Why do motors generate shaft currents?

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

Causes of shaft currents:[1]

1. Asymmetry of the magnetic field;
2. Harmonics in the supply current;
3. Poor manufacturing and installation, uneven air gap due to rotor eccentricity;
4. Gaps between two semicircles of the detachable stator core;
5. There is a fan-shaped stacked stator core of the number of pieces of collocation is not selected appropriately.

Hazards: the motor bearing surface or ball erosion, the shape of the course of the point-like microporous, so that the bearing running performance deterioration, friction loss, and heat increase, and eventually cause bearing burn.

Why the general motor can not be used in highland areas?

Altitude on the motor temperature rise, motor corona (high-voltage motor), and DC motor commutation have adverse effects.

The following three aspects should be noted:

1. At High altitudes, the greater the temperature rise of the motor, the smaller the output power. But when the temperature decreases with the rise in 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 high-voltage motors are used in the plateau;
3. Altitude is not good for DC motor commutation, pay attention to the choice of carbon brush material.

Why should the motor not run at a light load?

When the motor is running at a light load, it will cause

Low power factor of the motor;
Motor efficiency is low.

It will cause equipment waste and uneconomical operation.

What are the causes of motor overheating?

1. Excessive load;
2. Lack of phase;
3. Blockage of air ducts;
4. Excessive low-speed running time;
5. Excessive power supply harmonics.

What do I need to do before putting the motor into use for a long time?

1. Measure the insulation resistance of the stator and winding between each phase and the winding to the ground. Insulation resistance R should meet the following formula:

Un: rated voltage of motor winding (V)
P: motor power (KW)
For the motor with Un = 380V, R>0.38MΩ.
If the insulation resistance is low, it can be:
a: no-load operation of the motor for 2 to 3h drying;
b: using 10% of the rated voltage of low-voltage alternating current into the winding or three-phase winding in series with DC baking, keeping the current at 50% of the rated current;
c: use the fan to send in hot air or heating elements to heat.

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

Why can’t you start the motor in a cold environment arbitrarily?

A motor in a cold environment for too long will:

1. Cracking of motor insulation;
2. Freezing of bearing grease;
3. Solder chalking of lead wire connectors.

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

What are the causes of motor three-phase current unbalance?

1. Unbalanced three-phase voltage;
2. Poor welding or poor contact of a phase branch within the motor;
3. Short circuit between turns of motor winding or short circuit to ground or phase to phase;
4. Wiring errors.

Why 60 Hz motor can not be connected to the 50 Hz power supply?

The motor design generally makes the silicon steel sheet work in the magnetization curve of the satiation area, when the supply voltage is certain, reducing the frequency will increase the flux, and excitation current, resulting in improved motor current, increased copper consumption, and ultimately lead to higher motor temperature rise, and in severe cases may also burn the motor due to coil overheating.

What are the causes of motor phase loss?

Power supply:

Poor switch contact;
Transformer or line disconnection;
Fused insurance.

Motor side:

Loose motor junction box screws poor contact;
Poor welding of internal wiring;
Motor winding disconnection.

What are the causes of abnormal vibration and sound of the motor?

Mechanical aspects:

Poor bearing lubrication and bearing wear;
Loose fastening screws;
There is debris in the motor.
Electromagnetic aspects:

Motor overload operation;

Unbalanced three-phase current;
Lack of phase;
Stator, rotor winding short-circuit fault;
Broken bars caused by open welding of the cage rotor part.

What do I need to do before starting the motor?

Measure insulation resistance (should not be less than 0.5MΩ for low voltage motors);
Measure the power supply voltage. Checking whether the motor wiring is correct and whether the power supply voltage meets the requirements;
Checking whether the starting equipment is good;
Check whether the fuse is suitable;
Check whether the motor grounding and zeroing are good;
Checking whether the transmission device is defective;
Check whether the motor environment is suitable, and remove flammable materials and other debris.

What are the causes of motor bearing overheating?

Motor itself:[2]

Bearing inner and outer rings fit too tight;
Problems with the tolerance of the shape of parts, such as the seat, end cover, shaft, and other parts coaxially are not good;
Improper bearing selection;
Poor bearing lubrication or poor bearing cleaning and debris in the grease;
Shaft current.


Improper installation of the unit, such as the shaft coaxially of the motor shaft and the shaft of the device being dragged a requirement;
The belt pulley pulling too tight;
Poor bearing maintenance, insufficient grease or over-the-use period, dry and deteriorated.

What are the causes of the low insulation resistance of the motor?

Moisture or water intrusion in the winding;
accumulation of dust or oil on the winding;
Insulation aging;
Motor lead or terminal board insulation damage.

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