1) Shake Table Method: For a 6kV motor, if the insulation resistance between the winding and ground is below 6MΩ, it indicates that the winding may be wet or the insulation has deteriorated. If the resistance is zero, it means there is a direct ground fault.
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2) Lamp Test Method: Remove the end cover, take out the rotor, and disassemble the phase windings. Connect a 60W, 220V bulb in series between the end of the measured winding and the motor casing. Then apply 220V AC across the winding. If the bulb glows brightly, it indicates poor insulation—often referred to as "virtual connection." If the bulb shines very brightly, it suggests a real grounding fault. In such cases, the insulation at the grounded point may show cracks or blackened areas. You can visually inspect the slot for these signs. If it's a "virtual connection," you can apply a high voltage to the affected area to cause breakdown and identify the grounding point based on sparks or smoke marks.
3) Current Orientation Method: Apply a DC voltage (6–12V) to the first end of the faulty phase in parallel. Use a battery with an adjustable resistor in series, keeping the current under 5A. Move a small magnetic needle through each slot. The point where the needle changes direction is likely the grounding location. Once located, move the needle axially along the slot to confirm the exact failure point. If possible, you can also use current burn-through or voltage drop methods to locate the fault.
From past experience, most winding grounding faults occur at the slots or ends, rather than within the slots themselves. This is because the ends are typically the weakest points in the insulation system. As motor capacity increases, the electromagnetic forces acting on the stator winding ends become more intense. If the natural frequency of the winding ends is close to 100 Hz, resonance can develop during operation, leading to significant mechanical stress. The resulting motion is often elliptical and can cause severe damage over time.
In recent years, large motors have experienced issues due to resonance in the stator winding ends, as well as other factors like loose binding ropes, improperly tightened bracket bolts, or worn insulation on the wire rods. These problems can lead to unexpected failures and require careful inspection and maintenance.