Analytical Model of a High-Frequency Rotary Transformer

The permanent magnet synchronous motors are widely used for the propulsion of electric vehicles. These motors have good performance, but result expensive because the permanent magnets installed on their rotor contain rare earths. A promising solution to reduce the cost of the motors is to replace the magnets with an excitation coil, which requires to transfer its own supply power to the rotor of the motor. The power transfer is performed by means of a rotary transformer. This paper develops a simplified steady state model of the rotary transformer which allows to asses with good accuracy the position of some points in the waveforms of the currents in the primary and secondary coils of the transformer and in the excitation coil. The inputs of the model are the inductive parameters of the transformer itself and the electrical parameters of the power switches that compose the inverter that supplies the primary coil and of the diodes that form the rectifier connected to the secondary coil. The accuracy of the analytical expressions that give the position of the points on the current waveforms is verified by comparison with the results obtained from simulations and with the current waveforms acquired from a prototypal transformer available in the lab.