Slotless Lightweight Motor for Drone Applications Md Sariful Islam*, Iqbal Husain Department of Electrical and Computer Engineering (ECE) North Carolina State University (NCSU) Raleigh, USA *mislam8@ncsu.edu Rajib Mikail US Corporate Research Center ABB Inc. Raleigh, USA Abstract— This paper proposes a slotless, lightweight permanent magnet (PM) motor for drone applications. This research analyzes the steps through FEA to achieve the torque speed profile required for drone applications with optimum pole count for the base speed. A comparison on slotless vs slotted stators is presented. In addition, an optimization using halbach configuration proves to give a superior design over conventional rotor designs. Finally, the analysis leads to a slotless stator with halbach PM configuration in the rotor for a 4 kW system achieving high power density, very low cogging torque and torque ripple, sinusoidal BEMF, and good system efficiency. The resulting low inductance (µH range) of the proposed machine opens up the opportunity of using wide band gap devices as the enabling technology. Keywords—slotless; halbach; power density; drone; torque- speed; Lightweight I. INTRODUCTION The applications of drones and unmanned aerial vehicles (UAVs) are growing day by day for the purpose of transportation, delivery, and surveillance usages. Due to the limitation of low energy density of today’s battery technology, the system weight is the most critical parameter to minimize for a given power requirement. In order to increase the fuel efficiency and flight time, the power density of the electric motors needs special consideration. Permanent magnet (PM) motors are suitable for use in UAV electric propulsion systems where weight and space are critical factors [1]. Higher number of magnetic poles helps to reduce radial thickness of the yoke [2], and thus, the mass of the machine. Additionally, higher base speed (with high fundamental frequency) for a fixed power further reduces the size and weight of the machine. The air-gap winding is another key enabling concept where the teeth from the stator can be removed to help reduce iron loss and system mass. Air-gap winding combined with halbach magnetization have the advantages of high power density and low iron loss. This topology facilitates the benefit of zero rotor iron loss, high power factor, negligible cogging torque, and very small torque pulsation. Torque pulsation develops mechanical vibration, speed oscillation, and acoustic noise which significantly affects system stability [3]. This machine has the disadvantage of higher effective air-gap, which reduces the nominal torque output. The use of outer-rotor topology prevents the increase of effective air-gap caused by retainer ring thickness [2]. The stator consists of mostly conductors, which in this case is a significant percentage of the total mass. The density of copper is approximately three times of aluminum where the latter costs only 10% of the former for the same volume. Apart from lower mass and cost, Al has higher specific heat, which allows eventually better overload capability [4]. Therefore, Al conductor is also investigated here to further improve the power density of the machine. In this paper, a 4 kW system is analyzed for a drone application. The system is designed with four 1 kW motors for a base speed of 3600 rpm in a limited space and weight allowance. This paper presents a systematic design methodology for a slotless-halbach topology, and compares the design with a slotted-radial configuration in terms of power density, ripple, and core loss for two types of winding materials - copper and aluminum. The theoretical limits for the slotless- halbach configurations are discussed. It is shown that motors designed for drones with a high-speed propeller can achieve maximum continuous power density of approximately 2.4 kW/kg for the natural air-cooled case. The optimum design provides sinusoidal air-gap flux distribution without magnet pole-arc shaping. The final design features an increase in power density, zero cogging torque, low torque pulsations, and low harmonic contents while maintaining the required efficiency. II. DESIGN ASPECTS A. Torque Speed Characteristic A drone propulsion system consisting of four motors is designed for lifting a total mass of 25kg including payload, electrical system, frame, and battery. The drone altitude d is 110 m, which is required to be reached within 10s (t). The lifting force is, ܨ ௟௜௙௧ െ െ ܨ ௗ௥௔௚௣௥௢௙௜௟௘ ൌ , where  is the total mass of the system. To meet the required time specification, acceleration is required initially to achieve the required velocity. The considered height in achieving the required speed is 8m. The velocity ݒ ௖ ൒ ௗ ௧ ൎ ͳʹ ݏ ଵ , acceleration  ௖ ൌ ௩ ೎ మ ଶு ݐ,ൌ ௩ ೎ ௔ ೎ ൅ ሺ െ ͺሻ/ ݒ ௖ . During the vertical lift, force is ܨ ௟௜௙௧ ൌ ሺ ൅  ௖ ሻ, and power is  ௟௜௙௧ ൌ ܨ ௟௜௙௧ ݒ. ௖ . Considering an efficiency of 80% for the motor-drive system, the peak power required from the individual motor is,  ௟௜௙௧ ൌ ଴.ଶହ௉ ೗೔೑೟ ଴.଴ ൌ ͳ,͹͸ͲW. During cruising, ܨ ௟௜௙௧ ൌ ,  ݐݏݑݎൌ ܨ ௗ௥௔௚௣௥௢௙௜௟௘ . In order to provide the cruising speed, the thrust-to-weight ratio should be greater than one and the 978-1-5090-2998-3/17/$31.00 ©2017 IEEE 5041