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Introduction to POWER IN CIRCUITS

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POWER % ACCrcut!Instantaneous Power

The instantaneous power supplied to a device issimply the product of the instantaneous voltageacross its terminals and the instantaneous currentthat flows through it.

  p = v x i

Instantaneous power is always expressed in Watts

and may be positive or negative. A positive powervalue means that power flows into the device, whilsta negative power indicates that power is flowing outof the device.

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POWER % ACCrcut!

Active (Real) Power

The simple circuit shownin (a consists of a resistorconnected to an ac source.

The effective voltage andcurrent are ! and I and fora resistive circuit, phasors! and I will be in phase.

~!

I

"

!I

(a

(b

average power(#$! x (#$I = $!I 

 EI  EI 

 P    ==2

2

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-ue!to%

A%!$er!

-ue!to% 2

A%!$er!

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-ue!to% .

A%!$er!

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A%!$er!

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1. Sample Questions on AC Poer

!. Sample Questions on AC Poer

http://var/www/apps/conversion/tmp/Class%202/AC_Circuit_Analysis_Tutorial.pdfhttp://var/www/apps/conversion/tmp/scratch_3/57821_AC_Power_-_Tutorial.pdfhttp://var/www/apps/conversion/tmp/scratch_3/57821_AC_Power_-_Tutorial.pdfhttp://var/www/apps/conversion/tmp/scratch_3/57821_AC_Power_-_Tutorial.pdfhttp://var/www/apps/conversion/tmp/Class%202/AC_Circuit_Analysis_Tutorial.pdf

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"undamentals o# P$ASORS

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P#a!or! a%+ P#a!or Notato%-  AC voltages and currents can be represented diagrammatically by

phasors, the length of which corresponds to the rms value of the voltage

or current.

 

- Voltage and current values have a magnitude and direction (or phasedisplacement - like vectors.

-  An arrow is used to signify the direction of the phasor. !he angle between

two phasors is e"ual to the electrical phase angle between the "uantities.

- !wo phasors are said to be in phase when they are parallel to each other

and point in the same direction. !he phase angle between them is then

#ero.

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P#a!or! a%+ P#a!or Notato%

Parallel %ectors Collinear %ectors

%ectors at an an&le'Out o# P(ase )ectors*

The phase angle between them is theangle through which one of the phasors

has to be rotated to ma-e it point in thesame direction as the other.

+%ector %, leads +%ector I, - an&le+%ector I, le&s +%ector %, - an&l

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P#a!or! a%+ P#a!or Notato%!xample2raw the phasor diagram of the voltage and current in 3igure 4.

/

I

rms )alue o# )olta&e

rms )alue o# current

I la&s % - /0 de&rees

 

V

I

$%o$56/

76A

0 $2

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&outo%

%s

%R%3 4 %r 5 %l

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RE&1LT&

OR

P#a!or Da,ram et#o+

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&outo%

%ector Representatio

END o3 LECT1RE