Design and analysis of the modular variable flux reluctance linear machine

Abstract

Considering the fluctuating price of permanent magnet, variable flux reluctance linear machine (VF-RLM) with simple structure, good robustness, and high dynamic performance, is one of the potential candidates for linear direct-drive application replacing PM-excited linear machine. To ameliorate the fault-tolerance performance of VF-RLM, modular or segmented design has been come up with, denoted as modular variable flux reluctance linear machines (MVF­-RLM). However, suffering from large thrust ripple, the design of MVF-RLM still needs to be improved. Meanwhile, its low thrust force density due to poor excitation ability of DC windings, has been a long-existing bottleneck. To relieve the force ripple of MVF-RLM, a novel H-shaped modular variable-flux linear reluctance machine (HMVF-LRM) is proposed in this thesis. The principle of force ripple suppression by using parallel-complementary structure is analyzed deeply. In this thesis, the analytical calculation of induced voltage is developed quantitively with equivalent magnetic circuit method (EMC) and harmonics analysis. Further, the performance and fault-tolerant capability of the proposed machine are simulated and evaluated, and the prototype is tested. To further suppress the odd-order harmonics in the induced voltage and detent force of HMVF-LRM, a fractional pole- pair unequal module arrangement (FP-UMA) design, in which the distances of adjacent modularized mover segments are not equal, is proposed to VF-RLM in this thesis. The key is that the modularized movers are artificially designed to be unequally distributed regarding to spatial distribution to eliminate the odd-order harmonics in the induced voltage along with the thrust ripples they caused based on the quantitative analysis on the thrust ripple components. It is revealed that, with the proposed FP-UMA design, the thrust ripple ratio of the machine has been effectively relieved from 4.6% to 2.2% under copper loss of 450W. Aiming to boost the output thrust force of the VF-RLM, a novel high-order harmonic non-overlapped toroidal winding design utilizing magnetic gear effect is proposed for double sided variable flux Vernier reluctance linear machine (VF­-VRLM). The key is that the proposed armature winding makes full use of working harmonics modulated from both fundamental order and third-order harmonics generated by DC excitations, contributing to enhanced winding factor. Based on the FEA, with the proposed winding design approach, DS-DC-VRLM could achieve 2.26 times higher thrust force than those with conventional concentrated winding under the same copper loss. To further enhance the fault tolerance of VF-VRLM, an H-shaped double-sided modular variable flux Vernier reluctance linear machine (H-DS-MVF­-VRLM) with complementary structure is proposed. The novelty of MVF-VRLM is that an enhanced modulation effect utilizing flux densities modulated from the third-order harmonics in the air gaps and a toroidal armature winding is designed to make full use of them to boost the output thrust density of the machine. Meanwhile, the complementary structure is artfully constructed to provide a magnetic path for working harmonics. Based on the finite element analysis, the proposed H-DS-MVF-VRLM with novel winding configuration could improve 21.78% higher thrust density, under the fixed copper loss comparing with the MVF-RLM with concentrated winding.