Design, analysis and control of permanent-magnet vernier machines

Abstract

Electric machines are indispensable part in modern industry and they are widely used in many applications such as power generation, vehicles, elevator, robot, and so on. Owing to the diversity of the application occasions, different requirements have been proposed. In the applications, such as wind power generation, low-speed, high-torque and high-efficiency electric machines are required to match with the low-speed load. Currently, this problem is handled by either adopting a speed-boost mechanical gear, or using low-speed machine design. The former one causes mechanical wear and tear, audible noise and low efficiency, whereas the latter one increases the generator size and weight as well as raw material cost.

In recent years, another option, namely integrating a coaxial magnetic gear into a permanent-magnet (PM) machine has been proposed. This option allows for directly mounting the outer, low-speed shaft with the load while enables the electric machine coupled with the high-speed inner gear to operate at high speeds. However, this magnetic-geared electric machine desires a complex structure, involving two rotating bodies and three air-gaps, which increases manufacture difficulty and cost. The research work of this thesis attempts to solve the speed-matching problem by developing a new electric machine.

The incorporation of vernier concept with PM machine gives birth to the permanent-magnet verier (PMV) machine. Different from traditional synchronous machine, the rotor of the PMV machine rotates at a definite fraction of the synchronous speed, as if it were geared down from the high rotating field set up by the stator. The PMV machine, therefore, can be regarded as a combination of a gear with fixed gear ratio and an electric machine. This kind of machine is attractive in applications which require low speed and high torque, and mechanical gearing is undesirable.

The main objective of this thesis is to present the design, analysis and control of the proposed PMV machine. After the introduction on mechanical gears, magnetic gears, and low-speed machines, the design details of the proposed outer-rotor PMV machine are provided. Moreover, the working principle, stable torque generation mechanism and structural features are presented.

Due to the introduced vernier effect, it is important to perform the finite element analysis (FEA) for the proposed PMV machine. The electric circuit equation and the motion equation are coupled with the Maxwell’s equation to calculate the key parameters of the proposed machine. The analysis results of the magnetic field distributions, air-gap flux density distributions, flux linkages, winding inductances, back electromotive forces (EMFs), cogging torque and static torque are presented in detail. The performances of PMV machine are compared with other machines to show its advantages and disadvantages.

The control strategy of the PMV machine as a brushless DC machine is also presented. The conventional 120-degree conducting, dual-closed-loop control strategy is used for speed and torque control and the experimental setup are given. The results are obtained and compared with the simulation results, thus verify the validity of the design. Finally, the potential applications for the proposed PMV machine are suggested.