What are the common faults of Large Size DC Motor?

1. Brush and commutator faults
Abnormal brush wear
Reason: The brush may wear too fast due to improper pressure between the brush and the commutator. Excessive pressure will accelerate the mechanical wear of the brush; too little pressure will cause poor contact between the brush and the commutator, generate electric sparks, and then burn the brush. In addition, poor brush quality, rough commutator surface or impurities can also cause excessive brush wear.
Performance: The brush length is significantly shortened and the wear rate is much higher than the normal level. For example, under normal circumstances, the brush may be used for several months, but it may need to be replaced in a few weeks when it is abnormally worn.
Excessive brush sparks
Reason: First, the brush position is incorrect and is not in the neutral line position, so that the brush cannot smoothly transition the current during the commutation process; second, the commutator surface is uneven, such as convex marks, pits or protruding mica sheets, which destroys the good contact between the brush and the commutator; third, the armature winding is short-circuited or open-circuited, affecting the normal commutation of the current.
Performance: When the motor is running, obvious sparks appear between the brush and the commutator. In severe cases, the sparks are in the shape of ring fires, and a strong discharge sound can be heard, accompanied by increased motor vibration and unstable speed.
2. Armature winding failure
Armature winding short circuit
Reason: Long-term overload operation causes the winding temperature to be too high, resulting in insulation damage; the motor is damp, which reduces the insulation performance; mechanical damage, such as the motor is subjected to severe vibration, collision, or accidentally damages the winding insulation during maintenance.
Performance: When the motor is running, the current increases, the speed increases abnormally, and the motor heats up seriously. By measuring the resistance of the armature winding, it will be found that the resistance value is lower than the normal range. At the same time, the brush spark may increase.
Armature winding open circuit
Reason: Usually due to loose welding points or cold welding, the armature winding connection wire is disconnected; or the wire inside the winding is broken due to long-term mechanical stress, excessive current, etc.
Performance: The motor cannot start or stops suddenly during operation. If part of the winding is open circuit, the motor may experience a decrease in speed, torque, and increased brush sparks. When using a multimeter to measure the resistance of the armature winding, it will display infinity or the resistance value will be significantly larger.
3. Field winding failure
Field winding short circuit
Reason: The insulation is aged, damp or mechanically damaged, which destroys the insulation of the field winding and causes some coils to short-circuit.
Performance: The magnetic field of the motor is weakened, and the speed increases and the torque decreases. Because the winding of the short-circuited part cannot generate a magnetic field normally, the magnetic field distribution of the motor is uneven. At the same time, since the short-circuit of the field winding will cause the current to increase, the motor may overheat.
Field winding open circuit
Reason: The wire connecting the field winding is broken, the welding point falls off, or a break point appears inside the winding.
Performance: The motor cannot establish a magnetic field, so the motor cannot start normally. During operation, if the field winding is suddenly disconnected, the magnetic field of the motor disappears, which will cause the motor to stop running immediately, and may cause large sparks between the brush and the commutator.
4. Motor overheating failure
Reason: First, the motor is overloaded, and the actual load exceeds the rated load of the motor, resulting in excessive motor current and excessive heat; second, poor ventilation, such as fan damage and blocked ventilation ducts, which prevent the heat generated by the motor from being dissipated in time; third, winding failure, including short circuit or open circuit of the armature winding and the field winding, increases copper loss and causes the motor to heat up.
Performance: The temperature of the motor housing rises significantly, and it feels hot when touched by hand. If a temperature sensor is installed, it can be found that the temperature reading exceeds the normal operating temperature range (generally 20-30℃ higher than the ambient temperature). Long-term overheating may cause the insulation material of the motor to age faster, the grease to fail, and even damage the windings and other parts of the motor.
5. Mechanical failure
Bearing damage
Reason: Poor lubrication, such as grease drying, deterioration or insufficient filling; bearings are subjected to excessive axial or radial loads for a long time, exceeding their load-bearing capacity; improper bearing installation, such as deflection, overtightening or over-loosening during installation; impurities enter the bearing, causing increased wear.
Symptoms: The motor emits an abnormal humming or friction sound during operation, and the sound frequency and intensity may change with the change of motor speed. At the same time, the vibration of the motor increases, and in severe cases, the motor shaft can be seen to jump. By checking the radial and axial clearances of the bearings, it will be found that the clearances are beyond the normal range.
Excessive motor vibration
Reason: The motor installation foundation is not firm, causing the motor to shake during operation; the connection part between the motor and the load (such as the coupling) is improperly installed, there is a concentricity deviation or loose connection; the internal components of the motor are damaged, such as rotor imbalance, bearing damage, poor contact between the brush and the commutator, etc.
Symptoms: The motor produces obvious vibration during operation, the shaking of the motor body can be felt, and it may cause resonance of surrounding equipment. Excessive vibration may cause the motor components to loosen, the welding points to fall off, and even damage the mechanical structure and electrical components of the motor.