Journal of Power Electronics, Vol. 17, No. 5, pp. 1223-1230, September 2017 1223 https://doi.org/10.6113/JPE.2017.17.5.1223 ISSN(Print): 1598-2092 / ISSN(Online): 2093-4718 JPE 17-5-10 Torque Ripple Reduction of an Interior PM Synchronous Motor by Compensating Harmonic Currents Based on Flux Linkage Harmonics Myung Joon Nam * , Jong Hyun Kim * , Kwan-Yuhl Cho † , Hag-Wone Kim * , and Younghoon Cho ** *,† Department of Control and Instrumentation Engineering, Korea University of Transportation, Chungju, Korea ** Department of Electrical Engineering, Konkuk University, Seoul, Korea Abstract The back emf harmonics of a permanent magnet (PM) synchronous motor is a major source of torque ripple. For torque control applications including column fitted MDPS (motor driven power steering) systems, it is essential to reduce the mechanical vibrations due to torque ripples at low speeds. In this paper, a torque ripple reduction algorithm for interior PM synchronous motors is proposed. The harmonic currents that cancel the 6 th order torque harmonic are added to the nominal dq currents for MTPA (maximum torque per ampere) operation. The compensated harmonic currents are derived from flux linkage harmonics based on a FFT analysis of the back emf harmonics. Simulation and experimental results verify that the 6 th order torque harmonic and THD of the torque ripple are reduced by compensating the dq harmonic currents. Key words: Back emf harmonics, Flux linkage harmonics, MDPS, PMSM, Torque ripple I. INTRODUCTION For energy savings and the reduction of CO 2 emissions, electric vehicles and hybrid electric vehicles are an issue nowadays. In addition, some hydraulic controlled mechanical systems have been replaced by electric motor driven systems, such as power steering and brake systems. Compared with hydraulic power steering system, MDPS (motor driven power steering) has shown better fuel efficiency and better steering feeling in addition to being more environment friendly offering more space in the engine compartment due to the reduced components [1], [2]. In small and middle size vehicles, a column fitted type is normally used, where the electric motor is mounted on the steering column. In MDPS systems, the high torque capability, low torque pulsations, and energy efficiency are the key factors. Therefore, brushed DC motors have been replaced by PM synchronous motors. In column fitted MDPS systems, very low mechanical vibrations are required since the motors of such systems are directly attached below the steering handle. Mechanical vibrations are mainly generated by the torque ripples of the PM synchronous motor so that reducing the torque ripples is essential, especially in the applications of torque control such as MDPS and in the main traction of electric vehicles. The sources of torque ripples in PM synchronous motors are the cogging torque, the offset of the current sensors, the resolution of the rotor position sensors, and the interaction of the back emf harmonics and the distorted current waveforms due to dead-time and back emf harmonics. Many techniques have been presented to reduce torque ripples [3]-[17], and they can be categorized into two approaches. These approaches are the motor structure approach and the motor control approach. The motor structure approach focuses on cogging torque reduction by the skewed rotor, the fractional slots, increasing the air-gap, adding notches in the surface of stator, and optimization of the stator teeth shape [3]-[5]. The motor control approach deals with sensors for detecting the current and rotor position, current waveforms, and back emf harmonics. The offset of the measured currents in the current sensors makes torque ripples with the fundamental frequency, which can be removed by current offset compensation and current sensor calibration [6]. In [7], an offset current compensation to reduce the phase currents distortion caused by switching device voltage drops and dead time is presented. Manuscript received May 2, 2017; accepted Jun. 11, 2017 Recommended for publication by Associate Editor Kwang-Woon Lee. † Corresponding Author: kycho@ut.ac.kr Tel: +82-43-841-5329, Fax: +82-43-841-5320, Korea Nat’l Univ. of Transportation * Department of Control & Instrumentation Engineering, Korea National University of Transportation, Korea ** Department of Electrical Engineering, Konkuk University, Korea © 2017 KIPE