US3031768A - Control system for clothes dryers - Google Patents

Description

May 1, 1962 v. J. KUROWSKI CONTROL SYSTEM FOR CLOTHES DRYERS 3 Sheets-Sheet 1 Filed July 9, 1959 INVENTOR. J. Ku Rows K I F l G. i

HIS ATTORNEY May 1, 1962 v. J. KUROWSKI CONTROL SYSTEM FOR CLOTHES DRYERS 3 Sheets-Sheet 2 Filed July 9, 1959 INVENTOR. VICTOR 3'. KUROWSYKI H15 ATTORNEY May 1, 1962 v. .1. KUROWSKI CONTROL SYSTEM FOR CLOTHES DRYERS 5 Sheets-Sheet 5 Filed July 9, 1959 F'IG.4

INVENTOR VICTOR J. KUROWSK\ H l 5 ATTORNEY United States Patent IGNTRGL SYSTEM Edi CLQTHES DRYERS Victor .l'. Kurowshi, Louisville, Ky., assignor to General Eiectric tjornpany, a corporation of New York Fiied .luly s, 1959, fier. No. $25,972 it Claims. (Cl. 34--45) My invention relates to clothes drying machines, and more particularly to automatic control systems for use in such machines for controlling the drying operation.

The wide variety of modern fabrics used in the manufacture of clothing ha resulted in the fact that a substantial spread of drying temperatures represent the optimum for vmious fabrics. Experience has shown that the more delicate fabrics which are best dried at lower temperatures are also generally those which dry more quickly. For instance, nylon and Dacron clothing is best dried at relatively low temperature, but nonetheless it dries much more rapidly than cottons such as sheets and towels, even though the cottons be dried at a substantially higher temperature. Because of these factors, it is highly desirable that drying cycles for the more delicate fabrics provide for relatively low temperatures and that the longer drying cycles desirable for the drying of cottons provide higher drying temperatures. However, while it is desirable that materials such as cottons be dried at higher temperatures, it is also desirable that these temperatures be decreased toward the end of a drying cycle so that the clothes may be more comfortably handled after the drying sequence is finished and they are being removed from the machine.

It has been found that the foregoing, as to the desirability of a spread of drying temperatures which varies proportionately with the drying time, holds true whether the dryer be of the straight timed variety or of an automatic cycle type such as those where the total time of the cycle is controlled jointly by the timer mechanism and by a thermostatic device.

To summarize, it is desirable that where the material to be dried is such that it requires only a short cycle, a low temperature drying operation be provided and that the temperature of the drying be increased as the type of clothing requires a longer cycle. A further factor to be considered is the desirability of decreasing the drying ten perature towards the end of the drying cycle to improve the ease of handling the clothes.

it is therefore an object of my invention to provide economical means for providing relatively low drying temperatures for short cycles and relatively high drying temperatures for the longer cycles.

A further object of the invention is to achieve this effect by suitable variation of the heating effect of a biasing heater positioned in heating relation to a main thermostat arranged to sense the temperature of clothes in the drying machine, and to achieve this variation by means of a timer control mechanism.

A more specific object of the invention is to achieve this variation of the heating effect of the biasing heater by providing at least two resistances in parallel with each other with a switch in at least one of them, with means controlled by the timer mechanism for opening the switch when a relatively long cycle is selected so that a higher resistance is in series with the biasing heater which is consequently less effective; when the biasing heater is less eifective the main thermostat requires more heat from the clothes before it trips, and thus the opening of the switch causes a higher temperature drying operation to be provided by the selection of a longer drying time.

Yet a further object of the invention is to provide an improved control arrangement whereby longer drying cycles, which call for relatively high temperatures to start with, provide for a decrease in the temperature as the 3,031,768 Patented May 1, I962 cycle continues in order to facilitate handling of the clothes at the end of the cycle.

In carrying out my invention in one form thereof, I provide a clothes drying machine which has a conventional clothes container with a main heater positioned to heat the clothes in the container. Thermostatic means are positioned to be responsive to the temperature of the clothes in the container, and are also acted on by an electric resistance-type biasing heater which is positioned in suitable heating relation to the thermostatc means. When the thermostatic means senses a predetermined high temperature, due to the combination of the clothes temperature and the biasing heater, it causes a first switch to open to stop operation of the main heater. As thus far described, it is clear that the more effective the biasing heater, the lower the clothes temperature which will trip the thermostatic means.

