Design techniques for cogging torque reduction in a fractional- slot PMBLDC motor Lidija Petkovska Faculty of Electrical Engineering and Information Technologies, Ss Cyril and Methodius University in Skopje, Skopje, Macedonia Paul Lefley School of Engineering, University of Leicester, Leicester, UK, and Goga Vladimir Cvetkovski Faculty of Electrical Engineering and Information Technologies, Ss Cyril and Methodius University in Skopje, Skopje, Macedonia Abstract Purpose – This paper presents the design techniques applied to a novel fractional-slot 6/4 pole permanent magnet brushless direct current (PMBLDC) motor, for cogging torque reduction. The notable feature of this motor is the simplicity of the design and low production cost. The purpose of this paper is to reduce the peak cogging torque of the motor. The focus is put on the stator topology tuning, and a new design for the stator poles is proposed. By determining the optimum stator pole arc length and the best pole shoe thickness, the cogging torque is significantly reduced. This new optimised motor design has been analysed in detail. The validation of the results is documented with respective figures and charts. Design/methodology/approach – At the beginning, the design data for the 6/4 pole PMBLDC motor with concentrated three phase windings and asymmetric stator pole arcs is presented. In the study, this motor is taken as a reference model (A 0 , T 0 ). A full performance finite element analysis of the reference motor has been carried out, and the weak points in the motor design have been identified. By simple design techniques, tuning the stator pole geometry, a two-stage design optimisation for cogging torque minimisation has been performed and the solution array has been derived. The optimised model is selected and proposed (A opt , T opt ). The comparative analysis of the reference and optimised motors show the advantages of the proposed novel design and prove the methodology. Findings – The results of the work demonstrate how simple design techniques can minimise the peak of the cogging torque profile, while maintaining the specified electromagnetic torque value. The sensitivity of the cogging torque profile because of changes of the stator pole design inside the prescribed constraints is apparent. The stator poles of the reference motor have an arc length of 85° and pole shoe thickness of 6 mm. The newly shaped stator poles have an arc length of 78.5° and pole shoe thickness 4.8 mm. The peak-cogging torque has been reduced from 0.158 Nm to a respectable value of 0.066 Nm. However, to reduce electromagnetic torque ripple and pulsations, further investigations are required. Originality/value – The paper presents an approach to cogging torque reduction for a 6/4 PMBLDC motor. A two-step original design procedure is introduced and an optimised stator pole geometry is defined. The minimised cogging torque has been demonstrated with improved usage of the active materials. This work could serve as a good basis for further optimisation of the motor design. Keywords Permanent magnet machine, Finite element analysis, Shape optimisation, Optimisation, Design optimisation methodology, Torque calculation, PM brushless DC motor, Asymmetric motor, Fractional-slot motor, Cogging torque, Electromagnetic torque, Torque ripple Paper type Research paper Cogging torque reduction Received 11 January 2020 Revised 12 April 2020 Accepted 13 April 2020 COMPEL - The international journal for computation and mathematics in electrical and electronic engineering © Emerald Publishing Limited 0332-1649 DOI 10.1108/COMPEL-01-2020-0015 The current issue and full text archive of this journal is available on Emerald Insight at: https://www.emerald.com/insight/0332-1649.htm