signal so generated being supplied to a pulse generator circuit 9 more specifically the transfer function of the arithmetic unit is described as t at b where t represents the duration in which the coil 11a or 11b is to be energized and t represents the interval detected by the period detector 6 a is a constant that determines the response characteristic of the speed control system and b is a parameter which as described later is manually variable when the sewing machine is operated in a low speed range while the frequency dividing factor of the variable frequency divider 4 is maintained constant typically at unity the pulse generator 9 comprises a programmable counter 9a of which the full count value is preset to the binary signal from the arithmetic unit 8 and is clocked by source 6b through a gate 9b to increment its count to generate an output when the preset count is reached the pulse generator circuit 9 further includes flip flops 9c and 9d and an or gate 9e flip flops 9c and 9d respond respectively to acceleration and deceleration signals from arithmetic unit 8 to activate coils 11a and 11b through linear amplifiers 10a and 10b the outputs of flip flops 9c and 9d are coupled via or gate 9e to the control terminal of gate 9b to pass clock pulses from source 6b to programmable counter 9a gate 9b.