Understanding the Power Factor

Power factor is a measure of how effectively you are using electricity. Various types of power are at work to provide us with electrical energy. Here is what each one is doing.

Working Power – the “true” or “real” power used in all electrical appliances to perform the work of heating, lighting, motion, etc. We express this as kW or kilowatts. Common types of resistive loads are electric heating and lighting.

An inductive load, like a motor, compressor or ballast, also requires Reactive Power to generate and sustain a magnetic field in order to operate. We call this non-working power kVAR’s, or kilovolt-amperes-reactive.

Every home and business has both resistive and inductive loads. The ratio between these two types of loads becomes important as you add more inductive equipment. Working power and reactive power make up Apparent Power, which is called kVA, kilovolt-amperes. We determine apparent power using the formula, kVA2 = kV*A.

Going one step further, Power Factor (PF) is the ratio of working power to apparent power, or the formula PF = kW / kVA. A high PF benefits both the customer and utility, while a low PF indicates poor utilization of electrical power.

Here is an example.

A steel stamping operation runs at 100 kW (Working Power) and the Apparent Power meter records 125 kVA. To find the PF, divide 100 kW by 125 kVA to yield a PF of 80%. This means that only 80% of the incoming current does useful work and 20% is wasted through heating up the conductors. Because Laurens Electric must supply both the kW and kVA needs of all customers, the higher the PF is, the more efficient our distribution system becomes.

Improving the PF can maximize current-carrying capacity, improve voltage to equipment, reduce power losses, and lower electric bills.

The simplest way to improve power factor is to add PF correction capacitors to the electrical system. PF correction capacitors act as reactive current generators. They help offset the non-working power used by inductive loads, thereby improving the power factor. The interaction between PF capacitors and specialized equipment, such as variable speed drives, requires a well designed system.

PF correction capacitors can switch on every day when the inductive equipment starts. Switching a capacitor on can produce a very brief “over-voltage” condition. If a customer has problems with variable speed drives turning themselves off due to “over-voltage” at roughly the same time every day, investigate the switching control sequence. If a customer complains about fuses blowing on some but not all, of their capacitors, check for harmonic currents.

Correction of power factor with capacitors

Power factor is the relationship (phase) of current and voltage in AC electrical distribution systems. Under ideal conditions current and voltage are “in phase” and the power factor is “100%.” If inductive loads (motors) are present, power factor less than 100% (typically 80 to 90% can occur).

Low power factor, electrically speaking, causes heavier current to flow in power distribution lines in order to deliver a given number of kilowatts top an electrical load.

The effects?
The power distribution system in the building, or between buildings, can be overloaded by excess (useless) current.

Generating and power distribution systems owned by Laurens Electric have their capacity measured in KVA (kilo amps).

KVA = VOLTS X AMPS X 1.73 (three phase System) / 1,000

With unity power factor (100%), it would take 2,000 KVA of generating and distribution network capacity to deliver 2,000 KW. If the power factor dropped to 85%, however, 2, 353 KVA of capacity would be needed. Thus we see that lower power factor has an averse effect on generating and distribution capacity.

Low power factor overloads generating, distribution, and networks with excess KVA.

If you own a large building, you should consider correcting poor power factor for either or both of these reasons:

  • To reduce the possibility of additional power factor charges in event that Laurens Electric starts billing for PF corrections and
  • To restore the (KVA) capacity of overloaded feeders within the building or building complex.

There are several methods of correcting lower power factor. Commonly used are:capacity.

Capacitor Banks

The most practical and economical power factor correction device is the capacitor. It improves the power factor because the effects of capacitance are exactly opposite those of inductance.

The var of KVAR rating of a capacitor shows how much reactive power the capacitor will supply. Since this kind of reactive power cancels out the reactive power caused by inductance, each kilovar of capacitance decreases the net reactive power demand by the same amount. A 15 KVAR capacitor, for example, will cancel out 15 KVA of inductive reactive power.

Capacitors can be installed at any point in the electrical system and will improve the power factor between the point of application and the power source. However, the power factor between the load and the capacitor will remain unchanged. Capacitors are usually added at each piece of offending equipment, ahead of groups of motors (ahead of motor control centers or distribution panels) or at main services.

Switched Capacitors

Plants equipped with very large, intermitted inductive loads, such as large motors, compressors etc., may require switched capacitors; that is, capacitors are connected to individual motors or groups of motors. Therefore, they are only in action when the motor load is turned on. Or, capacity may be switched on and off at the substation, depending on measured power factor. The switching feature is only required if the capacitors needed are so large that they cause an undesirable leading power factor during times when large motors are turned off.

For more information, read the “Reducing Power Factor Cost” Fact sheet (pdf) published by the U.S. Department of Energy. Note: Adobe Acrobat Reader is required to view and print pdf files.