Wednesday, 10 June 2015

John Bennett, Director of iDrives, writes: What is safe torque-off? In a list of drive features it’s not exactly a headline-grabber like, say, direct torque control (DTC) or efficiency and productivity gains, but it’s still a hugely important feature. Indeed, it’s one that could potentially prevent workplace accidents and save lives. But what is safe-torque off? In order to explain exactly what it is let’s begin by breaking safe-torque off (or STO for short) down into its component parts.

Safe - Safety is of course paramount in all industrial processes, however with any plant machinery the moving parts and electrical input will always create some risk. It may sound grisly, but sadly every day across UK industry accidents occur, which can result in injury or even death. This is why most machines have features such as an emergency stop function which when activated, either manually or through sensors detecting a person’s proximity to the machine or its moving parts, will cut off torque to the motor by triggering a safety contactor.

Torque – The problem here is that the safety contactor not only requires installation, maintenance and regular testing, but if these duties are carried out incorrectly the contactor can itself fail, meaning that the machine might not stop or torque may resume unexpectedly. This of course is the last thing you want if you are, for instance, attempting to carry out machine maintenance. Safe torque-off brings the stopping function of the contactor into the drive, so that when those proximity sensors are triggered, the drive itself shuts the power to the motor down, instantly performing a coast stop. It cannot then be restarted until the operator instructs it to.

Off – Hence the “off” part of safe-torque off means just that. Once STO has been activated, an operator can be certain that no torque-generating energy can reach the motor, and if the motor cannot turn then the machine cannot move. This enables safe access to the machine’s moving parts for maintenance, cleaning etc. It should be noted that, whilst STO prevents torque in the motor, it does
NOT disconnect the drive from the electrical power supply, and so any access to the electrical parts of the drive or the motor should only be attempted once the drive system has been isolated from the main supply. However this function in itself means that, once STO is activated, the drive is ready at a moment’s notice to resume normal operations.

For a more visually pleasing explanation, as well as some other excellent points that I haven’t covered here, check out this video that I’ve narrated (with some help from ABB):

So there you have it. Safe torque-off. To recap, it’s SAFE, and it brings the motor’s TORQUE to a complete STOP. Easy when you think about it. It’s also one of many cutting edge safety features found in ABB drives. Stay tuned, for in the coming weeks we’ll be discussing in more detail what STO means in practice for you and your processes.

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