An alternator system has two pairs of diodes 21 and 22, and 23 and 24 connected between respective outputs 11 and 12 of an alternator 1 and respective outputs 4 and 5 of the system, the diodes being connected in opposite senses. A switch 32 and 34 is connected in shunt across each of the diodes connected to the zero volts output 5 of the system. The switches 32 and 34 are controlled by a control unit 3, which receives a sample of the signal appearing at the other output. The output voltage of the system is reduced by closing the switches 32 and 34 in phase with the output voltage while current is being supplied to the positive output 4 of the system. The output voltage is increased by closing the switches 32 and 34 while current is not being supplied to the positive output 4 and opening the switches 32 and 34 after current starts to be supplied to the positive output so that energy stored in the alternator 1 is transferred to the system output.