Abstract:
A low voltage terminal board interfaces a fan motor with a field thermostat. The board includes a power supply providing a 24 volt DC power source for driving an electronic air cleaner relay and fan motor control lines and a 5 volt DC power source for providing optical isolation and timing logic. Several heater stage signals are connected to a diode interlock network so that no matter which signal is generated the first electric stage heater is energized. A first optically coupled isolator receives a 240 volt AC input whenever the first stage electric heater is energized with the isolator output controlling energization of a heat or medium speed operation of the fan motor. Second and third optically coupled isolators receive a 24 volt AC input indicative of continuous fan operation and compressor operation respectively with the outputs controlling energization of low (on/off) fan speed operation and high speed fan operation respectively. A timing circuit is interconnected between the third optically coupled isolator and the high speed fan terminal to provide a delayed off period of fan operation. Diode interconnections ensure that the on/off terminal is energized anytime the high speed terminal or the first stage electric heater is energized and anytime the on/off line is energized the electronic air cleaner relay is energized.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates generally to heating, ventilating and air conditioning (HVAC) systems and more specifically to an interface circuit for use in controlling the speed of a variable speed electrically commutated fan motor. 
     2. Brief Description of the Prior Art 
     Electrically commutated motors or ECM&#39;s are DC motors having internal inverters in which low voltage control signals are used to select the motor speeds. However, the use of such a motor in HVAC systems such as a heat pump blower assembly, for example, leads to certain problems. In HVAC systems there are many paths which can conduct current and since this type of motor used for the fan will run, albeit at reduced efficiency, on incomplete portions of a sine wave, prior art approaches have required various discrete components to avoid spurious signals from causing unintended energization of the motor. In a heat pump it is necessary to energize the fan motor whenever electric heat has been energized. This requires the detection of 240 VAC power being applied to the electric heating element and subsequently energizing the fan motor. Further, when a &#34;continuous fan&#34; mode of operation is selected the lowest fan speed, or an on/off signal to the ECM, must be the only signal energized. 
     A prior art method for accomplishing this employs a transformer for transforming the 240 VAC signal to 24 VAC which is used to drive the ECM control lines. However, it is necessary to isolate the low voltage signal from the continuous (low) speed fan operation from another low voltage signal calling for high speed fan operation. Improper isolation can result in high speed fan operation when the low speed fan is selected. This system, which uses an additional transformer for a heat detection circuit, is inherently costly using two separate transformers as well as being unreliable. 
     It is an object of the present invention to provide a circuit which does not have the above limitations of the prior art, a circuit which is of low cost and yet reliable for controlling the variable speeds of an ECM motor. 
     SUMMARY OF THE INVENTION 
     Briefly, in accordance with the invention an interface circuit used to control the energization of an electrically commutated fan motor having low, medium and high speed operations in a heating, ventilating, air conditioning (HVAC) system comprises a power supply for providing a 24 volt DC and a 5 volt DC source. A timer provides a delay in deenergization of the fan motor once energized other than when the fan is energized in the continuous fan mode. Input signals include a Y signal indicating that the HVAC compressor is running and when present it energizes an optically coupled isolator turning on the high speed of the fan motor. Another signal G, signifying the continuous fan mode, is used to energize another optically coupled isolator which in turn causes an on/off terminal to be energized turning the fan motor on at its low speed. According to a feature of the invention if any of the speed taps of the motor is energized then the on/off terminal, and therefore the fan motor, is also energized through diode interconnections which also serve to block other selected voltages. The circuit also uses a diode interlock arrangement with several electric heat control signals, W2, W3 and E to ensure that even if miswired the first stage electric heat control signal is energized, i.e. the signals W2DC and W2GND. This in turn ensures that whenever a stage of electric heat is energized the fan is also energized. According to another feature of the invention whenever the first stage of electric heat is energized a line called AC interlock (ACINT) a 240 volt AC input, is energized in turn energizing another optically coupled isolator which will turn on the heat (medium) speed of the fan. According to yet another feature of the invention a relay which controls the energization of an electronic air cleaner is energized whenever any of the signals for the fan are energized. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic circuit of a fan coil control module made in accordance with the invention; 
     FIG. 2 is an enlarged schematic of the power supply circuit portion of FIG. 1; 
     FIG. 3 is an enlarged schematic of a diode interlock circuit portion of FIG. 1; 
     Fig. 4 is an enlarged time delay circuit portion of FIG. 1; 
     Fig. 5 is an enlarged input isolation circuit portion of FIG. 1; and 
     FIG. 6 is an enlarged output circuit portion of FIG. 1. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     With particular reference to FIG. 1 numeral 10 designates a control module having a low voltage terminal board 12 with terminals R, Y, O, L and C, G, E, W2 and W3. These are all 24 volt AC signals with respect to common C. A signal at Y signifies that the compressor motor of the system is running. R represents the control voltage which is connected to the room thermostat. R and C are connected to the 24 volt taps of a 240/24 volt transformer (not shown) with C being the common of the transformer which is tied to chassis ground. A signal at G will result in continuous fan energization. A signal at W2 signifies energization of a first stage of electric heat. A signal at W3 signifies energization of a second stage of electric heat and a signal at E signifies energization of emergency heat. Terminals O and L are wiring points for control signals which do not involve the control circuit of the instant invention. 
