Contactor, SSR, or SCR: Understanding the Differences and Choosing the Right Solution

The Control of Power

Today more than ever, engineers are designing electric process heat systems using SCR (thyristor) power controls. The advantages of using an SCR are many; more precise control of the heating process, extended heater life, improved product quality at faster production speeds and reduced maintenance costs.

An SCR is a solid-state switching device that can provide fast, infinitely variable proportional control of electric power. Unlike a mechanical relay or contactor, an SCR power controller has no mechanical parts to wear out. The SCR will not arc or be effected by dirty contacts. A mechanical relay will have to be replaced after a certain number of cycles. Due to the slow cycle time inherent in mechanical relays, control can be poor so the heated process may be damaged and heater life could be shortened due to thermal shock.

Mercury displacement relays can cycle faster than mechanical relays. However, if overheated due to excessively fast cycling or overloading, the mercury relay will explode.

This results in hazardous material problems and due to more stringent governmental regulations, shipping and disposing of mercury relays also are becoming increasingly difficult.

Solid-state relays (SSR’s) are a popular alternative to contactors but they are not typically supplied with terminations that make a solid connection for higher power levels. Also, they are not always sold with heat sinks, voltage protection or the fuses needed to protect and safely operate the relay.

Take note of rating values

Further problems could also arise from the ratings of the SSR. Almost all SSR’s are rated at 25℃. In real-world operating conditions where electrical enclosure’s internal temperatures reach over 40℃ the SSR may fail if used at full output. Most manufacturers have a derating chart to compensate for this discrepancy. Unfortunately, many users rely only on the maximum rating when selecting an SSR resulting in poor chose for their application. Most SCR power controllers are rated for ambient temperatures of 45℃ at full power ratings.

It’s worth noting that in real life situations using a Thyristor Stack or a Solid State Relay from CD Automation can help eliminate downtime by up to 95% and reduce your overall purchase cost by up to 90% over the product lifetime compared to using a traditional mechanical contactor. Thats a huge cost saving that cannot be ignored if you’re seriously looking at running an efficient process.

Further reading

You may also find our free PDF ‘5 Mistakes Made When Optimising Machine Performance’ an interesting read. This looks at how using a thyristor (SCR) power controller in the right way can save you time and money when it comes to optimising your electric thermal machine. Click below to receive a copy.