Rotary Transformer for Contactless Excitation of Synchronous Machines Fed Through Neutral Conductor

This paper presents a brushless excitation system for inverter operated electrical excited synchronous machines without the necessity of an additional inverter for the excitation system. In order to fulfill the requirement of a brushless system, a rotary transformer transfers the power to the rotor field winding inductively. A rectifier connected to the rotating secondary winding of the transformer provides a DC-voltage to the rotor winding. The neutral conductor runs from the star point of the three stator phases trough the stationary primary winding of the rotating transformer and back to the center point between the intermediate circuit capacitors. A common mode voltage (CMV) with fixed frequency and variable amplitude is added to the 3-phase modulation for the inverter working with pulse width modulation (PWM). Since the star point is connected, a current driven by the CMV will flow in the neutral conductor. Through the amplitude of the CMV the voltage supplying the primary winding of the transformer can be regulated, which has a direct impact on the DC-voltage of the field winding i.e. the excitation.