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Section 5.7.3 Power Factor Correction: Greening Federal Facilities; Second Edition Section

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5.7.3
Power Factor Correction
Power factor correction capacitors are designed
to provide the reactive current needed by inductive
loads. Capacitors may be installed to improve the power
factor of a single load or an entire power system and
come in sizes from 1 to 600 kVARs.
Automatic power factor correcting equipment
switches banks of capacitors on- and off-line depend-
ing on the power factor. These may provide good solu-
tions in applications where reactive loads vary in mag-
nitude over time.
Locate capacitors upstream of motor controllers
unless full-voltage, nonreversing, across-the-line start-
ers are used.
Replace standard motors with energy-ef-
ficient motors that have high power fac-
tor ratings. Note that even high-efficiency mo-
tors will have poor power factors under low load
conditionsand that efficiency is more impor-
tant than power factor. Be sure not to sacrifice
efficiency for power factor. Avoid operating equip-
ment above its rated voltage. Minimize opera-
tion of lightly loaded or idling motors.
Shut down a lightly loaded motor in situations
where a smaller, parallel motor can do the same
job. For example, when chilled water demand drops,
parallel pumps may be removed from service until loads
increase.
Be aware that installing power factor correction
capacitors on the load side of a motor-overload pro-
tection device may require reducing the overload size.
The capacitor manufacturer will have tables to assist
you in resizing.
Avoid oversizing capacitors installed on the load
side of motor controllers because they can discharge
into the motor when the controller is turned off. Dam-
aging voltages may occur if kVAR current exceeds
motor no-load current.
Note that power factor correction saves money
in three basic ways:
Avoided power factor penalties from the utility
(where applicable).
Freed capacity in supply transformers if such ca-
pacity is needed.
Induction motors, magnetic ballasts, and transform-
ers require two types of power to operate. Active power
(also called true or real power) produces work or heat,
is used by all electrical devices, and is expressed in
kilowatts. Reactive power is used by inductive devices
to generate magnetic fields. It does not perform useful
work and is expressed as kVARs (kilovolt-amps reac-
tive). Total power, or apparent power, is the vector sum
of active and reactive power and is expressed in kVA
(kilovolt-amps). A power factor is the ratio of active
power to total power and quantifies the portion of power
used by a facility that does electrically useful work.
Power companies generally charge an additional fee
to facilities having power factors less than 8595% in
order to capture costs to the utility company that are
not reflected by the electric energy (kWh) meter. Im-
proving the power factor can increase current-carry-
ing capacity, improve voltage to equipment, reduce
power losses, and lower electric bills.
Opportunities
Efforts should be made to improve power factors if (1)
power factors are below 9095% and penalties charged
by the electrical utility are high, (2) electrical prob-
lems within the facility can be eliminated by improv-
ing the power factor, or (3) installing larger transform-
ers for capacity needs can be deferred. Power factor
improvements should be considered whenever electri-
cal equipment such as motors and lighting are being
upgraded or replaced.
Technical Information
Electric motors are large contributors to poor
power factors because many generally operate un-
der light loads. Lower power factors do not necessarily
increase peak kVA demand because of the reduction in
load. For example, the power factor of an electric mo-
tor is lowest when the motor is lightly loaded. This
occurs when both its power draw and contribution to
the electrical peak demand is the least. Section

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Reduced I
2
R resistive losses in wiring, etc., provided
the capacitors are located close to the inductive
loads. Kilowatt-hour savings of less than 0.5% are
typical, and savings of 12% would be the high range
for typical commercial and industrial systems.
Beware of applications where there are
significant harmonics (VFDs and other
nonlinear loads). The harmonics can cause reso-
nances with the capacitors and damage them.
If harmonics exist, consider harmonic filters,
which also typically improve power factor.
Do not exceed manufacturers recommendation
on maximum capacitor size.
Install high-power-factor lighting and electronic
equipment. While motors garner most of the atten-
tion regarding power quality, lighting equipment and
other electronic products can also have a significant
impact on power factor. With lighting, ANSI classifies
ballasts with power factors above 0.90 as high power
factor (HPF). Magnetic ballasts often have far lower
power factors (0.50 is typical with some products), as
do many types of office equipment (desktop comput-
ers, monitors, laser printers, etc.). When data on power
factor are available, specify and buy high-power-fac-
tor products.
Power factor is less than one when en-
ergy is quickly stored and released in a
piece of equipment so that the voltage and cur-
rent are out of phase by the angle .
Power factor = watts = Cos volts*amps
Additional power is not consumed, but bigger
wires and transformers are required to handle
the additional amps needed by the load. Low
power factors of large inductive loads, such as
motors, can be improved by adding capacitors
to the load. Current through a capacitor has the
effect of cancelling out the lagging current.
References
Energy-Efficient Motor Selection Handbook (DE-B179-
93-B08158), Bonneville Power Administration, 1993.
Reducing Power Factor Cost, Technology Update,
Bonneville Power Administration, April 1991.
Morgan, Robert, Improving Power Factor for Greater
Efficiency, Electrical Construction and Maintenance,
September and November 1994.
Power Quality, Lighting Answers, Vol. 2, No. 2, Feb-
ruary 1995; National Lighting Product Information
Program, Lighting Research Center, Rensselaer Poly-
technic Institute, Troy, NY; (518) 276-8716; www.
lrc.rpi.edu/NLPIP. (This report is available as a down-
loadable file.)
This automated control and protection system includes
capacitor banks and harmonic filter banks.
Photo: Northeast Power Systems, Inc.