Excitation is a process to create a magnetic field from electric current. This process is used in electric machines (generators or motors) whose rotor spins in a magnetic field. This magnetic field can come from a permanent magnet or field coils. In the case of electric machine with field coils, a current must flow to the coil to generate the magnetic field, so that the field coils can act as a "magnet", like in the case of electric machine with permanent magnet. The device in which excitation happens is called the exciter.