Abstract:
A fan motor speed control system for controlling the fan motor speed of an air conditioning system includes a power output circuit including a power triac which is turned on and off by an opto-isolator connected to a pulse generator circuit for varying an AC voltage waveform imposed on the fan motor. The pulse generator circuit is connected to heating and cooling ramp circuits and a minimum speed circuit to provide a variable voltage signal imposed on the pulse generator circuit corresponding to the temperature difference sensed by a return air sensor and a heating or cooling sensor or by separate heating and cooling sensors disposed adjacent respective heating and cooling heat exchangers of the air conditioning system. An adjustable minimum speed circuit and a cutoff circuit are provided to control motor minimum speed or motor shutoff when a predetermined minimum speed is reached to prevent motor bearing failure or overheating. Sensor protection circuits in the control system operate to drive the motor to full speed if any of the temperature sensors experience an open or short circuit condition. The control system circuit maximizes air conditioning system efficiency by capturing additional heating or cooling effect, reduces noise associated with motor startup and shutdown, and reduces rapid change in the sensed temperature in the air conditioned space during motor startup and shutdown.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation-in-part of U.S. patent application Ser. No. 09/570,880, filed May 15, 2000, which is a continuation of U.S. patent application Ser. No. 08/801,560, filed Feb. 18, 1997, now U.S. Pat. No. 6,070,660, issued on Jun. 6, 2000. 
    
    
     FIELD OF THE INVENTION 
     The present invention pertains to a control system for continuously varying the speed of a fan drive motor for a forced air indoor space heating/cooling system during startup and after shutdown of a heating/cooling cycle. 
     BACKGROUND OF THE INVENTION 
     Conventional controls for forced air heating and cooling systems often provide for delayed startup of the fan drive motor at a single operating speed and delayed shutdown of the drive motor from a single operating speed after shutdown of the heat exchangers of the heating/cooling system. Conventional controls are designed to minimize unpleasant cold or hot drafts of air and to capture residual heat/cooling effect. However, changing motor speed abruptly from a deenergized or shutoff state to full speed usually generates unpleasant noise, does not preclude stratification of air in the system ductwork or in the space being heated or cooled, nor does such operation maximize the capture of residual heat/cooling effect of the system heat exchange equipment. 
     Control systems have been developed for forced air heating/cooling systems wherein the indoor space air circulating fan drive motor is driven at reduced speed for a period of time during startup and at a reduced speed for a period of time during the run-on or shutdown phase of the heating/cooling system operating cycle. Again, this type of control system does not minimize the stratification of warm or cold air in the ductwork or the space being heated or cooled nor does such a system maximize the capture of residual heating/cooling effect. 
     Prior U.S. patent applications Ser. Nos. 09/570,880 and 08/801,560 (now U.S. Pat. No. 6,070,660) assigned to the assignee of the present invention and referenced hereinabove are directed to an improved fan or blower drive motor control system and method for forced air heating/cooling systems wherein the fan drive motor speed is continuously varied during a starting phase and a shutdown phase of operation of the heating/cooling system. In one embodiment of the control system disclosed in the aforementioned patent application and patent, the system senses temperature in the airflow circuit of the heating/cooling system and prevents premature or unwanted operation of the fan drive motor. The present invention is directed to improvements in control systems of that general type. The subject matter of U.S. Pat. No. 6,070,660 issued Jun. 6, 2000 to Howard P. Byrnes, et al. is incorporated herein by reference, in its entirety. 
     SUMMARY OF THE INVENTION 
     The present invention provides an improved fan or blower drive motor control system for a forced air heating/cooling system wherein a control circuit is provided which substantially continuously varies the speed of the fan drive motor during a starting phase and a shutdown phase of operation. The control system may be easily adapted to conventional heating/cooling system controls to vary the forced air fan or blower drive motor speed in response to temperatures sensed in the heating/cooling system airflow circuit. The control system is particularly adapted for but not limited to use with permanent split capacitor or shaded pole blower or fan drive motors. 
     The control circuit includes an onboard power supply, an ac voltage wave crossover detector circuit and a control circuit for firing a triac to control the drive motor speed. The control system also includes a minimum speed detector circuit and a circuit which provides for continued operation of the fan drive motor at the minimum speed, if desired, or motor shutoff after reaching the minimum speed. 
     The control system of the present invention includes one embodiment which comprises a temperature sensor disposed in an airflow ductwork on the so-called return air side of the heating and/or cooling equipment and a temperature sensor on the downstream or so-called supply air side of the heating and/or cooling equipment. 
     In another embodiment, three sensors are disposed in the ductwork including the return air sensor which is disposed upstream with regard to the direction of airflow from an air heater heat exchanger, a heat sensing sensor which is disposed downstream of the air heater heat exchanger and a third sensor which is disposed downstream of an air cooling heat exchanger, such as an evaporator coil, for example. In this way a more versatile control system is provided and more accurate sensing of temperature is obtained, depending on the operating condition of the system, heating versus cooling. 
     The control systems of the present invention advantageously reduce energy consumption of conventional forced air heating and cooling systems, improve recovery of residual heating/cooling effect in conventional forced air heating/cooling systems, minimize stratification of air in the airflow circuit and the space being heated or cooled and reduce cold or hot air drafts during operation of the heating/cooling system. Moreover, by substantially continuously varying the fan or blower drive motor speed during startup and shutdown, noise associated with fan or blower operation is reduced and the circulation of air at a temperature other than normally sensed or preferred by occupants of an indoor space being heated or cooled is also reduced. 
    
