True power, VA power and reactive power

Chapter True power, VA power and reactive power

Teach Yourself Electricity and Electronics Third Edition Book
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Summary of Contents

Teach Yourself Electricity and Electronics Third Edition Book

  • A reactance causes ac current to shift in phase, so that it is no longer exactly in stepwith the voltage. In a circuit with inductive reactance, the current lags the voltage by upto 90 degrees, or one-quarter cycle. In a circuit with capacitive reactance, the currentleads the voltage by up to 90 degrees.In a resistance-reactance circuit, true power is dissipated only in the resistive com-ponents. The reactive components cause the VA power to be exaggerated comparedwith the true power.Why does reactance cause this discrepancy between apparent (VA) power andtrue power? In a circuit that is purely resistive, the voltage and current march rightalong in step with each other, and therefore, they combine in the most efficient possi-ble way (Fig. 17-4A). But in a circuit containing reactance, the voltage and currentdon’t work together as well (Fig. 17-4B) because of their phase difference. Therefore,the actual energy expenditure, or true power, is not as great as the product of the volt-age and the current.True power, VA power, and reactive power30917-4At A, current (I) andvoltage (E) are in phasein a nonreactive accircuit. At B, I and E arenot in phase whenreactance is present.True power, VA power, and reactive powerIn a circuit containing both resistance and reactance, the relationships among truepower PT, apparent or VA power PVA, and imaginary or reactive power PX arePVA2PT2PX2PT < PVAPX < PVAIf there is no reactance in the circuit, thenPVAPTPX0