The motor efficiency can be incresed by choosing correct lemination cores. for a given core to drive a certain load you cant do anything to improve. For motors if you increse the voltage current reduces to compansate electrical power taken to drive the constant load and vise versa ( if voltage drop occurs it takes more current and can lead to winding damage), simple application of V=IR will not work for motors. core losses (iron megnatizing loss & eddy current loss), loss in conductor, loss at bearing, air friction etc are kind of consideration for improvement