Excitation (magnetic)
![A 100 kVA direct-driven power station AC alternator with a separate belt-driven exciter generator, date c. 1917.](/Images/godic/202501/12/Murray_Alternator_with_Belt-Driven_Exciter3410.jpg")
![A self-excited shunt-wound DC generator is shown on the left, and a magneto DC generator with permanent field magnets is shown on the right. The shunt-wound generator output varies with the current draw, while the magneto output is steady regardless of load variations.](/Images/godic/202501/12/Self_Excited_vs_Magneto3410.png")
![A separately excited DC generator with bipolar field magnets. Separately excited generators like this are commonly used for large-scale power transmission plants. The smaller generator can be either a magneto with permanent field magnets or another self-excited generator.](/Images/godic/202501/12/Separately_Excited3410.png")
![A field coil may be connected in shunt, in series, or in compound with the armature of a DC machine (motor or generator)](/Images/godic/202501/12/Serie_Shunt_Coumpound3410.png")
An electric generator or electric motor consists of a rotor spinning in a magnetic field. The magnetic field may be produced by permanent magnets or by field coils. In the case of a machine with field coils, a current must flow in the coils to generate the field, otherwise no power is transferred to or from the rotor. The process of generating a magnetic field by means of an electric current is called excitation.