Disc Brushless Dc Motor Torque Ripple Control

The availability of high performance magnets, like samarium cobalt and neodymium-boron-iron, has made it possible to improve the performance of brushless dc motors significantly. Design engineers now selecting PMBLDC motors over conventional dc or induction motor drives for the following reasons: 1) The absence of brushes results in an essentially maintenance free operation and eliminates the undesirable effects of commutation such as sparks, brush-loss, radio-frequency interference, etc. 2) The low inertia of the rotor enhances the quality of the mechanical response of the motor, shortening the acceleration and deceleration time. 3) Utilizing rare-earth permanent magnet materials with high coercive force results in substantially higher efficiency than their counterpart with brushes of the same size. 4) Because of the absence of commutation segments, the stator construction is simple.Basically, a brushless dc motor consists of a permanent magnet (PM) synchronous motor, an inverter and a position sensor mounted on the motor shaft. The inverter drive signals are synchronized with the position sensor output signals so that synchronism between the stator winding currents and the rotor flux is ensured for any speed.The electrical structure and mechanical structure of the non-ripple torque control system of brushless motor is almost the same as that of the control system of conventional brushless motor. The difference is that the non-ripple torque control system of brushless motor has a incremental encoder which the control system of conventional brushless motor has not, which results in a great change of control method. Because the controller is easy to get the position of rotor by the encoder, we can continuously rotate the flux of the armature rotor by applying SPWM method to the control system. The continuous rotation of the flux of armature make the output torque of motor has no ripple. At the same time, we can get the speed value of motor by differential computing the position value, so it is possible to accurately control the speed of motor.In this paper, the research of the non-ripple torque control system of brushless motor is an emphasis. Considering the non-ripple torque control system of brushless motor is a member of the control system of conventional brushless motor family, firstly this paper introduces the control strategy of the conventional permanent magnet brushless dc motor, then the particularity of the non-ripple torque control system of brushless motor is analyzed.