Definition:
Safety Torque Off (STO) is a safety feature built into variable frequency drives (VFDs). While it can be used in functional safety applications, STO is primarily a machinery safety function, widely used in discrete manufacturing, robotics, and automated equipment. It disables the drive’s ability to produce torque by blocking power to the motor windings—typically by removing gate control signals to the power stage—while still maintaining power to the control electronics of the VFD.
In IEC 61511-1 process safety contexts, STO can serve as a final element in a safety instrumented function (SIF), but only if it is SIL rated.
Importantly, STO does not disconnect electrical power from the VFD itself—it only disables the motor’s ability to generate torque. This is different from opening a contactor or breaker on the inverter side of the VFD. Opening a breaker cuts power completely (which may be required in certain fail-safe designs), whereas STO disables motion but retains control system awareness and diagnostics.
Many VFDs include STOs, though not all are SIL rated. However, certified SIL 2 or SIL 3 STO options are available—either by default or as configurable add-ons.
Key Points:
- Disables torque output from a motor by cutting gate signals—not by cutting power to the drive. Thus can be a final element in a SIF.
- Primarily used in machinery safety; can be adapted for process safety applications.
- Many VFDs do not include STO, and among those that do, many are not SIL rated.
- Offers faster, safer stops than mechanical braking or power isolation in some applications.
Example:
In a chemical packaging line, a VFD-powered conveyor moves filled drums to a sealing station. If a misalignment is detected at the sealing station, a spill and a major accident could occur. A SIL 2 SIF triggers an STO to the conveyor VFD. This disables the motor, preventing further movement and allowing the misalignment to be resolved. The STO function would be the final element and is SIL 2 rated.
See also: final element, machinery safety
Cited Source:
- IEC 61800-5-2:2016 – Adjustable speed electrical power drive systems – Part 5-2: Safety requirements