DC series motors produce very high torque at zero or low speed. It is for this reason and its simplicity that these motors can still be found in oil drilling applications, among others. In oil drilling, the environment can be very harsh, leading to violent vibrations and shocks. Position sensors, needed to perform closed loop speed or position control, can break easily under these conditions, causing production losses. Moreover, they increase the complexity and cost of the system. Therefore, it is desirable to avoid them. In this work, we propose the use of a step-by-step super twisting observer in order to determine the speed of the motor. These observers have the property of finite time convergence. However, two inherent properties of DC series motors could make the use of observers impossible. On the one hand, there exists an observability singularity at zero current. On the other hand, there is the magnetic saturation which leads to incorrect speed observation if not taken into account. To overcome these limitations, we propose the use of an observer/estimator scheme and introduce the magnetic saturation into the observer model. An industrial application is conducted to highlight the performance of the proposed solution in the context of a sensorless speed control of a DC series motor.