79 Key Professional Knowledge Areas in Electrical Engineering （61-70）

By clyde March 9th, 2025 15 views

61.What should be checked if the bearing temperature of a motor is too high during operation?

① Inappropriate lubricant grade;
② Poor-quality or degraded lubricant;
③ Excessive or insufficient lubricant in the bearing housing;
④ Contaminants in the lubricant;
⑤ Contact between rotating and stationary parts;
⑥ Bearings slipping on the inner or outer race;
⑦ Incorrect or poor-quality bearings;
⑧ Misalignment of the coupling;
⑨ Belts tightened excessively;
⑩ Excessive vibration of the motor.

62.Why must the rotor of a motor be balanced? Which types of motors can have only static balancing?

During manufacturing, various factors such as material inconsistencies, casting voids, part weight errors, and machining inaccuracies can cause imbalance in the rotor's weight. Therefore, after assembly, the rotor must be balanced. For motors with six poles or more, or rated speeds of 1000 rpm or less, the rotor can be statically balanced. For other motors, dynamic balancing is required.

63.What precautions should be taken when using a welder?

New or long-unused welders may experience reduced insulation resistance due to moisture, leading to short circuits or grounding faults that could cause equipment or personal injury. Before use, check the insulation resistance with a megohmmeter to ensure it meets standards. Before starting a new welder, inspect the electrical system and contactor for proper functionality. Test-run the welder under no-load conditions to confirm there are no electrical hazards before testing under load. Finally, put it into normal operation. DC welders should rotate in the specified direction, and fans should blow air from above to cool the welder.

64.What protections are typically provided for medium and small capacity induction motors?

① Short-circuit protection: Fuses are commonly used as short-circuit protection devices.
② Undervoltage protection: The electromagnetic coil in the magnetic starter provides undervoltage protection in the motor control circuit. Automatic air switches and autotransformers generally include undervoltage release mechanisms to protect the motor against overload in such cases.
③ Overload protection: Thermal relays serve as overload protection devices for motors.

65.What should be noted during the operation and maintenance of induction motors?

① Keep the motor surroundings clean.
② Use instruments to monitor voltage and current fluctuations. Motors typically allow voltage variations within ±5% of the rated voltage, with phase voltage differences not exceeding 5%. Phase current imbalances should not exceed 10%, and check for single-phase operation.
③ Regularly check the motor's temperature rise using a thermometer, ensuring it does not exceed the maximum allowable value.
④ Listen for abnormal noises in the bearings, ensure good sealing, and regularly replace lubricating oil. The oil change cycle is generally 1000 hours for sliding bearings and 500 hours for rolling bearings.
⑤ Monitor the motor's sound, smell, vibration, and drive system condition. Under normal operation, the motor should produce uniform sound without unusual noise or abnormal sounds.

66.How to correctly dismantle and repair an induction motor?

Before dismantling the motor, prepare all necessary tools and perform pre-dismantling inspections and record-keeping. Steps for dismantling:
① Remove the pulley or coupling. Mark them before removal to facilitate reinstallation. Clean rust and debris before reinstalling.
② Remove the end cover: First remove the bearing cover, then the end cover, marking the front and rear covers for correct reassembly.
③ Remove the rotor: Place wear-resistant paper between the stator and rotor to prevent damage to the stator winding. If the rotor is heavy, use lifting equipment. Before installation, check for debris inside the stator, then install the rotor along with the fan and rear cover.

67.How to diagnose fault causes from abnormal vibrations and sounds in induction motors?

Abnormal vibrations and sounds in induction motors are mainly caused by mechanical and electromagnetic issues.
Mechanical causes:
① Damaged or loose fan blades causing collision with the fan shroud, producing intermittent noise.
② Worn or improperly mounted bearings causing severe rotor eccentricity, resulting in rubbing between the stator and rotor, generating intense vibration and uneven collision sounds.
③ Loose foot bolts or unstable foundation causing abnormal vibration under electromagnetic torque.
④ Lack of lubrication or damaged steel balls in long-used motors causing abnormal grinding or rumbling sounds in the bearing chamber.
Electromagnetic causes:
① Unbalanced three-phase currents, excessive load, or single-phase operation causing a drop in speed and low-pitched howling sounds.
② Short circuits in the stator or rotor windings or broken bars in squirrel-cage rotors causing fluctuating buzzing sounds and body vibration.

68.Why does the bearing temperature of an induction motor exceed the casing temperature?

① Long-term lack of lubrication increases frictional wear, overheating the bearing. Excessive or overly viscous lubrication during normal operation can also cause overheating.
② Hard particles in the lubricant or improper cleaning during replacement accelerate bearing wear and overheating, potentially damaging the bearing.
③ Improper assembly, such as uneven tightening of end cover screws, misaligning the two bearing centers or unbalancing the outer bearing race, reduces bearing flexibility and increases friction under load.
④ Tight belts or misalignment between the motor and driven equipment increase bearing load and heat generation.
⑤ Poor-quality or inappropriate bearings, such as rusted inner/outer races or irregular steel balls, contribute to overheating.
⑥ Damaged bearings during operation lead to excessive heating.

69.Why does the squirrel-cage rotor winding of an induction motor not require insulation against ground, while the wound rotor winding does?

A squirrel-cage rotor can be considered a multiphase winding where the number of phases equals the number of rotor bars per pole pair, with each phase having 1/2 turn. Since the induced rotor voltage is typically very low and silicon steel resistance is much higher than copper or aluminum, most current flows through the conductors without requiring insulation.
In contrast, wound rotors have the same number of phases as the stator and more turns per phase. According to

E_{2} = 4.44 K_{2} F_{2} W_{2} Φ

, the induced voltage in each phase is much higher. Without insulation, ground faults could occur, potentially damaging or burning out the motor.

70.How to repair common faults in the rotor shaft of an induction motor?

① Shaft bending: If the shaft extension exhibits jumping during operation, the shaft is bent. Severe bending can cause stator-rotor friction. Remove the rotor and correct it based on the specific situation.
② Shaft core seat wear: Long-term operation can loosen the shaft and core seats. Consider knurling the mating surfaces. If core displacement is possible, add keyways in grooves on both ends of the shaft and weld them in place.
③ Shaft journal wear: Repeated bearing removal can wear the shaft journal. Knurling can address minor wear, while severe wear requires welding and machining to restore dimensions.
④ Shaft cracks: If transverse cracks do not exceed 10-15% of the diameter or longitudinal cracks do not exceed 10% of the shaft length, they can be repaired with welding. Severely cracked or broken shafts require replacement.

79 Key Professional Knowledge Areas in Electrical Engineering （51-60）

79 Key Professional Knowledge Areas in Electrical Engineering （71-79）