Power factor

Chapter Power factor

Teach Yourself Electricity and Electronics Third Edition Book
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Summary of Contents

Teach Yourself Electricity and Electronics Third Edition Book

  • Engineers often strive to eliminate, or at least minimize, the reactance in a circuit.This is particularly true for radio antenna systems, or when signals must be sent overlong spans of cable. It is also important in the design of radio-frequency amplifiers. To alesser extent, minimizing the reactance is important in audio work and in utility powertransmission.Power factorThe ratio of the true power to the VA power, PT/PVA, is called the power factor in an accircuit. If there is no reactance, the ideal case, then PTPVA, and the power factor(PF) is equal to 1. If the circuit contains all reactance and no resistance of any signifi-cance (that is, zero or infinite resistance), then PT0, and PF0.If you try to get a pure reactance to dissipate power, it’s a little like throwing afoam-rubber ball into a gale-force wind. The ball will come right back in your face. Apure reactance cannot, and will not, dissipate power.When a load, or a circuit in which you want power to be dissipated, contains someresistance and some reactance, then PF will be between 0 and 1. That is, 0 < PF < 1. PFmight be expressed as a percentage between 0 and 100, written PF%. Mathematically,PFPT/PVAPF%100PT/PVAWhen a load has some resistance and some reactance, a portion (but not all) of thepower is dissipated as true power, and some is “rejected” by the load and sent back tothe source as VA power.Calculation of power factorThere are two ways to determine the power factor in an ac circuit that contains reac-tance and resistance. Either method can be used in any situation, although sometimesone scheme is more convenient than the other.Cosine of phase angleRecall that in a circuit having reactance and resistance, the current and the voltage arenot in phase. The extent to which they differ in phase is the phase angle. If there is noreactance, then the phase angle is 0 degrees. If there is a pure reactance, then the phaseangle is either 90 degrees or −90 degrees.The power factor is equal to the cosine of the phase angle. You can use a calculatorto find this easily.Problem 17-1A circuit contains no reactance, but a pure resistance of 600 Ω. What is the power factor?Without doing any calculations, it is evident that PF1, because PVAPT in apure resistance. So PT/PVA1. But you can also look at this by noting that the phase310 Power and resonance in ac circuits