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In this paper an accurate loss model-based controller to calculate the optimal air gap flux is proposed. The model includes copper losses, iron losses, harmonic losses, friction and windage losses, and stray losses. These losses are represented as a function of the air gap flux. By using the calculated optimal air gap flux compared with rated flux for speed sensorless indirect vector controlled induction motor, an improvement in motor efficiency and power factor is achieved especially at light load. By adapting the motor voltages and currents signals, load torque meter signal, and position sensor signal the average electrical and mechanical motor powers are calculated in a FPGA. The accuracy of the calculated electrical power is calibrated by using advanced power meter with accuracy equal 0.1% to verify the recommendation of the standard IEEE 112B for measuring the efficiency of the three phase induction motor. The improvement of the efficiency, the power factor and the motor stability under fast load variations by using the proposed optimal flux control method is compared with the rated flux control method experimentally.