Active du/dt filter dimensioning in variable speed AC drives

Active du/dt is a new output filtering method to mitigate cable reflection based motor overvoltages. The target in active du/dt is to minimize the passive filter in terms of size and weight, and to provide better filtering performance than with conventional passive approaches. The active du/dt method uses a passive LC filter circuit at the output of the inverter with additional active control for controlling the filter output voltage. The active control is carried out using the same output stage as in the fundamental frequency modulation, therefore, no additional components are required. In addition, considerations of dimensioning the filter du/dt value according to the cable length and allowed voltage overshoot at the motor terminals resulting from cable reflection are presented.