In this paper a non-linear rigurous model of a d.c. motor fed from single phase thyristor bridge is presented. The dig-ital current and speed control are realized by internal current loop with proportional integral controller and external speed loop with a non-zero proportional integral one. This model is linearized around the operating point to study the behaviour of the system for small perturbations. Analytical expressions are given for the controller parameters for any desired response. A general method of simulation to study the behaviour of the system, is also presented taking into account the possible system discontinuities.