Overload Analysis of Three-phase Asynchronous Motor

During the use of three-phase asynchronous motor, if the drag load exceeds its rated capacity, it will overload. The following abnormal phenomena will generally occur when the motor is overloaded: ① the motor is overheated ② the running current of the motor exceeds the rated current ③ the motor itself produces abnormal sound during operation ④ the motor and its driven equipment emit abnormal vibration, and the speed of the motor decreases.

The main reasons for motor overload are as follows:

1. The power supply voltage is too low. When the motor starts or operates with full load, the voltage at the incoming line terminal is not the no-load voltage of the line. When the load is on, if the power supply voltage decreases, such as the 380 V motor voltage is lower than 360 V, the current will inevitably increase, causing the motor temperature rise to be too high. The reason for the low voltage of the power supply is that the high-voltage power supply itself is low, and the transformer tap changer needs to be adjusted, which can be divided into two types: on load voltage regulation and no-load voltage regulation. Second, the motor is too far away from the transformer, the section of the power supply cable is too small, the load is heavy, and the line voltage drop is too large. At this time, it is necessary to increase the section of the conductor or reduce the line load.

2. The connection method is incorrect. For example, if the 380 V star connected motor is wrongly connected to a triangle, the phase voltage will increase to √ 3 times the original value, and the current will increase greatly; otherwise, if the 380 V delta connected motor is wrongly connected to a star Y, the phase voltage will drop to 220 V, and the current will increase greatly when the motor is fully loaded, and the motor will be burned in both cases.

3. Mechanical reasons. There are many kinds of mechanical failures, and the failures are complex. Common failures include bearing damage and equipment stalling.

4. Improper motor selection and long starting time. For example, centrifugal dehydrators, ball mills, etc. have large starting resistance torque, long starting time, and are easy to burn the motor. Such machinery shall be double squirrel cage, deep slot asynchronous motor or winding asynchronous motor with small starting current and large starting torque. In addition, there is margin for power selection. Frequency converter or soft starter can be selected according to the electrical control mode.