A motor has a stator and a rotor. The stator has 2P poles wherein P is an integer greater than one. The rotor has a shaft, a rotor core and a commutator fixed to the shaft. The commutator has m segments where m is an even integer greater than P. Every two adjacent segments are electrically connected by a winding unit so that the rotor winding comprises m winding units with at least one of the winding units comprising P coils connected in series; and each coil of each winding unit is directly connected to a corresponding two segments.