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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.

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.

References

Bimetal Temperature Switches, BEDIA Motorentechnik
Derating For Real World Conditions, Portage Electric Products
Thermal Switches for Harsh Environments, Control Products, Inc.
Thermostatic Bimetal Designer’s Guide, Engineered Materials Solutions

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Two dissimilar metals are joined together to form a bimetallic element. The important feature of the metals is their different rates of thermal expansion. The element consists of layers of dissimilar metals like brass, copper, or steel, joined together by welding, soldering, or another method. When initially fabricated, the two metal strips are usually the same length.

When the element is exposed to changes in heat, the metals react differently. If the temperature increases, the metal with a higher coefficient of thermal expansion gets longer compared to the metal with a lower coefficient of expansion. That causes the element to bend toward the metal with the lower coefficient.

The reverse happens for a decrease in temperature. The metal with the higher coefficient decreases more in length, and the element bends towards the metal with the higher thermal coefficient of expansion. The bimetallic element can be produced in a linear shape for cantilever devices or as a disk (Figure 1).

Cantilever

In cantilever bimetallic thermal control devices, one or both ends of the bimetallic element are restrained. That enables the control device to extract the maximum energy from the bimetallic element as it bends up and down in response to changes in temperature. In some designs, two bimetal elements are combined for increased sensitivity. Cantilever devices are split into categories based on their physical and electrical design characteristics including creep action versus snap action and conductive versus non-conductive, respectively.

A creep action device uses a single bimetal element that can close or open a circuit based on changes in temperature. The bimetallic element is fabricated to operate over a narrow temperature range between opening and closing.

The bimetal elements in snap action devices operate over a wider temperature range between the point where the switch opens and closes. These devices literally produce a snapping sound as the bimetallic element bends in response to temperature changes.

In conductive controls, the bimetallic element is also a circuit element and carries current. That means that the element’s self-heating from the current flow must be considered when designing and using these devices. When integrating these devices into an application, they must be derated to account for the self-heating effect.

In non-conductive thermal controls, the bimetallic element does not carry current. It provides the force to activate a mechanical switch, sometimes called a shunt. The shunt can get hotter when it’s carrying current, and since the shunt is in contact with the bimetal element, some amount of derating is needed. However, the derating for non-conductive bimetallic control devices is less than that needed for conductive designs.

Bimetallic disks

When the bimetallic element is formed into a hemispherical dome shape, it becomes a disk device. Disk bimetallic thermal control devices are inherently simpler in design than cantilever designs, and as a result, they are widely used.

The size of the bimetal disk varies. Common sizes include ½ and ¼ inches. While cantilever devices are available in conductive and non-conductive designs, disk devices are normally implemented as nonconductive designs and provide some level of electrical isolation between the switching action and the system.

For example, a snap action disk thermal device made using an ½ inch disk is available in a variety of hermetic packages and is rated for switching a maximum of 16A at 125 Vac for resistive loads (Figure 2). The differential range between switching on and off is less than 30 K. These devices are available with a choice of ±3 or ±5 °C operating temperature tolerances and have a dielectric withstand rating of 1,000 Vac for 1 minute or 1,800 Vac for 1 second.

Both cantilever and disk bimetallic thermal control devices are highly reliable. Cantilever devices are typically rated for 100,000 switching cycles at rated load and over a million switching cycles when switching a small control current. The 16A disk device pictured above Is rated for 100,000 switching cycles at 16A.

Summary

Cantilever and disk bimetallic thermal control devices are both based on harnessing the different thermal coefficients of expansion of two different metals like brass, copper, or steel. The strips of the two metals are mechanically connected using soldering, welding or another process and can be used to activate highly reliable switching actions based on increases or decreases in temperature.

References

A Complete Guide to the Working, Types, and Applications of Thermal Switches, Langir
Characteristics of Temperature Power Sensor and Disk Type Thermostat, Matsuo
The Bimetallic Strip Explained, Fictiv
The How-to Guide for Thermal Controls, Portage Electric Products

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