Electricity: motive power systems – Induction motor systems – Primary circuit control
Patent
1986-10-21
1988-07-12
Shoop, Jr., William M.
Electricity: motive power systems
Induction motor systems
Primary circuit control
318800, 318803, 318806, H02P 528
Patent
active
047572485
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
This invention relates to a system for controlling the torque of an induction motor.
Recently, vector control generally has been employed for controlling the velocity of induction motors. When controlling the velocity of an induction motor in accordance with such vector control, a torque command, excitation current, secondary current and slip frequency become non-linear due to such effects as the secondary leakage impedance and core loss of the motor in cases where velocity control is performed up to a region of high rotational velocity, as in the spindle motor of a machine tool, or the like, or in cases where control is performed to weaken excitation in accordance with the torque command at a constant rpm. As a result, the output torque also becomes non-linear with respect to the torque command and an accurate torque command cannot be produced.
A conventional approach for dealing with this is to use external equipment such as a torque sensor to measure motor torque as a means for accurately ascertaining the output torque of the motor. A problem that results is the high cost of equipment.
SUMMARY OF THE INVENTION
The present invention solves the aforementioned problem of the prior art, and an object thereof is to provide an induction motor torque control system in which output torque is linearly controlled in response to a torque command without requiring special torque measurement equipment.
A system for controlling the torque of an induction motor according to the present invention includes: means for deciding a torque command for an induction motor and an excitation flux command from an excitation frequency; means for deciding an excitation flux component of excitation current from the excitation flux; means for deciding a component, in the direction of an electromotive force, of the excitation current from the excitation flux and excitation frequency; means for deciding a component, in the direction of the electromotive force, of a secondary current of the motor from the torque command and excitation flux command; means for deciding a slip frequency from the component, in the direction of the electromotive force, of the secondary current and the excitation flux command; means for deciding an excitation flux component of the secondary current, from the component in the direction of the electromotive force, of the secondary current and from the slip frequency; and means for obtaining an excitation flux component, and a component in the direction of the electromotive force of the primary current of the motor, a the excitation flux component and from component in the direction of the electromotive force of the excitation current and secondary current, respectively, and for limiting a primary current command for the motor upon combining these components.
Accordingly, by obtaining a slip frequency command which will cause the required load current to flow, the present invention enables the torque command itself to be substituted for the output torque in order to linearly control the motor torque, this being performed without requiring special torque measurement equipment.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram illustrating the interconnection of devices in an embodiment of the present invention;
FIG. 2 is a flowchart illustrating a torque control operation of the present invention;
FIG. 3 is a schematic diagram illustrating an equivalent circuit of an induction motor; and
FIGS. 4 and 5 are vector diagrams showing the relationship of various characteristics of an induction motor.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
An embodiment of the present invention will now be described in detail with reference to the drawings.
First, the present invention is based on the relationship among various characteristics of an induction motor, including winding resistance R.sub.1, primary leakage reactance X.sub.1, excitation resistance R.sub.o, terminal voltage V, etc., and will be described with reference to the equivalent circuit of FIG. 3 and the v
REFERENCES:
patent: 4456868 (1984-06-01), Yamamura et al.
patent: 4499414 (1985-02-01), Fujioka et al.
patent: 4503376 (1985-03-01), Okuyama
patent: 4558269 (1985-12-01), Ishida et al.
patent: 4672287 (1987-06-01), Fujioka et al.
patent: 4672288 (1987-06-01), Abbondanti
Fujioka Yoshiki
Kouno Shin-ichi
Fanuc Ltd
Shoop Jr. William M.
Young Brian
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