In series with the biasing heater I provide at least two conductors connected in parallel with each other, with a second switch member in one of the two conductors. The resistances in the conductors are such that with the sec ond switch open a higher resistance is presented by the two conductors, and with the second switch closed a lower resistance is provided by the two conductors.

Manually presettable control means are provided for variably limiting the length of operation of the main heater. The control means, in addition, moves the second switch means to an open position when the control means is set for a relatively long period of operation and to a closed position when set for a relatively short period of operation. In this manner the biasing heater has a smaller effect on the thermostatic means when a long operation is selected and a relatively large eifect on the thermostatic means when a short operation is selected. The net result of this is to cause high temperature drying automatically in response to selection of a long timer operation and low temperature drying in response to selection of a short timer operation.

As well as having the timer control open the second switch means when moved to provide a selectively long cycle, the timer may readily be arranged to cause the second switch to close when the time decreases back to that provided for delicate loads. As a result, even if a long cycle is selected the clothes temperature will decrease toward the end thereof to facilitate the handling of the clothes when the cycle comes to an end.

The subject matter which I regard as my invention is particularly pointed out and distinctly claimed in the concluding portion of this specification. My invention, however, both as to organization and method of operation, together with further objects and advantages thereof, may be best understood by reference to the following description taken in conjunction with the accompanying drawings.

In the drawings, FIGURE 1 is a side elevational view of a clothes dryer incorporating my improved dryer control arrangement, the view being partly broken away and partly sectionalized to illustrate details;

FIGURE 2 is a horizontal sectional view of the drye with certain surfaces broken away and partly sectionalized to illustrate further details;

FIGURE 3 is a schematic illustration of my improved control circuit; and

FIGURE 4 is a cam chart showing the sequence of operation of the cam controlled switches of FIGURE 3.

Referring now to FIGURES l and 2 of the drawings, I have shown therein a domestic clothes dryer 1 including a clothes tumbling container or drum 2 provided within a suitable outer casing or cabinet 3 which completely encloses it on all sides. The drum is mounted for rotation within cabinet 3 on a substantially horizontal axis, and is generally cylindrical in shape, having a first outer Wall portion 4, second and third outer wall portions and 6 located respectively adjacent the ends of the drum, a back wall 7 and a front wall 8. Outer wall portions 4, 5 and 6' are imperforate over their entire length, so that the entire outer shell of the basket is imperforate; on the interior surface of central portion 4 there is provided a plurality of clothes tumbling ribs 9.

The front of drum 2 is rotatably supported Within the outer casing 3 by means of a pair of idler roller wheels it) and 11 which are rotatably secured to the top of an upwardly extending member 12 secured at its bottom to the base 12a of the machine (FIGURE 1). It will be observed that rollers and 11 are disposed beneath the drum in contact with portion 6 thereof. The rear end of the drum receives its support from a roller wheel 13 which is positioned beneath portion 5 of the drum in supporting and frictionally driving engagement therewith.

Roller 13 is mounted on a shaft 14 supported in bearings 1'5. Shaft 14 is secured to a pulley 16 which is driven from a belt 17 in turn powered by a pulley mounted on the end of a shaft 19 of an electric motor 20. The motor, pulleys, and roller 13 are so proportioned to drum 2 and to each other that drum 2 is rotated by roller 13 at an appropriate speed to provide tumbling action for articles of clothing placed therein.

in order that a stream of drying air may be introduced into and passed through the clothes drum, the drum is provided with a central aperture 21 in its front wall 3 and with a plurality of perforations 22 extending in an annulus around back wall 7. Rigidly secured to the rear wall 23 of casing 3 by any desired means such as, for instance, welding at suitable points 24, is a bathe member 25 which has secured thereto heating means 26 appropriately insulated from the battle member. Heating elements 26 may be annular in shape so as to be generally coextensive with the perforations 22 in drum 2. A baffie member 27 is rigidly secured to the back wall 7 of the drum outside the ring of perforations 22 and within the stationary bafile 25 so that an annular air inlet 23 is, in effect, formed by the two baflles 25 and 27. Baffle 27 is further provided with an annular series of openings 29; in this manner a passage is provided for air to enter annular opening 28 between the bafides, pass over the heating elements 26, through openings 29 and then through perforations 22 into the interior of drum member 2.

