A basic function of controlling an electric motor is the limiting of the starting current. There are two ways to reduce tension in starting. One is to supply current to the normal voltage, causing the motor to be temporarily connected to the mains, with the winding for a higher voltage, angular contact ball bearings using the star-delta starting system and supplying below-normal voltage current through resistors, inductors or autotransformer.
All of the starting systems with reduced tension present as opposed to the advantage of reducing the current, the disadvantage that the starting torque or torque is reduced in the proportion of the square of the voltage reduction supplied to the motor. The connection of the motors to a public electrical network must warn the prescriptions for this purpose, established by norm. Normally, we try to start a motor at full voltage in order to make the most of the starting torque. When starting the full voltage of an electric motor generates a voltage drop higher than the maximum permissible, a low voltage starting device must be used, but care must be taken to verify that the torque is sufficient to drive the load.