Any motor manufacturer may encounter disputes with customers due to so-called quality problems. Mr. S, a service staff of Ms.’s participating unit, also encountered such problems and was almost kidnapped. The motor cannot start after power-on! The customer asked the company to go to someone to solve it immediately. On the way to the construction site, the customer was quite rude to the old S. After arriving at the site, the experienced old S determined that the customer’s line was missing phase! Under the customer’s monitoring state, the old S completely eliminated its line failure, and the electric motor started immediately! In order to express apology and thank old S for solving the problem, the boss specially hosted a banquet for old S in the evening!
The specific manifestations of motor phase loss are increased vibration, abnormal noise, increased temperature, decreased speed, increased current, strong humming sound when starting and cannot be started.
The reason for the lack of phase of the motor is the problem of the power supply itself or the connection problem. It may be that the fuse is improperly selected or press-fitted, the fuse is disconnected, the switch is in poor contact, and the connector is loose or broken. It is also possible that a phase winding of the motor is disconnected.
After the motor is burnt out of phase loss, the intuitive fault feature of the winding is regular winding burn marks, and the degree of burn is not too much. For inter-turn, inter-phase or ground faults, the location of the fault point is particularly serious, and the spread of the fault is relatively lighter. This is a feature that is different from other faults.
● When the electromagnetic and torque motors operate in phase loss, the rotating magnetic field of the stator is seriously unbalanced, so that the stator generates a negative sequence current, and the negative sequence magnetic field and the rotor electromagnetically induce a potential close to 100Hz, resulting in a sharp increase in the rotor current and serious heating of the rotor. ; When the phase is missing, the load capacity of the motor decreases, resulting in a sharp increase in the stator current, and the most direct manifestation is the motor heating. Due to the serious unevenness of the magnetic field of the motor, the motor vibrates seriously, resulting in damage to the bearing. If the motor runs with a load and lack of phase, the motor will stop rotating instantly, and the direct consequence is that the motor will burn out. In order to prevent the occurrence of this problem, general motors have phase loss protection.
●The change of current under different operating states
During normal starting or running, the three-phase electricity is a symmetrical load, and the three-phase currents are equal in magnitude and less than or equal to the rated value. After one-phase disconnection occurs, the three-phase current is unbalanced or too large.
If the phase is missing when starting, the motor cannot be started, and its winding current is 5 to 7 times the rated current. The calorific value is 15 to 50 times the normal temperature rise, and the motor burns out because it quickly exceeds the allowable temperature rise.
When the phase is missing at full load , the motor is in an overcurrent state, that is, the current exceeds the rated current, the motor will change from fatigue to locked rotor, and the line current that is not broken will increase more, causing the motor to burn quickly.
When the motor is out of phase in light-load operation, the winding current that is not out of phase increases rapidly, causing the winding of this phase to be burned out due to the high temperature rise.
The lack of phase operation is very harmful to squirrel-cage motors that operate in a long-term working system. About 65% of the accidents in which such motors are burned are caused by lack of phase operation. Therefore, it is very important to protect the phase loss of the motor.
Post time: May-31-2022