Comparison of Different Capacitor Tuning Criteria in Air-Cored Resonant Induction Machines

Air-cored Resonant induction machines may offer some interesting features such as reduced inertia and the possibility of operating with square-wave modulation. Their operation is very sensitive to the tuning of the capacitors: by using the equivalent circuit equations this paper analyses different criteria to tune the stator and rotor capacitors. In particular, the paper explores in detail the machine performance in the cases of both stator and rotor capacitors being tuned to resonate with the stator and rotor self-inductances, and stator capacitors only to cancel out the machine input inductance neglecting resistances, and compare these scenarios with two alternative tuning options analysed in a previous paper: both capacitors resonating with the leakage inductances, and stator capacitors tuned to compensate for the overall stator terminal reactance in the absence of rotor capacitors. Analytic expressions for the maximum efficiency and maximum torque and related slip values are also derived as functions of the machine parameters: these expressions provide a valuable tool for design optimisation.