Department of Electrical Engineering Southern Taiwan University Simple position sensorless starting method for brushless DC motor Student: Po-Jui Hsiao Adviser: Ming-Shyan Wang Date : 16th-Dec-2009 P. Damodharan, R. Sandeep and K. Vasudevan, IET Electr. Power Appl., Vol. 2, No. 1, January 2008

Department of Electrical Engineering Southern Taiwan University 2 Outline Abstract Introduction Proposed sensorless starting scheme Simulation of the proposed sensorless starting method Hardware implementation and test results Starting on no-load Starting on load Conclusions References

Department of Electrical Engineering Southern Taiwan University 3 Abstract A simple method by which the motor is started from standstill up to a speed wherein sensorless methods will be able to detect the correct commutation instants is proposed. The proposed method relies on a difference of line voltages measured at the terminals of the motor. It is shown that this difference of line voltages provides an amplified version of an appropriate back-EMF at its zero crossings. It is further demonstrated that this information can be used to trigger devices so as to develop an accelerating torque from zero speed. This method is simple to implement and it can reliably start the motor even with load.

Department of Electrical Engineering Southern Taiwan University 4 Introduction When the motor is at standstill, there is no back-EMF induced in the coil and thus a startup algorithm or an initial rotor position detection method is required to start the motor reliably from standstill up to a minimum speed where the conventional position sensorless control methods based on back-EMF information could take over. 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.

Department of Electrical Engineering Southern Taiwan University 5 Proposed sensorless starting scheme Fig. 1 BLDC motor drive along with typical phase current and back-EMF

Department of Electrical Engineering Southern Taiwan University 6 Proposed sensorless starting scheme where is the stator resistance of the ‘ A ’ phase, the phase inductance, the back-EMF and the phase current. Similar equations can be written for the other two phases, as in (2) and (3)

Department of Electrical Engineering Southern Taiwan University 7 Proposed sensorless starting scheme From this, the line voltage may be determined as 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.

Department of Electrical Engineering Southern Taiwan University 8 Proposed sensorless starting scheme Subtracting (5) from (4) gives

Department of Electrical Engineering Southern Taiwan University 9 Proposed sensorless starting scheme Consider the interval when phases A and C are conducting and phase B is open as indicated by the shaded region in Fig. 1. In this interval, phase A winding is connected to the +ve of the DC supply, phase C to the -ve of the DC supply and phase B is open. Therefore and. It can be seen from Fig. 1 (shaded region) that the back-EMF in phases A and C are equal and opposite. Therefore in that interval (7) may be simplified as

Department of Electrical Engineering Southern Taiwan University 10 Proposed sensorless starting scheme Fig. 2 Flow chart of the proposed startup scheme

Department of Electrical Engineering Southern Taiwan University 11 Proposed sensorless starting scheme

Department of Electrical Engineering Southern Taiwan University 12 Simulation of the proposed sensorless starting method

Department of Electrical Engineering Southern Taiwan University 13 Simulation of the proposed sensorless starting method Fig. 3 Functional sequence of the operations in the proposed algorithm From the sensed terminal voltages with respect to negative DC bus (,, ), line voltages and subsequently their differences (,, ) are determined.

Department of Electrical Engineering Southern Taiwan University 14 Simulation of the proposed sensorless starting method Fig. 4 Line-to-line voltage difference with back-EMF Fig. 4 shows the simulated back-EMF waveform of phase B and the line voltage difference during the first triggering of TC+ and TB-. It can be seen that the plot validates (8) in the region of zero crossing of the back- EMF.

Department of Electrical Engineering Southern Taiwan University 15 Simulation of the proposed sensorless starting method Fig. 5 Detection of back-EMF zero crossing

Department of Electrical Engineering Southern Taiwan University 16 Simulation of the proposed sensorless starting method Fig. 6 Inverter switching signals with the back-EMF zero crossings and speed

Department of Electrical Engineering Southern Taiwan University 17 Simulation of the proposed sensorless starting method Fig. 7 Speed and phase current waveform

Department of Electrical Engineering Southern Taiwan University 18 Simulation of the proposed sensorless starting method Fig. 8 Rotor prepositioning from different initial positions

Department of Electrical Engineering Southern Taiwan University 19 Hardware implementation and test results Fig. 9 Block diagram of the experimental setup

Department of Electrical Engineering Southern Taiwan University 20 Starting on no-load Fig. 10 Line-to-line voltage difference with back-EMF (experimental)

