Electricity: motive power systems – Induction motor systems – Primary circuit control
Patent
1986-04-25
1987-06-09
Smith, Jr., David
Electricity: motive power systems
Induction motor systems
Primary circuit control
H02P 540
Patent
active
046722874
DESCRIPTION:
BRIEF SUMMARY
DESCRIPTION
1. Technical Field
This invention relates to an induction motor digital control system and, more particularly, to an induction motor digital control system well-suited for application to digital control of an induction motor in which an excitation current serves as a reference.
2. Background Art
Various methods of controlling the velocity of an induction motor are available in the prior art. Typical methods are voltage/frequency (V/F) control, slip frequency control and vector control. Vector control is classified broadly into flux detection-type vector control and slip frequency vector control. There is need of late for quick-response control which, using a microprocessor, takes transient phenomena into account. For this reason, vector control has drawn attention. As opposed to a system which treats voltage and current as scalar quantities having amplitude and frequency, the vector control system controls instantaneous values in each of three phases by treating voltage and current as vector quantities having two components formed by the instantanteous values of three-phase voltage or current. With the vector control system, instability and a limitation upon response ascribable to the scalar treatment are eliminated to make possible control equivalent to that applied to DC motors.
However, the vector control system is such that control is exercised with secondary flux serving as a reference. (Taking the secondary flux along the d axis, the vector control system is based upon a q axis orthogonal to the d axis.) In such a control system, therefore, the secondary flux must be detected, problems are encountered in terms of the response for control, and costs are raised.
An object of the present invention is to provide an induction motor digital control system in which, in order to eliminate the aforementioned difficulties, control is effected in vector fashion on the basis of excitation current, thereby making possible control equivalent to that applied to a DC motor by raising the control response and, at the same time, making possible a reduction in cost.
Another object of the present invention is to provide an induction motor digital control system suitable for digital control using a microcomputer.
Still another object of the present invention is to provide an induction motor digital control system which corrects the back electromotive force of an induction motor and performs precise digital control based on a primary current command.
DISCLOSURE OF THE INVENTION
The present invention provides an induction motor digital control system in which the actual velocity of an induction motor is made to follow up a commanded velocity by correcting a difference between the actual velocity and the commanded velocity, and which possesses a current negative feedback loop. The system comprises means for obtaining an excitation current from the actual velocity of the induction motor and a torque command of the difference, means for obtaining a secondary current, which is a load current, from the torque command of the difference, means for obtaining a primary current amplitude and a current phase based on the excitation current and secondary current, means for obtaining a slip frequency from the actual velocity and torque command, means for obtaining an excitation phase of a rotating field from the slip frequency and actual velocity, means for obtaining a current command in each phase based on the primary current amplitude and current phase, means for obtaining a voltage command based on a current deviation between the current command in each phase and a feedback current from the current negative feedback loop, means for generating an electromotive force for correcting the back electromotive force based on the excitation current, and means for superposing the electromotive force, which is for back electromotive force correction based on the excitation current, upon the voltage command. The means for obtaining the excitation current from the actual velocity of the induction motor and a torque command of the
REFERENCES:
patent: 4451771 (1984-05-01), Nagase et al.
patent: 4456868 (1984-06-01), Yamamura et al.
patent: 4459534 (1985-07-01), Nagase et al.
patent: 4543520 (1985-09-01), Danz et al.
Fujioka Yoshiki
Kouno Shin-ichi
Fanuc Ltd
Smith Jr. David
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