Power Factor Correction, PFC, Basics

- summary or tutorial about the basics of linear power supplies, their design, operation and advantages and disadvantages.

The power factor and power factor correction, PFC for all forms of mains powered electronic and electrical equipment is an important factor in ensuring the overall system can operate at its maximum efficiency.

The effects of having a poor power factor can lead to inefficient use of power, and as a result they can lead to increase in operating costs and poor performance.

As a result many systems utilise schemes that enable power factor correction, PFC to be applied to ensure that an acceptable power factor can be achieved.

What is power factor?

The issue of power factor arises because for a system using alternating current, AC power, the voltage and current may not always be in phase. If the load is purely resistive, then the current and voltage will be in phase.

Power Factor = 1
Voltage and Current in Phase - Power Factor = 1

If the load is either inductive or capacitive, then the current will not be drawn in phase with the voltage.

Power Factor
Voltage and Current not in Phase

As motors use coils and are inherently inductive, and many electronic power supplies have input transformers, and chokes, they too can be inductive.

In view of these factors, it is necessary to define the power factor of an electronic or electrical system.

It is possible to define the power factor of an electrical system as the ratio of the real power flowing to the load to the apparent power in the circuit. This is a dimensionless number between 0 and 1.

In this definition the real power is the power that would be dissipated in real heat, i.e. measured in watts, and the apparent power is the voltage and current that would be measured by separate meters that would not take into account the phase of the current and voltage waveforms. This is measured in VA.

Simple shifting of the phase between the voltage and current waveforms occurs as a result of inductive or capacitive loads and these are called linear loads. However circuits that use rectifiers or other non-linear components produce non-linear loads and these too can cause power factor issues as they distort the current waveform shape. In this case the apparent power may be greater than the real power.

Power factor correction basics

As a poor power factor results in an increased costs and increased requirement for cabling and other issues, power factor correction is widely applied to large factors, motors and other items.

Even for individual computers and other items of equipment power factor correction can have benefits.

Power factor correction can take a variety of forms:

  • Linear load power factor correction:   Many loads such as motors, transformers and other items present a purely reactive load to the supply, where the phase of the voltage and current is different. Read more about Linear load power factor correction
  • Non-linear load power factor correction:   Many items of electronic and electrical equipment use active devices and not only alter the phase of the current and voltage waveforms, but also the shape of the current waveform. Often items such as bridge rectifiers draw different levels of current over the course of each cycle. This will also present problems to the generation plant and needs to be corrected to ensure the optimum load is presented. In order to correct these forms of power factor issues two approaches can be taken. . Read more about Non-linear load power factor correction

    • Passive power factor correction:   This form of power factor correction uses passive circuit techniques. As passive power factor correction often needs to the distortion products, filtering techniques can be used along with other approaches. Read more about Non-linear passive power factor correction
    • Active power factor correction:   As the name suggests, this form of power factor correction uses active power factor correction techniques. It involves the use of active technology to correct the power factor issues. Read more about Non-linear active power factor correction

The choice of the type of power factor correction used will depend upon the type of power factor correction needed and also the particular application. Performance, cost and the nature of the technical issues will all have a bearing on the approach required to ensure the optimum power factor is achieved..

Power factor correction benefits

There are many benefits to applying power factor correction. The actual benefits and the degree to which they provide a return on investment depends upon the applications, cost and degree to which the power factor correction is required. However the benefits include:

  • Power factor correction can result in a reduction in electricity charges. As charges will be made for VA usage, improving he power factor will mean that a higher wattage can be consumed for the same VA consumptions.
  • Some power companies impose levies on users that have a poor power factor - typically this is only applied to commercial users. Installing power factor correction can reduce or eliminate these levies.
  • An improvement in power factor will reduce the I2R losses of transformers and distribution equipment for a given consumption measured in watts.
  • Power factor correction can lead to a reduction in the heat in cables, switchgear, transformers, etc. because the current level is reduced for a given power level measured in watts.
  • An improved power factor will result in a lower current being consumed for a given wattage power consumption. As a result voltage drop in cables is reduced. This can have allow smaller cables to be used in some instances.

With a much greater emphasis being placed on efficiency, which is driven by cost and also green issues, there are definite benefits to being able to maintain a good power factor as close to unity as possible. Not only does it keep supplier costs down ,but there are benefits in terms of installation costs as well as the running costs in the form of the electricity.

By Ian Poole

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