When meggering a motor, a technician should read the megohmmeter's instructions to establish the time interval for energizing the megger when checking coils or winding. When motor windings are under a vacuum, do not use a megger. Running the motor for an hour or more, disconnecting power and all electrical leads, and then connecting the meter to the motor quickly produces about the same winding temperatures. The results are more accurate when comparing the same compressor's readings from different days.
Technicians who are megging compressor motors should be sure to disconnect all of the compressor motor terminal wires. Only experienced technicians should use meggers with a higher voltage output than 500 volts DC. When meggering motors, technicians should not use a high voltage for an extended period, because such use can cause motor windings to weaken or fail. High voltage testing can also damage the winding insulation.
Technicians typically use electrical meters called megohmmeters, or meggers, to reveal when motor winding insulation is weak, when moisture is accumulating and acid is forming. These conditions can damage motor winding insulation. A megger is a giant ohmmeter that creates 500 volts DC or more from its internal battery. The meter reading standard is in millions of ohms, or megohms. A reading between 100 megohms and infinity is a good motor winding resistance reading.