Thirteen questions about the motor 1. Why does the motor generate shaft current? The current in the shaft---bearing seat---base circuit of the motor is called shaft current.Causes of shaft current: (1) The magnetic field is asymmetric; (2) There are harmonics in the power supply current; (3) Poor manufacturing and installation, uneven air gap due to rotor eccentricity; (4) There is a gap between the two semicircles of the detachable stator core; (5) The number of pieces of the stator core with fan-shaped stacking is not suitable.harm: The surface of the motor bearing or the ball is eroded, and the shape range is point-like micro-holes, which deteriorates the running performance of the bearing, increases friction loss and heat, and eventually causes the bearing to burn.prevention: (1) Eliminate the pulsating magnetic flux and power harmonics (such as adding an AC reactor on the output side of the inverter); (2) When designing the motor, insulate the bearing seat and base of the sliding bearing, and insulate the outer ring and end cover of the rolling bearing. 2. Why can't general motors be used in plateau areas? Altitude has adverse effects on motor temperature rise, motor corona (high voltage motor) and commutation of DC motor. The following three aspects should be noted: (1) The higher the altitude, the higher the temperature rise of the motor, the lower the output power. However, when the temperature decreases with the increase of altitude enough to compensate for the influence of altitude on the temperature rise, the rated output power of the motor can remain unchanged; (2) Anti-corona measures should be taken when the high-voltage motor is used in the plateau; (3) The altitude is not good for the commutation of the DC motor, so pay attention to the selection of carbon brush materials. 3. Why is the motor not suitable for light load operation? When the motor runs at light load, it will cause: (1) The power factor of the motor is low; (2) The motor efficiency is low. It will cause equipment waste and uneconomical operation. 4. What are the reasons for the motor overheating? (1) The load is too large; (2) lack of phase; (3) The air duct is blocked; (4) The low-speed running time is too long; (5) Power harmonics are too large. 5. What work should be done before the long-term unused motor is put into use? (1) Measure the insulation resistance of the stator, each phase of the winding and the winding to ground. Insulation resistance R should satisfy the following formula: R＞Un/(1000+P/1000)(MΩ) Un: rated voltage of motor winding (V) P: Motor power (KW) For the motor with Un=380V, R>0.38MΩ. If the insulation resistance is low, you can: a: The motor runs for 2 to 3 hours without load for drying; b: Pass the low-voltage alternating current of 10% of the rated voltage into the windings or connect the three-phase windings in series and bake with direct current to keep the current at 50% of the rated current; c: Use a fan to send hot air or heating elements to heat. (2) Clean the motor. (3) Replace the bearing grease. 6. Why can't the motor in the cold environment be started arbitrarily? Motors in a low temperature environment for too long will: (1) Motor insulation cracks; (2) Bearing grease freezes; (3) The solder powder of the wire joint is powdered. Therefore, the motor should be heated and stored in a cold environment, and the windings and bearings should be checked before running. 7. What are the reasons for the unbalanced three-phase current of the motor? (1) The three-phase voltage is unbalanced; (2) Poor welding or poor contact of a certain phase branch inside the motor; (3) Short circuit between turns of the motor winding or short circuit to ground and phase to phase; (4) Wiring error. 8. Why can't a 60Hz motor be connected to a 50Hz power supply? When the motor is designed, the silicon steel sheet generally works in the saturation region of the magnetization curve. When the power supply voltage is constant, reducing the frequency will increase the magnetic flux and the excitation current, resulting in an increase in the motor current and copper consumption, which will eventually lead to an increase in the temperature rise of the motor. In severe cases, the motor may be burned due to overheating of the coil. 9. What are the reasons for the lack of phase of the motor? Power supply: (1) The switch is in poor contact; (2) Transformer or line disconnection; (3) The fuse is blown.Motor aspects: (1) The screw of the motor junction box is loose and the contact is poor; (2) Poor welding of internal wiring; (3) The motor winding is disconnected. 10. What are the reasons for the abnormal vibration and sound of the motor? Mechanical aspects: (1) The bearing is poorly lubricated and the bearing is worn; (2) The fastening screws are loose; (3) There are debris in the motor.Electromagnetic aspects: (1) Motor overload operation; (2) Three-phase current is unbalanced; (3) lack of phase; (4) The stator and rotor windings have short-circuit faults; (5) The welding part of the cage rotor is broken due to open welding. 11. What work should be done before starting the motor? (1) Measure the insulation resistance (not less than 0.5MΩ for low-voltage motors); (2) Measure the power supply voltage. Check whether the motor wiring is correct and whether the power supply voltage meets the requirements; (3) Check whether the starting equipment is in good condition; (4) Check whether the fuse is suitable; (5) Check whether the motor is grounded and the zero connection is good; (6) Check whether the transmission device is defective; (7) Check whether the motor environment is suitable, and remove flammables and other sundries. 12. What are the reasons for the overheating of motor bearings? The motor itself: (1) The inner and outer rings of the bearing are too tight; (2) There is a problem with the shape and position tolerance of the parts, such as the coaxiality of the machine base, end cover, shaft and other parts is not good; (3) Improper selection of bearings; (4) The bearing is poorly lubricated or the bearing is not cleaned properly, and there are sundries in the grease; (5) Shaft current.Usage: (1) Improper installation of the unit, such as the coaxiality of the motor shaft and the shaft of the device being dragged meets the requirements; (2) The pulley is pulled too tightly; (3) The bearing is not well maintained, the grease is insufficient or the service life is exceeded, and the bearing is dry and deteriorated. 13. What are the reasons for the low insulation resistance of the motor? (1) The winding is damp or has water intrusion; (2) Dust or oil is accumulated on the winding; (3) insulation aging; (4) The insulation of the motor lead or terminal board is damaged.
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