If you are specifying a variable frequency drive for an application where personnel need to access moving equipment, Safe Torque Off is not optional. It is one of the most important safety functions built into modern drives, and understanding how it works helps you specify the right solution and stay compliant. Here is what you need to know.
What Is Safe Torque Off?
Safe Torque Off is a drive safety function that prevents a motor from generating torque without physically disconnecting power from the drive. When STO is activated, the drive cuts the gate signals to the output transistors. The motor cannot restart without deliberately deactivating STO first.
The key distinction is that STO does not cut power to the drive itself. The drive remains energized and ready. Only the motor’s ability to drive is disabled. That means faster restart times than a full-power disconnect, while still meeting the safety requirements for personnel access.
Why the Safety Rating Matters
Not all STO implementations are equal. The ABB ACS580 carries STO certifications at the highest available levels:
- SIL 3 per IEC 61508
- SIL CL 3 per IEC 62061
- Performance Level e (PLe) per EN ISO 13849-1
These are the highest functional safety classifications available for this type of equipment. For facilities operating in regulated industries including food and beverage, pharmaceutical manufacturing, and chemical processing, these certifications are often required by both internal safety standards and external regulatory bodies.
When evaluating a drive for a safety-critical application, always verify the STO certification level. Some drives offer STO but at lower safety integrity levels that may not meet your application requirements.
Where STO Is Commonly Used
STO is required or strongly recommended in any application where personnel need to interact with equipment that is driven by a motor. Common examples include:
- Conveyor systems where operators need to clear jams or make adjustments
- Packaging machinery with frequent changeover and maintenance access
- Robotic cells and automated assembly lines
- Any application subject to EN ISO 13849 or IEC 62061 machinery safety standards
If your facility is subject to OSHA lockout/tagout requirements, STO can complement your LOTO program. However, it is important to work with your safety engineer to confirm whether STO alone satisfies your specific LOTO requirements or whether a full power disconnect is still necessary.
STO in the ABB ACS580
The ACS580 includes STO as a built-in standard feature, no additional safety module or external hardware required. That simplifies panel design, reduces component count, and lowers the overall cost of building a safety-rated machine.
The STO function is activated via dedicated STO terminals on the drive. Wiring is straightforward and ABB provides detailed application documentation for integrating STO into safety circuits alongside safety relays and safety PLCs.
Local Availability When You Need It
Lakeland Engineering stocks ABB ACS580 drives with built-in STO in our Minneapolis location. If you are specifying a drive for a safety-critical application and want to confirm the right configuration, our team can help.
Shop ABB Drives at Lakeland Engineering: https://shop.lakelandengineering.com/brands/abb