Motor speed control system

摘要

A speed control system for an a.c. electric motor (1) comprises a microcomputer (6) having an interrupt signal input (INT/TO) to which is coupled the outputs of a detector (10) for detecting zero-crossings of the a.c. supply voltage and a tachogenerator (11) driven by the motor. The microcomputer supplies firing pulses to a triac (2), connected in series with the motor, in response to an overflow of a clocked counter (CT). This counter is 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.