Compact Air-Core Inductors for Variable Frequency Soft-Switching in 3 Phase Inverters

An air-core inductor is designed and experimentally tested for use in a 3-phase 47kW grid-tied soft-switching inverter. The combination of switching frequencies in the hundreds of kiloHertz and peak currents over 100A, necessary for soft-switching over the entire grid cycle, causes significant iron loss in cored inductor designs. The novel use of air-core inductors in this application, with power levels of 8kW per inductor, eliminates these losses, improving overall converter efficiency. A comparison in design and performance with a ferrite-cored inductor is made and the air-core is shown to be 0.5% more efficient at rated power, with higher savings at partial power. The design reduces weight while increasing volume to maintain inductance. Radiated electromagnetic interference is analyzed in a simulation and is shown to be benign to the circuit’s operation in practice.