Reversible magnetically geared machine

Abstract

Magnetic gears have gained increasing interest over the past few years for replacing mechanical gears in different applications. With no contacting parts, the magnetic gear eliminates typical problems with friction, lubrication, noise and vibration.

This thesis investigates the feasibility of implementing the magnetic gear technology in a system for operating a low speed high torque active heave compensation winch. Most electrical systems for such applications today uses batteries or super-capacitors for storing of heave cycle energy, and it is interesting to investigate if this can be replaced by a mechanical energy storage, possibly reducing the costs. The aim has been to find a configuration that implements the magnetic gear and at the same time utilizes the potential of a mechanical energy storage in the form of a flywheel.

The proposed system integrates the magnetic gear with an electrical machine into a compact configuration known as Pseudo direct drive. The winch and flywheel are connected to separate shafts of the machine, having the stator control the flow of energy between them. An active heave compensation winch is required to rotate both in forward and backward direction, and as a flywheel is allowed to rotate only in one direction, it is found that a variable gear ratio is required.

With the winch required to hold the load, its torque should be fairly constant and in one direction. Analysis of the magnetic gear reveals that in steady state conditions, the torques on all shafts should add to zero. By having a constant direction of torque on the winch, the direction of torque on the shaft of the flywheel was then also found to be constant. This complicates the wanted operation of the system, as the flywheel must be able to vary its torque for energy to be supplied or extracted.

Finite element analysis is carried out by the software COMSOL Multiphysics, and is used to investigate the working principles of the system. Results of the simulations confirms that the direction of torques of both the winch and flywheel are constant, and therefore complicates the wanted flow of energy in the system.