With my kitty  niadebesis
One day, me and my beloved kitty went to the local railway station cafè to have breakfast. It was a rainy spring saturday, so it was nice to stay inside. In that occasion, he (that's an expert and competent electronic technician) explained me how choppers work. These are devices used for high voltages of DC current to convert a fixed imput to a variable output. Among various applications, there's railway traction. During the 80s locomotives (E.632, E.633 and E.652 Classes, for example) were equipped with these devices and DC traction motors. Choppers were used to control the current sent to the traction motors (replacing the previously used but less efficient shunts). Afterwards, since the beginning of the 90s (with the E.402 Class locomotives), choppers became obsolete, since were preferred asynchronus 3-phase AC traction motors, driven by inverters (to convert the 3kV DC standard current supply from catenary to AC) and GTO Thyristors.