When the motor is started missing words, abnormal sound will be issued, the motor can not be started. If the motor phase. At run time, the motor will be issued the abnormal sound, and the motor will be because the role of inertia to continue running, if not immediately stop, and continue to run it, the motor winding will be burned out. Therefore need to set the open-phase protection in the control circuit of the motor. Three-phase asynchronous motor in shutdown, if one phase winding open circuit or power supply is disconnected (or phase). When start the motor, rotating magnetic field winding magnetic field can be divided into two equal and opposite, they produce and the rotor torque is equal and opposite direction. Therefore starting torque to zero and will not start.. In the operation phase, winding magnetic field produced by the rotating magnetic field also can be divided into two equal and opposite direction. But with the rotation of the motor rotating magnetic field and the rotor instead of the relative speed greatly, the induced electromotive force and the current in the rotor of the frequency difference is several times the power frequency, rotor inductance is great, it was decided to active component of current torque size is very small, so the reverse torque is much smaller than the positive torque, therefore, the motor can continue to run. However, should pay attention to when the motor power, it is only half the rated power of about, if the rated load unchanged, current motor winding and between will exceed the rated current, will make the motor shell fever, long time operation will burn motor. If this kind of situation should be able to correctly judge the situation, because, at this time of the motor temperature, current, sound will be different with the normal.