In addition to the air guiding function, the bafiles 25 and 29 help the rollers 19, 11 and 13 support the drum 2. Secured to the central portion 3G of baffie 27 is a bracket 31 to which in turn is secured a stub shaft mem- L ber 32 arranged substantially coaxial with drum 2. The central portion 39 of baffie 27 has an opening 34 formed therein in the form of a slot. The slot is suitably formed (as more fully described in application S.N. 789,869 filed January 29, 1959 by William F. Simpson and assigned to the same assignee as this invention) so as to permit stub shaft 32 a limited amount of movement in the vertical direction but virtually no movement in a horizontal direction. -Thus, the slight vertical motions of the drum which result from the tumbling of the clothes can be accommodated While at the same undesirable horizontal motion is afiirmatively prevented by the engagement of stub shaft 32 in slot 34.

The front opening 21 of the drum is substantially closed by means of a stationary bulkhead generally indicated by the numeral 35. Bulkhead 35 is made up of a number of adjacent members including the inner surface 36 of an access door 37 mounted on the dryer cabinet, a stationary frame 38 for the door, the inner surface 39 of an exhaust duct 40, and an annular flange 41 mounted on the frame 38 and on the duct wall. It will be noted that a suitable clearance is provided between the inner edge of the drum opening 21 and the edge of bulkhead 35 so that there is no rubbing between the drum and the bulkhead during rotation of the drum. In order to prevent any substantial air leakage through opening 21 between the interior and the exterior of the drum a suitable ring seal 42, preferably formed of felt-like material, is secured to flange 41 in sealing relationship with the exterior surface of drum wall 8.

Front opening 21, in addition to serving as part of the air flow path through the drum, also serves as a means whereby clothes may be loaded into and unloaded from the drum. The door 37, whose inner surface forms part of the bulkhead closing the opening, is mounted on cabinet 3 and when the door is opened clothes may be inserted into or removed from the drum through the door frame 38. It will be noted that the door includes an outer flat imperforate section 43 and an inwardly extending hollow section 44 mounted on the flat outer section. Hollow section 44 extends into the door frame 38 when the door is closed, and the door surface 36 which comprises part of the combination bulkhead 35 is actually the inner wall of this hollow section.

The air outlet from the basket is provided by a perforated opening 45 formed in the inner wall as of hollow door section 4-4. The bottom wall section of door 34 and the adjacent wall of door frame 3% are provided with aligned openings 46 and 47, opening 47 providing the entrance to duct 4%. As shown, a lint trap '48, which may comprise a fine mesh bag, is preferably positioned in exhaust duct 44? at opening $7, the bag being supported by the door frame 38.

Duct 4i) leads to suitable air movingmeans which may, as shown, comprise a centrifugal blower 49 mounted on the motor shaft and thus driven by motor 2i The outlet of blower 49 communicates with an outlet duct 50 (FIGURE 2) which extends to an opening 51 in the back 23 of cabinet 3. During operation of motor 263, the rotation of blower 49 causes air to be drawn into cabinet 3 through a suitable opening 51a, through annular opening 28, over heaters 26, through openings 29 and perforations 22 into drum 2, across the drum, through perforated opening 45 and aligned openings 46 and 4-7 into duct 4% and then through the blower 49, the outlet duct so, and opening 51 to atmosphere.

The operation of the dryer 1 is controlled by a new and improved control system, one embodiment of which is shown in the circuit diagram of FIGURE 3. As shown there, the entire control system of the machine is energized across a three wire power supply'system including supply conductors 52 and 53 and a neutral conductor 54. For domestic use, the conductors 52 and 53 will normally be connected across a 220 volt power supply with 110 volts appearing between the neutral line 54 and each of these conductors.

