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
Small equal opposite rotating magnetic fields, the torques generated by them and the rotor are also equal in magnitude and opposite in direction. Therefore, the starting torque is zero and cannot be started.

Consequences of missing one phase of the motor
When the motor runs in phase loss, its power is only about half of the rated power. If the rated load remains unchanged, the current between the motor windings will inevitably exceed the rated current, which will cause the motor casing to heat up and burn the motor for a long time.

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.
(1) If it is stationary, there will be no starting torque and cannot be started (n=0 at the moment of starting, which is equivalent to a short circuit, and the motor will burn out after a long time);
(2) If the phase is missing during rotation, it can still be rotated, but the stator current of the motor increases sharply, the speed decreases, the loss increases, and it is easy to overheat. If it is not removed in time, the motor insulation will be damaged due to excessive temperature rise.
Single-phase operation of three-phase asynchronous motors

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.
(2) When the three-phase windings are not completely symmetrical, the phases of the currents in the two branches will be different, and a rotating magnetic field with unequal forward and reverse amplitudes will be formed in the motor. Although the difference in the amplitudes of the two magnetic fields is not large, However, the generated starting torque can still enable the motor to start automatically at no load.

Variation of motor current during phase loss:
When starting or running normally, the three-phase motor 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.
Phase loss when starting: The motor cannot be started, and its winding current is 4 to 7 times the rated current. The calorific value is 16 to 49 times the normal temperature rise, and the motor burns out because it quickly exceeds the allowable temperature rise.
Phase loss during operation:
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. 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.