Refrigeration system maintenance can be broken down into two distinctive tasks. First, the system’s current operating conditions need to be inspected and observed to ensure that the system is operating at or close to its design parameters. Second, the system needs to be inspected to help prevent or predict any future system issues.

Preventing or predicting system problems can be a major benefit to both a service contractor and the customer. This is especially true when dealing with the system’s compressor. Preventing a compressor failure is a major benefit.

A good maintenance program will include a thorough inspection of the compressor. One inspection task that is sometimes overlooked is an examination of the compressor’s motor winding insulation. The integrity of the compressor’s motor winding insulation can be obtained by measuring its resistance value. This is done by measuring the resistance between the compressor’s motor winding and its body. This resistance value is normally quite high (in the millions ohm range) so a megohmmeter must be used in place of a standard ohmmeter. Measuring a low resistance value would be an indication of a problem within the compressor.

A low resistance reading can be the result of several problems within a compressor. One possible cause is a breakdown in the motor’s winding insulation. This condition will normally result in a failed compressor at some time in the future. How long until the failure depends on the extent of the damage to the insulation. One benefit to discovering this condition early is that the customer can plan for or budget the replacement cost of the compressor.

A low resistance, however, does not always mean the motor’s winding insulation is damaged. There are other problems that can also result in a low resistance reading. Contaminated refrigerant, oil level, refrigerant in the oil, and current leakage through electrical fusites or terminal plates can also cause a low resistance reading. These other possibilities should be investigated first before condemning the motor’s winding insulation.

When using a resistance test to determine a problem within a compressor, a single reading gives little insight into the condition of a motor’s winding insulation or another problem. The readings should be trended over a period of time and should be part of the regular maintenance program. If a trend of lower resistance readings is observed over a period of time, then corrective action should be taken.

When taking these resistance readings, always follow the instructions provided by the megohmmeter manufacturer. Any external electrical components connected to the compressor terminals must be removed, as they will affect the resistance reading. The reading should be taken the same way each time, with the same instrument and using the same method. The resistance reading will also vary with temperature, so it should be taken when the compressor temperature is the same.

Always check with the compressor manufacturer to determine when a resistance reading is too low. For example, Copeland recommends the following limits on its installed and running compressors: A 20- to 50-megohm reading indicates the compressor is OK. When a 0.5- to 20-megohm reading is observed the company recommends cleaning up the system (replacing the system’s filter-driers and changing the compressor’s oil). A reading of less than 0.5 megohm indicates a defective compressor.

Although the procedure for measuring and trending the resistance value of the compressor’s motor winding insulation is time-consuming, the benefits of preventing or predicting a compressor failure makes it time well-spent.

Publication Date:09/03/2007