Patent Publication Number: US-6700102-B2

Title: Dryer control circuit

Description:
FIELD OF THE INVENTION 
     The present invention relates to an automatic clothes dryer and in particular to an electric control circuit for use during an automatic clothes drying cycle which provides ambient temperature compensation. 
     BACKGROUND OF THE INVENTION 
     Clothes dryers are known to employ an operating thermostat responsive to the exhaust air temperature from the dryer to cycle the dryer heater on and off during the drying cycle to maintain the temperature in the dryer within a set range. Small heating elements may be placed adjacent the thermostat to provide a thermal bias. The provision of a thermal bias causes the thermostat to operate at a lower exhaust air temperature and is commonly used to lower the maximum exhaust air temperature at which the operating thermostat reacts. 
     While the use of thermally biased thermostats is known in the art, these thermostats have been used in automatic dryer applications where 240 volts is applied across the thermostats and dryer heater coils to deliver energy to heat the clothes to temperatures well above ambient. However, in small and mid-size clothes dryer applications where lower voltages of 110 to 120 volts and currents of 15 amps are employed, an automatic dryer operation has not been an energy efficient feature because the thermostats typically cause the dryer heating elements to automatically cycle on and off at too low of an outlet temperature. As a result, the dryers either have to apply heat to the clothing for periods longer than necessary or terminate the drying cycle when the clothes are still wet. This inefficient automatic drying operation problem is further enhanced when the ambient temperature around the dryer is relatively high resulting in a small temperature difference between the ambient temperature and the temperature at which the outlet dryer cycles on and off. As a result, automatic drying cycles have not been offered for all small and mid-sized dryers operating with 120 volt power sources. Instead, these dryers are provided with a time selected drying operation where the dryer operates at about 140° F. for the time specified by the user before entering a cool down period. 
     Accordingly, there is a need for an energy efficient, ambient responsive, automatic dryer control circuitry suitable for use with small and mid-sized clothes dryers capable of operating with 120 volt power supply. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a clothes dryer operating with a 120 volt power supply and having an automatic control circuit that compensates for changes in the ambient temperature to compensate for premature advancement of the dryer timer motor during an automatic drying cycle. The control circuit comprises a low outlet thermostat and a high outlet thermostat which control the energization of a heater in the dryer. The low outlet thermostat and the high outlet thermostat each open circuit at respective lower and higher predetermined temperatures when the outlet air flow from the dryer drum respectively reaches the lower or higher predetermined temperatures to thereby de-energize the heater and advance the timer motor. The control circuit includes an ambient thermostat that switches control of the heater energization and timer motor advancement from the low output thermostat to the high outlet thermostat when the ambient temperature rises above a predetermined ambient temperature. The control circuit provides for more energy efficient drying in an automatic drying cycle for small and mid-size dryers operating with a 120 volt power source. 
     The ambient thermostat preferably has an internal biasing heater or resistor that is energized to permit the ambient thermostat to switch between the low output thermostat and the high output thermostat when the predetermined ambient temperature is reached. Preferably, the internal biasing heater of the ambient thermostat is energized when the low output thermostat is open circuited. 
     In accordance with a further preferred aspect of the present invention, both the low and high output thermostats each have internal biasing heaters that are normally energized during the automatic drying cycle and are de-energized during time selected drying cycles so that the low and high temperature thermostats cycle on and off at higher temperatures than they would during the automatic drying cycle. 
     In accordance with an aspect of the present invention there is provided a control circuit for controlling the operation of a clothes dryer having a dryer drum, a drum air inlet and a drum air outlet permitting an air stream to flow into and out of the dryer drum, a heater for heating the air stream before the air stream passes into the dryer drum, and a control circuit for controlling energization of the heater during an automatic drying cycle. 
