The Helios TSSD 4 is a transformer-based speed control unit featuring two integrated transformers mounted on base runners, designed for V‑connection configurations. It is intended for installation inside a switch cabinet and is supplied without an external housing. The unit includes terminals for selecting between five voltage levels: 0, 80, 140, 280, and 400 volts. Operating at 50/60 Hz, the TSSD 4 provides accurate and efficient speed regulation for 3‑phase fans in enclosed control systems.