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Advanced Engineering 2020

NEC, Birmingham(B40 1NT)

04/11/2020 - 05/11/2020

The UK's largest annual advanced manufacturing trade show, Advanced Engineering is your opportunity to (more)

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28/09/2021 - 30/09/2021

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Safety first

Safety first
The control capabilities of modern variable-speed drives make it relatively easy to incorporate safety functions, ensuring that machines and applications are always operated in a safe manner.

Safety is of overriding importance in the operation of machinery, a fact recognised by the European Union which requires machines to meet essential health and safety requirements. This means that all new machinery must meet the same legal requirements when supplied within the EU.

The development of electronic control systems has also affected the safety technology used with AC drives. Traditionally, automation systems involving drives have used electromechanical safety relays. These relays monitor various safety input devices such as limit switches and emergency stop buttons, and operate contactors to safely cut power to the power drive system when given parameters are exceeded.

These electromechanical systems are increasingly being replaced by electronics in modern safety systems for AC drive applications. The standard governing the safety related functions of variable-speed drives is EN 61800-5-2. The main requirements are those governing the stopping of machinery in a safe way. This standard sets requirements for design principles of safety-related drive systems, as well as defining a number of standardised safety functions for drives.

Perhaps the most essential of these is the safe torque-off or STO function, which temporarily cuts the voltage to the motor without the need to shut down the whole drive. This allows safe cleaning and maintenance of processing machinery, while allowing the drive to be in a standby mode ready to resume operations with the minimum of interruption to production. This also means that the drive diagnostics functions keep running during the shutdown, making it possible to analyse the event afterwards. 
The contactors to safely stop the motor movement in emergency situations or where start-up needs to be prevented can now be eliminated thanks to a new feature integrated into the drive's power section. The safe torque-off (STO) feature simply disables drive output modulation and safely eliminates the drive's ability to make the motor produce torque. STO also provides higher Probability of Dangerous Failure per Hour (PFHd) values than conventional contactor systems i.e. higher reliability.

Processes can become more productive when STO is used to safely stop the motor without disconnecting the power supply or the drive's DC circuit, as the drive can quickly be re-started without having to recharge the DC circuits or re-establish control parameters. For electrical maintenance, the conventional method of lock-off isolation would still be needed.

Advanced monitoring
When additional functionality is needed, STO benefits can be complemented by combining the function with more advanced monitoring functions. External offerings include, for example, time-delay relays or so called configurable safety systems. These are typically intermediate between PLCs and solid state safety relays. Related functions are Safe-Stop, which brings the machinery to a safe stopped state if certain conditions are detected. It decelerates the motor and initiates the STO function either when the motor speed reaches a specified level or after a time delay.

Other functions are available include safe brake control (SBC), which provides a safe output signal to control the motor's mechanical brake. The safely limited speed (SLS) function prevents the motor from exceeding the specified safe speed.

Such configurable systems become viable when several safety functions are implemented in the same system. For single safety functions, dedicated single purpose components such as time delay relays or two hand control relays are more practical.

External safety components are usually wired and configured to operate together with the drive. Developing an application usually requires wiring and configuration of the two devices individually so that they can work together. Clear instructions from the drive supplier are needed to support this configuration. The system often has two separate field buses, one for safety communication to the safety device and the regular fieldbus to allow for control signals. 

The next logical step in drive based safety is to integrate the safety functionality into the drive itself. This gives a number of benefits: wiring is reduced, drive inputs and outputs are freed up, space is saved, and the configuration can be performed through a single connection with one set of tools. Because integrated safety functions are drive specific, commissioning mainly consists of setting parameter values and behaviour options. There is also no need to actually programme the basic functionality. 

Another advantage is that a single fieldbus connection can be used for both regular and safety communication (if the fieldbus is a safe fieldbus). The overall safety functionality can be optimised when safety and control parts of the drive share status information over a bus connection. And, of course the system looks much cleaner without all the separate units and all the wiring. 

Developing drive-integrated safety is challenging for drive manufacturers, yet is still cheaper than achieving the same functionality with external safety components, whilst achieving the same or better PFHd figures.

Functional safety features can help boost performance and usability of machines while meeting the safety regulations. But it also takes time to really understand the opportunities that safety functions offer and their implications and to ensure that these match the real needs of the application. Buyers need to clearly understand and define their safety needs in advance (by risk assessment) and select product components that meet those needs. The best drives vendors are prepared to offer support and advice to help users get the drive that meets their needs with the correct safety functionality to meet their risk assessment, ensuring that safety is always the first priority.
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