Causes and protective measures of phase loss of three-phase motor
Phase failure operation of three-phase motor refers to the lack of one phase of three-phase power supply in the stator winding of the motor. When the motor lacks a phase, if the motor is powered on and started, the motor will generally make a "buzzing" sound, and the speed rise of the motor is relatively slow. When the load dragged by the motor is relatively heavy, the specified speed cannot be reached when starting, or even cannot be started. If this state continues for a long time, the motor will overheat due to long-term high current, and will smoke, catch fire and burn out in serious cases.
If there is a sudden lack of one phase during the operation of the three-phase asynchronous motor, the speed of the motor will soon decrease, and the current of the motor stator winding will also increase a lot at this time. At the same time, it will make an abnormal sound of "buzzing". In this case, the motor cannot run for a long time, otherwise the three-phase motor will be burned out.
Generally speaking, the causes of phase loss of three-phase motor can be divided into two types: external and internal:
External reasons for phase loss of three-phase motor: 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 supply circuit are loose or oxidized, such as poor contact or large contact resistance of some contactors, resulting in phase loss of the power supply; Also, the fuse in the fuse is disconnected or not firmly connected.
Internal causes of phase loss of three-phase motor: internal causes are generally rare, generally due to the broken outgoing line of three-phase motor. At this time, we can measure the DC resistance of the three-phase winding. If the resistance of one phase is large or infinite, then the winding is the open phase. If the three-phase resistance is basically balanced, the cause of the motor can be eliminated.
The phase failure operation of three-phase motor is very harmful to the motor. We can take some protective measures to protect the three-phase motor during phase failure operation. For example, the thermal relay with open phase protection can be used for open phase protection, and the thermal relay with two pole or three-stage structure can be used for open phase protection when the stator winding is connected into Y-type; If the stator winding of the motor is connected into a triangle, we can only use the three-stage thermal relay with open phase protection device, so as to realize open phase protection.