How to Prevent Solid State Relay Failure in Power Supplies for Industrial Systems

Rogelio Castaneda and Oscar Rivera, Sensata Technologies


New, emerging solid-state relay designs for power supplies offer integrated thermal protection that can prevent overheating of industrial machinery and improve system safety, efficiency and longevity.

Figure 1. Advantages of solid-state switching technology

What do large industrial/commercial HVAC and refrigeration systems, heavy duty transportation, conveyor belts, assembly lines, medical, energy and other complex manufacturing systems all have in common? Electrical motors. Big and expensive electrical motors that, if their power supply system overheats, can be severely damaged.

Most large motor driven machinery require a system attached to the motor’s power supply that can sense overheating and turn off the power to the motor in order to prevent this damage. In many cases, this power device is an electrical relay that turns the electrical power on and off. There are two main types of these relays – electro-mechanical (EMRs) and solid state (SSRs).

SSRS versus EMRS – What are the differences?

Electro-mechanical relays (EMRs) have been the standard solution for managing load circuits for over 150 years. However, in the last decade or so, solid state relays (SSRs) have captured an ever increasing share of the power supply control market.

One of the most significant differences between EMRs and SSRs is device life-span. Because EMRs are mechanical based and have moving parts, they are very susceptible to vibration, shock, magnetic noise, and other outside influences that can affect wear and life cycle. In contrast, because of their durable, all-solid state electronic construction with no moving parts to affect wear or accuracy, SSRs thereby offer more predictable operation and longer life.  The average lifespan of electro-mechanical relays is in the range of hundreds of thousands of cycles compared to 5 million hours for three-phasesolid state relays. With such maintenance-free durability, SSRs often outlast the equipment in which they are installed!

In addition to a longer life span that provides greater reliability and a reduction in potential replacement costs, SSRs can  switch on and off faster than EMRs, making them adaptable to a wider range of high power load applications.  They operate silently (without the undesirable loud clicking sound emitted by EMRs), consume less input power, and produce little electrical interference. Because they are both shock and vibration resistant, SSRs can withstand harsh environments and can continue to operate accurately and reliably. In contrast, EMRs often require frequent replacement, especially in harsh environmental conditions.

SSRs excel over EMRs in other areas as well.

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Figure 2. Solid-state relays like Sensata’s 53TP Series offer long operational life of over 5 million hours and can withstand harsh environments found in many industrial systems.

They are compatible with control systems,immune to magnetic noise, and are encapsulated to protect components.  Their solid-state design makes them relatively insensitive to positioning, thereby providing design engineers with much more flexibility to mount SSRs anywhere within an application– whether sideways or upside down.  SSRs can be installed in places where there is heavy vibration with no resulting interference or reduced performance, whereas mechanical-based EMRs are very sensitive to positioning, shock and vibration, thereby restricting design options.

Though SSRs are significantly more expensive than EMRs, the price difference becomes a non-issue when one factors in the over 5 million hours of life that SSRs provide.

How do SSRS help solve the thermal management challenges found in power supplies?

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

To illustrate how an SSR operates, consider its use in a temperature control application in environmental chambers.

Within this application, the SSR’s function is to turn a compressor on or off in order to maintain the system temperature within a specified range. The input control might be from 90-280 AC with a required internal tripping temperature set at 95°C within the SSR. Using a variety of components, a buffer is engineered into the circuitry to ensure that the desired tripping action occurs at the right temperature.

When the SSR 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 power supply.

A Thermostat inside the Solid-State Relay helps solve the thermal management challenge

To tackle this overheating challenge, Sensata developed a game-changing SSR technology that ensures that the power supply relay always operates in a safe or protected mode. Retaining the inherent advantages of standard SSR technology, the new design is differentiated by its ability to prevent the SSR itself from overheating, thus protecting component and system operation from potential damage or shut down.

The new SSR is able to cut off input circuit power when the temperature goes beyond the specified maximum as determined by the application requirements.  Power is then automatically turned back on when the temperature has cooled down to within the normal operating range.

This automatic thermal protection is accomplished by means of an integrated thermostat embedded in the SSR.  Thethermostat 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 SSR to turn off the power.  This built-in thermal protection completely prevents overheating conditions by providing a trip before equipment damage can occur, thereby saving replacement time and costs.

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Figure 3. Next-generation SSRs incorporate thermal protection via an embedded thermostat within the SSR, preventing overheating conditions.

The integrated thermal protection function can also help to identify design issues in the system, such as incorrect heat sinking capacity in the SSR or system, poor installation resulting in insufficient heat sinking contact, heat dissipation efficiency of the system, and other design problems.  This provides a valuable trouble-shooting tool to the engineer responsible for the process system’s operation.

The new SSR designs can be adapted to a wide range of power supply applications including industrial ovens, sterilization equipment, molding and extrusion machinery, and welding equipment. Other uses include material handling applications such as heavy-duty conveyor systems, such as those in construction, mining, packaging, etc. 

For example, consider an overloaded conveyor belt application where a belt could get stuck and cause potential damage to the motor.   In this case, the SSR with integrated thermal protection would prevent overheating from occurring by shutting down the conveyor belt as soon as a pre-determined heat threshold was met within the SSR’s thermostat.

In injection molding applications where limited space can cause the temperature in the cabinet to rise, thermal protection can prevent the SSR from overheating if the heat sinking is not adequate, thus avoiding costly repairs.  For heating systems, the thermally protected SSR 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.

What’s Next? Smart Solid-State Relays to Protect Power Supply Systems and Motors

Engineers are in the process designing and developing thermally protected SSR technology with decision-making capabilities integrated inside the SSR package. This technology will incorporate a microcontroller with firmware specific to the desired internal trip temperature that activates a decision from the pre-programmed software settings. This capability will make motor and system protection from over-heating and breakdown even more automated and effective.  

Sensata Technologies