Abstract:
An airflow blockage detection apparatus for a permanent split-capacitor single-phase cooling fan motor measures and compares electrical currents in main and auxiliary windings of the motor to detect airflow blockage. Main and auxiliary current sensors detect AC currents in the main and auxiliary windings, respectively, and a blockage detection circuit forms a difference between the detected currents. An airflow blockage alarm is activated when the current difference exceeds a specified setpoint indicative of abnormally low airflow.

Description:
TECHNICAL FIELD 
   This invention relates to apparatus for monitoring the operation of a fan motor, and more particularly to an electrical circuit for detecting fan airflow blockage when the motor is a split-capacitor single-phase induction motor. 
   BACKGROUND OF THE INVENTION 
   Cooling fans for heat-sensitive electrical equipment are frequently driven by single-phase induction motors having main and auxiliary windings with one or more capacitors connected in series with the auxiliary winding. Such motors, sometimes referred to as permanent split-capacitor motors because the capacitor is continuously in series with the auxiliary winding, are widely used in cooling fan applications due to their low cost of manufacture and starting ease. In many cases, it is necessary to provide an indication of cooling loss should the fan airflow become blocked by accumulation of dust or foreign objects. Mechanical airflow sensors such as vane switches are known, of course, but such switches require periodic calibration and are not particularly reliable. Accordingly, what is needed is more reliable and trouble-free apparatus for detecting airflow blockage. 
   SUMMARY OF THE INVENTION 
   The present invention is directed to an improved airflow blockage detection apparatus for a permanent split-capacitor single-phase cooling fan motor, where electrical currents in main and auxiliary windings of the motor are measured and compared to detect airflow blockage. Main and auxiliary current sensors detect AC currents in the main and auxiliary windings, respectively, and a bridge circuit forms a difference between the detected currents. An airflow blockage alarm is activated when the difference exceeds a specified set-point indicative of abnormally low airflow. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram of a fan blockage detection circuit for a permanent split-capacitor single-phase cooling fan motor according to this invention. 
       FIG. 2  is a graph depicting main and auxiliary windings currents of the motor of  FIG. 1  for various degrees of airflow blockage. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring to the drawings, and particularly to  FIG. 1 , the reference numeral  10  generally designates an airflow blockage detection circuit for a cooling apparatus including a fan  12  and a permanent split-capacitor single-phase induction motor  14 . The motor  14  has a rotor  16  mechanically coupled to the fan  12 , a stator supporting main and auxiliary electrical windings  18  and  20 , and a capacitor  22  (which may be external or internal) connected in series with the auxiliary winding  20 . The single-phase AC power supply for motor  14  includes hot (H), neutral (N) and ground (G) wires  24 ,  26 ,  28 . The hot (H) and neutral (N) wires  24 ,  26  are connected across both the main winding  18  and the series combination of auxiliary winding  20  and capacitor  22 , and the ground (G) wire  28  is connected to the motor housing. 
   In the illustrated embodiment, the main and auxiliary AC winding currents Imain, Iaux are measured with sensors  30 ,  32  responsive to the root-mean-square (RMS) winding currents Imain_rms, Iaux_rms in the main and auxiliary windings, respectively. Each of the sensors  30 ,  32  includes a precision resistor  30   a ,  32   a  connected 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 FIG.  2 . 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  FIG. 2  to provide reliable detection of airflow blockage in excess of 60%. 
   Referring again to  FIG. 1 , 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 as 5 volts, for example) across lines  34  and  36 . The thermistors  30   b ,  32   b  are coupled across the power supply output lines  34 ,  36  through respective shunt resistors  38 ,  40 , defining measurement junctions  42 ,  44 . Since the electrical resistances of thermistors  30   b  and  32   b  vary with their temperatures, which in turn vary with the RMS winding currents Imain_rms and Iaux_rms, the voltages at measurement nodes  42  and  44  provide an indication of the RMS currents Imain_rms and Iaux_rms. The nodes  42  and  44  are coupled to a bridge amplifier  46 , which provides a signal on line  48  indicative of the winding current difference (Iaux_rms−Imain_rms). The winding current difference signal on line  48  is supplied along with a calibrated setpoint SP to a hysteresis comparator  50 , which activates an alarm  52  if the current difference signal exceeds the setpoint SP. 
   In summary, this invention provide a reliable and inexpensive apparatus for detecting significant airflow blockage and issuing a warning to prevent overheating of heat-sensitive equipment such as electronic and computer circuitry. While described in reference to the illustrated embodiment, it is expected that various modifications in addition to those mentioned above will occur to those skilled in the art. For example, it is possible to measure average or peak-to-peak currents instead of RMS currents, and the current sensors  30 ,  32  will vary accordingly. For example, the sensors  30 ,  32  may be inductively or capacitively coupled to the lines  24 ,  26 , or the currents may be detected by simply measuring and rectifying the voltage across a series resistor. Also, the winding current difference may be detected directly in the inductivel coupled approach, if desired. Various other measurement techniques are also possible. Additionally, some or all of the signal processing may be performed by a suitably programmed microprocessor, if desired. Thus, it will be understood that circuitry incorporating these and other modifications may fall within the scope of this invention, which is defined by the appended claims.