TechnoSyndicate

November 1, 2012

How to diagnose and repair a refrigerator

Filed under: General technical reference — VIP @ 00:54

Be careful. Read manufacturer’s recommendations. Do this at your own risk. Make sure to research local regulations on ozone-depleting refrigerants.

There are many appliances in our life that have several standard configurations and haven’t changed for years. Refrigerators are one of those appliances. Of course there new designs with linear-action compressors and electronic control circuits but the majority of refrigerators has a very common scheme to them.

For the past seventy years this remains unchanged:

Compressor – an overview.

A refrigerator compressor motor is a rather economical motor that rotates a crank or a cam and that makes a piston reciprocate. A motor, a piston and the valve mechanism are encapsulated in a steel body that is welded shut and filled with oil to ensure constant lubrication.

The motor has to be switched on and off by the circuit to ensure that the temperature in the refrigerator is right. When the motor is energized it has to overcome the back pressure of the refrigerant fluid and some static friction. To make the motor more energy efficient, it is designed so it cannot overcome it by itself. There is an auxiliary coil in the motor that makes it more powerful temporarily, just to overcome that standstill friction and backpressure. Once the motor is running the power to this auxiliary coil is cut.

This is what you see if you take the connector off the compressor:

There are three pins, one for the Starter winding, that auxiliary winding that helps the motor start; another pin is for the main motor winding coil; the third pin is shared between them and supplies both windings with power. (or supplies ground to both windings)

The circuitry that controlls the compressor has to switch both, the Start Winding and the Run Winding for the motor to start and than deenergize the Start winding when the motor is running.

  • IF THE START WINDING IS NOT ENERGIZED, THE MOTOR WOULD NOT RUN. WHEN THE MOTOR IS ENERGIZED IT WILL STAND AND HUM WITH THE 60HZ FREQUENCY FOR A WHILE AND THEN THE OVERPOWER PROTECTION CIRCUIT WILL SHUT IT OF IF THIS MODEL OF REFRIGERATORS IS LUCKY TO HAVE ONE; OTHERWISE THE MOTOR WILL OVERHEAT
  • WHEN TRANSPORTING A REFRIGERATOR ON ITS BACK MAKE SURE TO LET IT SIT UPRIGHT FOR A DAY WITHOUT PLUGGING IT IN TO MAKE SURE THAT THE LUBRICATION OIL RUNS BACK DOWN INTO THE COMPRESSOR FROM THE COOLING PIPELINES. IF YOU FAIL TO DO SO THE COMPRESSOR WILL SIEZE UP AND FAIL.

The thermostat and how it works.

I would admit that I didn’t get a chance to fix many refrigerators in my life, but I had repaired a few. Thermostats do vary in design and I cannot list all variations of thermostats here but here is the basic electromechanical thermostat that you are going to find in the refrigerator:

The thin tube of the thermostat goes up into the freezer. It acts as a heat conductor. When the gas in the tube and the chamber expands beyond a certain point, the chamber closes a switch. That “point” when the switch is closed by the chamber is set by an adjustment knob that usually threads some part in and out to bring the switch contacts closer together or further apart thus changes the point when they become engaged.

Since a thermostat is a rather delicate part on most large refrigerators it does not close the compressor circuit directly. The reason for this is to reduce the contact spark erosion and to keep the power chords that carry high current from going into the refrigerator.

  • WHEN REPLACING A THERMOSTAT BE CAREFUL NOT TO KINK OR PUNCTURE THE TUBE OR THE GAS WILL LEAK OUT. MAKE SURE THAT THE TUBE IS ROUTED THE SAME WAY AS IT WAS BEFORE.
  • SOMETIMES REFRIGERATORS RUN FOREVER AND NO ADJUSTMENT CAN STOP THEM. THIS MAY BE BECAUSE THE REFRIGERANT LEVEL IS LOW IN THE COOLING SYSTEM AND IS NOT AN ELECTRICAL PROBLEM. IT HAPPENS BECAUSE REFRIGERATOR BECOMES INEFFICIENT AND CANNOT COOL TO THE TEMPERATURE LOW ENOUGH TO OPEN THE THERMOSTAT AND STOP THE MOTOR. IN THIS CASE A REFRIGERANT NEEDS TO BE ADDED TO THE SYSTEM.

