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Postby TGTS0907129» Power in AC circuits

Describe the power in AC circuits


In ac circuits the power is given by P=VI. In such circuits both the current and voltage are assumed to be steady and in phase. In ac circuits this not the case. During half of the cycle, energy is supplied to the reactive component of the circuit, but this energy is returned to the source during the other half of the cycle. Hence no power is required to maintain the current in the part of the circuit which is purely reactive. All the power is used in the resistance portion of the circuit.
P = IVR = I(IR) = I2 R   

Since VR = V cos∅, the power equation (P = IVR) can be written
P = VI cos∅      

where P is the average power in watts when V is the effective value of the voltage and I is the effective value of the voltage and I is the effective value of the current. The angle ∅ is the angle of lag of the current behind the voltage. The quantity cos ∅ is called the power factor of the circuit. Note that the power factor can vary anywhere from zero for a purely reactive circuit to unity for a pure resistance.

It may be seen that
cos ∅= IR/IZ = R/Z

By TGTT170912123 on 9/29/2015 10:16:20 PM
TGTS0907129 on 9/28/2015 11:56:00 PM

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