Difference between revisions of "Power factor"
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Some loads, eg electric heaters, have a power factor of 1. The current drawn is proportional to the voltage at any instantaneous moment, ie maximum current flows at voltage peaks of the mains supply. | Some loads, eg electric heaters, have a power factor of 1. The current drawn is proportional to the voltage at any instantaneous moment, ie maximum current flows at voltage peaks of the mains supply. | ||
− | So a 1kW heater on 240v consumes 1000/240 = 4. | + | So a 1kW heater on 240v consumes 1000/240/1 = 4.2 Amps |
But some loads, such as motors, behave a bit differently, having a power factor of less than 1. With a 0.8 PF motor, the current drawn lags behind the voltage by a tiny fraction of a second, so the peaks in current draw occur after the voltage peaks. | But some loads, such as motors, behave a bit differently, having a power factor of less than 1. With a 0.8 PF motor, the current drawn lags behind the voltage by a tiny fraction of a second, so the peaks in current draw occur after the voltage peaks. | ||
− | A 1kW motor with 0.8 pf on 240v draws 1000/240 | + | A 1kW motor with 0.8 pf on 240v draws 1000/240/0.8 = 5.2A |
Notes: | Notes: | ||
* Power factor (PF) is a concept that only applies to electrical loads being powered from an AC supply. To try to apply it to a dc supply is meaningless. | * Power factor (PF) is a concept that only applies to electrical loads being powered from an AC supply. To try to apply it to a dc supply is meaningless. | ||
− | * Domestic electricity users are only charged for that part of the current that produces real power, so in the motor example above the user only pays for 1kW, not 5.2A. | + | * Domestic electricity users are only charged for that part of the current that produces real power, so in the motor example above the user only pays for 4.2 or 1kW, not 5.2A. |
* It quantifies what proportion of the apparent power flowing into a load, is actually dissipated as real power. | * It quantifies what proportion of the apparent power flowing into a load, is actually dissipated as real power. | ||
Revision as of 13:48, 27 April 2012
What is Power Factor
Power = Voltage x Current x Power factor.
Some loads, eg electric heaters, have a power factor of 1. The current drawn is proportional to the voltage at any instantaneous moment, ie maximum current flows at voltage peaks of the mains supply.
So a 1kW heater on 240v consumes 1000/240/1 = 4.2 Amps
But some loads, such as motors, behave a bit differently, having a power factor of less than 1. With a 0.8 PF motor, the current drawn lags behind the voltage by a tiny fraction of a second, so the peaks in current draw occur after the voltage peaks.
A 1kW motor with 0.8 pf on 240v draws 1000/240/0.8 = 5.2A
Notes:
- Power factor (PF) is a concept that only applies to electrical loads being powered from an AC supply. To try to apply it to a dc supply is meaningless.
- Domestic electricity users are only charged for that part of the current that produces real power, so in the motor example above the user only pays for 4.2 or 1kW, not 5.2A.
- It quantifies what proportion of the apparent power flowing into a load, is actually dissipated as real power.
What is the relevance of this to DIY?
There are occasions where it may crop up in a DIY setting. The most common cases are when designing circuits, and when taking electrical measurements. In circuit design, a low power factor increases circuit currents flow, and hence larger cable conductors sizes will be needed. When attempting to measure current flow or voltage drop, a low power factor may result in you getting erroneous readings, as it may fool many test meters.
Its relevant to:
- Loads such as motors and LPF fluorescent lighting on circuits where their uncompensated current draw can exceed the circuit's ampacity
- Tool use on generators
- Plug-in motorised appliances that can exceed 13A without PFC
- Use of droppers
- Large lighting circuits using CFLs
- Sales blurb of dubious energy saving gadgets
- Industrial electricity use where users pay for uncorrected PF
- Knowing which type of fluorescent lighting ballast to buy
- Use of motors and other below 1 PF loads on invertors
Terminology
- PF
- Power factor
- PFC
- Power factor correction
- LPF
- Low power factor
- HPF
- High power factor
Reactive power
With a DC supply, the power dissipated by a load is proportional to its resistance and the voltage applied to it. We have some simple relationships that we can apply.
In the simplest cases, the same calculations also apply to loads being powered from an AC source (like the mains). A resistive load being driven from the mains, will draw current that is in sympathy with the mains voltage; as it rises, the current will rise, and as it falls it will fall. At the zero crossing point it will be zero.
However things can get more complicated due to the effect of "reactive" elements in the load. These are typically components that have an inductance or capacitance. Inductors and capacitors actually store electrical energy (although in different ways). As you might imagine, having something that stores energy being fed from an AC supply, will cause it to charge and discharge in response to the ever changing applied voltage. So during one part of the mains cycle it can be "absorbing" energy, while in another it gives the stored energy back again. Reactive elements in a load can make the types of calculations that are easy to apply to simple resistive loads somewhat more difficult, because you can no longer assume that the current drawn will align with the alternating voltage.
Capacitive load
Inductive load
A similar situation exists with an "Inductive" load. Although inductors store energy like capacitors, they do so in a magnetic field around a coil of wire. Since the wire is coiled, the magnetic field produced by the current flowing in the wire will also interact with adjacent coils. As the current flowing through the coil changes, the associated magnetic field changes. This changing field will induce a current flow in the wire. However the induced current flow caused by the changing field, is in the opposite direction to the flow in the circuit. So the inductor tries to oppose changes in current flowing through it, by counteracting them using its stored energy. This means inductors will offer a low resistance path to stable voltages, but increase their resistance to changing ones - the opposite of a capacitor.
