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EE Design > In Technical
Title: Controlling AC power by phase angle Author: Vincent Wong Date: 29th March 2005 Phase control is a way of controlling the power of an AC load (resistive & inductive). An AC signal is a sinusoidal waveform where Pulse-Width-Modulation (PWM) method cannot be used to control the average power applied to the AC load. In a 50 Hz line, the sinusoidal waveform will swing from the positive half cycle to the negative half cycle (Period=20ms). Therefore, in order to control the power, we must control the turn on time for both the positive half cycle and the negative half cycle. The power output would then be the sum of both the on-time of each cycle. To synchronise with the power line frequency, we must first detect the zero crossing point. The zero crossing point is the point where the sinusoidal singnal changes signal from positive cycle to negative cycle, and from negative cycle to positive cycle. At this point, the voltage measured will be 0 volt. For the 50Hz line, the zero crossing point happens every 10ms. See below.     By varying the firing time of the triac (power control device) and with the referance of the zero-crossing point, the turn-on time of the device can be control and thus controlling the output power of the AC load. |