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The stator

  • The stator frame consist of a symmetrical and substantial casting, having feet cast integral with it. The stator core, consisting of high grade, low loss electrical sheet-steel stampings, is assembled in the frame under hydraulic pressure. The thickness of stampings/laminations is usually from 0.35 to 0.6 mm. The stator laminations are punched in one piece for small induction motor (Fig. 1). In induction machines of large size the stator core is assembled from a large number of segmental laminations.

The slots are sometimes of the ‘open type’ (i.e., having parallel walls) for the accommodation of former wound coils. But the usual practice is to have practically ‘enclosed slots’ in order to reduce the effective length of air-gap.

  • The stator windings are given the utmost care to make them mechanically and electrically sound, so as to ensure long life and high efficiency. After the winding is in position it thoroughly dried out whilst still hot and is completely immersed in a high grade synthetic resin varnish. It is then acid, alkali, moisture and oil proof.

For small motors working at ordinary voltages, single layer, mush winding is used. For medium size machines double layer lap winding with diamond shaped coils are used. Single layer concentric windings are used for large motors working at high voltages.

Stator stamping

Fig. 1 Stator stamping

  • Frames of electrical machines house the stator core. Frames of small and medium sizes of induction motors have hollow cylindrical form and that of larger motors have the shape of a circular box. In small induction motors, having a frame diameter of up to about 150 cm, the frame also supports the end shields. The frame should be strong and rigid as rigidity is very important in the case of induction motors of large dimensions. This is because the length of the air gap is very small and if the frame is not rigid, it would create an irregular air gap around the machine resulting in production of unbalanced magnetic pull. Frames for small machines are made as a single unit and are usually cast. The frames of medium and large sized machines are fabricated from rolled steel plates.