Impact of Number of Phases on Electromagnetic Torque Characteristics of Transverse Flux Permanent Magnet Machines

Abstract

This paper presents an investigation on the influences of number of phases of Transverse Flux Permanent Magnet (TFPM) machines on the characteristics of developed electromagnetic torque. Electromagnetic torque is expressed in terms of the Fourier series of phase currents and internal voltages. After some algebraic and trigonometric calculations, a general equation is obtained that establishes a relationship between the electromagnetic torque ripples, number of phases and harmonic contents of both internal voltages and phase currents. This result is significant when it is required to design a few specific number of identical single-phase TFPM machines which finally they will be assembled such that to build a multi-phase machine with a minimum torque ripple. The design parameters of a case study Claw Pole TFPM machine are introduced with some details and a few FE based simulation results are given as the validations of the analytical approach of the present paper. The simulation results show clearly the impact of each harmonic of the internal voltages and phase currents on the torque ripple for various numbers of phases leading the designer to find out which number of single-phase machine combinations is the optimum one regarding the torque ripple.

A Review of Transverse Flux Machines Topologies and Design

“…C-core TFMs benefit from lower losses and higher torque at the cost of a more challenging manufacture [84,97]. • Claw-pole stator [8,16,25,87,95,98,[122][123][124] or claw-pole stator and rotor [85,86]. Compared to a radial machine and a mutual flux path TFM with the same volume, the claw-pole TFM offers higher torque densities and its weight is lower.…”

“…Ideally, there is no magnetic coupling between phases, so they are all independent of each other. The number of phases determines the harmonics content, which causes torque ripple: considering a maximum of 6 phases, it is shown that the ripple to average torque ratio of 2-and 4-phase TFMs is more than double the ratio of 3-and 6-phase motors [122]. It must be taken into account that even if only one phase is fed with current, the cogging torque on the rotor involves all phases [82]; however, the machine can continue operating with less specific torque and more torque ripple, leading to a fault-tolerant drive [114].…”

“…Figure 11 depicts the torque-speed and power-speed ratings of some TFMs found in the literature [6,8,67,69,71,72,80,81,90,93,95,104,109,113,116,122,131], both for TF motors and generators. As shown in Figure 11a,b, most of the reported TFMs have their ratings up to 100 Nm and 2 kW: in Figure 11c,d The efficiency is a key aspect of modern electric drives, and it is closely linked to the machine losses: they are formed essentially by copper losses, iron losses, PMs losses, stray load losses and even housing losses (for example, if it is made of aluminum, eddy currents can be induced) [129].…”

“…However, cogging torque is the only component that is present with no stator current (and it has zero average value, as shown in Figure 14 [93]), so it has a specific influence in the machine start up (e.g., wind turbine applications) [67]. Most of cogging reduction techniques result in lower average torque [122], this is why cogging minimization is a key aspect of TFMs design. Two main categories of torque ripple reduction techniques can be established: those in which the machine structure is changed and those that are based on control strategies.…”

A Comparative Study of PM Synchronous Generator for Micro Hydropower Plants

“…FEM is a method that is used to obtain the solutions of amounts that are continuous in a certain region whose changes in the region where they are continuous may be represented by partial differential equations. By using this method, it is possible to obtain the electromagnetic parameters of a generator with high accuracy [24,25]. The mesh structure in the generators was created automatically with the help of a computer program.…”