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Re: [Amps] thyristor controlled soft start?

Subject: Re: [Amps] thyristor controlled soft start?
Date: Sun, 23 Mar 2008 18:22:06 EDT
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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?
-John, N9RF

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