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In my current electrical engineering course the concept of power factor has been a large topic, defined as the ratio of real power to apparent power. It is a measure of resistive power consumption in a circuit. Other kind of loss present in the circuit is the reactive or "parasitic" power.

In my material it is presented as a quantity that always should be maximized. But what I don't understand is why resistive power consumed is always a good measure of efficient use of power? There are lots of components where reactive power consumption is desirable, like transformers. If I connect a power source to a transformer, it would have mainly reactive losses and resistive power loss would actually be undesirable. But still, if we use power factor as a measure of circuit performance, it would be poor in this case as we would have mainly "parasitic" losses.

Not that I think of it, I can't really come up with many applications where purely resistive losses are good (in a resistive heater for one). So what do we really consider as consuming "real power" in this sense? What do we count as consuming "real" power and "parasitic"/reactive power when the circuit receiving power is more complicated than a simple example circuit consisting just a resistor, capacitor and an inductor?

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  • \$\begingroup\$ Real Power does work. Imaginary Power just moves charges around without doing any useful work. Maximizing power factor is about increasing Real Power and/or decreasing Imaginary Power. \$\endgroup\$ – mkeith Feb 16 at 2:16
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Looking at it from outside the box, a device with a power factor lower than unity causes power to be wasted in the distribution of the power between the generation and the device itself. That's assuming resistive losses in the wiring dominate, so the \$I^2R\$ losses are higher than the minimum for a given amount of real power that has to be delivered if there is a reactive component. If there is a lot of reactance, the related energy sloshes back and forth between generator and device each cycle, causing unnecessary losses along the way, and not actually transferring any average power to the devices.

What goes on inside the device is another question altogether, but it certainly needs to receive at least the real power that it consumes (assuming no energy storage) to satisfy conservation of energy.

So if you were designing, say, a large switchmode power supply, you would optimize the power conversion and regulation portion to minimize the losses in the supply (within all your other constraints), but would also likely add power factor correction so that the input current is minimized in normal operation, as much as practical, but certainly enough to meet any relevant standards.

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The design of a power transformer is a trade off in several physical attributes, driven by either the POWER, or CURRENT.

POWER, that is, the real power, is transmitted through the core, and onto customers. So the core needs to be built to a certain size to transmit x amount of power magnetically. Similarly, only the POWER lost in the transformer will cause heating effects. The cooling system needs only be scaled for the Watts of power lost, not the VARs as well. So many pf the physical characteristics will be governed by the POWER. And in the old days, it was physical characteristics that engineers cared about.

In contrast however, the copper windings, they must be built of a correct gauge of wire, or metal-strap in some cases, so to conduct the power in the form of amperes; but the amperes are a function of apparent power, VA. So they will have to be built larger to handle both real and reactive power components of the load and losses.

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Power factor is an important concept in power generation transmission and distribution because it tells the the extent to which real power is being delivered at a higher current level than necessary. The extra current that is used allows energy to continually transfer back and forth between inductive devices such as induction motors and capacitive devices like capacitors and wound-field synchronous generators. If that current flows over a long distance, it results in losses in the inherent resistance of transmission lines and transformer windings. Excess current also means the assets like generators, transformers and transmission lines have some of their capacity wasted.

Induction motors are responsible for the majority of reactive volt-amperes (sometimes calmed reactive power). Transformers and other magnetic devices are generally not considered to be a significant issue in that regard. You might think it would be simple to routinely add power-factor compensation to every induction motor. If you consider all of the details, you will find that is not always the most attractive solution.

If we discontinued the use of induction motors in favor of permanent-magnet motors, that would do away with the power-factor problem. However, permanent-magnet motors generally require electronic controls. That means replacing the power factor problem with a harmonic current distortion problem. That also causes low power factor, but that cause of low power factor can not be simply compensated with capacitors.

As more electric power is provided by means other than wound-field synchronous generators, dealing with low-power factor loads will be increasingly important since WFSGs are the only generator that inherently has some capacity to supply reactive VA.

So what do we really consider as consuming "real power" in this sense?

Most of the electrical power produced is converted to mechanical power by electric motors, mostly induction motors. The resulting mechanical power is used to pump water, blow or compress air or gasses, move materials, change the shape of materials with metalworking and woodworking tools and perform many other tasks. A lot of electrical power is converted to heat for space heating, cooking and industrial processes. The conversion to heat can involve resistive, microwave or inductive heating. Quite a bit of electrical power converted to light for illumination, convey information and provide entertainment. The electricity used to transfer and process information as electrical signals is also significant.

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