Mid Sweden University

The aim of this thesis is to design a kilowatt three-phase step-down rotary transformer for

a permanent magnet DC motor. The permanent magnet DC motor has an on-rotor drive

system, and therefore requiring a power supply that can transfer power to its drive unit

without mechanical contact. The rotary transformer has a detached magnetic coupling

structure that qualifies it as a potential method for the wireless power transfer.

This thesis studies the rotary transformer as a static device, focusing on its core loss.

By using a transient finite element analysis of COMSOL Multiphysics and an iron loss

prediction model, the rotary transformer was optimized in terms of efficiency and power

density for the on-rotor drive system through proper material selection and geometry exploration.

After this, a mechanical design, which based on a literature review of the

influences of manufacturing processes on electrical steels, was proposed for realizing the

core fabrication and the rotary transformer assembly.

The results show that the rotary transformer can step down 400 V/50 Hz three-phase

voltage to 13.15V in a Delta-wye connection and output 1.17kW power over an air-gap

of 0.3mm with 95.94% overall efficiency. The proposed mechanical design enables the

transformer to minimize the core loss and the manufacturing cost. Without using resonant

inductive coupling, this transformer design simplifies the power supply for the motor,

thereby decreasing the motor manufacturing and maintenance cost.