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Why does the motor produce shaft current?

Why does the motor produce shaft current?

The current in the motor’s shaft-bearing seat-base circuit is called shaft current.


Causes of shaft current:


Magnetic field asymmetry;

There are harmonics in the power supply current;

Poor manufacturing and installation, resulting in uneven air gaps due to rotor eccentricity;

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

The number of pieces of the stator core formed by stacking sectors is inappropriate.

Hazards: The motor bearing surface or balls will be eroded and point-like micropores will be formed, which will worsen the bearing operating performance, increase friction loss and heat, and eventually cause the bearing to burn out.

Why can’t general motors be used in plateau areas?

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


The following three aspects should be noted:


The higher the altitude, the greater the temperature rise of the motor and the smaller the output power. However, when the 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;

Corona prevention measures must be taken when high-voltage motors are used on plateaus;

Altitude is not good for DC motor commutation, so attention should be paid to the selection of carbon brush materials.


Why should the motor not be operated with light load?

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

The motor power factor is low;

Motor efficiency is low.


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

The motor power factor is low;

Motor efficiency is low.

It will cause waste of equipment and uneconomical operation.

What are the causes of motor overheating?

The load is too large;

missing phase;

Air ducts are blocked;

Low speed running time is too long;

The power supply harmonics are too large.

What work needs to be done before putting a motor that has not been used for a long time into use?

Measure the stator, winding phase-to-phase insulation resistance and winding-to-ground insulation resistance.

The insulation resistance R should satisfy the following formula:


Un: rated voltage of motor winding (V)

P: Motor power (KW)

For Un=380V motor, R>0.38MΩ.

If the insulation resistance is low, you can:

a: The motor runs without load for 2 to 3 hours for drying;

b: Use low-voltage alternating current of 10% of the rated voltage to pass into the winding or connect the three-phase windings in series and then bake them with direct current to keep the current at 50% of the rated current;

c: Use a fan to send hot air or a heating element for heating.

Clean the motor.

Replace bearing grease.


Why can’t I start a motor in a cold environment at will?

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

Motor insulation cracked;

Bearing grease freezes;

Solder powder at wire joints.


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

What are the causes of unbalanced three-phase current in the motor?

Three-phase voltage imbalance;

A certain phase branch inside the motor has poor welding or poor contact;

Motor winding turns-to-turn short circuit or short circuit to ground or phase-to-phase;

Wiring error.


Why can’t a 60Hz motor be connected to a 50Hz power supply?

When designing the motor, the silicon steel sheet is generally made to work in the saturation area 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 loss, which ultimately leads to an increase in the temperature rise of the motor. In severe cases, the motor may be burned due to overheating of the coil.

What are the causes of motor phase loss?
Power supply:

Poor switch contact;

Transformer or line break;

The fuse is blown.


Motor aspect:

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

Poor internal wiring welding;

The motor winding is broken.


What are the causes of abnormal vibration and sound of motors?
Mechanical aspects:
Poor bearing lubrication and bearing wear;
The fastening screws are loose;
There are debris inside the motor.
Electromagnetic aspects:
Motor overload operation;
Three-phase current imbalance;
missing phase;
Short circuit fault occurs in stator and rotor windings;
The welding part of the cage rotor is open and causes broken bars.
What work needs to be done before starting the motor?

Measure the insulation resistance (for low-voltage motors, it should not be less than 0.5MΩ);

Measure the supply voltage. Check whether the motor wiring is correct and whether the power supply voltage meets the requirements;

Check whether the starting equipment is in good condition;

Check whether the fuse is suitable;

Check whether the motor is grounded and the zero connection is good;

Check the transmission for defects;

Check whether the motor environment is suitable and remove flammable materials and other debris.


What are the causes of motor bearing overheating?

The motor itself:

The inner and outer rings of the bearing are too tight;

There are problems with the shape and position tolerance of parts, such as poor coaxiality of parts such as the machine base, end cover, and shaft;

Improper selection of bearings;

The bearing is poorly lubricated or the bearing is not cleaned cleanly, and there are debris in the grease;

axis current.



Improper installation of the unit, such as the coaxiality of the motor shaft and the driven device does not meet the requirements;

The pulley is pulled too tight;

The bearings are not well maintained, the grease is insufficient or the service life has expired, and the bearings dry out and deteriorate.


What are the reasons for low motor insulation resistance?

The winding is damp or has water intrusion;

Dust or oil accumulates on the windings;

Insulation aging;

The insulation of the motor lead or wiring board is damaged.

Post time: Nov-03-2023