Electricity: motive power systems – Induction motor systems
Reexamination Certificate
2000-12-20
2002-04-23
Masih, Karen (Department: 2837)
Electricity: motive power systems
Induction motor systems
C318S802000, C318S801000, C318S811000, C318S813000
Reexamination Certificate
active
06377017
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a control device for an alternating current motor. More specifically, the present invention relates to a technique used in an electric current feedback control system for carrying out a follow-up control of a measured value of the electric current with respect to an ordered value of the electric current.
2. Description of Related Art
In an alternating current (AC) motor, such as a permanent magnet type motor which utilizes a permanent magnet in the field, a control device for an AC motor is generally known which measures the electric current of an armature of the AC motor, converts the measured value into rectangular coordinates which rotate in synchronization with a rotor, i.e., the dq coordinate system, and carries out a feedback control so that the deviation between the ordered value and the measured value of the current on the dq coordinate becomes zero.
In such an AC motor, a change in the magnetic flux density is generated in the field magnetic flux which penetrates through a coil of the armature when, for instance, a rotor having a permanent magnet rotates, and a back electromotive voltage Er, which acts to cancel the supplied voltage of the coil electric current, is generated. The back electromotive voltage Er increases as the number of rotations of the rotor increases and, when Er becomes equal to the supplied voltage of the coil electric current, the coil electric current becomes zero and the rotation torque of the rotor also becomes zero.
A so-called weak field control is known, which makes it possible to increase, for instance, the operable range of the number of rotation, the rotation torque which may be generated, the number of rotations at which the motor can operate with high efficiency, and the range of rotation torque, by weakening the magnetic flux of the field equivalently.
FIG. 4
is a vector diagram showing a stationary state of an example of a conventional control device for an AC motor when a vector control is performed. In the figure, the direction of the magnetic flux of the field is indicated by the d-axis and the direction which is perpendicular to the d-axis is indicated by the q-axis. Ld and Lq indicate the inductance of the d-axis and the q-axis, respectively; R indicates an each phase resistance of the alternating current motor; &ohgr;re indicates a velocity of the electrical angle of the AC motor; &phgr; indicates a main magnetic flux of the field of the AC motor; id and iq indicate the electric current along the d-axis and the q-axis, respectively; vd and vq indicate the voltage along the d-axis and the q-axis, respectively, and Vmax indicates a maximum voltage which may be supplied to each phase of the AC motor.
In this case, the voltage, vd, in the d-axis and the voltage, vq, in the q-axis may be expressed by the equations (1) shown below. In the equation, &ohgr;re×Ld×id, which is a q-axis interference term, becomes a weak field component when the back electromotive voltage Er exceeds the maximum voltage Vmax, which may be supplied to each phase of the AC motor, and a weak field control is performed. Accordingly, the vector of the q-axis interference term extends downwardly in
FIG. 4
when the electric current id in the d-axis is increased and, hence, the AC motor may be actuated by using a voltage smaller than the back electromotive voltage Er=(&ohgr;re×&phgr;) at the voltage vq in the q-axis. In this manner, a desired rotational torque may be output by increasing the number of operational rotations.
Vd=R×id−&ohgr;re×Lq×iq
(where
&ohgr;re×Lq×iq
indicates d-axis interference term)
Vq=R×iq+&ohgr;re×&phgr;+&ohgr;re×Ld×id
(1)
(where
&ohgr;re×Ld×id
indicates q-axis interference term)
In the above-mentioned example of the conventional control device for the AC motor, after the back electromotive voltage, Er, of the AC motor exceeds the maximum voltage, Vmax, which may be supplied to each phase of the AC motor, and the rotation number is further increased, the actuation of the AC motor becomes impossible if &ohgr;re×Ld×id, which is the q-axis interference term and becomes a weak field component, is not increased in accordance with an increase in the back electromotive voltage Er=(&ohgr;re×&phgr;).
In the state described above, the magnitude of the voltage vq at the q-axis tends to be dominated by the magnitude of the electric current id at the d-axis. Further, because the d-axis interference term: −&ohgr;re×Lq×iq is present at the d-axis, the voltage vd on the d-axis tends to be dominated by the magnitude of the electric current iq on the q-axis.
In the above control device for an AC motor, however, the control of the electric current feedback is separately carried out for the d-axis and the q-axis. Hence, at the d-axis, the control is executed so that the deviation between the ordered value for the d-axis and the measured value of the electric current becomes zero and, at the q-axis, it is controlled so that the deviation between the ordered value for the q-axis and the measured value of the electric current becomes zero. For this reason, in a state in which one of the voltages vd and vq becomes dominant to the other voltage, there is a danger that the current control for stabilizing the AC motor may be destroyed. Thus, the current control may be destabilized and the electric current may be varied rapidly or a desired torque may not be obtained.
On the other hand, in the region where the back electromotive voltage Er of an AC motor is smaller than the maximum voltage, Vmax, which may be supplied to each phase of the AC motor, a so-called non-interactive control is known by which a d-axis compensation term and a q-axis compensation term that counteract each interference component for the d-axis and q-axis, respectively, are input so that an independent control of the d-axis and the q-axis becomes possible by counteracting the speed electromotive force components which interfere with each other between the d-axis and the q-axis.
However, in the region where the back electromotive voltage Er exceeds the maximum voltage Vmax, which may be supplied to each phase of the AC motor, the non-interactive control cannot be carried out since a power source, for instance, a battery or fuel cell, which supplies a voltage to the coil current, cannot provide the extra voltage. Thus, problems such as an instability in the control of the AC motor may be generated.
SUMMARY OF THE INVENTION
Accordingly, one of the objectives of the present invention is to provide a control device for an AC motor which can perform a stable control of the motor even when the back electromotive force of the AC motor is increased.
The above objectives may be achieved by a control device for an alternating current motor according to the present invention (for example, a control device
10
for an alternating motor explained in the embodiment described later), including: a target current generating unit (for instance, a target current computing unit
22
in the embodiment described later) which generates a current order value, based on a torque order (for example, a torque order, *T, in the embodiment described later), as a d-axis target current (for example, a d-axis target current, *id, in the embodiment described later) and a q-axis target current (for example, a q-axis target current, *iq, in the embodiment described later) on dq coordinates which are of a rotating rectangular coordinate system; a current detection device (for example, electric current detectors
16
and
17
in the embodiment described later) which detects an alternating current supplied to each phase (for example, a U-phase, V-phase, and W-phase in the embodiment described later) of a polyphase alternating current motor (for example, an AC motor
11
in the embodiment described later); a coordinate transforming unit (for example, a three-phase ac-d
Itoh Tomoyuki
Jinzai Hideyuki
Kondou Hajime
Saito Yuuji
Shinoki Hiroaki
Arent Fox Kintner & Plotkin & Kahn, PLLC
Honda Giken Kogyo Kabushiki Kaisha
Masih Karen
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