The drive motor 2e includes a main winding 55 and a start winding 56 which are connected in parallel with each other between supply conductor 52 and neutral conductor 54. Specifically, the circuit, starting at supply conductor 52, passes through a switch 5'7, then through the windings 55 and 56 in parallel with each other, a standard motor protector device 58, a switch 59 positioned (FIGURE 1) to be opened when the door 37 is opened and closed when the door is closed, and neutral conductor 54. Included in the parallel branch circuit of auxiliary winding 56 is a switch 60 controlled by a centrifugal mechanism 61 rotatable with the motor; when the motor comes up to a certain speed upon energization of the windings 55 and 56, the device 61 will cause switch 60 to open thereby de-energizing winding 56 and permitting the motor to continue running on winding 55 alone.

Returning to switch 57 which is in series with both windings of the motor, this switch is under the control of a cam member 62 which in turn is driven by a timer motor 63 when energized. The timer motor 63 also controls cams 64, 65 and 66 which respectively control switches 67, 68 and 69. As well as being driven by the timer motor 63 when energized, the cams 62, 64, 65

and 66 are manually presettable to a desired position by means of a control member 70 which is generally provided in a readily accessible position such as for instance, as shown in FIGURE 1, on the backsplasher 71 secured to the top of cabinet 3 of machine '1. As also shown in FIGURE 1, the timer motor 63 and the cams and switches controlled thereby form together a control assembly 72 which is generally positioned within backsplasher 71 as shown. The energizing circuit for timer motor 63, starting with supply conductor 52, passes through switch 67, a switch 73 controlled by centrifugal mechanism 61 and closed thereby at the same time that switch 6t is opened, a main thermostatic device 74, preferably in the form of a bimetal element as shown, and timer motor 63, to conductor 54. The bimetal thermostatic means has a high temperature trip position in which it moves into engagement *with a contact 75 and a low temperature reset position in which it moves into engagement with a contact 76. The circuit through the timer motor 63 is completed when element 74 engages contact 75. Alternatively, when element 74 engages contact 76, a circuit is completed, starting at supply conductor 5'2, through switches 67 and 73, bimetal element 74, contact 76, protective device 77, and the heaters26 to conductor 53.

It will be observed in FIGURE 1 that the thermostatic device 74 is positioned at the front of the dryer on the bulkhead 35 directly below the air outlet 45 from the drum. The thermostatic element 74 is effective to sense the temperature of the exhaust air leaving the drum, which temperature is very close to the actual clothes temperature. In addition to the clothes temperature as a factor affecting the operation of thermostatic means 74, a biasing heater 78 is provided in close physical relationship with element 74 so as to be in heating relation thereto. The heater 78 is of relatively high resistance compared to heater 26 (for instance, approximately 110% ohms for heater 73 and ohms for heater 26 may be provided) so that its power consumption compared to the main heater is exceedingly small. In fact, the small size of heater 78 and the desirability of having it close to the thermostatic element 74 generally causes the preferred construction, in practice, to provide for a unitary assembly within a single casing, which assembly includes both the biasing heater 78 and the thermostatic element 74.

The biasing heater 78 is energized through a circuit which, starting at conductor 52, extends through a first group 79 of parallel connected conductors, a second group 80 of parallel connected conductors in series with the first group 79 and the biasing heater, the biasing heater 78 itself, and neutral conductor 54.

The first group 79 includes three conductors 81, 82 and S3. Conductor 81 has therein a relatively high resistance 84, conductor 82 has therein a smaller resistance 85 in series with switch 69, and conductor 83 has the smallest resistance which may be simply its own almost negligible resistance, together with switch 68 therein. It will be clear that when both switches 68 and 69 are open the circuit for biasing heater 78 must of necessity pass through resistance 84; this has the result that relatively little voltage drop occurs across the biasing heater 78 which therefore does not provide as much heat as it would if a smaller resistance were in circuit with it. Of course, the less heat provided by biasing heater 78, the more heat must be provided by the clothes in drum 2 for thermostat 74 to trip so that in effect the opening of switches 68 and 69 has the efiect of increasing the clothes temperature which causes tripping of thermostat 74.

If switch 68 is maintained open but switch 69 is closed, then a smaller resistance 85 is available as an alternative path for the energizing circuit of biasing heater 78, more power will be available to the biasing heater, and it will furnish more heat to thermostat 74 so that less heat from the clothes is necessary for tripping of the thermostat. In the same manner, closing of switch 63 provides in effect a short circuit across conductors 81 and 82 through which the energizing circuit for heater 73 will pass. With this arrangement the heater 78 will receive a maximum of power, and the temperature of the clothes which causes tripping of thermostat 74 will be at a minimum.

