Bimetallic Thermal Control: Design Factors, Current Derating

Several design factors relate to specifying and using bimetallic thermal control devices and how they are used in systems. The metals and structure of the bimetallic element are of first importance. The selection of those metals impacts the device’s sensitivity and other performance parameters. Derating is especially important for conductive-type devices.

The coefficients of linear thermal expansion (CTE) of the two metals measure how the metals respond to changes in temperature. They are fundamental to bimetallic thermal control device design and performance (Equation 1). For an increase in temperature, the strip will bend towards the metal with the lower CTE; for a decrease in temperature, the strip will bend towards the metal with the higher CTE. In addition, material selection will determine the amount of bending and the sensitivity of the bimetallic strip.

Equation 1. Equation for linear thermal expansion. (Equation: Fictiv)

Some important material properties of the metals used in bimetallic thermal control devices include:

Stiffness and ductility are essential characteristics of the metal strips. Stiffness is a material’s ability to limit deformation, while ductility is a material’s ability to deform plastically. Ductile materials can sustain large deformations before failure. These qualities must be balanced for the specific application to ensure the required performance.

The modulus of elasticity is the ratio of stress to strain for a material undergoing elastic deformation and is related to stiffness and ductility. A lower modulus of elasticity means that the material is more flexible and ductile, while a higher value means that the material is stiffer and more resistant to deformation.

Maximum operating temperature — it is determined by combining the materials used to make the bimetallic thermal control device and the design. For example, some snap action bimetallic disk devices are rated for operation from 0 °F to 300 °F (-17.8 °C to 148 °C). Some slow-make-and-break devices that support close tolerance temperature sensing with a small differential are rated for operation from 0 °F to 650 °F (-17.8 °C to 343 °C). Rod and tube bimetallic thermal control devices offer rapid response times and temperature ratings up to 1750 °F (954 °C).

Electrical resistivity — the metals’ resistivity is especially important in conductive-type bimetallic thermal control devices. In these devices, the application load current flows through the bimetallic strip. The resistivity causes self-heating due to I²R losses, where I is the current and R is the resistivity. The self-heating is very sensitive to increases in current (I²).

The ability to match the sensitivity of the bimetallic element to the needs of the application is an important benefit of using conductive bimetallic controls. A wide range of metals can be used in the bimetallic element, like high conductivity (low resistance) copper and much lower conductivity (high resistance) steels. By changing the combined resistivity of the two metals that make up the bimetallic structure, designers can control the control’s sensitivity to current and self-heating effects. More sensitive conductive bimetallic thermal controls have a higher resistance, meaning they heat up faster for a given current flow.

Current derating

To ensure the accurate and rapid operation of conductive bimetallic thermal control devices, it’s important to apply the correct current derating factors. Current derating enables designers to anticipate how the device will perform under actual operating conditions. Derating is described by a family of curves that show the current versus the reduction in ambient tripping temperature because of the device’s self-heating (Figure 1). Properly used derating can increase the safety margin for applications that can experience unanticipated increases in electrical loads.

The derating curves illustrated above are for a snap-action thermal protection device designed for motor, transformer, and lighting applications that operate from 120, 240, and 277 Vac. The bimetallic element conducts the load current to ensure maximum sensitivity under short-circuit conditions in the load. Six bimetal variations are available from the low-sensitivity model J(B) to the maximum-sensitivity model J(J). The standard temperature differential between opening and closing can vary from 10 °C to 70 °C depending upon the no-load calibration temperature.

Summary

Several material parameters, including stiffness and ductility, modulus of elasticity, maximum operating temperature, and electrical resistivity, are useful for matching the performance of bimetallic thermal control devices to application requirements. In the case of conductive thermal control devices, current derating is an important factor to ensure robust application performance.