How Electric Dryers Work
This page provides a description as well as simple testing methods for each of the parts and components related to an electric dryer. All tests should be preformed with the dryer unplugged and all wires connected to the part being tested unplugged.
(Note: Label all terminals and connecting wires before disconnecting them for proper reinstallation.)
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The dryer's thermal fuse is a non-resettable safety fuse that is designed to break electrical contact to the dryer's heater if the dryer becomes too hot. This fuse will often burn out due to clogged dryer ducting or a defective cycling thermostat. The thermal fuse along with a broken door switch is the most common cause of electric dryers that will not start.
Test: How-To Test A Dryer Fuse Video.
Disconnect all connecting wires and check for continuity across the two wire contacts. There should be no resistance measured (a closed circuit, allowing electricity to flow).
The hi-limit thermostat is a safety switch that will break electrical contact to the dryer's Heating Element if it senses that the dryer has become too hot. The hi-limit thermostat will cycle the dryer's burner or heater off if the ducting has become clogged blocking proper airflow.
Symptoms are usually a very short heating cycle with a low drum temperature, or no heat at all. The hi-limit thermostat is a fairly reliable dryer component and is rarely the cause of dryer heating problems.
Test: Disconnect all connecting wires and check for continuity across the hi-limit thermostat or sensor set.
The dryer's thermal cut-off is similar to the thermal fuse and the high limit thermostat. It will break electrical contact to the dryer's burner or heating element if it senses that the dryer is reaching an unsafe temperature. The thermal cut-off is a set of two thermostats, one of which will not reset once cool. The thermal cut-off needs to be replaced as a set. If this set of dryer parts fails you will need to check your dryer and home ducting for clogging. And check to make sure that the dryer's cycling thermostat is operating properly. This type of safety thermostat does not just fail for no reason. If the non-resetable fuse has blown out, you need to check and clean your ducting!
Test: Disconnect all connecting wires and check for continuity across the terminals. The thermal cut-off should be closed (allowing current to flow) at room temperature.
The cycling thermostat is responsible for cycling the dryer's heat source on and off to keep a target temperature, set at the timer or with a temperature selection switch. The dryer's cycling thermostat is a very reliable bi-metal thermostat that rarely fails. This thermostat is normaly closed, allowing electricity to flow freely. When the dryer heats up it will open, breaking electrical contact to the dryer's heat source and allowing the drum temperature to drop slightly, causing the thermostat to close again and reenergizing the dryer's heat source.
The thermostat's operating temperature range is identified on the part with an "L" followed by the temperature. For example, L130 would open (cycling the dryer's heat off) at 130ºF. A dash followed by another number, for example L155-10, would mean that the dryer thermostat would close after having cooled 10ºF. Faulty cycling thermostat symptoms could take the form of very high dryer temperatures, blown out thermal fuses, or no heat. The cycling thermostat works in connection with a thermostat heater to achieve lower dryer temperatures, such as low and medium heat settings.
Test: Remove all connecting wires and test for continuity. There should be no resistance measured when the dryer is cool (a closed circuit, allowing electricity to flow). Cycling temperatures can be roughly calculated with a meat thermometer held next to the dryer's exhaust duct. As the dryer heat source is cycled on and off, temperatures can be observed.
Dryer Repair Videos
The thermostat heater is often located within the cycling thermostat. However, it may sometimes be a separate component mounted to the dryer's cycling thermostat. Depending on the dryer's temperature setting, more or less voltage is supplied to this heater. Low settings supply more voltage and create more heat, while medium settings supply slightly less voltage, generating less heat. High heat settings will not energize the thermostat heater at all. In this way the thermostat is tricked into thinking that the dryer is hotter than it actually is, so it opens at a lower drum temperature. Problems with this system can occur when the temperature selection switch fails to send the proper amount of voltage to the thermostat heater, or the heater itself fails to heat the thermostat.
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Test: Remove all connecting wires and check for resistance across the heater contacts. Approximately 3200-4000 ohms of resistance should be measured.
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