Armature reaction effects in a microturbine set with a wound rotor synchronous generator: Experimental insights

This study presents experimental investigations of a microturbine with a wound rotor synchronous generator (WRSG) using compressed nitrogen as the working fluid. Dynamic load-step experiments revealed a novel, repeatable effect: increasing the electrical load caused a decrease in angular speed at low turbine inlet pressures, whereas at higher pressures the same load increase led to rotor acceleration. Reducing the load resistance from 66Ω to 25Ω at constant excitation current resulted in a reversal of the speed response above an inlet pressure of approximately 2.5bar, with measured speed variations reaching several hundred rad/s. To interpret this behaviour, a dynamic model of the microturbine system was developed and validated against the experimental data. The model accounts for a current-dependent reduction of effective magnetic flux, representing the demagnetising armature reaction, and reproduces the observed pressure-dependent inversion of the transient response with deviations of about 10%. Unlike previously studied microturbine sets equipped with permanent magnet synchronous generators (PMSG), the WRSG configuration exhibits qualitatively different dynamic behaviour. Although the reported pressure levels are specific to nitrogen operation, the results demonstrate that generator type can fundamentally alter microturbine dynamics, with important implications for modelling and control of Organic Rankine Cycle microturbine systems.