VSDs with safety built in

Paul Boughton

A 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 has been 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.