     With particular respect to FIG. 2, power supply circuit 14 includes a full wave bridge comprising diodes CR18, CR19, CR8 and CR9 which converts the 24 volt AC power from the transformer to a 24 volt DC signal. Diode CR7 isolates a filter capacitor C5 from the signal and zener diode CR5 provides a 5 volt DC supply which is used for optical isolation and timing logic to be described below. 
     With respect to FIG. 3, diode interlock circuit portion 16 comprises diodes CR10-CR17 connected between terminal board 12 and a connector 18. The interlock functions to ensure that even if the thermostat wires are miswired with respect to the electric heat controls, W2, W3 and E that the first stage of electric heat control signal W2DC, W2GND will be energized. As will be explained below, this will ensure that whenever a stage of electric heat is energized the fan will also be energized. Terminal W2 is connected to diodes CR12 and CR13 with the cathode of CR13 connected to pin 2 of connector 18 through a line W2DC and the anode of diode CR12 connected to pin 3 through a line W2GND. Terminal W3 is similarly connected to diodes CR10 and CR11 with the cathode of diode CR10 connected to pin 2 through line W2DC and the anode of diode CR11 connected to pin 3 through line W2GND. W3 is also coupled directly to pin 6 of connector 18. Terminal E is connected to diodes CR14 and CR15 with the cathode of diode CR15 connected to pin 2 through line W2DC and the anode of diode CR14 to pin 3 through line W2GND. Terminal E is also directly coupled to pin 1 of connector 18. Terminal C is connected to the transformer tap through a 5 amp fuse and through line CFUSED to diodes CR17, CR16 with the cathode of diode CR17 connected to pin 2 through line W2DC and the anode of diode CR16 to pin 3 through line W2GND. Terminal C is also connected to pin 4 of connector 18. Signals W2DC, W2GND, pins 2 and 3 respectively of connector 18, are adapted for connection to a time delay relay (not shown) which, after a selected time delay, connects line 22 to line ACINT whenever electric heat is called for as well as energizing the first stage of electric heat. 
     Jumpers JW1 and JW2 are merely connections to wiring taps Y and R respectively on the circuit board 10. Dummy terminal T2 located in FIG. 1 above connector 18 provides a place to connect the unused tap on the 24 volt AC transformer. FIG. 3 also shows a series of dash lines between E and W2 and between W2 and W3 which are breakaway tabs on board 10. This is provided so that the module can be used with various types of room thermostats available in the market. That is, some thermostats have multiple heat stages and others have a single heat stage. With a single heat stage thermostat terminals E and W2 are common to each other so a signal in either line would energized both stages. The tab between terminals W2 and W3 is useful with an outdoor thermostat, that is, the tab would be broken and such a thermostat would be connected between W2 and W3 so that additional heat could be obtained when ambient temperature goes below a selected level. 
     Further information on the diode interlock and its function can be obtained in copending application Ser. No. 07/580,747 assigned to the assignee of the present invention and which is incorporated herein by this reference. 
     The time delay circuit portion 20 shown in FIGS. 1 and 4 comprises a programmable timer U4, a CMOS CD4541B having an internal oscillator coupled to pins 1, 2 and 3 (RTC, CTC and RS respectively). The oscillator frequency is set by resistor R7 and capacitor C3 and stabilized by resistor RS. The rate at which the time expires is selected by the two input signals at pins A and B. If pin A is at logic 1 and pin B is at logic 0 then the time delay is at a first, relatively short period, whereas if pin A and pin B are both at a logic 1 the time delay is at a second, relatively long period. When power is first applied to the circuit, capacitor C4, coupled to pin B, will be discharged. Pin 5, the AR or automatic reset line is also tied to capacitor C4. If AR is at a logic 0 then the timer will do an automatic reset. Resistor R12 and capacitor C4 form an RC timing charge so that pin B and AR will lag the power supply coming up resulting in the first, short time delay. When capacitor C4 becomes charged normal operation of the circuit takes place with the timer providing the second, relatively long time delay for a purpose to be explained infra. 
