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STO (Safe Torque Off) finds its roots with European manufacturers and has been driven into the foreground by the European safety codes. As of 2017, STO is widely accepted in both the European and Asian markets as a basic safety component. This feature has started to become more prevalent on drives in the US market as newer models of VFD’s are developed and the technology is included.

Since this is not yet a North American requirement most low cost drives are not equipped with this feature, however, that will most likely change as time moves forward.

Now down to the basics…

The STO function is the most basic drive-integrated safety function. It ensures that no torque-generating energy can continue to be applied to a motor and prevents unintentional starting. 

The Bottom Line


This safety function stops the VFD from restarting in accordance with EN 60204-1, Section 5.4. The Safe Torque Off function basically stops any pulses at the IGBT (insulated-gate bipolar transistor) switching gates. The drive now secured, qualifies against the EN60204-1 code and the motor or other machining actuator cannot restart until the STO has reset.

Why would I want this?


Rather than using separate input contactors or safety relays, which require additional installation and maintenance, a VFD with STO can be brought to a standstill in a sufficiently short time. This is done through the load torque or friction and is used when coasting down the drive is not relevant to safety.

STO also allows safer equipment functionality for equipment that has to stop frequently to either be serviced or stopped as part of the production process. This enables safe working when an item, like a protective door, is open (restart interlock) and can be used in wide range applications with moving axes, e.g. handling, conveyor technology.

*It is important to remember that although the STO halts torque in the motor, this does NOT mean the VFD has been electrically disconnected (it could still shock you if you poke it with a fork!)

STO Added benefits


Outside of simplifying installation by having the safety component and VFD in one device, the advantage of no longer needing potentially costly professional contactor servicing is huge in our opinion. Rapid switching times compared to its safety relay competitors, ease of installation and maintenance, better EMC behavior, and no acoustic noise make the STO function an excellent option to include in many applications.

*Note: STO is not a Safe Stop 1 level protection.

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