Cage type induction motors

When the electric motor is energized it acts as a transformer with the secondary being shorted and therefore will require a line much larger than the rated current, which can reach seven times the nominal current. The companies that supply electric power require that there is a limitation of the starting current of the motors according to the conditions of your system, ie the available installed power (generated or purchased) and the dimensioning of the conductors.

This requirement is made so as not to impair the quality of the power supplied, because at the time of departure of a large motor there will be a voltage drop in the feeders and other consumers will receive the power under a lower voltage. A concessionaire in a small city will require a decrease in starting current in small engines while concessionaires in large cities will be able to accept the direct departure of much larger engines.

As the rotating field “drags” the rotor increasing its speed, the current decreases until reaching the nominal current when the rotation reaches its nominal value. If the engine is idle, it will quickly get its rated speed and the current will decrease accordingly.