HUMIDITY RESPONSIVE DRYER CONTROL

3 205 590

HUMIDITY RESPONSIVE DRYER CONTROL Homer W. Deatou, Dayton, Ohio, assignor to General Motors Corporation, Detroit, Mich., a corporation of Delaware

Filed Feb. 23, 1962, Ser. No. 175,071
8 Claims. (CI. 34—45)

This invention relates generally to an improved power termination control circuit and more particularly to a point relative humidity.

In the clothes drying art, it has been recognized that different fabric loads (size and type) have different drying characteristics when considering dryer exhaust air humidity versus time for drying. Therefore, any device which automatically terminates a drying cycle on exhaust humidity alone cannot produce consistent drying results when the load varies both as to size and character of fabric. Note has been taken of the fact that the relative humidity of the outlet or exhaust air drops as the temperature thereof rises. Using this fact, this invention is directed to a control circuit which progressively lowers the humidity control point at which the drying cycle will terminate. Thus, this invention recognizes that smaller loads and lighter fabrics are properly dry at lower exhaust air relative humidity readings.

Accordingly, it is an object of this invention to provide a dryer termination control circuit equally effective in terminating the drying cycle of small or large loads of different fabrics.

Another object of this invention is the provision of a dryer termination control which is effective to select the control termination point in accordance with the type of load being dried.

A more specific object of this invention is the provision of a humidity element controlled dryer termination control in which the relative humidity termination point is continuously reduced whenever the sensible temperature of the dryer exhaust exceeds a predetermined value to indicate the load is approaching a dry condition.

Another object of this invention is the provision of an amplifier for a dryer control for terminating the drying cycle, said amplifier being actuated by a voltage divider circuit including a humidity sensing element responsive to the dryer exhaust air and a variable resistance element which is continuously mechanically increased to lower the relative humidity control point at which the exhaust air at the sensing element would be effective to terminate the drying cycle.

Another object of this invention is the provision of an improved amplifier in a dryer control which includes relay coil winding in parallel with a transistor—the transistor being selectively operated by a voltage divider circuit having a humidity sensing element and a variable resistance.

A more general object of this invention is the provision of a relay control circuit having a relay coil winding in parallel with a transistor or semiconductor switch means.

Another object of this invention is the provision of a control for a power supply including a circuit having a relay coil actuatable for connecting and disconnecting the circuit from said power supply, a transistor in parallel with said coil and a voltage divider circuit in parallel with said coil and connected to said transistor for biasing said transistor to actuate said coil.

Another object of this invention is provision of a voltage divider circuit in a clothes dryer control, said voltage divider circuit including a variable resistance which is progressively increased throughout the drying cycle to change the point at which the drying cycle is terminated.

Further objects and advantages of the present inven

2

tion will be apparent from the following description, reference being had to the accompanying drawings wherein preferred embodiments of the present invention are clearly shown.

In the drawings:

FIGURE 1 is a cross-sectional view of a clothes dryer suitable for use with this invention;

FIGURE 2 is a schematic wiring diagram of the control circuit of this invention; and

j FIGURE 3 is a graph of relative humidity versus time for the last part of a clothes drying cycle controlled in accordance with the teachings of this invention.

In accordance with this invention and with reference to FIGURE 1, a clothes dryer 10 is illustrated. The dry

jg er includes a front wall 12, a rear wall 14 and a top panel 16 having a control console 18 at the rear thereof. The walls 12, 14 and 16 partially form a casing for enclosing a tumbling drum 20 which is journaled in a bearing 22 carried by a rear bulkhead 24. The tumbling dram

2Q 20 has a perforated rear wall 26 and a flanged collar 28 which surrounds an inwardly directed collar 30 on a front port plate 32. The front port plate 32 is fixed to a recessed shoulder 34 in the front wall 12 of the dryer cabinet. Thus, the port plate 32 and its integral collar

2g 30 form the access passage for placing clothes within the tumbling drum 20.

