Solid-state switching means interruption of power by nonmechanical electronic means. A solid-state contactor is a power-switching device designed to replace magnetic contactor for applications involving both resistive and inductive loads.
Solid-state three-phase contactors are especially suited to high-cycling applications owing to the absence of arc-producing air gap contacts. Size range from 10 to 600 A, with input voltage from 240 to 550 Vac. Solid-state contactors now replace three-pole electromechanical contactors in industrial furnaces and ovens, mining and materials handling, and other industrial heating applications.
Silicon-controlled rectifiers (SCRs) enable reliable control of electric power from 1 kW to 1000 kW for most types of resistance heaters, motors, and other inductive loadds. It consists of a specially treated silicon disc enclosed in a plastic or ceramic housing with metal power leads arranged for the anode and cathode connections and a smaller tab or wire for the gate connection. The SCR, like a contact, is in either the ON state (closed contact) or the OFF state (open contact). The SCR is analogous to a “latched relay” circuit. Once the SCR is triggered, it will stay ON until its current decreases to zero. When current through the SCR stops, the “SCR switch” will open and stay open until retriggered.
In contrast to a magnetic contactor, an electronic contactor is absolutely silent, and its “contacts” never wear out. Inductive loads and voltage transients are both seen as problem areas in solid-state ac control includes a resistor and capacitor in series connected in parallel to each power pole. These “RC” or “snubber” networks divert the charging current from the SCRs and help prevent unwanted turn-on.
All silicon semiconductors, when in the ON state, still have a small voltage drop accross the junction of 1 to 2 V. The resultant 1 to 2 W of heat per conducted ampere through the device must be removed to the outside environment. Properly designed heat sinks acccomplish this normally by conduction or convection heat transfer to maintain the silicon below its maximum temperature level.
The abrupt switching of the SCRs from the blocking state to the conducting state, particularly ath higher current levels, may sometimes cause objectionable transients on the power line and create radio frequency interference (RFI). Zero-fired control refers to turning ON the SCRs at the zero voltage crossing for full cycles, applying full power or no power with the proportion of full cycles, or determining the resulting power to the load. This is sometimes called integral cycle mode or “burst firing.” The result is eliminaton of power line disturbances and RFI.
– Petruzella, Frank D. Industrial Electronics. 1996. McGraw-Hill International Editions. pg. 231-233.
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