in series between the hot h power supply wire 24 and the respective winding 18 20 and a thermistor 30 b 32 b disposed in close proximity to the respective resistor 30 a 32 a the resistors 30 a 32 a each have an electrical resistance on the order of approximately 2 ohms for example and dissipate power in the form of heat due to the respective winding currents imain iaux so that the temperature rises detected by the respective thermistors 30 b 32 b provide a measure of the respective rms winding currents imain_rms iaux_rms for purposes of the present invention however it is not necessary to know the magnitude of either imain or iaux only their difference since airflow blockage is indicated by a winding current difference in excess of a calibrated setpoint sp the relationship of the ac winding currents imain and iaux for a given forced air cooling system and various degrees of airflow blockage is graphically depicted in fig2 the data was obtained by variably restricting inlet airflow area airflow intake blockage and measuring the resulting airflow flow and winding currents imain iaux for the test system a current differential of approximately 120 ma is observed for airflow blockages of approximately 0 50 however the currents imain and iaux diverge as the blockage increases above 50 with imain decreasing and iaux increasing in the illustrated example the highest degree of divergence occurs with blockage above 60 allowing the setpoint sp to be calibrated substantially as shown in fig2 to provide reliable detection of airflow blockage in excess of 60 referring again to fig1 the detection circuit 10 includes a power supply ps 33 connected across the hot h and neutral n wires 24 26 for supplying a low level dc voltage such.