Giacomo Bacco

Geometry Analysis and Optimization of PM-Assisted Reluctance Motors

In this paper, a detailed geometry analysis of the rotor structure is presented, for both synchronous reluctance and PM-assisted reluctance motor, in order to suggest an automatic procedure to draw the rotor structure. The shape of flux barriers is selected to achieve both high d-axis inductance and low q-axis inductance, in order to obtain high output torque. Methods to properly design the geometry of the end of each barrier and PMs are adopted. In order to draw a rotor with proper shape, different modifications are discussed. After that, such a drawing procedure is used to rapidly analyze the impact of some geometry changes on the machine performance. The analyzing process starts from a reluctance motor, considering the number of barriers, insulation ratio, split ratio and slots per pole per phase. Then, the PMs are inset into flux barriers and the effect of PM width on torque, power factor and flux weakening capability is investigated. At last, the demagnetization limit under overload operations is analyzed.

Analytical approach to determine the power limit of high-speed synchronous reluctance machines

The design of synchronous reluctance machines for high-speed applications is an issue that nowadays remains almost not dealt with. The thickness of the iron ribs that mechanically sustain the structure increases with the rotor speed, so that a high magnetic flux flows through them. Therefore, the available torque decreases with the rated speed required by the application. The aim of this paper is to tackle the design of synchronous reluctance machines for high-speed applications. The potentials of a high-speed synchronous reluctance machine are investigated. For a given operating speed, the mechanical structure is firstly designed, so as to achieve a robust rotor. Then the maximum torque is investigated according to the thermal limits. A tradeoff between increasing speed and decreasing torque is obtained, up to obtain an optimum value of the electromagnetic power. An advantage of the proposed procedure is its analytical nature: all the design steps are based on analytical relationships, so as to allow a rapid extrapolation of the obtained results to different machine size, operating requirements, and so forth. Finite element analysis is used only to validate the final results.

High speed motors: A comparison between synchronous PM and reluctance machines

In high speed systems, the recent trend is to couple the electric machine directly to the mechanical shaft, in order to avoid the use of a gear-box and to increase the overall efficiency and reliability. Surface Permanent Magnet mounted (SPM) machines are usually adopted in such systems for their high torque density thanks to rare-earth PMs. The cost of rare-earth PMs together with the risk of their demagnetization induces to consider other alternatives such as the Synchronous Reluctance (REL) machine. The design of REL machines for high speed applications has to be carried out considering both magnetic and mechanical aspects. In particular, the iron ribs must be accurately designed with the aim of achieving the minimum flux leakage in the ribs and ensuring the structural integrity. The present paper deals with the design and comparison of an SPM and a REL motors with fixed stator geometry and winding. Such an approach allows to investigate the potential of the synchronous reluctance machine in comparison to the SPM one. It is shown that the synchronous reluctance machine represents an interesting alternative to SPM machines for its low cost and insignificant susceptibility to demagnetization. In fact the lower torque density achieved by the reluctance motor can be compensated with a limited increase of the stack length or with a higher electric loading.

Nonlinear Analytical Computation of the Magnetic Field in Reluctance Synchronous Machines

This paper deals with an analytical model of reluctance synchronous motor, able to consider both stator slotting effect and magnetic saturation in the stator and rotor iron paths. It joints the field distribution at the air gap to a lumped-parameter magnetic network. Local saturation factors are defined so as to adjust the flux-density distribution. The analytical approach exhibits great potentiality, since it is a formulation based on an harmonic study, and it is easily adaptable to any motor geometries. As an example, 36-slot 4-pole machine is analyzed, considering three flux barriers per pole. Finite element analysis confirms the results achieved by means of the analytical model.