    
     Those skilled in the art will further appreciate the important features and advantages of the invention, together with other superior aspects thereof upon reading the detailed description which follows in conjunction with the drawing. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagram of air temperature versus flow and motor speed indicating the change in airflow with increasing temperature sensed in the airflow circuit as well as decreasing flow with decreasing temperature in the airflow circuit in accordance with the control system of the present invention; 
     FIG. 2 is a schematic diagram of one preferred embodiment of a control system in accordance with the invention; and 
     FIG. 3 is a schematic diagram of another preferred embodiment of the invention and comprises FIGS. 3A,  3 B and  3 C, which may be viewed when arranged in accordance with the map diagram of FIG. 3; and 
     FIG. 4 is a somewhat schematic illustration of an air conditioning system showing one preferred arrangement of the locations of the sensors for the control system of FIG.  3 . 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     In the description which follows like elements are marked throughout the specification and drawing with the same reference numerals, respectively. Conventional elements are shown in somewhat generalized or schematic form in the interest of clarity and conciseness. 
     Referring to FIG. 1, the diagram illustrates a preferred change in motor speed and airflow rate through a conventional forced air heating/cooling system when the system thermostat senses the need for heating, for example, at a temperature setpoint of 78° F. in the space being heated. When the temperature sensed by the conventional system temperature sensor or thermostat drops below the setpoint of 78° F., for example, the furnace or heater turns on and the control system of the invention energizes the blower or fan drive motor at a minimum speed. When the air temperature sensed in the system ductwork increases, primarily at a location just downstream of the heater heat exchanger, as compared with the temperature in the return air duct upstream of the heater heat exchanger, the motor speed is increased. 
     Motor speed is proportional to airflow increase, and generally follows curve  10  in FIG. 1 until the temperature sensed by the sensor which is downstream of the heater heat exchanger reaches a setpoint of 110° F. At this time the blower motor continues to operate at full speed until the thermostat in the space being heated indicates that the demand for heating has been satisfied and the heater or “burner” is turned off so that the heater heat exchanger begins to cool. Accordingly, as the temperature sensed by the sensor which is disposed downstream in the direction of flow of air through the system decreases in relation to the return air temperature sensed at a point upstream of the heat exchanger, the control system of the invention varies the fan speed by continuously decreasing the fan drive motor speed. Airflow provided by the motor driven fan decreases along curve  12  in FIG. 1 until a minimum speed of the motor is reached which may result in continuous operation at the minimum speed or, at a slightly lower temperature, motor shutoff occurs. 
     Accordingly, motor operation and the airflow characteristic, as a function of the sensed temperature, provides for delivery of residual heat from the heater heat exchanger to the space being heated with increased efficiency, airflow increases and decreases gradually on start and stop of the heater or burner for quiet operation of the system and stratified air layers at various temperatures are substantially eliminated in the heating/cooling system ductwork and in the space being heated or cooled. More efficient operation of the heating/cooling system is obtained and a greater comfort level is provided for persons occupying the space controlled by a system in accordance with the invention. 
     Referring now to FIG. 2, there is illustrated a schematic diagram of one preferred embodiment of a temperature sensing, variable speed fan or blower motor control system in accordance with the invention and generally designated by the numeral  20 . The control system  20  is operable to sense the temperature in a ductwork of a conventional forced air heating and cooling system, a section of which ductwork is illustrated in FIG.  2  and generally designated by the numeral  22 . Ductwork  22  includes a return air duct part  24  whereby airflow from a space being heated or cooled is being returned for heating by a heater heat exchanger  26  or cooled by a cooling heat exchanger  28 . Accordingly, a return air temperature sensor R 11  is disposed in the ductwork  22  upstream of the heat exchanger for the heater  26  and a so-called supply air sensor R 9  is disposed in ductwork  22  downstream, with respect to the direction of airflow, of the air cooling heat exchanger  28  whereby supply air treated by the heating/cooling system is then returned to the control space via a supply air duct  30 . In fact, the ductwork  22  may comprise a conventional forced air furnace/air conditioning system wherein the heat exchanger  26  includes a gas fired burner or electrical resistance heater, not shown, and the heat exchanger  28  is an evaporator coil of a conventional vapor compression refrigeration circuit, not shown. The illustration of FIG. 2 with respect to the heating/cooling system is exemplary. 
     Referring further to FIG. 2, a HEAT/COOL SELECT circuit is indicated whereby, for example, when a heater associated with heat exchanger  26  is energized, such as by opening a gas burner valve, for example, 24 volt AC electrical power is applied across terminals P 5  and P 6 . Alternatively, when an air cooling system is operable, such as a vapor compression refrigeration system, and the compressor thereof is energized, 24 volt AC power is applied across terminals P 4  and P 6 . Power for the control system  20  is supplied by a 120 volt AC source at terminal P 2  and a neutral conductor P 2 ′. Alternatively, 24 volt AC power may be applied at terminals P 7  and P 2 ′. A fan or blower drive motor  32  may be connected at terminals P 2 ′ and P 2 ″ as indicated in FIG.  2 . The motor  32  may be of a type described in U.S. Pat. No. 6,070,660 which is incorporated herein by reference. The control system  20  is preferably connected to the motor medium speed winding as in the system of the &#39; 660  patent. 
