8.16 Bi-metal thermostats 8.16.1 General characteristics This type of thermostat, which exists in various versions with one or two contacts (normally closed or normally open) is utilized for a number of applications: ѓ control of the temperature of the washing water ѓ control of the temperature of the drying air ѓ as a safety device for overheating during washing and/or drying. 8.16.2 Operating principle When the bi-metal disk reaches the temperature at which the thermostat has been calibrated, it trips, causing a lever to open (or close, depending on the version) the contacts. During the cooling phase, the bi-metal disk returns to its original position at a pre-determined reset temperature.
1. Steel capsule 2. Bi-metal disk 3. Contact 4. Spring 5. High-temperature lever 6. Low-temperature lever 8.16.3 Electrical symbols Normally-closed Normally-open Manual reset 8.16.4 Manual-reset thermostat In certain applications (such as washer-dryers), manual-reset safety thermostats are fitted. In this case, the bi-metal element, after tripping as a result of overheating, does not reset automatically to its original position when it cools. It is necessary to press the corresponding button manually to reset the bi-metal element. 8.16.5 Checking for efficiency 1. Use a tester to check that, when cold, the contacts are in the correct position. 2.
Heat the thermostat bulb and check for correct switching of the contacts. 3. Allow the thermostat to cool and check that the contacts return to their original position. SOI/DT 2002-05 eb 87 599 35 23-17