Is the compressor running hot?

A common sign is when the compressor is too hot to touch. Take the temperature reading of the compressor with a thermometer. The temperature should not exceed 70°C as a nominal rule.

Ensure that the Starter and Capacitor are working as this could cause the compressor to overload during start.

If you have checked all the troubleshooting guides, you may have to replace the compressor at some point if the compressor parts have already begun to wear out prematurely.

Compressors usually operate between 35°C and 70°C. Any hotter than 75°C could indicate that there is a problem with the compressor.

There is an exception to this rule, such as rotary compressors which can operate as high as 95°C, however, the nominal temperature should be at around 70°C as they do rely on cooling from the refrigerant.

Sometimes the cause for overheating may be caused by a defective capacitor which is preventing the motor from engaging the Run cycle. If the PTC is faulty it may not detect the overheating or current draw, or the Starter may not be engaging correctly. Check if the Capacitor, Starter and PTC are working before diagnosing further.

High temperatures can be caused by low refrigerant, where the oil is forced out of the sump. #

In some cases the system may have been over-charged and left to run for too long, causing stress on the compressor.

Cooling and sufficient airflow are other factors that should be looked at, for instance, if the condensing fan had failed or if there is not enough air ventilation in the area.

Usually, the secondary cause for an overheating compressor lies in the PTC where the overload failed to detect if the compressor overheated or was pulling a higher current than what is normal.

In most cases, if the compressor is overheating and the refrigerant levels are in spec, this is just a matter of time before the compressor could start to fail.

Overheating compressors place strain on the mechanical parts as well as begin to decay the oil quality which is needed to lubricate these parts. This then becomes a cycle where the parts deteriorate faster.

Performing the compressor winding test

If the compressor is not running, this test will determine if the compressor is faulty from an electrical aspect.

DISCONNECT ALL POWER FROM THE FRIDGE BEFORE WORKING!

If the windings short out (Low Ohm Reading or have no electrical reading (OL or no reading), this will prevent the compressor from starting.

Common causes cause faulty windings in the compressor range from

• High acidity levels are caused by poor evacuation, incorrect refrigerant or contaminants in the pipe system from old blow-outs, and even moisture.
• Undervoltage causes the compressor to turn at a lower speed while drawing a higher current.
• Overvoltage causing higher than the rated voltage supply
• Compressor overheating from overworking, poor cooling, and incorrect refrigerant.
• Low refrigerant causes ineffective cooling and possibly sucks out the oil from the sump.
• High refrigerant levels cause unnecessary torque on the pistons and on the drive motor.
• Physical strain on the piston can cause the motor drive to overheat, along with a faulty PTC, which can prevent turning the motor off if the overload protection switch is faulty.
• A faulty capacitor or faulty start-up relay can cause the windings to burn out due to the incorrect switching to the Run winding, or even, overloading the Start windings.

The image below indicates the connectors and what they mean. It also shows the winding configuration inside the compressor.

In some compressors you may have the configuration upside down, nevertheless, the Start will always be on the LEFT-HAND SIDE.

C = Common is the common line which can either be Live or Neutral.

S = Start winding which helps to start turning the compressor. 

R = Run winding which uses torque to run the compressor at maximum operation.

This is an example of the winding test.

Please note that each compressor will have different readings, below is not a standard it is only an example.

Take the reading from Common to Start: C to S = 9 ohm

Take the reading from Common to Run: C to R = 20 Ohm

Take the reading from Start to Run: S to R = 27 Ohm

If you add C to S and C to R you will get approximately 27 to 29 Ohm.

Testing for Earth Leakage

Typically an earth fault can occur where the winding insulation has deteriorated. This causes an “Earth Leak” between the winding and the chassis of the compressor. When this occurs, the breakers usually trip.

1. Test from Common to the Compressor Chassis
2. Test from Start to the Compressor Chassis
3. Test from Run to the Compressor Chassis

You should not have any Ohm reading from either test.

If there is an earth leak, replace the compressor IMMEDIATELY.

Checking the Oil Condition

On compressors with a sight glass, you can notice the colour of the oil. On smaller compressors, to determine if the oil is contaminated, you will have to remove the compressor and turn it upside down in order to get a sample.

You can drain the oil and replace it with the same type of oil as per the specification of the compressor. This is not a guarantee that the compressor will last.

When replacing the oil, ensure that the grade matches the specifications for the compressor and the refrigerant. Mixing oils with incorrect refrigerants will damage the compressor and cause premature acidity.

Dark, murky or sludgy oil is an indication that the oil has been contaminated either by poor installation or inefficient evacuation, but also it could be a sign that the compressor may have been overworked at some point and has degraded the oil.

On compressors with a sight glass, you can notice the colour of the oil. On smaller compressors, to determine if the oil is contaminated, you will have to remove the compressor and turn it upside down in order to get a sample.

Honey coloured
Usually the general colour for new oils. Depending on the type of oil the shade may vary, however, the oil should look clear.

Dark syrup
This is an acceptable colour as other factors such as the age of the oil could cause the oil to be darker.

Dark, murky colour
The oil may have decayed or been contaminated. This is also an indication that the compressor reached excessive temperature.

Contaminated oil can be replaced, however, it is never recommended as there is never a guarantee that it will work as the damage may have already occurred on the compressor’s mechanical parts.

Degraded oil can also form a hard sludge which could slow the compressor causing sluggish starts. This could draw a high current to trip the PTC or the breakers.

When to replace the compressor:

If you get no readings on either of the windings, this indicates that the compressor winding has burnt out.

If you get very low Ohm readings or a direct short, the winding insulation has decayed and is shorting.

If you get any Ohm reading from either winding to the compressor body, this indicates that there is an earth leakage.