POWER FACTOR IN APPLIANCES

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A poor power factor can cause voltage fluctuations and power quality issues for neighbouring facilities, which negatively affects their equipment. It also limits the capacity of power lines to deliver energy to you and other customers. To counteract these effects utilities need to install capacitors on power lines system to maintain power quality for all customers on the line.

When your power factor is below 100% your facility is drawing both reactive power and real power.

Power capacitors are used in distribution systems to supply reactive volt amperes (Vars) to the system. When applied to a system or circuit having a lagging power factor, utilities can obtain several beneficial results. These results include power factor increase, voltage increase, system loss reduction, and an increase of electric system capacity.

To find out how much power an appliance consumes, energy auditors occasionally multiply line voltage by the current reading obtained from a clamp-on ammeter. However, depending on the appliance, this simple calculation will not always reflect true power usage.

The problem is that some loads draw more current than others to deliver a given amount of usable power. To determine the actual power consumed by an appliance, it is necessary to take account of its power factor.

The power factor is the value of the actual (or true) power used by an appliance divided by the expected (or apparent) power–the latter being the product of the line voltage (V) and current (I).

PF = true power 
(Vx I)

Power factors range between zero and one and are usually expressed as a percentage. A value of 100% indicates that all the electricity drawn by a load is being consumed. Electric-resistance heaters and incandescent bulbs are examples of appliances that have a power factor of 100%. Taking a 60W light bulb as an example, the wattage represents the actual power consumed by the bulb. Given light bulb connected to a standard 120V outlet with an ammeter measuring 0.5 amperes flowing through the circuit. In this case, the 60 watts of power could be calculated by multiplying 120 volts by the 0.5 ammeter reading.

EquipmentPower FactorLagging/Leading
Incandescent lights100%In phase
Fluorescent lamps (ballast)60-80%Lagging
Fluorescent lamps (solid state)80–90%Lagging
Heating devices (all types)100%In phase
Induction motors (loaded)80%Lagging
Induction motors (light load)20%Lagging
Neon lights30-70%Lagging
Synchronous motors (underexcited)VariesLagging
Synchronous motors (overexcited)VariesLeading
Static condensers0%Leading

Small air conditioners used in homes and small business have power factors as good as 98% and as bad as 55%. The “ old school” designs with pure single phase induction motors are the ones with poorest power factor (ranging 55% to 75%). The ones with run capacitors are much better, they usually work near PF = 1 (PF ranging 85% to 100%). The modern variable capacity air conditioner will present very good power factor in any condition, they have built in power factor correctors (they will work with PF over 90% all the time). Some of the variable capacity air conditioners produced more than 10 years ago will present low power factor (70%) because they have no means of power factor correction.

Early computer power supplies were found to have such a low power factor that new designs were created so that the industry could reduce the wastage. The EPA Energy Star program mandated even more redesign and computer power supplies today have a power factor that is 99.5% or better.

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The typical high end PC PSU now uses a LC filter to prevent high frequencies from being fed back into the grid. Then the AC is fed into a bridge rectifier before it is routed into main circuit. There is a large capacitor the is charged by a pulse width modulation that keep the voltage perfectly set regardless of load.

Electric motors, fluorescent lamps, and consumer electronics (such as televisions and computers) are examples of appliances that have power factors of less than 100%. This is because they include some type of storage element–such as a capacitance or inductance–or create distortions in the voltage/ current waveforms.

Neon lights used to be very common but they were expensive and tended to break down frequently. The very low power factor wasted so much power that new signs using fluorescent lights paid for themselves quickly. Now LED lighting is slowly replacing the fluorescent lights in homes and offices.

In addition to providing a quantity of horsepower, the motor used to run a refrigerator compressor will temporarily store electricity in its inductor–that is, in the turns of wire wrapped around the metallic core. The motor’s power factor is reduced because the load draws current that is not temporally in phase with the voltage waveform.

The supply voltage for a motor at 120 volts and the measured current at 5 amperes. Notice that the product of these values (600 volt-amps) does not equal the power measured by the wattmeter (400 watts). This is because the motor inductor is storing current and then returning it back to the line. The amount of returned current depends on how much work is being done–that is, how much mechanical energy (horsepower) is being performed by the motor. In this case, the power factor is 0.67 or 67% (400/600). This means that the motor requires 50% more current for a given amount of power than a load with the same true power and a power factor of one.

Inductor motors can be corrected with synchronizing devices, such as capacitors. However, low power factor can also be caused by distortions in the shape of the waveforms, known as harmonic distortion. Such problems are commonly associated with consumer electronics and electronic ballasts and are not as easily corrected.

If you want to measure true power, and you are unsure of an appliance’s true power factor, it is better to use a wattmeter than to try to calculate power usage from an ammeter reading. Low cost monitoring devices are widely now available.