Research On The Sensorless Control System Of High-speed Fan Motor

With more higher integration of the telecommunication equipments, moreheat need to be sent out of the equipment. In order to solve this problem, thepaper conducted the research of high-speed Fan. The brushless DC motor(BLDCM) offers many advantages including high reliability, high efficiency,convenience maintenance and good speed control performance. BLDCM hasbeen widely used in communication equipment fan; but the position sensor inBLDCM not only increases the cost of motor but also reduces the reliability, sothe developed fan in this paper adopt BLDCM with sensorless control method.At present, the Y-connected three-phase full-bridge drive BLDCM iscommonly used. Under sensorless control mode, three-phase full-bridge drivecan’t accurately detect the rotor position signal from the back electromotiveforce(BEMF) because of other phases’ interference. The three-phase H-bridgedrive structure of BLDCM can solve the problem because the three phases arecontrolled independently so as to the BEMF can be detected without interference.Meanwhile, the motor windings of three-phase H-bridge are parallel connected,so greater torque or rotational speed can be provided within the same voltage.The mathematical derivation and simulation about these two advantages areimplemented in this paper.The commutation point of three phase H-bridge drive is also30°electricalangle lag of BEMF zero-crossing. Lead or lag of commutation will produce largecurrent fluctuation, which will increase the motor torque ripple and lower theefficiency of system. In order to maintain the high efficiency, commutationpoints in different speed are adjusted by programming to ensure the windingcurrent is always in the middle of the BEMF.The BEMF is zero or very small when the motor is stationary or at lowspeed, and the information of rotor position can’t be obtained by detecting theBEMF which will cause the failure of motor start. The paper adopted the‘three-step starting method’ and carried out detailed mathematical derivation ofthe startup process, and a slope map of commutation time interval betweendifferent steps are developed and successfully used in prototype.The control chip used in the BLDCM fan drive system is XC886manufactured by Infineon. Experimental results show that the design of the paperusing three-phase H-bridge drive can accurately detect the BEMF signal, and themotor can start up and run smoothly by the obtained commutation time slope map. The results prove the feasibility of designed scheme.