What are the common causes of motor malfunctions

Electric motors are widely used in manufacturing and household equipment, and it is inevitable that they will malfunction during long-term use. It is important to quickly determine the cause of the malfunction and eliminate it, which is one of the important tasks of equipment maintenance.
The three-phase asynchronous motor is currently the most widely used type of motor, and there are also many types of faults. Next, we will analyze several typical faults:

After being powered on, the motor cannot rotate, but there is no abnormal noise, odor, or smoke.

Such faults are basically caused by the lack of action of the main circuit control equipment (frequency converter, contactor, soft starter, etc.). It is possible to investigate whether the main circuit control equipment has received control commands in a timely manner, such as whether the main contactor is engaged, whether the frequency converter is powered on and in operating mode, etc.

If the main circuit control equipment does not operate, then the status of the control circuit components should be checked:
1. Check whether the power supply of the control circuit is normal by measuring the voltage value at the output terminal of the control switch. If it is not normal, check whether the fuse is blown; Whether the control switch is turned on;
2. Whether the control circuit is open or not. If it is a general relay control circuit, the voltage at both ends of the start button can be measured when powered on. When controlling the circuit path, the measured voltage should be the control circuit voltage, otherwise the measured voltage value will be lower than the control circuit voltage value.

It is possible to check whether the protective device is activated, such as whether the thermal relay has not reset after activation, whether the normally closed contacts of the relevant relays are connected, whether there are any broken wires, poor contacts, screw damage in the circuit, whether the relay and contactor coils are open, and whether the intermediate relay contacts are burned.

If the main circuit control equipment has been powered on but the motor has not moved, it can be suspected that the main circuit circuit part:
1. The main circuit fuse is blown in two phases;
2. The main contact of the main circuit contactor is burnt out in two phases;
3. Motor phase failure;
4. The main circuit is disconnected.
These faults can be determined by measuring the resistance value in the event of a power outage.

After being powered on, the motor does not rotate and emits a "buzzing" sound
Faults like this are usually caused by single-phase or locked rotor faults, and can be judged by turning the motor in case of power failure. If the turning motor is not sinking, then it should be considered whether it is single-phase.
If the preliminary judgment is that it is a single-phase fault, then it should be checked whether it is a single-phase motor or a single-phase power supply. The motor winding can be measured to distinguish it. If the motor winding is in normal condition, the power circuit should be checked in sequence.
1. Check and measure the main fuse and main power switch;
2. Check and measure the contacts of the main contactor;
3. Check the wiring of the main circuit and measure the circuit of the main circuit.
If the rotating motor is too heavy, it should be distinguished whether it is caused by the load or the motor itself. The motor should be separated from the load and when the motor is turned in an unloaded state, the blockage caused by the motor itself may be due to bearing damage or rotor deformation.

Overcurrent protection action after power on
This type of fault is generally caused by overcurrent, or it may be due to the overcurrent setting being too low. It should be judged by measuring the current value. If the current value is indeed large, further investigation can be carried out.
1. If the load is too heavy, it can be judged by turning the motor rotor or running the motor without load;
2. Defective bearings, judged by rotating the motor rotor;
3. Motor failure, which is difficult to determine, often requires measuring the winding resistance or inter turn insulation value of the winding to determine. However, it usually shows low speed or insufficient torque when unloaded;
4. Poor wiring.

The motor is difficult to start, and at rated load, the motor speed is significantly lower than the rated speed
This fault is mainly caused by motor faults, mainly problems with the motor winding. Low power supply voltage or heavy motor load may also cause such faults.
1. The Δ connection method motor is mistakenly connected to Y;
2. Welding or fracture of cage rotor;
3. Wrong or reversed connection of local coils in the stator and rotor;
4. Excessive increase in turns when repairing motor windings;

Unbalanced no-load current of the motor with large three-phase difference
This type of fault can generally be judged as power imbalance, and the winding problem of the motor itself can also cause this type of fault phenomenon. It can be distinguished and judged by measuring the voltage at the motor terminal. If it is not a problem with the power supply, check the motor winding.
1. When rewinding, the number of turns in the stator three-phase winding is not equal;
2. Wrong connection of winding ends;
3. The winding has faults such as inter turn short circuit and coil reverse connection.
If it is a problem with the power supply voltage, because this fault is different from single-phase, it should be mainly suspected of poor contact. The focus should be on checking whether each wiring is burnt, whether the main contactor hoe is burnt, and whether the power switch has poor contact.

