Fault inspection of the motor(3)

When the motor is running, there is noise. The fault occurred in the mechanical and electromagnetic parts of the motor.

Here is the professional English translation of the motor fault diagnosis text, in standard technical English:

The method of differentiation is as follows: run the motor first and listen carefully to the noise during operation, then cut off the power supply. If the abnormal noise disappears, the fault lies in the electromagnetic part of the motor; otherwise, it is a mechanical fault.

Mechanical Noise

(1) Noise from bearings. Possible causes include broken bearing balls or insufficient lubricating oil. The inspection method is to place the tip of a screwdriver against the outside of the bearing cap and press your ear against the handle. If a “gurgling” sound is heard (a suitable hollow tube is recommended for clearer sound detection), it indicates a bearing fault.

(2) Noise caused by air friction is uniform and not intense, which is a normal phenomenon.

(3) Noise caused by motor rotor rubbing against the stator core is a “scraping” sound. If abnormal noise is found during operation of a newly repaired motor, check whether the motor current is balanced, whether rotation is smooth, and whether the rated speed is reached. If none of the above problems exist, the cause may be that insulation paper or bamboo wedges in the stator slots protrude beyond the slot openings, causing friction between the rotor and certain parts. This noise is sharp and high-pitched.

Electromagnetic Noise

Poor matching between the rotor and stator (usually occurs in new motors or when rotors of the same model are interchanged). Under normal conditions, the stator should be slightly longer than the rotor. This noise is a low, dull hum (also known as idle hum).

Axial Displacement of the Rotor

This causes electromagnetic noise, increases no-load current, and reduces the electromagnetic performance of the motor.

Causes include:

Improper matching of stator and rotor slot numbers (common in new motors);

Wrong installation of a rotor from another motor (or emergency replacement), resulting in uneven air gap between stator and rotor;

Out-of-round stator or rotor, or slightly bent shaft;

Phase loss, inter-turn short circuit, or phase-to-phase short circuit in the motor winding;

Overload operation.

All of the above can lead to electromagnetic noise.

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