An alternator 1 has a bridge rectifier and regulator R connected to the rotor winding. The rotor winding has sufficient remanent magnetism so that when the rotor spins, a speed signal that can be analysed is induced in the stator windings. A circuit S within the regulator only allows the rotor to be connected to earth on one side. Thus it permits an excitation current to flow when the magnitude of the speed signal reaches a certain threshold. It reaches the threshold at a speed of between 500 and 1000 rpm. The regulator is internally connected to the alternator.
