Research On Single-phase Brushless DC Motor And Its Sensorless Control System

The single phase brushless DC motor (SPBLDCM) has the advantages of low-cost, easy to fabricate and so on, so it is widely used in low-cost and power applications such as fan, bump which require low starting torque and operating performance. The excellent bidirectional operational performance of SPBLDCM is necessary in many applications. However, the bidirectional starting capability of SPBLDCM is worse compared to multiphase brushless DC motor (BLDCM).A tapered asymmetric air gap is usually used to eliminate the dead point where the excitation net torque is zero. In order to research the starting capability of SPBLDCM with different asymmetric air gap, a small power outer-rotor SPBLDCM for fans with tapered airgap is designed in this paper, the2D FEM model is established in Ansoft Mawell2D. Its bidirectional starting capability is researched by means of FEM under low voltage conditions. Later, the circuit for soft starting simulation is established in Ansoft Simplorer9, we researched its bidirectional starting capability by field-circuit coupled method. Then, the stator structure of motor is improved, there is a dip angle on the stator thin end, the bidirectional starting capability of SPBLDCM with a improved tapered air gap structure is researched by means of simulation. Finally, we designed the soft-start control circuit of SPBLDCM based on controller BD6974, the bidirectional starting capability of the two motors with the two different airgap structures through the experiment method. Simulation and test validate that the SPBLDCM with a improved air gap structure has a better starting capability.The sensorless control method for SPBLDCM based on winding time-sharing principle can keep the current and BEMF in phase exactly, which avoids the current reverse while commutating caused by freewheeling, thus, greatly reduces the vibration and noise of motor, improve the efficiency of motor. In the second part of the paper, a sensorless control system based on winding time-sharing method for SPBLDCM with the tapered airgap designed previously is researched, the key control parameters was analyzed in detail, and the experiment platform for sensorless control system of SPBLDCM is built. The experiment results show that the motor speed can be regulated directly by adjust the advance off-time and the motor current waveform can be significantly improved while phase commutation advance, furthermore, phase commutation advance can raise the motor speed compared to the motor speed while phase commutation normal.