     The control circuit comprises: 
     a supply line and a neutral line adapted for connection to a 120 volt power supply source; 
     a series connection across the supply line and the neutral line of a low output temperature thermostat, a high output temperature thermostat and the heater, the low output temperature thermostat opening at a first predetermined temperature and the high output temperature thermostat opening at a second predetermined temperature higher than the first predetermined temperature thereby controlling energization of the heater; 
     a timer motor connected between the supply line and a first point located between the high output temperature thermostat and the heater, the timer motor advancing to end the automatic drying cycle during periods when either one of the low output temperature thermostat and high output temperature thermostat is open; 
     an ambient thermostat for sensing ambient temperature and being electrically connected in parallel with the low outlet thermostat and in series with the supply line and a second point between the low output temperature thermostat and the high output temperature thermostat, the ambient thermostat providing a bypass circuit around the low temperature thermostat when a predetermined ambient temperature is reached; and, 
     wherein, the low output temperature thermostat opens when the temperature of the air stream exiting out of the drum reaches the first predetermined temperature to control cycling on and off of the heater and to control the advancement of the timer motor, and wherein control cycling on and off of the heater and control of advancement of the timer motor in response to the high output temperature thermostat opening at the second predetermined temperature occurs when the first outlet thermostat is bypassed out of circuit by the ambient thermostat. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a better understanding of the nature and objects of the present invention reference may be had to the following detailed description when taken in conjunction with the accompanying electrical diagrammatic drawings wherein: 
     FIG. 1 is a side view of a clothes dryer having the control circuit of the present invention; and, 
     FIG. 2 is a schematic wiring diagram showing circuitry for the control circuitry of the present invention. 
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Referring to FIG. 1 there is shown a clothes dryer  10  having a rotating drum  12  mounted therein. The rotating drum  12  has an open front through which access can be gained through door  14  of the dryer  10  for the insertion and removal of clothing and other articles from the drum  12 . 
     The clothes dryer  10  has a rear panel  16  provided with a series or plurality of louvers  18  through which air may be drawn into the interior of the dryer  10 . The airflow is shown by arrows  20  passing through the louvers, through a series of openings  23  in the rear end head  22  of the dryer drum  12  through front ducting  24  and out through exhaust ducting  26  that defines an exhaust air stream path. Motor  28  rotates blower  30  to draw the air  20  through the drum  12 . The motor  28  through pulley  35  and belt  33  also causes the rotation of the dryer drum  12 . 
     A heater housing assembly  32  is mounted by suitable bolts  34  to the rear panel  16  of the dryer  10 . The heater assembly  32  is adjacent the end head  22  of the drum  12 . The heater assembly  32  has a rear wall  36  that is spaced from the rear end head  22 . The heater assembly has an electrical heating element  50  mounted on insulators  54 . It should be understood that an alternative construction for the dryer drum  12  and heater assembly  32  may comprise a stationary rear end head with a heater box mounted to the rear end head for supplying heated air into the dryer drum. Further, the drum shows an axial air flow therethrough and alternatively the air flow may return through an outlet exhaust in the rear wall of the dryer drum. 
     An ambient temperature thermostat  58  is shown mounted below the heater assembly  32  of the electric dryer. This thermostat  58  is designed to open when the ambient temperature of the air flow into the dryer drum exceeds about 25° C. Second and third temperature thermostats  60 ,  62  are mounted in the dryer outlet air duct to respond to a rise in the temperature of the air exiting the clothes dryer drum  12 . 
     On top of the dryer  10  is a control panel  66  which includes control dials  64 , or touch sensitive key pads, for controlling the operation of the dryer  10 . It should be understood that these dials are utilized to provide for automatic control of the dryer  10  through a warm up cycle, one or more drying cycles and a cool down cycle. These dials can also select timed drying cycles. 
     Referring to FIG. 2 the control circuit of the present invention in relation to an electric dryer is shown. A 120 volt power source is fed to terminal  150  designated as L 1  The neutral terminal is shown at  159 . Terminal  150  is connected to one pole  156  of timer motor control switches  153  and  158 . The terminal  154  of switch  153  is connected to ambient thermostat  58  and a low output thermostat  60 . Terminal  160  of timer control switch  158  is connected to timer motor  108  and to terminal  164  of motor start switch  166 . Low output thermostat  60  is connected in series with high output thermostat  62  which in turn is connected in series with heater  50 . These three devices,  60 ,  62  and  50  are found in series branch  47  connected between the supply line  150  and neutral line  159 . Also connected in the series branch between resistance heater  50  and thermostat  62  is safety thermostat  80 . Safety thermostat is set to open should the dryer drum air inlet temperature exceeds the maximum preset inlet air temperature. 
