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Inverter Drive Systems Ltd

IDS are ABB Inverter specialists. We specify, install, commission, repair, service and hire variable speed drives.



Many ABB inverters have a feature called Safe Torque Off or STO, but what is it?

Safe Torque Off is a function that prevents the inverter from generating torque in the motor. It ensures that no torque generating energy continues to act on the motor and that the motor cannot be re-started until the STO is removed. What this means is that once the Safe Torque Off is activated the motor will not turn after it has come to a stop.

The Safe Torque Off function can be used when the natural inertia of the load can bring the motor to a safe standstill in an acceptably short time with no safety implications. STO then allows safe working on a machine for example to clear mechanical blockages: the Safe Torque Off circuit could incorporate proximity switches associated with mechanical guard. An important point to remember is that if braking is a safety requirement, it must be accompanied by an independent fail-safe braking mechanism.

Safe Torque Off

One of the major advantages compared to normal safety practices, both in terms of reduced cost and simplicity is the elimination of the need for input contactors or separate safety relays. Fewer components mean less effort in wiring and servicing. In addition electronic switching times are significantly quicker than electromechanical devices such as contactors or relays.

ABB’s Safe Torque Off has a certified SIL 3 / PL e safety rating and complies with EN/IEC 61800-5-2 which is the standard that defines the Safe Torque Off feature.

Finally, it is important to remember that Safe Torque Off DOES NOT provide electrical isolation. The motor must be electrically isolated by cutting the supply to the inverter and by local isolation to the motor before any access to the motor power connections.

If you require further information on Safe Torque Off or any aspect of ABB Inverters, please contact us on 0115 944 1036 or e-mail [email protected]

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