AiVibeSafe Torque Off (STO)
Safe Torque Off is a drive-integrated safety function, defined in IEC 61800-5-2, that prevents a variable frequency drive or servo drive from supplying torque-generating energy to the motor, without disconnecting the drive from its power supply.
When STO is activated, typically through dual-channel safety inputs or a safety network, the drive blocks the switching signals to its power semiconductors so the motor cannot produce torque. STO corresponds to an uncontrolled stop, stop category 0 in IEC 60204-1: a rotating motor coasts to a standstill unless a separate braking function acts. Combined with a prior controlled deceleration, the safe stop 1 (SS1) function achieves a category 1 stop.
Machine builders use STO to implement emergency stops and to prevent unexpected start-up while operators access hazardous areas, replacing or reducing the motor contactors otherwise needed. Because the drive remains powered, restarts are faster and contactor wear is avoided. STO is the foundation for further drive safety functions such as safely limited speed and safe operating stop.
STO does not provide electrical isolation; dangerous voltages remain in the drive and motor, so it is not a substitute for isolation and lock-out during electrical work. Loads that can move under gravity, such as hoists and vertical axes, need additional measures such as safety-rated brakes. The achieved SIL or PL depends on the whole safety function, and installation must follow the drive's safety manual and be validated by qualified personnel.
Key points
- Defined in IEC 61800-5-2 as a drive safety function
- Removes torque-producing energy from the motor while the drive stays powered
- Equivalent to a stop category 0 in IEC 60204-1
- Does not isolate the supply and is not a substitute for lock-out
- Vertical or gravity-loaded axes need additional braking measures
Where AiVibe comes in
AiVibe designs and manufactures the AiAmbA AI Factory, whose edge devices and AI agents let people talk to PLC, CNC and robot controllers in plain language. Agents only propose changes, a trained operator confirms each one, and the AI Factory never replaces a machine's control or safety systems.
Related terms
- Variable Frequency Drive (VFD)PLC & Industrial Control
- Servo Drive and Servo MotorPLC & Industrial Control
- Functional SafetyPLC & Industrial Control
- Safety PLCPLC & Industrial Control
- ISO 13849 and Performance Level (PL)PLC & Industrial Control
- IEC 62061PLC & Industrial Control