Thirteen Questions about Electric Motors
1. Why does a motor generate shaft current?
The current in the motor's shaft-bearing housing-base circuit is called shaft current.
The cause of shaft current:
(1) Magnetic field asymmetry;
(2) There are harmonics in the power supply current;
(3) Poor manufacturing and installation, resulting in 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 stator core pieces stacked in a fan shape is not appropriate.
harm:
This can cause erosion of the motor bearing surface or balls, forming pinhole-like micropores, which deteriorates the bearing's operating performance, increases friction loss and heat generation, and ultimately causes the bearing to burn out.
prevention:
(1) Eliminate pulsating magnetic flux and power supply harmonics (e.g., by adding an AC reactor to the output side of the frequency converter).
(2) When designing the motor, the bearing housing and base of the sliding bearing are insulated, and the outer ring and end cover of the rolling bearing are insulated.
2. Why can't regular electric motors be used in high-altitude areas?
Altitude negatively impacts motor temperature rise, motor corona (in high-voltage motors), and commutation in DC motors. The following three aspects should be considered:
(1) The higher the altitude, the greater the temperature rise of the motor and the lower the output power. However, when the temperature decreases sufficiently with increasing altitude to compensate for the effect 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 at high altitudes;
(3) Altitude is detrimental to the commutation of DC motors, so attention should be paid to the selection of carbon brush materials.
3. Why is it not advisable for motors to operate under light loads?
When a motor is running under light load, it will cause:
(1) The motor has a low power factor;
(2) The motor has low efficiency.
This will result in equipment waste and uneconomical operation.
4. What are the causes of motor overheating?
(1) Overload;
(2) Phase loss;
(3) Air duct blockage;
(4) The low-speed operation time is too long;
(5) Excessive power supply harmonics.
5. What work needs to be done before putting a motor that has been idle for a long time into operation?
(1) Measure the insulation resistance between each phase of the stator and winding and the insulation resistance between the winding and ground.
The insulation resistance R should satisfy the following formula:
R > Un/(1000+P/1000)(MΩ)
Un: Rated voltage of motor windings (V)
P: Motor power (KW)
For a motor with Un = 380V, R > 0.38MΩ.
If the insulation resistance is low, you can:
a: Drying is done by running the motor unloaded for 2-3 hours;
b: Apply low-voltage AC current of 10% of the rated voltage to the winding or connect the three-phase windings in series and then use DC current to heat them, keeping the current at 50% of the rated current.
c: Use a fan to deliver hot air or a heating element for heating.
(2) Clean the motor.
(3) Replace the bearing grease.
6. Why can't you start a motor in a cold environment?
If a motor is kept in a low-temperature environment for an extended period of time, it will:
(1) Cracking of motor insulation;
(2) The bearing grease has frozen;
(3) The solder of the wire joint is powdered.
Therefore, motors should be heated and stored in cold environments, and the windings and bearings should be inspected before operation.
7. What are the causes of unbalanced three-phase current in a motor?
(1) Three-phase voltage imbalance;
(2) Poor welding or poor contact in a certain phase branch inside the motor;
(3) Short circuit between turns of the motor winding or short circuit to ground or between phases;
(4) Wiring error.
8. Why can't a 60Hz motor be connected to a 50Hz power supply?
In motor design, silicon steel sheets are generally designed to operate 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, which will lead to an increase in motor current and copper loss, ultimately resulting in a higher motor temperature. In severe cases, the motor may even burn out due to overheating of the coil.
9. What are the causes of phase loss in a motor?
Regarding the power supply:
(1) Poor switch contact;
(2) Transformer or line disconnection;
(3) Circuit breaker.
Regarding the motor:
(1) Loose screws in the motor junction box, resulting in poor contact;
(2) Poor internal wiring soldering;
(3) The motor winding is broken.
10. What are the causes of abnormal vibration and noise in a motor?
Mechanical aspects:
(1) Poor bearing lubrication, bearing wear;
(2) The fastening screws are loose;
(3) There are foreign objects inside the motor.
Electromagnetic aspects:
(1) The motor is operating under overload;
(2) Three-phase current imbalance;
(3) Phase loss;
(4) A short circuit fault occurred in the stator and rotor windings;
(5) The welded part of the cage rotor is broken, causing the bar to break.
11. What work needs to be done before starting the motor?
(1) Measure the insulation resistance (for low-voltage motors, it should not be less 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 if the starting equipment is in good working order;
(4) Check if the fuse is suitable;
(5) Check the motor grounding and whether the neutral connection is good;
(6) Inspect the transmission device for defects;
(7) Check whether the motor environment is suitable and remove flammable materials and other debris.
12. What are the causes of motor bearing overheating?
The motor itself:
(1) The inner and outer rings of the bearing are too tightly fitted;
(2) There are problems with the form and position tolerances of the parts, such as poor coaxiality of parts such as the base, end cover, and shaft;
(3) Improper bearing selection;
(4) Poor bearing lubrication or unclean bearing, with impurities in the grease;
(5) Axis current.
In terms of usage:
(1) Improper installation of the unit, such as the motor shaft and the shaft of the driven device not meeting the coaxiality requirements;
(2) The pulley is pulled too tight;
(3) The bearing is not well maintained, the grease is insufficient or has exceeded its service life, and it has become dry and deteriorated.
13. What are the reasons for low insulation resistance in motors?
(1) The winding is damp or water has entered;
(2) Dust or oil accumulates on the windings;
(3) Insulation aging;
(4) Damage to the insulation of the motor leads or terminal block.
Disclaimer: This article is a reprint. If it involves copyright issues, please contact us promptly for deletion (QQ: 2737591964). We apologize for any inconvenience.
