Power factor correction (PFC) is a technique of counteracting the undesirable effects of electric loads that create a power factor (p.f.) that is less than 1. Power factor correction may be applied either by an electrical power transmission utility to improve the stability and efficiency of the transmission network; or, correction may be installed by individual electrical customers to reduce the costs charged to them by their electricity supplier. The power factor of an AC electric power system is defined as the ratio of the real power to the apparent power. ...

Explanation

When an electric load has a p.f. lower than 1, the apparent power delivered to the load is greater than the real power that the load consumes. Only the real power is capable of doing work, but the apparent power determines the amount of current that flows into the load, for a given load voltage. Usually hidden to the unaided eye, the 60Hz blinking of (non-incandescent) lighting powered by AC mains is revealed in this motion-blurred long exposure of city lights. ... Real Power is often described as true power (or effective power) and is used by engineers to describe the actual amount of power present in a system, it is expressed in watts (W). ... Electric current is by definition the flow of electric charge. ...

Energy losses in transmission lines increase with increasing current. Power companies therefore require that customers, especially those with large loads, maintain the power factors of their respective loads within specified limits or be subject to additional charges. Engineers are often interested in the power factor of a load as one of the factors that affect the efficiency of power transmission. Engineering is the design, analysis, and/or construction of works for practical purposes. ...

Power factor correction returns the power factor of an electric AC power transmission system to very near unity by switching in or out banks of capacitors or inductors which act to cancel the inductive or capacitive effects of the load. For example, the inductive effect of motor loads may be offset by locally connected capacitors. It is also possible to affect power factor correction with an unloaded synchronous motor connected across the supply. The power factor of the motor is varied by adjusting the field excitation and can be made to behave like a capacitor when over excited. City lights viewed in a motion blurred exposure. ... shading-coils within the magnetic circuit of the field coil Shaded-pole synchronous motors are a class of AC motor. ...

It is not possible to cancel out harmonic current using these techniques, so different techniques must be used to correct nonlinear loads.

Electricity industry aspects

PFC is desirable because the source of electrical energy must be capable of supplying real power as well as any reactive power demanded by the load. This can require larger, more expensive power plant equipment, transmission lines, transformers, switches, etc. than would be necessary for only real power delivered. Also, resistive losses in the transmission lines mean that some of the generated power is wasted because the extra current needed to supply reactive power only serves to heat up the power lines. It has been suggested that Electric reactance be merged into this article or section. ... A power station (also power plant) is a facility for the generation of electric power. ... A transmission line is the material medium or structure that forms all or part of a path from one place to another for directing the transmission of energy, such as electromagnetic waves or acoustic waves, as well as electric power transmission. ... For other uses, see transformers. ... Electrical switches. ...

The electric utilities therefore put a limit on the power factor of the loads that they will supply. The ideal figure for load power factor is 1, (that is, a purely resistive load), because it requires the smallest current to transmit a given amount of real power. Real loads deviate from this ideal. Electric motor loads are phase lagging (inductive), therefore requiring capacitor banks to counter this inductance. Sometimes, when the power factor is leading due to capacitive loading, inductors (also known as reactors in this context) are used to correct the power factor. In the electricity industry, inductors are said to consume reactive power and capacitors are said to supply it, even though the reactive power is actually just moving back and forth between each AC cycle. A public utility is a company that maintains the infrastructure for a public service. ... Rotating magnetic field as a sum of magnetic vectors from 3 phase coils An electric motor converts electrical energy into mechanical energy. ... Please wikify (format) this article or section as suggested in the Guide to layout and the Manual of Style. ... An inductor is a passive electrical device employed in electrical circuits for its property of inductance. ... Capacitors: SMD ceramic at top left; SMD tantalum at bottom left; through-hole tantalum at top right; through-hole electrolytic at bottom right. ... In typography, leading (IPA , rhymes with heading) refers to the amount of added vertical spacing between lines of type. ... Various types of capacitors A capacitor is a device that stores energy in the electric field created between a pair of conductors on which equal but opposite electric charges have been placed. ...

Electricity utilities measure reactive power used by high demand customers and charge higher rates accordingly. Some consumers install power factor correction schemes at their factories to cut down on these higher costs. This article is about consumers in economics. ...

Application

Electricity industry: power factor correction of linear loads

Power factor correction is achieved by complementing an inductive or a capacitive circuit with a (locally connected) reactance of opposite phase. For a typical phase lagging p.f. load, such as a large induction motor, this would consist of a capacitor bank in the form of several parallel capacitors at the power input to the device. An inductor is a passive electrical device employed in electrical circuits for its property of inductance. ... Please wikify (format) this article or section as suggested in the Guide to layout and the Manual of Style. ... ...