     As further shown in FIG. 2, a 12 volt DC power supply circuit is made up of capacitors C 2  and C 10 , resistor R 15 , a diode D 2  and a Zener diode D 5 . A four diode bridge BR 1  takes either the 24 volt AC signal from a step down transformer, not shown, or the 120 volt AC source at terminals P 2  and P 2 ′. A RESET circuit comprising resistors R 21 , R 24 , R 25 , R 26 , R 27 , R 44 , diodes D 4  and D 6 , capacitor C 4  and amplifier U 2 :A is operable to receive full wave voltage from the diode bridge BR 1  through resistors R 25 , R 26 , diode D 6  and amplifier U 2 :A to capacitor C 6  for the purpose of discharging capacitor C 6  every half cycle. Thus an output pulse always starts at the proper moment on each half cycle. If 24 volt AC power is input to the power supply and RESET circuits, jumper JP 2  is open and is shorted if there is no 24 volt AC supply. 
     Sensors R 9  and R 11  are preferably thermistors which are substantially similar and interposed in a HEAT/COOL RAMP GEN circuit to generate signals as the temperature differences change between each sensor location. If both sensors are at the same temperature the output of the sensors will be one-half of the 12 volt DC supply voltage. If the downstream or so called supply air sensor R 9  senses a temperature greater than the return air sensor R 11 , the output voltage at conductor  34  increases. If the temperature sensed by sensor R 9  is less than that sensed by sensor R 11 , voltage at conductor  34  will decrease. The output signal from the sensors R 9  and R 11  is input to the ramp circuits indicated in FIG. 2 as the COOL RAMP and the HEAT RAMP. If the output signal voltage is increasing the HEAT RAMP circuit is activated which comprises resistors R 10 , R 12 , R 13 , R 14 , R 16 , R 17 , R 18  and R 20 , capacitors C 8  and C 13 , diode D 3 , buffer amplifier U 1 :C and amplifier U 1 :A arranged in circuit as shown in FIG.  2 . The output signal of the HEAT RAMP circuit is imposed on conductor  36 . 
     The COOL RAMP circuit is also connected to conductor  34  to receive the resultant output signal from sensors R 9  and R 11  and if the signal magnitude is decreasing, a voltage output at conductor  36 ,  37  is increasing. The ramp output voltage generated by the COOL RAMP circuit is provided by circuit components including resistors R 1 , R 2 , R 3 , R 4 , R 5 , R 6 , R 7 , R 8 , capacitor C 14 , diode D 1  and amplifiers U 1 :B and U 1 :D. Capacitors C 13  and C 14  slow the change in the output signal of amplifier U 1 :B or U 1 :C which will minimize the chance of lockup of motor  32 . Capacitors C 13  and C 14  also minimize unwanted electrical noise from entering the ramp circuits previously described. 
     The control system  20  further includes a pulse generator or PULSE GEN circuit including resistors R 35 , R 36 , R 37 , capacitors C 5 , C 6  and C 9 , opto-isolator U 3  and diode D 10 . Ramp output voltage is input through resistors R 19  and R 28  to the PULSE GEN circuit and operational amplifier U 2 :B which has a reference voltage set at its negative input. When the ramp voltage exceeds this reference voltage, the output of amplifier U 2 :B goes “high”. Capacitor C 6  connects to the ramp voltage signal on conductor  36  also. Accordingly, a sawtooth waveform is input at the positive (+) terminal of amplifier U 2 :B. Therefore, the output of the PULSE GEN circuit is a square wave whose width varies as the ramp voltage signal varies. Since the RESET circuit discharges capacitor C 6  every half cycle, the output pulse of the PULSE GEN circuit always starts at the correct time on each half cycle. 
     A POWER OUTPUT circuit is shown in FIG. 2 comprising resistors R 30 , R 32 , power triac Q 1  and capacitor C 7 . A square wave output signal from the PULSE GEN circuit is imposed on capacitor C 5  which causes a voltage pulse to turn on the input diode of opto-isolator U 3  and when the diode in opto-isolator U 3  conducts its output triac turns “on”. This action causes current to flow into the gate of the power triac Q 1  which is connected to motor  32 . When current flows through the power triac Q 1 , motor  32  is energized to rotate to drive fan or blower  33  which is operably associated with the ductwork  22 . A snubber comprising resistor R 32  and capacitor C 7  are connected to power triac Q 1  to protect triac Q 1  from unexpected line voltage surges. 
     Referring still further to FIG. 2, a CUTOFF circuit includes resistors R 33 , R 34 , R 38 , R 39  and R 40 , diode D 8  and amplifier U 2 :C. Amplifier U 2 :C is operable to receive a variable voltage signal at its negative terminal via conductor  37  and, when the ramp voltage drops to a predetermined value, amplifier U 2 :C goes “high” and provides a signal coupled through diode D 8  to the PULSE GEN circuit. When a “high” signal is imposed on the negative (−) terminal of amplifier U 2 :B, the output signal of U 2 :B goes “low” shutting off an output signal from power triac Q 1  and motor  32  stops. Accordingly, when the ramp voltage at conductors  36 ,  37  increases slightly above a dropout value, the CUTOFF circuit output at amplifier U 2 :C goes low. This allows the ramp voltage to resume normal output to control the motor  32  through the triac Q 1 . 
     The CUTOFF circuit, including amplifier U 2 :C, is operably connected to a jumper JP 1  in a MIN SPEED circuit as shown in FIG.  2 . The discussion hereinabove regarding the CUTOFF circuit assumes that the jumper JP 1  is open. When jumper JP 1  is closed, control system supply voltage is coupled through diode D 11  to the negative input terminal of amplifier U 2 :C. The output signal of amplifier U 2 :C is then forced “low” regardless of the ramp voltage input to amplifier U 2 :C and therefore the pulse generator is not shutoff due to the CUTOFF circuit. The MIN SPEED circuit of the control system  20  includes resistors R 19 , R 22 , R 23 , R 28 , R 46 , diodes D 9  and D 11  and amplifier U 2 :D. When the jumper JP 1  is open, the output of the MIN SPEED circuit is low at the output of amplifier U 2 :D and the adjustable resistor R 22 , which is operable to adjust the minimum speed of the motor  32 , is inoperable. When JP 1  is closed, the CUTOFF circuit previously described is disabled through diodes D 11  and resistor R 46  and the MIN SPEED circuit is enabled. Adjustment of the minimum speed resistor or potentiometer R 22  enables the motor  32  to be set to run from approximately 180 rpm to 620 rpm, for example. The motor minimum speed will hold even though there may be a zero difference between supply air and return air temperatures as sensed by the sensors R 9  and R 11 . 
     The aforementioned HEAT/COOL SELECT circuit includes a cooling condition input circuit including resistors R 31  and R 45 , diodes D 7  and D 14  and capacitor C 11 . A 24 volt AC signal on the aforedescribed circuit will deactivate motor  32  by deactivation of triac Q 1 . This signal overrides any signals produced by the sensors R 9  and R 11 . Consequently, when the control system  20  is connected to the medium speed winding of a motor, such as the motor  32 , and the conventional control system for the motor applies power to the high speed winding and the 24 VAC COOL signal is provided at terminals P 4  and P 6  only the desired motor winding will be energized. However, when the thermostat is satisfied in the space being cooled and a signal is removed from terminals P 4  and P 6  the control system  20  will be operable to energize the motor  32  at the medium speed winding and gradually reduce the motor speed as the temperature difference between the sensors R 9  and R 11  decreases. 
