What is Power Factor?
It is the mathematical ratio ofACTIVE POWER (w) to APPARENT POWER (VA)
ACTIVE POWER
pf =
APPARENT POWER
-- - - -
*-
I
cos0
VA
pf angle in degrees = cos 0
ACTIVE POWER = W = “real power” = supplied by the power system to actually turn the motor.
REACTIVE POWER = VAR = (W)tan 0 = is used strictly to develop a magnetic field within the motor.
NOTE: Power factor may be “leading”or “lagging”depending on the direction of VAR flow.
CAPACITORS can be used to improve the power factor of a circuit with a large inductive load. Current through
capacitor LEADS the applied voltage by 90 electrical degrees (VAC), and has the effect of “opposing” the inductive
“LAGGING” current on a “one-for-one” (VAR) basis.
WHAT IS POWER FACTOR (pf)?
ACTIVE POWER (W)
FIGURE 24 POWER FACTOR DEFINED
Note: The growing use of Adjustable Speed Drives (ASD s) (nonlinear loads) has increased the complexily of system power
factor and its corrections. The application of pf correction capacitors without a thorough analysis of the system can
aggravate rather than correct the problem, particularly iftheflifth and seventh harmonics are present.
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20HP 3PH 6QHZ 176ORPNI 460V BALDQR
.-
FIGURE 25 AMPS, EFF, KW, PF, RPM, TQ CURYES FOR 20 HP MOTOR.
2OOHP 3PH 60HZ 1760RPM 460V BALDQR
RPM AMPS KW
%PF
%EFF
1800
480
300-
1790
400
250
100
1780
320
200
80
!77@
240
?50
5C
1760
150
100
40
1750
80
50
20
1740
0
0
0
I
FIGURE 26 AMPSs E F E KW, PF, RPM, TQ CURVES FOR 200 HP MOTOR.
MOTORMANAGER
Page 24 of 89
I
&USE POWER FACTOR (pf)?
i-WHY
Low (or “unsatisfactory”) power factor is caused by the use of inductive (magnetic) devices and can indicate possible low
-.-..-
system electrical operating efficiency. These devices are:
non-power factor corrected florescent and high intensity discharge lighting fixture ballast (40%-80% pf)
arc welders (50%-70% pf)
t
I
-
solenoids (20%-50% pf)
induction heating equipment (60%-90% pf)
lifting magnets (20%-50% pQ
small “dry-pack” transformers (30%-95% pf)
and most significantly, induction motors (55%-90% pf)
Induction motors are generally the principal cause of low power factor because there are so many in use, and they are usually
not fully loaded. The correction of the condition of LOW power factor is a problem of vital economic importance in the
generation, distribution and utilization of a-c power.
M% JOR BENEFITS OF POWER FACTOR IMPROVEMENT ARE:
increased plant capacity
reduced power factor “penalty” charges for the electric utility
--
improvement of voltage supply
less power losses in feeders, transformers and distribution equipment.,
\
WHERE TO CORRECT POWER FACTOR?
\
WHERE TO CORRECT POWER FACTOR
.-.
t
i
5
r
$
2
h
1
FIGURE 2 7 WHERE TO CORRECT POWER FACTOR
t
Capacitor correction is relatively inexpensive both in material and installation costs. 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 utilization equipment and the capacitor will remain unchanged. Capacitors are usually added at
each piece of offending equipment, ahead of groups of small motors (ahead of motor control centers or distribution panels) or
at main services. Refer to the National Electrical Code for installation requirements.
The advantages and disadvantages of each type of capacitor installation are listed below:
it
Page 25 of 89
MOTORMANAGER
Capacitor on each piece of equipment (1,2)
ADVANTAGES
increases load capabilities of distribution system.
can be switched with equipment; no additional switching is required.
better voltage regulation because capacitor use follows load.
capacitor sizing is simplified
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.L
capacitors are coupled with equipment and move with equipment if rearrangements are instituted.
DISADVANTAGES
small capacitors cost more per KVAC than larger units (economic break point for individual correction is generally at 10
HP).
Capacitor with equipment group (3)
ADVANTAGES
increased load capabilities of the service,
reduced material costs relative to individual correction
reduced installation costs relative to individual correction
DISADVANTAGES
switching means may be required to control amount of capacitance used.
Capacitor at main sewice (43, & 6)
ADVANTAGES
low material installation costs.
DISADVANTAGES
switching will usually be required to control the amount of capacitance used.
does not improve the load capabilities of the distribution system.
OTHER CONSIDERATIONS
Where the loads contributing to power factor are relatively constant, and system load capabilities are not a factor, correcting
at the main service could provide a cost advantage. When the low power factor is derived from a few selected pieces of
equipment, individual equipment correction would be cost effective. Most capacitors used for power factor correction have
built-in fusing; if not, fusing must be provided