High-Efficiency Converter for Vari-able Speed Drives
Switching losses reduced by more than 30 %
Technical University of Munich
Electric motors power industry automation, robotics, drilling, compressors, pumps, fans and many other machines. Whenever the load on the motor is changing with time, a Variable Speed Drive (VSD) is preferable due to its high operational efficiency and flexibility. If the motor is interfaced with a three-phase grid and requires bidirectional power flow, the VSD is commonly designed as a Voltage source Back-to-Back Converter (V-BBC). This invention further improves the efficiency of a V-BBC.
The V-BBC enables bidirectional power flow at a high conversion efficiency with a rather simple topology of only 12 semiconductor switches. Thereby, the Voltage-Source Rectifier (VSR) facing the grid, and the Voltage-Source Inverter (VSI) driving the load are usually both run with a 2/3 discontinuous pulse-width modulation (DPWM), where the switches in two out of three legs of both the VSR and VSI are active at any given time and the DC-link voltage is fixed at a constant (high) value.
The invention introduces a new form of swit-ching pattern of the semiconductor switches of the V-BBC. As in the regular case, either the VSR or VSI runs with a 2/3 DPWM, with two legs switching and one leg clamped to one of the DC-link rails. However, on the other side of the V-BBC, only one leg is switching, while the remaining legs are clamped to the negative and positive rail of the DC link respectively. The voltage of the DC link follows the highest line voltage. This still allows for synthesizing AC currents in all phases while reducing the number of switching transitions, maintaining the minimum possible DC-link voltage, and thus minimizing the switching losses.