Electric motors are widely used in manufacturing and household appliances. Failures are inevitable in long-term use. It is one of the important tasks of equipment maintenance to determine the cause of the failure and eliminate the failure as soon as possible.
Three-phase asynchronous motors are the most widely used type of motors at present, and there are many types of failures. Next, several typical failures are analyzed:
The motor cannot rotate after power is turned on, but there is no abnormal sound, no odor or smoke.
Such failures are basically caused by the main circuit control equipment (inverter, contactor, soft starter, etc.) not working. You can check whether the main circuit control equipment receives the control command in time, such as whether the main contactor is energized, whether the inverter is powered on and in operation mode, etc.
If the main circuit control device does not work, then the status of the control circuit components should be checked:
1. Whether the control circuit power supply is normal, judge by measuring the voltage value of the control switch output terminal. If it is not normal, check whether the fuse is blown; whether the control switch is connected;
2. Whether the control circuit is connected. If it is a general relay control circuit, the voltage at both ends of the start button can be measured when the power is on. When the control circuit is connected, the measured voltage should be the control circuit voltage, otherwise the measured voltage value will be lower than the control circuit voltage value.
You can check whether the protection device is working, such as whether the thermal relay is not reset after operation, whether the normally closed contacts of the relevant relays are connected, whether the line is broken, poor contact, screw burnout, whether the relay and contactor coil are open, whether the intermediate relay contacts are burned out, etc.
If the main circuit control device has been powered on, but the motor does not work, the main circuit circuit part can be suspected:
1. The main circuit fuse is blown in two phases;
2. The main contacts of the main circuit contactor are burned in two phases;
3. The motor is disconnected;
4. The main circuit is disconnected.
These faults can be judged by measuring the resistance value when the power is off.
After the power is on, the motor does not rotate and makes a "buzzing" sound. Faults like this are generally caused by single-phase faults or stall faults. They can be judged by turning the motor when the power is off. If the motor does not sink when it is turned, then it should be considered whether it is single-phase.
If it is initially judged to be 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. If the motor winding is in normal condition, the power supply circuit should be checked in turn.
1. Check and measure the main fuse and the main power switch;
2. Check and measure the main contactor contacts;
3. Check the main circuit wiring points and measure the main circuit line.
If the 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 turned under no-load conditions. The stall caused by the motor itself may be caused by bearing damage or rotor deformation.
Overcurrent protection after power-on
Such faults are generally caused by overcurrent, or the overcurrent setting value is too low. The current value should be measured for judgment. If the current value is indeed large, further investigation can be carried out.
1. The load is too heavy, which can be judged by rotating the motor rotor or running the motor at no load;
2. The bearing is bad, which can be judged by rotating the motor rotor;
3. Motor failure. This fault is difficult to judge. It is often necessary to measure the winding resistance or the insulation value between the winding turns. However, it usually shows low speed or insufficient torque when no load;
4. Poor wiring.
The motor is difficult to start. When the rated load is applied, the motor speed is much lower than the rated speed. This fault is mostly caused by motor failure, mainly due to problems with the motor winding. Low power supply voltage or heavy motor load may also cause such faults.
1. The motor of Δ connection method is mistakenly connected as Y;
2. The cage rotor is welded or broken;
3. The local coil of the stator and rotor is connected wrongly or reversely;
4. Too many turns are added when repairing the motor winding;
5. The no-load current of the motor is unbalanced, and the phase difference of the three phases is large
This kind of fault can generally be judged as the power supply imbalance. The winding problem of the motor itself can also cause this kind of fault phenomenon. It can be distinguished by measuring the voltage of the motor terminal. If it is not a power supply problem, the motor winding should be checked.
1. When rewinding, the number of turns of the three-phase stator winding is not equal;
2. The first and last ends of the winding are connected incorrectly;
3. There are faults such as inter-turn short circuit and reverse connection of the coil in the winding.
If it is a problem of power supply voltage, because this fault is somewhat different from single-phase, it should be suspected that the contact is poor. It should focus on checking whether each connection point is burned, whether the main contactor hoe is burned, and whether the power switch has poor contact.
When the motor is running, the pointer of the ammeter is unstable and swings.
The instability of the ammeter is generally caused by rotor failure, and the rotor should be checked.
1. The cage rotor bars are welded or broken;
2. The winding rotor is faulty (one-phase circuit break) or the brush and collector ring short-circuit device have poor contact.
The motor has a large no-load current
This type of fault is generally caused by the winding and the motor structure, such as poor bearing assembly, poor lubrication, etc.
1. During repair, the number of stator winding turns is reduced too much, which can be judged by measuring the winding resistance;
2. The Y-connected motor is mistakenly connected to Δ;
3. During the motor assembly, the rotor is installed in reverse, causing the stator core to be misaligned and the effective length to be shortened. This requires disassembly inspection;
4. The air gap is too large or uneven;
5. When overhauling and removing the old winding, the thermal removal method is improperly used, causing the core to burn.
There is an abnormal sound when the motor is running
This type of fault is relatively common, and the causes of the fault are also diverse, and it requires a certain amount of experience to troubleshoot.
1. The rotor and stator insulation paper or slot wedges rub against each other;
2. The bearing is worn or there are foreign objects such as sand in the oil, and the bearing is short of oil;
3. The stator and rotor cores are loose;
4. The air duct is blocked or the fan rubs the wind shield,
5. The power supply voltage is too high or unbalanced;
6. The stator winding is connected incorrectly or short-circuited.
Large vibration during motor operation
This type of fault is also a common fault, and there are many reasons, including load reasons and motor reasons.
1. Due to excessive wear of the bearing clearance, it is generally accompanied by heat;
2. Uneven air gap will produce regular vibration;
3. Unbalanced rotor will produce regular vibration;
4. The shaft is bent, which is generally easy to cause sweeping;
5. The iron core is deformed or loose, which will be accompanied by heat;
6. The center of the coupling (pulley) is not corrected, which will be accompanied by the problem of large motor current;
7. The fan is not well balanced;
8. The casing or foundation is not strong enough;
9. The motor anchor screws are loose;
10. Motor rotor circuit failure.
Bearing overheating
The bearing is the main component of the motor, the focus of daily maintenance of the motor, and the part with high fault incidence.
1. Too much or too little grease, or poor oil quality containing impurities;
2. The bearing and the shaft neck or end cover are not properly matched (too loose or too tight), or the motor end cover or bearing cover is not installed flat;
3. The inner hole of the bearing is eccentric, rubbing against the shaft, or the motor shaft is bent;
4. The coupling between the motor and the load is not calibrated, or the belt is too tight;
5. The bearing clearance is too large or too small.
The motor overheats or even smokes
This is the most common phenomenon of motor damage. Generally, the circuit and load should be fully checked.
1. The power supply voltage is too high, the iron loss increases, and the core heats up greatly;
2. The power supply voltage is too low, and the current is too large, which makes the winding heat up;
3. The core is damaged, the insulation between the silicon steel sheets is destroyed, the eddy current loss increases, and the iron loss increases;
4. The stator and rotor cores rub against each other;
5. The motor is overloaded or started frequently;
6. The rotor winding fails;
7. The motor runs in single phase;
8. The insulation between the motor turns is poor;
9. The motor has poor heat dissipation;
10. The winding is short-circuited between phases and turns, and the internal connection of the stator winding is wrong.
11. The motor bearing is damaged.
The above are just common fault phenomena of AC asynchronous motors. With the upgrading of motor control technology, the control method is more complicated. Many control devices have perfect protection measures, and many motor faults can be predicted in advance, but more fault types are derived, which need to be judged and handled through experience summary.
