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Inverter Drive Systems Ltd

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

ABB ACS880 Safe Torque Off

ABB ACS880 Safe Torque Off

In this informative article, ABB Area Sales Manager, Stuart Ruskin talks us through the innovative ABB ACS880 Safe Torque Off feature.

What is Safe Torque Off (STO)?

Inverter drives control motor applications that sometimes need to be stopped in an emergency. Safety should always be a priority concern when designing a machine. To achieve the safety regulations this often meant adding in a suitably rated contactor or sometimes 2 contactors upstream of the drive so that you could cut power to the inverter, in case of emergency or failure. Interrupting the power supply can mean losing position data or worse. This could cause issues like the motor could be freewheeling when a failure / emergency happened while running. Inverter drive manufacturers responded by providing contactors and other components built into their drives, creating a feature called Safe Torque Off (STO).

So How Do We Utilize Safe Torque Off???

STO is a function that is built into ABB drives, the function doesn’t allow the drive to generate any torque and will subsequently not turn the motor. The drive would display an STO Fault on the screen to advise what the current status of the inverter drive. IDS, one of ABB VP Value Providers explains STO here on their website. 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, it is important to know safe torque off does not provide electrical isolation. The motor must be electrically isolated by cutting the supply to the inverter and a form of local isolation to the motor before any access to the motor power connections.

Safe Torque Off (STO)

STO is typically used for a prevention of an unexpected startup (EN 1037) of machinery or for an emergency stop, fulfilling stop category 0 (EN 60204-1).

Safe Torque Off (STO)

This feature allows you to cut panel costs of unnecessary components including the contactors, panel wiring & therefore can lead to a reduced size panel. These all provide cost savings on the design and build of the machine / panel.


Whilst this innovative safety feature is available on the ABB ACS880 variable speed drive, the above benefits also apply to ALL ABB inverter drives fitted with safe torque off (STO). For advice on the right inverter drive for your environment and applications contact IDS on 0115 944 1036 or email [email protected]

Further Inverter Drive Assistance

As an ABB Value Provider, we at IDS undertake regular stringent training to ensure our knowledge of  ABB variable speed drives and all of their features and functionality are current and up to date. This includes a full working understand of the Safe Torque Off feature. We are available 24/7 for technical advice and breakdown support, which means in the unlikely event you encounter an issue we have engineers on hand to get your drives back up and running. Our engineers carry a full range of ABB repair suitcases and we have a substantial hire drive range if the issue cannot be immediately rectified. We also have a large range of inverter stock at our centrally located facility in Derbyshire, making us the perfect drives partner for a full wrap around service. From specifying, installation, commissioning and servicing your drives, to emergency repair, technical advice and hire drives – Inverter Drive Systems are here for you!

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