Abstract:The main shaft hoist system is powered by #1 and #2 electric current closed loop drive systems. The #2 rectifier panel could start when the electric current was under 1000A. However, as the current increased to 1800A, a “fast-acting fuse” alarm was registered, with one fast-acting fuse blown out, followed by abnormal sound of the motor. An investigation was made and the rectifier panel and the main drive system were checked. The wave forms of both primary and secondary sides of the panel were measured. Judging from the wave aberration, the 6RA70 digital speed regulating device was found to be broken, as the pulses distributed to the six sets of thyristors were uneven. Consequently the thyristors were unable to change phases normally, and the DC EMF and AC voltage were in series aiding, forming a short circuit, which blew out the fast-acting fuse.
