Electricity: motive power systems – Induction motor systems – Primary circuit control
Reexamination Certificate
2000-11-07
2001-08-14
Dang, Khanh (Department: 2837)
Electricity: motive power systems
Induction motor systems
Primary circuit control
C318S810000, C318S801000
Reexamination Certificate
active
06275000
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of controlling an induction motor for increasing an output torque upon starting operation thereof. Further, the present invention is concerned with an apparatus for carrying out the control method mentioned above.
2. Description of Related Art
According to a hitherto known or conventional scheme for controlling the induction motor, an exciting current component (also referred to simply as the exciting current) and a torque current component (also referred to simply as the torque current) of an AC electric power supplied to the induction motor is controlled independently from each other by resorting to a control method known as the vector control or field-oriented control method. More specifically, upon starting of the induction motor (i.e., when operation of the induction motor is to be started), the exciting current is first supplied to the induction motor. Subsequently, after a sufficient or effective amount of magnetic flux has been generated, the torque current component is supplied to the induction motor for starting the operation of the same.
In addition, there is also known a so-called forcing control method according to which the exciting current of a magnitude which exceeds that of the rated exciting current is forcibly caused to flow through the induction motor for thereby generating the magnetic flux very speedily, i.e., at a very high rate. At the time point when the magnetic flux has reached the rated value, the exciting current is then decreased to the rated value thereof, whereupon the torque current is caused to flow through the induction motor for thereby allowing the operation thereof to be started.
However, in the case of the control apparatus for the induction motor which is equipped with an inverter constituted by a plurality of switching elements connected in the form of a bridge circuit, current limitation is necessarily imposed in view of the current withstanding capability of the switching elements. More specifically, the square root of a sum of a square of the exciting current component and that of the torque current component can not exceed the current limitation value imposed on the switching element. If otherwise, the switching element may unwantedly be injured. Such being the circumstances, there is the necessity of determining proportions of the exciting current component and the torque current component which can ensure a maximum torque within the limited current range. Of course, the attempt for generating the torque of greater magnitude will necessarily be accompanied with the need for employment of expensive switching elements each having a high current capacity and thus being expensive.
SUMMARY OF THE INVENTION
In the light of the state of the art described above, it is an object of the present invention to provide a method of controlling an induction motor which allows the torque generated by the induction motor upon starting operation thereof to increase significantly without need for employment of the expensive switching elements each having a large current capacity.
Another object of the present invention is to provide an induction motor control apparatus for carrying out the method mentioned above.
In view of the above and other objects which will become apparent as the description proceeds, there is provided according to an aspect of the present invention a control apparatus for performing a vector control of an induction motor, which apparatus includes a current control unit for controlling each of an exciting current component (ids) and a torque current component (iqs) of an AC power supplied to the induction motor on the basis of a command value, a current command value arithmetic unit for generating an exciting current component command value (ids*) and a torque current component command value (iqs*) for the exciting current component and the torque current component on the basis of a torque command value (&tgr;m*), and a starting unit designed for decreasing at least the exciting current component command value (ids*) while increasing at the same time the torque current component command value (iqs*) upon starting of the induction motor.
By virtue of the structure of the induction motor control apparatus described above, the torque generated by the induction motor upon starting operation thereof can be increased without need for employment of the switching elements of expensiveness and large current capacity, to a great advantage.
In the induction motor control apparatus described above, the starting unit should preferably be so designed as to decrease at least the exciting current component command value (ids*) while increasing simultaneously the torque current component command value (iqs*) to a level given by iqs*=(Imax
2
−ids*
2
)
½
upon starting of the induction motor, where Imax represents an allowable maximum current.
With the structure of the induction motor control apparatus described above, the torque generated by the induction motor upon starting operation thereof can also be increased without making use of expensive and large-capacity switching elements, to a great advantage.
According to another aspect of the present invention, there is provided a control apparatus for performing a vector control of an induction motor, which apparatus includes a current detector for detecting a primary current of the induction motor, a current arithmetic unit receiving as inputs thereto an output of the current detector and a primary frequency (&ohgr;) to thereby determine arithmetically a d-axis current component and a q-axis current component of the primary current, a slip frequency arithmetic unit receiving as inputs thereto a d-axis current component (ids) or alternatively a d-axis current component command value (ids*) of the primary current, a q-axis current component (iqs) or alternatively a q-axis current component command value (iqs*) of the primary current and the primary frequency (&ohgr;), to thereby determine arithmetically a slip frequency (&ohgr;s) of the induction motor in accordance with a predetermined functional arithmetic operation by using a constant of the induction motor, a rotational speed detector for detecting a rotational speed (rpm) of the induction motor, an adder circuit for adding together the slip frequency (&ohgr;s) outputted from the slip frequency arithmetic unit and an output of the rotational speed detector to thereby generate the primary frequency (&ohgr;), a current control unit for controlling the primary current of the induction motor so that the d-axis current component and the q-axis current component of the primary current can follow the d-axis current component command value (ids*) and the q-axis current component command value (iqs*), respectively, and a starting unit designed such that upon starting of the induction motor, the starting unit sets the d-axis current component command value as a predetermined value and decreases the d-axis current component command value while increasing the q-axis current component command value simultaneously, when the d-axis current component becomes substantially equal to the d-axis current component command value.
Owing to the structure of the induction motor control apparatus described above, the torque generated by the induction motor upon starting thereof can be increased without need for using the switching elements of large current capacity and expensiveness, to an advantage.
Further, in the induction motor control apparatus described above, the starting unit should preferably be so designed as to decrease the d-axis current component command value (ids*) to a predetermined value after the d-axis current component (ids) has become substantially equal to the d-axis current component command value (ids*).
With the arrangement described above, the time duration for sustaining the magnetic flux can be extended in correspondence to the predetermined value mentioned above, whereby the torque can be caused to decrease only slowly.
Furtherm
Dang Khanh
Mitsubishi Denki & Kabushiki Kaisha
Sughrue Mion Zinn Macpeak & Seas, PLLC
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