Phase and
Power Control Using SCR:
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
In ac circuits the SCR can be turned-on by the gate at any
angle α with respect to applied voltage. This angle α is called the firing
angle and power control is obtained by varying the firing angle. This is known
as phase control. A simple half-wave circuit is shown in figure a. for
illustrating the principle of phase control for an inductive load. The load
current, load voltage and supply voltage waveforms are shown in figure b. The
SCR will turn-off by natural commutation when the current becomes zero. Angle β
is known as the conduction angle. By varying the firing angle a, the rms value
of the load voltage can be varied. The power consumed by the load decreases
with the increase in firing angle a. The reactive power input from the supply
increases with the increase in firing angle. The load current wave-form can be
improved by connecting a free-wheeling diode D1, as shown by the dotted line in
fig-a. With this diode, SCR will be turned-off as soon as the input voltage
polarity reverses. After that, the load current will free wheel through the
diode and a reverse voltage will appear across the SCR. The main advantage of
phase control is that the load current passes through a natural zero point
during every half cycle. So, the device turns-off by itself at the end of every
conducting period and no other commutating circuit is required
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