Copyright 2003-2013 CSM CO.,LTD
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Understanding SCR
Power Controls
When selecting SCR Power Controls, it is important to have a basic understanding of how
SCRs work. This will help you select the right SCR for your application.
•
How SCR Power Controls Work
A silicon controlled rectifier (SCR) is a solid state switching device which can provide fast, infinitely
variable proportional control of electric power. Not only does this give maximum control of your heat
process, but it can extend heater life many times over other control methods. Since the SCR is solid
state, it can cycle on and off over a billion times, if properly used.
•
PHASE ANGLE FIRED
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 changes and provide maximum heater life.
Options including Voltage Limit, Soft Start and Current Limit are only available with Phase Angle
Fired SCR's.
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ZERO VOLTAGE SWITCHED ( ZERO VOLTAGE
CROSSING
)
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.
•
ON/OFF CONTROL
works like a mechanical or mercury relay. When the SCR is turned on it fires full voltage into the
heater until the command signal turns it off.
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The advantages of SCR Controls over other temperature control methods
- Improved response time
- Closer process control
- Extended heater life
- Reduced maintenance costs
- Silent operation
- Reduced peak power consumption
Remember, a heating element is made of wire in most cases. If you use a mechanical relay to turn
on your heater, it will cycle on or off within 30 seconds or longer. The heater will expand and
contract, getting more and more brittle each time. This is called thermal shock. Temperature
overshoot may also occur. Relays also arc, spark and will burn out on a regular basis.
