Design of Novel Axial-Flux Dual Stator Doubly Fed Reluctance Machine

This paper proposes a novel machine termed the axial-flux brushless doubly fed reluctance machine (AF-BDFRM) suitable for high-torque, low-speed direct drive applications. The main potential advantages of AF-BDFRM are its higher torque and power density compared with the radial-flux BDFRM. In addition, the proposed machine utilizes a bidirectional power converter of much lower rating as compared with the machine rating depending on the range of speed variation. The proposed machine axial-flux topology is composed of two stators and a reluctance iron rotor. The upper stator winding which is connected directly to the power grid, transfers active power to the grid and lower stator winding creates excitation for the machine and connects to the grid through a partially rated converter. The design principles of the proposed AF-BDFRM are studied in this paper. Furthermore, the transient 3-D finite-element analysis result of the improved design is presented.

I. Introduction

Brushless doubly fed reluctance machines (BDFRMs) are a class of machines that can be controlled using a power converter that has a lower rating compared with the total power rating of the machine. These machines are an attractive solution for applications where speed control over a limited operating range is required. A bidirectional power converter rated 20% of the machine rating can provide a speed control range of 40% with constant average torque output. One of the windings, connected directly to the power grid is called power winding, while the other winding is connected through a variable-speed drive to control and sustain a constant frequency and voltage at the power winding, is called the control winding. The schematic of the operation of doubly fed machines is shown in Fig. 1. Quantities and are the number of pole pairs of the power and control winding, respectively. Typically, the rating of the power converter is significantly smaller than the total power rating of the machine, which makes its use more economical as compared with the conventional synchronous generators employed in wind energy applications [1], [2]. Doubly fed induction generators (DFIGs) employed in wind generating systems maybe the most well-known application of the doubly fed machines. Although, a clear drawback to the use of DFIGs compared with the BDFRM is the need for slip rings to supply power to the rotor circuit, which results in increased maintenance cost. In contrast, the BDFRM, rotor is winding free and made only of iron, which makes it more robust and easier to manufacture. Fig. 1.

Schematic of BDFM.