HVAC School - For Techs, By Techs
Bi-metal & Magnetic Switches - Short #252
· Summarized
Key takeaways
- Bimetal switches are made of two different types of metals that bend with temperature changes, often found in older heating systems.
- Magnetic switches are faster and typically external to protected equipment, making them essential for critical applications.
- Bimetal switches have inherent time delays and can nuisance trip, while magnetic switches respond immediately to overload conditions.
- Understanding the interaction between ambient temperatures and switch types is crucial for ensuring the proper function and reliability of systems.
- Both switch types can be used together in applications to leverage their respective advantages.
Summary
Introduction to Bimetal and Magnetic Switches
The podcast episode introduces listeners to bimetal and magnetic switches, emphasizing the differences and applications of each type. Bimetal switches are made of two dissimilar metals bonded together, which bend with temperature changes. They serve various functions, including overload protection and control in older systems like thermostats and fan limit switches. Magnetic switches, on the other hand, are generally external to the equipment they protect and react more quickly, making them suitable for mission-critical applications.
How Bimetal Switches Function
Bimetal switches operate by bending and opening circuits in response to temperature changes, making them effective for overload conditions. This section delves into their historical use in devices like mercury thermostats and explains how they function in modern applications. Bimetallic strips and coils provide a built-in delay, which can be advantageous in certain situations, such as preventing cold air from being blown through heating systems.
Comparative Analysis of Bimetal and Magnetic Switches
The podcast contrasts bimetal switches with magnetic switches in terms of speed, operational behavior, and design. Bimetal devices tend to auto-reset, can nuisance trip, and are sensitive to ambient temperature, which might lead to premature tripping under certain conditions. Magnetic switches, while more complex, are less likely to fail and respond more rapidly to overcurrent conditions, providing better protection for critical machinery.
Application and Failure Modes
This section discusses the typical applications where bimetal and magnetic switches are used, highlighting the importance of understanding the nuances between internal and external overloads in motors. It also covers the failure modes of both types of switches, with bimetal switches often failing closed and magnetic switches failing open, impacting service reliability and maintenance protocols.
Conclusion and Learning Opportunity
The podcast wraps up encouraging continuous learning and discussion about these technologies, emphasizing the importance for technicians to understand the basic principles of bimetallic and magnetic switches. Listeners are prompted to engage further by discussing insights with apprentices or diving deeper themselves.
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Show notes
In this short podcast episode, Bryan covers some basics on bi-metal and magnetic switches.
Bi-metal switches are made of two dissimilar metals that are bonded together and are often integrated into motors. When the temperature changes, the metals bend. In an overload circuit, they're designed to bend and open the circuit in an overheat condition. Mercury thermostats also use these; they have a bulb attached to a bi-metallic coil, which causes it to wind or unwind when heated or cooled. Fan limit switches also use bi-metallic coils. Bi-metallic discs also exist, and they snap open to make or break a circuit.
Bi-metallic switches have a time delay and often auto-reset (with exceptions like rollout switches on furnaces), which is advantageous in some applications. However, they're also affected by the ambient temperature and are more likely to trip in warm weather and less likely to trip in cold weather. Nuisance trips are more common than in magnetic switches, and they may weld themselves shut and fail closed (though failing open is relatively common as well).
Magnetic switches are usually more external to the equipment or parts they're protecting (such as in the contactor or starter assembly instead of inside a motor itself). These switches are also more instantaneous and are better for mission-critical applications. These also respond to amperage and aren't affected by ambient temperature. They're more likely to fail open than closed.
Compared to bi-metallic technology, magnetic switches trip faster and are better for sudden issues rather than long-term operational overheating. In many cases, we use both of them to get the benefits of each.
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