EE209
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Title: Three Phase AC Circuits
Aim: To verify the relationships between phase
and line voltages and currents in three phase balance systems and To Measure
power in three phase systems using One wattmeter and two wattmeter methods.
Apparatus:
See your manual for apparatus on page 35
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THEORY:
Three-phase
electric power is a common method
of alternating-current electric
power generation, transmission, and distribution.[1] It is a
type of polyphase system and is the most common method used
by electrical grids worldwide to transfer power. It is also used to
power large motors and other heavy loads.
A three-phase system is usually more economical than an
equivalent single-phase or two-phase system at the same
line to ground voltage because it uses less conductor material to
transmit electrical power.
In a symmetric three-phase power supply system,
three conductors each carry an alternating current of the same
frequency and voltage amplitude relative to a common reference but with a phase
difference of one third the period. The common reference is usually connected
to ground and often to a current-carrying conductor called the neutral. Due to
the phase difference, the voltage on any conductor reaches its peak at one
third of a cycle after one of the other conductors and one third of a cycle
before the remaining conductor. This phase delay gives constant power transfer
to a balanced linear load. It also makes possible to produce a rotating
magnetic field in an electric motor and generate other phase
arrangements using transformers.
The symmetric three‐phase
systems described here are simply referred to as three‐phase
systems because, although it is possible to design and implement
asymmetric three‐phase power systems (i.e., with unequal voltages or
phase shifts), they are not used in practice because they lack the most
important advantages of symmetric systems.
In a three‐phase system
feeding a balanced and linear load, the sum of the instantaneous currents of
the three conductors is zero. In other words, the current in each conductor is
equal in magnitude to, but with the opposite sign of, the sum of the currents
in the other two. The return path for the current in any phase conductor is the
other two phase conductors.
Compared to a single-phase AC power supply that uses
two conductors (phase and neutral), a three-phase supply with no neutral,
the same phase-to-ground voltage and current capacity per phase can transmit
three times as much power using just 1.5 times as many wires (i.e., three
instead of two). Thus, the ratio of capacity to conductor material is doubled.
The same (but not the other properties of three-phase power) can also be
attained with a center-grounded single-phase system.[3]
Three-phase supplies have properties that make them
very desirable in electric power distribution systems:
·
The
phase currents tend to cancel out one another, summing to zero in the case of a
linear balanced load. This makes it possible to reduce the size of the neutral
conductor because it carries little or no current. With a balanced load, all
the phase conductors carry the same current and so can be the same size.
·
Power
transfer into a linear balanced load is constant, which helps to reduce
generator and motor vibrations.
·
Three-phase
systems can produce a rotating magnetic field with a specified
direction and constant magnitude, which simplifies the design of electric
motors.
For
Procedure and Circuit Diagram, See your Electrical Laboratory Manual on pages
35-36.
Fill Up
Tables in Your Manual and Continue To….
PRECAUTIONS:
For
Precautions, See General Electrical Lab
Precautions
Answers
to Questions:
1.
Three-phase Phasor Diagram
The phase voltages
are all equal in magnitude but only differ in their phase angle. The three
windings of the coils are connected together at points, a1, b1 and
c1 to produce a common neutral connection for the three individual phases.
Then if the red phase is taken as the reference phase each individual phase
voltage can be defined with respect to the common neutral as.
Three-phase
Voltage Equations
If the red phase
voltage, VRN is taken as the reference voltage as stated earlier then
the phase sequence will be R – Y – B so the
voltage in the yellow phase lags VRN by 120o, and the voltage in the
blue phase lags VYN also by 120o. But we can also say the blue phase
voltage, VBN leads the red phase voltage, VRN by 120o.
One final point
about a three-phase system. As the three individual sinusoidal voltages have a
fixed relationship between each other of 120o they are said to be
“balanced” therefore, in a set of balanced three phase voltages their phasor
sum will always be zero
as: Va + Vb + Vc = 0
2.
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