     Pole  168  of start switch  166  is connected to pole  116  of centrifugal switch  110  which in turn is connected to “run” winding  118  of the blower and drum drive motor  28 . The other end of run winding  118  is connected to terminal  114  which is connected to terminal  126  of door switch  122 . The other terminal  124  of switch  122  is connected to neutral terminal  159 . 
     Pole  164  of start switch  166  is also connected to pole  126  via poles  107  and  111  of belt break switch  109 . The blade  126  of centrifugal switch  110  is shown in its “start” position, i.e. bridging poles  116  and  128 . Pole  128  is connected to “start” winding  130  of motor  28 . The other end of start winding  130  is connected to terminal  114 . 
     The timer motor  108  is connected to switch  170  which toggles between poles  172  and  174 . Switch  170  is ganged with switch  178  at the bottom of the circuit drawing and the functioning of these switches for a timed drying cycle selection is discussed hereinbelow. However, in the positions shown for switches  170  and  178 , the control circuit is in the timed drying cycle selection where the dryer operates for a predetermined amount of time during which the heater  50  is cycled on and off controlled by either of outlet thermostats  60  or  62  and timer motor  108  continuously advances. When the switch  170  is connected to pole  174 , switch  178  is also closed and connected to pole  180 . In this arrangement, the timer motor  108  is in an automatic dry cycle and is connected to series branch  47  at point  191 . The timer motor  108  does not advance as long as the heater coil  50  remains energized from L 1 . 
     As the clothes begin to dry, the temperature of the air exiting the drum begins to increase and outlet thermostats  60  and  62  together with the ambient thermostat  58  control energization of heater coil  50 . In the control circuit, the ambient thermostat  58  is connected from pole  154  between the outlet thermostats  60  and  62  at point  190 . The ambient thermostat  58  has an internal biasing heater (resistor)  59  connected across pole  154  and point  190  between outlet thermostats  60  and  62 . The outlet thermostats  60  and  62  each have respective internal biasing heaters  61 ,  63 . Internal biasing heater  61  is connected from a point  190  between the low output thermostat  60  and the high outlet thermostat  62  to the neutral terminal  159  through switch  178 . Internal biasing heater  63  is connected between point  191  in the series branch  47  to the neutral terminal  159  through switch  178 . 
     During the automatic dry setting, the level of energization (i.e. current flow along series branch  47  and through) heater coil  50  is controlled by the closed or open states of thermostats  58 ,  60  and  62 . The following describes the functioning of the present invention as it relates to an “automatic” cycle for the 120 volt dryer. 
     The preferred aspect of the present invention is in the use of a biasable low drum outlet control thermostat  60  to provide the automatic cycling required for the automatic cycle at low ambient temperature below 25° C. and, a biasable, normally open, ambient air thermostat  58 , to switch from the biasable low drum outlet control thermostat  60  to a biasable regular drum outlet thermostat  62 , when the ambient temperature around the dryer is above 25° C. Both the outlet thermostats  60 ,  62  are positioned to sense the temperature of the air exiting the dryer drum  12 . The ambient air thermostat  58  is positioned to sense the temperature of the air entering the dryer cabinet  10 . 
     When the dryer is operated on an automatic cycle in an ambient temperature of 15 to 25° C., the low outlet thermostat  60  and the high outlet thermostat  62  are closed at the beginning of the cycle. The timer contact or switch  170  is open to pole  172  and is closed to pole  174 . Also switch  178  is closed to pole  180 . As a result, the internal biasing heaters  61  and  63  of both the low outlet temperature thermostat  60  and the high outlet temperature thermostat  62  are energized. The ambient thermostat  58  is open and its internal biasing heater  59  is not energized since the low outlet temperature thermostat  60  is closed and same voltage is applied to both sides of the biasing heater  59 . 
