Power Factor Principles
Power factor (PF), as described briefly on the Power Quality page, can be represented by the power triangle to show the relationship between real power in kilowatts (A), reactive power in kilovolt-amps reactive (B) and apparent power in kilovolt-amps (C):
Real power performs actual work such as heating a burner element or illuminating an incandescent bulb. Reactive power does not perform work but energizes magnetic fields in motor windings or power supplies which create inductive loads. Apparent power is the result of combining real power and reactive power. It measures the true load of an electrical distribution system.
If real power and reactive power exist simultaneously, why can't we just add them together to get apparent power? The reason is that purely reactive current (inductive load) is ninety degrees out of phase with real current (resistive load). Thus, we use the power triangle vectors to graphically represent this 90 degree relationship.
Another way to look at this relationship is to compare the sine waves, or oscillograms, of voltage and current for resistive and inductive loads. In a purely resistive load the current sine wave and voltage sine wave are in sync with one another. The PF in this case is 100 percent or unity.
In a purely inductive load the current sine wave lags 90 degrees behind the voltage sine wave. The PF in this case would be zero.
In the real world there are no purely inductive loads because there is always some amount of work being done by the device even if it is only the generation of heat. On the other hand, purely resistive loads do exist in the real world such as a burner element or incandescent bulb. When energized they have a PF of unity.
Impact of Low Power Factor
Low PF causes an inefficient utilization of electric power. In other words, you are using more current to do the same amount of work when the PF is low. If we take the basic equation for single phase power:
and solve for current we get:
Volts x PF
Voltage is assumed to remain constant in this example. If power is to be maintained, current must go up when PF decreases. This increased requirement for current is where the electrical inefficiency occurs.
Lets look at it graphically with two power triangles:
In the example on the left the PF was measured at 70 percent. If our goal is to produce 100 kilowatts of real power we find that 141 KVA are required. The power triangle on the right shows a PF of 95 percent. In this instance only 105 KVA are required to produce the same amount of real power. Since voltage remains constant, the current must increase by 35 percent to deliver the desired power when the PF is at 70 percent.
Power Factor Correction
Is there a way to correct this inefficient use of current? The answer is yes, by using power factor correction capacitors. These capacitors are wired in parallel with the load. They may be installed at the service entrance of the building or be dedicated to a specific device with a low power factor.
PF correction capacitors are sized by the amount of KVAR they are able to correct. To determine proper sizing, the PF for the building or the device must be measured under normal operating conditions. A target PF such as 95 percent is selected. Using the Pythagorean theorem we can calculate the proper amount of correction as shown here:
If your home energy metering system can measure PF, take advantage of this information. Check the PF on the larger motors in your home such as HVAC compressors and fans or a pool pump if you have one. If it is below 80 percent you may want to consider power factor correction capacitors for these motors.
Limit correction to only your larger motors as power factor correction capacitors can introduce additional harmonic currents into your electrical system. Harmonics can interfere with power line carrier communications which may affect your home energy monitor system. Click here for more information about how PF and harmonics interrelate.
Running motors with a higher PF has its benefits. Using more efficient power can lower operating temperatures which extends bearing and motor life. It also reduces the load on your transformer and decreases the amount of reactive currents circulating in your household wiring although you probably won't see any direct savings on your electric bill.
This is because most residential utility rates only charge for kilowatt-hours (KWh), not kilovolt-amp hours (KVAh) nor do they apply a specific penalty for low power factor. Check the fine print on your power bill to be sure. This may change in the future as residential customers add more inductive loads with electronic power supplies and home automation equipment.
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