Electricity: motive power systems – Synchronous motor systems – Field winding circuits
Reexamination Certificate
2000-02-08
2001-12-11
Ip, Paul (Department: 2837)
Electricity: motive power systems
Synchronous motor systems
Field winding circuits
C318S721000, C318S139000
Reexamination Certificate
active
06329781
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a synchronous electric motor control apparatus employed in a system for driving a synchronous electric motor by using a discharge output of a battery and used for controlling a synchronous electric motor in accordance with a torque command.
In a system for driving a synchronous electric motor by using a discharge output of a battery, it is quite within the bounds of possibility that the voltage of the electric motor exceeds the voltage of the battery at a high-speed region. In order to solve this problem, field weakening control to forcibly weaken the field magnetic flux of the electric motor is adopted in a variety of motor driving systems. To put it concretely, the field weakening control reduces the intensity of a field magnetic flux generated in a winding or a magnet by controlling a command of a d-axis current which is a part of an electric motor current contributing to generation of the field magnetic flux. Depending on factors such as the performance of the battery, a load condition and a residual electric charge, however, it is still quite within the bounds of possibility that the voltage of the electric motor exceeds the voltage of the battery, damaging the control apparatus even if the field weakening control is executed.
In order to solve the above problem, there has been introduced a technique of executing field weakening control after waiting for variations in battery voltage caused by a load of the electric motor to get stabilized as is disclosed in Japanese Patent Laid-open No. Hei 7-246000.
Japanese Patent Laid-open No. Hei 7-107772 discloses a technique of field weakening control which, for a rotational speed of an electric motor exceeding a base rotational speed, comprises the steps of computing a maximum voltage that can be output by the electric motor from the voltage of a battery; computing a weakening field current command Id* from the computed maximum voltage of the electric motor and executing the field weakening control based on the computed current Id*. Japanese Patent Laid-open No. Hei 10-136699 discloses a technique of correction and reduction of a torque command given to the electric motor so as to merely make an inferred value of the electric motor voltage below a value corresponding to a detected value of the battery voltage.
The conventional technology is effective for execution of field weakening control with the voltage of the electric motor never exceeding the voltage of the battery due to correction of a torque command. When it is desired to output a desired torque at a desired rotational speed, however, depending on the voltage of the battery, a sufficiently large torque may not be obtained for some cases. In addition, since the base rotational speed changes when the voltage of the battery changes, optimum field weakening control can not be executed in some cases due to the variable base rotational speed.
SUMMARY OF THE INVENTION
It is thus an object of the present invention to provide an electric motor control apparatus capable of executing field weakening control keeping up with variations in battery voltage and field weakening control capable of always outputting a desired torque at a desired rotational speed without a risk of for example damaging the apparatus due to the fact that the terminal voltage of the electric motor exceeds the voltage of the battery even if the voltage of the battery varies.
It is another object of the present invention to provide an electric motor control apparatus which is capable of implementing field weakening control in proper extent, allows the configuration to be simplified, and reduces a decrease in efficiency to a minimum.
It is a further object of the present invention to provide an electric motor control apparatus which is capable of implementing highly efficient field weakening control and sustaining a required torque in spite of a variation in battery voltage.
In order to achieve the objects described above, there is provided a control apparatus of a permanent-magnet synchronous electric motor driven by power supplied by a battery through an inverter circuit, the control apparatus comprising: a current-command generation means for outputting a d-axis current command, a torque current command or a so-called q-axis current command to the permanent-magnet synchronous electric motor; a current detection means for detecting a d-axis current and a q-axis current of the synchronous electric motor from phases and 3-phase alternating-current values of the synchronous-electric motor; a compensation computation means for computing a d-axis voltage compensation value and a q-axis voltage compensation value on the basis of the d-axis current command, the q-axis current command, a detected value of the d-axis current and a detected value of the q-axis current; a voltage-command generation means for generating a 3-phase alternating-current voltage command from the d-axis voltage compensation value, the q-axis voltage compensation value and phases, as well as the rotational speed of the synchronous electric motor; and a PWM (Pulse Width Modulation) control means for generating a PWM signal for driving a power device of the inverter circuit from a value of the 3-phase alternating-current voltage command.
The present invention is characterized in that, in execution of the field weakening control, a start rotational speed of the field weakening control is corrected in accordance with a detected value of the voltage of the battery so that, when the voltage of the battery is high, the start rotational speed is increased while, when the voltage of the battery is low, the start rotational speed is decreased.
In addition, the present invention is further characterized in that at least one of the d-axis current command and the q-axis current command is corrected in accordance with a ratio (V
B0
/V
BB
) of a rated battery voltage V
B0
in a normal operation to a detected battery voltage V
B
.
Furthermore, the present invention is further characterized in that at least one of the d-axis current command and the q-axis current command is generated by interpolation based on either a table of d-axis current commands or a table of q-axis current commands respectively which includes at least two interpolation values for usable different values of the battery voltage corresponding to a detected value of the battery voltage.
According to the present invention, in execution of the field weakening control without generating a torque, the start rotational speed of the field weakening control is computed from a ratio of a battery voltage used in a normal operation to a detected battery voltage. By correcting the start rotational speed of the field weakening control in this way, it is possible to execute field weakening control which keeps up with a variation in battery voltage. As a result, it is possible to implement field weakening control, in proper extent, that never allows the terminal voltage of the electric motor to exceed the voltage of the battery.
Another characteristic of the present invention is that, in addition to the correction of the start rotational speed of the field weakening control described above, a current of a weakened field is corrected in accordance with a ratio of a battery voltage used in a normal operation to a detected battery voltage. As a result, it is possible to implement field weakening control, in proper extent, which allows the configuration to be made simple, allows a decrease in efficiency to be reduced to a minimum, and never allows the terminal voltage of the electric motor to exceed the voltage of the battery.
In addition, a current of a weakened field is selected from a plurality of current-command tables corresponding to voltages of the battery and by linear interpolation based on values read out from the tables. As a result, it is possible to execute highly efficient current control and to assure a required torque even if the voltage of the battery varies.
REFERENCES:
patent: 4823251 (1989-04-01), Kawabata et al.
patent: 5038092 (
Katada Hiroshi
Matsui Hirokazu
Sawada Takefumi
Crowell & Moring LLP
Hitachi , Ltd.
Ip Paul
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