A Look at Final Control Elements, Part 1
Final control elements are those devices that obey the controller output and provide the muscle to modulate the process heat. Although on/off devices figure prominently in power modulation, I'll ignore the uses for on/off control that do not count as modulation. Instead, I'll look at where they are used in modulating control with time-proportioning action.
Electric Heat. Previously I've dealt with time-proportioning control of electric heat, cycle time, feed-forward and line voltage compensation. From this point, I'll move on to compare common switching devices.
Magnetic Contactors. With this control, energizing the operating coil pulls in the magnetic core that actuates the contacts. The coil typically takes 120 VAC and can consume anything from 10 to 200 VA, depending on the contactor's size. A typical contact arrangement is three normally open and three normally closed contacts, and ratings can go to 200 A at 600 VAC. Optionally, an auxiliary low-current contact can be added for control or logic functions. Frequent switching of high currents causes erosion damage, so the contacts usually are designed to be replaceable. Magnetic contactors can be clunky and somewhat noisy, but they are economical in first cost. To conserve service life, cycle times faster than 10 sec are not recommended.
Mercury Displacement Contactors. With this control, the coil takes typically 120 V at about 50 VA. This operates a plunger that raises the level of mercury in a sealed capsule to bridge a pair of contacts. One-, two- and three-pole normally open models are available with a common coil, or with one coil per capsule. Silent operation and a long reliable life are two advantages of this design, but you will need to evaluate any environmental hazards regarding disposal or in the event of ruptured capsules, especially if the control will be used in a food or container plant.
Solid-State Contactors. With this control, a logic circuit that can take 120 VAC or low-voltage DC is used in place of the coil. Cycle times can be as short as a half-cycle of the AC supply. Advantages of this design include the fact that the mechanism does not wear out due to the number of operations, and you can operate fast-responding heaters beyond the reach of magnetic contactors. Ratings usually go from 10 to 1,000 A at up to 660 VAC. Unlike metal-to-metal contactors, you need to allow about 1 W per ampere when sizing the enclosure for heat dissipation. If your process allows you a choice of heater voltage, selecting a higher voltage permits you to reduce current correspondingly. This reduces heat dissipation requirements and heat sink size. Also, the control logic signal wires can double as a data route to feed back signals such as the heater current value readout, heater faults and low current.
One solid-state contactor -- the silicon controlled rectifier -- is offered with many other features and firing modes such as smooth phase-angle control. This topic would fill a separate article, so it will not be covered here.
Gas And Fuel-Oil Heating. Time-proportioning control also is utilized for gas and fuel heating, using a solenoid valve or equivalent on/off valve or burner contactor. As with electric heat, power can be modulated in a precise linear manner, following the control signal as the controller adjusts the "on" time to cycle time. This method has two advantages: First, control loop gain and, therefore, control loop stability, does not change with power output, as might be the case with a throttling valve. Second, the optimum fuel/air ratio can be maintained because you don't have to cope with many different flow rates. Note that with gas controls, off usually means low fire and on means high fire. This avoids the purge and ignition cycle that follows a period of fuel off.
Steam or Heat Transfer Fluid. Time-proportioning control also can be used in applications delivering heat by steam or heat transfer fluid. These heat sources must be reasonably stable in pressure and temperature to avoid changes in control loop gain.