The paper is concerned with the stability of closed-loop control systems incorporating controlled rectifiers. A suitable small-signal model for a convertor is developed which takes into account the discrete behaviour of the controlled rectifier. This model permits the use of standard sampled-data control-system theory in the stability analysis of the system. Contrary to usual practice, the a.c. system reactance is not neglected in the modelling. It is shown that the commutation angle is an important factor in the determination of stability boundaries. Finally, again-correction factor is employed to allow for the effect of the ripple under finite-commutation-angle conditions. Discrete Laplace transformation (z transformation) is used throughout the analysis together with root-locus techniques. Stability boundaries derived from the new model and other existing models are compared, and it is shown that experimental results are in good agreement with the theory developed in this paper.