On 09/21/2010 03:18 PM, Ignoramus25344 wrote:
If you recall, a month or so ago I posted about a refrigerator not
working.

The compressor unit in it would keep trying to start and would be
unable to start, tripping an overload relay.

After some "time off" to let the system cool, the fridge started just
fine. I also replaced a capacitor and relay (which was very cheap and
possibly not needed) and thought that I was done.

However, since yesterday it started having trouble again. Letting it
cool down for a couple of hours would get it started, only to find it
unable to restart a few hours later.

What could be the culprit here? Unlike last time, there is no
emergency, as we have less food and all our food easily fits in
another fridge.

Yeah, I thought this might happen!

Well, this thermistor capacitor start scheme sounds pretty crummy to me.
If the power gets a short glitch, even a couple cycles of the line, that
may allow the compressor to nearly stop when there is high pressure in
the condenser side. Then, power comes back and the thermistor is hot,
so it doesn't restart. There is a thing called a "sinpac" switch,
currently manufactured by Stearns. It uses a voltage sensing chip and
a triac to control the start winding. It is made as a replacement for
shot centrifugal switches, but is also great on motors where such a
switch can't be used, like refrigerator compressors. At least through
the channels I got one a while ago, they are expensive, but I'll bet a
refrigeration shop will carry them much cheaper.

There are also electronic modules, often used in the heating/cooling
trade, to prevent "short-cycling". They are just wired in series with
a motor, and will shut it off for some number of minutes whenever there
is a power interruption.

Of course, you may just have a defective compressor, and it is slowly
seizing up, and will eventually lock up for good.

I converted a central air conditioning system from capillary tube to
expansion valve some years ago, and had problems with the compressor
failing to start. I installed a "hard start kit" from my friendly local
A/C supply shop. This was a potential relay and a BIG starting cap.
It augments the run cap when starting, and greatly increases starting
torque. It solved the problem. I don't know if this is the kind of
problem this unit is having, and you might have to instrument it to find
out what the cause is. It could be a bad compressor, it could be a
bad starting system design, it could be a program problem in the
computerized controller that is occasionally trying to stop and restart
the compressor too quickly, it could be a mechanical timer on the
defrost that has that same effect, or it could even be low line voltage
or a worn-out wall socket making poor contact.

The friendly local A/C supplier isn't so friendly anymore, I have to
show them my EPA "green card" to get them to sell anything to me.
The trade prices on stuff just blows me away. I needed a run cap for
our current A/C unit a couple years ago, showed them my green card, and
got a HUGE run cap for about $7.50. I walked in there expecting to pay
$50 for it.

Jon