Variable-speed drives comprising static inverters and squirrel-cage induction machines have attractive characteristics and are suitable for use in many industrial applications. A major attribute of some of these drives is their very high energy efficiency, and this paper is a presentation of the results of an experimental investigation into the factors affecting cage induction-machine efficiency on static inverter supplies. It is demonstrated that the conventional `constant V/Hz’ strategy must be modified if the best possible drive efficiency is to be realised. The sensitivity of the machine to the high-frequency distortion components present in some inverter waveforms is illustrated and the major loss mechanisms are discussed. The emphasis is on inverter drives using sinusoidal pulse-width modulation, and it is concluded that the properties of an `asynchronously’ generated pulse width modulated sequence can be used to improve the drive efficiency. Other inverter waveforms are also briefly considered. A control strategy, which requires a microcomputer for implementation in a practical drive but can run the drive at the maximum achievable efficiency under all operating conditions, is discussed.