machine is desirable the micro processor starts to drive the rotor reversely with a current whose strength directly proportional to the square of the instantaneous rotational speed of the motor at that moment so as to brake the rotor shaft as the initial speed of the motor is already lower than the positioning cruising speed or it is decelerated down to that speed and the needle positioning signal nd generated from the needle positioning signal detector 59 has been detected the rotor will stop being reversed at the moment that the rotation direction being changed has been detected by the rotation direction detector 57 two position signals with 90 degree phase difference generated by the position detector 58 with comparatively fine optical grid can serve as the two input signals of the rotation direction detector 57 with which to detect the exact point of termination of the braking process precisely so as to quickly stagnate the motor and consequently positionedly stagnate the sewing needle in order to more clearly describe the operative sequence of fig7 please refer to fig8 which in a wave form shows the variation of signals representing specific blocks shown in fig7 as time equals t1 t t1 an initial pedal signal pn from the control pedal of the sewing machine activates the rotational speed setting apparatus so as to make it set up a dead slow speed nl since at that moment the output signal fv of the rotation speed setting apparatus 56 equals zero the micro processor will then sent out the current in accord with the dead slow speed nl since the value of fv is zero.