Motor related questions (Part 1)

1. Why does the motor produce shaft current?

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

Causes of shaft current:

(1) magnetic field asymmetry;
(2) There are harmonics in the power supply current;
(3) Manufacturing and installation are not good, and the air gap is uneven due to the eccentricity of the rotor;
(4) There is a gap between two semicircles of the detachable stator core;
(5) The number of pieces selected for fan-stacked stator cores is inappropriate.

Damage:

The motor bearing surface or ball is eroded, forming spot-like microholes, worsening the bearing performance, increasing friction loss and heat, and eventually causing the bearing to burn.

Prevention:

(1) Eliminate pulsating flux and power harmonics (such as installing an AC reactor on the output side of the inverter);
(2) When the motor is designed, the bearing seat and base of the sliding bearing shall be insulated, and the outer ring and end cap of the rolling bearing shall be insulated.

2, why the general motor can not be used in plateau areas?
Altitude has adverse effects on motor temperature rise, motor corona (high voltage motor) and DC motor commutation. Attention should be paid to the following three aspects:

(1) The higher the altitude, the greater the motor temperature rise, the smaller the output power. However, when the temperature decreases with the increase of altitude enough 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 at high altitude;
(3) The altitude is unfavorable to the commutation of the DC motor, and attention should be paid to the selection of carbon brush materials.

3. Why should the motor not run with light load?
When the motor is lightly loaded, it will cause:

(1) Low motor power factor;
(2) The low efficiency of the motor will cause waste of equipment and uneconomical operation.

4. What are the causes of motor overheating?

(1) The load is too large;
(2) lack of phase;
(3) The air duct is blocked;
(4) Low speed operation time is too long;
(5) The power supply harmonics are too large.

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

(1) Measure the stator, winding phase and winding insulation resistance to the ground.

Insulation resistance R should meet 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, it can:

a: motor no-load operation for 2 ~ 3h drying;

b: Use 10% of the rated voltage of low-voltage AC electricity into the winding or the three-phase winding series after the direct current to dry, keep the current at 50% of the rated current;

c: Use a fan to feed hot air or heating elements to heat.

(2) Clean the motor.
(3) Replace bearing grease.

6, why can not arbitrarily start the motor in a cold environment?

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

(1) Motor insulation cracking;
(2) freezing of bearing grease;
(3) Wire joint solder powder.

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

Source: Chongqing Bashu Vocational School