ANALYSIS OF THE REASONS FOR THE LACK OF PHASE OPERATION OF THE MOTOR 2
If the stopped motor lacks one phase of the power supply and closes, generally only a buzzing sound occurs and cannot be started. This is because the symmetrical three-phase alternating current of the motor will generate a circular rotating magnetic field in the stator iron core, but when one phase is missing, a circular rotating magnetic field will be generated. After the power supply, the single-phase pulsating magnetic field is generated in the stator iron core, which cannot make the motor generate starting torque. Therefore, the motor cannot be started when the power supply phase is missing.
When the Y THREE PHASE ASYNCHRONOUS ELECTRIC MOTOR is out of operation, if one phase of the winding is open or one phase of the power supply is disconnected (or missing). When the motor is started, the magnetic field generated by the winding can be divided into two large
Consequences of missing one phase of the motor
Motor lacks two phases
When the two phases are broken, the Y THREE PHASE ASYNCHRONOUS ELECTRIC MOTOR cannot be started because the rotating magnetic field cannot be established, and there is no harm to the motor.
One-phase disconnection operation (loss of phase)
If the three-phase asynchronous motor is star-connected, after one phase is disconnected, the other two-phase windings are connected in series to form a single circuit to access the line voltage.
After the phase loss, the motor winding has two phases in series to form a single circuit and the other phase winding is connected in parallel to the power supply voltage.
(1) When the winding parameters are completely symmetrical, the current phases of the two circuits are the same, but the amplitudes are not equal, and the combined magnetic potential is still a single-phase pulsating magnetic potential, and there is no starting torque.
Variation of motor current during phase loss:
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. Lack of phase operation is very harmful to squirrel-cage motors that operate in a long-term working system. 60% to 70% 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.
According to the principle of electrical machinery. When the motor is out of phase. The stator winding is no longer a three-phase alternating current. but single-phase current. The magnetic field in the air gap changes from a circular rotating magnetic field to a single-phase pulsating magnetic field. On the one hand, when the motor starts out of phase, its starting torque is zero. The motor is actually in a two-phase short circuit. The motor windings are seriously hot. Destruction of motor insulation, so as to burn the motor, affect production, and even cause accidents. In addition, when the motor is running in phase loss. The overload capacity has been significantly reduced. The slip rate increases. The increase of the stator and rotor current will inevitably make the windings heat up, which is extremely unfavorable for the operation of the motor.