Power Factor Correction

Here is my attempt to present an explanation of Power Factor Correction (PFC).  

This is not meant to be a doctorate so please do not hold me responsible for any wee errors – just email me and discuss and if you wish to use the material please ask my permission first.

Power Factor (PF)

PF is a measure of how efficiently electrical power is consumed.  

The ideal Power Factor is unity – or one. 

Anything less is <100% efficient which means that extra power is required to achieve the task.  

The extra energy is called Reactive Power and it is often necessary to provide the magnetising effect required by motors and other inductive loads to perform their jobs. 

Reactive Power can also be interpreted as watt-less, magnetising or wasted power and an extra burden on the electricity supply.

PFC is the term given to a technology that has been used since the turn of the 20th century to restore Power Factor to as close to unity as is economically possible.  

This is normally achieved by the addition of capacitors to the electrical network, which compensate for the Reactive Power demand of the inductive load and thus reduce the burden on the supply.

Benefits of Power Factor Correction

  • Power Consumption Reduced
  • Electricity Bills Reduced
  • Electrical energy efficiency improved
  • Extra kVA availability from the existing supply
  • Transformer and distribution equipment losses reduced
  • Voltage drop reductions in long cables

How is Power Factor caused?

Most electrical equipment such as motors, compressors, welding sets and even switch start fluorescent lighting, create what is known as an inductive load on the supply.  

An inductive load requires a magnetic field to operate, and in creating such a magnetic field causes the current to “lag” the voltage (that is, the current is not in phase with the voltage).

Power Factor Correction is the process of compensating for the “lagging” current by applying a “leading” current in the form of capacitors.  

This way Power Factor is adjusted closer to unity and energy waste is minimised.

Power factor explained

3-phase AC supplies can be measured in three ways.

  • Active Power – the real usable power available – measured in kW
  • Reactive Power – that part of the supply that creates the inductive load, measured in kVAr
  • Apparent Power – the resultant of the other two components, measured in kVA
  • Power Factor is best expressed as Active Power kW / Apparent Power kVA

A good analogy of Power Factor is to imagine a horse pulling a barge along a canal

Power factor Correction explained

The useful work is the force acting along the line of the canal, which moves the barge by overcoming the resistance of the water in which it floats, analogous to the useful power (kW) in an electrical circuit.  The horse can’t walk on water and must move along the towpath, so the towrope is at an angle to the direction along which the barge must move.

The towrope is always trying to pull the horse sideways into the water and at the same time trying to pull the barge into the canal bank. The horse must walk along at a slight angle and the bargee uses the rudder to keep the craft in the centre of the canal.

The sideways forces act at right angles to the direction the barge intends to travel and are “useless” – analogous to reactive current in the electrical circuit (kVAr).

The towrope itself shows the resultant force created by the useful pull and useless side pull and the length is representative of size of force, or kVA in the electrical version.

The actual value of the term “power factor” is the ratio of the force represented by the diagonal line divided by the total force represented by the towrope. 

Cosine of the angle B of the triangle forces, so power is sometimes also referred to as “cosine B”. 

Lets look at a hypothetical example – grossly over-simplified (and my apologies to the Power Quality Engineers out there):

  • We find a 100kW motor operating at a Power Factor of 0.80
  • The total or apparent power required by the motor is actually 125kVA  (100kW / 0.80).
  • If we can improve Power Factor to 0.95 the total power draw from the supply will be reduced
  • 100kW / 0.95 = 105kVA

Now it is important to make clear that this is nothing to do with true power.  True power is kW and the motor is still rated at 100kW, however, the Apparent Power in this case will be cut by 16% and more importantly, the user of the motor will not be charged by his supplier for Reactive Power, so there may not be an energy saving as such but there will be a financial saving.  Don’t take this as an example of energy savings because I haven’t given you enough information – savings will depend on many factors such as resistance of the wire and transformer supplying the motor.   

The Hidden Cost of Power Factor

Electrical networks with poor Power Factor draw more power than strictly necessary, forcing electricity generators to increase output.

This extra power means extra generating costs which is always passed on to the end user in one form or another.  Most often you will be charged for Reactive Power and it doesn’t come cheaply.  Power Factor Correction will usually show payback within a couple of years.  However, this is not a simple technology and there is much to consider before you install PFC – you need to talk to a specialist. 

There are lots of PFC firms out there are I recommend Enspec Power from personal experience of their service.  If you have any concerns or queries over your electricity charges, or if you know your Power Factor is poor contact me today.