OPTIMAL EFFICIENCY CONTROL OF INDUCTION MACHINES (FIELD, ORIENTATION)

Induction machines operate most efficiently in the vicinity of their nominal operating conditions since the balance between the load losses and the magnetizing losses is designed to be optimum near that point. At partial loads or at non-nominal speeds the motor efficiency drops off considerably due to the improper distribution of the losses. The purpose of this thesis is to show that a significant amount of energy can be saved by adjusting the flux level according to the load and the speed of an induction motor and to develop means for implementing such control while maintaining acceptable dynamic response.; The first part of this dissertation is devoted to a theoretical analysis of this problem using an improved steady state equivalent circuit of an induction machine. It is shown that the role of saturation in determining the optimal operating point is dominant and that the power losses in an induction motor can be reduced by increasing the flux level for large loads and reducing this level for light loads. Laboratory experiments confirm these theoretical results which are then extended to include such effects as harmonic losses and the influence of the motor size and type.; The second part of this dissertation is devoted to the design of an optimal efficiency controller based upon the adaptive adjustment of the flux level in a field oriented controller. An implementation using a controller of the direct type is analyzed using digital simulation. Experimental results obtained with a laboratory prototype using indirect field orientation are presented and discussed. A method for maintaining acceptable dynamic performance is proposed and experimentally demonstrated.