Department of Electrical Engineering Southern Taiwan University 21 Starting on no-load Fig. 11 Phase current and speed waveform on no-load

Department of Electrical Engineering Southern Taiwan University 22 Starting on no-load Fig. 12 Switching signals for inverter with 50% duty ratio PWM on no-load

Department of Electrical Engineering Southern Taiwan University 23 Starting on no-load Fig. 13 Phase current and speed waveform on no-load with 50% duty ratio PWM

Department of Electrical Engineering Southern Taiwan University 24 Starting on load Fig. 14 Phase current and speed waveform on load with 50% duty ratio PWM

Department of Electrical Engineering Southern Taiwan University 25 Conclusions This method makes use of line-to-line voltage differences to detect and amplify back-EMF 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.

Department of Electrical Engineering Southern Taiwan University 26 References 1 Kenjo, T., and Nagamori, S.: ‘Permanent-magnet and brushless DC motors’ (Clarendon Press, Oxford, 1985) 2 Miller, T.J.E.: ‘Brushless permanent-magnet and reluctance motor drives’ (Clarendon Press, Oxford, 1989) 3 Iizuka, K., Uzuhashi, H., Kano, M. et al.: ‘Microcomputer control for sensorless brushless motor’, IEEE Trans. Ind. Appl., 1985, IA-21, (4), pp. 595–601 4 Chen, H.-C., and Liaw, C.-M.: ‘Current-mode control for sensorless BDCM drive with intelligent commutation tuning’, IEEE Trans. Power Electron., 2002, 17, (5), pp. 747–756 5 Cheng, K.-Y., and Tzou, Y.-Y.: ‘Design of a sensorless commutation IC for BLDC motors’, IEEE Trans. Power Electron., 2003, 18, (6), pp. 1365– Su, G.-J., and McKeever, J.W.: ‘Low-cost sensorless control of brushless DC motors with improved speed range’, IEEE Trans. Power Electron., 2004, 19, (2), pp. 296–302 7 Jung, D.-H., and Ha, I.-J.: ‘Low-cost sensorless control of brushless DC motors using a frequency-independent phase shifter’, IEEETrans.Power Electron., 2000, 15, (4), pp. 744–752

Department of Electrical Engineering Southern Taiwan University 27 References 8 Moreira, J.C.: ‘Indirect sensing for rotor flux position of permanent magnet AC motors operating over a wide speed range’, IEEE Trans. Ind. Appl., 1996, 32, (6), pp. 1394– Shao, J., Nolan, D., Teissier, M. et al.: ‘A novel microcontroller-based sensorless brushless DC (BLDC) motor drive for automotive fuel pumps’, IEEE Trans. Ind. Appl., 2003, 39, (6), pp. 1734– Ogasawara, S., and Akagi, H.: ‘An approach to position sensorless drive for brushless DC motors’, IEEE Trans. Ind. Appl., 1991, 27, (5), pp. 928– Kim, T.-H., and Ehsani, M.: ‘Sensorless control of BLDC motors from near-zero to high speeds’, IEEE Trans. Power Electron., 2004, 19, (6), pp. 1635– Tursini, M., Petrella, R., and Parasiliti, F.: ‘Initial rotor position estimation method for PM motors’, IEEE Trans. Ind. Appl., 2003, 39, (6), pp. 1630– Jang, G.H., Park, J.H., and Chang, J.H.: ‘Position detection and start-up algorithm of a rotor in a sensorless BLDC motor utilising inductance variation’, IEE Proc.,- Electr. Power Appl., 2002, 149, (2), pp. 137–142

Department of Electrical Engineering Southern Taiwan University 28 References 14 Lee, W.-J., and Sul, S.-K.: ‘A new starting method of BLDC motors without position sensor’, IEEE Trans. Ind. Appl., 2006, 42, (6), pp. 1532– Acarnley, P.P., and Watson, J.F.: ‘Review of position-sensorless operation of brushless permanent-magnet machines’, IEEE Trans. Ind. Electron., 2006, 53, (2), pp. 352– Fairchild Semiconductor: ‘Using the ML4425/ML4426 BLDC motor controllers’, Application note 42004, June Micro Linear: ‘ML4435 Sensorless BLDC motor controller datasheet’, May Allegro MicroSystems: ‘ Phase brushless DC motor controller/ driver with back-EMF sensing’, datasheet, Microchip: ‘Sensorless BLDC control with back-EMF filtering’, Application note AN1083, STMicroelectronics: ‘BLDC motor start routine for the ST72141 microcontroller’, Application note AN1276, 2000