The main difference resulting from this is that the current waveform lags the voltage waveform rather than leads it. (see diagram to the right)
How is a power factor expressed?
A power factor is expressed as a number between 0 and 1. A power factor of 1 (aka a "unity power factor") basically says the power in a load can be treated like a normal resistive load, and ohms law applies. A PF of 0 is a perfect reactive load, or one that actually dissipates no real power at all.
The power factor of our load can be expressed as:
Real Power Power Factor = -------------- VA So in our capacitive and inductive examples above, the power factor would actually be zero.
A load like this that is totally "reactive" would be unusual. In real world situations, loads can have both resistive and reactive components (and of those reactive components, the capacitive ones will have leading, and the inductive ones lagging phase shifts).
To compute the actual current flow into a load like this at any given time therefore requires "vector" arithmetic to take into account not only the reactance of the leading and lagging components, but also their phase shifts.
Composite loads
In the real world, electrical loads are rarely just reactive, they are usually combined with a resistive element as well. This can give rise to the situation where there is a misalignment between the alternating voltage and the current drawn as before, but not as pronounced. If you look at the resulting power graph, you can see that in this example, most of the power flowing into the load is power transfer that is doing some useful work, but a small proportion is being returned as reactive power.
An appliance like an electric fan heater may present a load like this. The bulk of the current drawn will be supplying the resistive heater element. However a small amount will be driving the fan typically driven by an induction motor.
Other causes of low power factor
The above examples show the classical cause of non unity power factors. A typical real word example of a load that has a less than unity power factor as a result of these phase shift effects is an induction motor. Here a significant proportion of the current flow into the motor is actually reactive and does not get dissipated as work. So a power factor of 0.5 would not be uncommon. Standard linear strip lights are another common load with a poor PF.
There are other causes of low power factor. One such example is where current only flows in the load during some but not all of the mains cycle. giving rise to a non sinusoidal current waveform.
Can you change or compensate for a poor power factor?
Yes, you can do what it called power factor correction (PFC). While this is worthwhile in an industrial setting where customers are usually charged based on their VA loading rather than their real power loading in watts, it is less often useful in a domestic one where the meter will give a reasonable indication of the actual power consumption regardless of the PF.
In the case of poor PFs caused by current phase shifts, you can add other reactive components to the load to try and offset the effects of the reactive components in the load. So if you had a reactive bank of fluorescent strip lights (which have an lagging power factor due to their inductive ballasts), adding a capacitor to create a leading reactive element can actually cancel out the effects of the inductors.
- Note that recent EU legislation has stipulated that larger SMPS (i.e. over 25W) must now include PFC. However there will be lots of legacy equipment in use that does not include this for some considerable time.
How to correct PF
- Inductive lagging load: add the right sized capacitor in parallel with the load. If the load can't be relied on to discharge the cap when unplugged, also add a bleeded resistor across the cap.
- Capacitive leading load: add a suitably specced inductor in parallel with the load. Its very rare that capacitive loads need compensation
- Rectifier capacitor load: These can be corrected with either active electronic circuitry or use of relatively large passive components. Both require electronics expertise and are outside the scope of DIY.
Note that components used for PFC need to be calculated correctly. Do not just hook up whatever's in the junk box.
Does a low power factor mean I am using more electricity?
Sort of. In the sense you will be drawing more current that you would with a high PF. However in a domestic situation a poor power factor will not result in you being charged for more electricity.
Poor power factors are bad for distribution efficiency though, and can result in the mains supply waveform getting misshapen and noisy - so power supply companies tend to penalise big industrial users if they don't control their power factors.
If you want an analogy, imagine riding a bike up hill. You stick a certain about of push into the pedals to keep it moving overcoming resistance, and more to add the energy you are acquiring by climbing the hill. Imagine someone attaching a big spring to one pedal and the seat post, such that every time you push the right pedal down you also need to stretch the spring. As you can imagine this will take more "push" from you to keep riding. However that extra push is only required on the right pedal. When you push the left pedal you have the energy stored in the spring pulling up on the right pedal and hence working for you. So the result is the bike is harder to ride, but the total energy required to get up the hill is actually the same. This is similar to the effect of having a poor power factor as a result of large reactive elements in the load - the load still dissipates the same amount of energy, but it is harder to drive (i.e. needs more peak current flow).
Generators
Generators are rated by VA rather than power in watts. So a 1kW 0.8pf drill consumes 1.25kVA under load, and the generator needs to supply this.
Connecting a suitable capacitor would allow the above tool to run on some generators that couldn't quite run it without PFC.
Use of tools on generators is more complex than this, as
- many tools also consume well above run current during startup.
- invertor and non-electronic generators behave quite differently with overcurrents, the latter handling them much better.
What about this device that claims to save money?
For industrial users of electricity, PFC can and does save them money. This has lead to some makers of gadgets and gizmos to promote them to domestic customers for the same reason. Alas, PFC alone is unlikely to save you anything in electricity, since the standard UK electric meters only measure the real power used, and aren't fooled by reactive currents that may also be drawn from (and returned to!) the supply.
See also
- For a very comprehensive explanation of this topic, I recommend this site.
- Category: Electrical
- Category: Appliances
- Wikipedia article