The working principle of the three-item asynchronous motor should be：
When symmetrical three-term alternating current is passed into the three-term stator winding, a rotating magnetic field is generated that rotates clockwise along the inner circular space of the stator and rotor at a synchronous speed n1. Since the rotating magnetic field rotates at n1 speed, the rotor conductor is stationary at first, so the rotor conductor will cut the stator rotating magnetic field and generate an induced electromotive force (the direction of the induced electromotive force is determined by the right-hand rule). Since both ends of the conductor are short-circuited by the short-circuit ring, under the action of the induced electromotive force, an induced current will be generated in the rotor conductor that is basically consistent with the direction of the induced electromotive force. The current-carrying conductors of the rotor are acted upon by electromagnetic forces in the stator’s magnetic field (the direction of the force is determined by the left-hand rule). The electromagnetic force generates electromagnetic torque on the rotor shaft, driving the rotor to rotate along the direction of the rotating magnetic field.
Through the above analysis, it can be concluded that the working principle of the motor is: when the three stator windings of the motor (each with a phase difference of 120 degrees in electrical angle) are supplied with three alternating currents, a rotating magnetic field will be generated. An induced current is generated in the winding (the rotor winding is a closed path). The current-carrying rotor conductor will generate electromagnetic force under the action of the stator’s rotating magnetic field, thereby forming an electromagnetic torque on the motor shaft, driving the motor to rotate, and the motor’s rotation direction is consistent with the rotating magnetic field. Same direction.
Reasons: 1. If one or two phase windings of the motor are burned out (or overheated), it is usually caused by phase loss operation. There will be no in-depth theoretical analysis here, only a brief explanation. When the motor loses a phase for whatever reason, although the motor can still continue to run, the speed drops and the slip becomes larger. The B and C phases become a series relationship and are connected in parallel with the A phase. When the load remains unchanged, If the current of phase A is too large, if it runs for a long time, the winding of this phase will inevitably overheat and burn out. After the power phase is lost, the motor can still continue to run, but the speed also drops significantly, the slip becomes larger, and the rate of the magnetic field cutting the conductor increases. At this time, the B-phase winding is open-circuited, and the A and C phase windings become in series and pass Excessive current and long-term operation will cause the two-phase windings to burn out at the same timeIt is important to point out here that if a stopped motor lacks one phase of power supply and is switched on, it will generally only make a buzzing sound and cannot start. This is because the symmetrical three-phase alternating current supplied to the motor will generate a circular rotating magnetic field in the stator core. However, when one phase of power supply is missing, a single-phase pulsating magnetic field is generated in the stator core, which cannot cause the motor to generate starting torque. Therefore, the motor cannot start when the power supply phase is missing. However, during operation, an elliptical rotating magnetic field with high three-phase harmonic components is generated in the air gap of the motor. Therefore, the running motor can still run after a phase loss, but the magnetic field is distorted and the harmful current component increases sharply. , eventually causing the winding to burn out.
Corresponding countermeasures: No matter whether the motor is static or dynamic, the direct harm caused by phase loss operation is that one or two phase windings of the motor will overheat or even burn out. At the same time, the overcurrent operation of power cables accelerates insulation aging. Especially in the static state, the lack of phase will produce a locked rotor current several times the rated current in the motor winding. The winding burnout speed is faster and more serious than the sudden phase loss during operation. Therefore, when we perform daily maintenance and inspection of the motor, we must conduct comprehensive inspection and testing of the corresponding MCC functional unit of the motor. In particular, the reliability of load switches, power lines, and static and dynamic contacts should be carefully checked. Prevent phase loss operation.
Post time: Dec-04-2023
