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Daily inspection of motor and its maintenance - Solutions - Huaqiang
**Motor Maintenance and Inspection Guide**
The motor is composed of a stator frame, windings, insulation materials, a rotor, two end bearings, and an end cover. It has a relatively simple structure. Common causes of motor failure include phase loss, incorrect voltage or frequency, winding short circuits, open circuits, or grounding, poor bearing condition, internal or external dirt leading to poor heat dissipation, damaged cooling fans, inadequate ventilation, mechanical issues, long-term high-load operation, and high ambient temperatures.
More than 90% of the damage to shipboard electric motors results from insufficient daily inspection and maintenance by personnel. By consistently observing, listening, touching, measuring, and taking action, most faults can be prevented, reducing spare parts costs, repair expenses, and potential schedule delays.
1. **Observe**: During daily inspections, not only motor crew but also engineers and fuelers should monitor the motor’s current levels, check for leaks or drips around the motor, which could lead to reduced insulation and motor burnout. Also, ensure that the surrounding environment allows proper ventilation and heat dissipation. Clean the fan cover, blades, and motor exterior regularly. Any issues found should be addressed immediately.
2. **Listen**: Pay close attention to any abnormal sounds from the motor. In noisy engine rooms, use tools like a screwdriver or stick to detect unusual vibrations or bearing noises. Regular monitoring helps identify bearing oil levels, allowing timely refueling or replacement to prevent dry running, overheating, and damage.
Most manufacturers prefer open bearings for easier replacement. When using an oil gun, always use special bearing oil (-35°C to +140°C) and remove the bulkhead screws on the opposite side to allow old oil to drain. Improper oiling during refueling can cause pressure buildup, splashing oil onto the rotor, and affecting the motor's cooling function.
3. **Touch**: Use the back of your hand to feel the temperature around the motor. Under normal conditions, the bearing ends should be cooler than the middle winding area. If the bearing area feels excessively hot, check it in conjunction with the sound. If the entire motor is too hot, check the load, equipment, and ventilation based on the working current.
Depending on the insulation class of the motor, you can determine the maximum allowable operating temperature. Insulation classes are generally divided into Y, A, E, B, F, H, and C. The Y class has the lowest temperature rise (45°C), while the C class allows the highest (over 135°C). For bearings and other components, the measured temperature should ideally stay below 85°C. Most onboard motors are E or B class, while generators are often B or F class.
4. **Test**:
- When the motor is off, use an insulation tester to measure the resistance between phases and ground. If the insulation is low, use a baking lamp to improve it. The recommended value should be above 1 MΩ. Motors with built-in heating should not be turned off unless necessary. During docking, rainy weather, or winter, ensure the motor is waterproof, moisture-proof, and properly dried. Before departure, test the insulation of key motors. Protect motors in wet areas with canvas covers, but remove them during hot weather to avoid overheating.
- If a motor is soaked in seawater, disassemble it, rinse the rotor with hot fresh water (60–70°C), blow it dry with compressed air, and bake it inside with a baking lamp until the insulation returns to normal.
The resistance of the three-phase windings after repair should not differ by more than 4% from the average. Insulation resistance is typically measured when cold. After running for 30 minutes, thermal insulation should be checked. If there's a large difference, the winding paint may not be fully dry. The insulation resistance between the winding and ground (or casing) must meet the formula:
$$
\text{Insulation Resistance (MΩ)} = \frac{3 \times \text{Motor Rated Voltage (V)}}{\text{Motor Rated Power (KVA)} + 1000}
$$
For motors with only three leads, insulation between windings cannot be tested. The insulation resistance of rewound motors should be at least 5 MΩ.
- While the motor is running, measure the three-phase voltage and current to ensure balance. Voltage should be roughly equal, and each phase current should not deviate by more than 10% from the average. Large differences may indicate a turn-to-turn short circuit. Disconnect the load to measure no-load current, which varies depending on the number of poles and motor capacity. Ensure the motor is connected correctly as Y or Δ. Mistakes in connection can increase or decrease the working current. Poor winding design or incorrect wire diameter can also affect no-load current. These checks help assess the motor's condition.
5. **Act**: Not only address problems found during inspections, but also follow scheduled maintenance tasks such as tightening screws, checking wiring, cleaning, and inspecting. For example, loose motor cover screws on a "XX Sea" air conditioner fan led to a burned-out broom motor. On the "X Strong" boiler fan, a missing fan blade caused a blockage and motor burnout due to lack of inspection. On the "Kang X" wheel, a motor burned out because its bearing was not replaced in time. On the "X Strong" wheel, a main oil pump motor failed due to damp weather and low insulation, which wasn't baked in time. These incidents show that even if problems are identified, failure to act leads to breakdowns.
When replacing bearings, try to use imported ones, as many Chinese-refurbished bearings may have unreliable quality. If the bearing outer race doesn’t fit tightly with the end cap, adjust the inner cover with copper pads or bushings. Ensure the center point is fixed to prevent damage. Before starting, confirm that the shaft extension and end cover are tight, the rotor rotates smoothly, and the winding connections are correct.
Many engineers pressurize the packing box when a pump leaks. Experienced engineers manually check the packing while adjusting the cap screw, then run the motor for a few cycles to ensure normal current. Inexperienced engineers may tighten the brake without considering the motor, causing excessive starting current and burning out the winding. When repairing mechanical devices driven by motors, carefully align the motor and driven machine to ensure smooth rotation and flexible coupling. Only through careful, thorough, and meticulous work can equipment integrity be improved.