A washing machine with a direct drive motor system includes a plastic tub, a drum rotatably mounted inside the tub and a shaft connected to the drum for transmitting a driving torque of a direct drive motor. The motor has a rotor assembly fastened to the shaft and a stator assembly fastened to the tub. The plastic tub includes a central protruding portion where a central opening of the stator assembly is centered before it is fastened to the tub, the central opening of the stator assembly presenting a plurality of centering surfaces configured to cooperate with an outside cylindrical surface of the central protruding portion of the tub