Motors are the common denominator between large industrial/commercial HVAC and refrigeration systems, heavy-duty transportation, conveyor belts, assembly lines, medical, energy, and other sophisticated manufacturing systems.
Electrical motors are large and costly, and if they overheat, they can incur damage or even be destroyed. Most large-motor-driven machinery demands a system linked to the motor’s power supply to notice overheating and switch off the motor to guard against such damage. This device is often an electrical relay that switches power on and off.
These relays occur in two main varieties: Electromechanical relays (EMRs) and solid-state relays (SSRs).
Image Credit: Sensata Technologies, Inc.
EMRs vs. SSRs
For more than 150 years, EMRs have been the go-to solution for managing load circuits. In the last 30 years, however, SSRs have taken a significant portion of the market share. Critical differences exist between EMRs and SSRs, particularly regarding life span.
EMRs are mechanical and have moving parts, making them susceptible to magnetic noise, vibration, shock, and additional external influences that can impact their wear and life cycle.
SSRs provide a robust, all-solid-state electronic construction with no moving parts that may affect wear or accuracy. They provide predictable operation and an enhanced lifespan.
The average life of EMRs is in the range of hundreds of thousands of cycles, compared to five million hours for three-phase SSRs. Due to their maintenance-free durability, SSRs frequently outlive the components in which they are installed.
In addition to a longer life span that enables more reliability and replacement cost savings, SSRs offer more rapid switching than EMRs, making it possible to calibrate them according to more high-power load applications. They work without making any noise (like the unpleasant clicking sound emitted by EMRs) with low input power consumption. They also transmit little electrical interference.
Shock- and vibration-resistant SSRs can withstand harsh conditions and continue to function accurately and reliably, while EMRs require frequent replacement, making them extremely undesirable in harsh environments.
SSRs also excel over EMRs in other domains. They work alongside control systems, are resistant to magnetic noise, and are covered to protect crucial components. Their solid-state design means they are position-insensitive. It also gives design engineers greater flexibility to mount SSRs anywhere inside an application, whether sideways or upside down.
It is possible to set up SSRs in areas with significant vibration without performance being affected, while mechanical-based EMRs are extremely sensitive to positioning, shock and vibration, limiting design possibilities.
Considering SSRs' benefits, it is understandable that they are more costly than EMRs. The price difference becomes insignificant when considering the over five million hours of life SSRs offer.
The Thermal Management Challenge
SSRs emit heat when conducting current, so they have a thermal management component, like the motors they control. Should overheating occur, diagnosing and replacing a damaged SSR may require time while the assembly line or manufacturing system is not running, leading to additional expenses.
To understand an SSR's operations, consider how the product is used in commercial refrigeration applications in the building equipment market. In a refrigeration application, the SSR turns the compressor on or off to maintain the system temperature within a pre-selected range.
The input control may be from 90-280 AC, with a necessary tripping temperature calibrated to 95 °C. Multiple components build a buffer into the circuitry to ensure the desired tripping action occurs.
Figure 2. Solid state relays like Sensata’s 53TP Series offer a long operational life of 5 million hours and can withstand harsh environments in many industrial systems. Image Credit: Sensata Technologies, Inc.
Each time the SSR switches on to conduct load current, it generates internal heat. Failure to properly protect the SSR may damage the relay or the load.
Next-Generation SSRs
To resolve overheating, Sensata built an innovative and unique SSR technology that includes a thermostat to ensure the relay always functions in a safe or protected mode. This game-changing design integrates the benefits of standard SSR technology and protects the SSR from overheating, defending components and system operation from potential damage or shutdown.
The new SSR cuts off input circuit power when the internal temperature exceeds the maximum as specified by the application criteria. Power is spontaneously switched on again when the temperature has reduced to within the normal operating range.
This spontaneous thermal protection is achieved via an integrated thermostat, which identifies the mechanical interface’s internal temperature using a metal plate where the internal power-switching device is mounted.
If the heat is above the normal range, it transmits a signal to the SSR to switch off the power. This built-in thermal protection completely guards against overheating conditions by offering a trip before equipment may be harmed, saving time and money.
The integrated thermal protection function prevents overheating and can troubleshoot system design issues. It can help distinguish incorrect heat sinking capacity within the SSR or system, poor installation stemming from inadequate heat sinking contact, and heat dissipation efficiency of the system, among other issues. This makes it a valuable instrument for the engineer in charge of system operation.
Figure 3. Next-generation SSRs incorporate thermal protection via an embedded thermostat within the SSR, preventing overheating conditions. Image Credit: Sensata Technologies, Inc.
Although built for commercial refrigeration applications, this novel SSR design may be used in other industrial and manufacturing applications for similar system benefits.
A conveyor belt application where a motor sticks, for example, could lead to overload alongside potential system damage. With integrated thermal protection, the SSR would disallow overheating by closing off the conveyor belt when a predetermined heat threshold is reached within its thermostat.
Injection molding use cases with limited space may raise a cabinet’s temperature. Thermal protection guards the SSR from overheating if the heat sinking is insufficient, avoiding expensive repairs.
For heating systems, the thermally protected SSR may help shut down the heating component if a problem with the temperature controller causes a temperature runaway, safeguarding the whole system.
Thermally Protected “Smart” SSRs with Decision-Making Capabilities
Sensata engineers aim to develop thermally protected SSR technology with even more in-built functionality, redefining the term “smart SSRs.” This technology integrates a microcontroller with firmware specific to the desired internal trip temperature, which triggers a decision from the pre-programmed software settings.
With a decision-making capability within the SSR package, safeguarding motors and systems from overheating and breakdown will be more automated than ever.
Since the 1970s, traditional SSRs have provided a proven electronic switching solution for multiple use cases. They offer a longer-lasting, more versatile, flexible, and dynamic solution than EMRs.
For the first time, new SSRs equipped with integrated thermostats can prevent overheating and detect heat-sinking issues to enhance system safety, efficiency, and longevity. This product evolution will indubitably improve thermal protection for numerous applications.
This information has been sourced, reviewed, and adapted from materials provided by Sensata Technologies, Inc.
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