Electricity: motive power systems – Induction motor systems – Primary circuit control
Patent
1982-12-14
1985-12-10
Shoop, Jr., William M.
Electricity: motive power systems
Induction motor systems
Primary circuit control
318802, 318806, H02P 534
Patent
active
045582696
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
This invention relates to an induction motor drive apparatus and, more particularly, to an induction motor drive apparatus which produces three-phase current commands through digital processing executed by use of a microcomputer.
Proposed methods of controlling induction motors include a so-called vector control method and field acceleration control method. Using such methods of control, the primary current of an induction motor can be controlled in terms of the instantaneous magnitude thereof to enable fine control having comparatively good response.
FIG. 1 shows an equivalent circuit of an induction motor and is useful for describing vector control. In FIG. 1, l.sub.m denotes excitation reactance, r.sub.2 represents equivalent resistance, and s designates slip. If we consider the equivalent circuit of the induction motor in this fashion, the generated torque T will be expressed by: ##EQU1## Note that .omega..sub.s represents the slip frequency. If we assume that I.sub.2 is proportional to s.multidot..omega..sub.s, then the torque T will be proportional to the secondary current and will have a torque generating mechanism similar to that of a DC motor. The following will hold from FIG. 1: ##EQU2## Accordingly, to make I.sub.2 and s.multidot..omega..sub.s proportional to each other, the excitation current I.sub.o must be made a constant quantity.
Therefore, according to vector control, the excitation current I.sub.o is held constant and only the secondary current I.sub.2 is varied, in proportion to the load torque, while perpendicularity between the excitation current I.sub.o and secondary current I.sub.2 is maintained. Also, since an error (speed error) ER between a commanded speed and the actual speed may be regarded as a torque command, the primary current I.sub.1 is defined so as to satisfy the following, in accordance with the speed error ER:
In the prior art, the primary current is generated in accordance with the following method. Specifically, the method includes generating mutually perpendicular, two-phase primary current commands i.alpha. and i.beta. in accordance with the speed error ER and actual speed n, and converting these two-phase primary current commands i.alpha. and i.beta. into three-phase commands through the use of a two-phase to three-phase converter, whereby three-phase primary current commands i.sub.u, i.sub.v and i.sub.w are generated. FIG. 2 is a block diagram of an example of the prior art, in which numeral 10 denotes an induction motor, 11 a two-phase primary current generator for generating current commands i.alpha. and i.beta. of an amplitude and frequency which conform to the speed error ER and actual speed n, and 12 a two-phase to three-phase converter. As illustrated in FIG. 3, the two-phase to three-phase converter 12 is composed of resistors R.sub.1 through R.sub.4 each having a resistance of 20 K.OMEGA.., a resistor R.sub.5 of 11.55 K.OMEGA., a resistor R6 of 10 K.OMEGA., and operational amplifiers OPA.sub.1 and OPA.sub.2. In accordance with the resistance values set in this fashion, the two-phase to three-phase converter 12 performs the following vector operations to convert the mutually perpendicular two-phase currents into three-phase currents: ##EQU3##
Returning to FIG. 2, numerals 13 and 14 denote current transformers for sensing phase currents i.sub.ua and i.sub.va which flow in the U and V phases of the three-phase induction motor 10, respectively, and numeral 15 denotes an arithmetic circuit which performs the following addition operation for producing a phase current i.sub.wa to flow in the W phase of the induction motor 10: to each of the three phases, for computing and then amplifying current differences (i.sub.u -i.sub.ua), (i.sub.v -i.sub.va) and (i.sub.w -i.sub.wa). Numeral 17 denotes a drive unit which includes a pulse-width modulator and an inverter comprising transistors.
As illustrated in FIG. 4, the pulse-width modulator PWM in the drive unit 17 includes comparators COM.sub.u, COM.sub.v, COM.sub.w, NOT gates NOT.sub.1 throu
REFERENCES:
patent: 4099109 (1978-07-01), Abbondanti
patent: 4266176 (1981-05-01), Fulton
patent: 4357569 (1982-11-01), Iwakane
patent: 4361794 (1982-11-01), Kawada
patent: 4364109 (1982-12-01), Okado
patent: 4377779 (1983-03-01), Plunkett
patent: 4384244 (1983-05-01), Matsumoto
patent: 4456868 (1984-06-01), Yamamura
Patent Abstracts of Japan, vol. 7, No. 8, Jan. 13, 1983, p. (E-152) (1153)-for Japanese Pat. No. 57-166890.
Supplementary European Search Report, the Hague, Jul. 20, 1984, Examiner: Houillon.
Fujioka Yoshiki
Ishida Hiroshi
Ota Naoto
Fanuc Ltd
Keane Patrick C.
Shoop Jr. William M.
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