Thyristors, (both SCR's and Triacs are thyristors) are notoriously noisy and
suffer from a poor power factor. Triacs are SCR's back-to-back, and so
conduct AC in one package, where an SCR is more like a simple diode; conducting
one direction. They both work on the same principle: Supply a positive gate
voltage with respect to cathode (more precisely a gate to cathode current),
and they conduct; only turning off if the voltage potential between anode an
cathode causes the anode current to fall below the holding current. For all
practical purposes this is nearly 0 v. In an AC circuit, this would be the 0
crossing point of the sine wave.
The real problem with thyristors as a variable AC power control, is that to
reduce voltage (say on the transformer), the thyristor is turned on during a
180 degree portion of the positive (with respect to cathode) portion of the
AC wave. So, instead of a gradually increasing voltage (and resulting
current), the thyristor (and the rest of the circuitry) is subjected to a jolt
power (the leading edge of the once sine wave, now resembles the leading edge
a square wave). If the thyristor is turned on at the 90 degree point of the
AC cycle, the inrush current is at maximum.
Another way of turning off a thyristor is by reversing the voltage across
the device itself - called commutation. This eliminates the abrupt inrush if
the thyristor is turned on just after the 0 crossing point, but is still very
noisy and requires complicated circuitry. This type of control has been used to
control DC motors with much success for decades.
The voltage drop across a thyristor is great enough to warrant a bypass
contactor, for with high enough currents, the device does dissipate power as
heat. As you already have to use a contactor, why not stick with the tried and
true dropping resistor and by-pass contactor?
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