can be significantly increased due to eddy-current effect. There are two different ... of fault positions along the coil as illustrated in fig2.b, are also given in fig 3.
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A Winding Topology in Permanent Magnet Machine – A compromise Between Losses and Fault-tolerant Capability Puvan Arumugam, Tahar Hamiti and Chris Gerada Abstract—This paper looks at a systematic investigation on split ratio to minimize the AC losses of vertically placed rectangular conductors in concentrated-wound surface mounted PM synchronous machines having fault tolerant (FT) capability. The repeated numerical design optimization is carried out in Finite Element (FE) to achieve the required torque and FT capabilities for minimum level of electromagnetic losses. In addition, the influence of the tooth width to slot pitch ratio, slotting effects and tooth height are investigated. The proposed winding design is verified with FE analysis and validated experimentally. It will be shown that the overall losses can be minimised by adequate design. Index Terms-- AC losses, Fault tolerant, Permanent magnet, Proximity, Split ratio, Tooth-width ratio.
I. INTRODUCTION
P
ermanent magnet (PM) machines are increasingly being used in safety critical applications, where the necessary system reliability level can be reached by designing for fault tolerance. However, there are no perfect design solutions for an inter-turn short-circuit (SC) fault which can be critical due to the resulting fault currents which can be excessive even though remedial action is taken [1]. This current can be restrained by employing copper strip vertically placed in the slot instead of adopting a traditional stranded configuration [1]. Even if the vertically placed windings can be a solution for the SC faults, generated losses in the windings are significantly higher than the stranded ones [1]. Hence, necessary measures are needed to produce a design which captures the fault-tolerant requirements with minimum losses. Generally the winding copper losses in electrical machines can be significantly increased due to eddy-current effect. There are two different phenomena that contribute the total eddy-current effect: skin effect and proximity effect which is also named as double sided skin effect in previous literature [3]. The skin effect is the tendency for very high frequency currents to flow on the surface of the conductor and the proximity effect is the tendency for current to flow in other undesirable patterns that form a localized current distribution in the slot due to the presence of an external field
which can be produced by either the magnets or adjacent surrounding conductors. The effects are dependent on the physical structure of winding, conductor’s length, slot dimensions and operating frequency. These can result in significant amount of AC copper loss in addition to the DC loss. Naturally the overall losses and their distribution influence the efficiency and power density of the machine. Litz conductor can be used to minimize the eddy current effect which results in a reduced fill factor and thus DC loss will be increased. The DC loss can be reduced by adopting a higher copper fill factor in the slot by using single strand coils. However, this generally increases eddy-current loss due to the increased conductor area. In this paper an effort will be made to optimize the design of the machine in terms of the split ratio, ie. the ratio of rotor outer diameter to stator outer diameter (Dr /Do). It will be seen that this ratio significantly influences the power density and efficiency of the machine.
Fig. 1. Perspective view of the vertical winding
Previous works [4 - 6] have investigated the optimum split ratio, accounting for DC copper loss and the effect of stranded winding design. In this paper, AC loss is accounted for and the optimal split ratio is systematically investigated to minimize the overall losses of vertically placed rectangular conductors (as shown in fig.1) in concentrated-wound surface mounted PM synchronous machine having fault tolerant
2 capability. The repeated numerical design optimization is carried out with FE to achieve the required torque for minimum level of losses. In addition, slotting effects are also considered. II. VERTICAL WINDING CONCEPT In the conventional fault tolerant PM machine, winding short-circuit fault remains critical because of the resulting excessive current which can cause further failure. As a remedial action, this current can be partially controlled by shorting the whole phase through the power converter switches [7]. As a result, the current in the shorted turns reduces because all the turns in the faulty phase share the induced electromotive force (emf). However, this action is ineffective for a single turn short-circuit fault due to its low impedance and, as a consequence, a significantly high fault current compared to the rated current results [3]. Additionally, the level of short-circuit current has a strong dependency on the position of the shorted turn within the slot and the slot geometry itself [1]. As a solution to the large short-circuit current and conductor position dependency, the vertical winding concept is introduced. This limits the shortcircuit current intrinsically to its rated value without any dependency on the position of the shorted turn [1].
(a)
(b)
Fig. 2. Representation of the short-circuit fault location of : (a) stranded winding, (b) vertical winding
For the sake of clarity, the results of FE-computed interturn short-circuit currents in a stranded winding after the application of a remedial control strategy are shown in fig. 3. These results show the current values when the fault is located at nine different positions (as shown in fig.2.a) within the slot of a 12-slot 14-pole per-unit inductance permanent magnet synchronous machine (FT-PMSM) machine which parameters are given in Table I. The turn-turn short circuit current of a corresponding vertical wound coil for a number of fault positions along the coil as illustrated in fig2.b, are also given in fig 3.
Fig. 3. Short-circuit current vs. shorted turn position. Comparison between vertically placed and stranded windings
