Electricity: motive power systems – Induction motor systems – Primary circuit control
Patent
1982-11-26
1984-06-26
Witkowski, Stanley J.
Electricity: motive power systems
Induction motor systems
Primary circuit control
318806, 318802, 318809, H02P 126
Patent
active
044568681
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention relates to an AC motor control method and an apparatus therefor. More particularly, the invention relates to a method and apparatus for controlling AC motors wherein torque irregularity is suppressed and an excellent response obtained by executing control which take secondary leakage reactance into consideration.
BACKGROUND ART
It is well known in the art that variable frequency (VF) and variable voltage-variable frequency (VVVF) are available as control methods for converting direct current into alternating current by means of an inverter circuit to drive an induction motor through use of the alternating current. With the VF control method, a primary frequency, which is the output of the inverter circuit, is varied in accordance with a speed command. With the VVVF control method, the amplitude of the primary voltage also is varied in proportion to the change in primary frequency, with the output torque being held constant. These control methods deal with the voltage and current applied to the induction motor in terms of amplitude and frequency, but both of them are mean value control methods. It is not possible, therefore, to achieve fine control with good response. Accordingly, in order to improve upon this disadvantage, a so-called "vector control method" has recently been developed and put into practical use. According to such method, a pulse-width control method is employed to control the momentary value of the stator current of an induction motor, enabling torque generation similar to that seen in a shunt-wound DC machine. The vector control method applied to induction motors is based on the torque generating principle of a shunt-wound DC machine and controls the momentary value of a stator current to generate a torque in the same manner as said DC machine.
A brief description of the vector control method will now be set forth.
In general, the torque generating mechanism of a shunt-wound DC machine is such that a current switching operation is effected by a commutator in order that the magnetomotive force of an armature current I.sub.a will lie perpendicular to the main magnetic flux .phi. at all times, as shown in (A), (B) of FIG. 1. The generated torque T is expressed by the following equation, the torque T.sub.a being proportional to the armature current I.sub.a if the main magnetic flux .phi. is constant:
In FIG. 1(A), FM denotes field poles, AM an armature, and AW the armature winding.
In order to apply the foregoing relation to an induction motor, correspondence is established between .phi. and the magnetic flux vector .phi..sub.2 of a rotor, and between I.sub.a and a secondary current vector I.sub.2. Accordingly, to drive an induction motor in accordance with a principle resembling the generation of a torque by means of a shunt-wound DC machine, control should be effected in such a manner that the relation between the rotor flux vector .phi..sub.2 and the secondary current I.sub.2 remains as shown in FIG. 1(B) at all times, that is, in such a manner that these vectors are made to cross each other perpendicularly.
Thus, in accordance with vector control, the equivalent circuit of an induction motor may be considered to have the configuration shown in FIG. 2. That is, the perpendicular relation between the magnetic flux .phi..sub.2 and the secondary current vector I.sub.2 is assured by neglecting secondary leakage reactance. As a result, the generated torque T.sub.a, neglecting secondary leakage reactance, is expressed by: .apprxeq.k.multidot.I.sub.2 .multidot..phi..sub.m ( 2) current I.sub.o). FIG. 3 is a vector diagram of a two-phase induction motor, in which the C-D axes represent a coordinate system which coincides with the phase of the main flux .phi..sub.m, and the A-B axes represent the static coordinate system of the stator. Furthermore, I.sub.1 denotes the stator current (primary current), I.sub.o an excitation current component, and I.sub.2 a secondary current. I.sub.1a, I.sub.1b denote the A and B axis components of the stator current I.sub.1, namely
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patent: 4392100 (1983-07-01), Stanton et al.
Kawada Shigeki
Yamamura Sakae
Evans A.
Fanuc Limited
Witkowski Stanley J.
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