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The question does not mention the exact type of motor as ac and dc comes in brush less and with brush as well.
But if we take for example conventional dc motor it uses cooling blower specially in variable speed drive applications where as ac induction motor uses tube axial fan thus it is proven that dc motor emits more heat losses in comparison to ac induction motor and this is also because of carbon brushes and the commutator as well .
So in my opinion dc motor is more inefficient but if we take BLD type dc motors they have more efficiency than ac induction motor but less efficiency than ac synchronous reluctance motor
There are many different types of AC and DC motors and a wide range of power ratings. In addition, some types of motors are used only with electronic speed controllers or have electronic components built in. Some types of motors are suitable for using either AC or DC. They are series-connected, commutator-type DC or universal motors and permanent-magnet, brushless-type "DC" motors without a built-in electronic commutator. For very small motors, those producing less than100 watts or1/8 horsepower of mechanical power, all types of DC motors are probably more efficient than all types of AC motors. For small motors producing more than100 watts up to about10000 watts or15 horsepower, the efficiency advantage shifts to AC motors somewhere in that range depending on the specific motor configuration. Single phase AC motors are less efficient than3-phase motors. Among single-phase motors, shaded pole motors are less efficient than capacitor motors. Permanent-magnet DC motors with simple electronic commutators probably have the efficiency advantage compared3-phase induction motors up to a higher power level than any any other comparison of DC vs AC types. If variable speed is a requirement, permanent-magnet motors probably have the advantage over3-phase induction motors in all power ratings for which permanent-magnet motors are available. That may extend somewhat above10000 watts. The speed control unit may be essentially the same for either type of motor and the permanent-magnet motor essentially operates on AC.
I Would Say, DC Motor Is More Efficient Than AC Motor.
DC motor is more efficeint then AC.
DC motor is more powerful.