     When a 24 VAC signal occurs on Y optically coupled isolator U3 becomes energized and a signal is driven through resistor R16 that charges capacitor C6 (FIG. 1) which serves as a filter network to prevent false triggering of the timer. Pin MR, serially connected through resistor R16, is then driven high causing output Q at pin 8 to go high turning on the fan motor as will be explained in more detail below. When pin MR goes low upon removal of the signal on Y output Q will remain high until the expiry of the second, relatively long time delay, selected by the AB input, typically 60 seconds keeping the fan on for that extended period of time. 
     Diode CR6 in parallel with resistor R12 allows the circuit to continue to operate in the event that power from the power supply is interrupted by dumping the voltage stored therein into the 5 volt supply on line 22. Resistor R11, a 10K ohm resistor connected between line 22 and ground ensures that the 5 volt supply is decreased in a timely manner thereby avoiding the possibility of attempting to turn on the fan when there is insufficient power. Resistor R9 coupled to the MR signal is a pull down resistor to ground to provide discharge of capacitor C6 as well as serve as an emitter resistance for the output transistor Q3 to ensure that if transistor Q3 is off that MR goes to ground. Resistor R10 is a current limiting resistor for transistor Q2. 
     Input isolation circuit 24, FIG. 5, and output circuit 26, FIG. 6, comprise optically coupled isolators U2 and U3, PS2505-1 (NEC) having an AC input. A pair of back to back zener diodes CR23, CR22 and CR21, CR20 are provided in the respective signal lines of G and Y to act as voltage discriminators set at 12 volts apiece so that anytime the input signal between G and C is above 12 volts U2 will be energized and in like manner anytime the input signal between Y and C is above 12 volts U3 will be energized. Resistors R6 and R4 serve as current limiters to prevent excessive input into the optically coupled isolators. Resistors R15 and R14 for isolators U2, U3 respectively act as pull down resistors for the G and Y signals and provide compatibility of the circuit for use with electronic thermostats. When a signal appears on line G or Y an RMS AC current of 10 milliamperes are conducted through the infrared LED internal to the isolator which causes emission driving the photo transistor into the on state thereby converting the AC input signal to DC without any filtering, rectification or the like and avoiding problems associated with improper ground connections. 
     Optically coupled isolator U3 is coupled to the MR pin of timer U4 through resistor R16, as stated supra, with output Q coupled to the base of NPN transistor Q2 through current limiting resistor R10. Optically coupled isolator U2 is coupled to NPN transistor Q3 through current limiting resistor R5 and is adapted to drive 5 volts DC into resistor R5 and turn on transistor Q3. 
     A detector circuit 28, FIG. 1, comprises optically coupled isolator U1 in a somewhat different manner. Isolator U1 is used to detect a 240 volt AC input signal between lines L1 and ACINT. An AC interlock line, ACINT, is a common contact of the first stage electric heat control (driven by W2DC and W2GND) so that whenever the first stage heat control is energized, ACINT is also energized. The input of isolator U1 is serially connected through resistor R2, capacitor C1 back to L1. Capacitor C1 is an AC line rated capacitor which serves as impedance to limit current flow into isolator U1. Resistor R2 is also connected to the input of isolator U1 to prevent excessive current surges, which may be caused, for example, by a partially energized capacitor on contact bounce of the ACINT signal, from damaging isolator U1. Resistor R1 connected between line L1 and line ACINT provides a path for any long term voltage stored in capacitor C1 to discharge to avoid the possibility of shocking someone servicing the board. 
     As mentioned above, the ACINT line becomes energized whenever the first stage electric heat comes on and when this occurs the output transistor of isolator U1 becomes saturated which connects 5 volts to resistor R3 which is a current limiting resistor in the base of an NPN transistor Q1. This charges capacitor C2 which is connected between the base of transistor Q1 and ground. Capacitor C2 serves as a filter maintaining a continuous charge to drive transistor Q1 keeping it on when the AC wave passes through zero current level. When transistor Q1 is on the heat or medium speed of the fan motor is selected (tap F3). 
