Analysis of a two phase brushless motor used in a CPU cooling fan

Cooling fans for PC microprocessors often use two phase brushless motors to reduce electrical noise and to extend fan life. They are made with the winding on the inside of the motor (stator), and the magnet on the outside (rotor). This paper describes the analysis of an existing motor using a lumped parameter model. The flux paths are described and a circuit model is developed. From this model, startup and running torque are derived using the change in magnetic energy with angle. The derived torque is then compared to experimental measurements.