a) DC "Micro Cooling Fan" is to provide the use of a DC power source: DC motor stator and rotor two major components, there is a stator pole (winding or permanent magnet), rotor windings, the power, the rotor is also a magnetic field (magnetic), there is an angle between the "Micro Cooling Fan" of the stator and rotor poles attract each other in the rotor magnetic field (N and S poles between) under the motor rotation. Change the brush seat, you can change the angle between the rotor and stator poles (assuming that is the angle between the stator pole starting side to the other side of the rotor poles, the poles of the rotor stator poles pointing direction is the direction of rotation of the motor) direction, thereby changing the direction of rotation of the motor.
b) the exchange of "Micro Cooling Fan" is to provide the use of an alternating power supply: single-phase AC motor is only one winding, a squirrel-cage rotor is. When single-phase sinusoidal current through the stator windings, the motor will produce an alternating magnetic field, the strength and direction of the magnetic field at any time intercropping sine law, but in the spatial orientation is fixed, so called the alternating magnetic field pulsating magnetic field. This alternating pulsating magnetic field can be decomposed into two at the same rotational speed, the rotational direction opposite to each other to produce a rotating magnetic field in the rotor in two equal and opposite torque, so that the net torque is zero, so the motor can not rotate.