FAN MOTOR MONITORING CIRCUIT

Abstract: A circuit for monitoring the operation of a fan motor comprises a first circuit for monitoring the voltage supplied to the motor and a second circuit for monitoring the current supplied to the motor. An indicator circuit indicates a fault if the monitored voltage falls below a predetermined threshold or the monitored current falls below a predetermined threshold. The indicator circuit may cyclically turn the fan motor on and off to indicate a fault.

Description: FAN MOTOR MONITORING CIRCUIT The present invention relates to a fan motor monitoring circuit. Fan motors are used to deliver air to a wide range of units. For example, fan motors are used to blow air through filters in masks worn by persons operating in areas where the atmosphere contains potentially dangerous dust such as asbestos dust. To maintain the mobility of users of the masks the fans are battery powered. With the known mask, the fan is designed to supply surplus air to the user so that even when the user breathes in hard the supply pressure, that is the pressure beneath the mask in the region of the user's nose and mouth, is greater than atmospheric pressure. If this excess pressure is not maintained, dust-laden air can be drawn in around the edges of the mask, bypassing the filter. The air supply pressure can drop as a result of the batteries becoming discharged. This is a real problem as although the state of charge can be checked when a mask is first put on there is no guarantee that after several hours use the battery will not become sufficiently discharged to fail to drive the fan motor sufficiently fast. Even if for example a high quality nickel-cadmium battery is left on charge for many hours so that one is sure it is fully charged, one cannot be sure of the total amount of recoverable energy stored as this is a function of the internal condition of the battery which is difficult to assess. The air supply pressure can also drop as a result of the filter becoming clogged. This problem can only be avoided by frequently checking and cleaning the filter during use. It is easy to over-look this check. The air supply pressure can be directly monitored but this involves the provision of an extra 5 circuit, involving increased cost, weight and complexity. These problems are considerable because it is difficult to provide a simple and economic pressure transducer which can reliably detect relatively small differences between the supply • *» 10 pressure and atmospheric pressure. It is an object of the present invention to provide an improved fan motor monitoring circuit. According to the present invention, there is provided a circuit for monitoring the operation of a 15 fan motor, comprising a first circuit for monitoring the voltage supplied to the motor, a second circuit for monitoring the current supplied to the motor, and an indicator circuit for indicating a fault if the monitored voltage falls below a predetermined 20 threshold and/or the monitored current falls below a predetermined threshold. Battery failure is detected by the voltage monitor. Filter blockage is detected by the current monitor as the current supplied to a motor reduces as 25 the rate of air flow delivered by the fan reduces. The current monitor will also detect electrical faults in the fan motor and its connections to the battery. Thus potentially dangerous failures or reductions in efficiency of the equipment are 30 detected as soon as they occur. The primary application of the present invention is in the field of battery powered face masks but it will be appreciated that the invention will have utility in other applications of fan motor.-. 35 Preferably the indicator circuit comprises means for cyclically turning the fan motor on and off if the voltage and/or current fall below the thresholds. This is a particularly effective way of alerting the us.er of a face mask to problems with the fan motor circuit of his mask. The fan motor may drive a centrifugal fan. With such fans the motor current falls significantly as the air flow is reduced by for example clogging of a filter. Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, ' in which: Fig. 1 shows in block schematic form a circuit in accordance with the invention; and Fig. 2 illustrates in greater detail a circuit of the type illustrated in Fig. 1. Fig. 1 shows in block schematic form a circuit in accordance with the invention for monitoring the operation of a fan motor 1. The fan motor will be arranged to drive a fan which blows air through a • filter (not shown) to a mask (not shown) . The motor 1 is supplied from a battery 2 via lines 3 and 4. A voltage comparator circuit 5 is connected across lines 3 and 4, and a current comparator circuit 6 is connected across a low ohmic value resistor 7 in series with line 4. The voltage and current comparators receive respective inputs from voltage threshold setting circuit 8 and current threshold setting circuit 9. If the monitored voltage drops below the threshold set by circuit 8, circuit 5 provides an input to OR gate 10. If the monitored current drops below the threshold set by circuit 9, circuit 6 provides an input to O'd gate 10. If the OR gate receives an input from circuit 5 and/or circuit 6, it energises an alarm output circuit 11. Thus the user is immediately alerted if a fault condition is established. The alarm .output circuit could be for example a light emitting diode, buzzer, or the like. Preferably however the alarm output circuit comprises a circuit for turning the fan on and off cyclically and light emitting diodes giving a visual indication of the condition of the battery. Turning the fan on and off is a very effective way of alerting the user to a failure or an impending failure of the fan motor circuit. Referring to Fig. 2, the illustrated circuit is adapted to turn the fan on and off when the battery voltage or the motor current drops below preset thresholds. A battery 12 supplies DC current to a motor 13 of a centrifugal fan (not shown) via a series transistor 14 and a series resistor 15 of low ohmic value (e.g. 0.22 ohms). A diode 16 is connected across the motor so that when the motor is turning but disconnected from the battery the voltage generated, by the motor is short circuited by the diode. In normal operation, once the motor is turned on by closing an appropriately positioned mechanical switch (not shown), the semiconductor switch formed by transistor 14 remains conductive and current flows continuously in the battery circuit. A stable reference voltage is derived from the battery supply by a zener diode 17 and a parallel capacitor 18. A current threshold potentiometer 19 is used to apply a selected proportion of the reference voltage to an operational amplifier 20 wired as a comparator, the selected voltage setting the current threshold which is considered to be the minimum acceptable motor current. Tne current threshold setting voltage is compared by the amplifier 20 with the voltage developed across the resistor 15. If the voltage across the resistor 15 drops below the threshold, the output of amplifier 20 swings positive and the voltage at the positive input of an operational amplifier 21 swings positive. The amplifier 21 is also configured as a comparator, and thus the voltage at the output of amplifier 21 rises to close to the voltage of the positive terminal of the battery. This turns off transistor 14 via transistor 22. The supply to the motor is thus turned off, and the motor begins to slow down. A capacitor 23 charges up via a resistor 24 until the voltage on the negative input of amplifier 21 is- approximately equal to the battery positive supply voltage. The output of amplifier 21 then falls to zero, and transistor 14 is turned on again. The motor supply is thus turned on and off at a frequency determined by the difference between the voltages compared by the amplifier 20 and the time constant of the capacitive / resistive netw.rk 23, 24. The frequency increases as the sensed current falls. The cycling of the motor and the rate .at which its frequency increases is immediately apparent to the user who can take appropriate action before the circuit fails totally. The positive supply voltage is also monitored continuously by two operational amplifiers 25, 26 each configured as a comparator. Resistors 27, 28, 29 and 30 form a potential divider which applies two sensed voltages to the amplifiers, the higher sensed voltage being applied to amplifier 25, and the lower to amplifier 26. If the lower sensed voltage is greater than the reference voltage across diode 17, a green light emitting diode (LED) 31 is illuminated as a result of the output of amplifier 26 being approximately equal to the battery positive supply voltage. If the lower sensed voltage is less than the reference voltage but the upper sensed voltage is greater than the reference voltage, the output of amplifiers 25 and 26 are approximately zero volts, transistor 32 is turned on and a yellow LED 33 is illuminated, indicating a reduced but not yet dangerously low battery voltage. If the upper sensed voltage is below the threshold voltage, the output of amplifier 25 is approximately equal to the battery positive supply voltage and a red LED 34 is illuminated, indicating a dangerously low battery voltage requiring immediate action. When the output of the amplifier 25 swings positive, - illuminating diode 34, a diode 35 is forward biased and thus the amplifier 21 causes the transistor 14 to turn oh and off as described above with reference to the operation of the current sensing portion of the circuit. Accordingly, the user is alerted to both dangerously reduced battery voltage and motor current by cycling of the motor, and can assess at a glance the condition of the battery.

