Guide to selection of electric motors

To avoid shor circuits between conductors, it is necessary to insert insulating material. For each kind of materials, there is a limiting temperature beyond which it detriorates and loses its insulating characteristics. The specifications select the materials in the different classes according to their temerature limit, as the life of the winding insulation ( And so of the motor) reduces with the increase of temperature. The rules fix temperature limits to allow a normal lifetime with a further safety margin on the temperature limit. The standard insulation classes for electric motors are class E-B-F and H.


Characteristics torque/RPM in a 3 phase motor
The torque generated by the motor as a function of the speed, has a course similar to those indicated in the figure (A). The characteristics of the curve are the static torque, max. torque and nominal torque. To start and to necessary, this means that for each speed the torque must reach nominal running speed. And accereration torque is necessary, this means that for each speed the torque must be higher than the corresponding load.
At the intersection point of the torque characteristics the acceleration torque disappears and there is a balanced state corresponding to the point of nominal running of the motor.
The curves of the torque and of the absorbed current, as a function of the speed, change with the various types of cage motors used.