What is STO ?
(for AC drive)
STO (Safe Torque Off) is a safety function that is used in control and automation systems to shut off the torque (rotational force) of a motor in a safe and controlled manner. It is typically used in applications where the motor needs to be stopped quickly in the event of an emergency, such as in a machine tool or a conveyor system.
STO (Safe Torque Off) — Dual Channel Safety
Interactive demo · DERELL
Controls
Rule: If both channels are healthy and EM-STOP is not pressed, torque is ON → motor rotates.
If any channel fails or EM-STOP is pressed, torque is OFF → motor stops.
Drive Status
STOPPED
STO Status
Both Channels Healthy
Motor Torque
OFF
Function Diagram
Explanation: In dual channel design, both channels must be healthy for torque ON (shaft rotates).
If any channel fails or EM-STOP is pressed, STO activates and shaft stops (torque = 0).
Note: This demo explains the principle only. Real wiring, testing and validation must follow
safety standards (e.g. EN ISO 13849-1, IEC 61508).
STO is an important safety feature in control and automation systems, as it helps to prevent accidents and injuries by stopping the motor in a controlled and safe manner. It is often used in combination with other safety features, such as emergency stop buttons and safety guards, to create a comprehensive safety system.
In the field, STO is often misunderstood as just “cutting the power.” However, it is a certified safety function that complies with international standards like EN ISO 13849-1 and IEC 61508. This makes it not only a practical feature but also a legal requirement for machine builders who want to achieve the right safety category or performance level.
Another key point is that STO does not completely power down the drive electronics. The control section remains active, allowing for fast restart after the safety event is cleared. This provides a big advantage in industrial environments where downtime is critical.
In practice, STO circuits are often designed with dual-channel wiring. This redundancy ensures that even if one channel fails (for example, due to a cable break or relay fault), the second channel will still guarantee safe torque off. Using a dual-channel safety relay is strongly recommended to achieve reliable performance in the long term.
To conclude, STO is more than a simple stop function — it is a vital safety layer that protects both people and machines. Especially in high-speed motors, heavy conveyor systems, or presses, STO has proven to be a lifesaver by eliminating torque instantly without compromising the control system’s readiness.
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