Rotary transformer with electrical steel core for brushless excitation of synchronous machines

Salient pole synchronous machines (SPSYM) feature a high efficiency over a wide speed range and a high power density. Furthermore, the adjustable excitation as an additional degree of freedom can be used to optimize the overall efficiency and to control the temperature distribution between rotor and stator. Therefore, the SPSYM is suitable for drive trains of electric vehicles. For the excitation of the rotor with a DC current, a power transmission system is needed. The disadvantages of conventional slip rings such as the limited lifetime and noise emissions increase the interest in the development of brushless transmission systems. This paper presents a novel rotary transformer design with a core made of electrical steel. The innovative design offers the possibility to arrange the electrical steel along the transformer's circumference in order to axially direct the magnetic flux. Furthermore it offers the opportunity to place the transformer inside the machine's housing instead of placing it in a seperate housing. That leads to a more compact machine design. Considering the transient effects, the leakage inductances, iron losses, and copper losses, the electromagnetic design of the rotary transformer is discussed. Furthermore, analytical calculations are validated with measurements on a prototype.