OPTIMIZATION OF LOSSES iN THREE-PHASE VARIABLE FREQUENCY INVERTER-FED INDUCTION MOTORS

Abstract

The losses of a 3-phase squirrel-gage induction motor excited from a bridge inverter are investigated here. Formulation of the losses for variable operating frequency taking into account the presence of time harmonies in the voltage waveform is carried out based on the well-known relations available for fixed 'frequency operation. Next, flow charts have been developed for determi-nation of the losres with constant Volts / Hz and constant-flux modes of operation of the motor on sinusoidal as well as non-sinusoidal supply voltage waveforms.. Computer programs based on the flow charts have been used to calculate the losses for a 5 h.p. motor with constant, full load torque over a frequency range from 0.2 to 1.0 per unit normal (rated) frequency. The effect of variation of equivalent-circus t parameters on the losses of the motor is also studied from the computed results. Finally, a flow chart for optimization of the losses of the inverter-fed induction motor has been developed using the incremental search technique. The quantities which are considered as variables are the nunber of conductors per stator slot, the area of cross-section of the stator conductor and the core length. A program prepared on the basis of this flow chart, has been used to ,obtain suitable values of the design variables for the 5 h.p. motor taken as ate ex.=plo so that the total losses of the motor are minimized.