Fault analysis and handling methods for three-phase asynchronous motors

Three phase asynchronous motors are the most widely used electrical equipment in production. Their function is to convert electrical energy into mechanical energy. In enterprises, the electrical energy consumed by motors accounts for more than 60% of the energy consumption. Among them, squirrel cage asynchronous motors are the most commonly used. They have a simple structure, easy starting, small size, reliable and durable operation, and are easy to maintain and repair. Asynchronous motors have many faults, which can generally be divided into electrical and mechanical parts. Electrical faults include stator winding phase loss operation, reverse connection of stator winding ends, three-phase current imbalance, winding short circuit and grounding, winding overheating and rotor bar breakage, open circuit, etc; Mechanical failures include rotor sweeping, vibration, bearing overheating, damage, etc. In order to ensure the safe operation of asynchronous motors, it is necessary to quickly and accurately identify the cause of the fault after it occurs, in order to repair it as soon as possible.

Several common faults and troubleshooting methods of three-phase asynchronous motors:

1、 Poor voltage withstand strength of motor stator

Experience has shown that the main reason for poor voltage withstand strength of the stator of an electric motor is mostly due to defects in the insulation of the winding, such as broken insulation sleeves of the lead wires, damage to the winding ends, aging insulation between phases, and moisture damage to the motor, which can cause the insulation of the winding to break down and burn out the motor.

2、 The no-load current of the motor is too high

The no-load current of an electric motor is related to factors such as the design of magnetic materials and manufacturing level, as well as the power and number of poles of the motor. In general, there is a certain proportional relationship between the no-load current and the full load current of an electric motor. When the power is small, the number of poles is large, and the ratio of no-load current to full load current is large, the Y series motor is commonly used in the factory for the ratio of no-load current to full load current (%). For new motors or motors with changed windings, it is necessary to test the no-load current. If the measured no-load current of the motor exceeds the normal range, it indicates that there is a problem with the motor and the cause needs to be investigated for handling. Table 2 lists the reasons and solutions for the excessive no-load current of asynchronous motors.

3、 Unbalanced three-phase current of electric motor

When the three-phase power supply is symmetrical, the three-phase no-load current of an asynchronous motor at rated voltage shall not deviate from the average value by more than 10%. Only when the three-phase voltage imbalance is too large or the motor itself has a fault, will the motor produce a significant three-phase current imbalance. The imbalance of three-phase current not only causes additional heat generation in the motor, but also results in an imbalance of the three-phase rotating magnetic field, causing the motor to produce a special low pitched sound and the body to vibrate as a result. Table 3 shows the analysis of the causes of three-phase current imbalance in asynchronous motors.

4、 High temperature of electric motor

When the motor is operating normally under rated working conditions, its temperature should not exceed the temperature limit. The causes of motor overheating are very complex. Abnormal conditions in the power supply, motor itself, and load can all cause motor overheating. Poor ventilation and heat dissipation can also cause motor overheating. Figure 1 shows the causal relationship diagram of motor overheating. Long term overheating of the motor can cause insulation aging and affect its service life. For motors in use, if the temperature rises, they should be stopped to identify the cause and troubleshoot before use.

5、 Vibration and noise of electric motors

When the electric motor is running normally, the body should be stable, the sound should be low and uniform, and the vibration of the electric motor should be distinguished from whether it is caused by the motor itself, poor installation of the transmission device, or transmitted from the mechanical load end, and then eliminated according to the specific situation. The vibration caused by the electric motor itself is mostly due to poor dynamic balance, poor bearings, bent shafts, uneven motor installation foundations, loose fasteners, and other reasons in practical production. Vibration generates noise and additional load.

If the sound is abnormal, there may be several situations: 1. When a loud buzzing sound is emitted, it indicates that the current is too high, which may be caused by overload or three-phase current imbalance. When the motor is running in single-phase, the buzzing sound will be even louder. 2. When making a gurgling sound, it may be caused by damage to the bearing ball. 3. When there is an uneven rubbing sound, it is often caused by the sound of the rotor rubbing against the stator, which is called the sweeping sound. The machine should be stopped immediately for processing.

6、 Electric motor sweeping chamber

When an electric motor rotates, the rotor collides with the inner circle of the stator, which is called electric motor sweeping. When the electric motor is sweeping the bore, scratches may appear on the outer surface of the rotor and the inner circle of the stator, indicating uneven clearance between the rotor and stator. Severe sweeping of the chamber can cause local high temperatures in the inner circle of the stator, and the insulation on the groove surface becomes brittle at high temperatures, resulting in winding grounding or short circuits. It can also cause motor vibration and noise, and reduce motor performance. There are many reasons that can cause electric motors to sweep the chamber, and they are intertwined with each other.

7、 Overheating of motor bearings

In small motors, ball bearings are generally used for both the front and rear ends. In medium-sized motors, roller bearings are generally used for the transmission end, ball bearings are used for the other end, and sliding bearing structures are also used. Bearing heating is the most common fault of asynchronous motors. Bearing heating can cause mild dilution and leakage of lubricating oil, and severe damage to the bearing.

8、 Phase deficient operation

In daily work, phase loss operation is one of the common faults. If one phase of the three-phase power supply is disconnected, it will cause the motor to operate in phase loss. Phase loss operation may be caused by the melting of fuses on the circuit, poor contact of switch contacts or wire connectors, and other reasons. If a three-phase motor is in a stopped state due to a lack of one phase power supply, it will stall (unable to start) due to zero synthesized torque. The locked rotor current of an electric motor is much higher than the normal operating current, so in this case, prolonged or frequent power on starting will cause the motor to burn out. When the running motor is missing one phase, if the load torque is very small, it can still maintain operation with only a slight decrease in speed and abnormal noise. When the load is heavy and the running time is too long, it will cause the motor winding to burn out.