Copyright Goodheart-Willcox Co., Inc. 382 Motors and Electric Control Systems Electrical insulation testers, also called megohmmeters, are often used to test motor winding insulation, but can also be used to test the insulation on other electri- cal devices. Like an ohmmeter, a megohmmeter mea- sures resistance by applying a known voltage supplied by the meter. However, because the resistance of the tested insulation is often in the megohm (MΩ) range (1 MΩ = 1,000,000 Ω), a megohmmeter must apply a very high voltage to obtain an accurate reading, usu- ally between 500 Vdc and 1000 Vdc. Due to this high voltage, a megohmmeter should only be used by an experienced technician because the motor windings and insulation may be further weakened or damaged if a high voltage is applied for an excessive period of time during testing. Insulation Damage from Overloads It is important to test a motor’s winding insulation after an overload. Overloads can heat motor windings to the point of breaking down the insulation. If possible, review a system’s service records. Be sure to ask the system’s owner how often overloads have occurred in the past. Pro Tip Insulation Testing under Vacuum When a system is under vacuum, do not perform an insulation test. Without refrigerant and oil in the system and the compressor, the readings will be inaccurate. In addition, electrical arcing may occur, burning the windings. Caution Procedure Testing a Hermetic Compressor’s Winding Insulation If possible, operate the compressor for at least one hour before testing its winding insulation. By doing this, a technician can compare readings taken on different days because the winding temperatures will be approximately the same. 1. Disconnect power from the motor circuit. 2. Isolate the compressor electrically by disconnecting all wires from the compressor terminals. 3. Wipe the terminals clean with a dry towel. 4. Connect a shunt or jumper wire across all of the compressor terminals. To ensure that the only resistance tested by the meter is between the winding insulation and the ground, all three terminals are wired together. 5. Connect the black meter lead to any bare metal part on the compressor dome. Be sure this spot has no paint or coating on it. If necessary, wipe or gently file a spot to expose bare metal. 6. Connect the red meter lead to one of the motor terminals, Figure 17-10. 7. Activate the megohmmeter. Refer to the manufacturer’s instructions to verify how long to energize the megohmmeter to produce an accurate reading. 8. Read the meter and write the results of the measurement on a service record. 9. Remove both of the meter leads (red first, then black) and then remove the terminal shunt or jumper wires. 10. Reconnect the proper system wiring and restore power to the Pro Testing a Hermeti If p ossible, operate the compr before testi ng its windi ng insulation. B y do can compare readin gs taken on different da ys windin g temperatures will be approximately the sam 1. Disconnect p ower from the motor circuit. 2. Isolate the compressor electricall y by disconnecting all wires from the compressor terminals. 3. Wipe the terminals clean with a dr y towe l. 4 4 . Connect a shunt or jumper wire across all of the compressor co terminals. To ensure that the onl y resistance tested t ed b y th e meter is between the windi ng insulation an d th e e ground, gr al l three terminals are wired together. 5 . Connect Co th h e e black meter lead to an y bare metal p art on t h e compressor co or dome. B e sure this sp ot has no pa int m 8 . Re ad t h measurement o 9. Remove both of the m and then remove the terminal 10. Reconnect the proper sy stem wirin g a to t he ccircuit..itcuir Red lead attached to motor terminal Black lead attached to compressor dome Megohmmeter Sealed Unit Parts Co., Inc. Figure 17-10. Checking the winding insulation on a hermetic compressor. The black lead is connected to a bare metal portion of the compressor dome. The red lead is connected to one of the motor terminals.