Reactive Power Primer Video Lecture - Electrical Engineering (EE)

I'll bet I can teach you about an
element of electric power that you don't
know even exists or if you do know give
you a new perspective on it
I'm sure you've heard of a watt which is
the basic measure of work in an electric
circuit it's your usage of watts that
drives your electric meter and results
in your monthly bill but did you know
that there are two other forms of
electrical energy circulating in your
wires
called reactive current or wot less
power or foam on beer or any one of a
number of colorful names these two
elements are circuit loads that heat up
wires you know system capacity and cause
voltage problems although you don't
notice them in your household circuits
they're there all the time
the first of these loads comes from the
requirement of some equipment to operate
with magnetic fields mainly motors and
transformers
when an electrical current flows through
a wire two things happen to the wire it
heats up you know that right and the
wire becomes a magnet as this video clip
shows that's right the wire becomes
magnetic and that's a good thing mostly
because that's why motors work and it's
what allows transformers to do their
thing which is to raise or lower voltage
okay quick review what happens to a wire
when current flows through it which
choice do you think is correct
remember it warms up from the friction
of moving charges and it gets magnetic
because the moving charge aligns
magnetic forces already in the wire
well it takes energy to create that
magnetic field around the wire so when
current starts to flow in a circuit part
of the energy that went into that
circuit goes into creating the magnetic
field instead of showing up as watts
part of the energy is dispersed in the
heat too
if the circuit is DC that's pretty much
the end of the story for the magnetic
field but most circuits are not DC
they're AC which means the voltage and
current rise and fall periodically 60
times a second in this neighborhood
if the voltage and current rise and fall
so does the magnetic field since it
comes from the movement of current so
now 60 times a second energy is taken
from the circuit to make the field and
then added back in when the AC current
changes direction and the field
collapses
but it's not an even exchange the energy
is added back in alright
but it's added back in right at the time
the current is trying to change
direction and it delays the change
you heard me in an AC circuit current
and voltage changed direction sixty
times a second but the addition of this
magnetic reactive energy delays the
change in current actually throws
voltage and current out of sync
the voltage and current sine waves are
no longer rising and falling in step and
part of the energy applied to the
circuit doesn't make watts
it makes the magnetic field the reactive
current or voltage reactive VARs as
they're called
the more coils of wire there are in the
system the more demand there is for this
magnetizing energy
so during the day when motors are
running air conditioners are fired up
and transformers are heavily loaded var
flow is high power system operators say
the load is inductive because the moving
magnetic fields are said to induce the
opposing current flow in the lines so
the magnetic field causes a kind of load
in AC circuits that operators call
inductive because the action of the
magnetic field is said to induce current
flow and make the change in current lag
the change in voltage so Watts from
straight resistance or heat is the kind
of load we'd all heard about the second
kind is inductance from the moving
magnetic field that's just explained the
third kind comes from another thing that
happens when AC power changes direction
as you probably know electrical energy
comes in two flavors positive and
negative current flows when positives
and negatives are separated like when
you scrape your feet across a carpet
John Travolta CH shows us how here you
know physically separating the positive
and negative charges and when you touch
something like a doorknob that has a
different charge current flows
in an AC circuit the positive and
negative sides of the circuit change 60
times a second because the generator
that's separating the charges is
rotating 60 times a second
so that means that each side of the
circuit changes polarity when that
happens the side that has positive
charges becomes negative and decide that
was negative becomes positive let's look
at a simple example here's an electric
circuit with a battery as the power
source notice the open switch on the
right side since the polarity of the
battery doesn't change its DC one side
has a bunch of positive charges and the
other side has a bunch of negative
charges with the switch open they're
just kind of sitting there built up on
either side of the gap we know there's a
kind of pressure built up between the
points of that gap called voltage that's
right and that if we close the switch
current would flow in the circuit from
negative to positive but now think about
this what would happen if we flip the
battery around
what do you think
it's be the charge runs the other way
let's look at it when the polarity
switches the battery forces all the
negative charges to run around the
circuit to the other side and vice-versa
for the positives it's like there's a
reservoir of charges trying to keep that
side of the circuit the way it was while
the battery is trying to change it
of course the battery eventually does
but in the same way that the effect of
inductance causes a delay in the change
of current flow this effect causes a
delay in the change in voltage
substitute an AC voltage source and this
is happening 60 times a second the
effect is called capacitance and equal
amounts of it cancel out the effect of
inductance so capacitance causes a delay
in the change of voltage in an AC
circuit unfortunately the system does
not have equal amounts of inductance and
capacitance since there are millions of
coils of wire out there every motor
every transformer all acting like little
reactive power generators the system
tends to be inductive especially in the
daytime when factories are working and
air conditioners are running the effect
of all those inductors is to overload
lines and make the voltage drop because
of the added resistance the solution
of course if capacitance is added to the
circuit near the source of inductance
the effect is canceled locally and
reactive power doesn't have to be
transmitted through the lines saving the
capacity for watts each capacitor acts
like a temporary battery storing charge
and part of the cycle and releasing it
in another causing a delay in the change
of voltage if planned in size correctly
the delay and voltage change will match
the delay in current change caused by
the local inductance and the circuit
will have a minimum of reactive power
flow
so that's my story and I'm sticking to
it
aside from the watts used in your
circuits there are two other types of
load inductive from the magnetic fields
and capacitive from the changing
polarity in all AC circuits these extra
loads use capacity heat up lines and
cause other problems your power supplier
takes various steps to handle them but
they're a fact of life for your
electrical system
thanks for watching