     As the drying cycle advances, the clothes humidity goes down and the temperature of the air exiting the drum  12  goes up. At a certain predetermined temperature, established as the proper calibration of its sensing disc and the proper wattage level of internal biasing heater  61 , the low outlet temperature thermostat  60  opens. When thermostat  60  opens, the dryer heating element  50  and the internal biasing heaters  61  and  63  of both the low outlet temperature thermostat  60  and high outlet temperature thermostat  62  are de-energized. Also, the internal biasing heater of the ambient thermostat  58  is energized. 
     The calibration of the ambient thermostat  58  and its internal biasing heater  59  are calibrated so that, when the ambient temperature is below 25° C., the thermostat  58  remains open. After a period of time, since the dryer heating element  50  is off, the temperature of the air exiting the drum will go down and the low outlet temperature thermostat  60 , with its own internal biasing heater  61  de-energized, closes. The dryer heating element  50  and the internal biasing heaters  61  and  63  of the low outlet temperature thermostat  60  and the high outlet temperature thermostat  62  become re-energized. 
     When the low outlet temperature thermostat  60  is open and the timer switch  170  is closed to pole  174 , the timer motor  108  advances extra space. When the low outlet temperature thermostat  60  closes the timer motor  108  stops. This cycling open and closed of the low outlet temperature thermostat  60  and the corresponding timer  108  continues until the timer  108  advances to the cool down cycle and then to the “off” position. With the proper calibration of the thermostats  58 ,  60 , and  62 , the timer  108  advances to the “off” position when the clothes load reach the proper degree of dryness. 
     With the same “automatic” cycle selection, if the ambient temperature is above 25° C., the opening of the low outlet temperature thermostat  60  would occur sooner and thermostat  60  stays open longer before closing again. To prevent the timer motor  108  from advancing to the “off” position too soon and the dryer stopping when the clothes are still too damp, the present invention further comprises a high outlet temperature thermostat  62  that operates in co-operation with the ambient thermostat  58 . Thus, when the ambient temperature is above 25° C., the automatic drying cycle runs initially as described above, however, when the low outlet temperature thermostat  60  opens, the internal biasing heater  59  of the ambient thermostat  58  becomes energized and after a short while, the ambient thermostat  58  closes before thermostat  60  cycles closed. When the ambient thermostat  58  closes, the dryer heating element  50  and the internal biasing heaters  61  and  62  of both the low outlet temperature thermostat  60  and the high outlet temperature thermostat  62  are energized. This prevents closing of the low outlet thermostat  60 . The dryer heater  50  continues to heat the air entering the dryer drum  12  until the temperature of the air exiting the drum  12  reaches a second higher predetermined temperature that causes the biased high outlet temperature thermostat  62  to open. Once the thermostat  62  opens, the cycling of the heating element  50  and timer motor  108  advancement continues with opening and closing of the thermostat  62  until the end of the drying cycle is reached. The control circuit now cycles at a higher temperature associated with the higher temperature at which thermostat  62  opens, when biased, compared to the lower temperature at which thermostat  60  opens, when biased. With the circuit cycling at a higher temperature, the clothes are properly dried even though the dryer  10  is operating in higher ambient air temperature. 
     It should be understood that when the “automatic” cycle is selected, the dryer automatically stops when the clothes load is dried but, to get the automatic cycle termination, the thermostats  60  and  62  must cycle on-off to get the timer motor  108  to advance. With the 120 volt dryer, the energy input is relatively low and the low outlet temperature thermostat  60  and the high outlet temperature thermostat  62  are set to cycle at relatively low temperatures. If it was not for the automatic termination requirement, there would be no need to cycle the heater on-off at these low temperatures. Therefore, to further improve the drying performance, the present invention further provides another preferred feature that automatically prevents this low temperature cycling of the thermostats  60  and  62  when the “timed” cycle is selected. That is when a user sets the drying time for the dryer to operate. In the “timed” selection, the timer controlled contact or switch  178  is open from pole  180  and, as a result, no current flows through internal biasing heaters  61  and  63 . Accordingly, the low outlet temperature thermostat  60  and the high outlet temperature thermostat  62  are no longer biased and the exhaust air temperature from the dryer drum  12  has to reach a higher temperature before the low outlet temperature thermostat  60  and the high outlet temperature thermostat  62  start cycling.