Oscillations in single convertor drives

A conventional regulating system for a convertor-fed d.c. machine drive has a Nyquist locus close to the negative real axis, if the loop is opened to isolate the convertor. Under certain conditions, this locus shows the system to be conditionally stable, and if the drive has provision for controlled current in one direction only, sustained oscillations in the form of convertor current pulses can be produced. These oscillations, and the conditions necessary to excite them, are examined using a describing function, and the analysis is compared with analogue-computer results.