Referring now to group of parallel connected conductors, it includes three conductors 86, 87 and 88. Conductor 86 includes a relatively high resistance 89, conductor 37 has a lower resistance 9%, and the third conductor 88 has a very small resistance which may be simply its own. In series with the resistances in conductors 87 and 88 are a pair of thermostatically responsive devices 92 and 93 respectively which may, as shown, he simply small bimetallic cantilever switch arms. Thermostats 92 and 93 are positioned, as shown in FIGURE 2, to be responsive to the ambient temperature, that is, to the temperature of the air surrounding the dryer which will be drawn into the dryer by the blower for heating and circulation through drum 2. Thermostats 92 and 93 are calibrated so that they each close at a different ambient temperature; the thermostat 92 closes at the higher temperature, preferably on the order of 90 F., while the other thermostat 93 closes at a substantially lower temperature, which may be on the order of 70.

With this arrangement a relatively high ambient temperature causes a relatively high clothes temperature to be required before thermostat 74 trips, and the clothes temperature required to efiect tripping decreases as the ambient temperature decreases and the small secondary thermostats close. The construction providing the ambient temperature compensation, while described herein to provide complete understanding of a complete and operative control circuit, does not form part of my invention but is the specific invention of Donald W. Lynch and is claimed in his application SN. 825,976 filed concurrently herewith (now Patent 3,009,256 issued November 21, 1961) and assigned to General Electric Company, assignee of the present invention.

For a more complete understanding of the invention, let it now be assumed that a load of ordinary cottons is to be dried, and that the machine is located. in a room where the temperature of the air to be drawn into the machine is 75. With an average type load such as ordinary cottons, the user sets the dial 70 so that approximately two thirds of the time available is provided. At this setting, as can be seen by reference to FIGURE 4, switch 69 is closed by cam 66 and switch 68 is opened by cam 65; also, with a temperature of 75, thermostat 93 is open and thermostat 92 is closed. In addition, since the cycle is just starting, the main thermostat 74 is in its cool or reset position where it engages contact 76 to provide an energizing circuit for the heaters 26 and to prevent the timer motor 63 from operating.

It will further be observed from FIGURE 4 that the operation of manual member 76 also causes closing of switches 57 and 67 and, provided the door is closed so that switch 59 is closed, drive motor 20 will start in operation. When it has come up to speed, the start winding 56 is de-energized and switch 73 is closed by centrifugal mechanism 61. The closing of switch 73 completes the energizing circuit for heaters 26 which thereupon start to heat the air as it is drawn into the drum 2 by the blower 49.

At the same time, the biasing heater 7% is energized in series with resistances and 99. The resistance 99 is that which is selected for connection in the circuit when the ambient temperature is between 70 and so that the provision of resistance 96) in the circuit has a compensating effect on biasing heater 78. The provision of intermediate value resistance 85 in the circuit also has a compensating effect on biasing heater 78.

The condition of the thermostats 92 and 93 is not, of course, affected by the progression of the drying cycle, and if, as is normally true, the ambient temperature remains constant within a few degrees during the drying cycle, resistance 99 Will continue to be the resistance determining the ambient temperature compensation effect on biasing heater 78. 7

As stated, the heater 26 is energized; after a period of such energization, in accordance with the efiect of biasing heater 78 as modified by groups '79 and it the ther mostat 74 will trip in response to a rise in the clothes temperature and will engage contact '75. This de-energizes heater 26 and energizes timer motor 63 which then continues to operate until such time as the thermostatic element 74 resets and engages contact 76. At this point of course the heater 26 is re-energized and the timer motor 63 is de-energized. This causes the temperature to rise again, and when the predetermined point is again reached the thermostat again causes de-energization of the heater and re-energization of the timer. It will be seen that this sequence will continue until such time as the timer has caused the cams to rotate to the off position at which point all circuits within the dryer will be opened, and the sequence will be terminated, With the clothes within the drum being completely dry.

