17-10-2016, 10:24 AM
1459331530-ACFrOgCNx38JD9506s9Od8ZOy0VTtOHP32SvpSkueb0WOFjnoY3W3EN6BZbfbccDQ5sOdFyWdjFGAImO5AnOGiXp4zLZjvu0m754OzlAwHSVA (Size: 366.72 KB / Downloads: 4)
INTRODUCTION
Power instability in developing countries creates
the need for automation of phase selection or
alternative sources of power to back-up the utility
supply .
Most industrial and commercial applications are
dependent on power supply .
If the process of changeover is manual, not only
considerable time is wasted but also the device
or machine may get damaged from human error
during the changeover connections, incurring
massive losses.
www.pediain.com
Digital phase selector that can be installed in residential
and office premises where single-phase equipment are
used. When any of the mains phase lines fails, it
automatically selects the available phase line (out of three
phase lines or backup lines).
CIRCUIT DESCRIPTION
It comprises phase (or inverter) sensing,
control logic and relay driver sections .
The phase-sensing circuitry has R,Y,B and
inverter sensing sections, which sense the
presence of R,Y,B and inverter supply,
respectively.
The control logic circuitry decides the
phase priority for one out of four lines .
The order of phase priority is R-phase
followed by Y-phase , B-phase and then
backup (inverter )as shown in the truth table
given below .
The relay-driver section drives the relay
according to the signal received from the
control logic circuitry .
Power supply provides the power to control
the logic circuit and relay driver sections.
Explanation of control logic
The control logic circuit comprises NOT
gate, AND gate, diodes and a few passive
components.
It is basically a priority encoder and works
according to the truth table.
If an input variable is at logic ‘0’ it means
that particular phase (line) is absent in the
phase selector .
If an input variable is at logic ‘0’ it means
that particular phase (line) is absent in the
phase selector .
if an input variable is at logic ‘1’ that
particular phase (line) is present in the
phase selector.
Circuit Description
Figure shows the circuit of the digital phase selector. The R-phase
of AC mains supply is rectified by half-wave rectifier IN4007 (D1).
The rectified signal is limited to 9 volts by the zener diode and
filtered by a 10úF capacitor. The 15-kilo-ohm resistor acts as the
current limiter.
The cathode of the zener diode is connected to pin 1 of optocoupler
4N35 through a 1-kilo-ohm resistor. The 1-kilo-ohm resistor acts as
the current limiter for 4N35. Each optocoupler consists of a galliumarsenide
infrared LED and a silicon npn phototransistor. When Rphase
is present, the internal infrared LED drives the internal
phototransistor of 4N35. Emitter
Pin 4 of 4N35 is used for the control logic circuitry.
All the line-/phase-sensing circuits are similar as explained above.
The control logic circuit is isolated from the phase-sensing circuit by
optocoupler 4N35.
RELAY
Relay is a common, simple application of electromagnetism. It uses an
electromagnet made from an iron rod wound with hundreds of fine copper
wire. When electricity is applied to the wire, the rod becomes magnetic. A
movable contact arm above the rod is then pulled toward the rod until it
closes a switch contact. When the electricity is removed, a small spring
pulls the contract arm away from the rod until it closes a second switch
contact. By means of relay, a current circuit can be broken or closed in one
circuit as a result of a current in another circuit.
RESISTANCE
Resistance is the opposition of a material to the current. It is measured in Ohms (W). All
conductors represent a certain amount of resistance, since no conductor is 100% efficient. To
control the electron flow (current) in a predictable manner, we use resistors. Electronic circuits use
calibrated lumped resistance to control the flow of current. Broadly speaking, resistor can be
divided into two groups viz. fixed & adjustable (variable) resistors. In fixed resistors, the value is
fixed & cannot be varied. In variable resistors, the resistance value can be varied by an adjuster
knob. It can be divided into (a) Carbon composition (b) Wire wound © Special type. The most
common type of resistors used in our projects is carbon type. The resistance value is normally
indicated by colour bands. Each resistance has four colours, one of the band on either side will be
gold or silver, this is called fourth band and indicates the tolerance, others three band will give the
value of resistance.
Optocoupler
In electronics, an opto-isolator, also called an optocoupler,
photocoupler, or optical isolator, is "an electronic device designed
to transfer electrical signals by utilizing light waves to provide
coupling with electrical isolation between its input and output". The
main purpose of an opto-isolator is "to prevent high voltages or
rapidly changing voltages on one side of the circuit from damaging
components or distorting transmissions on the other side."
Commercially available opto-isolators withstand input-to-output
voltages up to 10 kV and voltage transients with speeds up to
10 kV/μs.
Conclusion
The DIGITAL PHASE SELECTOR is an advanced
technique which is not only used for automation.
It is one of the techniques for the power quality
improvement.
This is a safe mode of change over of phase as it uses ic
4N35 optocoupler which is high frequency switch.
It does not contain any mechanical contacts and hence
losses are negligible.