Common sense of air compressor motor | Why does the motor generate shaft current?

1. Why does the motor generate shaft current?

（1）Causes of shaft current:

— Magnetic field asymmetry;

— There are harmonics in the supply current;

—The manufacturing and installation are not good, and the air gap is uneven due to the eccentricity of the rotor;

— There is a gap between the two semicircles of the detachable stator core;

— The number of pieces of the fan-shaped stacked stator core is not selected properly.

（2）Hazards of shaft current:

The surface of the motor bearing or the ball is eroded, forming point-like micropores, which deteriorates the running performance of the bearing, increases friction loss and heat generation, and eventually causes the bearing to burn out.

（3）Prevention of shaft currents:

— Eliminate pulsating magnetic flux and power harmonics (such as installing an AC reactor on the output side of the inverter);

— 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 motors. The following three aspects should be paid attention to:

(1) The higher the altitude, the greater the temperature rise of the motor and the lower the output power. However, when the air 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 remain unchanged;

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

(3) The altitude is unfavorable to the commutation of the DC motor, so attention should be paid to the selection of carbon brush materials.

 

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

When the motor is running with light load, it will cause:

(1) The power factor of the motor is low;

(3) The efficiency of the motor is low. It will cause waste of equipment and uneconomical operation.

 

4. What are the reasons for the overheating of the motor?

(1) The load is too large;

(2) lack of phase;

(3) The air duct is blocked;

(4) Low speed running time is too long;

(5) The harmonics of the power supply are too large.

 

5. What work needs to be done before the long-term unused motor is put into operation?

(1) Measure the insulation resistance between the phases of the stator and windings and the windings 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 the motor of Un=380V, R>0.38MΩ. If the insulation resistance is low, you can:

a: Dry the motor with no load for 2 to 3 hours;

b: Use 10% of the rated voltage of low-voltage alternating current to pass into the winding or connect the three-phase windings in series and then use direct current to bake, keeping 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 exposed to low temperatures for too long, it will:

(1) Motor insulation cracking; (2) Bearing grease freezing; (3) Solder powdering of 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) Unbalanced three-phase voltage;

(2) Poor welding or poor contact of a certain phase branch inside the motor;

(3) Turn-to-turn short circuit of motor winding or short circuit to ground or phase to phase;

(3) 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 made 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 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?

（1）Power supply:

— Poor contact of the switch;

— The transformer or line is disconnected;

— The fuse is blown.

（2）Motor aspect:

— The motor junction box screws are loose and in poor contact;

— Poor internal wiring welding;

— The motor winding is disconnected.

 

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

（1）Mechanical aspects:

— The bearing is poorly lubricated and the bearing is worn;

— The fastening screws are loose;

— There are sundries in the motor.

（2）Electromagnetic aspect:

— The motor is overloaded;

— Unbalanced three-phase current;

— lack of phase;

— The stator and rotor windings have short-circuit faults;

— The welding part of the cage rotor is separated and caused broken bars.

 

11. What work should be done before starting the motor?

(1) Measure the 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 in good condition;

(4) Check whether the fuse is suitable;

(5) Check the grounding of the motor and whether the zero connection is good;

(6) Check whether the transmission device is defective;

(7) Check whether the environment of the motor is suitable, and remove flammables and other sundries.

 

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

（1）The motor itself:

— The fit of the inner and outer rings of the bearing is too tight;

— There is a problem with the shape and position tolerance of the parts, such as the poor coaxiality of the machine base, end cover, shaft and other parts;

— Improper selection of bearings;

— The bearing is poorly lubricated or the bearing is not clean, and there are sundries in the grease;

— Shaft current.

（2）Usage:

— Improper installation of the unit, such as the coaxiality of the motor shaft and the driven device;

— The pulley is too tight;

— The bearing is not well maintained, the lubricating grease is insufficient or exceeds the service life, 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 has water intrusion;

(2) Dust or oil is accumulated on the winding;

(3) Insulation aging.

 

Sollant Group-Professional Compressed Gas Solutions Provider