The special symptoms of 3-phase asynchronous motors

The unloading condition of the motor is  normal,  in this artcle some special cases are mentioned:

1. Unloading motor

Unload motor (spin without load) or very  weak load, weak currents in the motor that do not harm the engine, but waste resource, and low  capacity  factor and output, causing waste electrical energy . Therefore,  the motor had not better to be  worked  under load or unload .

2. Short-circuited case 

Short-circuited phenomena can be caused by: 

When you turn on, rotor can not spin, curent on the windings is very high , if  this symptoms is ignored in long time, motor will be broken. This is called short-circuited phenomena
Before operating motor, you should measure insulating resistor. To small or medium capacity motor, if insulating resistor is less than 0,5 M, you have to dry motor before operation. While operating , if motor is clogged or the load is too heavy, immediately disconnect power( if not the motor will be burnt).  

3. Working case with asymmetric voltage 

If the 3-phase voltage is installed in an unbalanced motor, in the motor outside the magnetic field (in the same direction as the rotor rotation), or in inverse magnetic field. The magnetic field that causes an inverse  torque to rotation direction reducing  the torque  motor, increasing the loss, electric currents   and the heat of motor. The working condition of asymmetric voltages that often occurs in production is the loss of an electric phase. If this situation is not corrected for a long time, the motor will be  burnt.