The HVAC guy told my wife the motor run capacitor on my AC unit needed
changing because it measured 4 instead of 5. He didn't say 4 or 5
what, I'm assuming uF. Does this really need to be replaced?
Jimmie you can reduce capacitor in value as long as motor start up
but you can not increase, increase will cause start windings draw
excessive currents and eventually possible burn out, I have found
some motors may even run much cooler by reducing run cap. in value.
A new capacitor that size is not that expensive and from the value of
5uf I have to assume it's the run capacitor for the blower or condenser
fan motor. If the tech tries to charge you more than $20.00 for the cap
which wholesales for $3 to $5, he may be trying to rip you off. The cap
value dropping from 5uf to 4uf is quite a bit and could be an indication
of a capacitor that's failing. I've had to replace a lot of defective
capacitors for customers in the last decade because all the damn things
are coming from China. If you decide to have it replaced, ask the tech
to replace it with a higher voltage rated capacitor. If it rated at 370
volts AC ask the tech to install one rated at 440 volts AC.
Ive never heard of anyone measuring the things before. While I am not
officially an HVAC tech, I work on radar systems, I wind up doing a
lot of the preventative maintenance on the ones at work and our tech
orders dont call for checking caps for capacitance. What they do call
for is measuring the start and run currents and recording them to
show trends and verifying the right capacitor is being used. If the
current is out of spec the first thing on the trouble chart is change
the cap. Over 25 years Ive changed a lot of them but never because I
measured the capacitance.. I know the environmental techs dont measure
capacitance either, they do check leakage current along with all the
other checks. I also know that even the best multimeters tend not to
be that accurate at measuring capacitance and I doubt like heck he is
carrying a capacitor analyzer around with him. Just didnt sound like a
good reason to fork out $100.
I check the darn things all the time because power surges and near
lightning strikes pop a lot of capacitors in AC systems. For folks
living in rural areas, I've been installing surge arresters on the
furnace and outdoor condensing units because of not only capacitor
damage but newer units have a lot of expensive circuit boards where
the older systems relied on relay logic controls. The more conscientious
HVAC techs use a capacitor tester on a regular basis because more damage
is caused to very expensive motors and compressors when the Chinese caps
go "POP!". Most of the HVAC techs who have a capacitor tester carry
this one or a similar one:
My own test gear tends to be more elaborate thus more expensive since I
also do circuit board repair work on a lot of different things. If you
have electrical/electronic repair experience, you should be able to get
your own 5uf capacitor at any motor rewind/repair shop, HVAC supply
house or W.W. Grainger. I always replace them with the 440 volt rated
On Sun, 12 Feb 2012 00:50:46 -0600, The Daring Dufas wrote:
So do motor capacitors typically fail in a way where it's only the actual
capacitance that drops, rather than the ESR which climbs (the latter
being what often causes problems* in electronic circuits)? The
description of the meter in the link you posted doesn't seem to make any
mention of ESR, so I assume it's not relevant in motor appications?
* the real issue being that the capacitance can still check out OK on a
meter, but the high ESR will affect the circuit and cause it to fail.
Motor run/start capacitors are an extremely inexpensive commodity and I
tend to look at them like disposable batteries here lately because so
many of the Chinese caps fail. It seems like every little hit from a
voltage spike coming from the electrical service punches holes in the
dielectric of the darn things. A simple test on every service call is
the prudent thing to do because of the nature of the equipment. I have
seen sources for very high quality American manufactured capacitors but
they're very expensive. o_O
On Sun, 12 Feb 2012 17:16:17 -0600, The Daring Dufas wrote:
Sure, I don't doubt that they're under high stress and that testing of
capacitance is wise - I just wondered if it was different to the
electronics world, where high ESR normally warrants replacement long
before a meter capable of measuring capacitance alone might show up any
Maybe it's that high ESR just isn't an issue for motor start/run like it
(usually) is for electronic circuits, so capacitance is the only thing
there that techs really need to check.
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