What is Safe Torque Off? The LED Controls guide to STO

Safe Torque Off is a safety feature built into many Variable Speed Drives (VSDs) and servo drives. It prevents the motor from generating torque - which means it can't turn - even if it’s still powered.

When STO is active, the drive’s output stage is shut down so no current can reach the motor windings. The motor simply coasts to a stop rather than braking sharply. This is known as an uncontrolled stop or “Stop Category 0”.

Understanding Stop Categories and Safety Ratings

When you're flipping through VSD or servo drive specifications, you might notice references to Stop Category 0, Stop Category 1, or acronyms such as SIL or PL. These define how the motor behaves when a stop or fault command is given, and how it measures up to current safety standards.

Stop Categories (EN 60204-1 Standard)

The Stop Category defines how quickly and safely a motor is brought to rest when a stop signal is issued. There are three main categories:

• Stop Category 0 – An uncontrolled stop. Power to the motor is immediately cut, and the motor coasts to a halt naturally. This is the simplest (and fastest) method, but may require a separate breaking system if a full, immediate stop is required.
• Stop Category 1 – A controlled stop before power is removed. The drive actively decelerates the motor using its control circuitry until it reaches a standstill, then isolates power. It’s safer for machinery that must come to a complete, managed stop without coasting.
• Stop Category 2 – The drive maintains power to hold the motor in a controlled state even after stopping. This is often used where the motor must hold position (for example, in servo systems).

In short: Category 0 stops instantly; Category 1 stops smoothly; Category 2 stops and holds.

Safety Ratings (Functional Safety Standards)

As well as STO, modern drives often use Functional Safety features such as Safe Stop 1 (SS1), or Safe Limited Speed (SLS). These are all assessed under two main safety rating systems:

• SIL (Safety Integrity Level) – Defined in IEC 61508/62061, this ranges from SIL 1 to SIL 4. The higher the number, the lower the probability of failure on demand. Many industrial drives achieve SIL 2 or SIL 3 for critical safety functions.
• PL (Performance Level) – Defined by ISO 13849-1, this ranges from PL a to PL e. Again, the higher the letter, the safer and more reliable the system.

A VSD with STO to SIL 2 / PL d means that when the safety circuit is triggered, it safely removes torque from the motor (Category 0 stop) without needing to cut the main power supply.

Functional Safety Modules

Functional Safety Modules (FSO) extend a drive’s safety capabilities beyond Safe Torque Off (STO). They can be built-in as standard or optional extras, and enable advanced safety functions such as Safe Stop 1 (SS1), Safely-Limited Speed (SLS), Safe Brake Control (SBC), and Safe Direction (SDI), allowing controlled motion rather than a full stop. By monitoring and managing these parameters directly within the drive, FSOs make machinery compliant with higher safety standards like SIL 3 and PL e.

Why is Safe Torque Off important?

STO is vital for keeping both people and machinery safe. It prevents motors from starting unexpectedly when maintenance is taking place, protecting workers and equipment from injury and damage. It uses a dedicated, fail-safe hardware circuit to ensure stoppage, meaning it works faster than methods that rely on software or input/output circuits. And an Uncontrolled Stop ensures a gentle, coasting stop rather than a jolting halt.

Because STO is built directly into the drive, it removes the need for external safety relays and contactors in many applications, reducing cost and complexity and improving reliability. And integration means quicker and easier restarts, helping limit downtime. So it's a quick, slick and cheap method of keeping your people and machines safe!

How is STO used in real life?

You’ll find STO used anywhere motors power moving parts: conveyors, mixers, packaging machines, and robotics systems. For example:

• In a bottling plant, STO stops the conveyor motors instantly when an emergency stop is pressed, without cutting total power to the system.
• On a robotic arm, STO allows the machine to hold position without resetting.
• In HVAC systems, drives with STO can safely power down fans for inspection without isolating the whole control panel.

Drives with STO from LED Controls

At LED Controls, many of the Variable Speed Drives we supply already feature Safe Torque Off as standard:

Brand	Series
Danfoss	FC-280 Range
Danfoss	FC-302 Range
ABB	ABB ACS Range
Unitronics	UMI-B1 Range
Unitronics	UMI-B7 Range
TECO	A510 Range

By choosing a drive with STO built in, you’re simplifying your wiring, reducing the need for extra hardware, and increasing overall safety reliability. Still more of our products are available with STO as an optional upgrade, so if you don't see the drive you want, just give us a shout and we'll see what we can do for you.

A small feature with a big impact

Safe Torque Off is a key player in making modern automation efficient and safe. It’s quick to install, simple to use, and gives you peace of mind that your systems will stop safely when needed.

So, if you’re exploring drives for a new project or upgrading an existing system, check out our range of Variable Speed Drives or get in touch for more information:

01706 242050
[email protected]
ledcontrols.co.uk