Wiring 25 — VFD Safe Torque Off and Status Feedback

VFD Safe Torque Off and Status Feedback

Integrate a dual-channel guard switch, safety relay and VFD Safe Torque Off inputs. Add the ordinary run command and hardwired running/fault feedback to a PLC. STO removes motor-producing torque; it does not isolate the drive’s mains or DC bus.

The device in the real world

Safe Torque Off is a drive feature that stops the drive from producing torque, so the motor cannot start accidentally while a guard is open. It uses two separate input channels, STO1 and STO2, normally fed from the contacts of a safety relay. The ordinary run command from the PLC is a separate, non-safety signal. The drive also reports its running and fault status back to the PLC through simple outputs.

Why it is wired this way

Both STO channels must be independently controlled and monitored. The normal run command cannot be part of the safety function. Maintenance still requires lockable mains isolation, waiting the DC bus discharge time stated in the drive manual, and proof of dead.

Worked example

With the guard door closed, both guard channels feed the safety relay, which closes its two output poles and supplies 24 V to STO1 and STO2. The PLC can now send a run command on STF, and the drive reports RUN to input X0. Open the door and both poles drop together, torque stops and the drive shows STO active. Note that the mains and DC bus are still live inside the drive, so maintenance still needs isolation. Wire this lab by building the power path first, then the safety chain, then the PLC signals.

Common mistakes

• Feeding both STO inputs from one relay pole, which loses the independent second channel.
• Treating the run command as part of the safety function.
• Assuming STO makes the drive safe to open, when the mains and DC bus remain energised.
• Connecting the status outputs to the wrong common.

What you can now do

You can wire an STO chain from a guard switch through a safety relay to the drive, and keep the run command and status signals separate from it.

Commissioning standard

Verify each STO channel on its own, then both together, and prove that the motor cannot turn with either channel open.

Training material. Follow your site procedures, local electrical code and the manufacturer's instructions. Lockout/tagout and a qualified person are required for real equipment.