Instead of using a capacitor, it is possible to use an unloaded synchronous motor. This is referred to as a synchronous condenser. It is started and connected to the electrical network. It operates at full leading power factor and puts VARs onto the network as required to support a system’s voltage or to maintain the system power factor at a specified level. The condenser’s installation and operation are identical to large electric motors. Capacitors: SMD ceramic at top left; SMD tantalum at bottom left; through-hole tantalum at top right; through-hole electrolytic at bottom right. ... shading-coils within the magnetic circuit of the field coil Shaded-pole synchronous motors are a class of AC motor. ... A synchronous condenser is fundamentally an AC synchronous motor that is not attached to any driven equipment. ... This article or section does not adequately cite its references or sources. ... The power factor of an AC electric power system is defined as the ratio of the real power to the apparent power. ... volt-amperes reactive (VAR): In alternating-current power transmission and distribution, the product of the rms voltage and current, the apparent power, multiplied by the sine of the phase angle between the voltage and the current. ... International safety symbol Caution, risk of electric shock (ISO 3864), colloquially known as high voltage symbol. ... Rotating magnetic field as a sum of magnetic vectors from 3 phase coils An electric motor converts electrical energy into mechanical energy. ...

The reactive power drawn by the synchronous motor is a function of its field excitation. Its principal advantage is the ease with which the amount of correction can be adjusted; it behaves like an electrically variable capacitor. Reactive power is an abstract quantity, typically used by power engineers to describe a certain type of energy flow in an electric distribution system. ...

Switched-mode power supplies: power factor correction of non-linear loads

A typical switched-mode power supply first makes a DC bus, using a bridge rectifier or similar circuit. The output voltage is then derived from this DC bus. The problem with this is that the rectifier is a non-linear device, so the input current is highly non-linear. That means that the input current has energy at harmonics of the frequency of the voltage. A switched-mode power supply, switch-mode power supply, or SMPS, is an electronic power supply unit (PSU) that incorporates a switching regulator — an internal control circuit that switches power transistors (such as MOSFETs) rapidly on and off in order to stabilize the output voltage or current. ... A diode bridge is an electronic circuit that provides the same polarity of output voltage and current for both possible polarities of input power. ... AC, half-wave and full wave rectified signals A rectifier is an electrical device, comprising one or more semiconductive devices (such as diodes) or vacuum tubes arranged for converting alternating current to direct current. ... In acoustics and telecommunication, the harmonic of a wave is a component frequency of the signal that is an integer multiple of the fundamental frequency. ...

This presents a particular problem for the power companies, because they cannot compensate for the harmonic current by adding capacitors or inductors, as they could for the reactive power drawn by a linear load. Many jurisdictions are beginning to legally require PFC for all power supplies above a certain power level.

The simplest way to control the harmonic current is to use a filter: it is possible to design a filter that passes current only at the frequency of the voltage (e.g. 50 or 60 Hz). This filter kills the harmonic current, which means that the non-linear device now looks like a linear load. At this point the power factor can be brought to near unity, using capacitors or inductors as required. This filter requires large-value high-current inductors, however, which are bulky and expensive. In acoustics and telecommunication, the harmonic of a wave is a component frequency of the signal that is an integer multiple of the fundamental frequency. ... Television signal splitter consisting of a hi-pass and a low-pass filter. ... The word linear comes from the Latin word linearis, which means created by lines. ...

It is also possible to perform active PFC. In this case, a boost converter is inserted between the bridge rectifier and the main input capacitors. The boost converter attempts to maintain a constant DC bus voltage on its output while drawing a current that is always in phase with and at the same frequency as the line voltage. Another switchmode converter inside the power supply produces the desired output voltage from the DC bus. This approach requires additional semiconductor switches and control electronics, but permits cheaper and smaller passive components. It is frequently used in practice. Due to their very wide input voltage range, many power supplies with active PFC can automatically adjust to operate on AC power from about 100 V (Japan) to 240 V (UK). That feature is particularly welcome in power supplies for laptops and cell phones. An Active Power Factor Corrector (active PFC) is a power electronic system that controls the amount of power drawn by a load in order to obtain a Power factor as close as possible to unity. ... A boost converter (step-up converter) is a power converter with an output dc voltage greater than its input dc voltage. ...

Types

Active PFC

An Active Power Factor Corrector (active PFC) is a power electronic system that controls the amount of power drawn by a load in order to obtain a Power factor as close as possible to unity. In most applications, the active PFC controls the input current of the load so that the current waveform is proportional to the mains voltage waveform (a sinewave). Power electronics is the technology associated with the efficient conversion, control and conditioning of electric power by static means from its available input form into the desired electrical output form. ... The power factor of an AC electric power system is defined as the ratio of the real power to the apparent power. ...

Some types of active PFC are

1. Boost
2. Buck
3. Buck-boost

Active power factor correctors can be single-stage or multi-stage.

Passive PFC

This is a simple way of correcting the nonlinearity of a load by using capacitor banks. It is not as effective as active PFC. Switching the capacitors into or out of the circuit causes harmonics, which is why active PFC or a synchronous motor is preferred.

Synchronous

Synchronous motors can also be used for PFC. In the past, shaftless motors were made to which no load could be connected and which ran free on the line at capacitive (leading) power factor for the purposes of PFC.

See also

• Power factor correction unit

A power factor correction unit is equipment used to improve power factor, in industrial networks. ...

References

• A. K. Maini "Electronic Projects for Beginners", "Pustak Mahal", 2nd Edition: March, 1998 (India)

External links

• Lessons In Electric Circuits - POWER FACTOR
• ON Semiconductor Power Factor Correction Handbook