Adjustable flux three phase a.c. machines with combined multiple-step star-delta winding connections

In the paper it is developed a complete theory and analysis method of combined star-delta three-phase windings from the point of view of MMF spatial harmonics content and equivalent winding factors calculation. The method allows for any space angle and coils percentage distribution between the star and delta balanced winding systems. It is proven that connecting the inner delta in the CW or CCW direction leads to two different space angle between the star and delta systems and two different steps in the flux level control. It means that, compared to what is known and applied up to now, using n winding sections in each phase, it is possible to obtain 2n steps (instead of n+1) in the flux level. To check the analysis validity, two experimental tests are reported on both squirrel cage integer winding induction motor and PM low speed fractional synchronous generator with a specially designed stator winding, allowing multiple-step air-gap flux regulation.