PLC Simulator
S-01 · vendor-neutral function

VFD Safe Torque Off (STO)

Safe torque off prevents the drive from generating motor torque when its safety channels are de-energized. It does not isolate mains power and does not necessarily stop a moving machine quickly.

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Why it matters

The behavior behind the parameter.

STO belongs to the machine safety function, not the ordinary run/stop logic. Commissioning must prove both channels, diagnostics, reset/restart behavior and the required risk reduction using the actual safety design.

  • STO removes torque-producing energy from the motor output stage.
  • The motor can coast and hazardous voltage can remain inside the drive.
  • Dual channels are not permission to bridge or bypass the safety chain.

Interactive proof

Test vfd safe torque off (sto) in three passes.

Keep the motor, load and starting condition constant. Change one function at a time, then use the electrical and mechanical evidence to explain the result.

  1. 01

    Run at low speed, open one STO channel and observe torque removal and coast.

  2. 02

    Confirm the drive cannot restart while either channel remains open.

  3. 03

    Restore the complete safety chain and prove the designed manual reset/restart behavior.

Load F-code S-01 in the function lab

Evidence to watch

The function is proved by changed behavior, not by a parameter value alone.

  • STO channel A and B status
  • Torque-enabled and ready status
  • Residual speed and DC-bus voltage

Common commissioning mistakes

These errors can make a healthy drive appear faulty or create a misleading test.

  • Treating STO as electrical isolation.
  • Expecting STO alone to provide a controlled fast stop.
  • Resetting the safety chain without checking for an active run demand.

Primary technical references

Transfer the concept; verify the exact drive.

This guide uses vendor-neutral terminology. For real equipment, use the manual for the installed model, option cards and firmware, plus the machine risk assessment and approved commissioning procedure.

Rockwell PowerFlex 520-series user manualSiemens SINAMICS G120 function manualDanfoss VLT AutomationDrive programming guideABB ACS580 manual directory

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A VFD can ramp the frequency down, remove torque and coast, use a faster quick-stop ramp, or apply DC injection braking. Each produces a different current, torque, stopping time and DC-bus response.

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VFD Fault Codes and Diagnostic Method

A VFD fault code identifies which protection function operated; it does not by itself prove the failed component. Diagnose with the code, first-changing measurement, trip history and machine condition together.

Read and test

From explanation to evidence

Change the function and watch the motor respond.

Open the VFD function lab