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Drives’ built-in safety functions give machine builders more scope

Safety functionA range of built-in safety features for variable-speed drives (VSDs) that make it easier to achieve safe operation of processes and protect maintenance staff working on motor driven machinery is introduced by ABB.

A built-in safe torque-off (STO) function is provided. This temporarily cuts the voltage to the motor without the need to shut down the drive. Safe cleaning and maintenance of processing machinery can then be carried out, while the drive remains ready to resume normal operations.

The STO function conforms to the machinery directive and offers a dual channel safety interface that meets the dual redundancy requirements of SIL 3/PL e (Safety Integrity Level 3/Performance Level e). The most practical way of achieving PL e is to employ continuous testing or monitoring techniques in the safety circuit (test pulses). This technique is not achievable using mechanical or electro mechanical components such as contactors or relays as the continuous testing is not possible.

TUV certified, the STO function allows machine builders to interface machinery directive compliant (EN IEC 62061 and EN ISO 13849-1) emergency stop functions, without the need for contactors or safety relays.

The STO function is also ATEX certified, allowing it to be used with confidence in processes operating in explosive atmospheres when interfaced to an ATEX safety relay.

Further safety features are available by including the functional safety option module, allowing the drive to meet certified requirements under the Machinery Directive. These are: SS1 (Safe Stop 1), which ramps the controlled motor down to zero speed; SSE (Safe Stop Emergency), emergency stop ramped; SBC (Safe Brake Control), used for proper safe management of a crane brake; SMS (Safe Max Speed), typically used on centrifuges to prevent damage; and SLS (Safe Limited Speed), which ensures safe limited speeds for motions like jogging and threading.

Programming of the functional safety option module is parameter driven. This makes it easier to build compliant safety systems, as it removes the need for complex and expensive programmable safety relays. A totally separate speed feedback signal within the drive, a feature only available in an ABB drive, achieves SIL 3 and means the module's functions can be performed without the need for expensive feedback devices such as encoders.

The functional safety option module is available as a separate kit, as well as a pre-purchased option. This is a major advantage for engineers, allowing them to retrofit VSDs in the field as their guarding or function changes.

Further IEC 61800-5-2 drive related safety functions are available on a second module derivative: SSM (Safe Speed Monitor), SS2 (Safe Stop 2), SOS (Safe Operating Speed), SAR (Safe Acceleration Range) allow engineers to build different styles of safety related installations.

Secure, authorised configuration of safety functions is achieved using the Drive Composer Pro tool. Parameter edits are made within a special safety configuration area of the Composer Pro tool. The parameter configuration is loaded to the safety module internally via the standard PC tool connection and is password protected.

The entry level tool comes free to users and can be used to view parameter edits but not change them. Parameters can also be viewed via the keypad.

Expensive hardwiring can be avoided by using PROFIsafe, the safe version of the fieldbus system Profibus. The ABB AC500 PLC can be purchased as a safety PLC and has safety I/O and PROFIsafe support and can therefore be combined with a PROFIsafe equipped drive to design and build a complete system.

The drive itself communicates with the internal functional safety option module via a standard Profibus module that supports PROFISafe (black safety channel), cutting unnecessary expense while still maintaining the highest safety levels.

ABB can also supply a certified safety encoder interface for systems that require encoder feedback and checking for absolute safety. These are often systems where people are involved, for example, cranes and elevators, or where there is a significant risk of overhauling loads.

The new PC-based safety design tool will allow process engineers to combine ABB components such as safety PLCs and VSDs and determine if a system achieves the desired safety rating. The tool also assists with the upfront risk assessment and determination of the "target" safety level required for the system.

All these features are incorporated in or will work with the ABB industrial drive, ACS880.


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