Solid-State Relays vs. Electromechanical Relays Choosing the Right Solution

Solid-State Relays vs. Electromechanical Relays

Choosing the Right Solution for Smart Building Automation

Relays are devices that switch electrical loads and are considered the heart of smart building automation. There are two types of relays: solid-state relays (SSRs) and electromechanical relays (EMRs). You must choose the best relay suitable for your application, as the choice impacts smart building automation systems' performance, reliability, and longevity. This article can guide you in selecting the right solution for advanced building automation applications. It examines the key differences, strengths, and limitations of SSRs and EMRs.

Figure 1: Structure and operating principle of SSRs and EMRs (image source)

Key differences and a Comparative Analysis Between SSRs and EMRs:

Both relay types have distinctive advantages and trade-offs. An informed choice is essential for optimizing smart building operations

Features	Solid-State Relays (SSRs)	Electromechanical Relays (EMRs)
Operating mechanism	Operate using semiconductor devices like triacs, thyristors, or MOSFETs to perform switching without physical contact. This ensures silent operation and eliminates the degradation typically associated with mechanical components.	Use an electromagnetic coil to move mechanical contacts for switching. While simple and practical, this mechanism results in wear over time due to physical movement and arcing.
Switching speed	Deliver fast and precise switching, making them ideal for applications requiring rapid cycling or real-time adjustments.	Have slower response times due to the mechanical movement of contacts, which can be a limitation in dynamic systems.
Durability	Provide a longer operational lifespan since they lack moving parts. They are particularly reliable in environments where frequent switching is required.	Prone to mechanical degradation, limiting their lifespan in high-usage scenarios.
Load compatibility	Handle non-linear and low-current loads efficiently, such as LED drivers and low-power control circuits. They excel in smart building applications where load variability is common.	Better suited for high-current and high-voltage resistive loads, such as heating elements or industrial-grade motors.
Power consumption	Consume less control power, aligning with the energy-saving goals of smart buildings.	Require more power to energize their coils, leading to higher operational energy costs.
Noise and interference	Operate silently, an advantage in settings prioritizing occupant comfort. However, they can generate electrical noise that requires mitigation in sensitive circuits.	Produce audible clicks during operation, which may be unsuitable for noise-sensitive environments such as offices or healthcare facilities.
Heat generation	May require heat sinks at high currents.	Generate less heat but experience more mechanical degradation.

Strengths and limitations of SSRs and EMRs for Smart Building Applications:

Understanding the strengths and limitations of SSRs and EMRs is essential for their optimal use. Each relay type has unique benefits and, unfortunately, a few trade-offs.

Type	Strength	Limitations
SSRs	Silent operation, fast switching, long lifespan, low maintenance, and compatibility with modern smart control systems.	Higher initial costs, heat dissipation challenges, and potential for electrical noise in certain setups.
EMRs	Affordable for high-power applications, robust for straightforward automation tasks, and effective in systems with low switching frequency.	Noisy operation, shorter lifespan due to mechanical wear, and higher power consumption.

Applications in Smart Building Automation:

Solid State Relays (SSRs) and Electromechanical Relays (EMRs) integrate advanced technologies making them crucial to the Smart building automation ecosystem. SSRs and EMRs higher operational efficiency, comfort, and security, with each providing unique advantages suited to various applications.

Solid State Relays (SSRs)

