Design optimisation of a slotless brushless permanent magnet DC motor with helically-wound laminations for underwater rim-driven thrusters

Abstract

Rim (or tip) driven thrusters with structurally integrated brushless PM motors are now an established technology with an increasing range of applications. In these thrusters, the stator of the motor is housed within the thruster duct, and the rotor forms a ring around the tips of the propeller. Such high pole number motors tend to be very thin radially, have very small axial length to diameter ratios, and have relatively large airgaps to accommodate corrosion protection layers on the surfaces of the rotor and stator. The relatively large diameter stator laminations of such machines tend, therefore, to have a very thin back of core and narrow teeth, which make them expensive and difficult to manufacture. This thesis proposes an alternative motor topology featuring a toothless stator whose laminations are manufactured from a single strip of steel that is edge wound into a spiral.
The electromagnetic design of the motor was optimised for maximum efficiency for a given propeller torque and speed. The airgap flux density in was obtained from an analytical solution of Laplace and Poisson's equations of scalar magnetic potential. Electromagnetic torque was calculated for ideal square wave current distribution. Copper and core losses were estimated in the usual manner. Design of the machine was refined using transient finite element analysis, allowing for rotation of the rotor. The design optimisation revealed that there is an optimum radial thickness for the permanent magnet and number of poles at which the efficiency is maximum. A demonstrator machine was built and tested, and yield a 10% lower efficiency when compared with an existing slotted machine of the same diameter, with an increased volume in the slotless machine of 15%. A cost analysis yielded that the slotless edge-wound laminations are cheaper to manufacture than slotted laser-cut laminations, however the costs of the increased magnet material required are higher. This project has demonstrated a potential cost savings in the manufacture of laminations, however, for this specific thruster application the costs are offset by the need for more magnet material.

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