Automatic clothes dryers consist of a motor driven revolving basket, an electric heating element or gas flame, thermostats, and a timer. Some models also have a selector switch. Air, heated by the electric element or gas flame, is forced through the tumbling clothes by a fan. The fan is driven by the drive motor. The temperature of the heated air, entering and leaving the basket, is controlled by thermostats which maintain a balance between the air velocity, air volume, and air temperature. The temperature of the exhaust air is a measure of the dryness of the clothes.
After the dryer is started and loaded with damp articles, the temperature inside the basket will rise rapidly. When the temperature reaches approximately 130F, the evaporation of the moisture in the load will absorb the heat as quickly as the heat is generated by the heater. The temperature will not rise appreciably above 130F until the load is near dry.
When the load is near dry, there will not be enough moisture in the clothes to absorb the heat, and the basket and clothing temperatures will rise. This heat rise will continue until the discharge air reaches approximately 160F. At this point the heater circuit is disconnected by a thermostat. The dryer basket will continue to revolve in what is called the overrun period, and cool, fresh air will enter the basket to cool the clothes for handling. The overrun time may last from 3 to 10 min and is controlled by either the timer or an overrun thermostat which will stop the dryer.
Modern dryers of today, Maytag, Kenmore, GE, Whirlpool, Frigidaire, have a moisture sensor which relays moisture conditions inside the dryer to a control board which controls the cycle time and cool-down modes. The moisture sensor consistors of two conductive metallic wires located inside the dryer and mounted to the plastic lint filter housing. One wire has a very low-current AC voltage applied to it and the other one is grounded. When moisture is present a conduct path exists intermittently between the two wires. When the clothes are dry, there is no conduct path. When the dryer is in the automatic dry cycle, the control board will then advance to the cool-down mode.