Influence of Stator Asymmetry on Cogging Torque of Permanent Magnet Brushless Machines

Dept. of Electron. & Electr. Eng., Univ. of Sheffield, Sheffield
IEEE Transactions on Magnetics (Impact Factor: 1.39). 12/2008; 44(11):3851 - 3854. DOI: 10.1109/TMAG.2008.2001322
Source: IEEE Xplore


The paper investigates the influence of stator asymmetry on the cogging torque of permanent magnet brushless motors. It shows that high localized magnetic saturation due to such asymmetry can significantly increase the magnitude of the cogging torque and introduces cogging components which have an order lower than the least common multiple between the stator slot number and the rotor pole number. The findings are confirmed by both finite element analyses and measurements on an 18-slot, 6-pole prototype motor.

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    • "Recently, much work has been done on design techniques to minimize these remaining harmonics by determining analytical expressions for cogging torque [9]–[11]. However problems arise when manufacturing variations [8] or motor design [12] leads to incomplete cancellation. In that situation, harmonics much lower than than the lowest common multiple are present in the cogging torque. "
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    ABSTRACT: Theoretically, the layout of fractional pitch permanent magnet motors should ensure the cancellation of most cogging torque harmonics. In reality, manufacturing errors lead to incomplete cancellation and higher than expected cogging torque. One potential manufacturing error is the variation in angle between neighboring magnets. This paper uses the superposition technique to analytically determine the expected magnitude of each cogging torque harmonic based on a Gaussian distribution of angular magnet placement variation. A simple expression is presented for the expected value of each harmonic's magnitude based upon the harmonic number, the number of magnets, and the standard deviation of the Gaussian placement distribution. The derived expression is validated using Monte Carlo simulations and experimental implementation.
    Full-text · Article · Sep 2011 · IEEE Transactions on Magnetics
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    • "Similar to other kinds of permanentmagnet motors, cogging torque is an inherent problem and results in mechanical resonance, vibration, and noise. For surface-mounted permanent-magnet motors, many methods have been proposed to reduce cogging torque, such as a fractional number of slots per pole [1], slot skewing [2], magnet skewing [3], [4], " goodness " of slot number and pole number combination [4], slot or tooth pairing [5], [10], magnet segmentation [6], pole-arc coefficient adjusting and its optimization [7], [9]–[11], adjusting the shape of the slots or the tooth width [8], [13], [14], auxiliary slots or teeth [12], [14], magnet displacing and shifting [3], [15]–[17], etc. However, there is no study for solid rotor permanent-magnet motors with the interior permanent magnet (IPM) by now. "
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    ABSTRACT: In this paper, the method of magnetic poles shifting was combined with optimization method to reduce cogging torque in solid-rotor permanent-magnet synchronous motors. Although the finite-element method (FEM) can calculate the cogging torque accurately, to find the peak value of cogging torque, the cogging torque for different relative positions between permanent magnets and slots must be calculated; thus, the optimization will take a long time. To reduce optimization time, a novel analytical method was proposed to determine the initial value and feasible range of the shifting angles. Then the optimization method and FEM were used to minimize the cogging torque. Two prototype motors were analyzed and optimized, respectively. It was proved that the cogging torque can be greatly reduced by the proposed method.
    Preview · Article · Jun 2010 · IEEE Transactions on Magnetics
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    ABSTRACT: A Various technique is available in reducing the cogging torque in Brushless DC machine (BLDC). Cogging torque is the main hindrance in BLDC motor, which results in direct impact on the performance of the Permanent Magnet (PM) motor. In this paper the PMs has various structures such as surface mounted, interior and buried permanent magnet. The cogging torque is compared for the various designs of the rotor structures. The CAD package validates the design and shows the performance enhancement of BLDC motor.
    Full-text · Article · Apr 2007
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