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Current-control-apparatus-for-electric-power-systems

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Details

Inventors: Otsuka, Kenichi; Shimamura, Takeo;
Assignee: Tokyo Shibaura Denki Kabushiki Kaisha (Kawasaki, JP)
Primary Examiner: Smith, Jr.; David
Assistant Examiner:
Attorney, Agent or Firm: Oblon, Fisher, Spivak, McClelland & Maier

A current control apparatus includes an instructor for providing a DC amplitude instruction and an AC phase instruction, and a cycloconverter for supplying an induction motor with AC load currents. These load currents are detected by sensors. The sensed signals from the sensors are applied to a detector. The detector generates a DC phase error signal and a DC amplitude signal according to the load currents and the phase instruction. The amplitude instruction, amplitude signal and phase error signal are applied to a comparator circuit. The comparator circuit generates a first voltage instruction and a second voltage instruction. These first and second voltage instructions are applied to a control circuit. The control circuit supplies the cycloconverter with voltage instructions. The comparator circuit, control circuit, cycloconverter, sensors and detector constitute a closed DC feedback control loop which utilizes a vector control method.

DETAILED DESCRIPTION It is accordingly the object of the present invention to provide a current control apparatus for electric power systems in which stationary phase and amplitude errors between a current instruction input and each of the actual polyphase AC load currents are eliminated.
A further object of the invention is to provide a current control apparatus which is not only free from stationary phase and amplitude errors but also possesses good transient response characteristics.
The present invention is derived from the following concept.
An AC feedback control system generally has a certain phase error between a control instruction value and an actual controlled value, even in a stationary state.
On the other hand, a certain DC feedback control system can completely avoid the phase error of an AC control system.
This is because in a DC control system, a control instruction input and a feedback signal representing an actual load current are both DC, and thus, a detected error signal for feedback control is also DC.
DC does not depend on the frequency of the load current.
Further, when the DC feedback loop includes an integration element, it is also possible to avoid a stationary amplitude error between the control instruction input and the actual load current.
A frequency-free DC data for the DC feedback control is obtained by utilizing a relation: A.
sup.
2 sin.
sup.
2 .
omega.
t+A.
sup.
2 cos.
sup.
2 .
omega.
t=A.
sup.
2 (constant) where .
omega.
denotes an angular frequency of the AC load current and t denotes time.
Data A(=.
sqroot.
*A.
sup.
2) indicates the amplitude of AC load current but is independent of the AC component or time component.
That is, data A or A.
sup.
2 is a kind of DC.
A frequency-free DC phase error data for the DC feedback control is obtained from the phase difference between an AC load current and an AC control instruction whose angular frequency is the same as that of the AC load current.
The frequency-free DC amplitude data (Eq.
(12)) can be independent of the frequency-free DC phase error data (Eq

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