Two-minute explainer: Power Factor and LEDs

Power Factor is one of those esoteric subjects spoken about by electrical engineers in gloomy rooms once the bars have closed. It first came to my attention back in the early 1970s when I was learning my trade. It made sense at the time, but it does require that you to believe electricity can go sideways, because it’s all about triangles.

There’s power, and then there’s Power

There is ‘useful’ power, which is what we need for things to work, known by convention as ‘Real Power’ and there is energy that is wasted because of inefficiencies within the electronic circuitry. Combining this wasted energy with Real Power gives us ‘Apparent Power’. Dividing Real Power by Apparent Power gives us the Power Factor value. Obviously, we would prefer there to be no wasted energy, so that Real Power and Apparent Power have the same value and the Power Factor would be 1.0. From a practical standpoint that’s not always possible, but we can do what is necesary to reduce the difference between Real and Apparent values by applying Power Factor correction.

This is where the triangle and the sideways electricity comes in:

Draw an imaginary line representing Real Power  in the middle of your computer screen, running from the bottom of the screen towards the top. This represents our (useful) Real Power. If we then draw a horizontal line from the bottom of the vertical line for a short distance, that line represents the energy being wasted in the circuitry. The hypotenuse of those two lines gives us the Apparent Power value.

In LED luminaires, poor Power Factor is a consequence of poor circuit design in the drivers controlling the LEDs. Distortions are created in the waveforms between the voltage and current within the system and it is the level of distortion that determines the degree of energy loss.

This effect is not an inevitable feature of LED lighting, its a result of commercial decisions taken to keep down costs. 

And here’s the practical upshot of all of this. 

Power Factor has always been a tricky concept to explain. Here’s my favourite anaology: the barge being towed along the canal. This involves a barge, a boatman, a horse and a length of rope. The horse is on the towpath pulling the barge along the canal, fastened to the bow by the rope. The boatman is at the stern with the tiller, steering the barge along the middle of the cut. Because the horse is at an angle to the barge, the action of the tiller makes pulling the barge that much harder. The extra work having to be done by the horse represents the Power Factor of the barge.Now, if the rope was longer, the horse would not have to work so hard because it would be nearer to the centre-line of the barge. The additional length of the rope represents Power Factor Correction. 

A domestic electricity meter will only read the Real Power load, so Power Factor doesn’t appear to have any impact on our bills. But any wasted energy due to poor Power Factor does have to be accounted for back at the power plant by the burning of more fuel. And as we look more towards renewable energy and the potential for battery storage of electricity, the Apparent Power requirement of a device becomes vitally important. And we can’t afford to waste energy in this way.

How does this relate to LEDs? Many low wattage LEDs in the domestic range have no Power Factor correction. The ‘natural state’ of an LED circuit could be as low as 0.3, certainly less than 0.5. That means, using the cosine formula, that a 5W LED lamp (claimed wattages represent only the Real Power, of course) could actually be anything from 10W to 16W, which is a scandalous situation.

Away from the generating station, there is a local consideration. While the electricity meter may not measure the wasted energy, the circuit protection will. Lets consider two 20 watt LED flood lights; one with a Power Factor of 0.55, and the other with a Power Factor of 0.95. Factoring in the effect of Power Factor, the current drawn from the mains supply will be 0.092 Amps for the high power factor fixture, and 0.16 Amps for the low power factor fixture.

Let’s assume that the electrical circuit has been designed using a 6 Amp circuit breaker. Using the high Power Factor fixture means that 65 fixtures could be installed, against only 37 fixtures at the low Power Factor.

Considering that current also dictates the cable size and voltage drop, we understand that low Power Factor can have a serious impact on the size of cables, length of cable runs and the number of circuits needed. As we move more towards a totally LED lighted environment, these are issues that need to taken very seriously.

Conclusion

Power Factor Correction is not difficult to achieve. No LED should have a Power Factor of less than 0.9 and we look forward to that being the absolute minimum for all LED fixtures. We may have a wait on our hands, but the argument is under way with some passion.

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