Copyright Goodheart-Willcox Co., Inc. Chapter 17 Electrical Troubleshooting Fundamentals 383 6 In general, a megohm reading of 100 MΩ or higher indicates that the motor winding insulation is in excel- lent condition. Readings less than 100 MΩ indicate that there may be an issue with the insulation. In addition to testing the winding insulation in a hermetic compressor, a megohmmeter provides a snapshot of the level of mois- ture and contaminants within the refrigerant and oil mix- ture. As contaminants in the refrigerant and oil mixture increase, they cause the motor winding insulation to break down, decreasing the insulation’s electrical resistance. Figure 17-11 shows the motor conditions indicated by dif- ferent insulation resistance measurements. After performing any maintenance on a motor, wait a few weeks to take another megohmmeter reading. If the reading remains the same or rises to a higher resistance, then another megohmmeter reading is not necessary for a few months. If the megohmmeter resistance reading has decreased, warn the owner that the compressor motor may fail soon, and list other parts of the system (such as fans and switches) that could be damaged from such a break- down. Replacing a compressor in this condition before it fails, rather than waiting for the compressor to fail, will reduce downtime and save money for the customer. Motor Insulation Measurement Record Record a system’s motor insulation measurements in both column and graph format. These two records provide an easy method of observing insulation resistance trends over a long period of time. After installing a new motor, take an insulation resistance measurement as the first in a series of important system records. Review and compare these records each time the system is serviced. Pro Tip 17.2 Electrical Diagrams Electrical diagrams can be thought of as a road map. They show you how the various electrical devices in an HVACR unit are connected to create the desired heating and cool- ing output. Electrical diagrams show each component in a system and how it is wired to the next component in a system. Electrical diagrams are prepared by manufacturers for a specific piece of equipment. These diagrams are often adhered to the inside of equipment access panels. This practice ensures that diagrams are conveniently located for the technician performing service on the equipment. In addition, electrical diagrams are available in the equip- ment’s service and installation manuals, which are gen- erally available on the manufacturer’s website or in a manufacturer’s app. HVACR equipment may be manufactured in one country and then installed in another country. In order to ensure a server technician can read an electrical diagram created in a different country, manufacturers use fairly standard symbols and conventions in electrical diagrams. In this respect, electrical diagrams can be thought of as being written in their own language. However, electrical diagram symbols and conven- tions include some variation. The international body that standardizes electrical symbols is the International Electrotechnical Commission (IEC). In the United States, electrical drawing standards and symbols specified by the National Electrical Manufacturers Association (NEMA) are often used in electrical diagrams. In some instances, the electrical symbols specified by NEMA differ from those specified by IEC. In addition, the electrical symbols used in diagrams may vary slightly from one manufacturer to another. Goodheart-Willcox Publisher Figure 17-11. Hermetic compressor motor conditions and suggested maintenance procedures as indicated by different ranges of motor winding resistance. Motor Insulation Conditions and Maintenance Procedures Insulation Resistance Measurement Indicated Condition Suggested Maintenance Over 100 MΩ Excellent insulation. No action necessary. 60–100 MΩ Some insulation breakdown, some moisture present. Change filter-drier. If possible, clean dust and debris from windings using a solvent to prevent further insulation breakdown. 40–60 MΩ Overheated winding, contaminated oil, moisture in the system. Change filter-drier. Check the oil for a burnt odor, which indicates an overheated winding. If there is no odor, the oil is contaminated and needs to be changed. 20–40 MΩ Severe oil contamination, system likely to fail soon. Change oil and filter-driers. If old oil has a burnt odor, check the system’s motors, including fan motors, for worn bearings.