preferred embodiments, it is to be understood that such disclosure is not 
to be interpreted as limiting. Various alternations and modifications will 
no doubt become apparent to those skilled in the art to which the present 
invention pertains, after having read the above disclosure. Accordingly, 
it is intended that the appended claims be interpreted as covering all 
alterations and modifications as fall within the true spirit and scope of 
the invention.