Underground three-phase direct-current permanent magnet closed-loop straight motor

The invention relates to a straight motor, in particular to an underground three-phase direct-current permanent magnet closed-loop straight motor. The straight motor comprises an outer cylinder tube, wherein iron cylinders are arranged at both ends of the outer cylinder tube; an iron shaft tube is arranged in the outer cylinder tube; the both ends of the iron shaft tube are connected with pull shafts; the iron shaft tube and the pull shafts are hollow; a silicon steel sheet is arranged on the inner wall of the outer cylinder tube; a coil is arranged on an iron core of the silicon steel sheet; a magnetic ring and a stainless steel ring are sleeved on the outer wall of the iron shaft tube; the magnetic ring and the stainless steel ring are alternatively sleeved on the iron shaft tube; the pull shafts at the both ends of the iron shaft tube pass through an iron cylinder; and a gap is reserved among the magnetic ring, the stainless steel ring and the coil. The problems in the conventional induction direct-current