     Module 10 is provided with an electronic air cleaner relay coil K1 which is connected to the 24 volt DC power supply through transistor Q3. Also connected to the coil of relay K1 is an on/off tap F2 through diode CR1. Tap F2 is connected to tap F3 through diode CR2 and to transistor Q2 and tap F4 of the high speed signal through diode CR3. Relay K1 has a normally closed contact connecting tap EAC2 with line ACINT and when energized the movable contact connects EAC2 with line L2. Relay K1 is driven with full wave rectified 24 volt power, i.e. without any filter capacitor which facilitates the use of the referenced diodes in driving the several control signals. 
     Whenever 24 volts AC is applied only from line G to C current flows through R6 and the input to isolator U2 turning it on and in turn turning transistor Q3 on. Current through transistor Q3 will sink current through diode CR1 from the on/off terminal F2 of the fan motor thereby activating the low speed. 
     If 24 volts AC is applied to line Y, isolator U3 is activated in a corresponding manner resulting in pin MR of timer U4 going high and in turn output Q going high thereby turning on transistor Q2 which will sink current from the high speed terminal F4 resulting in high speed of the fan ECM. Transistor Q2 will also sink current through diode CR3 from the on/off terminal F2 if line G has not been activated. This ensures that the motor is on. If 240 volts AC is applied to line ACINT energizing the electric heat then isolator U1 will be activated turning on transistor Q1 which will sink current from the heat terminal F3 resulting in medium speed of the fan motor. If line G is not energized then current will sink from the on/off terminal F2 through diode CR2 so that the fan motor will be on when the electric heat is energized. 
     Diode CR1 also serves as a blocking diode ensuring that in the event that another signal is turned on that the relay K1 is not turned on without energizing transistor Q3 turning on the fan motor. Whenever the fan is on the electronic air cleaner, terminal EAC2, is on. The normally closed contact of relay K1 connects EAC2 to ACINT line. Thus, if the first stage of the electric heat is on the circuit ensures that the fan is energized with the electronic air cleaner being energized through normally closed contacts of relay K1. Upon energization of the relay then line L2 is connected to terminal EAC2 which occurs only when the fan on/off signal is present. 
     The following components were used in a module made in accordance with the invention: 
     
         ______________________________________Dower supply circuit portion 14diodes CR7, CR8, CR9 CRI8, CR19                 1N4007capacitor C5          2.2 uf, 63VDC                 electrolyticresistor R13          1K ohm, 1 watt,                 5% TOL, metal filmdiode CR5             5.1 volt zener,                 1N523Bdiode interlock circuit portion 16diodes CRIO through CR17                 1N4007connector 18          12 pin connectorterminal board 12     low voltage                 terminal boardjumper JW1, JW2       low voltage jumpertime delay circuit portion 20resistors R12, R8     402K ohm, 1/4 watt,                 1% TOL, metal filmdiode CR6             1N4007capacitor C4          2.2 uf, 63VDC                 electrolyticcapacitor C3          .0056 uf, 5OVDC,                 monolithicresistor R7           215K ohm, 1/4 watt,                 1% TOL, metal filmresistor R10          10K ohm, 1/4 watt,                 1% TOL, metal filmIC U4                 CD4541 CMOS                 programmable timerinput isolation circuit portion 24resistors R14, R15    470 ohm, 3 watt,                 5% TOL, metal filmresistors R4, R6      3.24K ohm, 1/4                 watt, 1% TOL,                 metal filmdiodes CR20 through CR23                 12V zener IN5242BIC&#39;s U2, U3           PS 2505-1                 optically coupled                 isolatorsdetector circuit portion 28IC U1                 PS 2505-1                 optically coupled                 isolatoroutput circuit portion 26terminals F1 through F4                 low voltage quick                 connectsterminal EAC1         high voltage quick                 connecttransistors Q1 through Q3                 MPSA06 NPNrelay K1              AZ8, 24VDCresistors R3, R5, R10 3.24K ohm, 1/4                 watt, 1% TOL,                 metal filmdiodes CR1 through CR4                 1N4007capacitor C2          2.2 uf, 63VDC                 electrolytic______________________________________ 
    
     Though the invention has been described with respect to a specific preferred embodiment thereof, many variations and modifications will immediately become apparent to those skilled in the art. It is therefore the intention that the appended claims be interpreted as broadly as possible in view of the prior art to include all such variations and modifications.