     Conversely, when a 24V AC HEAT input signal is provided at the HEAT/COOL SELECT circuit, the COOL RAMP circuit is disabled and only the temperature of sensor R 9  rising above the temperature of sensor R 11  will affect motor speed. The sensor R 9  temperature, when below the sensor R 11  temperature, will maintain the HEAT/COOL RAMP GEN circuit at its minimum voltage. Motor  32  will either then be at zero speed or a minimum speed depending on the selection of the connection of jumper JP 1  for cutoff or minimum speed. The heat input side of the HEAT/COOL SELECT circuit includes capacitor C 12 , diodes D 12 , D 13  and resistors R 41  and R 42 . 
     Lastly, the control system  20  includes a SENSOR PROTECTION circuit including resistors R 47 , R 48 , R 49 , R 50 , R 51 , capacitors C 1 , diode D 15  and amplifier U 4 :A. A positive input signal to amplifier U 4 :A of the SENSOR PROTECTION circuit is provided by the ramp output voltage signal and the protection circuit negative input to amplifier U 4 :A is connected to a reference voltage available from resistors R 49  and R 50 . When the ramp output voltage exceeds the reference voltage, the output signal of amplifier U 4 :A goes high and this DC voltage signal is connected to the opto-isolator U 3  through diode D 15 . This action causes the triac Q 1  to be on full at all times and avoid the possibility of motor lockup. 
     Still further, there are three ways for the ramp output voltage signal to exceed the reference voltage signal at amplifier U 4 :A, which reference is established by resistors R 49  and R 50 , namely (1) if either of the sensors R 9  or R 11  are open, (2) if both of sensors R 9  and R 11  are open or are shorted, or (3) if the design parameter for the temperature difference between sensors R 9  and R 11  has been exceeded. If any of the above noted conditions occurs the motor  32  will be fully on until the condition goes back to the system normal mode of operation or power is removed from the control system  20 . 
     An alternate embodiment of a control system in accordance with the invention will now be described in conjunction with FIGS. 3 and 4. In certain air conditioning systems it may be necessary to monitor a change in cooling air temperature and heating air temperature at different locations in the air conditioning system and the reference or return air sensor may be required to be mounted in a return air duct to monitor temperature in a third location. A major advantage of having the flexibility of being able to choose the location of the temperature sensors is with regard to certain installations wherein, for example, during a cooling phase of operation air is routed through a different duct than for the routing of air during heating operation. Still further, in other applications the heating/cooling equipment may be arranged such that the location of the so-called supply air sensor may be suitable for the heating mode of operation but not the cooling mode or vice versa. 
     By way of example, and referring to FIG. 4, there is illustrated a vertical or updraft air conditioning system  110  which includes duct or cabinet  111 . Cabinet  111  is mounted on a return air plenum  112  whereby air being returned from an air conditioned space enters the system  110  and flows upward over surfaces of a heat exchanger  114  and then further upward through an evaporator coil or air cooling heat exchanger  116  before being discharged into a supply air plenum  118  for distribution through suitable supply air ducts  120 ,  122  and  124 , for example. The motor and blower or fan  32 ,  33  for the system  110  is shown in one preferred location in plenum  112  in the somewhat schematic illustration of FIG. 4, but may also be located, alternatively, in the cabinet  111 , for example. 
     A control system  200  is illustrated in FIGS. 3A,  3 B and  3 C which is advantageous for use with the air conditioning system  110  of FIG.  4 . The system  110  of FIG. 4 is provided to illustrate that a typical location of a return air sensor  130  would be in the return air duct or plenum  112 . A heated air sensor  132  should be disposed just downstream in the direction of airflow through the system  110  of the heat exchanger  114  for sensing the temperature of heated air, and a third or cooling air sensor  134  is shown located just downstream, in the direction of airflow, from the evaporator or cooling coil  116 . With this arrangement more accurate and timely readings of the heated air temperature and the cooled air temperature is provided even though the flowpath for the air during the heating mode or the cooling mode is not through separate ducts in the exemplary system  110 . 
     Referring now to FIG. 3C, the control system  200  includes a POWER SUPPLY circuit substantially like the power supply for the control system  20 . Power at 120 volts AC may be applied at terminals Pi and P 3  or alternatively 24 volt AC power may be applied at terminals P 1  and P 2 . As indicated in FIG. 3C, a jumper JP 2  is applied if 120 volt AC power is connected to the control system. Motor  32  is connected between terminals P 1  and P 4  also, as indicated. The POWER SUPPLY circuit is made up of capacitors C 60  and C 50 , resistor R 200 , diodes D 50  and D 80  and bridge circuit BR 1 . Bridge circuit BR 1  is a set of four diodes which cause a fullwave bridge output signal to be developed. Since the voltage developed falls to near zero every half cycle, the output is used to synchronize a pulse generator by resetting capacitor C 140  every half cycle. A RESET circuit including diode D 70 , resistors R 280 , R 420 , R 350 , capacitor C 100 , resistor R 460 , resistor R 410 , resistor R 400 , amplifier U 2 :A 1  and diode D 110  provides an output pulse to amplifier U 2 :B 1  at the correct moment on each cycle. 
     Control system  200  also includes a PULSE GEN circuit, as shown in FIG. 3C, comprising resistors R 510 , R 600  and R 610 , diode D 150 , amplifier U 2 :B 1 , and capacitors C 130 , C 140  and C 110 . A voltage signal from the ramp circuits shown in FIG.  3 A and to be described further herein is provided via reference terminal  212  in FIG. 3A to reference terminal  214  in FIG.  3 C. This voltage is applied to the positive terminal of amplifier U 2 :B 1  and a reference voltage provided through resistors R 510  and R 600  is applied to the negative terminal of amplifier U 2 :B 1 . When the ramp voltage at reference terminal  214  exceeds the reference voltage, amplifier U 2 :B 1  provides a “high” output signal. Capacitor C 140  connects to the ramp voltage signal imposed on reference terminal  214  and a sawtooth waveform results at the positive terminal of U 2 :B 1 . Accordingly, the output signal of amplifier U 2 :B 1  is a square wave whose width varies as the ramp voltage imposed on reference terminal  214  varies. 
