The motor will malfunction due to various reasons during operation. The common faults of the motor are mainly divided into mechanical and electrical aspects.
(1) Mechanical failures such as bore sweeping, vibration, bearing overheating and damage.
The air gap between the stator and the rotor of the asynchronous motor is very small, which easily leads to collision between the stator and the rotor. Generally, due to the wear of the inner hole of the end cover shaft or the wear and deformation of the stop of the end cover and the stop of the machine base, the machine base, the end cover, and the rotor are not on the same axis to cause bore sweeping.
The vibration should first be distinguished whether it is caused by the motor itself, caused by a defective transmission device, or transmitted by the mechanical load end, and then eliminated according to the specific situation. It belongs to the vibration caused by the motor itself, mostly due to poor rotor dynamic balance, poor bearing, bending of the rotating shaft, or different shafts of the end cover, frame, and rotor, or uneven installation foundation of the motor, improper installation, and loose fasteners. of. Vibration generates noise and also generates additional load.
(2) Electrical faults include lack of phase operation of the stator windings, reverse connection of the stator windings, unbalanced three-phase currents, short-circuit and grounding of the windings, overheating of the windings, and broken rotor bars or open circuits.
Phase failure operation is one of the common faults. As long as one phase of the three-phase power supply is open, the motor will run without phase. The lack of phase operation may be caused by the fusing of the fuse on the line, the poor contact of the switch contact or the wire connector, etc.
After a three-phase motor lacks one-phase power supply, if it is in a stopped state, it will stall (cannot start) because the combined torque is zero. The locked-rotor current of the motor is much larger than the normal working current. Therefore, in this case, the power-on time is too long or the power is switched on frequently and the motor is started to burn out. When the running motor lacks one phase, if the load torque is small, it can still keep running, only the speed drops slightly, and an abnormal noise is emitted; when the load is heavy, the motor winding will be burnt if the running time is too long.
When the three-phase winding is connected wrongly from the end to the end, serious imbalance of the three-phase current will occur after the power is turned on, the speed will drop, the temperature rise will increase sharply, the vibration will increase, and the sound will change suddenly. If the protection device does not operate, it is easy to burn the motor windings. Therefore, it is necessary to distinguish the beginning and the end of the motor outlet before it can be powered on.
The fault of the unbalanced three-phase current is often caused by the unbalanced voltage of the external power supply of the motor; the internal reason is mainly the short circuit between the winding turns or the wrong number of coil turns or the wrong wiring when the motor is rewinded and repaired.
Grounding and short-circuiting of windings will cause excessive current. The ground fault can be checked with a megohmmeter. The short-circuit fault can be judged by measuring the current when the voltage of the stator winding power supply is reduced, or by measuring the DC resistance.
The main reason for motor overheating is that the dragging load is too heavy, and the voltage is too high or too low to overheat the motor. Severe overheating will cause the insulation scorching smell inside the motor. If it is not handled in time or the protection device does not operate, it is easy to burn the motor.
When the cast aluminum conductor of the cage motor rotor is broken or the rotor winding of the wound motor is broken, it will cause the stator current to be abnormal, with high and low periodic changes, as well as noise and vibration. The heavier the load, the more obvious this phenomenon.
Check eight methods for judging the short circuit of three-phase asynchronous motor windings
(1) External observation method.
Observe whether the junction box and the ends of the windings are burnt, and the windings leave dark brown and smelly after overheating.
(2) Temperature detection method.
Run for 20 minutes with no load (stop immediately when an abnormality is found), and check whether the motor housing part exceeds the normal temperature by hand.
(3) Power-on experiment method.
Measure with an ammeter. If the current in a phase is too large, it means that there is a short circuit in that phase.
(4) Bridge inspection.
Measure the DC resistance of each winding. Generally, the difference should not exceed 5%. If it exceeds, the phase with the smaller resistance will have a short-circuit fault.
(5) Short-circuit detector method.
If the tested winding has a short circuit, the steel sheet will vibrate.
(6) Multimeter or megohmmeter method.
Measure the insulation resistance between any two phase winding phases. If the reading is extremely small or zero, it means that there is a short circuit between the two phase winding phases.
(7) Voltage drop method.
Measure the voltage drop of each phase winding after connecting the three windings in series and then have a short circuit fault.
(8) Current method.
Measure the three-phase current first when the motor is running under no load, and then measure and compare the two phases. If it does not change with the power supply, the one-phase winding with the larger current is short-circuited.
The above methods should be used comprehensively to judge the faulty motor.
