For the troubleshooting of motor faults, various materials present a wide range of methods. From the perspective of combining theory with practice, the following specific analysis is conducted.
1. The motor fails to start.
1) The motor does not rotate and there is no sound. The reason is that there is a two-phase or three-phase open circuit in the motor’s power supply or winding. First, check if there is power voltage. If there is no voltage for all three phases, the fault lies in the circuit; if the three-phase voltage is balanced, the fault is in the motor itself. At this time, measure the resistance of the three-phase windings of the motor to find the broken winding.
2) The motor does not rotate, but there is a “buzzing” sound. Measure the terminal of the motor. If the three-phase voltage is balanced and equal to the rated value, it can be determined as a severe overload.
The inspection steps are as follows: First, remove the load. If the motor’s speed and sound are normal, it can be determined as overload or there is a fault in the mechanical part of the load. If it still does not rotate, turn the motor shaft by hand. If it is very tight or cannot be turned, measure the three-phase current. If the three-phase current is balanced but greater than the rated value, it may be that the mechanical part of the motor is stuck, the motor is lacking oil, the bearing is rusted and severely damaged, the end cover or oil cover is installed too obliquely, or the rotor and the inner cavity collide (also called scavenge). If turning the motor shaft to a certain angle feels difficult or hearing periodic “chattering” sounds, it can be judged as scavenge. The reasons are: (1) The gap between the inner and outer rings of the bearing is too large, and a new bearing needs to be replaced; (2) The bearing chamber (bearing hole) is too large, and long-term wear causes the inner diameter to be too large. Emergency measures are to electroplate a layer of metal or add a sleeve, or make some small holes on the inner wall of the bearing chamber; (3) The shaft is bent, the stop edge of the end cover is worn.
3) The motor rotates slowly and accompanied by a “buzzing” sound and shaft vibration. If the current of one phase is zero, and the current of the other two phases is much higher than the rated value, it indicates that two phases are rotating. The reason is that one phase of the circuit or power supply is open or one phase of the motor winding is open.
For small motors, a single-phase open circuit can be checked using an ohmmeter and multimeter or by testing with a lamp. When checking a star or delta-connected motor, the three-phase windings’ connections must be disconnected and each phase checked for open circuit. For medium-capacity motors, their windings mostly use multiple conductors wound in multiple branches and connected in parallel. If several conductors are broken or one parallel branch is disconnected for inspection, it is relatively complex. The commonly used methods are the three-phase current balance method and the resistance method. Generally, if the three-phase current (or resistance) values differ by more than 5% from each other, the phase with the smaller current (or larger resistance) is the open circuit phase.
Practice has proved that motor open circuit faults often occur at the end, connection point, or lead of the winding.
Low voltage, uneven motor air gap, blocked ventilation holes of the iron core, and damaged fan blades, etc.
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Post time: Apr-15-2026