A door 36 is hingedly mounted on the front wall of the dryer and carries a lint collecting housing 38 which nests within the port plate 32 in sealing engagement thereto

30 when the door is closed. The lint collecting housing carries a lint collecting screen 40 which is interposed between the perforations 42 of the lint collector housing 38 on one side of the screen 40 and an outlet passage 44 on the other side of the housing. The port plate 32 carries

35 an arcuate series of ports 46 at its lower edge which directs air into a front duct 48 leading to a blower 50 within the dryer cabinet. This blower 50 exhausts the air to the atmosphere through a duct 74. Between the front duct 48 and the blower 50 is a control conduit 52 in which

40 a humidity sensing element or sensor 54 resides. An amplifier 56 in the dryer console is electrically connected to the sensor 54 and adapted to signal the end of the automatic drying cycle. A heater 60 is included in the annular space between the bulkhead 24 and the perfo

45 rated rear wall 26 of the tumbling drum. A motor 62 is positioned in the lower part of the dryer casing and, adapted to drive the blower 50 and rotate the tumbling drum 20 through a conventional pulley and belt arrangement 64.

50 Air flow through the tumbling drum 20 for drying the clothes is thus afforded as follows. Air enters the back wall 14 of the dryer casing through openings, such as 66 and 68. The air is drawn through the perforations 70 of the rear bulkhead 24 and across the heating element

55 go into the tumbling drum 20. Air leaving the tumbling drum after vaporizing moisture from the clothes enters the lint collecting housing 38 through the perforations 42 thereof and has the lint filtered therefrom by the screen 40. This exhaust air flowing from the tumbling drum by

60 way of the front duct 48 and the conduit 52 passes the humidity sensing element 54 prior to its being exhausted through the outlet duct 74 to the atmosphere. This invention is directed to a control for terminating the drying cycle in the dryer describer hereinabove.

65 The control circuit is best seen in FIGURE 2 and includes a push-actuated start button 76 in an accessible location such as on the control console of the dryer. The start button 76 is depressed momentarily to energize a holding circuit which includes a holding relay coil 78 and a switch 80 actuated by the holding coil. A delay means may be provided for the push button 76 to prevent o

its opening before moisture is reflected at the sensing element 54 in the dryer exhaust. It should be recognized, however, that a more sensitive sensing element 54 will preclude the necessity for this time delay in the start switch.

The dryer termination control includes an improved amplifier 56 having a direct current relay coil or winding 82 for actuating a switch 84 in series with the holding circuit coil 78. The switch 84 is normally open and is closable so long as the relay coil 82 is energized. By-passing the relay coil 82 is a transistor 86 having its emitter 88 connected to one side of the coil 82 and its collector SO connected to the other side of the relay coil. Since the relay coil 82 is actuatable by direct current, a diode 92 is included in parallel relationship with the coil to pass current in one direction only. A capacitor 94 may also be included for relays which have a tendency to hum when used on A.C. or pulsating D.C A resistance 96, in series with the relay coil 82, must be small enough to permit the energization of the coil 82 with the available power supply. Grouping the foregoing components, the only electrical connections to the amplifier 56 are a conductor 98 to one side of the domestic 115 v-230 v. power supply line Li and a conductor 100 to the neutral side of the line N. The base connection 102 for the transistor 86 extends out of the amplifier 56 and is connected to a voltage divider circuit which includes the humidity sensing element 54 and a motorized resistance unit 104 in juxtaposition to the amplifier in the console. The unit 104 has a variable resistance 108 and a positioning motor 106 which is thermally responsively energizable for progressively increasing the resistance 108. Since the resistance 108 is gradually and progressively increased during the drying cycle whenever the motor 106 is energized, a mechanical connection (shown schematically at 110) is provided between the positioning motor shaft and the start button 76 so that the variable resistance 108 may be mechanically repositioned to its lowest value each time a new drying cycle is initiated.

The resistances 108 and 54 in the voltage divider circuit are proportioned such that the resistance 108 is much larger, on the order of 100 to 1, than the sensing element resistance 54. Insofar as the voltage divider circuit is concerned, therefore, when the motor 106 runs, the increase in resistance 108 tends to cancel out or offset a portion of the increased voltage drop across the sensor 54 due to the drying of the clothes load (resistance 54 increases as the clothes become dry). The partial offsetting action tends to delay the point at which a sufficient voltage potential exists, base 102 to emitter 88, to fire the transistor 86 and terminate the drying cycle. In effect, the gradually increasing resistance 108 acts to continually decrease the percent relative humidity requirement of the exhaust air necessary to terminate the drying cycle.

