The 13th motor asked, which one is difficult for you?

07/10/2019

1. Why does the motor generate shaft current?

    The shaft of the motor---bearing---the current in the base circuit is called the shaft current.

    Reasons for shaft current generation:
    (1) Asymmetry of the magnetic field;
    (2) There are harmonics in the supply current;
    (3) The manufacturing and installation are not good, and the air gap is not uniform due to the eccentricity of the rotor;
    (4) There is a gap between the two semicircles of the detachable stator core;
    (5) The number of tiles having the stator cores stacked in a fan shape is not properly selected.

    harm
    The surface of the motor bearing or the ball is eroded, and the shaped micro-holes are formed, which deteriorates the running performance of the bearing, increases the friction loss and heat, and eventually causes the bearing to burn.

    prevention:
    (1) Eliminate the pulsating magnetic flux and power supply harmonics (such as adding an AC reactor on the output side of the inverter);
    (2) When designing the motor, the bearing housing and the base of the sliding bearing are insulated, and the outer ring and the end cover of the rolling bearing are insulated.

    2. Why can't general motors be used in highland areas?

    Altitude has an adverse effect on motor temperature rise, motor corona (high voltage motor) and DC motor commutation. The following three aspects should be noted:
    (1) The altitude is high, the greater the temperature rise of the motor, the smaller the output power. However, when the temperature decreases with increasing altitude to compensate for the effect of altitude on temperature rise, the rated output power of the motor can be unchanged;
    (2) Anti-corona measures should be taken when the high-voltage motor is used in the plateau;
    (3) Altitude is unfavorable for DC motor commutation, and attention should be paid to the selection of carbon brush materials.

    3. Why should the motor not run at light load?

    When the motor is running at light load, it will cause:
    (1) The motor power factor is low;
    (2) The motor efficiency is low.
    It will cause equipment waste and uneconomical operation.

    4. What are the causes of motor overheating?
    (1) The load is too large;
    (2) lack of phase;
    (3) blocked air ducts;
    (4) The low speed running time is too long;
    (5) The power supply harmonics are too large.

    5. What work does the motor need to do before it is put into use?
    (1) Measure the insulation resistance between the stator and winding phases and the winding to ground.
    The insulation resistance R should satisfy the following formula:
    R>Un/(1000+P/1000)(MΩ)
    Un: motor winding rated voltage (V)
    P: motor power (KW)
    For motors with Un=380V, R>0.38MΩ.
    If the insulation resistance is low, you can:
    a: The motor is running at no load for 2~3h;
    b: use 10% rated voltage of low-voltage alternating current to enter the winding or connect the three-phase windings in series and then dry with DC to maintain the current at 50% of the rated current;
    c: Heated by hot air or heating element.
    (2) Clean the motor.
    (3) Replace the bearing grease.

    6. Why can't I start the motor in a cold environment at will?

    The motor is too long in a low temperature environment and will:
    (1) Motor insulation cracking;
    (2) bearing grease freezing;
    (3) Wire joint solder powder.

    Therefore, the motor should be stored in a cold environment and the windings and bearings should be inspected before operation.

    7. What are the reasons for the motor three-phase current imbalance?
    (1) Three-phase voltage imbalance;
    (2) The welding of a certain phase branch inside the motor is poor or the contact is not good;
    (3) Short circuit between motor windings or short circuit to ground and phase;
    (4) Wiring error.

    8. Why can't a 60Hz motor be connected to a 50Hz power supply?

    When designing the motor, the silicon steel sheet is generally operated in the saturation region of the magnetization curve. When the power supply voltage is constant, lowering the frequency will increase the magnetic flux, and the excitation current will increase, resulting in an increase in the motor current and an increase in copper consumption, eventually resulting in an increase in the temperature rise of the motor. In severe cases, the motor may be burnt due to overheating of the coil.

    9. What are the reasons for the lack of phase of the motor?

    Power supply:
    (1) Poor contact in the switch;
    (2) Transformer or line disconnection;
    (3) Insurance fuses.

    Motor:
    (1) The motor terminal box screw loose contact is poor;
    (2) Poor internal wiring;
    (3) The motor winding is broken.

    10. What are the causes of abnormal vibration and sound of the motor?

    Mechanical aspects:
    (1) poor bearing lubrication and bearing wear;
    (2) The fastening screw is loose;
    (3) There are debris in the motor.

    Electromagnetic aspects:
    (1) Motor overload operation;
    (2) Three-phase current imbalance;
    (3) lack of phase;
    (4) The stator and rotor windings have a short circuit fault;
    (5) The broken part of the welded part of the cage rotor is broken.

    11. What work needs to be done before starting the motor?
    (1) Measure the insulation resistance (for low-voltage motors should not be lower than 0.5MΩ);
    (2) Measure the power supply voltage. Check if the motor wiring is correct and the power supply voltage meets the requirements;
    (3) Check if the starting equipment is good (public number: pump housekeeper);
    (4) Check if the fuse is suitable;
    (5) Check the grounding of the motor and whether the zero connection is good;
    (6) Check the transmission for defects;
    (7) Check if the motor environment is suitable and remove flammable materials and other sundries.

    12. What are the causes of motor bearing overheating?

    The motor itself:
    (1) The inner and outer rings of the bearing are too tight;
    (2) There are problems with the shape and position tolerance of the parts, such as the coaxiality of the parts such as the base, end cover and shaft;
    (3) improper selection of bearings;
    (4) The bearing is poorly lubricated or the bearing is not cleaned, and there is debris in the grease;
    (5) Shaft current.

    Use aspect:
    (1) Improper installation of the unit, such as the coaxiality of the shaft of the motor shaft and the driven device;
    (2) The pulley is pulled too tightly;
    (3) The bearing is not well maintained, the grease is insufficient or exceeds the service period, and the hair is deteriorated.

    13. What are the reasons for the low insulation resistance of the motor?
    (1) The winding is damp or has water intrusion;
    (2) accumulation of dust or oil on the windings;
    (3) Insulation aging;
    (4) The insulation of the motor lead or terminal block is broken.