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Everything you wanted to know about VSD functional safety

07 October 2019

Modern variable speed drives (VSDs) are packed full of safety features. But the sheer number of similar sounding functions and acronyms mean that they can be overlooked, leading to missed opportunities to improve functional safety. ABB’s Martin Richardson, explains.

In any industrial process it is critically important that when something goes wrong the machinery is quickly and safely brought to a safe state, or in many cases, stopped. Once stopped it must not start again unexpectedly.

The traditional way of building a safe machine involves connecting safety limit switches, relays and other external safety monitoring devices together with the VSD. These were necessary in the days when VSDs contained only basic safety functions. However modern VSDs now include a wide range of integrated functional safety features, moving external electromechanical safety solutions which may have included contactors inside the drive. This not only simplifies the overall design process, but with fewer parts and less wiring to fit into the control panel, the complexity, cost of configuration and installation, as well as development time, is also significantly reduced.

While dispensing with external safety devices and consolidating their functions inside the drive may seem like a win-win, it can also beg the question: “what if the drive itself fails?” Firstly, drives very seldom fail when properly installed and maintained, but even in a situation where multiple components within the drive have failed completely, functions such as safe torque off would still be activated. This is because of the way a VSD operates. Torque in the motor cannot be generated without the operation of the VSD, and so in the event of a serious fault, a no-torque state would effectively be the motor’s default state. This means that the safe torque off function is a failsafe last line of defence, unlike a mechanical safety contactor which can fail over time. Even in a worst-case scenario, the motor will stop and stay stopped.

Read the full article in the October issue of DPA.

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