Speed control of a series excited d.c motor is primarily achieved through a regulated flux in the field winding. A well-adjusted flux is considered very effective in voltage and speed regulation. This process can be achieved through a field and armature current adjustment using a shunt diverter resistance. Tapped field control and parallel field coils also serve as a veritable measure for the speed control of a dc series motor. Although this method is scarcely used for the speed control of dc series motor. This paper is aimed at investigating the effect of applying a diverter resistance with well-adjusted thyristor firing angles for a controlled rectifier on dc series motor speed control. In this study, the steady state and the dynamic state equations that correlate the speed of dc series motor with diverter resistance and supplied voltage as a function of a thyristor firing angles were derived. Speed characteristics based on the derived equations were determined through simulation in MATLAB 7.14 for different values of diverter resistance and thyristor firing angles under a varying machine load. The simulation results showed that the series dc motor runs at a very high speed above the rated value in the reverse braking mode by increasing the thyristor firing angle beyond 900. Adjusting the firing angle below 900, speed values below the rated speed were obtained as the load current is increased in the forward motoring mode. A proper adjustment of the firing angle regulates the supply voltage as well as the high starting current.