According to the working principle of a three-phase motor, during normal operation, the current provided by the three-phase power supply generates a rotating magnetic field in the stator winding of the motor. This magnetic field interacts with the rotor magnetic field, allowing the motor to rotate normally. When a three-phase motor loses phase, that is, when one phase of the three-phase power supply is disconnected or has poor contact, the operating state of the motor will undergo serious changes.

In the case of phase loss, the magnetic field in the stator winding of the motor is no longer a uniform rotating magnetic field, but becomes a pulsating magnetic field. This will cause a significant decrease in motor torque, a decrease in motor speed, or even a stop in rotation. But at this time, the motor is still powered on, and due to the unchanged load, the current in the two-phase windings without phase loss will increase sharply.

An increase in current means that the motor winding is heating up more severely. Under such high current and high temperature conditions for a long time, the insulation layer of the motor winding will rapidly age and be damaged, until it is short circuited and burned out.

Taking a three-phase motor with a star connection as an example, when there is a phase loss, it is equivalent to two windings connected in series to withstand the line voltage. Due to phase loss, the voltage divided between these two windings is insufficient, and in order to maintain the output power of the motor, the current will increase significantly. Moreover, the current in the two windings connected in series is the same, and the heating situation is also similar. Long term operation will cause both windings to burn out at the same time.

 For three-phase motors with angular connections, a phase loss is equivalent to only one winding in operation. This standalone winding has to bear the load that was originally shared by the three phases, causing a sharp increase in current and quickly heating up and burning out.

At the same time, phase loss can also cause severe vibration and noise in the motor. This not only accelerates the mechanical wear of the motor, but may also cause damage to the equipment and components connected to it.

The main methods of phase loss protection for three-phase motors are as follows:

Using thermal relay protection: The thermal relay can detect the magnitude of the motor current. When the motor loses phase and causes an abnormal increase in current, the thermal relay will activate and cut off the power supply.

Zero sequence current transformer protection: Determine whether there is a phase loss by detecting the vector sum of three-phase currents. When normal, the vector sum is zero, but when there is a phase loss, the vector sum is not zero, which triggers the protection action.