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Chapter 12 Three Phase Circuits - PowerPoint PPT Presentation

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Chapter 12 Three Phase Circuits

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Know the difference between balanced and unbalanced circuits. Learn about power in a balanced three-phase system ... We look at one phase, say phase a and ... – PowerPoint PPT presentation

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Title: Chapter 12 Three Phase Circuits


1
Chapter 12Three Phase Circuits
  • Chapter Objectives
  • Be familiar with different three-phase
    configurations and how to analyze them.
  • Know the difference between balanced and
    unbalanced circuits
  • Learn about power in a balanced three-phase
    system
  • Know how to analyze unbalanced three-phase
    systems
  • Be able to use PSpice to analyze three-phase
    circuits
  • Apply what is learnt to three-phase measurement
    and residential wiring

Huseyin Bilgekul Eeng224 Circuit Theory
II Department of Electrical and Electronic
Engineering Eastern Mediterranean University
2
Three phase Circuits
  • An AC generator designed to develop a single
    sinusoidal voltage for each rotation of the shaft
    (rotor) is referred to as a single-phase AC
    generator.
  • If the number of coils on the rotor is increased
    in a specified manner, the result is a Polyphase
    AC generator, which develops more than one AC
    phase voltage per rotation of the rotor
  • In general, three-phase systems are preferred
    over single-phase systems for the transmission of
    power for many reasons.
  • 1. Thinner conductors can be used to transmit
    the same kVA at the same voltage, which reduces
    the amount of copper required (typically about
    25 less).
  • 2. The lighter lines are easier to install, and
    the supporting structures can be less massive and
    farther apart.
  • 3. Three-phase equipment and motors have
    preferred running and starting characteristics
    compared to single-phase systems because of a
    more even flow of power to the transducer than
    can be delivered with a single-phase supply.
  • 4. In general, most larger motors are three
    phase because they are essentially self-starting
    and do not require a special design or additional
    starting circuitry.

3
Single Phase, Three phase Circuits
b) Single phase systems three-wire type. Allows
connection to both 120 V and 240 V.
a) Single phase systems two-wire type
Two-phase three-wire system. The AC sources
operate at different phases.
4
Three-phase Generator
  • The three-phase generator has three induction
    coils placed 120 apart on the stator.
  • The three coils have an equal number of turns,
    the voltage induced across each coil will have
    the same peak value, shape and frequency.

5
Balanced Three-phase Voltages
Three-phase four-wire system
Neutral Wire
A Three-phase Generator
Voltages having 120? phase difference
6
Balanced Three phase Voltages
Neutral Wire
a) Wye Connected Source
b) Delta Connected Source
a) abc or positive sequence b) acb or
negative sequence
7
Balanced Three phase Loads
  • A Balanced load has equal impedances on all the
    phases

a) Wye-connected load b)
Delta-connected load
8
Three phase Connections
  • Both the three phase source and the three phase
    load can be connected either Wye or DELTA.
  • We have 4 possible connection types.
  • Y-Y connection
  • Y-? connection
  • ?-? connection
  • ?-Y connection
  • Balanced ? connected load is more common.
  • Y connected sources are more common.

9
A balanced Y-Y system, showing the source, line
and load impedances.
Balanced Wye-wye Connection
Line Impedance
Source Impedance
Load Impedance
10
Balanced Wye-wye Connection
Line current In add up to zero. Neutral current
is zero In -(Ia Ib Ic) 0
  • Phase voltages are Van, Vbn and Vcn.
  • The three conductors connected from a to A, b
    to B and c to C are called LINES.
  • The voltage from one line to another is called a
    LINE voltage
  • Line voltages are Vab, Vbc and Vca
  • Magnitude of line voltages is v3 times the
    magnitude of phase voltages. VL v3 Vp

11
Balanced Wye-wye Connection
Line current In add up to zero. Neutral current
is zero In -(Ia Ib Ic) 0
  • Magnitude of line voltages is v3 times the
    magnitude of phase voltages. VL v3 Vp

12
Balanced Wye-wye Connection
  • Phasor diagram of phase and line voltages

13
Single Phase Equivalent of Balanced Y-Y Connection
  • Balanced three phase circuits can be analyzed on
    per phase basis..
  • We look at one phase, say phase a and analyze
    the single phase equivalent circuit.
  • Because the circuiit is balanced, we can easily
    obtain other phase values using their phase
    relationships.

14
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15
Balanced Wye-delta Connection
  • Three phase sources are usually Wye connected
    and three phase loads are Delta connected.
  • There is no neutral connection for the Y-?
    system.
  • Line currents are obtained from the phase
    currents IAB, IBC and ICA

16
Balanced Wye-delta Connection
  • Phasor diagram of phase and line currents

  • Single phase equivalent circuit of the balanced
    Wye-delta connection

17
Balanced Delta-delta Connection
  • Both the source and load are Delta connected and
    balanced.

18
Balanced Delta-wye Connection
Transforming a Delta connected source to an
equivalent Wye connection
Single phase equivalent of Delta Wye connection
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