Motor speed control arrangement

Claim 1

A motor speed control arrangement for changing the speed of a motor to, and/or maintaining the speed of the motor at, any required speed within a given range in dependence upon the difference (if any) between the actual speed of the motor, as represented by the periodicity of signals from a tachogenerator driven by the motor, and the required speed, the speed being varied by controlling the phase angle in each half-cycle of a mains voltage supply at which a controlled semiconductor switch is fired by the control arrangement, the switch being connected in series with the motor across the mains voltage supply, said arrangement comprising a clock-driven cycling counter the cycle frequency of which is a plurality of times the frequency of individual half-cycles of the mains voltage supply, which counter is arranged to supply an interrupt signal each time it overflows, first storage means for a number representative of the time elapsing between the arrival instants of a first and a subsequent second tachogenerator signal, second storage means for a number representative of the time which would elapse between the first and second tachogenerator signals if the motor were running at the correct speed, and third storage means for receiving a number determined by the firing period from the beginning of a mains half-cycle to the instant in that half-cycle at which the switch is required to be fired in order to provide the appropriate speed control, said arrangement being arranged to perform the operations of (a) responding to the arrivals of the first and second tachogenerator signals by reading the contents of the counter at the relevant instants, (b) responding to the occurrence of at least some of said interrupt signals by adjusting the content of the first storage means in such manner that said content eventually depends upon the values of the counter contents so read and the number of counts executed by the counter between the interrupt immediately following the occurrence of the first tachogenerator signal and the interrupt immediately preceding the occurrence of the second tachogenerator signal and thereby becomes representative of the period between said instants, (c) calculating said firing period in dependence upon the contents of the first and second storage means when the content of the first storage means has become representative of the period between said instants, (d) storing a number determined by a firing period calculated in an operation (c) in said third storage means, (e) responding to the occurrence of each zero-crossing in the mains voltage waveform by, each time a said interrupt signal occurs thereafter, decrementing a number stored in the third storage means in an operation (d) by an amount corresponding to the number of counts executed by the counter since the immediately preceding interrupt occurred or, in the case of the first said interrupt after said subsequent zero-crossing, since that zero-crossing occurred, until the number stored in the third storage means becomes smaller than that which corresponds to a complete cycle period of the counter, (f) responding further to the interrupt signal which results in the number stored in the third storage means becoming smaller than that which corresponds to a complete cycle period of the counter by setting the counter content to a value determined by the difference between the then value of the number stored in the third storage means and the total counter capacity, and (g) responding to the occurrence of the next said interrupt signal by firing said switch.