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Powder Metallurgy has been used for many years as a cost effective method of producing high volumes of consistent magnetic components for DC and hard magnetic applications. The method allows high material utilization, precise material control and the ability to produce relatively complex shapes. Recent advances in materials research have produced a soft magnetic composite material which offers AC performance approaching that of steel laminations for a similar material cost [123,124]. Therefore the manufacturing advantages of powdered metallurgy may be exploited in the manufacture of electrical machines [125-128]. There have been a number of occasions where powdered iron has been considered as a direct replacement for lamination steels. In virtually every case, the machine has been shown to give poorer performance because the powdered iron has a lower unsaturated permeability, lower saturation flux density, and increased iron loss. Due to the segmentation of the stator core, it is possible to use pre-made coils in the stator. The coils are made up of rectangular copper conductors manufactured under tension. The result is a high slot fill factor of 0.78.

Due to the relatively large cross-section area of the conductors, the eddy-currents induced at increasing or fixed frequency are not negligible. So, one of the key advantages of FSCW is the ability to achieve significantly higher copper slot fill factor (compared to conventional laminated stator structures) if coupled with segmented stator structures particularly if the windings are prepressed.

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This can have a significant impact on the machine power density. The copper slot fill factor is defined as (3.1).

(3.1)

where is the total copper area and is the total slot area. Several methods have been proposed to achieve this goal. This section will cover the key papers addressing various concepts of segmenting the stator structure to significantly increase the slot fill factor and reduce manufacturing cost.

The windings were made with the aim of maximizing the copper cross- sectional area within a slot, in order to minimize the per-unit resistance of the machine. The turns were machine wound onto a former coated with epoxy in a wet layup process. They were then pressed in a die and cured under load. The finished coils were then tape wrapped to form a layer of ground wall insulation. Polymeric wire insulation was found to survive compaction under high pressure up to 800 MPa.To optimize the performance of the powdered iron machine the design may be radically altered as follows [129].

1. The number of teeth was reduced to three teeth per pole pair. This improved the tooth aspect ratio for pressing and aided the winding process. 2. Instead of a fully pitched winding, the windings enclosed a single tooth (120° electrical). Thus, they formed a bobbin shape which was simple to wind and easy to assemble into the stator.

3. The powdered iron stator core was split to form tooth and core back sections, each of which could easily be pressed. The two sections allowed a preformed coil to be placed over the tooth, before the tooth was slotted into its core back section, as shown in Fig. 3.2. The teeth segments were then bonded together and shrunk into a standard aluminum frame as show in Fig 3.3 (a).

4. The axial ends of the stator teeth were rounded to maintain close thermal contact with the winding, while also maximizing the stator tooth area. The smooth tooth profile removed the need for a thick slot liner.

5. The core back was axially extended over the end winding, thereby allowing a shallower core back and larger slot area.

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6. The coil was performed and then pressed to 450 MPa to give a copper fill factor (ratio of copper area to total slot area) of 78% as shown in Fig. 3.3 (b) and Fig. 3.4.

7. The rotor or stator should be skewed by one half slot pitch to reduce cogging torque.

(a) (b)

Figure 3.2 : Stator components; (a) Manufactured core components and coil, (b) Tooth assembly [129].

(a) (b)

Figure 3.3 : Complete stator and coil sections; (a) Assembled stator before insertion into core, (b) pressing trial results-coil sections [129]. A copper fill factor of around 64% was achieved by machine winding alone. The copper fill factor increases with applied pressure, reaching a practical maximum value of approximately 81% at around 400 MPa in practice, above which point there is little to be gained by further increasing pressure. Fig. 3.3 (b) shows sections

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through coils pressed to 200, 400, 600, and 800 MPa. Clearly, the turns are deformed from circular toward hexagonal cross section at around 400 MPa. Above this pressure (600, 800 MPa), the sections appear almost fully dense, supporting the graphical results of Fig. 3.4.

Figure 3.4 : Pressing trial results-copper fill factor variation with pressing pressure (dotted line is theoretical maximum) [129].

Figure 3.5 : Joint-lapped core machine; (a) Cross section of a joint-lapped core machine (75% fill factor reported). (b) Joint-lapped core after winding. Similar values of slot fill factor values have been reported in case of segmented laminated stator structures using plug-intooth technique [84]. Such configuration is

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shown in Fig. 3.5 [84]. In [120], more recently, Akita et. al. reported a 75% slot fill factor using a “joint-lapped core”. There have been several publications addressing the use of FSCW in conjunction with segmented stator structures for various types of PM machines. It was shown that using rectangular wires instead of conventional round wires reduces the end coil region by 15% and a higher slot fill factor can be achieved. It was shown that using an improved stator tooth configuration where the air gap is larger at the high stress points helps reducing the vibrations and noise in the machine [130]. It was shown that by appropriately adjusting the pole-arc to pole- pitch ratio, the optimum ratio for cogging torque minimization that was derived for SPM machines is equally applicable in the case of IPM machines. It was also shown that the cogging torque in case of the non-overlapping concentrated windings is almost half that in the case of full-pitch overlapping windings [131].