Large industrial temperature control systems, assembly lines, energy and other complex manufacturing systems all rely on motors for operation. If that motor overheats, it can be damaged or even destroyed. To prevent this from happening, either an electromechanical relay (EMR) or a solid-state relay (SSR) generally is used in these systems to sense whether the motor is overheating, and if so, turn off the motor in time to prevent damage.

 Electromechanical relays have been the standard solution for managing load circuits for more than 150 years. However, in the last decade or so, solid-state relays have taken a great deal of the marketshare.

One of the most significant differences between electromechanical relays and solid-state relays is lifespan. Electromechanical relays are mechanical based and have moving parts, making them highly susceptible to magnetic noise, vibration, shock and other outside influences that can affect wear and lifecycle.

By contrast, solid-state relays offer a durable, all-solid-state electronic construction with no moving parts to affect wear or accuracy (figure 1). They offer predictable operation and longer life. The average lifespan of electromechanical relays is in the range of hundreds of thousands of cycles. Three-phase solid-state relays offer a lifespan of 5 million hours. With such maintenance-free durability, solid-state relays often outlast the equipment in which they are installed.

In addition to a longer lifespan that provides greater reliability and replacement cost savings, solid-state relays provide faster switching than electromechanical relays. This makes them adaptable to a wider range of high power load applications. They operate silently (without the clicking sound emitted by electromechanical relays) with low input power consumption and produce little electrical interference. Shock- and vibration-resistant, solid-state relays can withstand harsh environments and continue to operate accurately and reliably.

Solid-state relays offer advantages over electromechanical relays in other areas as well. They are compatible with control systems, immune to magnetic noise and are encapsulated to protect components. Their solid-state design makes them position insensitive, which provides design engineers more flexibility to mount them anywhere within an application — whether sideways or upside down. Solid-state relays can be installed in places where there is heavy vibration without interference in performance. Mechanical-based electromechanical relays are sensitive to positioning, shock and vibration, thereby restricting design options.

One disadvantage is that solid-state relays are more expensive than electromechanical relays. In many applications, that difference in cost is mitigated by the 5 million hours of life that solid-state relays provide.

solid-state relays

FIGURE 1. Solid-state relays offer a number of advantages over electromechanical relays for process heating applications.

Thermal Management Challenge

As solid-state relays generate heat when conducting a current, there is a thermal management component to their operation. Should overheating occur and damage a solid-state relay, diagnosing and replacing the solid-state relay can take time while the assembly line or manufacturing system is down and out of service, running up even more costs.

To illustrate how a solid-state relay operates, consider its use in a temperature control application in environmental chambers. In this example, the solid-state relay’s function is to turn a compressor on or off to keep the system temperature within a specified range. The input control might be from 90 to 280 AC with a required internal tripping temperature set at 203°F (95°C) within the solid-state relay. A buffer is engineered into the circuitry using a variety of components to ensure that the desired tripping action occurs.

When the solid-state relay turns on to conduct load current, internal heat is generated. Failure to adequately protect the solid-state relay can cause damage to the relay or to the load.

solid state relays

FIGURE 2. Solid-state relays incorporate thermal protection via an embedded thermostat, preventing overheating conditions.

Solid-State Relays Engineered to Avoid Overheating

To tackle the overheating challenge, modern solid-state relay technology was developed that integrates a thermostat inside the solid-state relay. This helps ensure that the relay always operates in a safe or protected mode. Retaining the inherent advantages of standard solid-state relay technology, the new design is differentiated by its ability to prevent the solid-state relay from overheating; thus, it protects component and system operation from potential damage or shutdown.

The solid-state relay cuts off input circuit power when the temperature goes beyond the specified maximum as determined by the application requirements. Power is automatically turned on again when the temperature has cooled down to within the normal operating range.

solid-state relays

Solid-state relays offer a long operational life of more than 2 million cycles and can withstand the harsh environments found in many industrial systems.

This automatic thermal protection is accomplished by means of an integrated thermostat embedded in the solid-state relay. The thermostat senses the internal temperature of a mechanical interface with a metal plate where the internal power-switching device is mounted. If the heat exceeds the normal range, it sends a signal to the solid-state relay to turn off the power. This built-in thermal protection completely prevents overheating conditions by providing a trip before equipment damage can occur (figure 2).

The integrated thermal protection function also can help identify design issues in the system such as:

  • Incorrect heat-sinking capacity in the solid-state relay or system.
  • Poor installation resulting in insufficient heat-sinking contact.
  • Heat dissipation efficiency of the system.

This provides a troubleshooting tool to the engineer responsible for the process system’s operation.

The solid-state relay designs can be adapted to many temperature control applications such as industrial ovens, sterilization equipment, molding and extrusion machinery, and welding equipment. Other uses include heavy-duty conveyor systems such as those in construction, mining, packaging and material handling. For example, consider a conveyor belt application where a motor could stick and cause overload and potential damage to the system. In this case, the solid-state relay with integrated thermal protection would prevent overheating from occurring by shutting down the conveyor belt as soon as a predetermined heat threshold was met within the solid-state relay’s thermostat.

In injection-molding applications, where limited space can cause the temperature in the cabinet to rise, thermal protection prevents the solid-state relay from overheating if the heat sinking is not adequate, thus avoiding costly repairs. For heating systems, the thermally protected solid-state relay can help shut down the heating element if there is a problem with the temperature controller that causes a temperature runaway, thereby protecting the entire system.

 In conclusion, engineers are now working to develop thermally protected solid-state relay technology with decision-making capability inside the solid-state relay package. This technology incorporates a microcontroller with firmware specific to the desired internal trip temperature that activates a decision from the preprogrammed software settings. This capability will make motor and system protection from over-heating and breakdown more automated than ever.

Since the 1970s, traditional solid-state relays have offered a proven electronic switching solution for a variety of applications. They provide a longer lasting, more versatile, flexible and robust solution than electromechanical relays.

Solid-state relays with integrated thermostats can prevent overheating and identify heat sinking issues to help improve system safety, efficiency and longevity. This product evolution may change thermal protection for a range of applications.