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A recent Motor Doctor article on the subject of capacitors generated
correspondence that raised some interesting issues related to servicing
capacitors in the field.
The original article gave a basic description of capacitor design and
function, then moved on to some suggestions for diagnosing a defective
capacitor in the field and how to deal with the problem.
One point I made in the article had to do with emergency field replacement
of a capacitor when you don’t have the correct size available. I
stated that it is an acceptable temporary fix to go up one rating size
(for example, from a 7.5 microfarad original up to a 10 microfarad replacement)
so long as the voltage of the replacement capacitor is equal to or greater
than the original and the current draw under load does not exceed the
nameplate amps. I stressed the importance of installing a capacitor of
the correct size as soon as possible.
Fred Baron of Motors & Armatures, Inc. offers the following refinement
and caution to the original article:
“The general rule of being able to replace a capacitor of equal
microfarad rating but higher voltage rating is certainly true as far as
the capacitor is concerned. There is a circumstance in which a run capacitor
failure may indicate an application problem, in which case replacing the
capacitor with one of higher voltage may worsen that problem. Here it
is:
“When either the discharge or inlet of a squirrel cage blower is
blocked, the blower motor’s RPM’s increase. (M.D.--Remember,
the less air moved, the less work is done. This leads to a decreased load
on the motor and higher RPMs.) As the RPMs increase, the regenerated voltage
increases and tends to stress the run capacitor. The run capacitor, in
air-moving applications, acts almost as a fuse, in that its failure could
be an indication of a more significant problem. If the replacement capacitor
is of higher voltage, the ‘fuse effect’ is diminished.”
Fred goes on to add that he agrees about our prescription for emergency
fixes, but stresses that nothing is as good as finding a replacement that
exactly matches the specifications of the original.
Larry Johnston, a knowledgeable service technician from Madison, MO, offers
some additional insights into suggested capacitor testing procedures.
Larry suggests that when using an ohmeter to troubleshoot capacitors,
you should have a capacitor of similar value to compare how far the needle
swings as it shows conformity on that particular test equipment. He points
out that many defective capacitors will show some degree of continuity.
Larry relates a story in which he was asked to check on a “stuck
compressor” diagnosis from a service technician who also had experience
as a TV technician. The customer was pleasantly surprised to learn his
problem was a weak capacitor and that he didn’t need a new compressor.
Larry, in turn, was puzzled that a TV technician did not have more expertise
in capacitor diagnosis. In his experience, motor start capacitors and
certain round type run capacitors seem most prone to show continuity but
still exhibit diminished capacitance.
Larry offers the following insights into testing capacitors: “I
always discharge with a 500-ohm, 11-watt resistor. The needle swing test
depends on quick connection of the probe. Note, I have repaired multiple
motors in air conditioning equipment where the technician has discharged
the capacitor with a screwdriver and the screwdriver brushed against the
return leads. I have also seen situations where the capacitor has fallen
down onto an evaporator, burning a hole in it.”
The Motor Doctor welcomes your letters and always appreciates your thoughts
and comments. They help everyone expand their knowledge of issues in the
field and the best way to service motors and keep them in operation.
by Neil Simon, aka the Motor Doctor - regional sales
manager for A. O. Smith Electrical Products Company.
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