ac power control with thyristor


Posted on Feb 7, 2014

The photo above clearly illustrates phase angle control: output voltage controlled by the gate drive signal applied to a thyristor. What is phase angle control That is what I`m going to talk about in this article. Phase angle control is a method of PWM applied to AC input voltages, usually the mains supply. Of course, the AC supply could be from


ac power control with thyristor
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a transformer or any other AC source, but the mains supply is the most common input this gives the phase angle control method its greatest usefulness. It has of course become quite obvious from the title (and I`m sure most of you reading will already know this) that the purpose of phase angle control is to control or limit power to the load. The power device used in phase angle controllers is a thyristor mostly triacs or SCRs. (There are methods of phase controlling employing high frequency switching using a MOSFET or IGBT, but here I`ll talk about phase angle control with thyristors only). The power flow to the load is controlled by delaying the firing angle (firing time each half-cycle) to the power device. We know that the thyristor is a latching device when the thyristor is turned on by a gating signal and the current is higher than the holding current and the latching current, the thyristor stays on, until the current through it becomes sufficiently low (very close to zero). The thyristor turns off when current through it becomes zero, as happens at the AC mains zero crossing. This is the natural line commutation. (Another method of turning the thyristor off is by forced commutation. I won`t go into that now. ) The assumption here is that the load is resistive and has little to no inductance. Of course, this is not always the case, as inductive loads are often used. However, I`ll work with this assumption for now. I`ve...




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