Take for example ceiling fans: the newer DC motor ones use up to 70% less electricity than conventional AC motor fans for the same size/output (although cost more to purchase).
In: Physics
I don’t think this is necessarily true if the AC motor is used correctly.
The problem with AC motors is that the speed the motor runs at generally has to be tied to how fast the alternating current alternates, we call this speed its synchronous speed. At its synchronous speed, the motor is generally pretty efficient, the problem is that at non synchronous speeds there are a lot of problems.
AC motors don’t really run at all at non synchronous speeds, you have to convert frequencies which is actually a big challenge in electronics, and this is where much of the waste comes from. House fans tend to like having variable control for their speeds and this is where much of the problem results.
Modern brushless DC motors actually run basically using alternating current, they just use the trick that converting DC to AC current is actually pretty easy, you just switch the current directions every other moment and smooth things out a little bit, and you can make any AC frequency you want by changing how often you switch the DC current, though the speed of the motor is still tied to the voltage supplied and the load it has to turn.
For DC house fans, you of course have to eat the efficiency penalty of converting from AC to DC, but done properly this isn’t that inefficient. Its just older motors that try to go directly from AC to other AC tend to be pretty inefficient in this regard.
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