09-11-2016, 10:53 AM
1467382087-acvoltagecontroller.pdf (Size: 1.05 MB / Downloads: 6)
An ac voltage controller is a converter that controls the voltage,
current, and average power delivered to an ac load from an ac
source.
Some applications;
Light-dimmer circuits
Speed control of induction motors
There are two control methods
Phase control
Integral-cycle control
(On-off control)
1- On - off control
• This type of control is applied in applications which have high
mechanical inertia and high thermal time constant.
• Typical examples are industrial heating, refrigiration and speed
control of motors.
• If the input voltage is connected to load for n cycles and is
disconnected for m cycles, the output load voltage is found from
Principle of Phase Control
• The principle of phase control can be
explained with the following circuit.
• Due to the presence of diode D1
, the
control range is limited.
• The rms output voltage can only be varied
between 70.7 to 100%.
• The output voltage and input current are
asymmetrical and contain a dc component.
• This unidirectional controller is suitable
only for low power resistive loads, such as
heating and lighting.
Bidirectional Phase Control
Switch is composed of two antiparallel
SCRs
Current is bidirectional.
Triac can be used instead of the
two thysistors
Other controlled switches can
be used instead of SCR
2- Bidirectional Phase Control
Notes
•The two SCRs can not conduct simultaneously
•The load voltage is the same as the source voltage when either
SCR is on.
•Switch voltage is zero when SCR is on, and is equal to source
voltage when SCR is off.
•Average current in the source and load is zero.
•Average current in each SCR is not zero
•rms current in each SCR is 1/√2 times the rms load current
Static Tab Changer
Thyristors can be used as static switches for ON-load tap changing of trans.
The static tap changers have the advantage of very fast switching action.
The changeover can be controlled to cope with load conditions and is smooth.
Commonly used for resistive heating loads.
v1 = √2V1
sinωt
v2 = √2V2
sinωt
If T3
and T4
are alternately fired with α
= 0°
, the load voltage is held at reduced
level of V1
.
If T1 and T2 are alternately fired with a
delay angle of α = 0° V0 = V1 + V2
.
The gating pulses of Thyristors can be
controlled to vary the load voltage.
The RMS Vo, can be varied within three
ranges