Logistics and warehouse systems use conveyors, lifts, sorters, and automated storage equipment that are powered by electric motors. Safe Torque Off, or STO, is a safety function built into many modern motor drives to prevent a motor from producing torque when a hazardous stop is required. It matters because workers may need to clear jams, inspect equipment, or enter guarded areas near moving machinery.
STO helps reduce the risk of unexpected motion without always removing all electrical power from the drive cabinet.
STO works by interrupting the drive signals that allow the inverter to send torque-producing current to the motor. The motor may coast to a stop unless a brake or controlled stop function is also used, so STO is not the same as instant stopping. In a warehouse conveyor, safety devices such as emergency stop buttons, interlock switches, light curtains, and safety PLC outputs can command STO.
A properly designed STO system uses redundant channels, monitoring, and safety ratings so a single fault is less likely to create a dangerous failure.
Key Facts
- STO prevents torque production by disabling the drive output stage, so motor torque T approaches 0 N m.
- STO is a safety function, not a power isolation method, because hazardous voltage may still be present inside the drive.
- Stopping distance can be estimated by d = v^2 / (2a) when deceleration a is approximately constant.
- Coast stop time can be estimated by t = omega / alpha, where omega is angular speed and alpha is angular deceleration.
- Redundant STO inputs often use two channels, such as STO A and STO B, to detect wiring faults and improve safety reliability.
- Risk reduction depends on the full safety chain: sensor, safety controller, wiring, STO input, motor drive, mechanics, and validation testing.
Vocabulary
- Safe Torque Off
- Safe Torque Off is a drive safety function that prevents a motor from generating torque by disabling torque-producing output signals.
- Motor drive
- A motor drive is an electronic device that controls motor speed, torque, and direction by adjusting electrical power to the motor.
- Torque
- Torque is the turning effect of a force and is measured in newton meters.
- Safety PLC
- A safety PLC is a programmable controller designed and certified to monitor safety devices and command safe machine responses.
- Coast stop
- A coast stop occurs when a motor is no longer driven and slows down due to friction, load, and drag rather than active braking.
Common Mistakes to Avoid
- Treating STO as a main disconnect is wrong because STO may leave dangerous voltage inside the drive and motor circuit.
- Assuming STO stops motion instantly is wrong because STO removes torque but the conveyor may coast unless braking or a controlled stop is provided.
- Using only one unmonitored safety wire is wrong because a short circuit or broken wire could defeat the safety function without being detected.
- Resetting the system without checking the hazard zone is wrong because STO can be removed and torque can return if the safety chain is reset while a person or object is still in danger.
Practice Questions
- 1 A conveyor belt is moving at 1.8 m/s when STO is triggered and the belt coasts with an average deceleration of 0.60 m/s^2. How long does it take to stop, and how far does the belt travel before stopping?
- 2 A motor shaft spins at 120 rad/s and coasts to rest in 8.0 s after STO is commanded. Assuming constant angular deceleration, what is the angular deceleration, and how many radians does the shaft rotate before stopping?
- 3 A technician says that opening a guard door connected to STO makes the conveyor electrically safe to repair because the motor cannot make torque. Explain what is correct about this statement and what important safety limitation is missing.