Power Factor
What is Power Factor?Power Factor is a measure of how efficiently electrical power is consumed. In the ideal world Power Factor would be unity (or 1). Unfortunately in the real world Power Factor is reduced by highly inductive loads to 0.7 or less. This induction is caused by equipment such as lightly loaded electric motors, luminaire transformers and fluorescent lighting ballasts and welding sets, etc.
One of the ways power factor may be defined is the ratio of true power to apparent power.
The effects of poor (low) power factor are:
- load currents will be higher than they would otherwise be and transformers, conductors and switchgear will have to be sized accordingly (i.e. increased in size)
- to increase the cost of electricity due to increased maximum demand and kVArh charges rendered by the electricity supplier.
In installations having a leading power factor, for example, where manually-switched capacitors are used to improve power factor, there may be an increase of voltage at the load. Where there is a leading power factor for significant periods of time, local switchgear may need to be uprated to account for the increase in current.
What does it do to my electricity bill?
In a 3 phase supply, kW consumed is (VOLTS x AMPS x 1.73 x Power Factor) / 1000. The Electricity Company supplies you VOLTS x AMPS and they have to supply extra to make up for the loss caused by poor Power Factor. When the power factor falls below a set figure, the electricity supply companies charge a premium on the kW being consumed, or, charge for the whole supply as kVA.
What causes Power Factor to change?
Inductive loads cause the AMPS to lag behind the VOLTS. The wave forms of VOLTS and AMPS are then "out of phase" with each other. The more out of phase they become then the lower the Power Factor. Power Factor is usually expressed as Cos Phi. (Ø)
In 3 phase power supplies the "power" can be measured as a triangle.
- ACTIVE Power is the base line and is the real usable power measured in kW.
- REACTIVE power is the vertical or that part of the supply which causes the inductive load. The reactive power in is measured in kVAr (kilo volt-amperes reactive).
- APPARENT Power is the hypotenuse. This is the resultant of the other two components and is measured in kVA.
The effects of power factor
Consider a canal boat being pulled by a horse. If the horse could walk on water then the angle (Phi) Ø would be zero and COSINE Ø=1. Meaning all the horse power is being used to pull the load.
However the relative position of the horse influences the power. As the horse gets closer to the barge, angle Ø1 increases and power is wasted, but, as the horse is positioned further away, then angle Ø2 gets closer to zero and less power is wasted
Why do I need Power factor correction?
Capacitive Power Factor correction (PFC) is applied to electric circuits as a means of minimising the inductive component of the current and thereby reducing the losses in the supply. The introduction of Power Factor Correction capacitors is a widely recognised method of reducing an electrical load, thus minimising wasted energy and hence improving the efficiency of a plant and reducing the electricity bill. It is not usually necessary to reach unity, i.e. Power Factor 1, since most supply companies are happy with a PF of 0.95 to 0.98
How does it work?
By installing suitably sized switched capacitors into the circuit, the Power Factor is improved and the value becomes nearer to 1 thus minimising wasted energy and improving the efficiency of a plant.
Once in place, the cost of PFC can usually be recovered in less than 1 year.
For further information please contact XS Engineering in Paphos, Cyprus.