Referring now to FIGURES 1 and 2, the dryer control circuit includes such normal safety features as a high limit thermostat 112 which opens upon an abnormal rise in temperature within the heater chamber and a motor switch 114 which is closed by a centrifugally actuated mechanism (not shown) in the main motor 62 to insure that the heater 60 will not be energized unless the motor 62 is operating to rotate the tumbling drum 20. In addition to the safety controls, this circuit includes a termination conrol thermostat 116 responsive to the temperature of air leaving the tumbling drum. The thermostat 116 has a switch blade 118 which closes on a heater circuit contact 120 when the temperature of the exhaust air is between 130° and 140° F. or below. Above this predetermined temperature range, the switch blade moves to the contact 122 in which position it energizes the positioning motor 106 to increase the resistance 108 in the voltage divider circuit. Note that the termination control thermostat 116 is first moved to its positioning motor contact 122 when the sensible temperature of the exhaust air increases suddenly over the said predetermined temperature range. This generally occurs when the clothing is reaching a dry condition as reflected by the moisture content of the dryer

4

exhaust air. This dry bulb temperature jump has been used repeatedly in prior art devices to terminate the drying cycle. However, such prior art systems fail to compensate for the occasional or LIGHT loads, such as a shag

6 rug or four to five pounds of assorted clothing and hardto-dry items wherein fabric seams resist giving up moisture evenly. In these LIGHT loads, the point at which the sensible temperature jumps above the predetermined temperature range is in advance of the proper dryness

10 end point for the clothes load and, thus, this invention seeks to prolong the cycle by operating the motor 106 to gradually reduce the percent relative humidity at which the cycle will terminate. As has been explained hereinbefore, this is accomplished by increasing the resistance

15 108 in the voltage divider circuit to delay the point at which the humidity sensing element 54 is effective to bias transistor 86 to by-pass the power and shunt the termination relay coil 82. An operating drying cycle will be explained with refer

90 ence to the control circuit of FIGURE 2 and the graph in FIGURE 3. Note that the graph in FIGURE 3 takes into account only the last portion of the drying cycle which shows the manner in which the various type loads terminate. Consider first a NORMAL eight pound load which

2g is placed in the tumbling drum 20 and the dryer energized by pushing the button 76. At the start, the temperature in the tumbling drum is below 130° F. and the thermostat 116 is positioned to energize the heater 60 and the prime mover or main motor 62. The amplifier 56 is actuatabie

30 by humidity sensed by the sensing element 54 in the exhaust duct 52. It is possible that the system may terminate the NORMAL or regular eight pound load cycle before the motor 106 is even energized—assuming that the sensor 54 fired the transistor 86 in advance of the point at which

35 the sensible temperature jumps to actuate the thermostat 116. In this situation, the NORMAL drying cycle will terminate when the resistance of the element 54 is increased to the point at which the transistor 86 is biased to shunt the power from the relay coil §2 to cause the switch 84

40 to open. However, most loads do permit the temperature to increase enough to actuate the thermostat 116 and run the positioning motor 106 some. In graphic form, this more usual condition is represented by curve 136 which is shown dropping off to about to 35% R.H. mark at which point 138 the relay coil 82 is deenergized and the

45 cycle terminated before the clothes are overdried. Thus, at point 138 a NORMAL eight pound load of clothes is dry when the resistance of the sensor 54 increases to create a voltage at the base 102 which wiil fire the transistor.

50 Let us assume now that a LIGHT load, such as a shag rug or a small load (under five pounds) of light fabrics, is being dried. The moisture will be very quickly vaporized from the exposed fabric surface in the LIGHT load. However, in the depths of the shag rug and in the folds

5a and seams of the light fabrics, the moisture will still be retained so that an exhaust air indication of thirty-five percent relative humidity would be a premature indication that the load is dry. Consequently, and in accordance with this invention, when the sensible temperature

00 first jumps and moves above the predetermined setting of thermostat 116, say 140° F., the switch blade 118 will move to the contact 122 to energize the positioning motor 106. The motor then starts to gradually increase the resistance 108 to hold off the point at which the

G5 sensing element 54 would normally fire the transistor 86 to terminate the drying cycle. This, in effect, lowers the relative humidity setting which is necessary to terminate the drying cycle.

_0 A load controlled in accordance with this invention

'would exhibit three curves of interest. The drying rates of a typical LIGHT load and NORMAL load are evidenced on the drying curves 140 and 136 respectively, and the rate of depressing termination point relative

75 humidity is shown by the control curve 142. The re