High-performance low-pass filter utilising a mechanical energy store

A novel low-pass power filter is proposed, in which a d.c. machine is used as a reactive element, replacing the conventional electrostatic capacitor. The equivalent capacitance of a suitably operated d.c. machine is, typically, several times larger than commercially obtainable electrostatic capacitors, and, therefore, a filter using a machine as one of its elements should have exceptional characteristics. The primitive machine filter, i.e. one in which the machine directly replaces the capacitance, is shown to be unsatisfactory because of the rotor´s leakage inductance and resistance. The critical parameters in the compensation circuit necessary to overcome the effects of these factors are derived together with the required VA ratings of the machine and inductance in a typical L-type filter. Based on the derived principles, a machine filter, using a standard solid-iron-frame d.c. machine, has been assembled and tested, and the results show an output-voltage waveform which contains less than 0.333% total distortion. A theoretical study of the filter is presented using the proposed equivalent circuit. Finally, the machine filter is compared with classical passive-element systems under steady-state and transient conditions, and possible improvements are suggested. It is concluded that the machine filter is somewhat better than the classical filter in terms of cost and size, and that the noise and vibration problem may be offset in a number of applications by the machine filter´s superior transient performance and extended temperature range.