When the motor is running, the ammeter pointer is unstable and swings
Instability of ammeters is generally caused by rotor faults, and should be investigated specifically for the rotor.
1. Welding or breakage of cage rotor guide bars;
2. Winding rotor fault (one phase open circuit) or poor contact of brush and collector ring short circuit device.

The no-load current of the motor is high
This type of fault generally has reasons for the winding as well as motor construction, such as poor bearing assembly, poor lubrication, and so on.
1. When repairing, the number of turns in the stator winding decreases too much, which can be judged by measuring the winding resistance value;
2. The Y-connected motor was mistakenly connected as Δ;
3. During motor assembly, the rotor is reversed, causing the stator core to be misaligned and the effective length to be reduced. This requires disassembly and inspection;
4. Excessive or uneven air gap;
5. Improper use of hot dismantling method during major overhaul and dismantling of old windings resulted in burning of the iron core.

Abnormal noise during motor operation
This type of malfunction is quite common, and the causes of the malfunction are also diverse, requiring some experience to troubleshoot.
1. Friction between rotor and stator insulation paper or slot wedges;
2. Wear of bearings or foreign objects such as sand particles in the oil, resulting in a lack of oil in the bearings;
3. Loose iron core of stator and rotor;
4. Filling the air duct or wiping the fan cover,
5. High or unbalanced power supply voltage;
6. The stator winding is connected incorrectly or short circuited.

Excessive vibration during motor operation
This type of fault is also a common fault with many causes, including load and motor itself.
1. Due to excessive clearance between worn bearings, it is generally accompanied by heating;
2. Uneven air gaps can cause regular vibrations;
3. Unbalanced rotor will generate regular vibrations;
4. Bending of the shaft can easily cause bore cleaning;
5. The deformation or loosening of the iron core will be accompanied by heating;
6. The center of the coupling (pulley) is not calibrated, which may be accompanied by a problem of high motor current;
7. Poor dynamic balance of the fan;
8. Insufficient strength of the casing or foundation;
9. Loose motor foundation screws;
10. Motor rotor circuit malfunction.

Overheating of bearings
Bearings are the main components of motors, the focus of daily maintenance, and also the high-risk area for faults.
1. Excessive or insufficient lubricating grease, or poor oil quality containing impurities;
2. Improper fit between bearings and shaft journals or end caps (too loose or too tight), or motor end caps or bearing caps not installed flat;
3. Eccentricity of the bearing inner hole, friction with the shaft, or bending of the motor shaft;
4. The coupling between the motor and the load is not calibrated, or the belt is too tight;
5. Excessive or insufficient bearing clearance

Electric motor overheating or even smoking
This is the most common phenomenon of motor damage, and a comprehensive investigation of the circuit and load should generally be carried out.
1. If the power supply voltage is too high, iron loss increases, greatly increasing the heating of the iron core;
2. If the power supply voltage is too low and the current is too high, the winding will heat up;
3. Iron core damage, insulation between silicon steel sheets is damaged, eddy current loss increases, and iron loss increases;
4. Friction between the stator and rotor iron cores;
5. Motor overload or frequent starting;
6. Fault in rotor winding;
7. Single phase operation of the motor;
8. Poor insulation between motor turns;
9. Poor motor heat dissipation;
10. Short circuit between windings and turns, internal connection error in stator winding.
11. Motor bearings damaged;

The above are just common faults in asynchronous motors. With the upgrading of motor control technology, control methods have become more complex, and many control devices have complete protection measures. Many faults in the motor can be predicted in advance, but more types of faults have also emerged, which need to be judged and handled through experience summary.