Back to Blog
Applications and How-Tos

What is Safe Torque Off (STO)?

Jonathan Bullick | August 22nd, 2019

STO Function helps achieve safe operation of VFDs

The functional description for Safe Torque Off (STO) is:

“The safety-related disconnection according to STO is reached by a two-channel opto-coupler blockage. The supply of the opto-couplers, which are responsible for the commutation of the connected drive occurs via transformation coupling of the input voltage. This ensures at input voltage loss that no supply of the opto-couplers is possible. If the opto-couplers are no longer supplied, no insulated-gate bipolar transistor (IGBT) can be controlled and thus no energy can be supplied to the drive. The two channels are reached by way that input STO1 prevents the voltage supply (VTRO) of the upper opto-couplers of the inverter bridge and input STO2 the lower opto-couplers (VTRU).”


What this functional description means, is that a 2-channel safety-rated circuit is implemented inside of KEB Variable Frequency Drives (VFD) products. When 1 of these 2 channels is disconnected, lost, or opened, the drive will immediately remove modulation and electrical supply to the motor. If the Safe Torque Off is removed, the motor will coast to a stop based on its inertia and system friction. The motor will also be unable to start until the STO channels are reset.

EtherCAT wiring diagram of KEB Safety IO module
KEB safety IO module offers STO functionality to machine builders

Unlike other KEB functions such as quick stop, DC injection braking, or power-off mode, the STO functionality is safety-rated. KEB’s STO functionality meets the requirements of performance level e (PLe, ISO 13849-1) and SIL 3 (IEC 61508 and IEC 62061).


Applications of STO

STO can be incorporated into and used with any application. While the use of STO may not be explicitly required for your machine or application, it can be a useful safety feature to protect personnel and mechanical equipment.

This is a generic example of a Safe Torque Off application

For applications with large moving equipment, STO can be connected to a safety circuit, or a light curtain for example, to immediately stop and prohibit motor function if a specific safety barrier is triggered.

fsoe decentralized safety
STO linked to a light curtain using KEB functional Safety PLC and IO modules

Another example of using STO is to stop a conveyor or horizontal axis machine. Even without a dedicated safety circuit, an emergency stop switch can be activated which directly controls the integrated STO on the VFD.

STO functionality can also be used in vertical axis applications that are carrying load and/or people. If an emergency situation arises, STO functionality can remove power to the motor which at the same time can allow for a brake to engage to maintain and hold the load. In addition, a disconnected STO prevents any unexpected starting of the motor.


How to set up Safe Torque Off

STO is available in the F5 VFD product line, however, STO is supplied as standard in our new generation 6 VFDs.

There are many ways and options to set up the STO functionality for your system. The exact nature of how to wire the STO channels is at the discretion of the machine builder; however, KEB does have some wiring examples that may provide some insight and assistance. These examples are shown below.

Wiring diagram for direct switching off with emergency stop switch
Figure 1 – Direct switching off with emergency stop switch

This is a most basic wiring scheme – there are two separate wiring paths that supply power to STO1 and STO2.

How to Wire an Emergency Stop Switch monitoring
Figure 2 – Direct switching off with emergency stop switch and monitoring of the wiring

This layout will trip the 24VDC supply fuse if there is a wiring short circuit.

Wiring a Safe Torque Off module with Test pulses
Figure 3 – Direct switching off by safety module with test pulses

Some installations may require the use of a safety system that consistently checks the signal paths with OSSD test signals.


Safety Considerations

STO is a certified safety function that helps achieve safe operation of a VFD. STO serves as the backbone for VFD-based safety and the building block for other safety functions that are available on KEB’s safety products.

safe motion drive
KEB Drive Safe Motion

While STO improves the safety performance of a machine, the STO functionality is not a catch-all. For example, consider the situation of operating a saw blade with high inertia. While the STO will immediately remove modulation to the blade motor, the blade will still coast to a stop under friction. Therefore, STO and the inclusion of other safety measures need to be analyzed for the specific application.

STO on its own provides an “uncontrolled” stop by immediate removal of power to the output transistors. STO can be coupled with other KEB safety features to provide any level of safety functionality that your application may require.



Safe Torque Off is a functionality that has become a standard offering on our generation 6 VFD products. KEB is dedicated to the commitment of safety and will serve as your partner in safety automation. Do you require STO or safety in your application? Contact an Application Engineer today to discuss

Contact Us

Let's Work Together

Connect with us today to learn more about our industrial automation solutions—and how to commission them for your application.

  • This field is for validation purposes and should be left unchanged.