The single-phase capacitor-run induction motor is generally used for fractional power in applications where the price is an important criterion. In order to save costs, one tries to estimate the speed on the basis of voltage and current measurements, and thus avoid a speed sensor. The techniques used are very varied. In the last few years, they have included numerous patents and often the use of microcontrollers. First of all, this paper shows the analytical characteristics of the induction motor with its capacitor. It explains the different possible methods to assess the speed. Then, the paper suggests a realization using one of these methods on a motor of a rolling shutter. The setup tested implements a microcontroller that cuts off the supply as soon as an abnormal reduction in speed is observed on a basis of a voltage measurement. This paper details the implementation on the industrial application.