Uninterruptible power supply system

A power supply system comprises first electrical conversion means (22) for converting an alternating current electrical output from a generator (24) to a direct current electrical output for supply to an electrically powered motor (14), second electrical conversion means (26) for converting a direct current electrical output from a battery (28) to a direct current electrical output for supply to the motor (14), and control means (30) operable in the event of an interruption in the output from the first electrical conversion means (22) to supply power from the second electrical conversion means (26) to the motor (14).