Electricity: motive power systems – Induction motor systems – Primary circuit control
Reexamination Certificate
2000-05-24
2001-02-27
Ip, Paul (Department: 2837)
Electricity: motive power systems
Induction motor systems
Primary circuit control
C318S807000, C318S808000, C318S825000
Reexamination Certificate
active
06194864
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a control device for an induction motor and, more particularly, to a control device with which it is possible to keep the efficiency of an induction motor at the maximum value irrespective of its load condition.
2. Description of the Prior Art
FIG. 1
is a block diagram depicting a conventional control device disclosed, for example, in Japanese Pat. Appln. Laid-Open Gazette No. 89493/87. Reference numeral
1
a
denotes an inverter,
2
an induction motor,
3
a current sensor,
21
a power rectifying part,
22
a capacitor,
23
a power inverting part,
24
a rectifier,
25
a low-pass filter,
26
an A/D converter,
27
a control circuit formed by a microcomputer, and
28
a PWM (Power Width Modulation) circuit. The inverter
1
a
provided with the capacitor
22
is combined with the PWM circuit
28
to constitute a known voltage shift PWM inverter which functions to supply a variable voltage variable frequency, three-phase AC voltage to the induction motor
2
.
FIG. 2
is a diagram explanatory of the principle of operation of a high-efficiency control scheme in the conventional induction motor control. The amplitudes of an input current (primary current) and an input voltage (primary voltage) of the induction motor bear such a relationship as indicated by a characteristic curve A in
FIG. 2
when the on-load torque is constant. With an input voltage raised higher than it needs to be, an exciting current increases, which causes an increase in a primary copper loss or iron loss, hence inevitably impairing the efficiency of the induction motor. Conversely, when the input voltage is lower than it needs to be, slip power increases and a secondary current increases, which causes an increase in primary and secondary copper losses, also decreasing the efficiency of the induction motor. Since the primary current of the induction motor is expressed as the sum of vectors of the exciting current and the secondary current, the loss becomes minimum at the point where the primary current becomes minimum (the point B in FIG.
2
). Accordingly, the induction motor can be driven with the maximum efficiency by controlling the amplitude of the primary voltage to minimize the amplitude of the primary current.
Next, the operation of the prior art example will be described.
The primary current detected by the current sensor
3
is applied to the low-pass filter
25
, from which is provided the average value of the primary current. The average value is provided via the A/D converter
26
to the control circuit
27
. Based on the above-mentioned principle of operation, the control circuit
27
calculates a primary voltage command value which decreases the average value of the primary current, and provides the calculated value to the PWM circuit
28
. As a result, the voltage shift PWM inverter formed by the PWM circuit
28
and the inverter
1
a
supplies the induction motor
2
with a primary voltage that matches the primary voltage command value provided from the control circuit
27
.
Since the conventional induction motor control device detects, as described above, the primary current of the induction motor and controls the amplitude of the primary voltage in a manner to minimize the amplitude of the primary current, information necessary for control is the primary current alone, hence permitting control with an inexpensive device configuration. In the case of variable speed driving of the induction motor, however, the amplitude of the primary voltage varies with changes in a frequency command value of the voltage shift PWM inverter, it is impossible to control the amplitude of the primary voltage in such a manner as to minimize the amplitude of the primary current. On this account, the conventional control device is incapable of driving the induction motor with the maximum efficiency during variable speed operation.
Furthermore, during constant speed driving, too, when the induction motor-generated torque goes below the on-load torque due to too rapid changes in the amplitude of the primary voltage and the rotational speed of the induction motor begins to decrease, induced voltage also drops in proportion to the speed of the motor. In consequence, further reduction of the amplitude of the primary voltage leads to a decrease in the amplitude of the primary current, giving rise to a problem that the induction motor stops at the worst. To avoid this, it is necessary to make the amplitude variation of the primary voltage gentle, but in a particular use of the induction motor that involves frequent changes in the on-load torque during fixed speed driving, so it is impossible to achieve maximum efficiency driving that follows in variations in the on-load torque well.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a control device which detects the primary current and allows high efficiency control of the induction motor during accelerating and decelerating driving as well as during constant speed driving and which permits high efficiency control of the induction motor without decreasing control stability, even in an abrupt or substantial changes in the on-load torque.
According to an aspect of the present invention, there is provided an induction motor control device which comprises: a power conversion circuit for generating a variable voltage variable frequency, AC primary voltage to drive an induction motor; a current sensor for sensing the primary current which is supplied from the power conversion circuit to the induction motor; a current component calculation circuit for calculating first and second current components from the primary current and a preset frequency command value of the AC primary voltage; a magnetic flux command calculation circuit for a magnetic flux command value such that the amplitude ratio between squared values of the first and second current components takes a predetermined value; a voltage component command calculation circuit for calculating a primary voltage component command value from the frequency command value and the magnetic flux command value; and a primary voltage command calculation circuit for calculating a primary voltage command value for the induction motor from the frequency command value and the primary voltage command value and for providing the calculated value to the power conversion circuit. The current component calculation circuit is adapted to calculate first and second current components in phase and 90° out of phase with the primary voltage component command value, respectively.
With this control device, it is possible to effect high efficiency control of the induction motor during accelerated and decelerated driving as well as during constant speed driving. In the use of the induction motor which involves abrupt or substantial changes in the on-load toque, too, high efficiency driving of the induction motor can be achieved without decreasing stability. Moreover, since information necessary for control is only the primary current that is supplied to the induction motor, the control device can be constructed at low cost.
According to another aspect of the present invention, there is provided an induction motor control device which comprises: a power conversion circuit for generating a variable voltage variable frequency, AC primary voltage to drive an induction motor; a current sensor for sensing the primary current which is supplied from the power conversion circuit to the induction motor; a current component calculation circuit for calculating first and second current components from the primary current and a preset frequency command value of the AC primary voltage; a magnetic flux command calculation circuit for calculating a magnetic flux command value such that the amplitude ratio between squared values of the first and second current components takes a predetermined value, and a magnetic flux differentiation command value based on the differentiation of the magnetic flux command value; a voltage component command ca
Kinpara Yoshihiko
Koyama Masato
Okuyama Miho
Sakurai Hisao
Ip Paul
Leydig , Voit & Mayer, Ltd.
Mitsubishi Denki & Kabushiki Kaisha
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