| 摘要 |
PURPOSE:To make an operation of an electromagnet with a current limiting capacitor which limits the current applied to a coil smooth by providing a discharge circuit in order to discharge residual charge of the capacitor. CONSTITUTION:When a semiconductor switch 21 for controlling a magnet is turned off, residual voltage identical to a peak voltage of an AC source voltage remain between both terminals of a capacitor 3. Therefore, the resultant voltage of the residual DC voltage of the capacitor 3 and the AC source voltage is applied to the semiconductor switch 21 and the maximum voltage reaches twice of the peak value of the source voltage. If the voltage exceeds a break-over voltage, a break-over phenomenon is produced and the voltage between the terminals becomes almost null and a break-over current flows. A current larger than a holding current of the electric magnet is applied to a trigger resistance 11 and a coil 2. At this time, a thyristor 10 is turned off and the residual charge of the capacitor 3 is discharged successively through a discharge resistor 9, the thyristor 10, the trigger resistance 11, a full wave rectifier 1, the coil 2 and a resistance 4 so that the residual voltage of the capacitor 3 becomes null. |
