The necessary condition for the normal operation of three-phase asynchronous motors is the symmetry of voltage. However, in actual operation, voltage asymmetry always occurs due to faults in the power grid or the motor itself. For example, when there is a large single-phase load connected to the power grid, or accidents such as phase to phase short circuits, one phase to ground or one phase to ground disconnection in the power grid, as well as quality problems such as phase to phase, broken or ground to ground wiring in the motor itself, all can cause three-phase voltage asymmetry in the motor winding. When a three-phase asynchronous motor operates under asymmetric voltage, the starting torque, overload capacity, and efficiency of the motor will all decrease.

When the three-phase voltage is asymmetric, there will be both positive sequence magnetic field and negative sequence magnetic field in the motor air gap. The positive sequence magnetic field generates positive electromagnetic torque, while the negative sequence magnetic field generates reverse electromagnetic torque and negative mechanical power.

Due to the negative torque generated by the negative sequence magnetic field, it is equivalent to a braking torque, which hinders the normal rotation of the motor rotor, resulting in a decrease in the overall torque of the motor. As a result, the large torque of the motor also decreases, and naturally, the overload capacity of the motor deteriorates.

From the analysis of motor slip, when the motor is running normally, the slip rate of the three-phase asynchronous motor is very small, so the braking torque caused by negative sequence magnetic field is not a big problem. However, the heating problem of the winding will be very serious, which may cause the winding to burn out in a short period of time. Meanwhile, the negative sequence magnetic field will increase motor losses, resulting in a decrease in motor efficiency and an increase in temperature rise.

Based on the above analysis, it can be found that when the power supply voltage of a three-phase asynchronous motor is unbalanced, the overload capacity and efficiency level of the motor decrease, while the winding heating is a major problem. This is also the key to the requirements for power supply voltage unevenness in the technical specifications of motor products.

We can demonstrate our understanding of this issue through problem analysis. Whether it is the testing or use of the motor, there is a possibility of phase loss in the motor.

From the theoretical analysis of three-phase voltage asymmetry, we can draw a conclusion that if the grid voltage is asymmetric and the motor cannot operate at rated load, the motor will experience catastrophic electrical faults due to winding heating. When there is a phase loss problem during the operation of the motor, due to the overheating of the winding, a burnt odor will permeate the environment, accompanied by a noise similar to low-frequency electromagnetic sound, and the motor speed will decrease; Once the motor stops running, it will no longer be able to start.

For many equipment factories, necessary adjustments should be made to the load distribution due to the large number of equipment working. Especially when there are many large single-phase loads, the symmetry of three-phase voltage should be monitored to prevent motor failures due to voltage asymmetry.