EE209 - Three Phase AC Circuits

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 threephase systems described here are simply referred to as threephase systems because, although it is possible to design and implement asymmetric threephase 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 threephase 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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About Uniben Engineering

Stephen Djes is a passionate Graduate of Engineering from the University of Benin, and he is geared towards helping fellow engineering students in the great institution of UNIBEN to do better at academics.
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