A novel starting method for BLDC motors without the position sensors Behzad Asaei, Alireza Rostami * School of Electrical and Computer Engineering, Faculty of Engineering, University of Tehran, Tehran, Iran University of Yazd, Yazd, Iran article info Article history: Received 13 February 2008 Accepted 12 September 2008 Available online 1 November 2008 Keywords: Brushless dc (BLDC) motor Hard disk drive (HDD) Saturation Sensorless Starting torque abstract This paper presents a novel method to estimate the rotor position of a brushless dc (BLDC) motor at standstill. Moreover, a method for startup and acceleration of the motor up to a certain speed is intro- duced. The principle of the estimation method is based on the variation of the BLDC motor current in the magnetic axis due to the magnetic saturation of the stator core. An advantage of this method is that the maximum estimated error of the initial rotor position is 6°. Therefore, the motor starting torque is increased significantly. However, to implement this method, a current sensor at the dc link of the inverter is needed. Ó 2008 Elsevier Ltd. All rights reserved. 1. Introduction Sensorless operation of a brushless dc (BLDC) motor using the back EMF information (electromotive force), such as back EMF zero crossing, is one of the methods that has been used for hard disk drives (HDDs). However, at standstill or at low speeds, it is impos- sible or very difficult to estimate the position of the rotor by using the back EMF method. Therefore, an improved new starting meth- od is needed [1]. Recently, HDDs are widely used in mobile appli- cations. Thus, it is necessary to obtain a stable starting and acceleration up to the nominal operation speed under severe mechanical disturbances. In these applications, estimation of the initial rotor position is important not only for stable starting but also for maximum starting torque to reduce the start up time of the HDD. To start from standstill, various methods were proposed. In or- der to measure the stator inductance that varies with rotor posi- tion, a number of voltage pulses are applied periodically to the motor [2–6]. Since the applied voltage pulses make the system inefficient, these methods are not adequate. In a conventional method, a predetermined voltage has been applied that rotates the rotor synchronously with the applied voltage from standstill [7]. In that method, the motor parameters, such as the torque con- stant, friction coefficient and inertia of the rotor, should be known for a stable start of the motor [8]. However, a sudden change of the load may cause motor start failure. A popular method to estimate the initial rotor position at standstill is to utilize the saturation ef- fect of the stator iron core because it acts as a permanent magnet. The inductance of the stator winding has its minimum value where the north pole of the rotor magnet lies in the corresponding stator winding axis of a HDD motor. Several voltage vectors are applied to the stator winding of the motor to identify the rotor position. The vector that has a minimum inductance can be considered as the d- axis direction. In the conventional method, a 60° resolution in the estimation of the rotor position is possible by using a current sen- sor in the dc link of the inverter as the available voltage vectors to measure the inductance with the pulse width modulation (PWM) inverter are six nonzero vectors. By using the novel method that is proposed in Ref. [9], the rotor position can be estimated with the maximum error of ±15°. In this paper, a new method to esti- mate the initial rotor position is proposed that can estimate the ro- tor position with a maximum error of 6  . Therefore, the starting torque is increased significantly compared to the previous method [9] without a need to add any additional hardware. 2. Initial rotor position estimation method 2.1. Principal of estimation The estimation of the rotor position is based on the nonlinear magnetization characteristic of the stator core. The stator core close to the pole of the magnetic field is magnetized more than the other. Therefore, for the stator winding close to the magnetic pole of the rotor, the current is large compared to the absolute va- lue of the stator current flowing in the other directions. Fig. 1 shows the eight voltage space vectors in a voltage source inverter (VSI) in the d–q plane (direct–quadrature plane). By applying the 0196-8904/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.enconman.2008.09.020 * Corresponding author. Tel.: +98 21 82084936; fax: +98 21 88633029. E-mail address: a.rostamy@ece.ut.ac.ir (A. Rostami). Energy Conversion and Management 50 (2009) 337–343 Contents lists available at ScienceDirect Energy Conversion and Management journal homepage: www.elsevier.com/locate/enconman