Design and Electromagnetic Feature Analysis of AC Rotating Machines

The chapter is aimed at the basis of the design of rotating AC machines with emphasis on their air gap magnetomotive force (MMF). The formulation of the MMF is firstly treated using the Amperes’ theorem and the conservation law. Then, the harmonic content of its spatial repartition is investigated. In a second part, different case studies are considered including single and three phase concentrated windings. The third part is devoted to the distributed windings with the superposition approach-based formulation of their MMFs. A special attention is paid to the investigation of the capabilities of the distributed windings in reducing the harmonic content of the MMF.

The principle of operation of AC machines lies on the rotating field concept. Basically, it consists in a flux density moving in the cylindrical trajectory of the air gap of AC rotating machines. This chapter deals with the formulation of the rotating fields that could be generated considering different techniques, such as: (i) moving a single phase winding linked to the rotor and fed by a DC current (case of the field of the synchronous machines), and (ii) feeding a poly-phase winding by poly-phase sinusoidal currents. Then, the effects of the rotating field on the winding located in the vicinity, on the other side of the air gap, are analyzed. These effects are initiated by the induction of back-EMFs that could lead to the circulation of poly-phase currents in the involved winding. Consequently, a second rotating field is generated which is synchronized with the initial one, resulting in the production of an electromagnetic torque.

The arrangement of either distributed or concentrated windings in slots around the air gap of AC machines represents a crucial design issue, in so far as it deeply affects their features. This statement is much more critical in fractional-slot concentrated winding permanent magnet synchronous machines, also named fractional-slot permanent magnet machines (FSPMMs). These are characterized by fractional slot per pole and per phase lower than unity, resulting in dense harmonic contents of the armature MMF with a number of pole pairs lower than the field one. This chapter is aimed at the arrangement of the armature winding of FSPMMs, which is achieved considering the star of slots approach. Following the winding arrangement, the star of slots is used for the investigation of the back-EMF harmonic content as well as for the determination of the winding factors of the fundamental and harmonic backs-EMFs. Finally a case study is treated with emphasis on the MMF spatial repartition and harmonic content.