ON THE ISSUES OF STARTING AND COGGING FORCE REDUCTION OF A TUBULAR PERMANENT MAGNET LINEAR GENERATOR

Abstract In this paper methods to start a dual piston linear generator are proposed. The key issue of starting a linear generator is finding efficient methods to provide initial force to move the translator that is strong enough to overcome opposing forces like cogging, friction, and compression in the combustion chamber. The cogging force is reduced using techniques listed in the paper. The finite element method (FEM) is used to simulate the force calculation, energy equation and cogging force reduction. The results of the calculation of cogging force and the electromotive force at the stator coils terminals are subsequently verified by the experimental data collected from testing a real linear generator. 1. INTRODUCTION The linear machines are being employed increasingly in applications ranging from transportations, manufacturing, and office automation to material processing and generation systems [1]. For linear generators that are powered by internal combustion engine, the dual piston configuration has been suggested to be superior as it is more suitable for high speed applications and avoids complexity on control [2]. In order to initiate combustion, force must be applied to move the translator to compress fuel in the combustion chamber and ignite it. The existence of the cogging force poses a problem for starting as it opposes the movement of the generator translator wherever it travels from one side to the other. This cogging force results from the interaction between the iron-structure stator teeth and the permanent magnets mounted on the translator part. This paper will mainly focus on the aspect of delivering enough initial force to move the translator from standstill to top dead center of a combustion engine cylinder. The energy is delivered by the interaction of the magnetic field of the permanent magnets attached to the translator and the field produced by the coils attached to the stator as suitable current is injected into them. Two methods are used with varying currents to produce enough propelling force to overcome the opposing force mainly from cogging and compression. Hence, the cogging force is reduced to the minimum. Methods to reduce cogging force are explained briefly. The stroke length, outer stator diameters are constrains in this tubular generator design.