     A POWER OUTPUT circuit of the control system  200  includes resistors R 490 , R 570 , amplifier or opto-isolator U 60  and power triac Q 1  as well as capacitor C 120 . The aforementioned squarewave output signal from the amplifier U 2 :B 1  is connected to capacitor C 110 . A voltage pulse is formed by capacitor C 110  and diode D 150  to the input diode of opto-isolator U 60 . When the aforementioned input diode of opto-isolator U 60  conducts, an output triac of the opto-isolator U 60  turns “on” which causes current to flow into the gate of power triac Q 1 . Motor  32  is connected to the power triac Q 1  at terminal P 4  and when current flows through the triac, the motor is energized to drive fan or blower  33 . A snubber resistor-capacitor combination comprising resistor R 570  and capacitor C 120  are connected to the power triac Q 1  to protect the triac from unexpected line voltage surges. 
     Referring now to FIG. 3B also, a fan motor speed CUTOFF circuit is shown including resistors R 400 , R 150 , R 160 , R 180 , R 90 , R 120 , diode D 300 , diode D 200  and amplifier U 2 :C 1 . The negative terminal of amplifier U 2 :C 1  is connected to a HEAT/COOL RAMP circuit shown in FIG.  3 A through resistor R 400  via conductor  221 . When the COOL RAMP circuit output voltage drops to a predetermined level, as determined by the reference voltage at the positive terminal of amplifier U 2 :C 1 , this amplifier provides an output signal to diode D 200  and the PULSE GEN circuit by way of conductor  223  which is coupled to the negative input terminal of amplifier U 2 :B 1 . When a “high” output signal is applied via conductor  223  to amplifier U 2 :B 1 , the output signal of amplifier U 2 :B 1  goes “low” shutting down the output signal of opto-isolator U 60  and power triac Q 1  thereby deenergizing motor  32 . When the HEAT/COOL RAMP voltage signal from conductor  221  increases slightly above a so-called dropout voltage, the output signal from amplifier U 2 :C 1  goes low and allows the ramp voltage signal to resume normal action to control the motor speed through the power triac Q 1 . It should be noted that the CUTOFF circuit just described has a hysteresis equivalent to approximately 10 rpm on motor  32 . 
     The above description with respect to the CUTOFF circuit assumes that the jumper JP 3 , FIG. 3A, is in an open condition. When jumper JP 3  is closed, the supply voltage provided thereby enables a reference voltage to the noninverting input of amplifier U 2 :D 1 , the output voltage of which is coupled through diode D 140  to the output conductors of the HEAT/COOL RAMP and HEAT RAMP circuits and the reference terminals  212 ,  214 , which determines the motor speed by biasing amplifier U 2 :B 1  as previously described. By adjusting an adjustable resistor R 540  of a MIN SPEED circuit, FIG. 3A, the minimum speed of the motor  32  can be preset. 
     However, when jumper JP 3  is open, switch U 5 :A 1  is connected across the JP 3  contacts. Switch U 5 :A 1  is energized through resistor R 450  which connects to a 24 volt AC HEAT signal of the HEAT/COOL SELECT circuit, FIG. 3A, by way of reference terminals  225  and  227  and by way of diode D 170 , resistor R 690 , diode D 190 , capacitor C 150  and resistor R 450 . With this arrangement, when a 24 volt AC HEAT signal is present, the MIN SPEED circuit is energized and if the MIN SPEED circuit is energized, the CUTOFF circuit, FIG. 3B, is deenergized by way of resistors R 580  and R 400 . Accordingly, when a signal is applied to the 24 volt AC HEAT input, switch U 5 :A 1  is immediately switched on and this action shorts jumper JP 3  from resistor R 530  to positive 12 volts DC. A bias voltage is applied to positive pin of amplifier U 2 :D 1  and the output of amplifier U 2 :D 1  is applied at the reference terminal  212  through diode D 140 . Therefore, as the control system  200  is operated in conjunction with the air conditioning system  110 , wherein a signal indicating a heat mode of operation is applied, the motor  32  runs at a minimum speed to provide better air circulation surrounding sensors  130 ,  132  and  134 . 
     Adjustment of the minimum speed MIN ADJUST resistor R 540  enables the motor speed to be set from approximately 180 rpm to 620 rpm. The minimum speed of the motor  32  will hold at its designated RPM even though there may be no difference between supply air and return air temperatures. 
     Referring further to FIG. 3A, the HEAT/COOL SELECT circuit is operable to provide a 24 volt AC input signal at 24 VAC COOL across terminals P 6  and P 7   a  and imposed on diode D 200 , resistors R 700 , R 710 , diode D 210  and capacitor C 160 . When 24 volt AC power is applied across terminals P 6  and P 7   a , motor  32  is turned off by deactivation of power triac Q 1  due to the application of an output signal at reference terminal  229  which is connected to reference terminal  231 , FIG.  3 C. In other words, when a voltage is applied to reference terminals  229 ,  231  and the negative terminal of amplifier U 2 :B 1  the output signal of amplifier U 2 :B 1  goes “low” and causes opto-isolator U 60  and power triac Q 1  to shut off power to motor  32 . A signal as described above applied at reference terminal  229  overrides signals provided by return air sensor  130  and supply air sensors  132  and  134 . Accordingly, the control system  200  is also operable to avoid supplying power to both the high speed winding and the medium speed winding of the motor  32  when the thermostat for the air conditioning system  110  has called for operation in the cooling mode and the motor is being separately controlled by the conventional motor control system to operate at a high speed. However, as with the control system  200  when a control signal is removed across terminals P 6  and P 7   a , the control system  200  will assume control over the motor  32  and will gradually decrease the speed of the motor as determined by the difference in temperatures sensed by the sensors  130  and  134 . 
