Variable speed drive comprising a main rectifier module (10) including thyristors (K1, K2, K3) to power a main continuous bus (15) from an external power network (5), an inverter module (30) delivering a variable control voltage to an electric motor from the main bus (15), an auxiliary rectifier module (20) connected to the external power network (5), an electronic control unit (50) controlling the thyristors, a powering module (29) supplying the control unit (50) from the auxiliary rectifier module (20) and from the main rectifier module (10). According to the invention, a switching device (40) controlled by the control unit (50) is used to disconnect the powering module (29) and the auxiliary rectifier module (20).