A multiple electric traction operated by both ac and dc current. They contain converter and inverter that divert the ac to dc and vice versa. When an ac supply has to be converted to dc it is called a rectifier and when a dc is converted to ac it is called an inverter.
A dc current always travels in one direction only along a conductor. Ac changes direction, flowing first one way along the conductor and than the other. Traction motors operated by both currents. Ac motors were almost exclusively used in constant speed applications. A modern railway 3-phase traction motor is controlled by feeding in three ac currents, which interact to cause the machine to turn. The three phases are most easily provided by an inverter, which supplies the three variable voltages, variable frequency motor inputs. The ac motor can be used by either an ac or dc traction supply system.
