In DC electric motors, the speed can be adjusted by inserting a rheostat into the field circuit to ensure flow adjustments. The connection of the motors to a public power grid must comply with the regulations established for this purpose.
Generally we try to start a motor at full voltage, so that you can make the most of the starting torque and another way to adjust the speed of AC motors, which can allow the acquisition of the speed axis, which can range from zero to twice the synchronous speed, is by the well-known rotor system with a switch, by means of brush off. 2BK80H
The possibility that exists when we deal with the change of speed in electric induction motors is achieved through the frequency inverter, which allows the control of the alternating current motor, thus changing the frequency, as well as making the change of the output voltage, in order to respect the characteristic of the V / F motor, which is the motor frequency voltage.