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 lacks a phase, if the motor is energized and started at this time, then the motor will generally emit a "buzzing" sound, and the motor will increase slowly. When the load dragged by the motor is relatively heavy, the specified rotation speed cannot be reached when starting, and even the starting cannot be started. If this state continues for a long time, the motor will overheat due to the long-term large current, and in serious cases, it will smoke and burn.
If a phase is suddenly missing during the operation of a three-phase asynchronous motor, the speed of the motor will quickly decrease. At this time, the current of the stator winding of the motor will also increase a lot, and at the same time, an abnormal sound of "humming" will be emitted. In this case, the motor can not run for a long time, otherwise it will burn the three-phase motor. We can use a simple sentence to remember the hazards of three-phase motors lacking phase operation, that is, "Motors lacking phases can not be idle, and eliminating disasters as early as possible".
Reasons for lack of phase of three-phase motor
The reasons for the lack of phases of three-phase motors can generally be divided into two types: three-phase motor external and three-phase motor internal:
External reasons for lack of phase in three-phase motors: common in a large number of maintenance cases is loose wiring screws in the motor junction box; followed by loose or oxidized connection points in three-phase power circuits, such as poor contact or contact of some contactors The large resistance causes the power supply to lose phase; the fuse in the fuse is disconnected or not fastened.
The internal cause of the lack of phase of the three-phase motor: the internal cause is generally rare, and the lead wire of the three-phase motor is generally 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 phase. If the three-phase resistance is basically balanced, then the cause of the motor can be ruled out.
Therefore, the lack of phase operation of the three-phase motor is very harmful to the motor. We can use some protective measures to protect the three-phase motor when the phase is missing. For example, a thermal relay with phase failure protection can be used for phase failure protection. For the stator winding connected to Y type, we can use a two-pole or three-stage thermal relay for phase failure protection; if the motor stator winding is connected to a triangle, then We can only use three-stage thermal relays with phase-off protection devices to achieve phase-off protection.