Which Temperature Control Device Should You Use?

By Advanced Energy, Inc.

Editor’s Note: This article is adapted from the whitepaper, “Top Advantages of SCR Power Controllers Over Contactors and Solid-State Relays,” from Advanced Energy. Download the free, comprehensive white paper to get in-depth information comparing electromechanical contactors and solid-state relays to SCR power controllers operating in a similar on-off manner. It includes details about on-off cycling, utility cost reduction, control accuracy, heater lifetime, cost-efficiency, noise and transient resistance and diagnostics.

Electricity can be controlled by a variety of switching devices. This article compares SCR power controllers to electromechanical contactors and solid-state relays (SSRs) operating in a similar on-off manner to regulate temperature

            An electromechanical contactor is a device that opens or closes a contact, allowing electricity to be switched fully on and fully off. An electrical control signal triggers the opening or closing of the contact or contacts.

In general, an electromechanical contactor is rated for a specific number of on-off cycles while in operation throughout its lifetime. This rating depends on the manufacturer and typically ranges from less than a million cycles at rated current and voltage to a few million cycles at rated current and voltage.

For a process cycle time of 30 seconds, a mechanical contactor performs 829,440 on-off cycles per year. At a process cycle time of 10 seconds, a mechanical contactor performs 2,488,320 on-off cycles per year. Based on the manufacturer’s recommendation, the electromechanical contactor electrical contacts should be inspected and cleaned, and possibly repaired or replaced, after reaching the specified number of on-off operations.

A solid-state relay (SSR) is an electronic switch that operates without an electromechanical relay’s moving parts. Because it does not have moving parts, the SSR’s life expectancy is greater. Typical cycle times are 2 to 5 seconds for reasonable SSR life.

The SSR can operate at a faster cycle (on-off) time than a comparable mechanical contactor. However, the SSR has a higher contact resistance and is susceptible to damage from surge currents. It also has limited switching arrangements compared to an electromechanical contactor (typically a SPST contact).

The SCR power control module is designed to switch the power applied to its output load quickly. Typical cycle times are 1 second, which means that the SCR will perform 24,883,200 operations in a year for the conditions stated.

Figure 1 compares the calculated total quantity of cycles of operation for the electromechanical relay, the SSR and the SCR over 48 weeks, operating six days per week.