Design and Thermal Analysis of a Rotating Transformer

A rotating transformer is a reliable, compact and brush-less method to power the field winding of a wound-rotor synchronous machine. In the interest of efficiency, safety and reliability, wound rotor synchronous machines remain the preferred generator topology for many applications. This is due to the ability to fully control and easily switch-off, the rotor magnetic field. When designing a rotating transformer for this application, the close proximity to the machine means effective cooling is paramount, along with the desire for high power density and efficiency. Accurate thermal and loss modelling is needed to develop an optimal design. However, numerous thermal properties are difficult to accurately model or predict in advance. In addition, multi-stranded litz wire, typically required when operating at high frequencies, is challenging to model. Both in terms of the AC loss and the effectiveness of heat extraction from the winding slot. To combat this, standard manufacturer cores are first used to allow a rapid and cost-effective investigation into the design. A liquid-cooled test facility allows a detailed thermal and loss analysis to be conducted on the candidate transformer prototype. Through these experimental tests and the use of thermal modelling analysis techniques, critical thermal and loss data is obtained and used to improve the performance prediction for a new, more optimised rotating transformer geometry.