<P> In this diagram, V t (\ displaystyle V_ (t)) is the terminal voltage . If we ignore the resistance as shown above, we find that the power can be calculated: </P> <P> Breaking the apparent power into Real and Reactive power, we get: </P> <P> In a permanent magnet generator, the magnetic field of the rotor is produced by permanent magnets . Other types of generator use electromagnets to produce a magnetic field in a rotor winding . The direct current in the rotor field winding is fed through a slip - ring assembly or provided by a brushless exciter on the same shaft . </P> <P> Permanent magnet generators (PMGs) or alternators (PMAs) do not require a DC supply for the excitation circuit, nor do they have slip rings and contact brushes . A key disadvantage in PMAs or PMGs is that the air gap flux is not controllable, so the voltage of the machine cannot be easily regulated . A persistent magnetic field imposes safety issues during assembly, field service or repair . High performance permanent magnets, themselves, have structural and thermal issues . Torque current MMF vectorially combines with the persistent flux of permanent magnets, which leads to higher air - gap flux density and eventually, core saturation . In this permanent magnet alternators the speed is directly proportional to the output voltage of the alternator . </P>

There real advantage to using a rotating field and a stationary armature on commercial generators