POWER FACTOR CORRECTION
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Let me attempt to present a simplified explanation of this terminology - this is not meant to be a doctorate so please do not hold me responsible for any wee errors - just email me and discuss! |
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Power Factor is a measure of how efficiently electrical power is consumed. The ideal Power Factor is unity - or one. Anything less than one, (or 100% efficiency), means that extra power is required to achieve the actual task at hand. This extra energy is known as Reactive Power, which unfortunately is necessary to provide a magnetising effect required by motors and other inductive loads to perform their desired functions. However, Reactive Power can also be interpreted as watt-less, magnetising or wasted power and an extra burden on the electricity supply. |
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Power Factor Correction 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. |
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Benefits of Power Factor Correction
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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. |
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Power factor explained. 3-phase AC supplies can be measured in three ways.
Power Factor is best expressed as Active Power kW / Apparent Power kVA |
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A good analogy of Power Factor is to imagine a horse pulling a barge along a canal. 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". |
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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. |
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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. |