Claims: CLAIMS : 1. A circuit for monitoring the operation of a fan motor, comprising a first circuit for monitoring the voltage supplied to the motor, a second circuit for monitoring the current supplied to the motor, and an indicator circuit for indicating a fault if the monitored voltage falls below a predetermined threshold and/or the monitored current falls below a predetermined threshold. 2. A circuit according to claim 1, wherein the indicator circuit comprises means for cyclically turning the fan motor on and off if the voltage and/or current fall below the said thresholds. 3. A circuit according to claim 2, wherein the indicator circuit comprises a semiconductor switch in series with the motor, an operational amplifier having an input connected to the voltage and current monitoring circuits and an output connected to the semiconductor switch, and a capacitive/resistive network- connected between an input and the output of the operational amplifier output, the operational amplifier and capacitive/resistive network switching the semiconductor switch on and off when the monitored voltage and/or current fall below the said thresholds. 4. A circuit according to claim 1, 2 or 3, wherein the indicator circuit comprises at least two light emitting diodes one of which is illuminated when the monitored voltage is above a predetermined voltage threshold and the other of which is illuminated when the monitored voltage is below a predetermined voltage threshold. 5. A circuit substantially as hereinbefore described with reference to the accompanying drawings.

Inventors: FEENEY JOHN ROBERT, FEENEY, JOHN, ROBERT
Applicants: PROTECTOR SAFETY LTD, PROTECTOR SAFETY LIMITED
Application year: 1985
Main classification: G01R19
IPC codes: G01R19