Detect the cause of the motor failure to start

Many companies use various three-phase asynchronous motors and encounter the problem of the motor not starting. Is the motor burned out? Or is the line broken but the motor is still intact? Is the motor overloaded? How to detect the cause of the motor not starting failure? The motor cannot start failure generally has the following situations:

1. The motor does not rotate and there is no sound: two or more phases of the power supply or winding are broken. First check whether the power supply has voltage. If the three-phase voltage is balanced, then the fault is in the motor itself. You can detect the resistance of the three-phase winding of the motor to find the broken winding.

2. The motor speed is slow and there is a buzzing sound: This fault is manifested as shaft vibration. If the current of one phase is zero, and the current of the other two phases greatly exceeds the rated current, it means that it is running in two phases. The reason is: one phase of the circuit or power supply is broken, or one phase of the motor winding is broken. Small-capacity motors can be directly measured with a multimeter to see if they are on or off. Medium-capacity motors are mostly made of multiple wires and connected in parallel with multiple branches. If several wires are broken or a parallel branch is disconnected, it is more troublesome to check. In this case, the phase current balance method or the resistance method is usually used. The resistance method uses a bridge to measure the resistance of the three-phase winding. If the three-phase resistance differs by more than 5%, the phase with larger resistance is the broken phase.

3. The motor does not rotate but has a buzzing sound: measure the motor terminals. If the three-phase voltage is balanced and is at the rated voltage, it can be determined that it is severely overloaded. The inspection steps are as follows: first remove the load. At this time, the motor speed and sound are normal, and it can be determined that it is overloaded or the mechanical part of the load is faulty. If it still does not rotate, you can turn the motor shaft by hand. If it is very tight or cannot be turned, measure the three-phase current again. If the three-phase current is balanced but larger than the rated value, it means that the mechanical part of the motor is stuck. The motor may be short of oil, the bearings are rusted, or severely damaged, the end cover or oil cover is installed too tilted, and the rotor and the inner bore collide (sweep the bore). When you feel more laborious when turning the motor shaft to a certain angle by hand or hear a periodic rubbing sound, it can be determined that it is sweeping the bore.

Experience has shown that most circuit breaker failures of electric motors occur at the ends of windings, joints or lead wires.