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The SCR control can deliver electrical power to heaters in several ways: phase angle fired, zero voltage switched and on/off control.
PHASE ANGLE FIRED
These controls proportionally turn on a percentage of each power line half cycle. This gives smooth, infinitely variable application of power to the heaters. Imagine a light dimmer and how it provides power to a light bulb. This method will provide the most precise control of heaters. Phase angle fired SCR’s will respond the fastest to load change and provide maximum heater life. It is the most precise method of control. Phase angle firing can increase heater life up to seven times depending on heater type. Phase angle firing also allows options such as soft start, voltage limit and current limit. These options are not available with any other means of control.
Equipment manufacturers and maintenance engineers need a reliable, flexible approach to controlling electric-heating processes in today’s competitive, cost-conscious, industrial environment. And making the right choices up front is vitally important.
Let us help you understand SCR Power Controls, compare power control methods in order to make the right selection, and determine the type of
electrical enclosure you need. Graphs below
shows phase angle fired voltage output on a 240V power line.
Graph shows 25% output or 60V RMS. |
Graph shows 50% output, 120V RMS. |
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Graph shows 99.5% output or 238.8V RMS |
ZERO VOLTAGE SWITCHED
Zero voltage switched controls proportionally turn on and off each full
cycle of the power line cycle. By varying the number of AC power line
cycles, the SCR provides power to the heaters. With a variable time
base, the optimum number of cycles turned on/off is achieved. The
switching is done at fractions of a second during the zero voltage
crossing of the power line cycles. For IR heating with a fast element,
you would see the elements flickering. On a ceramic heater, or more
massive medium wave heater you can control within very tight tolerances.
Zero voltage switching controls proportionally turn on and off each full
cycle of the power line. By varying the number of AC power line cycles,
the SCR provides power to the heaters. With a variable time base, the
optimum number of cycles turned on/off is achieved. This method produces
less RFI line noise than phase angle fired SCRs. Power factor is 100%,
while harmonic distortion is limited. Chart below shows 25% power output
using variable time based zero voltage switching.
Avatar Instruments variable time base SCR firing. Note how half
cycles can be turned on and off providing 0.08 second resolution. Above
chart shows 50% output.
100% output at 240V RMS.
On/Off controls function the same way as a mechanical or mercury
relay, with the advantage of much faster cycle times. Chart below shows
an Avatar Instruments model A1Z SCR output with variable base time
proportioning. Compare the output cycles against an SSR with a 1 second
time base and a 5 second time base. The superior firing resolution and
speed of response of the A1Z SCR is instantly apparent.
Avatar Instruments A1Z SCR 50% output with variable time base zero
voltage switching.
To show the full effect, the scope had to be set on a 500ms scale, or 5X
longer time base than the 100ms screen above.
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