The thermal relay

The thermal relay is another component that can be found in many refrigerators. The reason refrigerators use a Thermal relay is because it can be engaged by an Alternating Current unlike an electromagnetic relay that would require the current to be rectified. Also this relay reacts with a delay.

A thermal relay control circuit current heats up a small coil of wire that heats up the bimetallic strip. This bimetallic strip bends in one direction when heated to a point because bimetallic strips are made of two different metals with different heat expansion rates. This strip opens and closes the contacts of the power circuit that drives the load: a compressor in case of a refrigerator.

More modern thermal relays use a heating element that is incorporated into the bimetallic strip; therefore the control circuit is grounded into the power circuit since it is easier to build a relay like this.

On older refrigerators, a dual thermal relay will close the Run Winding and Start Winding circuit when thermostat circuit is closed; opening the Start Winding circuit after a certain time interval after the compressor starts to move.

Solid-state devices.

More modern refrigerators use a Solid-State device to shut off the Start Winding after the motor starts.

Those devices are commonly found to be built into the three pin female connectors that plug into the compressor, making this assembly very compact. They also commonly include a non-solid-state overpower protection that opens the circuit if too much current passes through it for too long. They are, once again, made out of bimetallic elements that open the circuit when they get hot, but stay closed when they are cold.

The “Solid-state” Start Winding shutoff relay looks like a puck of some material that gains resistance as it heats up. This puck is held in place by some contacts. When this puck gets hot, and its resistance increases and the current through it decreases, the Starter Winding gets less current and practically does not operate.

Usually, the Overpower protection is connected to the Common pin on the connector and it protects both windings, while this solid-state puck is connected to the Starter Winding pin. So if Overpower is in series, on the Line side of the circuit, the Start Winding shutoff delay is on the Neutral side of the circuit, or vice versa.

This image was taken from http://www.spudfiles.com/forums/how-connect-wires-to-my-fridge-compressor-t21724.html

  • Sometimes moisture gets in this plug/relay and corrodes the surface of the solid-state puck. The corrosion can be sanded off and the puck can be rotated 90˚ so contacts would not press against the same spots on the puck as before. This is one of the cheap ways to fix this problem without buying replacement parts.

Refrigerant – leakage and refilling.

Refrigerator cooling pipework is made from a variety of metals. The compressor inner casting is made of cast iron or aluminum; the pipes that come out of the compressor are steel or aluminum. To solder copper to aluminum some transition coupling is soldered between them that are made out of nickel. The freezer us usually aluminum and the heat exchanger on the back is steel. The pipes that connect everything are usually copper.

  • Because all metals have different thermal expansion coefficients, the connections between any dissimilar metals usually fail first. This also applies to places where copper pipe exits from some solid part that is made out of steel. Things heat and cool and expand and contract, causing places where they are joined to bend. This eventually causes those areas to crack and for the refrigerant to leak out. Connection areas are places to look for leaks.
  • If there is still some REFRIGERANT in the system a simple soap and water solution can be applied with a brush. If bubbles are forming, than there is a leak.
  • REFRIGERANTS can be FLAMMABLE. the lubrication oil that the pump is circulating along with the REFRIGERANT is always FLAMMABLE. if it is forced out of the system by leaking REFRIGERANT while there is a source of ignition, there will be an explosion. do not solder the pipes unless the system was depressurized and oil was allowed to drain.
  • Sometimes REFRIGERATORS loose REFRIGERANTS over time without any obvious leak spots. this happens because no system can be built perfectly sealed.
  • on every compressor there is a pipe that leads to nowhere in is crimped shut. this is how REFRIGERATORS get filled in the factory. to fill one at home, the manufacturer-recommended REFRIGERANT has to be filled through a different kind of opening.
  • a hole can be poked in the copper return line of the compressor. A thin tube that connects the heat exchanger with the compressor can be salvaged from some old REFRIGERATOR and inserted into that hole.
  • everything needs to be cleaned from corrosion and covered with flux before soldering is done. than the tube is soldered in place and a fitting is made to let in new REFRIGERANT.
  • after the system is filled (about 30 seconds, the tank is closed and the thin tube is crimped and soldered shut.
  • refrigerator oil is hygroscopic. it ABSORBS water from the atmosphere, just like brake fluid. avoid prolonged exposure of the cooling system to the outside air.
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