Slip-energy recovery techniques for control of induction machines.

This thesis describes two different techniques for efficient control of slip energy in a slip-ring induction machine. The static Kramer system merely recovers slip power and returns it to the a.c. supply. As a result only sub-synchronous motoring or super-synchronous generating is possible. In the static Scherbius system, however, the slip power can be controlled both into and out of the secondary circuit. This allows the machine to operate as a motor and generator at both sub- and super-synchronous speeds. For wide speed range operation a current source inverter was used as this can inherently provide reversal of power flow. The operating requirements for the current source inverter operating in the secondary circuit of an induction machine have been determined. These considerations show that the current source inverter control signal must be synchronised to the secondary e.m.f. of the machine. The machine can then operate in a stable manner over a very wide speed range. The conventional analysis of the current source inverter has been developed to include the effect of the secondary slip e.m.f. which is shown to have a major effect on the commutation behaviour of the inverter. The action of the commutation circuit is affected by the phase angle between the secondary current and the slip e.m.f. This angle can be controlled electronically and the effect of this has been predicted and observed. A detailed study of the Kramer system has included analysis of the d.c. link current waveform including Fourier harmonic prediction in terms of the circuit parameters and the operating slip. The operation of the Kramer and Scherbius systems has been studied for both motoring and generating modes of the induction machine and their relative merits have been compared. In particular the novel idea of using the Scherbius system for variable speed wind energy recovery has been considered and reported in a published paper. Finally suggestions have been made for further work particularly for application to wind energy recovery.