Three-phase motor lack of phase operation refers to the lack of one phase of the three-phase power supply of the stator winding of the motor. When the motor is missing one phase, if the motor is energized to start, the motor will generally make a "humming" sound, and the motor will increase slowly. When the load dragged by the motor is relatively heavy, the specified speed cannot be reached when starting, and it cannot even start. If this state continues for a long time, the motor will overheat due to the high current for a long time, and in severe cases, it will smoke and catch fire.
If a phase is suddenly missing during the operation of the three-phase asynchronous motor, the speed of the motor will decrease quickly, and the current of the stator winding of the motor will also increase a lot, and an abnormal sound of "humming" will be emitted. In this case, the motor cannot run for a long time, otherwise the three-phase motor will be burned.
The reasons for phase loss of Three-Phase AC Asynchronous Motor can be generally summarized into two types: external to the three-phase motor and internal to the three-phase motor:
External reasons for phase loss of three-phase motors: in a large number of maintenance cases, it is common that the wiring screws in the motor junction box are loose; secondly, the connection points in the three-phase power circuit are loose or oxidized, such as poor contact or contact of some contactors. The resistance is very large, which causes the phase loss of the power supply; and the fuse in the fuse is disconnected or not fastened.
Internal reasons for phase loss of three-phase motors: Internal reasons are generally rare, usually the lead wires of three-phase motors are broken. At this time, we can measure the DC resistance of the three-phase winding. If the resistance of one phase is very large or infinite, then this winding is the open-circuit phase. If the three-phase resistance is basically balanced, then the cause of the motor can be ruled out.
The lack of phase operation of the three-phase motor is very harmful to the motor. We can adopt some protective measures to protect the three-phase motor when the phase is lacking. For example, a thermal relay with phase failure protection can be used for phase failure protection. For the Y-shaped stator winding, we can use a two-pole or three-stage thermal relay for phase failure protection; if the motor stator winding is connected in a triangle, then We can only use a three-level thermal relay with a phase failure protection device, so that phase failure protection can be achieved.