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
DC motors are more efficient, if you already have DC, like in your car. If you have AC, like in your house, and the fan includes a low cost “wall wart” to make DC for the fan then the efficiency of the motor is wasted in the inefficiency of the conversion.
As the motor gets bigger, the efficiency flips. Electric cars and cruise ships all use AC induction motors for higher efficiency at a variety of speeds.
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