     Referring further to FIG. 3A, when a 24 volt AC signal is applied across terminals P 5  and P 7   a  and diode D 170 , resistor R 690 , diode D 190 , capacitor C 150  and diode D 180 , the COOL RAMP circuit is disabled by way of reference terminal  233  which is connected to reference terminal  235  in FIG.  3 A. Under these operating conditions only temperatures rising above the sensor  132  temperature compared to the return air sensor  130  will affect motor speed. Temperatures sensed by the heat sensor  132  and cool air sensor  134 , if less than the temperature sensed by the return air sensor  130 , will only maintain a HEAT RAMP circuit output voltage at its minimum. This will cause the motor  32  to operate at zero rpm or at its minimum speed, depending on whether a cutoff or minimum speed mode is chosen. 
     Referring now to FIG. 3B, an OVERVOLTAGE RAMP AND SENSOR PROTECTION circuit is provided which includes capacitor C 70 , resistors R 260 , R 270 , R 310  and R 340 , amplifier U 4 :A 1 , diode D 90  and resistor R 330  for the HIGH RAMP output. The OVERVOLTAGE RAMP AND SENSOR PROTECTION circuit further includes resistors R 470 , R 500 , R 440 , R 480 , amplifier U 4 :B 1  and diode D 120 , for the ZERO RAMP. Resistors R 550 , R 620 , R 560 , R 590 , amplifier U 4 :C 1  and diode D 130  comprise the circuit of a RETURN AIR SENSE OPEN output. Resistors R 660 , R 680 , R 650 , R 670 , amplifier U 4 :D 1  and diode D 160  comprise the circuit for the RETURN AIR SENSOR SHORT output. The purpose of these circuits is to cause the motor  32  to be driven at full speed if either of the sensors  130 ,  132  or  134  is in an open or a shorted operating condition, or if the HEAT/COOL RAMP is at zero or greater than the ramp voltage boundaries. 
     Referring further to FIG. 3B, amplifier U 4 :A 1  receives an input signal on its positive terminal by way of conductor  237  which is connected to the HEAT/COOL RAMP of FIG.  3 A. An output signal from amplifier U 4 :A 1  goes high when the COOL RAMP or HEAT RAMP circuit output signals equal or exceed a differential temperature trip point. In fact, amplifiers U 4 :A 1 , U 4 :B 1 , U 4 :C 1  and U 4 :D 1  are all operable, when providing a high output signal, to cause motor  32  to run at full speed. The output signals from any one of these amplifiers is conducted via reference terminals  239  and  241  to the opto-isolator U 60 . 
     Referring further to FIG. 3B, an output signal from amplifier U 4 :B 1  goes high when the output from the HEAT RAMP or COOL RAMP circuits goes to zero. This occurrence would be the result of the cool sensor  134  being shorted or the heat sensor  132  going to an open condition. The output signal from amplifier U 4 :A 1  goes high when the output signal from the HEAT RAMP circuit or the COOL RAMP circuit goes high. This occurs when the cool sensor  134  has an open circuit condition or when the heat sensor  132  experiences a shorted condition. Still further, the output signal from amplifier U 4 :C 1  goes high when the return air sensor  130  is in an open condition and the output signal from amplifier U 4 :D 1  goes high when the return air sensor is shorted. A signal from the return air sensor  130  is supplied to amplifiers U 4 :C 1  and U 4 :D 1  via reference terminals  243  and  245 . Reference terminals  247  and  249 , FIG. 3A, are also connected to reference terminal  245  and impose signals on amplifiers U 1 :C 1  and U 3 :B 1 . 
     Referring further to FIG. 3A, the HEAT RAMP circuit receives a variable voltage signal from the heat sensor  132  by way of a buffer amplifier U 3 :C 1 . The output of the HEAT SENSE circuit, the junction of resistors R 720  and R 430 , is connected to the HEAT RAMP circuit through resistor R 390  and buffer amplifier U 3 :C 1 . Amplifier U 3 :A 1  is a differential amplifier and its output voltage is determined by the difference between the heat sensor voltage output signal and the return air sensor voltage output signal which is the output signal from the junction of resistors R 190  and R 210  as imposed on reference terminal  249 . Amplifier U 3 :B 1  is also a buffer amplifier for the return air sensor voltage output signal. A variable voltage output signal from amplifier U 3 :A 1  and diode D 100  is thus imposed on reference terminals  212  and  214  through resistors R 580  and R 630 . 
     Referring still further to FIG. 3A, the COOL RAMP circuit includes resistors R 20 A, R 30 A, R 50 A, R 70 , R 80 , R 10 A, R 11 A, capacitors C 10 A and amplifiers U 1 :B 1 , U 1 :C 1  and U 1 :D 1  as well as diode D 10 A. Capacitors C 10 A and C 90  minimize the effect of a step function at the output of the cool sensor  134  or heat sensor  132  to prevent motor lockup and also to minimize unwanted electrical noise from entering the circuit. The output signal from the cool sensor  134  is connected to the COOL RAMP circuit through resistor R 70  to buffer amplifier U 1 :B 1  whose output is imposed on amplifier U 1 :D 1 . Amplifier U 1 :D 1  is a difference amplifier whose output signal is determined by the difference between the cool sensor voltage output signal and the return air sensor voltage signal at the junction of resistors R 190  and R 210 . Amplifier U 1 :C 1  is a buffer amplifier for the output signal of return air sensor  130 . The output signal from the COOL RAMP circuit is by way of amplifier U 1 :D 1  through diode D 10 A to conductor  221  and to reference terminal  212  by way resistors R 580  and R 630 . The output signal from the junction of resistors R 60  and R 100  is also imposed on amplifier U 1 :A 1  by way of reference terminals  253  and  255  to disable the HEAT RAMP circuit when the system  200  is operating in a cooling mode. Conversely, the output of the heat sensor  132 , as measured at the junction of resistors R 720  and R 430 , is imposed on reference terminals  257  and  259  and amplifier U 3 :D 1  to disable the COOL RAMP circuit. The ramp circuits will generate a voltage signal as the differences between the return air sensor voltage and the heat sensor voltage or cool sensor voltage pass outside of a dead band of approximately 5° F. The cooling temperature signal output must be below the referenced temperature by about 2.5° F. and the temperature signal from the heat sensor must be above the reference temperature by about 2.5° F. When either condition exists, the ramp voltage signal imposed on terminal  221  will increase starting just outside the deadband. 
     The operation of the control systems  20  and  200  to vary the speed of a fan motor for a forced air air conditioning system for the advantageous purposes set forth herein is believed to be understandable to those of ordinary skill in the art based on the foregoing description. A correlation table of the components of the systems  20  and  200  is set forth hereinbelow. Certain ones of the circuit components shown in the drawing and included in the correlation table are not discussed in detail but are believed to be understandable to those of ordinary skill in the art. Preferred values and commercial part numbers for certain components are identified also. 
     
