| 摘要 |
PROBLEM TO BE SOLVED: To extend the life of an electrode and thereby extend the intervals of maintenance by detecting a firing angle and moving 3 non-rotary electrode in a manner to make the closest gap distance adjustable so as to maintain the firing angle within a predetermined range. SOLUTION: A firing angle detection means 6 detects a firing angleθas a reference firing angle when the gap distance between one of four rotary electrodes 1 and one of two non-rotary electrodes 2 becomes not more than a predetermined dielectric breakdown distance (d) every time the four rotary electrodes make one-fourth rotation and both the electrodes are fired. Then, an electric actuator control circuit part 5c that has received signals S0 separately from each of the non-rotary electrodes 2 assigns four kinds of firing anglesθ1 -θ4 corresponding to the four rotary electrodes 1 to each of the two non-rotary electrodes 2. When the minimum valueθmin of the firing anglesθ1 -θ4 becomes lower than a predetermined lower limit valueθL, the non-rotary electrode 2 is so moved toward a rotary shaft 3 that the condition ofθmin >θL should be satisfied within the range where the maximum valueθmax of the firing anglesθ1 -θ4 does not exceed an initial firing angleθO. In this case, 0<θL<θO should be satisfied.
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