An aircraft main power generation system comprising: a rotor shaft; a main power generator including generator windings, the generator windings mechanically coupled to the rotor shaft such that the rotor shaft causes the generator windings to spin when the rotor shaft is spinning; a permanent magnet generator mechanically coupled to the rotor shaft, the permanent magnet generator configured to generate power when the rotor shaft is spinning; a rotating rectifier mechanically coupled to the rotor shaft; an exciter electrically coupled to the generator windings of the main power generator via the rotating rectifier, the exciter configured to provide a current to the generator windings, via the rotating rectifier, in response to receiving a control current, the control current being based upon an output voltage and/or speed of the main power generator; an aircraft power bus configured to receive power from the main power generator when the main power generator is generating electrical power; and a generator control unit comprising a DC to DC converter, a rectifier, a controller and a power supply, the generator control unit electrically coupled to the permanent magnet generator and configured to receive power generated by the permanent magnet generator, the generator control unit configured to monitor a speed of the main power generator and configured to: provide the control current to the exciter in response to the speed of the main power generator reaching a threshold speed, thereby causing the main power generator to generate and supply electrical power to the aircraft power bus, wherein the threshold speed is lower than a minimum operating speed; or provide the control current to the exciter when a time period has elapsed following the speed of the main power generator reaching a predetermined speed, thereby causing the main power generator to generate and supply electrical power to the aircraft power bus; the aircraft main power generation system is characterized in that the time period is between 5 seconds and 60 seconds, or wherein the time period is between 60 seconds and 100 seconds. a rotor shaft; a main power generator including generator windings, the generator windings mechanically coupled to the rotor shaft such that the rotor shaft causes the generator windings to spin when the rotor shaft is spinning; a permanent magnet generator mechanically coupled to the rotor shaft, the permanent magnet generator configured to generate power when the rotor shaft is spinning; a rotating rectifier mechanically coupled to the rotor shaft; an exciter electrically coupled to the generator windings of the main power generator via the rotating rectifier, the exciter configured to provide a current to the generator windings, via the rotating rectifier, in response to receiving a control current, the control current being based upon an output voltage and/or speed of the main power generator; an aircraft power bus configured to receive power from the main power generator when the main power generator is generating electrical power; and a generator control unit comprising a DC to DC converter, a rectifier, a controller and a power supply, the generator control unit electrically coupled to the permanent magnet generator and configured to receive power generated by the permanent magnet generator, the generator control unit configured to monitor a speed of the main power generator and configured to: provide the control current to the exciter in response to the speed of the main power generator reaching a threshold speed, thereby causing the main power generator to generate and supply electrical power to the aircraft power bus, wherein the threshold speed is lower than a minimum operating speed; or provide the control current to the exciter when a time period has elapsed following the speed of the main power generator reaching a predetermined speed, thereby causing the main power generator to generate and supply electrical power to the aircraft power bus; provide the control current to the exciter in response to the speed of the main power generator reaching a threshold speed, thereby causing the main power generator to generate and supply electrical power to the aircraft power bus, wherein the threshold speed is lower than a minimum operating speed; or provide the control current to the exciter when a time period has elapsed following the speed of the main power generator reaching a predetermined speed, thereby causing the main power generator to generate and supply electrical power to the aircraft power bus; the aircraft main power generation system is characterized in that the time period is between 5 seconds and 60 seconds, or wherein the time period is between 60 seconds and 100 seconds.