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Three-Phase Circuit

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Why Three-Phase Circuit is more superior than Single Phase?

1. The horsepower rating of three-phase motors and thekilovolt-ampere rating of three-phase transformers are aout1!"# greater than for single-phase motors or transformers witha similar frame si$e.

%. The power delivered y a single-phase system pulsates .Thepower falls to $ero three times during each cycle. The powerdelivered y a three-phase circuit pulsates also& ut it never

falls to $ero. 'n a three-phase system& the power delivered tothe load is the same at any instant. This produces superioroperating characteristics for three-phase motors.

.

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Why Three-Phase Circuit is more superior than Single Phase?

(. 'n a alanced three-phase system& the conductorsneed e only aout )!# the si$e of conductors for asingle-phase two-wire system of the same kilovolt-

ampere *k+, rating. This savings helps offset the costof supplying the third conductor reuired y three-phase systems.

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Single Phase

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Three-Phase Circuit

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Three-Phase Circuit

/iagi %& yaitu0

1. Wye Connection

%. /elta Connection

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Wye Connection

line 2 phase 3 1.)(%'line2 'phase

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/elta Connection

line2 phase

'line 2 'phase 3 1.)(%

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Three-Phase Power

'f linevalues of voltage and current are known& theapparent power of the circuit can e computed usingthe formula

+, 2 4 'line 4 line'f thephasevalues of voltage and current are known&

the apparent power can e computed using theformula

+, 2 ( 4 'phase 4 phase

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Watt and +,5s

Three-phase watts can e computed y multiplyingthe apparent power y the power factor0

P 2 4 'line 4 line4 P6

7r

P 2 ( 4 'phase 4 phase4 P6

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, delta-connected alternator is connected to a wye-connectedresistive load. The alternator produces a line voltage of %8" +and the resistors have a value of 9 each. 6ind the following

values0:*:oad;line voltage of the load

P*:oad;phase voltage of the load

'P*:oad;phase current of the load

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':*:oad;line current to the load

':*,lt;line current delivered y the alternator

'P *,lt;phase current of the alternator

P*,lt;phase voltage of the alternatorP;true power

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,nswer

:*:oad 2 %8" +olt

P*:oad 2 2 +olt

'P*:oad 2 2 %(."