Figure 1. After you have sized and specified the transformer, decide where to put the SCR.

The most common and economical arrangement of SCR power control is an SCR unit inserted between the AC power line and a constant resistance load having a voltage rating that matches an available standard power source. Sources with more than toaster capacity are usually in the 400 to 600 V range. Heaters that suit the process -- and are preferably rugged -- often demand lower voltages.

Here are some cases where you need a transformer between the power source and the load.

• The load has low resistance and needs a low voltage and high current. A typical example would be where you have to provide 10 V at 500 A to heat directly a length of steel pipe and the fluid inside.

• More difficult loads include silicon carbide, molybdenum, tungsten, molybdenum disilicide or graphite. These may demand a low and adjustable voltage AC power source.

After you have sized and specified the transformer, decide where to put the SCR (figure 1).

SCR in the Secondary

The most common and economical arrangement of a SCR power control is an SCR unit inserted between the AC power line and a constant resistance load having a voltage rating that matches an available standard power source.

SCR in the secondary usually is problem-free, but you probably will need a higher-current, lower-voltage rated SCR. This will be more expensive and call for extra heat-sink dissipation. Given reasonably constant resistance, you can use the lower cost fast-cycling mode or even a solid-state contactor (SSC) -- cheaper yet because your controller can provide the control pulses.

If your heaters have temperature- or age-dependent resistance, this calls for features like current-, voltage- or power-limiting. These are only found with phase-angle control. With silicon-carbide heaters, a tapped transformer often is used, combined with some phase-angle throttling. This lets you to move the available secondary voltage up to, say, double the starting voltage as the element resistance increases with age. Compared with having phase-angle perform all the voltage-limiting, the tapped transformer lets you run with a higher power factor.

Next month, I'll look at placing the SCR in the primary.