Figure 4 shows the Semikron power conversion module [5] including a diode rectifier and a three legs inverter, which can be configured to represent many different power converter topologies. For this laboratory experiment the three legs are controlled separately as individual buck converters, using the top IGBT and the bottom diodes on each leg. The bottom IGBTs (A-, B-, C-) are therefore off at all times, while the top IGBTs are controlled as follows:

• The duty cycle for the IGBT A+ is set by a combination of the DC bus voltage (Vdc) and the user defined “speed” command, which is discussed in the following section.
• The duty cycle for the IGBT B+ is set to a constant and determines the field excitation for the separately excited DC motor.
• The third buck converter is used to change the load of the DC motor, by changing the field voltage of the dynamometer.