1. Why Do Motors Generate Shaft Current?

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

Causes of Shaft Current:

(1) Asymmetric magnetic field;(2) Harmonics in the supply current;(3) Poor manufacturing or installation, leading to uneven air gap due to rotor eccentricity;(4) Gaps between the two semicircles of a detachable stator core;(5) Improper selection of the number of segments for fan-shaped laminated stator core sheets.

Hazards:

Erodes the motor bearing surface or balls, forming dotted micro-holes. This deteriorates the bearing’s operating performance, increases friction loss and heat generation, and may eventually cause bearing burnout.

Prevention Measures:

(1) Eliminate pulsating magnetic flux and power supply harmonics (e.g., install an AC reactor on the output side of the frequency converter);(2) During motor design, insulate the bearing housing and base of sliding bearings, and insulate the outer ring of rolling bearings from the end cover.

2. Why Cannot Ordinary Motors Be Used in Plateau Areas?

Altitude has adverse effects on motor temperature rise, motor corona (for high-voltage motors), and commutation of DC motors. Attention should be paid to the following three aspects:(1) The higher the altitude, the greater the motor temperature rise and the smaller the output power. However, if the temperature drop with increasing altitude sufficiently compensates for the impact of altitude on temperature rise, the rated output power of the motor can remain unchanged;(2) High-voltage motors must adopt anti-corona measures when used in plateau areas;(3) Altitude is unfavorable for the commutation of DC motors, so attention should be paid to the selection of brush materials.

3. Why Is Light-Load Operation Undesirable for Motors?

Light-load operation of motors causes the following issues:(1) Low power factor of the motor;(2) Low efficiency of the motor.This results in equipment waste and uneconomical operation.

4. What Are the Causes of Motor Overheating?

(1) Excessive load;(2) Phase loss;(3) Blocked air ducts;(4) Prolonged low-speed operation;(5) Excessive power supply harmonics.

5. What Work Needs to Be Done Before Putting a Long-Unused Motor into Operation?

(1) Measure the insulation resistance between each phase of the stator winding and between the windings and the ground.The insulation resistance R should meet the following formula:R>Un/(1000+P/1000)(MΩ)Where:Un: Rated voltage of the motor winding (V);P: Motor power (KW).For motors with Un=380V, R>0.38MΩ.If the insulation resistance is low, the following methods can be used:a. Run the motor at no load for 2–3 hours to dry it;b. Pass low-voltage AC (10% of the rated voltage) through the windings, or connect the three-phase windings in series and dry them with DC, maintaining the current at 50% of the rated current;c. Blow hot air into the motor using a fan or heat it with heating elements.(2) Clean the motor;(3) Replace the bearing grease.

6. Why Cannot Motors in Cold Environments Be Started Randomly?

Prolonged exposure of motors to low-temperature environments will cause:(1) Cracking of the motor insulation;(2) Freezing of bearing grease;(3) Powdering of solder at wire joints.Therefore, motors in cold environments should be stored with heating. Before operation, the windings and bearings must be inspected.

7. What Are the Causes of Unbalanced Three-Phase Current in Motors?

(1) Unbalanced three-phase voltage;(2) Poor welding or contact in a phase branch inside the motor;(3) Inter-turn short circuit of the motor winding, or short circuit to ground/between phases;(4) Incorrect wiring.

8. Why Cannot a 60Hz Motor Be Connected to a 50Hz Power Supply?

Motors are generally designed to operate the silicon steel sheet in the saturation region of the magnetization curve. When the power supply voltage is constant, reducing the frequency will increase the magnetic flux and excitation current, leading to higher motor current and copper loss. This raises the motor temperature, and in severe cases, may burn the motor due to overheating of the windings.

9. What Are the Causes of Motor Phase Loss?

Power Supply Side:

(1) Poor contact of switches;(2) Broken transformer or line;(3) Blown fuse.

Motor Side:

(1) Loose screws and poor contact in the motor terminal box;(2) Poor internal wiring welding;(3) Broken motor winding.

10. What Are the Causes of Abnormal Motor Vibration and Noise?

Mechanical Aspects:

(1) Poor bearing lubrication or bearing wear;(2) Loose fastening screws;(3) Foreign objects inside the motor.

Electromagnetic Aspects:

(1) Overload operation of the motor;(2) Unbalanced three-phase current;(3) Phase loss;(4) Short-circuit fault in the stator or rotor winding;(5) Broken bars caused by welding detachment in the squirrel-cage rotor.

11. What Work Needs to Be Done Before Starting a Motor?

(1) Measure the insulation resistance (should not be less than 0.5 MΩ for low-voltage motors);(2) Measure the power supply voltage. Check if the motor wiring is correct and if the power supply voltage meets the requirements;(3) Check if the starting equipment is in good condition (WeChat Official Account: Pump Manager);(4) Check if the fuse is suitable;(5) Check if the motor grounding or zero connection is in good condition;(6) Check if the transmission device has defects;(7) Check if the motor environment is suitable, and remove flammable materials and other debris.

12. What Are the Causes of Motor Bearing Overheating?

Motor Itself:

(1) Over-tight fit between the inner and outer rings of the bearing;(2) Geometric tolerance issues of components (e.g., poor coaxiality of the base, end cover, shaft, etc.);(3) Improper bearing selection;(4) Poor bearing lubrication or incomplete bearing cleaning (with impurities in the grease);(5) Shaft current.

Usage Aspect:

(1) Improper unit installation (e.g., insufficient coaxiality between the motor shaft and the shaft of the driven device);(2) Over-tightened pulley;(3) Poor bearing maintenance (insufficient grease, expired grease, or dry/deteriorated grease).

13. What Are the Causes of Low Insulation Resistance in Motors?

(1) Damp windings or water intrusion;(2) Accumulation of dust or oil on the windings;(3) Insulation aging;(4) Damaged insulation of motor leads or terminal blocks.