A DC motor is fixed with a ring-shaped permanent magnet inside, and the current flows through the coil on the rotor to generate an ampere force. When the coil on the rotor is parallel to the magnetic field, the direction of the magnetic field received by further rotation will change. Therefore, at this time, the electric brush at the end of the rotor and the conversion piece alternately contact, so the direction of the current on the coil also changes, and the direction of the generated Lorentz force remains unchanged, so the motor can maintain one direction of rotation.