This invention relates to stator windings for miniature brushless dc motors in general and more particularly to an improved stator winding of the type in which several component windings are designed as injection molded frame windings.
Stator windings for a miniature brushless dc motor comprising a plurality of component windings designed as frame windings injection molded in plastic and nested together to form a cylinder surrounding the permanent magnet rotor of the motor and which can be inserted into a ring-shaped magnetic return yoke without slots are disclosed in German Offenlegungsschrift No. 1,613,379. In the stator winding disclosed therein the reaction moment acting on the winding is transmitted through a frictional connection between the cylinder and the magnetic return yoke which is connected to the housing. In order to achieve the necessary positive transmission between the return yoke and the cylinder formed by the frame windings, the cylinder must rest firmly against the return yoke. This requires a high dimensional accuracy in the cylinder which is to be inserted into the return yoke. However, when using parts which are injection molded in plastic such dimensional accuracy can be achieved only with great difficulty. A further problem arises because the plastic materials typically used in such applications change their dimensions as a function of climatic influences. The plastics can shrink or swell depending on the ambient temperature and air humidity. Because of these properties it is difficult to insert the cylinder formed by the frame windings into the return yoke in such a manner that perfect transmission of the reaction moment acting on the winding is assured at all times.
Thus, the need for an improved design for stator windings of this nature which will ensure reliable transmission of the reaction momemnt acting on the windings to the motor housing without placing any special requirements as to dimensional accuracy of the frame windings becomes evident.