A Novel Low Cost Bipolar-Starting and Unipolar- Running Converter to Drive Spindle Motor at High Speed with High Starting Torque Narasimham PVRL*, Sarma AVRS ** , Srinivasarao D *** * E.E.E. Department , Gudlavalleru Engineering College, Gudlavalleru, India, 521356, pvrl2005@yahoo.co.in ** Department of Electrical Engineering, College of Engineering, Osmania University, Hyderabad, India. ***E.E.E. Department, Gudlavalleru Engineering College, Gudlavalleru, India. Abstract- A Hard Disk Drive (HDD) spindle motor is a brushless dc (BLDC) motor which has been used in high speed applications due to its high efficiency and good controllability. This paper presents a novel topology for a low cost converter which drives a spindle motor at high speed with high starting torque using the bipolar starting and unipolar running algorithm. This topology is simple and uses only eight switches in the converter to drive the spindle motor at high speed with high starting torque. The proposed scheme has been simulated on MATLAB/SIMULINK platform, the results are presented and discussed. The proposed topology is best suited for low power drives. Index Terms- Bipolar starting and unipolar running drive, HDD Spindle Motor, Unipolar drive. I. INTRODUCTION High efficiency, high power density and wide range speed controllability of BLDC motors make them suitable in various drive applications. In particular the spindle motors used in computer hard disk drives are to possess high speed characteristics for fast data access [1]. Several methods were proposed in literature [2]-[10] to achieve high speed operation. High speed operation could be achieved by any of the four schemes. The first method is to design the electromagnets in such a way that, the machine possess low back emf which offers high speed for a given system voltage [4]. But the disadvantage is that a low back emf constant results in low starting torque. The second method is winding method i.e., a series winding start and parallel winding run to run the motor at high speed with large starting torque [5]. This requires additional switching devices and more complex control logic. The third method is to use a higher dc bus voltage, where a high starting torque with high speed operation can be achieved. But this scheme has a problem where the switch voltage rating is to be enhanced and it requires a current protection to limit the current during low speeds. This also adds cost and safety hazard to the system. Apart from the above three methods, the forth method is to use a converter which can provide high speed with high starting torque. To get high torque BLDC motor needs to be operated in bipolar mode and to get high speed BLDC motor should be operated in unipolar mode. In [2], a new converter is proposed which can achieve high starting torque with high speed by using 14 switches of same rating. Drawback of this circuit is, using numerous gate drives and switches. In this paper a novel inverter topology is proposed which uses bipolar operation to achieve high torque during starting and unipolar operation later to achieve high speed using only 8 switches. The proposed inverter is similar to conventional 3 leg inverter with one additional leg. The model has been verified with an inverter-motor model developed using Matlab/Simulink. II. PROPOSED INVERTER OPERATION In conventional BLDC motor during bipolar operation, at any time across DC bus, two phases come in series. Only half of the DC bus voltage is applied to each phase, resulting in addition of torque constant on both phases there by achieving high starting torque. But speed will be limited. To get higher speed, full DC bus voltage is to be applied to each phase. This can be achieved in unipolar operation, where each phase conducts only in one direction which in turn reduces the starting torque. Thus in order to get high torque, motor should operate in bipolar mode and to get high speed motor should operate in unipolar mode. Shifting of modes between unipolar and bipolar operation is achieved based on speed requirement. The proposed inverter consists of 4 legs. The 3 phases of the BLDC motor are connected to first 3 legs and neutral point is connected to the fourth leg as shown in Fig.1. In bipolar operation first 3 legs are active and the 4th leg is inactive. In bipolar operation by switching on Q1 and Q4, phase A conducts in positive direction and phase B conducts in negative direction. By switching off Q4 and switching on Q6, a free-wheeling path is established through phase B, diode D3, switch Q1 and Phase A as shown in Fig. 2.