ABB Drive integrated functional Safe Torque Off (STO)
Reduce component count and cost without sacrificing machine safety performance.

ABB Drives have as standard SIL3 or PLe safety inputs for Drive-based safety functions that are used in applications that require risk reduction from unexpected and hazardous movement. The aim of this Tech Tip is to design machines that are safe to use via inbuilt readily available technology to reduce cost and component count without sacrificing machine safety performance level.

The safety standards require the drive output to the motor to be completely disengaged so that an unexpected restart cannot occur.

This is typically achieved by a couple of contactors on the drive output and some interlocking back to a safety relay or Safety PLC. However, the same can be achieved by using the inbuilt Safe inputs on the ABB ACS355 Safe Torque Off (STO) inputs.

Figure 1: Drive integration to E-stop and Limit switch using the STO inputs on ABB’s ACS355

In the example where a emergency stop switch is placed on a machine to safely stop a motor driven by a ABB ACS355 drive to Stop Category 0,the emergency stop can be wired directly to the STO drive inputs. Depending on the evaluated risk for the safety performance level further devices such as a safety relay can be added to provide diagnostics for the emergency stop button wiring thereby increasing the safety performance level. The relay also enables the use of a separate reset button, if required by the risk assessment.

Figure 2: Typical motor speed with a stop category 0 stop

Figure 3: Typical motor speed with a stop category 1 stop

To achieve Stop Category 1 for the system we can use a ABB RT7 Safety Timer Relay in series with the e-stop to the STO drive inputs which will produce a timed stop of the motor used in applications where asynchronized stop of multiple axes is required.

Figure 4: Connection example of the emergency stop function withACS355. Emergency stop reset button is not used in this example.

Figure 5: Emergency stop function details

Further modification can be made to the wiring and type of safety relay used to expand the safety functions the drive can control including Safe Direction SDI, Safely-Limited Speed SLS, and Safe Stop 1 SS1.

For more information on how the ABB drive can be used in your design to enable the machine safety level performance please contact IPD by clicking here.