After a predetermined length of timer operation, cam 65 will cause switch 68 to close; this bypassing of resistance 85 will cause increased power to be available to bias heater 78 with the result that the clothes temperature causing tripping of thermostat '74 will be lowered. This is a highly desirable feature since with a clothes load which requires an intermediate temperature, such as ordinary cottons, it is desirable that toward the end of the cycle the temperature be decreased to facilitate handling of the clothes.

It will thus be observed that a drying cycle has been provided which in an economical manner, provides for compensation for the ambient temperature (as claimed in the aforesaid Lynch application) and also by my invention provides a heating effect tailored to the particular type of load being dried While at the same time insuring that the temperature will decrease toward the end of the cycle to facilitate handling of the clothes.

If a relatively heavy load had been provided for drying, such as turkish towels for instance, the manual control 7t} would have been set for a maximum time of operation of timer motor 63. This would have caused both switches 69 and 63 to be open originally (see FIGURE 4) and a high clothes temperature would have been required for tripping the thermostat 74. As the timer motor operated the cams, the temperature would have first been decreased by the closing of switch 69 and then finally, toward the end of the cycle, by the closing of switch 68. If a relatively easily dried load such as delicate synthetics had been placed in the drum 2, the control 70 would have been set so that a relatively short amount of time was provided with switch 68 closed to provide a low temperature trip throughout the relatively brief period of operation.

It will thus be seen that, as an improvement of the basic concept of the aforesaid Lynch application, I provide an economical and elfective system for tailoring the operation of the clothes drying machine 1 to the type of clothes load. My invention, more specifically, resides in the improvement to the aforesaid Lynch application which provides timer controlled modification of the effectiveness of the biasing heater by means of a timer controlled switch in a circuit incorporating at least two parallel conductors, the conductors providing one resistance when the switch is open and a different resistance when the switch is closed.

It will be recognized that, while a particular automatic sequence has been described in which the operation of the heater and the timer are completely alternative to each other, many features of my invention will be of value in other types of drying cycles including those where operation of the heater and timer is concurrent part of the time, and those where the operation of the timer is continuous.

It will further be recognized that the biasing heater may be made to operate continually as shown, or that its operation may be contingent on some other factor such as the operation of the main heater 26. In the present embodiment it is deemed preferable that the operation of the biasing heater be continuous, but it is readily conceivable that in some cases, particularly where the operation of the heaters and timer motor is not completely alternative in nature, it might be desirable to have the biasing heater turned on and off at the same time as the main heaters.

Another point to be noted is that while each group of conductors has been shown to include three conductors including decreasing resistances, with switches in the two conductors having the lesser resistances, as few as two and as many conductors as desired may be provided without departing from the invention. Also, variation in the resistance presented by the conductors may be obtained other than by different resistances with switch means opening the lesser resistance. For instance, if equal resistances are provided in each of two conductors with a switch in one of them, the conductors will present a resistance when the switch is open which is twice as great as that which they present when the switch is closed; thus, exactly equal resistances may even be used to provide difierent resistances depending on the switch position. In addition, the switch means need not necessarily be provided in the conductor having the lesser resistance but may, particularly where the resistances are not exceedingly low as in the case of the almost short circuit provided by conductors 33 and 88 in the present invention, be provided in the conductor having the higher resistance, and a desirable compensating effect will still be obtained in response to movement of the switch between open and closed positions.

While in accordance with the patent statutes I have described what at present is considered to be the preferred embodiment of my invention, it will be obvious to those skilled in the art that various changes and modifications may be made therein without departing from the invention, and it is therefore aimed in the appended claims to cover all such equivalent variations as fall within the true spirit and scope of the invention.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In a clothes drying machine, a clothes container, a main heater positioned to heat clothes in said container, thermostatic means positioned to be responsive to the temperature of clothes in said container, an electric resistance type biasing heater positioned in heating relationship to said thermostatic means, first switch means controlling operation of said main heater, said thermostatic means causing said first switch means to open in response to a predetermined high temperature, at least two parallel connected conductors in series with said biasing heater, at least one resistor and at least one second switch means connected in said conductors, said conductors providing a greater resistance when said second switch means is open and a lesser resistance when said second switch means is closed, manually presettable automatic control means for automatically terminating operation of said main heater after a preselected period of operation of said control means, said control means moving said second switch means to an open position when said control means is set for a relatively long period of operation and to a closed position when said control means is set for a relatively short period of operation, whereby said biasing heater has a smaller effect on said thermostatic means when said relatively long operation is selected and a relatively large effect on said thermostatic means when said relatively short operation is selected.

