Self-excitation in inverter driven induction machines

If the dc supply to a voltage source inverter driving an induction machine is removed, self-excitation as a result of circulating currents in the inverter can occur whenever the inverter switching frequency is below the machine speed. In this mode the inverter-machine system functions as a stable self-excited induction generator capable of supplying dc or ac power to a load. The electrical output of the system is readily governed by slip control; thus the inverter makes practical the development of efficient induction generators capable of operating over wide speed ranges. A theoretical treatment based on a first harmonic approximation of inverter performance is presented. The results indicate that the magnetization characteristic and the stator and rotor resistances of the machine are the important parameters controlling system performance. Experimental results which confirm the validity of the analysis and illustrate the behavior of the self-excited systems are also presented.