Pole Changing Vernier Machines for Variable Speed Applications

Abstract

The fluctuating prices of the fuel, the limited amount of the natural resources, and the global warming issues has led to massive energy efficiency and sustainability research. The electrification of vehicles is a serious issue in this regard because a large volume of oil is consumed by vehicles. Hence, the electric vehicle (EV) industry is expanding. The electric machines for EV application are required to operate on wide speed range and have high power density. Additionally, they must have high efficiency during wide speed range operation, high torque at low speeds for driving in urban areas and high power at high speeds for cruising.

This thesis proposes pole-changing vernier machines for electric vehicles application. The proposed machine operates in a vernier mode at low speeds and a synchronous mode at high speeds to utilize the merits of both a vernier machine and a synchronous-machine in a single topology. Initially, PM pole changing vernier machine is proposed with two kinds of PMs: coercive force (LCF) i.e. Alnico and high coercive force (HCF) i.e. (NdFeB). The magnetization direction of the LCF PMs is changed to change the poles of the machine. However, the proposed machine suffers from complex control and high cost.

In order to solve the issues of a PM pole changing vernier machine, a wound field pole changing vernier machine is further proposed. The stator has 3-phase distributed armature winding whereas the rotor is wound with two sets of field windings. The direction of current in one set of field winding is changed which changes the poles of the machine. Hence, the machine works in two modes that is vernier mode and wound field synchronous machine mode. However, this machine suffers from high torque ripple. Therefore, the effect of skew is analyzed in the proposed machine. It is seen that skew decreases the torque ripple of the machine; however, it also decreases the torque of the machine specially in vernier mode. Therefore, there is a compromise between high torque ripple and high torque using skew.

Further, a wound field pole changing vernier machine is proposed using fractional slot concentrated armature winding. Using this structure, the poles of the machine can be turned to half and hence, it can also work in two modes that is vernier mode and a wound field synchronous machine mode. The torque ripple of the machine is low, and the maximum speed of the machine is increased. The machine is analyzed using finite element method initially. Then, the prototype of the machine is manufactured, and experiment is performed. During experiment, the back EMF of the machine in two modes at base speed of 300 rpm is measured. Further load experiments are performed, and the torque of the machine is measured in two modes. Additionally, the machine is also switched from one mode to another to measure the transients during mode switching. Further, wide speed range experiment is also performed. All the FEA results and the experiment results show good agreement. Hence, the proposed pole changing vernier machines are a promising candidate for EV application.