1. Why does the motor produce shaft current?

 

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

   Causes of shaft current:

   (1) Asymmetrical magnetic field;

   (2) There are harmonics in the power supply current;

   (3) Poor manufacture and installation, uneven air gap due to eccentric rotor;

   (4) There is a gap between the two semicircles of the detachable stator core;

   (5) The selection of the number of pieces of the stator core with fan-shaped stacks is inappropriate.

  harm:

   The motor bearing surface or balls are corroded, and the shape of the point-like micro-holes deteriorates the running performance of the bearing, increases friction loss and heat generation, and finally causes the bearing to burn.

   prevention:

   (1) Eliminate pulsating magnetic flux and power harmonics (such as installing 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 an adverse effect 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 greater the temperature rise of the motor, the smaller the output power. But when the temperature decreases with the increase of altitude enough to compensate for the influence of altitude on temperature rise, the rated output power of the motor can be unchanged;

   (2) Anti-corona measures should be taken when high-voltage motors are used on plateaus;

   (3) The altitude is unfavorable 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 under light load, it will cause:

   (1) The motor power factor is low;

   (2) The motor efficiency is low.

   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 running time at low speed is too long;

   (5) Power harmonics are too large.

 

 

  5. What work needs to be done before a motor that has been left unused for a long time?

 

   (1) Measure the insulation resistance between each phase of the stator and the winding and the winding to the ground.

   The insulation resistance R should meet the following formula:

   R＞Un/(1000+P/1000)(MΩ)

   Un: rated voltage of motor winding (V)

   P: Motor power (KW)

   For a 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;

   b: Use low-voltage alternating current of 10% of the rated voltage to pass into the windings or connect the three-phase windings in series and then bake them with a 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?

 

  If the motor is too long in a low temperature environment, it will:

   (1) The motor insulation is cracked;

   (2) The bearing grease is frozen;

   (3) The solder powder of the wire joints.

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

 

 

  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 phase branch circuit inside the motor;

   (3) Short circuit between turns of motor winding or short circuit to ground or 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 is generally used to work 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, an increase in copper consumption, and ultimately 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 of the switch;

   (2) Transformer or line disconnection;

   (3) Fuse blown.

  Motor aspect:

   (1) Loose screws of the motor junction box and poor contact;

   (2) Poor internal wiring welding;

   (3) The motor winding is broken.

 

 

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

 

  Mechanical aspects:

   (1) Poor bearing lubrication, bearing wear;

   (2) The fastening screws are loose;

   (3) There is debris in the motor.

   Electromagnetic aspect:

   (1) Motor overload operation;

   (2) The three-phase current is unbalanced;

   (3) Lack of phase;

   (4) The stator and rotor windings have a short-circuit fault;

   (5) The welding part of the cage rotor is open and welded and the bar is broken.

 

 

   11. What work needs to be done before starting the motor?

 

   (1) Measure insulation resistance (for low-voltage motors, it should not be lower than 0.5MΩ);

   (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 good;

   (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 flammable materials and other debris.

 

 

  12. What are the reasons for overheating of motor bearings?

 

   The motor itself:

   (1) The inner and outer rings of the bearing are too tight;

   (2) The shape and position tolerances of parts have problems, such as the coaxiality of the machine base, end cover, shaft and other parts;

   (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 motor shaft and the driven device;

   (2) Pulling the pulley is too tight;

   (3) The bearing is not well maintained, the grease is insufficient or the service life is exceeded, and it becomes dry and deteriorated.

 

 

   13. What are the reasons for the low insulation resistance of the motor?

 

   (1) The winding is damp or water invades;

   (2) Dust or oil stains accumulate on the windings;

   (3) Insulation aging;

   (4) The insulation of the motor lead or wiring board is damaged.