Cantilever Bimetallic Thermostats
How They Work:
These devices utilize a bimetallic element to make or break a circuit. The heart of cantilever bimetallic thermostat devices lies in the bimetallic element. This element is made from specially bonded materials that react differently under the influence of a thermal load. The principle behind how bimetals work makes use of the fact that different metals will expand or contract at different rates when heated or cooled. This is commonly referred to as the coefficient of expansion. The materials which are used to produce bimetal react differently when heat is induced into the bimetal element either through exposure to heat generated by an application or caused by the self-heating effect created by passing current through the bimetallic element, or a combination of both. This permits the bimetallic element to "bend" creating the work force required to make or break an electrical circuit.
The technical name for these thermal controls is to call them a cantilever type in order to differentiate them from disc-type bimetal controls. In cantilever bimetallic thermostats, one or both ends of the bimetallic element are restrained to take maximum advantage of the work force created by the bending effect of the bimetal that is created by increases or decreases in the ambient temperature of an application. In most cases, cantilever bimetal thermal controls are divided into two sub-categories, either conductive type controls, or non-conductive type controls or also referred to as a shunted type.
In conductive type controls, the bimetallic element carries the circuit current so the action of the bimetallic element is not only influenced by responding to any changes in temperature but also by the self heating effect caused by the electrical load passing through the bimetal. This dual action provides for circuit breaker characteristics that can enable the device to function on increases in current as well as in temperature. This self heating of the bimetallic element is commonly referred to as "derating." The derating effect can best be described as the difference between the operating temperature of a control under a no-load condition to that when an electrical load is applied. The derating effect increases with increases in the electrical load.
In non-conductive type controls, the bimetallic element does not carry the circuit current of the application, but provides work force against another internal component that does. The component that carries the electrical load is commonly referred to as a "shunt." Although the bimetallic element does not carry the circuit current, there is some derating caused by the electrical load passing through the shunt element.