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Robot and Servo Drive Lab. Simple position sensorless starting method for brushless DC motor Electric Power Applications, VOL. 2, Page. 49~Page. 55, Jan 2008, By Sandeep R, Vasudevan, K. Student: CIH-HUEI SHIH Department of Electrical Engineering Southern Taiwan University of Science and Technology 2016/7/14 Outline Proposed sensorless starting scheme Flow chart Block Diagram Switching sequence BLDC motor parameters Functional operations Detection of back-EMF zero crossing Line-to-line voltage difference with back-EMF Inverter switching signals Speed and phase current waveform Result 2016/7/14 Department of Electrical Engineering Robot and Servo Drive Lab. Southern Taiwan University of Science and Technology 2 Abstract Position sensorless methods for brushless DC motors based on back-EMF zero crossing suffer from a starting problem since there is no back-EMF at standstill. A simple method by which the motor is started from standstill up to a speed where in sensorless methods will be able to detect the correct commutation instants is proposed. 2016/7/14 Department of Electrical Engineering Robot and Servo Drive Lab. Southern Taiwan University of Science and Technology 3 Introduction This paper proposes a simple and reliable method to detect the back-EMF zero crossings. It is further shown in the paper that this method can be used to start the machine as well, once the initial rotational movement is established. In this work, the rotor is first brought to a known position through a prepositioning step. Subsequent rotation of the rotor is achieved by a 120 electrical degree triggering followed by a sequential triggering of the devices based on zero crossings of the back-EMF. 2016/7/14 Department of Electrical Engineering Robot and Servo Drive Lab. Southern Taiwan University of Science and Technology 4 Proposed sensorless starting scheme Consider a BLDC motor having three stator phase windings connected in star. PMs are mounted on the rotor. The BLDC motor is driven by a three-phase inverter in which the devices are triggered with respect to the rotor position as shown in Fig. 1. The phase A terminal voltage with respect to the star point of the stator Van is given in as 2016/7/14 Department of Electrical Engineering Robot and Servo Drive Lab. Southern Taiwan University of Science and Technology 5 where Ra is the stator resistance of the ‘A’ phase, La the phase inductance, ean the back-EMF and ia the phase current. Similar equations can be written for the other two phases, as in (2) and (3) 2016/7/14 Department of Electrical Engineering Robot and Servo Drive Lab. Southern Taiwan University of Science and Technology 6 where the symbols have their obvious meanings. From this, the line voltage Vab may be determined as 2016/7/14 Department of Electrical Engineering Robot and Servo Drive Lab. Southern Taiwan University of Science and Technology 7 These line voltages can, however, be estimated without the need for star point by taking the difference of terminal voltages measured with respect to the negative DC bus. Subtracting (5) from (4) gives 2016/7/14 Department of Electrical Engineering Robot and Servo Drive Lab. Southern Taiwan University of Science and Technology 8 Flow chart Start Commutate to TC+ TB- Excite Device TB+ TCPre-position No No End of time Tp? Commutate to TC+ TA- No 2016/7/14 Detect the +ve going Zero crossing of vabbc ? Detect the -ve going Zero crossing of vcaab ? Commutate to TA+ TB- End of time Tp? Exit Start-up Department of Electrical Engineering Robot and Servo Drive Lab. Southern Taiwan University of Science and Technology No Continue the Start-up Sequence 9 Block Diagram 2016/7/14 Department of Electrical Engineering Robot and Servo Drive Lab. Southern Taiwan University of Science and Technology 10 Switching sequence 2016/7/14 Department of Electrical Engineering Robot and Servo Drive Lab. Southern Taiwan University of Science and Technology 11 BLDC motor parameters 2016/7/14 Department of Electrical Engineering Robot and Servo Drive Lab. Southern Taiwan University of Science and Technology 12 Functional operations 2016/7/14 Department of Electrical Engineering Robot and Servo Drive Lab. Southern Taiwan University of Science and Technology 13 Detection of back-EMF zero crossing 2016/7/14 Department of Electrical Engineering Robot and Servo Drive Lab. Southern Taiwan University of Science and Technology 14 Line-to-line voltage difference with back-EMF 2016/7/14 Department of Electrical Engineering Robot and Servo Drive Lab. Southern Taiwan University of Science and Technology 15 Inverter switching signals 2016/7/14 Department of Electrical Engineering Robot and Servo Drive Lab. Southern Taiwan University of Science and Technology 16 Speed and phase current waveform 2016/7/14 Department of Electrical Engineering Robot and Servo Drive Lab. Southern Taiwan University of Science and Technology 17 Result Rotor prepositioning from different initial positions 2016/7/14 Department of Electrical Engineering Robot and Servo Drive Lab. Southern Taiwan University of Science and Technology 18 Line-to-line voltage with Back-EMF 2016/7/14 Department of Electrical Engineering Robot and Servo Drive Lab. Southern Taiwan University of Science and Technology 19 Phase current and speed waveform on no-load Switching signals for inverter with 50% duty ratio PWM on no-load Phase current and speed waveform on no-load with 50% duty ratio PWM Phase current and speed waveform on load with 50% duty ratio PWM Department of Electrical Engineering Robot and Servo Drive Lab. Southern Taiwan University of Science and Technology 20 Conclusion A simple technique to start the BLDC motor for position sensorless schemes is proposed. This method makes use of line-to-line voltage differences to detect and amplify backEMF signals so that even EMF zero crossings caused by initial rotor rotation can be easily detected. Subsequent device triggerings ensure acceleration and are based on further zero crossing detections. The motor is found to start smoothly from standstill and run up to a speed where a sensorless scheme can take over. Simulation and experimental results are shown, which validate the suitability of the proposed method. 2016/7/14 Department of Electrical Engineering Robot and Servo Drive Lab. Southern Taiwan University of Science and Technology 21 Thanks for your listening 2016/7/14 Department of Electrical Engineering Robot and Servo Drive Lab. Southern Taiwan University of Science and Technology 22