1.1 this method the ac power is supplied to

AC Phase Control

Basics of AC Phase Control

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AC phase control
is the process of controlling ac average power to a load. In this method the ac
power is supplied to the load for a controlled portion of each half-cycle. During
each positive half-cycle of the ac, the signal is turned-off/blocked for a
certain interval, known as the delay angle (measured in degrees/radians), and
then it is turned-on/allowed and conducts current through the load for the
remaining portion of the positive half-cycle, called the conduction angle. The
process is repeated on the negative half-cycle but now current is conducted in
the opposite direction through the load (2. Floyd).
This process is shown in fig… below:


In case of phase angle control if
the delay angle/firing angle is  () and rms input voltage is , then the rms output voltage
can be found from (4. 61412):

And the output power
delivered to a resistive load of resistance R can be found as:


To implement AC
phase control practically, gated thrysistors such Silicon-Controlled Rectifier(SCR)
and Triacs can be used. Control is performed by altering the conduction angle
of the thyristor, usually by delivering a trigger signal to a gate of the
thyristor such that the timing of the trigger signal varies with the required
power level (3. UP).




Triacs are members of the thyristor
family and are high-speed AC power switches. They are solid-state devices with no
moving parts to bend or wear out, and they can be operated at high voltages (up
to several hundred volts) with high current (up to tens of amps) handling capabilities.
Therefore, they can be used with advantage to replace conventional mechanical
switches and relays in many power-control applications such as ac phase control

These are Static devices used to
perform switching action. Here it is used to control voltage across heater
(resistive coil) by applying firing pulse. Firing pulse is generated by
micro-controller for particular voltage (1. ATCFVA).


Zero Crossing

A zero crossing
detector is a one type of voltage comparator, used to detect a sine waveform
transition from positive and negative, that coincides when the I/p crosses the
zero voltage condition. In alternating current, the zero-crossing is the
instantaneous point at which there is no voltage present. In a sine wave or
other simple waveform, this normally occurs twice during each cycle. It is used
to create firing pulse to the TRIAC for positive and negative cycle (1. ATCFVA).


Since the
conduction angle is dependent on the detection of the Zero crossing, it is
crucial that Zero cross detection be accurate and reliable. AC line conditions
are rarely ideal, and less than ideal conditions can cause inaccuracy in the detection
of Zero crossings, With consequent intensity variations and/or flickering, as
Well as other problems, especially at low levels of delivered power ( 3 UP).

The information
about the Zero crosses is used by the microprocessor 26 to determine the half
period of the AC fundamental 62, Which Will be used in order to properly gate the
thyristor. The microprocessor is programmed to attempt to maintain constant
duty cycle. Constant duty cycle means that even if the AC line frequency
changes (Which Will change the time betWeen Zero crosses), the amount of power output
to the load Will be held constant. In order to maintain constant poWer While
the AC line frequency is changing, the amount of time after the Zero cross that
the thyristor is to be?red must be adjusted. If the AC line frequency
increases, e.g., if the frequency changes from 50 HZ to 50.1 HZ, the time
between Zero crosses decreases and the amount of time after the Zero cross that
the thyristor is to be ?red must be decreased in order to keep the output poWer
constant (3. UP).


The Microcontroller








US patent, UP

6_1_412, 61412