An eccentric rotor creates an electromagnetic force between the rotor and stator of an electrical machine. This force tends to further increase the rotor eccentricity and may severely degrade the performance of the machine, causing acoustic noise, vibration, excessive wear of bearing, rotor and stator rubbing, and so forth. Parallel connections are known to be a simple yet effective remedy for the problems associated with rotor eccentricity. We have investigated two common types of electrical machines running with eccentric rotors. We examined operation over a wide whirling frequency range. We numerically evaluated and compared the effects of parallel connections in the stator and rotor windings on the eccentricity force. We found that the parallel stator windings can be more effective in mitigating the unbalanced magnetic pull than the rotor cage (or damper winding), which normally has many more parallel circuits.
"The 40 bar and 26 bar machines were studied under locked rotor conditions using magneto-static FEA (with the current in the rotor calculated using an analytical calculation tool—this technique was studied in ). It is possible to develop full transient time-stepped FEA models ; these are very time consuming both in terms of development and run. While magneto-static FEA should be used with prudence for an induction machine. "
[Show abstract][Hide abstract] ABSTRACT: In this paper, the radial forces in an induction motor are calculated using finite element analysis. These radial forces (or unbalanced magnetic pull-UMP) are generated when the rotor is eccentric. The work illustrates the importance of higher winding harmonics and rotor differential leakage in the starting UMP. Examples of a 6 pole machine with 26 and 40 bar rotors show that increasing the bar number and air-gap length will reduce the UMP. Further studies are carried out using parameter variation and a 10-pole machine is also addressed, where experimental results exist, in order to validate the calculation.
"NBALANCED MAGNETIC PULL (UMP) is usually associated with larger induction and synchronous machines under winding faults or rotor eccentricity conditions – and there is a large amount of literature on this subject. However, smaller machines are still subject to unbalanced magnetic pull ,  even without rotor anomalies. "
[Show abstract][Hide abstract] ABSTRACT: We report on an investigation into the unbalanced magnetic pull (UMP) in ferrite-magnet fractional-slot brushless permanent-magnet motors due to either magnetic asymmetry or static rotor eccentricity. We justify the work in terms of the establishment of total indicated runout (TIR) tolerance and use 10% eccentricity as the nominal tolerance. The UMP will generate force and vibration, which causes wear. We investigated several machines with different pole numbers, slot numbers, and winding arrangements. Some of the windings contain sub-harmonics, and we found these to be more susceptible to UMP when there is rotor eccentricity and also to produce vibrating UMP. We found that consequent rotor poles produce high UMP under centered and rotor eccentricity conditions. We conducted detailed finite-element studies to calculate the UMP.
"HE unbalanced magnetic force (UMF), which is also known as unbalanced magnetic pull, due to static/dynamic rotor eccentricities has been extensively studied in conventional electric machines, as well reviewed in –. However, the fractional-slot surface-mounted permanent magnet (SPM) brushless machine having diametrically asymmetrical winding distribution tends to exhibit more spatial and temporal harmonics in the field distribution than the conventional electric machine, which may result in the UMF even when the machine is ideally manufactured, without static/dynamic rotor eccentricities or uneven distribution of material properties . "
[Show abstract][Hide abstract] ABSTRACT: We present an analytical model of unbalanced magnetic force (UMF) in fractional-slot surface-mounted permanent magnet (PM) machines having diametrically asymmetrical winding distribution but no static/dynamic rotor eccentricities. It is based on a 2-D analytical field model and accounts for the influence of both the radial and tangential force waves under any load condition. It is capable of providing insight into the generation, harmonic contents, and characteristics of the UMF and accurately predicting the magnitudes, rotation directions, phase angles of its harmonics and various components, such as the UMFs due to armature reaction only, mutual interaction between both PM and armature reaction fields, and radial and tangential force waves. The cancellation and additive effects between the UMF components resulting from the radial and tangential force waves are revealed for the first time by the analytical model. We show that such effects strongly depend on the UMF harmonic order, slot/pole number combinations, and the internal/external rotor machine topologies. We have validated the analytical model by finite-element analyses and partially by experimental results.
IEEE Transactions on Magnetics 08/2010; DOI:10.1109/TMAG.2010.2044417 · 1.39 Impact Factor
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