       
         
               
             
               
               
               
               
             
               
               
               
               
               
             
           
               
                   
               
               
                 CORRELATION TABLE 
               
             
          
           
               
                   
                   
                 COM- 
                   
               
               
                   
                   
                 MERCIAL 
               
               
                   
                 ITEM 
                 PART NO. 
                 VALUE 
               
               
                   
                   
               
             
          
           
               
                   
                 C1 
                   
                 .1 
                 μF 
               
               
                   
                 C2 
                   
                 100 
                 μF 
               
               
                   
                   
                   
                 63 
                 V 
               
               
                   
                 C4 
                   
                 . 22 
                 μF 
               
               
                   
                 C5 
                   
                 .1 
                 μF 
               
               
                   
                 C6 
                   
                 .1 
                 μF 
               
               
                   
                 C7 
                   
                 .1 
                 μF 
               
               
                   
                   
                   
                 250 
                 V 
               
               
                   
                 C8 
                   
                 0.1 
                 μF 
               
               
                   
                 C9 
                   
                 .1 
                 μF 
               
               
                   
                 C10 
                   
                 .1 
                 μF 
               
               
                   
                 C10A 
                   
                 0.47 
                 μF 
               
               
                   
                 C11 
                   
                 10 
                 μF 
               
               
                   
                   
                   
                 25 
                 V 
               
               
                   
                 C12 
                   
                 10 
                 μF 
               
               
                   
                   
                   
                 25 
                 V 
               
               
                   
                 C13 
                   
                 .47 
                 μF 
               
               
                   
                 C14 
                   
                 .47 
                 μF 
               
               
                   
                 C20 
                   
                 0.1 
                 μF 
               
               
                   
                 C30 
                   
                 0.1 
                 μF 
               
               
                   
                 C40 
                   
                 0.1 
                 μF 
               
               
                   
                 C50 
                   
                 100 
                 μF 
               
               
                   
                   
                   
                 63 
                 V 
               
               
                   
                 C60 
                   
                 0.1 
                 μF 
               
               
                   
                 C70 
                   
                 0.1 
                 μF 
               
               
                   
                 C80 
                   
                 0.1 
                 μF 
               
               
                   
                 C90 
                   
                 0.47 
                 μF 
               
               
                   
                 C100 
                   
                 0.22 
                 μF 
               
               
                   
                 C110 
                   
                 0.1 
                 μF 
               
               
                   
                 C120 
                   
                 .1 
                 μF 
               
               
                   
                   
                   
                 250 
                 V 
               
               
                   
                 C130 
                   
                 0.1 
                 μF 
               
               
                   
                 C140 
                   
                 0.1 
                 μF 
               
               
                   
                 C150 
                   
                 10 
                 μF 
               
               
                   
                   
                   
                 25 
                 V 
               
               
                   
                 C160 
                   
                 10 
                 μF 
               
               
                   
                   
                   
                 25 
                 V 
               
               
                   
                 D1 
                 IN4148 
               
               
                   
                 D2 
                 IN4003 
               
               
                   
                 D3 
                 IN4148 
               
               
                   
                 D4 
                 IN4148 
               
               
                   
                 D5 
                 IN4742A 
               
               
                   
                 D6 
                 IN4148 
               
               
                   
                 D7 
                 IN4148 
               
               
                   
                 D8 
                 IN4148 
               
               
                   
                 D9 
                 IN4148 
               
               
                   
                 D10 
                 IN4148 
               
               
                   
                 D10A 
                 IN4148 
               
               
                   
                 D11 
                 IN4148 
               
               
                   
                 D12 
                 IN4148 
               
               
                   
                 D13 
                 IN4148 
               
               
                   
                 D14 
                 IN4148 
               
               
                   
                 D15 
                 IN4148 
               
               
                   
                 D40 
                 IN4148 
               
               
                   
                 D50 
                 IN4003 
               
               
                   
                 D60 
                 IN4148 
               
               
                   
                 D70 
                 IN4148 
               
               
                   
                 D80 
                 IN4742A 
               
               
                   
                 D90 
                 IN4148 
               
               
                   
                 D100 
                 IN4148 
               
               
                   
                 D110 
                 IN4148 
               
               
                   
                 D120 
                 IN4148 
               
               
                   
                 D130 
                 IN4148 
               
               
                   
                 D140 
                 IN4148 
               
               
                   
                 D150 
                 IN4148 
               
               
                   
                 D160 
                 IN4148 
               
               
                   
                 D170 
                 IN4148 
               
               
                   
                 D180 
                 IN4148 
               
               
                   
                 D190 
                 IN4742A 
               
               
                   
                 D200 
                 IN4148 
               
               
                   
                 D210 
                 IN47742A 
               
               
                   
                 D300 
                 IN4148 
               
               
                   
                 Q1 
                   
                 BTA 6 
               
               
                   
                 R1 
                   
                 1M 
               
               
                   
                 R2 
                   
                 475K 
               
               
                   
                 R3 
                   
                 10K 
               
               
                   
                 R4 
                   
                 475K 
               
               
                   
                 R5 
                   
                 1M 
               
               
                   
                 R6 
                   
                 1M 
               
               
                   
                 R7 
                   
                 26.7 
                 K 
               
               
                   
                 R8 
                   
                 1M 
               
               
                   
                 R9 
                   
                 10K 
                 THERMISTOR 
               
               
                   
                 R10 
                   
               
               
                   
                 R10A 
                   
               
               
                   
                 R11 
                   
                 10K 
                 THERMISTOR 
               
               
                   
                 R11A 
                   
               
               
                   
                 R12 
                   
                 10K 
               
               
                   
                 R13 
                   
                 825K 
               
               
                   
                 R14 
                   
                 1M 
               
               
                   
                 R15 
                   
                 750. ½ 
                 W 
               
               
                   
                 R16 
                   
                 1M 
                 (RP2.4) 
               
               
                   
                 R17 
                   
                 1.24K 
               
               
                   
                 R18 
                   
                 825K 
               
               
                   
                 R19 
                   
                 25.5K 
               
               
                   
                 R20 
                   
                 1M 
               
               
                   
                 R20A 
                   
                 1M 
               
               
                   
                 R21 
                   
                 499K 
               
               
                   
                 R22 
                   
                 5K 
               
               
                   
                 R23 
                   
                 634K 
               
               
                   
                 R23A 
                   
               
               
                   
                 R24 
                   
                 1M 
               
               
                   
                 R24A 
                   
               
               
                   
                 R25 
                   
                 1M 
               
               
                   
                 R26 
                   
                 2.74M 
               
               
                   
                 R27 
                   
                 100K 
               
               
                   
                 R28 
                   
                 22.1K 
               
               
                   
                 R29 
                   
                 44.2K 
               
               
                   
                 R30 
                   
                 470 ½ 
                 W 
               
               
                   
                 R30A 
                   
               
               
                   
                 R31 
                   
                 7.5K 
               
               
                   
                 R32 
                   
                 510 2 
                 W, 
               
               
                   
                   
                   
                   
                 METAL 
               
               
                   
                   
                   
                   
                 OXIDE 
               
               
                   
                 R32A 
                   
               
               
