Stability Analysis and Control of a Rotary Transformer-Based Synchronous Motor Field Excitation System

This paper analyses the stability and control of a rotary transformer-based field excitation system. Rotary transformers enable wireless and contactless power transfer to rotating components of a system such as the rotor of a wound field synchronous motor. Their use in the field excitation of synchronous motor for electric vehicle and similar applications overcomes the disadvantages associated with the traditional slip rings-brushes arrangement such as frequent maintenance requirement, significant friction losses at high speed, contact cooling issues, increased overall motor weight and size and reduced reliability and life. The rotary transformer studied in this paper is series-series compensated with the transmitter coil supplied from a single-phase inverter, and the receiver coil connected to a full bridge diode rectifier that feeds the rotor field winding. Harmonic balance technique is used to obtain a complex variable dynamic model of the system. Using this model and the concept of the bimatrix, the controllability and observability of the rotary transformer is analyzed for different output variables. The controllability and observability matrices are not of full rank indicating the existence of states which are unaffected by the control input. The concept of zero dynamics of the system is used to determine these states and the condition for the stability of the zero dynamics is determined.