02-01-2013, 10:00 AM
PRINCIPLE OF AC PHASE CONTROL
ABSTRACT
The basic principle of ac phase control technique is explained with reference to a single
phase half wave ac voltage controller (unidirectional controller) circuit shown in the below
figure.
The half wave ac controller uses one thyristor and one diode connected in parallel across
each other in opposite direction that is anode of thyristor 1 T is connected to the cathode of diode
1 D and the cathode of 1 T is connected to the anode of 1 D . The output voltage across the load
resistor ‘R’ and hence the ac power flow to the load is controlled by varying the trigger angle
‘α’.
The trigger angle or the delay angle ‘α’ refers to the value of ωt or the instant at which
the thyristor 1 T is triggered to turn it ON, by applying a suitable gate trigger pulse between the
gate and cathode lead.
The thyristor 1 T is forward biased during the positive half cycle of input ac supply. It can
be triggered and made to conduct by applying a suitable gate trigger pulse only during the
positive half cycle of input supply. When 1 T is triggered it conducts and the load current flows
through the thyristor 1 T , the load and through the transformer secondary winding.
By assuming 1 T as an ideal thyristor switch it can be considered as a closed switch when
it is ON during the period ωt =α to π radians. The output voltage across the load follows the
input supply voltage when the thyristor 1 T is turned-on and when it conducts from ωt =α to π
radians. When the input supply voltage decreases to zero at ωt =π , for a resistive load the load
current also falls to zero at ωt =π and hence the thyristor 1 T turns off at ωt =π . Between the
time period ωt =π to 2π , when the supply voltage reverses and becomes negative the diode 1 D
becomes forward biased and hence turns ON and conducts. The load current flows in the
opposite direction during ωt =π to 2π radians when 1 D is ON and the output voltage follows
the negative half cycle of input supply.
DISADVANTAGES OF SINGLE PHASE HALF WAVE AC VOLTAGE CONTROLLER.
• The output load voltage has a DC component because the two halves of the output
voltage waveform are not symmetrical with respect to ‘0’ level. The input supply current
waveform also has a DC component (average value) which can result in the problem of
core saturation of the input supply transformer.
• The half wave ac voltage controller using a single thyristor and a single diode provides
control on the thyristor only in one half cycle of the input supply. Hence ac power flow to
the load can be controlled only in one half cycle.
• Half wave ac voltage controller gives limited range of RMS output voltage control.
Because the RMS value of ac output voltage can be varied from a maximum of 100% of
VS at a trigger angle α = 0 to a low of 70.7% of S V at α =π Radians .
These drawbacks of single phase half wave ac voltage controller can be over come by
using a single phase full wave ac voltage controller.