SERVICE
How can you verify that the motor’s ground insulation is safe and reliable?
When an electric motor is left unused for extended periods in a humid environment, or exposed to rain and sunlight, or operated under prolonged overload conditions, it may be subjected to corrosive gases, leading to a decline or aging of its insulation performance. Additionally, damage to slot insulation can result in breakdown between the motor windings and ground, causing high‑current faults that energize the motor housing and pose a serious safety hazard.
Ground fault detection can be performed using a megohmmeter, which measures the insulation resistance of each phase winding to ground. If the insulation resistance of all three phases to ground is low but not zero, it indicates that the motor has become damp or contaminated with oil. However, after cleaning and drying, as long as the winding-to-ground resistance is restored to at least 0.5 MΩ, the motor may continue to be used.
