Jiangsu Qilong Electronic Technology Co., Ltd. , https://www.qilongtouch.com
Daily inspection of motor and its maintenance - Solutions - Huaqiang
**Motor Maintenance and Inspection Guide**
The motor consists of a stator frame, windings, insulation materials, a rotor, two end bearings, and an end cover, making it relatively simple in design. Common causes of motor failure include phase loss, incorrect voltage or frequency, winding short circuits, open circuits, grounding, poor bearing performance, internal or external dirt, poor heat dissipation due to thick paint or blocked cooling fans, mechanical issues, long-term high load, and high ambient temperatures. Over 90% of motor damage on ships is due to insufficient daily inspections and maintenance by the crew. By consistently observing, listening, touching, measuring, and acting, most faults can be prevented, reducing spare parts costs and potential schedule delays.
**1. Observe:**
Daily inspections are essential, not only for the motor crew but also for engineers and fuelers. They should monitor current levels, check for leaks or drips around the motor, which could lead to reduced insulation and motor burnout. Inspect the surrounding area for objects that may block ventilation and heat dissipation. Clean the fan cover, blades, and motor exterior regularly to ensure proper cooling. Any issues found should be addressed immediately.
**2. Listen:**
Carefully listen for abnormal sounds from the motor. In noisy environments, use tools like a screwdriver or stick to detect unusual vibrations or bearing noises. Regular listening helps identify bearing oil levels, allowing timely lubrication or replacement to prevent dry running, overheating, and eventual motor failure.
Most manufacturers replace large bearings with open bearings for easier maintenance. When using an oil gun, special bearing oil (-35°C to +140°C) must be used, and the bulkhead screws on the opposite side should be removed to allow old oil to drain. During refueling, pressure can cause oil to enter the motor, splashing onto the rotor and affecting its cooling function.
**3. Touch:**
Use the back of your hand to check the temperature around the motor. Under normal conditions, the temperature at both ends of the motor should be lower than the middle section. If the bearing areas are excessively hot, check them alongside the measured sound. If the overall temperature is too high, investigate the load, equipment, and ventilation. The ideal temperature for thermometers should be below 85°C. Most onboard motors are E or B grade, while generators are typically B or F grade.
**4. Test:**
When the motor is off, use an insulation meter to measure ground or phase resistance. If the reading is low, use a baking lamp to improve insulation and avoid winding burnout (recommended value >1 MΩ). Motors with electric heating should not be turned off unless necessary. Pay special attention to waterproofing, moisture protection, and drying during docking, wet weather, and winter. Insulation checks are required before departure and docking, especially for motors in wet areas. If a motor is suspected of being waterlogged, inspect it thoroughly before use. A canvas cover should be used to protect the motor, but it should be removed during high-temperature periods to prevent overheating.
The resistance of the three-phase windings of a repaired motor should not differ by more than 4% from the average. Insulation resistance is generally measured in cold conditions (normal temperature). After running for 30 minutes, thermal insulation should be checked. If the difference between cold and hot readings is significant, the winding paint may not be fully dry. The insulation resistance between windings and ground should not be less than the value calculated by the formula:
$$
\text{Insulation Resistance (MΩ)} = \frac{3 \times \text{Motor Rated Voltage (V)}}{\text{Motor Rated Power (KVA)} + 1000}
$$
Some motors have only three leads, so inter-winding insulation cannot be tested. The insulation resistance of rewound motors should be at least 5 MΩ.
When the motor is running, measure the three-phase voltage and current to ensure balance. Voltage should be equal, and the error between each phase current and the average should not exceed 10%. If the current difference is large, there may be a turn-to-turn short circuit. It may be necessary to disconnect the load and measure no-load current. For example, a 2-pole motor at 2800 rpm should draw about 1/3 of the rated current; a 4-pole motor at 1600 rpm should draw about 40%, and a 6-pole motor at 900 rpm should draw about 55%. The Y and Δ connections must be correct, as mismatching can cause overcurrent or undercurrent. Incorrect winding or wire diameter (0.2–1.0 mm) can also increase no-load current. These observations help assess the motor’s condition.
**5. Act:**
Not only should problems identified during inspection be addressed promptly, but regular maintenance such as tightening screws, checking wiring, disassembling, cleaning, and servicing should also be performed. For example, loose motor cover screws on a "XX Sea" air conditioner fan caused a broom operation to burn out. A "X Strong" boiler fan blade fell off due to lack of inspection, causing a fire. Another case involved a "Kang X" wheel where poor bearing performance led to motor burnout due to delayed replacement. A "X Strong" main oil pump motor failed due to damp weather and low insulation resistance, which wasn’t baked in time. These incidents highlight the importance of not just identifying issues but also taking action.
If bearings need to be replaced, it's best to use original imports, as many Chinese-refurbished bearings may not meet quality standards. If the bearing outer ring and end cap seat don’t fit tightly, remove the inner cover, copper pad, or bushing accordingly. Ensure the center point is fixed to prevent damage. Before starting, confirm that the shaft extension is tightened, the rotor rotates freely, and the winding connections are correct.
Many engineers pressurize leaking pumps without checking the motor. Experienced engineers manually adjust the packing, balance the cap screw, and test the motor by starting and stopping a few times to ensure normal current. Inexperienced engineers may tighten the brake too much, leading to excessive starting current and motor burnout. When repairing or replacing mechanical equipment driven by motors, carefully align the motor and driven machine to ensure smooth rotation and flexible coupling. Only through meticulous and serious work can equipment integrity be improved.