Q7.
What
is
Power
Factor
and
how
do
you
correct
a
bad
Power
Factor?
A7.
Power
Factor
Essentially,
power
factor
(PF)
is
a
simple
notion.
It
is
a
measurement
of
the
efficiency
of
an
alternating
current
power
system,
that
is,
the
proportion
of
its
energy,
which
is
available
for
useful
work
in
a
machine
or
electrical
load.
It
may
be
better
understood
by
taking
the
following
mechanical
analogy.
If
a
Barge
is
being
towed
along
a
channel
by
a
short
rope
then
the
large
angle
between
the
tow
line
and
the
direction
of
travel
means
that
much
less
of
the
applied
force
is
available
for
forward
travel
-
it
has
a
poor
"power
factor".
By
using
a
longer
towline,
the
angle
will
be
less
and
the
"power
factor"
will
be
improved.
The
problem
does
not
lie
with
the
power
supply
itself;
electrical
utilities
go
to
great
lengths
to
supply
"high
quality
electricity".
This
problem
is
caused
by
the
effects,
which
certain
types
of
load
have
on
the
power
supply.
The
theory
behind
this
effect
show
that
most
machinery
introduce
a
phase
difference
between
the
current
and
voltage
waveforms.
This
means
that
the
electrical
load
cannot
convert
all
the
supplied
electricity
into
useable
mechanical
energy.
To
make
up
for
this,
the
load
takes
more
current
than
it
really
needs.
In
theory
this
current
just
circulates
and
is
not
used
by
the
load
-
so
where's
the
problem?
Let's
look
at
another
analogy.
The
supply
authority
fills
our
bath
with
electrical
power,
we
drain
off
that
power
to
use,
but
the
bath
has
a
small
leak
-
bad
power
factor.
The
supply
authority,
in
the
past,
charged
the
consumer
for
the
power
drained
off
but
now
we
are
being
charged
for
the
power
supplied
to
fill
the
bath.
This
means
that
the
consumer
has
to
bare
the
cost
of
the
leakage
and
for
many
plant
managers
a
poor
power
factor
has
long
been
seen
as
just
one
of
the
necessary
evils
of
an
engineer's
life.
Now
he
has
to
improve
the
bad
power
factor
or
pay
the
extra
cost.
Power
Factor
Correction.
To
understand
power
factor
correction
we
need
a
quick
refresher
on
the
basics
of
a.c.
theory.
First
we
need
to
recall
that
in
a
purely
resistive
(
R
)
circuit,
carrying
an
alternating
current:
V
=
I
x
R
where
I
and
V
are
in
phase.
In
such
a
circuit,
the
current
and
voltage
are
exactly
in
phase
and
all
the
power
supplied
by
the
current
is
delivered
to
the
load
-
the
resistance.
However,
in
a
purely
inductive
(
L
)
circuit,
the
current
I
lags
the
voltage
V
by
90
degrees,
whereas
in
a
purely
capacitive
(
C
)
circuit,
the
current
I
leads
the
voltage
V
by
the
same
amount.
But
what
happens
when
we
mix
R,
C
and
L
in
a
simple
series
circuit?
The
resultant
phase
angle
between
I
and
V
will
be
a
function
of
the
total
impedance
(
Z
)
in
the
circuit,
and:
V
=
I
x
R
x
Pf
In
the
more
common
inductive
circuit
(resistance
R
plus
inductance
L),
the
inductance
element
causes
a
lagging
effect
(I
toV)
or
inefficient
phase
angle
-
normally
0.7
to
0.8
PF.
To
counter
this
effect
and
improve
the
phase
angle
closer
to
unity
(1.0),
we
introduce
a
capacitive
C
load
element.
This
capacitance
can
be
of
a
fixed
value
or
it
can
be
switched
into
circuit
when
the
individual
inductive
load
is
switched.
Clearly,
it
makes
sense
to
have
the
power
factor
as
close
to
unity
as
possible
to
keep
the
cost
of
power
used
as
close
as
possible
to
the
charged
cost
of
power
supplied.
It
boils
down
to
this:
the
addition
of
capacitors
to
a
network
supplies
the
reactive
energy
component
of
the
load.
Upstream
of
the
capacitors,
therefore,
reactive
demand
is
reduced.
The
result
is
the
reduction
of
total
power
and
an
improvement
in
power
factor.
contact
Controlpak
<<Back
