Preventing SSR overheating

What do complex industrial/commercial manufacturing systems all have in common? Motors. Electrical motors that, if they overheat, can be damaged or even destroyed.

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

EMRs versus SSRs

There are significant differences between EMRs and SSRs, especially in terms of life-span.

EMRs are mechanical-based and have moving parts, making them highly susceptible to magnetic noise, vibration, and shock. In contrast, SSRs offer a durable, all-solid state electronic construction with no moving parts to affect wear or accuracy, thereby offering predictable operation and longer life. The average lifespan of electro-mechanical relays is in the range of hundreds of thousands of cycles compared to five million hours for three-phase solid state relays. With such maintenance–free durability, SSRs often outlast the equipment in which they are installed.

SSRs provide faster switching than EMRs, making them adaptable to a wider range of high power load applications. They operate silently with low input power consumption and produce little electrical interference. SSRs can withstand harsh environments whereas EMRs need frequent replacement.

As SSRs generate heat when conducting current, there is a thermal management component to their operation, just like the motors that they control. Should overheating occur, diagnosing and replacing a damaged SSR can take time while the assembly line or manufacturing system is down.

Consider the product’s use in commercial refrigeration applications in the building equipment market. In a refrigeration application, the SSR’s function is to turn the compressor on or off to keep the system temperature within a specified range. The input control might be from 90-280 AC with a required tripping temperature set at 95°C. A buffer is engineered into the circuitry using a variety of components to ensure that the desired tripping action occurs.

When the SSR turns on to conduct load current, it also generates heat. Failure to adequately protect the SSR can cause damage to the relay or to the load.

Next generation SSRs

New SSR technologies are being developed that integrate a thermostat into the SSR itself to ensure that the relay always operates in a safe or protected mode. This design incorporates all the advantages of standard SSR technology, but is differentiated by its ability to prevent the SSR from overheating, thus protecting component and system operation from potential damage or shut down.

The new generation of SSR 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. 

This automatic thermal protection is accomplished by means of an integrated thermostat embedded in the SSR. 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 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 time and money.

Future technologies will incorporate a microcontroller, with firmware specific to the desired internal trip temperature that activates a decision from the pre-programmed software settings. With a decision-making capability inside the SSR package, protecting motors and systems from over-heating and breakdown will be more automated than ever.