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Today more than ever, engineers are designing electric process heat systems using SCR power controls. The advantages of using an SCR power control are many: more precise control of the heating process, extended heater life, improved product quality at faster production speeds and reduced maintenance costs.
As a decision maker in your company, you have to choose from many types of components used in your process. Take power controls, for instance. You might find yourself asking, "Why use a silicon-controlled rectifier (SCR) power control?"
An SCR is a solid-state switching device that can provide fast, infinitely variable proportional control of electric power. It can cycle as fast as 0.083 sec on a 60 Hz power line. If selected and used properly, an SCR power control can cycle on and off 1 billion times without any problems.
Unlike a mechanical relay or contactor, an SCR power control has no mechanical parts to wear out. The SCR power control will not arc or be affected by dirty contacts. A, mechanical relay will have to be replaced after a certain number of cycles. Due to the slow (30 sec minimum) cycle time inherent in mechanical relays, control is 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 a hazardous materials problem. Because of more stringent governmental regulations, shipping and disposing of mercury relays also are becoming increasingly difficult.
Solid-state relays are a popular alternative to mechanical power controls. Common to all solid-state devices, solid-state relays dissipate the heat that needs to removed, and they are capable of dissipating more heat than SCRs. But, solid-state relays typically are not supplied with lugs that make a solid electrical 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.
Further problems could arise from the ratings of the solid-state relay. Almost all solid-state relays are rated for maximum output at 77 deg F (25 deg C). In real-world operating conditions where electrical enclosures' internal temperatures reach...