Question about Refrigerators
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Posted on Jan 02, 2017
The method of starting the compressor on very old units was that a real relay would close its contacts for a few seconds shorting the connection of the start(S) post on the compressor with the run(R) post thus feeding power to the start coil in the compressor for a few seconds. Well, I guess the bright design boys/girls were told to design for less cost and eliminated the real relay altogether. In its place they made a little plastic holder that has a disc type thermistor inside it. Normal resistors have a negative temperature coeficient meaning that as temperature goes up the resistance goes down. Thermistors do the opposite so as temperature goes up the resistance also goes up. They stuck one of these little thermistor discs about the size of a penny and just about as thick between the S and R terminal posts on the compressor so that when the thermostat in the refrigerator calls for more cooling power goes to the R post and through the thermistor disc to the S (start coil) post thus energizing the start coil in the compressor and giving it the jolt it needs to get going. Since the thermistor is left connected across there it heats up from the current going through it. I measured the cold resistance of one disc at exactly 5 ohms. Using ohms law of voltage squared divided by resistance gives us for a 120 vac circuit quite a bit of power: 120 X 120/5 or around 2880 watts that might be needed to be dissipated by that little disc thermistor every time the compressor kicks on. Of course the thermistor gets really hot and its resistance goes up thus limiting the current and everyone is happy...for a while...years if you are lucky. Back in the 1960's I worked at Western Electric Co.'s Lees Summit, MO plant and was responsible for lines making thermistors including the disc type...they do a great job for the telephone company where not much current goes through them...but for high current applications such as controlling starting of refrigerator compressors the "black cookie dough" they are made of gets real hot...I mean really hot...and the dough dries out even more and finally breaks down often shorting out and leaving a black carbon mess all over everything plus the heat can cause a fire...so the old design was better as far as I am concerned using relays that often lasted 40 years with no maintenance. I digress...again... I just repaired yesterday a Kenmore side by side that was thrown away on the street...with melted run terminal post on the compressor from the blown up thermistor and it was thought by them and the service guy that the compressor was bad as well. An ohmmeter check showed indeed that all the compressor terminals were shorted to each other and to the compressor case ground. I then unbolted the compressor and gently bent the freon containing copper tubing allowing me to pull the compressor out to give good access to the terminals. I wire brushed the three terminals and around them on the compressor real good and cleaned it up using carburetor spray cleaner, allowed it to dry completely and retested with my meter and...behold...no more terminal shorts to anything and the resistance measured of the coils inside the compressor were good readings.... so... I then made a test cable which would plug into a 120 vac outlet. I then connected the 120 vac between the common(C) and run(R) compressor terminal posts while simultaneously using another wire through a push button switch to feed power also to the start (S) terminal for perhaps 2 seconds...alla kerzam...the compressor started and ran fine. I left it running for a while and found that a cup of water froze in the freezer compartment in about 1 hour. Then I installed a new thermistor unit and jerry rigged a way to make connection to the melted stub on the run(R) terminal, put the whole compressor etc. back in the unit, put the cardboard cover on and it has now been doing an excellent job of cooling ever since. You may not want to go to all this trouble...but..hey...I got a 2 year old side by side kenmore that the people said they paid over $1000 for and I only spent $22.88 on it on internet for a new start thermistor/overload module...even so, I would have preferred to use a start relay.
Posted on Aug 21, 2009
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