The motor comprises an internal rotor having an external lob; an external rotor outside the internal rotor and having an internal lob engaging the external lob; and an inlet port and an outlet port. An inner diameter of the inlet port is less than a diameter of a dedendum circle of the internal rotor, and an outer diameter of the inlet port is greater than a diameter of a dedendum circle of the external rotor. An inner diameter of the outlet port equals the diameter of the dedendum circle of the internal rotor. An outer diameter of the outlet port equals the diameter of the dedendum circle of the external rotor. The inner diameter of the inlet port is less than the inner diameter of the outlet port. The outer diameter of the inlet port is greater than the outer diameter of the outlet port.