D.C. motor starting system supplied from poly-phase mains - uses main and cut-off thyristors at lower than mains frequency

摘要

To avoid excessive starting currents of the d.c. motor the working frequency of the thyristors during the starting period is reduced. At first the main thyristors with their main diodes are blocked in the first instance and only the cut-off thyristors are fired to discharge or change the polarity of their capacitors. When the capacitors are brought to the right cut-off polarity through the diodes a lower than mains or pulse frequency fires the cut-off thyristors. The main thyristors are then fired with a lower frequency. All thyristors are then fired with a lower frequency. Finally the pulse or mains frequency is used with the lowest possible d.c. voltage.