Characteristics of motor phase loss

The specific manifestations of motor phase loss are increased vibration, abnormal noise, increased temperature, decreased speed, increased current, strong humming sound when starting and inability to start. The cause of motor phase loss is the power supply itself or the connection problem. It may be improper fuse selection or poor compression, fuse disconnection; poor switch contact; loose or broken joints. It is also possible that one phase of the motor winding is disconnected. After the motor is burned due to phase loss, the intuitive fault feature of the winding is regular winding burn marks, and the degree of burning is not much different. For turn-to-turn, phase-to-phase or ground faults, the fault point location is particularly severely burned, and the spread of the fault is relatively lighter. This is a feature that is different from other faults.

Theoretical analysis of motor running with missing phase

When electromagnetic and torque motors run with missing phase, the stator rotating magnetic field is seriously unbalanced, so the stator generates negative sequence current, and the negative sequence magnetic field and the rotor generate electromagnetic induction with a potential close to 100Hz, causing the rotor current to increase sharply and the rotor to heat up seriously; when the motor is missing phase, the load capacity of the motor decreases, causing the stator current to increase sharply, and the most direct manifestation is the heating of the motor. Due to the serious unevenness of the motor magnetic field, the motor vibrates severely, causing bearing damage. If the motor runs with a missing phase under load, the motor will stop rotating instantly, and the direct consequence is that the motor burns out. In order to prevent this problem from happening, general motors have missing phase protection.

Changes in current under different operating conditions

During normal starting or operation, the three-phase electricity is a symmetrical load, and the three-phase current is equal, less than or equal to the rated value. After one phase is disconnected, the three-phase current is unbalanced or too large. When starting, the motor cannot start due to phase loss, and its winding current is 5 to 7 times the rated current. The heat generated is 15 to 50 times the normal temperature rise, and the motor burns out because it quickly exceeds the allowable temperature rise.

When the motor is in overcurrent state when the phase is missing under full load, the current exceeds the rated current, the motor will change from fatigue to stall, and the line current of the unbroken phase will increase more, causing the motor to burn out quickly. When the motor is in phase failure under light load operation, the current of the unbroken phase winding increases rapidly, causing the phase winding to burn out due to excessive temperature rise. Phase failure operation is very harmful to squirrel cage motors that are operated for a long time. About 65% of the accidents of burning such motors are caused by phase failure operation. Therefore, it is very important to protect the motor from phase failure.

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