Typical values ​​Power factor

Typical values ​​Power factor

Typical Power Factor Values ​​(cosen φ)

This guide is an indispensable resource for anyone involved in the design, analysis, or management of electrical and energy systems. Power factor is one of the fundamental concepts in electrical engineering, and its understanding is crucial to ensuring the efficiency, stability and quality of electrical energy supplied or used in a wide range of applications.

Power factor measures the efficiency with which an electrical system transfers active (useful) energy relative to apparent energy (sum of active and reactive energy). A low power factor can lead to energy losses, inefficiencies in the use of electricity and overloads on electrical networks. On the other hand, a high power factor indicates efficient use of electrical energy.

Within this guide, we will explore a number of typical power factor values ​​for a wide range of electrical and electronic equipment and systems. We will provide you with a detailed overview of power factors commonly encountered in different contexts, including electric motors, electronic devices, industrial equipment and power systems.

These typical power factor values ​​are an essential resource for electrical system engineers, technicians, and operators, enabling them to make accurate estimates of energy consumption, identify potential inefficiencies, and plan strategies to improve energy efficiency.

Understanding power factor is critical to ensuring the proper functioning of electrical systems, reducing energy costs and promoting overall energy efficiency. This guide will provide you with practical knowledge and a clear view of typical power factors, helping you make informed decisions in managing electricity.

Typical values ​​Power factor

The power factor (cos φ) of an electrical load is defined as the cosine of the phase shift angle φ between the voltage V and the power supply current I of the load itself in an alternating current electrical system.

In an electrical system with a purely resistive load the phase shift is zero (cos φ = 1).

 This is the ideal situation: the apparent power VA corresponds to the active power W and the reactive power VAR is zero.

In an inductive type system (electric motor, fluorescent lamp, etc.) with the cos φ lower than 1, the reactive (parasitic) power is not zero. If it reaches high values, the possibility of proceeding with an appropriate power factor correction of the system must be taken into consideration (electricity suppliers charge additional costs to customers of industrial or commercial users who have a power factor below a certain limit - usually 0,9, since this affects the efficiency of the transmission lines - to consider that with cos φ = 0,7, the losses in the circuit would be almost doubled, since they are proportional to the square of the current).

Furthermore, all the components of the system (generators, cables, transformers) should be larger in size to carry the greater current necessary, with an obvious increase in costs.    

The power factor is measured with an instrument called "Cosphimeter".

Cosphimeter

Typical values ​​Power factor

Difference between Active, Reactive and Apparent power:

Active (real): it is the one actually consumed by a load – indicated by W;

Reactive: being an exchange energy between the power supply line and the inductive load, it does not generate consumption – it is indicated with VAR;

Apparent: it is the sum (in quadrature) between the active and reactive power – indicated by VA.

In purely resistive circuits with particular users (incandescent bulbs, water heaters, ovens, etc.), the apparent power absorbed is all active power (cos φ = 1).

In circuits with users that have internal windings capable of creating variable magnetic fields (motors, welders, fluorescent lamp power supplies, transformers, etc.), part of the absorbed power is not used as active power W but as reactive power VAR (cos φ <1).  

Triangle of powers

W - active power

VAR - reactive power

VA - apparent power

φ - phase shift angle

How to correct the power factor (cos φ tending to 1)

The power factor (cos φ) of linear loads can be corrected through a passive network of capacitors (power factor correction capacitors) in order to have a value as close as possible to 1, a necessary condition to indicate that all the energy supplied by the source is consumed by the load.

To correct the power factor (re-phase the system), it is necessary to supply reactive energy of opposite sign (adding capacitors that cancel the inductive or capacitive effects of the load).

The devices for the correction of the power factor (phase plugs) can be placed in a centralized position of the electrical system, scattered along it, or inserted inside the single inductive loads. 

Power factor correction

Typical values ​​of the power factor (cos φ)

The power factor (cos φ) measures a phenomenon that occurs when loads are not purely resistive (such as a lamp or stove) but include electrical windings or capacitors.

These loads store the energy in the form of a magnetic field (in the case of wire windings) or an electric field (for capacitors) and return it cyclically to each half-wave to the network, without actually “consuming” it.

A part of the current then passes "back and forth", can be measured, but does not really contribute to the true consumption in Watts (Active Power W).

This is why the true consumption is always lower than the value in VA (Apparent Power VA).

There is a phase shift between voltage and current which is expressed by a value called cos φ (the trigonometric cosine of the angle between the voltage and current vectors, called by the Greek letter φ).

The cos φ or power factor can vary between 0 and 1.
Active power (W) is expressed in Watts and is equal to:

Active Power (W) = Apparent Power (VA) x power factor (cos φ) - W = VA x cos φ - where V and A are the Volts and Amps respectively

Typical values

Equipment

Power factor cos φ

Asynchronous motor with load factor at 0% 0,17
Asynchronous motor with load factor at 25% 0,55
Asynchronous motor with load factor at 50% 0,73
Asynchronous motor with load factor at 75% 0,8
Asynchronous motor with load factor at 100% 0,85
Incandescent lamps 1
Uncorrected fluorescent lamps 0,5
Power factor corrected fluorescent lamps 0,93
Discharge lamp 0,4 – 0,6
Resistance ovens 1
Induction ovens 0,85
Dielectric loss furnaces 0,85
Spot welder 0,8 – 0,9
Arc welding powered by a single-phase static group 0,5
Arc welding powered by a rotating group 0,7 – 0,9
Arc welding powered by transformer / rectifier group 0,7 – 0,8
Arc furnaces 0,8

Formulas

V Voltage Volt V
I Current Ampere A
R Resistance Ohm W
P Potenza Watt W
Power factor Phase displacement n cos φ
P = V x I x cos φ
P = V x I x cos φ x 1,73
Power factor =  cos φ =  P active = VI cos φ
Apparent P VI

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Typical values ​​Power factor

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