Why is it not suitable for motors to run under light load? What are the reasons for motor overheating?

Why does the motor generate shaft current?

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

Causes of shaft current:

Magnetic field asymmetry;

There are harmonics in the supply current;

Poor manufacturing and installation, uneven air gap due to rotor eccentricity;

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

The number of segments of the fan-shaped stator core is not appropriately selected.

Hazards: The motor bearing surface or ball will be corroded, forming punctate micropores, which will deteriorate the bearing 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 an adverse effect on motor temperature rise, motor corona (high voltage motor) and commutation of DC motor.

The following three aspects should be noted:

The higher the altitude, the greater the motor temperature rise and the smaller the output power. However, when the temperature drops with the increase in altitude and is sufficient to compensate for the effect of altitude on temperature rise, the rated output power of the motor can remain unchanged;

When high-voltage motors are used in plateaus, anti-corona measures must be taken;

Altitude is not conducive to the commutation of DC motors, so pay attention to the selection of carbon brush materials.

Why is it not suitable for motors to run under light load?

When the motor runs under light load, it will cause:

The motor power factor is low;

The motor efficiency is low.

It will cause equipment waste and uneconomical operation.

What are the causes of motor overheating?

The load is too large;

lack of phase;

Air duct is blocked;

Low speed operation time is too long;

The power supply harmonics are too large.

What needs to be done before a motor that has been unused for a long time is put into use?

Measure the insulation resistance between the stator and winding phases and 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 winding (V)

P: Motor power (KW)

For a motor with Un=380V, R＞0.38MΩ.

If the insulation resistance is low, you can:

a: The motor runs at no load for 2 to 3 hours for drying;

b: Use low voltage AC with 10% rated voltage to pass through the winding or connect the three-phase winding in series and then use DC to dry it, keeping the current at 50% of the rated current;

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

Clean the motor.

Replace the bearing grease.

Why can't you start the motor in a cold environment at will?

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

Motor insulation cracking;

Bearing grease freezing;

The solder on the lead joints is powdered.

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

What are the reasons for unbalanced three-phase current of the motor?

Three-phase voltage is unbalanced;

The welding or contact of a phase branch inside the motor is poor;

The motor winding is short-circuited between turns or to the ground or between phases;

Wiring error.

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

When designing a motor, the silicon steel sheets are generally made 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 increased motor current and copper loss, and ultimately increase the motor temperature rise. In severe cases, the motor may be burned due to coil overheating.

What are the reasons for motor phase loss?

Power supply:

The switch has poor contact;

The transformer or line is broken;

The fuse has blown.

Motor:

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

Internal wiring welding is poor;

The motor winding is broken.

What are the causes of abnormal motor vibration and noise?

Mechanical aspects:

Poor bearing lubrication and bearing wear;

The fastening screws are loose;

There is debris inside the motor.

Electromagnetic aspects:

The motor is overloaded;

Three-phase current is unbalanced;

lack of phase;

The stator and rotor windings have short circuit faults;

The cage rotor welding part is open and causes broken bars.

What needs to be done before starting the motor?

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

Measure the power 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 grounding and zero connection are 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 tolerances of parts, such as the base, end cover, shaft and other parts have poor coaxiality;

Improper selection of bearings;

The bearing is poorly lubricated or not cleaned properly, and there are foreign objects in the grease;

Shaft current.

Usage:

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

The pulley is pulled too tight;

The bearings are not well maintained, lack grease or have exceeded their service life, causing them to dry out and deteriorate.

What are the reasons for low motor insulation resistance?

The winding is damp or water has penetrated;

Dust or oil accumulates on the windings;

Insulation aging;

The insulation of the motor leads or terminal block is damaged.