What is Power Factor and How Does It Affect My Choice of Energy Data Logger?

Sine Wave

Photo courtesy of wstryder(CC Attribution)

What is Power Factor?

Power Factor is a measurement affecting AC (Alternating Current) electrical circuits, which are the type normally used by everyone on the electricity grid (which is most of us). The Power Factor is an important part of electricity billing, and it plays a big part in electricity companies’ planning and design of grids.

To understand Power Factor you first need to understand a little about how electricity works, and in particular AC electricity.

Boy at Water Pump

Electricity works like pumping water through a pipe.. Photo courtesy of dok1(CC Attribution)

Understanding Electricity

The easiest way to understand basic electricity voltage and current, is by analogy with water pipes. Imagine water flowing through a pipe. The “electrical voltage” is then analagous to the “water pressure” in the pipe. The “electrical current” is analogous to the “water flow” through the pipe. 

For a more concrete example, imagine a pipe with one end open (analogous to connecting up the end of a wire to electrical “ground”), and the other end connected to a pump (analogous to connecting the other end of the wire to a battery), and water is sitting in the pipe. As soon as you switch on the pump (connect it to the battery) it applies pressure to the pipe (ie. voltage), then the water will flow out of the open end (ie. current will flow) and fall onto the ground. 

If you want the water (electricity) to do some work, say to turn a small paddle wheel mounted in the pipe (analogous to an electric motor), the water will need to flow through the wheel and make it turn. For this to happen you will need to apply pressure, and the water will need to flow through the load, to do the work. Hence we calculate “Power” as being Voltage (pressure) x Current (flow). “Power” basically represents the amount of work being done by the water (electricity) at any instant.

Understanding AC

Now for AC (Alternating Current) electricity, it works the same way but instead of a steady voltage (think pressure), its voltage is constantly swinging between positive and negative. So it’s like the water pump is repeatedly switching between pushing water forward and pulling water back. The water will then flow forward, then backward, then forward again, then backward again, and so on. Of course for this to work in a physical pipe, the pipe would have to be shaped as a closed loop – like a closed electrical circuit – then the water can flow back and forth freely.

If you then put your load (paddle wheel) in the circuit (water pipe), the paddle wheel will spin forwards, then backwards, then forwards etc. So the pump is transferring energy to the paddle wheel, via the water pipe. This is analogous to an electric power station transferring energy to a motor, via the cable in an electrical circuit.

Understanding Power Factor

Continuing the analogy, if the paddle wheel is heavy, you might find that it takes time to get it moving. So by the time the pump has finished its forward-turning cycle, the paddle wheel has barely started turning forwards. Then the pump will pull the water back, and the paddle wheel will have to slow down, then start turning backwards. Hence the paddle wheel will constantly “lag” behind compared to how the pump is pushing the water, and it will not turn as fast as the water wants it to.

This means that although it seems like there is “Power” being transferred to the paddle wheel, it is in fact not doing as much work as it should, because it is so slow to start spinning. 

Hence we have the concept of “Real Power”, which is the actual work being done by the paddle wheel, and this is less than the “Apparent Power” which is the pressure (voltage) multiplied by the flow (current), or the work that should have been done if the paddle wheel was not heavy.

This is exactly how it works in AC electrical circuits: the mains power applies an alternating Voltage (pressure), and when this is connected to a load (such as a motor), the load allows Current (water flow) to flow, but the current that actually flows might lag behind compared to the timing of the Voltage (pressure). When there is a lag like this, the amount of real work done by the motor is less than it would otherwise have been if there was no lag. So we have a difference between Real Power (the actual work done by the motor) and Apparent Power (the work that would have been done if there was no lag).

Power Factor, then, is defined as this ratio: the ratio of “Real Power” compared to “Apparent Power”. So if the Power Factor (PF) is 1, the Real Power is equal to the Apparent Power and there is no lag, and no reduction of work. If PF is less than 1 then there is a lag (or possibly a lead) which puts the timing out, and results in less work being done.

How does Power Factor affect me?

The main area in which Power Factor affects us is in electricity billing. Most homes and small businesses are billed on their Real Energy usage, which is Real Power over time. So if your average Power Factor is less than 1, say it might be 0.8, then you will only be billed for 80% of the actual measured current usage. Of course this is good for you, because you are only being billed for Real Power, in other words Useful Work done by the electricity.

Hence if you install fluorescent lighting, for example, these lights have a power factor much less than 1, so even though they might appear to be using a significant amount of electrical current, they are actually only using very little Real Power, and hence your bill will be cheaper as a result of the power factor.

Checking an Energy Data Logger

Photo courtesy of NAVFAC(CC Attribution)

How does Power Factor affect my choice of Energy Data Logger?

When choosing an energy data logger for measuring and recording your electricity usage, it is important to choose one that measures and takes into account Power Factor. 

Some low end instruments will only measure the Current, and they then assume that the Voltage is fixed (which it mostly is). They would also have to assume that the Power Factor is 1, as they are not connecting to the voltage and comparing its timing with the current flow. Many electrical appliances and light fittings have a power factor less than 1, so that kind of instrument can give you a very wrong impression of your electricity usage.

If you want to get an accurate measurement of power usage, then, you will need an energy data logger that measures both the Voltage and the Current usage, and correlates the timing between the two to measure the Power Factor. This is the only way to obtain a correct measurement of the True Power or True Energy.

About Chris Dobbie

Chris Dobbie is an experienced Systems Engineer focused on Industrial Technology. As the owner of Esis (Sydney, Australia) he has had exposure to a wide range of industrial electronic equipment in a variety of applications, and also has extensive system design and C/C++ programming experience. Contact Chris if you want to chat about your project!

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