Rotation-supervised d-c fan or blower system

Claim 1

Rotation-supervised d-c fan, blower, or ventilator system having a brushless d-c motor (G) arranged for connection to a source of d-c (10) providing intermittent, pulsed power to said motor to operate said motor at a predetermined speed, or within a predetermined speed range, current flow (i) through the motor (G) being in pulses undulating between upper and lower values, whereby the current through the motor will have an a-c component, said system comprising an impedance element (Z) connected in series with the motor (G); an a-c responsive evaluation circuit (K, T) sensitive to the frequency of an applied signal, said evaluation circuit being connected across said impedance element and sensing the voltage drop of the a-c component of the pulsating current flow through the motor; an a-c amplifier (V, V') connected across the impedance element (Z), and applying the a-c component of the voltage drop across the impedance element (Z) to the evaluation circuit (K, T); and a band pass filter means (F) connected to the evaluation circuit (K, T) and to the motor and matched to the frequency of the a-c component at said operating speed, or range, to enhance said a-c component at said speed, or range, with respect to other frequencies at other speeds; the evaluation circuit (K, T) providing an output signal (U AO ) if the frequency of the sensed a-c component of the pulsating current flow through the motor drops below a predetermined level.