World Grain - March 2024 - 58

MILLING OPERATIONS
Energy cost management:
POWER FACTOR
Exploring ways to improve PF and reduce penalties on electric bills
by Jeff Gwirtz
As presented in my article on energy cost management in the
November 2023 issue of World Grain, there are many hidden
charges in the electrical bill that impact milling energy costs.
Contractual discussions with the mill's electrical suppliers as
well as internal operational management decisions can reduce
demand and peak demand charges.
In addition to basic consumption and demand charges, there
are often fixed charges associated with simply being a customer
and having a meter that must be monitored and maintained.
Various taxes, fees and penalties may be incurred, including
late payment fees as well as Power Factor penalties, which are
the focus of this article.
What is Power Factor?
Power Factor (PF) is the ratio of true working power delivered
by the utility (kW) power and the apparent power (kVA),
which represents the demand of the plant performing useful
work. The formula is PF = kW/kVA where kW is measured as
kilowatt and kVA is kilovolt amperes, also known as demand.
PF is a measure of how efficiently incoming power is used.
A load with a low PF draws more current than a load with a
high PF for the same amount of useful or true working power
delivered. Energy loss in the distribution system is greater
and requires larger wires and other equipment. Electric utilities
usually will charge a higher fee or penalty to industrial
customers with PF lower than 0.95 and in some instances are
not required to provide power to customers whose PF drops
below 0.85.
Figure 1 (right) presents a graphical relationship between
working, apparent and reactive power. As shown, the efficiency
or PF is the cosine of the angle between working and
apparent power.
The relationship between the real or useful power and apparent
or demand power is shown in Table 1 (page 60). PF is
the ratio that is dimensionless proportion that some may convert
to a percentage to express efficiency of power utilization
based on power delivered by the utility. The phase difference
between the current and voltage measured in degrees is shown
in the last column.
As shown in Figure 2 (page 60), this phase difference is the
58
result of utilizing alternating rather than direct current.
PF is not applicable to direct circuits (DC) as current, or voltage
frequency, is zero. The current and voltage relationship
in alternating currents (AC) is shown in Figure 2. Alternating
currents have a frequency, generally 50 or 60 HZ (cycles per
second) PF that is always between unity -1 and 1, the cosine
of the phase difference between V (voltage) and A (current in
ampere).
A Leading Power Factor range of -1 to 0 occurs in purely
capacitive circuits where current leads the voltage. A lagging
Power Factor in the range of 0 to 1 occurs in purely inductive
circuits where the current lags the voltage.
Lagging Power Factor is where most industrial and milling
operations perform. When current and voltage are in this
phase, there is no leading or lagging PF as the PF is 1 or -1,
which is only possible in ideal circuits. Power Factors of -1 or
1 indicate there is no reactive power consumed, which is the
power used by the circuit, transformers and motors needed to
produce or result in magnetic fields.
Often it is desirable to compare AC circuits that are constantly
varying in both voltage and current to an equivalent steady
DC circuit. The comparison is accomplished using RMS (Root
Mean Square) to determine the DC power delivery equivalent
of an AC current based on power delivery capabilities.
RMS voltage is calculated by squaring the voltage waveform,
finding the average over a single period, and taking the
square root of the result. For this article, we can focus on a
single-phase AC current as shown in Figure 2. PF provides an
Figure 1: Relationship between working,
apparent and reactive power.
kVA (real power)
March 2024 / World Grain / World-Grain.com
kVA (apparent power)
kVA (reactive power)
http://www.World-Grain.com
World Grain - March 2024

Table of Contents for the Digital Edition of World Grain - March 2024
