Efficient induction motor drive with multilevel inverter and variable rotor flux

Multilevel inverters are an alternative for a wide range of low voltage electrical drives. They reduce power semiconductor losses, voltage transients at the motor windings, harmonic losses in the cables and common mode disturbance currents. In the low voltage range (<1000 V) new wide-bandgap semiconductors (SiC, GaN) have the potential to reduce the conduction and switching losses in multilevel inverters. For 550 V DC link voltage the power semiconductor losses of a two-level VSI with 1200 V-IGBT and a neutral-point-clamped three-level VSI with 600 V-IGBT are compared. At 20 kHz switching frequency and an electrical power of 35 kW the semiconductor losses of the three-level inverter are 38 % lower compared to the two-level inverter. Further energy saving potential for induction motors is the use of variable rotor flux instead of commonly used constant flux. The 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 look-up table. 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. The calculated and measured increase in efficiency at an 11 kW induction motor 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. An increase in efficiency can be reached below 2/3 of rated torque. Energy saving with variable rotor flux is possible in the whole speed range from 10 to 100 Hz.