Energy-efficient control of induction machines with variable rotor flux

The paper presents the energy saving potential of induction machines by using variable rotor flux instead of commonly used constant flux. The efficiency optimized rotor flux depends on the actual load and speed of the motor. At low load it has to be decreased with reference to a pre-calculated lookup table. This table can be included in a field oriented speed control structure as reference value of the rotor flux. The calculation of optimal rotor flux curves includes the non-linear main inductance, the frequency-dependent rotor resistance and rotor leakage inductance, the speed-dependent iron, friction and windage losses and the motor frequency. This distinguishes this paper from former scientific works. The calculations and measurements are carried out at an 11 kW high-efficiency (efficiency classification IE2) asynchronous machine with 2 pole pairs. An increase in efficiency can be reached below 2/3 of rated torque. The maximum increase is about 35 % in motor and 45 % in generator operation mode. The average efficiency improvement is 4,5 % in motor and 6,0 % in generator mode. Additionally, the power factor can be improved over the whole load range and the current consumption can be reduced. Energy saving with variable rotor flux is possible in the whole speed range from 10 to 100 Hz.