Operation Analysis Of Three-phase Asynchronous Motor

When the load torque T2 carried by the motor is less than the starting torque Tst, the motor can start with a load. From point c to point b, the torque of the motor increases with the increase of speed, causing the speed of the motor to rapidly increase. When reaching point b, the torque reaches its maximum value Tmax. After turning point b, the torque of the motor decreases with the increase of speed, but as long as the electromagnetic torque T is greater than the load torque T2, the speed of the motor continues to rise until T=T2, at which point the speed of the motor stabilizes. So, the stable operating point of the electric motor is located at a certain point in the interval b and a of the n=f (T) curve. Therefore, the AB area is called a stable working area. BC area is an unstable working area.
If the load suddenly increases or the power supply voltage suddenly drops to T2>Tmax, the motor speed drops rapidly and enters the bc segment. The electromagnetic torque of the motor decreases with the decrease in speed, causing the motor to quickly stop running. This phenomenon is called stalling. After stalling, the current in the motor immediately increases to several times the rated current. If there are no protective measures to cut off the power supply in a timely manner, the motor may be burned out.