2 In a clothes drying machine, a clothes container, a mam heater positioned to heat clothes in said container,

ace:

thermostatic means positioned to be responsive to the temperature of clothes in said container, an electric resistance type biasing heater positioned in heating relationship to said thermostatic means, first switch means controlling operation of said main heater, said thermostatic means causing said first switch means to open in response to a predetermined high temperature, at least two parallel connected conductors in series with said biasing heater, each of said conductors having a different resistance included therein, the one of said conductors including the lesser of said resistances further including a second switch means in series with said lesser resistance, manually preettable automatic control means for automatically terminating operation of said main heater after a preselected period of operation of said control means, said control means moving said second switch means to an open position when said control means is set for a relatively long period of operation and to a closed position when said control means is set for a relatively short period of opera tion, whereby said biasing heater has a smaller effect on said thermostatic means when said relatively long operation is selected and a relatively large effect on said thermostatic means when said relatively short operation is selected.

3. The apparatus defined in claim 1 wherein, said control means is arranged so that when said relatively long period of operation is selected, said control means moves said second switch means to closed position when the remaining length of operation of said control means has decreased to that which would be normally manually provided for said relatively short period of operation.

4. In a clothes drying machine, a clothes container, a main heater positioned to heat clothes in said container, first thermostatic means positioned to be responsive to the temperature of clothes in said container, an electric resistance type biasing heater positioned in heating relation to said thermostatic means, first switch means controlling operation of said main heater, said first thermostatic means causing said first switch means to open in response to a predetermined high temperature, two groups each including at least two parallel connected conductors, said groups being connected in series with each other and with said biasing heater, each group including at least one resistor and one second switch means connected in the conductors of that group, the conductors of each said group providing a greater resistance when said second switch means thereof is open and a lesser resistance when it is closed, second thermostatic means responsive to ambient temperature to open said second switch means of one of said groups when the ambient temperature rises to a predetermined level, and manually presettable control means for variably limitiii ing the length of operation of a drying sequence, said control means opening said second switch means of the other or" said groups when a relatively long cycle of operation is selected and closing it when a relatively short cycle of operation is selected, whereby the effect of said biasing heater on said first thermostatic means is variable both in accordance with ambient temperature and with the length of the cycle selected.

5. The apparatus defined in claim 1 wherein said thermostatic means comprises a bimetallic element.

6. The apparatus defined in claim 1 including air circulating means arranged to circulate ambient air over said heater and into said container, said thermostatic means being positioned to sense the temperature of the air after it has contacted the clothes in said container.

7. The apparatus defined in claim 1 wherein said biasing resistance is connected to operate continuously during a drying operation.

8. In a clothes drying machine, a clothes container, 21 main heater positioned to heat the clothes in said con tainer, thermostatic means positioned to be responsive to the temperature of clothes in said container, an electric resistance type biasing heater positioned in heating relationship to said thermostatic means, means controlling operation of said main heater, said thermostatic means causing said controlling means to shut said main heater off in response to a predetermined high temperature, at least two parallel connected conductors in series with said biasing heater, at least one resistor and at least one switch means connected in said conductors, said conductors providing a greater resistance when said switch means is open and a lesser resistance when said switch means is closed, manually presetta ble automatic control means for automatically terminating operation of said main heater after a preselected period of operation of said control means, said control means moving said switch means to an open position when said manually presettable means is set for a relatively long period of operation and to a closed position when said manually presettable means is set for a relatively short period of operation, whereby said biasing heater has a smaller efiect on said thermostatic means when said relatively long operation is selected and a relatively large effect on said thermostatic means when said relatively short operation is selected.

References Cited in the file of this patent UNITED STATES PATENTS 2,790,114 Perkins Apr. 23, 1957 2,851,799 Worst Sept. 16, 1958 2,878,579 Fuchs Mar. 24, 1959

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