I don't know much about HVAC, and I don't know the answer to that question.
But, here is what happened to me as a layperson in a house that I own. The
occupants said that the A/C wasn't working and they smell something burning.
I, of course, told them to turn the HVAC off and then I went there to check
things out. When I turned the A/C on, I could hear a humming sound and I
did smell a slight burning. No circuit breakers were tripped, and it seemed
like the humming would continue unless I turned the unit off. I turned it
off and called our utility company because we have what they call a
WorryFree service contract with them for the HVAC. They came out and it was
a bad start capacitor. They replaced the capacitor and everything was
The point is that I assume that if the start capacitor is bad, it does not
necessarily mean that any circuit breaker or other protective device will
shut things down. I don't know if the motor would have burned out if I left
the power to the HVAC on, and I don't know if the burning smell was from the
motor or the capacitor.
If a cap opens up completely and it is a start or run cap the motor
will not turn or may start then stop and start until some circuit
protection kicks in. A weak run cap may drag along before it comes up
to full speed.
In the case of an electrolytic which has lost some of it's capacity
due to age/heat it will pull more current and take longer when it
starts until the capacity falls low enough that it will not start.
You did mention something about not knowing the charge in the system
so that may affect starting too.
Fans typically don't have starting caps but they may have running
caps. The whole idea is to shift the phase angle of some of the
windings to emulate multiphase operation from a single phase motor. It
is more efficient then shifting the phase with shading poles
(essentially shorted windings that can also shift the phase/ cause the
motor field to rotate).
In the near future all AC compressors will be rotary styles and use
variable frequency drives (sometimes called "inverter technology" in
HVAC-speak) and three phase motors. They are the most efficient
drives and allow things like running at only the speed necessary to
meet demand, as well as "soft" start and stop and they convert single
phase to three phase.
Almost no new window AC's do this but many central air and most new
mini-split systems do. As efficiency standards tighten all AC's will
use this technology. It is already cost effective, unless you are a
manufacturer who has old stock piston compressors or hasn't learned
the new economies that can be realized from variable speed drives.
If I sound like an evangelist, it's because I ditched my former AC and
went to mini split and reduced my power bill by an order of magnitude.
The only advantage that central air has over mini split (other than
some unsightly wall mounted evaporator unit) is the noise of a mini
split - it can't match a central air system that is well isolated with
In my case the capacitor failed, what I saw was the compressor would not
turn over and the fan did not turn as well (two part cap), Lennox Elite
11 unit. My UPS software logged the load caused 120 volts in the house
to drop for a moment to 113 vac until the compressor safety tripped out.
Some caps have polarity specific, some don't. Their use is circuit
specific. Latter can replace former, not the other way around.
I always keep critical parts on the system as spares. HSI for furnace,
caps for ODU, etc. Mr. Murphy strikes me usually on week end, LOL!
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