                   
                 R33 
                   
                 1M 
               
               
                   
                 R34 
                   
                 1M 
               
               
                   
                 R35 
                   
                 10K 
               
               
                   
                 R36 
                   
                 6.04K 
               
               
                   
                 R37 
                   
                 499 
               
               
                   
                 R37A 
               
               
                   
                 R38 
                   
                 10.5K 
               
               
                   
                 R39 
                   
                 10K 
               
               
                   
                 R40 
                   
                 200K 
               
               
                   
                 R41 
                   
                 7.5K 
               
               
                   
                 R42 
                   
                 100K 
               
               
                   
                 R43 
                   
                 10K 
               
               
                   
                 R44 
                   
                 20.5K 
               
               
                   
                 R45 
                   
                 10K 
               
               
                   
                 R46 
                   
                 100K 
               
               
                   
                 R47 
                   
                 1M 
               
               
                   
                 R48 
                   
                 1M 
               
               
                   
                 R49 
                   
                 3.48K 
               
               
                   
                 R50 
                   
                 10K 
               
               
                   
                 R50A 
                   
                 10K 
               
               
                   
                 R51 
                   
                 2K 
               
               
                   
                 R70 
                   
                 7.5K 
               
               
                   
                 R71 
                   
                 100K 
               
               
                   
                 R80 
                   
                 { } 
               
               
                   
                 R90 
                   
                 200K 
               
               
                   
                 R100 
               
               
                   
                 R120 
                   
                 100K 
               
               
                   
                 R130 
               
               
                   
                 R140 
                   
                 1M 
               
               
                   
                 R150 
                   
                 1M 
               
               
                   
                 R160 
                   
                 10.5K 
               
               
                   
                 R170 
                   
                 1M 
               
               
                   
                 R180 
                   
                 10K 
               
               
                   
                 R190 
               
               
                   
                 R200 
                   
                 6K 
               
               
                   
                   
                   
                 2 
                 W 
               
               
                   
                 R210 
                   
                 10K 
               
               
                   
                 R220 
                   
                 1M 
               
               
                   
                 R250 
                   
                 1M 
               
               
                   
                 R260 
                   
                 10K 
               
               
                   
                 R270 
                   
                 3.48K 
               
               
                   
                 R280 
               
               
                   
                 R290 
               
               
                   
                 R300 
                   
                 1M 
               
               
                   
                 R310 
                   
                 1M 
               
               
                   
                 R330 
                   
                 2K 
               
               
                   
                 R340 
                   
                 1M 
               
               
                   
                 R350 
                   
                 2.74M 
               
               
                   
                 R360 
               
               
                   
                 R380 
               
               
                   
                 R390 
                   
                 1M 
               
               
                   
                 R400 
                   
                 1M 
               
               
                   
                 R410 
                   
                 1M 
               
               
                   
                 R420 
                   
                 100K 
               
               
                   
                 R430 
               
               
                   
                 R440 
                   
                 1M 
               
               
                   
                 R450 
                   
                 10K 
               
               
                   
                 R460 
                   
                 20.5K 
               
               
                   
                 R470 
               
               
                   
                 R480 
                   
                 1M 
               
               
                   
                 R490 
                   
                 470 ½ 
                 W 
               
               
                   
                 R500 
               
               
                   
                 R510 
                   
                 10K 
               
               
                   
                 R520 
                   
                 10K 
               
               
                   
                 R530 
                   
                 63.4K 
               
               
                   
                 R540 
                   
                 5K 
               
               
                   
                 R550 
               
               
                   
                 R560 
                   
                 1M 
               
               
                   
                 R570 
                   
                 510 2 
                 W. 
               
               
                   
                   
                   
                   
                 METAL 
               
               
                   
                   
                   
                   
                 OXIDE 
               
               
                   
                 R580 
                   
                 25.5K 
               
               
                   
                 R590 
                   
                 1M 
               
               
                   
                 R600 
                   
                 6.04K 
               
               
                   
                 R610 
                   
                 499 
               
               
                   
                 R620 
               
               
                   
                 R630 
                   
                 22.1K 
               
               
                   
                 R640 
                   
                 44.2K 
               
               
                   
                 R650 
                   
                 1M 
               
               
                   
                 R660 
               
               
                   
                 R670 
                   
                 1M 
               
               
                   
                 R680 
               
               
                   
                 R690 
                   
                 7.5K 
               
               
                   
                 R700 
                   
                 7.5K 
               
               
                   
                 R710 
                   
                 100K 
               
               
                   
                 R720 
                   
               
               
                   
                 U1:A 
                 LM2902 
               
               
                   
                 U1:A1 
                 LM2902N 
               
               
                   
                 U1:B 
                 LM2902 
               
               
                   
                 U1:B1 
               
               
                   
                 U1:C 
                 LM2902 
               
               
                   
                 U1:C1 
                 LM2902N 
               
               
                   
                 U1:D 
                 LM2902 
               
               
                   
                 U1:D1 
                 LM2902 
               
               
                   
                 U2:A 
                 LM2902 
               
               
                   
                 U2:A1 
                 LM2902 
               
               
                   
                 U2:B 
                 LM2902 
               
               
                   
                 U2:B1 
                 LM2902 
               
               
                   
                 U2:C 
                 LM2902 
               
               
                   
                 U2:C1 
                 LM2902 
               
               
                   
                 U2:D 
                 LM2902 
               
               
                   
                 U2:D1 
                 LM2902 
               
               
                   
                 U3 
                 MOC3052N 
               
               
                   
                 U3:A1 
                 LM2902 
               
               
                   
                 U3:B1 
                 LM2902N 
               
               
                   
                 U3:C1 
               
               
                   
                 U3:D1 
                 LM2902N 
               
               
                   
                 U4:A 
                 LM2904 
               
               
                   
                 U4:A1 
                 LM2904 
               
               
                   
                 U4:B1 
                 LM2902N 
               
               
                   
                 U4:C1 
                 LM2902N 
               
               
                   
                 U4:D1 
                 LM2902N 
               
               
                   
                 U5:A1 
                 CD4066 
               
               
                   
                 U60 
                 MOC3052N 
               
               
                   
                   
               
             
          
         
       
     
     Although preferred embodiments of the invention have been described in detail herein, those skilled in the art will also recognize that various substitutions and modifications may be made without departing from the scope and spirit of the appended claims.