Abstract: An uninterruptable power supply, operating normally under power supplied by a utility, has a main motor, a smaller induction motor, and a mechanical load such as a generator coaxially coupled together. A battery is connected to the smaller induction motor through an inverter. The induction motor is normally idled at a frequency lower than that of the utility, and therefore operates as an induction generator which is used to charge the battery through the inverter. Upon utility failure, the utility line is disconnected and the induction motor is powered by the battery through the inverter. The frequency of the inverter may be increased to maintain the rotational speed of the system at a nearly constant level.