An impeller, which is driven by a stator, includes a conducting shell, a hub and a plurality of blades. The conducting shell covers at least one side of the stator. The hub covers the conducting shell. At least one air gap is formed between the hub and the stator. A top portion of the hub has an opening to partially expose a top portion of the conducting shell. At least one airflow passage is formed between an inner wall of the hub and an outer surface of the conducting shell. The airflow passage has at least one inlet connected with the opening and at least one outlet connected with the air gap. The blades are disposed around the circumferences of the hub.