The electric motors belong to two main groups: the direct current and the alternating current. Electric induction motors fit into asynchronous AC motors precisely because they can not operate at synchronous speed. About 80% of the energy consumed by the industries is used to convert electrical energy into mechanics through electric motors.
DC electric motors have this name because they are constant in time, having their value well defined, circulating in the same direction in a conductor. Already the AC motors are variants in the time, alternating the direction that they circulate in a conductor.
Alternating Current is used in numerous applications, especially in high power systems, industries and electrical machines, usually in the electric motors that equip appliances such as electric kettles, refrigerators and washing machines.
All electronic circuits are powered by a source of electric power. Generally this source is a DC voltage source. For portable systems, batteries may be used, however, circuits need to be powered from the available local AC power grid. For these cases, there should be a part of the equipment that converts the alternating waveform of the network to DC power.