What is Power Factor, PF

Electrical Power Includes:
What is power     Understanding Power Factor    

The power factor, often abbreviated to PF is a basic concept for electrical systems of all types.

Although power factor is not often mentioned in many arenas it can be a concept that is of considerable importance in many areas of electrical and electronic engineering.

What is power factor

In order to understand what power factor is, it is worth stating with a more formal definition of what it is and then expanding on that to explain further what it is.

It is found that if a circuit has capacitance or inductance in it, then the current and voltage run out of phase with each other.

The actual power dissipated is determined by the instantaneous value of voltage and current, and if the current and voltage are out of phase, then the power dissipated will be less than if they were in-phase.

However the supply, still needs to supply the levels of current and voltage regardless of their phase, and this leads to inefficiency in the system.

A power factor can range between 0 and 1, where 1 is a fully efficient system, and 0 is totally inefficient.

Obviously most systems fall between these two as it's never possible to get a completely efficient system.

It's important to have a good power factor otherwise this can lead to considerably increased energy costs, and this, at a time when energy is not cheap, and wasting it causes additional greenhouse emissions.

What is VA

One of the terms often found on electrical and other items using an alternating supply, is the term VA. A motor may have a rating in terms of VA rather than watts.

The term VA is used as a shortened form to indicate the volts times amps, but without taking regard for the phase difference between them.

The term VA may often be referred to as the apparent power, as it takes the voltage and current readings that would be observed using a multimeter or other meter for measuring these parameters.

This can be applicable to many electrical machines and the readings are often given in terms of kBVA, i.e. thousands of VA.

Calculating power factor

Power factor is calculated by taking the real power, i.e. the power dissipated and dividing it by the apparent power, i.e. the consumption given in terms of VA.

For this equation the two terms are:

Real power (W): This is the actual power consumed by the item - it is converted into useful work, e.g. heat, movement, light, etc. It will be measured by a wattmeter or other similar instrument.

Apparent power (VA): This is the perceived power as gained from a knowledge simply of the current and voltage applied. It does not account for the effects of reactive inputs and the resulting phase shift between the current and the voltage waveforms.

Importance of power factor

Power factor is an important factor in many situations and especially where larger operations are being managed.

• Energy efficiency:   A poor power fact can lead to low levels of energy efficiency. The voltage in an incoming line or mains supply is maintained accurately by the electrical supplier, but with a poor power factor a larger level of current needs to be supplied for a given actual power (wattage) delivered. This can lead to difficulties in generation and low efficiencies in usage.

• Cost implications:   Often, if the power factor falls outside a certain range, then the electricity supplier may add a surcharge as they need to supply larger current values (the voltage is maintained accurately), and this costs them more.

• Equipment performance :   It is necessary to understand the power factor to be able to size the power requirements for overall plant consumption, or for the installation of new equipment. Equipment using motors, transformers, etc needs to be chosen and the supply chosen to match the requirements.

• Environmental impact :   A poor power factor leads to inefficient usage and generation of power. In today's energy conscious society it is important to ensure that maximum efficiencies are achieved.

• Legal requirements:   In some countries there are statutory requirements for operating within certain power factor limits.

Improving power factor

In most countries and with most utility suppliers it is a network requirement that users maintain a specific minimum power factor. The actual figure will depend on a number of factors including the country, supplier and application.

If the power factor of the power consumed by a certain site, etc falls below the minimum requirement, then utility companies may charge customers a penalty payment on top of consumption charges.

To overcome this possibility and as well as improving the efficiency of the site power consumption, it is possible to use items called power factor correction equipment.

Simple power factor correction solutions may be a banks of capacitors that correct the phase difference - most loads tend to be inductive and by adding reactive capacitance, this can reduce the power factor.

These simple power factor correction systems were types that were designed many decades ago when electrical environments were a lot simpler than they are today. These systems are very common, they are also very economical, but they are only suitable for what are termed linear loads: motors and other inductive linear loads.

Modern electrical environments tend to be mode complicated because of the use of many electronically controlled networks: LED/energy efficient lighting, switch-mode power supplies, uninterruptible power supplies or UPS’s, electronic equipment like computer systems and and a host of other items.

These are all far more complex int he loads they present and they are non-linear, meaning that simple capacitor banks are not applicable.

To achieve compensation for these scenarios far more sophisticated power factor correctors are needed. Equipment needs to provide instantaneous, dynamic step-less compensation. Several manufacturers are able to provide the latest in power correction equipment for these scenarios.

Power factor is a critical concept for people working with electrical systems. It can also affect general business because offices using large numbers of computers, etc can have issues with power factor if this is not accommodated suitably.

  Written by Ian Poole .
  Experienced electronics engineer and author.

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