Key points for the maintenance and inspection of motors

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I. Key points for visual inspection of motors 01. Four key points Point 1: Check whether the motor’s speed is normal, and whether there is any speed fluctuation or stalling (this can be checked using a tachometer). Point 2: Check whether the anchor bolts of the motor are loose, whether the foundation is intact, and whether there are any debris around it, etc. Point 3: Check whether the motor’s enclosure and coupling are in good condition, and whether the screws securing the coupling are loose. Check whether there are any foreign objects or dirt on the motor body, and whether the ventilation slots are blocked, etc. Point 4: For air-cooled motors, it is necessary to check whether the air ducts are unobstructed, whether all connections are secure, whether the valves on the ducts are in the correct position, and whether natural ventilation is adequate. For water-cooled motors, check whether the cooling water pipeline is unobstructed, if there is any leakage of cooling water, and whether the temperature and flow rate of the cooling water are normal. 02. Key points: Point 1 – For synchronous motors, wound-rotor motors, etc., it is also necessary to check whether the contact between the brushes and the slip rings is good, whether there is any commutation spark, and whether the color and shape of such sparks are abnormal. Point 2: Whether the excitation system of the synchronous motor is functioning properly. Point 3: Check whether the wiring of the motor and the connection of the ground wire are loose. 03. Key Points Point 1: Check whether the current of the operating motor exceeds the allowable value, whether it corresponds to the actual load, whether there are any sudden changes, whether the voltage is within the acceptable range, and whether there is a phase loss. Point 2: Whether the temperatures of various parts of the motor exceed the allowable values. Point 3: Check whether the bearing is overheating, whether there is sufficient lubricating grease, whether the oil level indicated by the oil level indicator is normal, and whether the oil ring rotates smoothly. Point 4: Check whether the motor is smoking and whether there is any overheating such as discoloration of the windings. II. Key points for auditory inspection of motors: Check whether the operating sound of the motor is normal and whether there are any abnormal sounds. III. Key points for checking the motor’s odor: Look for any abnormal odors, such as burnt smells or other unusual smells. IV. Key points for tactile inspection of motors Point 1: Determine whether the vibration during motor operation is normal, and check for any abnormal vibrations by feeling them with the hand; if the vibration is severe, use a vibration meter for measurement. (Generally, rolling or sliding bearings produce a uniform operating sound; the bearing does not exhibit significant vibration when touched. When vibration is high, it can be measured with a vibration meter – a value of less than 3.5 mm/s is considered good.) Point 2: Check whether the temperature of the motor during operation is normal, and whether there is any abnormal temperature rise (read the value using an embedded thermometer; if such a thermometer cannot be installed, judge based on tactile sensation, or use a rod-type thermometer or infrared temperature gun for measurement). Point 3: The maximum allowable temperature of the outer surface of a bearing when touched by hand is generally considered to be very hot (3–4 seconds of contact), meaning that the upper limit for the temperature of rolling bearings is usually 80°C. V. Key points for auditory inspection of motors Point 1: The protection measures for the transmission parts that are easily accessible to the inspector must be robust and reliable. Point 2: When the motor has not been in use for three months or more, its insulation resistance should be measured before it is put back into operation. For those with a rated voltage of 500V or higher, a megohmmeter rated at 1000V or 2500V should be used. Point 3: For motors that are supplied with cooling air from the outside, the supply of cooling air should be stopped immediately after shutdown to prevent the motor from getting damp. For motors that have been shut down for more than 24 hours, a heater should be turned on to prevent a decrease in the motor’s insulation resistance. Point 4: For motors that are in operation, if the insulation resistance value of their windings decreases by 50% or more when compared to the previous value (after conversion to the same temperature), the cause must be identified, and a dielectric strength test should be conducted if necessary. Point 5: The core temperature should be measured using an alcohol thermometer; in areas with a magnetic field, a mercury thermometer must not be used, to avoid heat generation due to eddy current losses in the mercury. Point 6: For wound-rotor motors, it is necessary to check whether the copper strands of the brushes and slip rings are in good condition, whether the contact is tight, and whether there is any short circuit or overheating with respect to the housing. There should be no dirt inside the brushes and brush holders; if carbon buildup is present, it must be removed promptly. The brushes should be replaced when they are worn down to 2/3 of their original length. Point 7: Key points for checking the external conditions after the motor is installed on-site: external wiring (whether the phase sequence is correct and the connections are secure), grounding wires (ensure they are reliable), thermometers, the rotation direction of the fan motor, connections of the cooling water pipes, the status of valves, etc. – in other words, whether the wiring and connections of all electrical components are in good condition.