HVAC systems control - SSRs are widely utilized in heating, ventilation, and air conditioning (HVAC) systems due to their ability to control compressors, fans, and auxiliary components with high precision. For example, in variable air volume (VAV) systems, SSRs provide precise and silent control of heating or cooling units. In data centers, SSRs manage the temperature by controlling the compressors and fans to maintain optimal conditions for sensitive equipment. Products like the celduc® SO9 and SUL9 series SSRs are specifically designed for such applications. SSRs also facilitate advanced control methods such as Pulse Width Modulation (PWM), optimizing energy use without sacrificing performance.
Lighting automation - Modern lighting systems, such as LED arrays and dimming controls, benefit significantly from SSRs because of their fast and noiseless operation. For example, the Crouzet GNRD-0 series SSR is utilized in commercial buildings to control fan motors for air circulation during lighting operations. SSRs can rapidly adjust lighting levels in response to sensor data or time-based schedules. Zero-crossing SSRs are particularly effective as they can reduce electromagnetic interference and ensure smooth operation, critical for maintaining occupant comfort.
Security systems - SSRs enhance security systems through automated door controls, intrusion alarms, and fire safety systems. Their compact design allows easy integration into existing infrastructures, providing reliable performance in critical safety applications.
Predictive maintenance - The integration of SSRs with IoT sensors enables real-time monitoring of electrical loads and system diagnostics. This capability is vital for facility managers as it helps to formulate predictive maintenance strategies, before they lead to system failures. Predictive maintenance reduces downtime and maintenance costs.
Energy Management Systems - Energy storage systems, load balancing, and renewable energy integration often require precise control of switching devices. The ability of SSRs to operate without mechanical wear increases reliability and longevity, and consequently contributes to energy efficiency in smart buildings. SSRs enable precise control over power loads. This ability makes them ideal for use in applications that require frequent switching. For example, Omega's SSRL series is used to manage large resistance heaters in conjunction with temperature controllers, optimizing energy usage based on real-time demand.

Electromechanical Relays (EMRs)

High-Power Applications—EMRs are often used when high power handling is necessary, such as controlling large motors, compressors, or heavy lighting loads that switch less frequency. Their robust construction allows them to manage substantial electrical currents effectively.
Simple and affordable - For typical applications where cost is a primary concern, EMRs provide a reliable solution. Their simpler design can be advantageous in less complex systems where high-speed switching is not critical. EMRs can still serve well, particularly in backup power systems and grid interface applications.

Selecting the right relay for your Smart Building:

When choosing between SSRs and EMRs, consider the following factors-

Load type - SSRs handle resistive and inductive loads effectively in high-frequency scenarios, whereas EMRs work better for complex, variable loads.
Switching frequency - Frequent switching requires SSRs to prevent mechanical degradation caused by the moving parts in EMRs.
Durability and maintenance - The solid-state nature of SSRs offers a longer lifespan and lower maintenance compared to EMRs, which require periodic replacement.
Cost - EMRs are usually more affordable in the beginning but may incur higher long-term costs due to maintenance and replacements.
Environmental Conditions - For noisy, high-vibration environments, SSRs are more reliable since they lack mechanical components.
Energy Efficiency - SSRs consume less power during operation, making them more suitable for energy-conscious smart building designs.

Case Study: Solid-State Relays in Building HVAC applications

Modern HVAC systems play a crucial role in maintaining thermal comfort and indoor air quality across commercial buildings. A prime example is the use of Air Handler Units (AHUs), which filter, heat, cool, dehumidify, and distribute air throughout large spaces. Historically, such systems relied on electromechanical relays (EMRs) or even mercury displacement relays, but advancements have led to the adoption of Solid-State Relays (SSRs), like those provided by Sensata Technologies.

SSRs offer numerous advantages for HVAC systems, including extended lifespan, silent operation, low power consumption, and rapid switching capabilities. Their suitability for handling diverse loads, such as fans, blowers, motors, and electric heating elements, makes them invaluable in applications demanding constant and precise operation. For example, proportional control methods like phase-angle or burst-fire control, achievable only with SSRs, ensure accurate temperature regulation in heating systems.

Sensata's product line, including the DR67 and PM67 series SSRs, exemplifies this evolution. These DIN rail and panel-mounted SSRs, equipped with built-in overvoltage protection and versatile input configurations, seamlessly handle fan/motor control and heating applications in AHUs. By replacing older technologies, these SSRs not only enhance reliability and efficiency but also contribute to quieter and more sustainable HVAC solutions.

Figure 2: Air handler unit controlled by several SSRs (image source)

Used DR 67 series SSRs by Crydom for Fan/motor start/stop heating control.
Used PM 67 series SSRs by Crydom for Fan/motor start/stop heating Control.
Used PMP series SSRs by Crydomfor heating control.
Used DR 22 series SSRs by Crydom for heating control.

Conclusion

SSRs and EMRs play different roles in smart building automation. SSRs offer precision, durability, and silent operation, making them perfect for advanced IoT systems. In contrast, EMRs are versatile and cost-effective for handling high-current and inductive loads. Selecting the right relay involves balancing speed, load type, and cost considerations. Evaluating project requirements, designers can harness the strengths of each relay type to create efficient, reliable, and future-ready smart building solutions that elevate automation capabilities.