3 phase a.c. circuits.pptx

8/10/2019 3 phase a.c. circuits.pptx

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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 .



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x

N

S

Motion is perpendicular to flux.

Induced voltage is maximum.


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Motion is 45 to flux. N

S

Induced voltage is 0.707 of maximum.


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N

S

Motion is parallel to flux.



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N

S

Notice current in the

conductor has reversed. Induced voltage is0.707 of maximum.



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N

S

Motion is perpendicular to flux.

Induced voltage is maximum.


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N

S


Induced voltage is 0.707 of maximum.


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Motion is parallel to flux. N

S


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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3 phase a.c. circuits.pptx

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