How Dryers Work

The Basic Components of an Electric Dryer
Although the location of components and control panel layouts vary by manufacturer and model, the basic operation of dryers are the same. Out of all major appliances, dryers are the easiest to work on. In fact, it is the one appliance most often repaired by homeowners. The basic function of a dryer is to accept wet clothes, tumble them, dry them, and then cool down before the end of the cycle to help keep the clothes from wrinkling.

Air Flow
In most cases a dryer draws in air from the rear through louvers on the back of the dryer near the motor. The purpose of this is so that the incoming air will pass over the motor and help keep it cool. This is why you should never operate a dryer without a vent. When a dryer is operated without a vent, the hot humid air filled with lint dust exits the dryer from the dryer vent, and is immediately sucked back in over the motor. A dryer motor cannot tolerate this type of dirt and will eventually fail. After the air enters the dryer, it passes through the heater and into the dryer drum, through the clothes, then enters the filter duct where it passes through the lint screen ( sometimes called the lint filter, or lint catcher) then exits the dryer via the dryer vent system. Not cleaning a dryer lint screen after every load will not only cause the dryer to run longer and use more energy, it will also decrease the life of the dryer by causing it to run unnecessarily hot. That's because a dryer blower system is designed at the factory to blow through a reasonably clean lint screen. If the screen is constantly restricted, the hot humid air spends too much time in the dryer drum, increasing the time it takes to dry the clothes. This puts unnecessary wear and tear on all of the components within the dryer.

Dryer Heater
The heater on electric dryers produces heat by using a resistance wire to slow down the flow of electrons, thereby producing friction. It is the friction of the electrons passing through the resistance wire that causes the heat. The amount of heat produced by a typical electric dryer is roughly equivalent to the heat produced by a typical bake element in an electric stove. By the way both the bake element in an electric stove, and the heater on a dryer produce heat the same way. On some dryers the electric heater is located in the rear behind the drum, while on other models the heater is located on the base of the dryer. Regardless of the location of the heater, the function is the same. Air is forced through the heater, into the dryer drum to dry the clothes. A dryer heater will show resistance when tested with an ohmmeter. The resistance of a dryer heater will vary depending on the manufacturer and model. A typical dryer heater resistance will range from about 10 ohms, to 30 ohms.

Dryer Motor
Although this is starting to change, most dryers used in the United States today have motors that operate on 120 volts, A/C. On most dryers in the United States, only the heater operates on 220 volts AC. A 120 volt AC dryer motor, has two sets of windings. The two sets of windings are called the start winding and the run winding. The start winding is disengaged after the motor comes up to speed, and is taken out of the circuit electrically. This is usually done by a centrifugal switch located on the motor shaft. The centrifugal switch has a set of weights that when rotated at a certain RPM, disengage a set of contacts on the motor start switch. This disengages the start winding, and energizes a set of contacts to complete the circuit to the dryer heater. A dryer motor is used to rotate the drum, and turn the blower that forces the air through the dryer. On one end of the dryer motor you will find a pulley that is used to rotate the dryer belt. The dryer belt is looped over the drum and is held tight by a dryer idler pulley that has some type of spring tension on it. By looping the dryer belt over the drum and attaching it to the pulley on the motor, dryer manufacturers can take the high RPM of the motor, and reduce that to the drum RPM necessary to tumble clothing. If a dryer drum were to turn too fast, it would not dry the clothes properly because centrifugal force would be forcing them to cling to the drum and not tumble. Clothes need to tumble in a dryer to dry properly. That's why overloading a dryer with too many clothes, produces unsatisfactory results. If the clothes can't tumble they will get wrinkled.

The blower on the other end of the motor, in most cases is attached by a nut molded into the blower, and screwed onto the motor shaft with threads. The threads on the nut molded into the blower are made to tighten up as the motor turns. This prevents the blower from unscrewing itself from the motor shaft. On some dryers however, the blower is attached to the motor shaft that has a flat side, with a clamp.

Timers And Electronic Controls
The amount of time that a dryer operates is controlled by a timer or an electronic control. A timer on an electric dryer is a mechanical device made up of a series of switches controlled by a motor, usually mounted on the outside, attached to gears that rotate cams that open and close switch contacts to operate specific loads on the dryer. For example, a timer will have a set of switch contacts dedicated to closing the circuit to the dryer heater. The timer has another set of contacts dedicated to the dryer motor, and will open and close those contacts based on how often the cam internally pushes the contacts closed completing the circuit to the motor.

Electronic controls are simply sophisticated timers. Just like a timer that is made up of a series of switches, electronic controls open and close relays to control the loads within a dryer. For example, an electronic control will decide how long the dryer motor should operate. This is sometimes based on time, or it could be based on a sensor inside the drum that determines the moisture content of the clothing. Fortunately timers and electronic controls are very well built and rarely fail.

Dryer Thermostats And Sensors
All dryers manufactured today have multiple thermostats or sensors on them. Some thermostats are used to open and close the circuit to the heater, to control the heat within the drum. This thermostat is called the control thermostat. Other thermostats are used as safety devices. The operating range of safety thermostats is much higher than the operating range of the control thermostat. On a normally functioning dryer, the contacts on a safety thermostat are always closed completing the circuit to the heater or the motor. In the event of a control thermostat failure, or a dryer vent restriction, the temperature within the drum can rise to a dangerous level. That's when the dryer safety thermostat opens the circuit to the heater or the motor. In some cases dryer safety thermostats will reset after the temperature within the dryer cools down. Some dryer safety thermostats do not reset when the dryer cools down and must be replaced. Any time you find a dryer open safety thermostat you need to find out what caused that to happen. Was it a vent restriction, or a dryer heater shorted to ground? In any event, whatever the cause a dryer safety thermostat is there to prevent a dryer fire. If it fails the cause must be determined. Another way to control the heat within a dryer, or the amount of time a dryer operates is to use sensors, also known as thermistors. On dryers with electronic controls, thermistors are used to control the heat. A thermistor is a temperature sensitive resistor where the resistance changes with temperature. An electronic control sends out a small electrical current through the thermistor and measures the amount of current that went out compared to the amount of current that comes back. The amount of current that returns to the electronic control is regulated by the temperature of the thermistor. As the temperature increases in the dryer drum, the resistance of the sensor (thermistor) usually goes up.

Other Mechanical Components
The drum within a dryer is supported by rollers, or bearings. When we say bearings here we do not mean the kind of bearing that is on your bicycle wheel. A bearing on a dryer is usually made up of some type of heavy duty felt, nylon or other material that supports the dryer drum. This type of bearing is attached to the front panel above the door and supports the dryer drum. Some manufacturers use rollers to support the weight of the dryer drum.

All dryer manufacturers today use some type of idler pulley to put tension on the drum belt. The idler pulley is usually located next to the dryer motor. The belt is looped through the idler pulley and over the dryer motor pulley. The idler pulley will be made up of a spring type material, or will have a spring attached to it. This spring action keeps tension on the belt. By the way most of the time when a dryer is squeaking, it is not the belt. Dryer belts rarely squeak, they just break when they fail.