A bimetallic strip is a composite material made of two or more metals or other materials with compatible properties.
Also known as a "thermal bimetallic strip," it functions based on the differing coefficients of thermal expansion among its constituent layers. When the temperature changes, the "active layer" (with the higher expansion coefficient) deforms more than the "passive layer" (with the lower expansion coefficient), causing the entire strip to bend toward the passive layer side. While traditional bimetallic strips consist of two layers, advancements in bonding technology and expanding application fields have led to the development of three-, four-, and five-layer versions; however, any composite material that changes shape in response to temperature fluctuations is still customarily referred to as a "thermal bimetal."
Applications
Bimetallic strips are widely used in relays, switches, and controllers. The starter in a fluorescent lamp is a prime example. They can also be used to manufacture thermometers capable of measuring high temperatures.
A bimetallic strip is created by mechanically rolling together two metal sheets with different linear expansion coefficients; one end is fixed, while the other remains free. As the temperature rises, the difference in expansion coefficients causes the strip to bend.
Consider a scenario where a thermal element is connected in series with a motor's stator winding; the motor's winding current is thus the same as the current flowing through the thermal element. During normal operation, the heat generated by the thermal element causes the bimetallic strip to bend, but not enough to trigger the relay. Under overload conditions, the heat output increases, causing greater bending displacement in the strip. Eventually, the strip pushes a guide plate which, via a compensating bimetallic strip and a pushrod, opens the relay's normally closed (NC) contact. This contact is wired in series with the contactor coil circuit; opening it de-energizes the contactor, causing the contactor's normally open (NO) power contacts to disconnect the motor (or other load) from the power supply.