Modeling and Steady-State Analysis of a Rotary Transformer-Based Field Excitation System for Wound Rotor Synchronous Machine

Wound rotor synchronous motors are gaining attention as an alternative to the permanent magnet-based synchronous motors for electric vehicle traction. Rotary transformers have been proposed for the field excitation of these motors to avoid the disadvantages of the slip rings and brushes arrangement. Conventional rotary transformers use small airgap and as a result can be limited in operating speed unless costly high precision manufacturing is used. This paper proposes a novel rotary transformer-based excitation system that overcomes these disadvantages. Modeling and analysis of the proposed system is provided. Simulation results using the developed model and PLECs shows very good agreement for fundamental frequency. Furthermore, a steady state study of the field excitation system using harmonic balance technique is carried out to show dependencies of the output controlled variables on the input variables in the presence of system parameter variations. With series resonant compensations, the open-loop control of the proposed excitation system is robust against variation of field winding resistance with temperature.