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Polyphase A.C. Circuits
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Poly-Phase Systems Many phases Multiple phasesproduces a smootherelectrical power
Polyphase A.C.Circuits
Why Use 3 Phase Power?: Easy to Generate
Provides ConstantInstantaneous Powerto Load
Uses Fewest Wires toDeliver ConstantInstantaneous Power
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Advantages of three phase system In single phase system ,the instantaneous power is
fluctuating in nature. However in in three phase system itis constant at all times.
The o/p of three phase system is greater than single
phase system.
Transmission and distribution of three phase system ischeaper than that of single phase system.
Three phase motors are more efficient and have higherpower factor than single phase motors.
Three phase motors are self starting whereas singlehase motors are not self startin .
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Generation of Three Ph.Voltages Three phase voltages are generated by a generator. Three sets of independent windings which are
physically spaced 120 degrees around thestator.
Voltages are labeled phase R, phase Y, andphase B are of same magnitude but differ in phaseangle by 120 degrees.
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THREE PHASE
GENERATION
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FARADAYS LAW Three things must be present in order to
produce electrical current:a) Magnetic fieldb) Conductorc) Relative motion
Conductor cuts lines of magnetic flux, avoltage is induced in the conductor
Direction and Speed are important
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GENERATING A SINGLE PHASE
Motion is parallel to the flux.
No voltage is induced.
N
S
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N
S
Motion is 45 to flux.
Induced voltage is 0.707 of maximum .
GENERATING A SINGLE PHASE
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GENERATING A SINGLE PHASE
x
N
S
Motion is perpendicular to flux.
Induced voltage is maximum.
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GENERATING A SINGLE PHASE
Motion is 45 to flux. N
S
Induced voltage is 0.707 of maximum.
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GENERATING A SINGLE PHASE
N
S
Motion is parallel to flux.
No voltage is induced.
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GENERATING A SINGLE PHASE
N
S
Notice current in the
conductor has reversed. Induced voltage is0.707 of maximum.
Motion is 45 to flux.
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GENERATING A SINGLE PHASE
N
S
Motion is perpendicular to flux.
Induced voltage is maximum.
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GENERATING A SINGLE PHASE
N
S
Motion is 45 to flux.
Induced voltage is 0.707 of maximum.
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GENERATING A SINGLE PHASE
Motion is parallel to flux. N
S
No voltage is induced.
Ready to produce another cycle.
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GENERATOR WORK The generator consists of a rotating magnet
(rotor ) surrounded by a stationary winding(stator ).
Three separate windings or coils withterminals a- a, b -b, and c -c are physicallyplaced 120 apart around the stator.
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As the rotor rotates, its magnetic field cuts theflux from the three coils and induces voltagesin the coils.
The induced voltage have equal magnitudebut out of phase by 120 .
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GENERATION OF THREE-PHASE AC
N
xx
S
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THREE-PHASE WAVEFORM
Phase 2 lags phase 1 by 120 Phase 2 leads phase 3 by 120
Phase 3 lags phase 1 by 240 Phase 1 leads phase 3 by 240
Phase 1 Phase 2 Phase 3
120 120 120
240
120 120 120
240
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Phase 1 Phase 2 Phase 3
GENERATION OF 3 VOLTAGES
Phase 1 is ready to go positive.Phase 2 is going more negative.
Phase 3 is going less positive.
N
xx
S
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Waveforms
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Phase sequence
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Interconnection of three phases
Star Connection or Wye Connections The wye, or star, connection is made by
connecting same end of each of the phasewindings together in a common node.
Delta Connection or Mesh Connection
Dissimilar ends of three phase windings are joined together to form closed loop.
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THREE PHASE QUANTITIES
QUANTITY SYMBOL
Phase current I ph
Line current I L
Phase voltage V ph
Line voltage V L
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BALANCED VOLTAGE AND LOAD
Balanced Phase Voltage : all phase voltagesare equal in magnitude and are out of phasewith each other by 120 .
Balanced Load : the phase impedances areequal in magnitude and in phase.
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PHASE VOLTAGES and LINE VOLTAGES
Phase voltage is measured between theneutral and any line: line to neutral voltage
Line voltage is measured between any two of
the three lines: line to line voltage.
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PHASE CURRENTS and LINECURRENTS
Line current (I L) is the current in each line ofthe source or load.
Phase current (I ) is the current in eachphase of the source or load.
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Definitions Phase sequence-The sequence in which the voltages in three phases
reach their maximum positive value is called phase sequence Phase voltage-The voltage induced in each winding is called phase
voltage. Phase current-The current flowing through each winding is called phase
current.
Line voltage-The voltage available between any pair of terminals or linesis called line voltage. Line current-The current flowing through each line is called line current Balanced system-A three phase system is said to be balanced if1.The voltages in three phases are equal in magnitude and differ in phase
from one another by 120 degrees.2.The currents in three phases are equal in magnitude and differ in phase
from one another by 120 degrees.
3.The loads connected across the three phases are identical i.e all the loadshave same magnitude and power factor.
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Comparison between star and delta connection
Star connection Delta connection V L= 3 V ph V L= V ph I L= I ph I L= 3 I ph Line voltage leads the
respective phasevoltage by 30 degrees
Line current lags therespective phase current by30 degrees
Power in star connectionis one third of power indelta connection
Power in delta connection isthree times of power in starconnection
Three phase ,three wireand three phase fourwire systems are
possible.
Only three phaes three wiresystem is possible
The phasor sum of all
th h t i
The phasor sum of all the
h t i
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