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Power Factor and Capacitors

Electrical Efficiency, Power Factor and Capacitors

Power Factor is the name given to reduced power output caused by inefficiency in electric motors. Capacitors are widely used by engineers to reduce efficiency losses (reactive power), power surges and save electricity. Power savers are capacitors that we can plug into our electrical circuits and are claimed to save power. What are they, how do they work and do they actually save power?

Attached to some people's power circuits you may have seen little plastic boxes with a plug poking out and the words “Power Saver” in friendly letters on the outside. You may also see claims that these devices can save up to 40% of your power bill. It all sounds like snake oil stuff doesn't it? Rather like that magic liquid you put in your petrol tank and it fully overhauls your engine, services the brakes and balances the wheels while you drive along. So what are these power savers and more importantly “do they work?”

Power Factor

Let us first consider a thing called “Power Factor”.

Imagine Fred is pulling a box along the ground. Fred is 6 feet tall and has a rope over his shoulder which is tied to the box which is only 1 foot high - the rope is sloping. The box is being pulled horizontally along the ground but he is pulling the rope at an angle. This means some of his energy is wasted pulling the box upwards. Now if Fred was a very small man and the height of his shoulder was the same height as the centre of the box then the rope would be horizontal and all his effort would be pulling in the direction the box is moving - this is more efficient. The horizontal rope has a power factor of 1 and the sloping rope has a power factor less than one - some power is wasted. If we apply this analogy to electrical circuits, Electricity running through an electric motor or transformer creates a magnetic field to:

  1. Create movement (in the case of the motor) or to
  2. Induce an electrical current (in the case of a transformer).

In both cases there is inefficiency, some power is needed to create the magnetic field. This lost power is called “Reactive Power”.

Electrical engineers talk in terms of KVA (kilo volt amps) as the power we put in, KW (kilo watts) as the useful power we get out and a thing called KVAR (reactive kilo volt amps) as the lost power through inefficiency. The ratio of the power we put in in KVA to the power we get out in KW is called the “Power Factor”. It is the efficiency.

Power Factor and Usable Power Output of Generators

In countries where the electrical supply is unreliable many people buy generators. They will have come to know that the size of a generator is given not in KW (kilowatts) but in KVA (kilovoltamps). If you buy a generator you need to know that you must buy one with a power rating greater than you need. Why? Well because a 10 KVA generator may give you an output of only 8 or 8.5 usable kilowatts of power. This is because of the “Power Factor” and the loss of some of the energy in creating the magnetic field to generate the electricity.

Electrical Capacitors

In large industrial plants where many large electric motors are used the loss of power through inefficiencies adds up and can cost a lot of money. Electricity supply companies in many parts of the world calculate the power factor of their customers and penalise them if they are inefficient. To save money and electricity electrical engineers have worked hard at making electric motors more efficient. They have also found a way of cancelling out the reactive power loss.

They use an electrical component which, as everyone knows, is called a capacitor. A capacitor is installed in the electrical circuit and collects spare power then, when fully charged, it can inject a surge of power into the elctrical circuits. The capacity of a capacitor is determined by the amount of power they can collect before they release it.

Capacitors are used in places where a surge of power is required to get things started. They are used to start electric motors and flourescent light tubes.

Capacitors also have the effect of reducing power surges and spikes that can damage equipment and cut out circuit breakers.

Where do we use capacitors?

In an electrical circuit adding a capacitor into the circuit has a tendency to balance out the lost reactive power. This reduces the power consumption and also reduces the power load on the circuit. Large industrial operations use capacitors extensively to reduce power loss in electric motors. It is important that the capacitors are carefully matched to the motors they are connected to. Incidentally electric motors that are not working very hard are a lot less efficient, they have a lower power factor than motors that are working at their hardest.

Anything electrical that works by creating a magnetic field may benefit from the addition of capacitors in the electrical circuit. This means anything with motors such as water pumps, pool pumps, air conditioners, electric fans and electric toothbrushes, or anything with transformers such as flourescent lights, computers and televisions.

Electrical devices that do not produce a magnetic field such as water heaters, electric kettles and hairdryers do not have this reactive power loss and so do not benefit from the use of capacitors.

Household Power Savers

For housing circuits you can buy capacitors branded as “Power Savers” which you simply plug into an electrical plug socket.

How well these devices work depends on many factors and is difficult to determine. Certainly the claim on the packaging of up to 40% is stretching the belief a little.

As has already been stated, in industrial situations electrical engineers calculate where and what should be installed.

A good friend of mine has a small house, he has 1,300 watts and is right on the limit of his power supply. If his well pump switches on while he has his air conditioner running the surge in power makes his PLN circuit breaker cut out. He recently bought a power saver but has not observed any notable difference since he started using it.

It should be noted that with the current PLN difficulties voltages are dropping to very low levels (I recently heard of 130 volts!) and this results in higher currents which can make even robust electrical circuits cut out.

Do Power Savers Work

So do Power Savers work? Well yes they do for Industrial purposes when they are correctly designed. The small boxes you buy for home use are another matter. Not designed for a specific application means they are unlikely to have much, if any, effect. In addition our electrical circuits already have capacitors installed in pumps, air conditioners and flourescent lights.

Savings on electrical bills may also be very questionable and, unless you are running many air conditioners and water pumps, very likely not “major'. My friend found no difference when running his air conditioner and has power cuts just as often as before. He feels he wasted the Rp400,000 investment.

Copyright © Phil Wilson December 2009
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