What is a power factor, and how does it affect your plant’s utility bill? In electrical engineering, power factor (PF) is the ratio of real power to the apparent power flowing to the load from the source. From a business standpoint, it’s important to understand how having a low PF raises your plant’s power bill. Identifying this value is the first step toward using corrective techniques to raise PF for substantial savings and greater equipment efficiency.
Power factor is measured between 0 and 1.0 — usually given as a percentage, with 100% or 1.0 being unity — and is usually judged as either leading or lagging, depending on the position of the current waveform with respect to the voltage. If your facility’s PF is below a certain level, typically 96%, your power provider will charge a reactive power fee. This is because a low PF represents an inefficient load source that is drawing reactive, or non-working, power. The utility needs to compensate for this. Unless your facility can raise its PF to 96% or above, you’ll continue to see this extra charge every month on your bill.
For maximum efficiency, power in an ac circuit is best used when the voltage and current are in alignment. However, in the real world much of your electrical equipment is probably delaying as it draws current, meaning that the current and voltage are instead in misalignment. In this case, your equipment has a level of inefficiency depending on how misaligned it is, causing it to draw more current to operate. Therefore your PF value reveals how efficiently your ac power system and equipment are using power.
How is power factor calculated?
An ac circuit’s PF is calculated using three aspects of its electrical power as they relate to one another, these being:
- real power — power used to run equipment, expressed in kW
- reactive power — power that doesn’t produce work, expressed in kVAr (as your reactive power use increases, your electric system loses more energy; hence, the reactive power fee)
- apparent power — the combination of real power and reactive power, expressed in kVA.
You can use these three aspects to derive the PF (Figure 1).
Figure 1. An ac circuit’s PF is calculated using the relationships between real power, reactive power, and apparent power.
In an electric power system, a load with a low power factor draws more current than a load with a high power factor (near 100%) for the same amount of useful power transferred. These higher currents increase the energy lost in the distribution system and also require larger wires and other equipment. In other words, your PF percentage shows you how much of the total current is being used to do real work. A PF of 80% means that a full 20% of the current is non-working power. Again, because of the costs of larger equipment and wasted energy, electrical utilities will usually assign a penalty fee to industrial or commercial customers if they have a low power factor.
A high power factor is generally desirable in a transmission system to reduce transmission losses and improve voltage regulation at the load, so it’s often beneficial to correct the power factor of a system to near 100%. When reactive elements supply or absorb reactive power near the load, the apparent power is reduced.
Motors driven by variable speed drives will use the same power as before, but may draw more current. Note that with reduced stored energy in the dc bus capacitors, they may be more vulnerable to power dips.
Benefits of PF correction
Lower electricity bills: PF correction is an actionable way to lower your utility bills. Savings can range from hundreds to tens of thousands of dollars per year, depending on the size of your facility.
Avoid utility reactive power fees: Utility companies routinely charge reactive power fees to consumers with low power factors (less than 96%). For example, this can result in your bills increasing by up to 20%, depending on which company is supplying your electricity.
Reduce carbon emissions: By utilizing power factor correction you can also lower the amount of carbon emissions released into the atmosphere. This can be another great source of savings.
Reduce power loss: Reduce I2R losses in transformers and electrical distribution equipment.
Reduce heat: Achieve heat reduction in cables, switchgear, transformers, and alternators, which in turn prolongs the lifespan of this equipment.
Reduce voltage drop: Allow the same cable to supply a larger motor and improve the starting of motors located at the end of long cable runs. This also helps to avoid motor failure and damage to your equipment.
How can you raise your power factor?
To avoid reactive power fees and improve equipment efficiency, you can raise your power factor by applying several different power factor correction techniques. Individual electrical customers who are regularly charged by their utility for a low PF often install correction equipment to reduce or remove these costs. Power factor correction brings the power factor of an ac power circuit closer to 100%, such as by supplying reactive power of the opposite sign by adding capacitors or inductors that act to cancel the inductive or capacitive effects of the load, respectively.
To begin with, there are a few simple methods you can use to raise your PF without buying expensive devices. For example, check your existing equipment to see if any pieces are operating above the voltage for which it’s been rated. You can also cut back on how often your plant is running motors with a light load and avoid running idling motors for extended periods.