A Review on the Rotary Transformer for Rotor Excitation in Electric Machines

The demand for sustainable and cost-effective electrical machines has increased interest in Electrically Excited Synchronous Machines (EESM) and Induction Machines (IM) as alternatives to Permanent Magnet Synchronous Machines (PMSM) due to their lower costs and reduced reliance on rareearth materials. A key challenge in these machines is power transmission to rotating components, traditionally done via slip rings, which suffer from wear, electrical noise, and maintenance issues. Rotary transformers (RTs) offer a reliable, contactless alternative by enabling power transfer through electromagnetic induction. This paper reviews RTs as replacements for slip rings, covering their principles, design, and performance while highlighting applications in EESM. Additionally, recent advancements and future research directions are explored to provide insights into RT technology development.