I looked into this a while ago, when I was replacing the pump on my swimming pool. This is a bit of a special case, as there are gains to be made from using a variable speed drive. Whilst a variable speed drive may increase electrical losses, the slower water flow may reduce losses due to turbulance by a greater amount, leading to a net increase in efficiency for volume of water moved.
As far as I can gather, for variable speed motors a brushless permanent magnet DC motor is more efficient for small power applications (< 1-2kW), but as the power goes up, a high efficiency three-phase induction motor with a variable-speed drive become more efficient than the DC motor. A high efficiency induction motor has extra copper in the rotor, to reduce resistive losses.
For fixed speed applications, you'd think the above variable speed performance would reflect the performance for a DC supply, as the DC supply requires switching in both cases. For a three-phase AC supply, you'd think the induction motor would win, due to the absence of switching.
As far as I can gather, for variable speed motors a brushless permanent magnet DC motor is more efficient for small power applications (< 1-2kW), but as the power goes up, a high efficiency three-phase induction motor with a variable-speed drive become more efficient than the DC motor. A high efficiency induction motor has extra copper in the rotor, to reduce resistive losses.
For fixed speed applications, you'd think the above variable speed performance would reflect the performance for a DC supply, as the DC supply requires switching in both cases. For a three-phase AC supply, you'd think the induction motor would win, due to the absence of switching.