Motor speed control system.

摘要

A speed control system for an a.c. electric motor (1) comprises a microcomputer (6) to an interrupt signal input ( @@@/TO) of which are coupled the outputs of a detector (10) for zero-crossings in the a.c. supply and a tachogenerator (11) connected to the motor. The microcomputer supplies firing pulses to a triac (2), included in series with the motor, in response to an overflow of a clocked counter (CT), this counter being suitably preloaded at each zero-crossing. The counter is read each time a tachogenerator pulse occurs and the time which this reading indicates has elapsed since the immediately preceding zero-crossing is stored. A record is also kept in a register (ZCSLTA) of how many zero-crossings occur between each tachogenerator pulse and the next and from this and the stored times relating to the relevant pulses the period of the tachogenerator pulses and hence the actual speed of the motor is calculated. The preloading value for the counter is calculated from this and the required speed.