A New Technique of Cogging Torque Suppression in Direct-Drive Permanent-Magnet Brushless Machines

Abstract

In direct-drive electric propulsion systems, where there is no reduction gear to minimize and absorb the adverse effects due to cogging torque generated by the permanent magnet (PM) machine, minimization of cogging torque generation is of particular importance. In this paper, a novel axial pole pairing method is proposed to minimize cogging torque generation in a special three-phase outer-rotor PM brushless machine, which uses uneven stator poles to enhance back electromotive force (EMF). Analytical formulas of the machine's cogging torque are first derived. The new technique is compared with conventional cogging torque suppression methods by means of analytical models and comprehensive finite element analysis (FEA). The FEA results show that the new method not only achieves effective cogging torque reduction, but also results in improved harmonic content of the back EMF. The validity of the FEA model is verified by experimental results from the prototype machines. Finally, the significance of optimizing both the load-independent machine design techniques and load-dependent driving techniques to achieve overall torque ripple minimization is discussed.  Index Terms-Axial pole pairing, cogging torque, finite element, magnet pole-arc width, permanent magnet brushless machine, stator teeth pairing, stepped rotor skewing.

Effect of stator permanent magnet thickness and rotor geometry modifications on the minimization of cogging torque of a flux reversal machine

“…This shows the cogging torque is caused by the attraction between the PM and rotor tooth. Based on R variation, the cogging torque changes periodically [10][11][12][13].…”

New Cogging Torque Reduction Methods for Permanent Magnet Machine

“…Skewing the slots increase the copper losses because of expanded slot length, bringing about the more drawn out wire. Also the slots skewing is related with high production cost [9]. In IPM machines, only stator skewing or rotor step (discrete) skewing is a feasible solution Furthermore, depending on the skewed angle the both skew methods decrease the output torque of the machine.…”

“…Soo-Gyung Lee in [17] proposed an improved pole pairing considering axial stack length using analytical formulas which reduces the cogging torque by 71.98%. Researcher in [9] mentioned, by selecting the optimal rotor pole arc, the effect of cogging torque not only can be reduced, but also results in improved harmonic content of the back EMF. For this study, an axial rotor pole pairing configuration with outer pole outer pole arc of 28.0mm and 33.0mm is as illustrated in Figure 5 [18], Bin Zhang combined teeth notching with rotor magnets skewing [19] and Gyeong-Chan Lee has combined the stator tooth pairing with stator displacement which results in reducing the cogging torque effect up to 93.3% [2].…”

“…Generally, the common stator side modification comprises the stator pole ratio [5], stator tooth pairing [2,6], dummy slots [7], displacement slot [7]. Conversely, the rotor side modification consists of rotor pole radially and axially paired [8,9], chamfering [10], magnet pole arc [11], magnet shaping or shifting [12], skewing [9,13], and notching [14]. Change in rotor part has been generally utilized compared to the stator part as the important drawback to minimize the cogging torque segment is that it confuses the stator producing and consequently expands the assembling expense of the machine.…”

Performance analysis of 20 Pole 1.5 KW Three Phase Permanent Magnet Synchronous Generator for low Speed Vertical Axis Wind Turbine

“…The multi-pole stator arrangement can be configured to generate high voltage at low revolution, and high current at faster speeds. The stator is designed to produce negligible cogging torque therefore causing the generator to start up and cut-in at low speeds to produce high current [3]. One significant advantage of PMSG, is that it is much lighter, smaller in size, and uses less constructional material so lowering cost and hub size [2] [3].…”