in this hvacr training video we're going
over why capacitors fail in air
conditioning units so we're going to
verify that this one is good and then
you're going to see it fail in real time
while the system is running
[Music]
next we're going to test this capacitor
to see if it's still good we're going to
test it while it's under load so i
already have my multimeter set up so
this one's going to read
the voltage and current for our
compressor coming off of the capacitor
and this one is measuring the voltage
and the
uh current for the fan motor and so you
can see
one probe is here and one is here so
we're measuring the voltage on the fan
and the common which is connected right
here on the capacitor
and we're going to measure the current
coming off of the capacitor right here
so this is on the fan terminal over here
you see we're measuring the voltage on
the herm and the common and then we're
measuring the current for the compressor
so i'm going to turn the system on let
it run for five minutes i'm going to
show you how to test this while it's
under load before we see this capacitor
fail in real time the system's been
running for about five minutes and you
see a current of 6.32 on the herm which
is the start winding of the compressor
and we measure 357.7
volts so what we do is we take
6.32
and then we multiply that by two six
five zero
and you divide that by the voltage which
is the back emf from the compressor and
we're at 358. so we have 46.78
microfarads on that capacitor so that
capacitor is good it's actually rated
for 45 now let's check the fan and you
see that we're measuring 0.7 amps at
363.5 volts so we're going to take 0.71
times
2650
divided by the back emf the back voltage
from the fan motor which is 364.7
5.15 microfarads on that capacitor for
the fan
and so that capacitor is good and so now
what i want to do is just go ahead and
watch the top of that capacitor
and we're going to see that capacitor
actually fail
so
that capacitor has failed on the
compressor side and also on the fan side
you can hear both the fan and compressor
are still running
now if i turn this off uh the fan and
compressor are not going to be able to
turn on this time so the only reason
that they're running was just because
capacitor was installed in the very
beginning
current is very high on both the fan
motor and the compressor right now in
order to keep them spinning now we're
going to allow power to the system and
you're going to see a high current draw
but you're not going to see the
compressor or the fan motor turn on
so i don't know if you can see this on
the capacitor but this one is still good
it's flat down in there but this one has
a mushroom on it right there and you can
see that
the terminals on the top are kind of
spread apart right here so this
capacitor failed actually due to
overheating on the bottom of that
capacitor you can see it's bulged
downwards here's a good capacitor it is
completely flat
and so you can see that this capacitor
actually failed and if we were to go
over to
mfd so that's microfarads you can see
that we are still reading zero so this
capacitor is bad now we're measuring the
microfarads for the fan motors we have
one on common and one on fan you can see
that we're reading 0.0 microfarads so
when a capacitor fails they tend to
bulge like this right here
or like this and so that's just due to
the heating of the dielectric fluid
inside the capacitor
and they don't always feel like that
there may be
some that feel with the fluid that
basically
comes out of the capacitor and other
times it'll just look completely fine it
may look a little rusty may look a
little old and it may be bad but it may
be good that's why you need to test them
we test them either under load or
isolated by themselves and discharged
and so what you need to remember is this
right here the capacitor so on a dual
capacitor there is two of them and a
dual means they have a shared common and
so this is the oval style uh capacitor
and then inside there's dielectric fluid
and so that's a heat absorption fluid
and so these are going to heat up while
it's running
and
on these you're going to see on the
inside it's just a little foil and then
there is
a another piece between each layer of
the foil in order to separate the
electrical sheets and so these are
called sheets or panels and you can see
there's a huge amount of wraps right
here
for this capacitor and so it's a little
different than a start capacitor because
the start capacitor will have
thicker metal sheets and so this is from
a a start capacitor that looked like
this
now i want to get into why a run
capacitor would fail and so there's one
big misconception about how run
capacitors fail and that's when tech say
that they failed due to high current
well that's not the case and you saw
that during while i was breaking the
capacitor due to high heat at the base
of this capacitor you saw our current
was falling
and so as we were losing the capacity
within a capacitor on this little foil
plate right here so this is a basically
the inside of a capacitor and so you
know i have a dual run capacitor right
here
and it's just sitting in the dielectric
fluid as i was heating this
i was making the plates fail and so we
had less and less surface area on the
foil in order to store the electrons and
so really what was happening is
the capacitor stored and released less
current
as i was heating up this capacitor and
so you saw as the current was falling
that capacitor was starting to fail so
it failed on the compressor part of the
capacitor first then after that on the
fan side of the capacitor let's get into
the reasons why a capacitor fails number
one is the capacitors fail due to high
heat and so if this shroud of the
outdoor unit is in the sun all the time
this capacitor is going to be really hot
and so what could happen is the
dielectric fluid could expand it could
press outward on the shell of the
capacitor like this or it could also
push down at the bottom at the base
right here and basically this is a
telltale sign that a capacitor has
failed here's another one where you see
a mushroom top
and so sometimes a capacitor fails and
you won't see any any reason why
sometimes when they overheat and the
dielectric fluid is trying to get out
basically it might come out of the side
and leak out of the side of the
capacitor
so that's reason one they could overheat
problem number two is that the lifespan
of a capacitor is limited and it's
really relative to the operating
temperature and so
the higher the temperature the lower the
lifespan the less amount of hours that
this is going to be able to to run and
so
what's going to happen is some of the
foil is going to break down over time
and that's why when you take a reading
on a capacitor and you notice a lower
microfarad reading than what the
capacitor is rated for
that means you've lost some of the
plating inside so it's just not going to
last forever number three
is that you have a voltage rating on the
side of the capacitor so if you have
exceeded that voltage rating that could
end up doing damage onto the plates
inside of the capacitor so
this uh the voltage on say the c and the
herm while this is running is actually
the back emf off of the compressor
and so it's higher than say just a 240
volts in so we measured around 365 volts
while this system was running and that's
because there's back emf off of the
compressor
and so you could also have a lightning
strike or some other reason where you
have some type of a voltage spike that
could do damage to your capacitor
another reason could be that you have
corroded connections right here so you
could have just a bad rusty connection
point and so this may or may not make
the capacitor fail because what's going
to happen is these terminals right here
are going to heat up i've seen the weld
basically attaching this to the
capacitor just melt right off and so
that's another way
that this could fail is due to a bad
connection point located at the top of
the capacitor itself so that's it you
could have overheated the capacitor you
could have reached the life cycle end of
the capacitor you could have had a high
voltage some type of a voltage spike
that has ruined the inside of the
capacitor or you could have a loose
connection at the top of the capacitor
itself and so these do not fail due to
high current because they control the
current going to the start winding of
the compressor and also the outdoor fan
motor if you want to learn more about
capacitors i have some other videos
linked down in the description section
below if you want to learn about hvac
electrical troubleshooting make sure to
check out some of the articles we have
over at ac servicestick.com articles and
while you're there make sure to check
out our book the refrigerant charging
and service procedures for air
conditioning and so we go over how to
check the refrigerant charge of a system
how to prepare a system for refrigerant
and also how to troubleshoot a running
system so that's available over our
website at acservicetech.com and also on
amazon hope you enjoyed yourself we'll
see you next time at ac service tech
channel