W Va

The leading contributor to low power factor in most facilities is the standard ac induction motor. Full-load power factor ratings typically range from 0.75 for small motors to 0.90 for large motors. Any other equipment with magnetic coils and fields, such as induction furnaces, also have power factors of less than unity, or 1.

Power factor varies with the load at any point in time. Most facilities have power factors ranging from 75 to 95%. When there is more motor power on line and less lighting, for example, power factor will be lower. Power factor will sometimes decrease with the load on a given motor. In general, for open drip-proof motors, power factor decreases as the percent full-load on the motor decreases. Because VAR from inductance and capacitance are in opposite directions, capacitors can be added to a line to cancel inductive VAR and improve power factor.

Purchased power from most utilities is measured in kW. A lower power factor penalizes the utility because it requires more current-carrying capacity in the generator for a given real-power demand, indicated by kW billing, and for resistance losses in the conductors on their system. This is why utilities often require a minimum power factor (ranging from 80 to 90%) from customers. Some utilities include power factor penalties as part of demand charges. Some utilities simply bill in kVA. Both strategies shift the burden of power factor correction onto the customer. Utilities that do not account for power factor simply compensate through the capacity and energy components of the rate structure. When electricity is generated on-site, poor power factor reduces the effective output of the generation equipment by reducing the generator efficiency.

Another advantage of improving power factor is that reduction of reactive current may allow for added load (growth) on the electric system without the need for upsizing transformers, main feeders, and bus ducts. Operating-cost savings potential also lies in the reduction of resistive power loss due to current flow in the conductors. All of these cases may provide sufficient impetus to cost-effectively maximize power factor.

A capacitor compensates for the inductive magnetizing current locally by producing capacitance kVARs that cancel out the inductance kVARs. Capacitors typically come in various voltages in discrete increments of 30 kVAR. Number and total capacity is determined by economic analysis. They can be installed together at any point past the utility metering device, in groups connected at the electrical center of power feeders or on individual motor terminals. Ideally, they should be as close to the load as economically possible.

Another way to improve power factor is with the use of synchronous motors. These motors can be designed with unity or leading power factor. When operating at a leading power factor, these motors are referred to as synchronous condensers. The leading power factor can be adjusted to balance the effect of the lagging power factor of induction motors. This practice, however, is quite costly relative to installing capacitors.

The benefits of improved power factor can be summarized as follows:

• Reduction in size requirements and capital cost for transformers, main feeders, and bus ducts.

• Reduced power losses.

• Reduced loads on transformers and motors, resulting in longer life and lower failure rates.

• Reduced rates where electricity is billed in kVA and elimination of power factor penalties where they exist.

• Reduction in voltage drop at the facility, which tends to improve the overall voltage profile.

Reactive Power and On-Site Generation

Power factor becomes more complicated with the interconnection of on-site generators. The power output of a generator is controlled by varying the torque applied by the prime mover to its shaft. The kVAR output is controlled by varying the field of excitation. If an on-site generator is overexcited, it produces kVAR as well as kW. This kVAR flows into the facility's load to provide excitation current, reducing the amount of kVAR the loads draw from the utility system. This serves the same function as installing capacitors to correct facility power factor.

Interconnected on-site generators that operate in parallel with the utility can use voltage regulators as power factor correction devices to constantly adjust their excitation according to the power factor. The excitation level is trimmed to control the flow of kVARs from the generator so that the generator's power factor rating is not exceeded.

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