73
Safe Torque Off (STO)
3ADW000488R0101 DCS880 Service manual e a
Safe Torque Off (STO)
Contents of this chapter
This chapter describes the acceptance test, repetitive function test and the STO revalidation test.
Note:
This is not the original operation manual of the STO function for the DCS880.
Description
The STO function can be used, for example, to construct safety or supervision circuits that stop the drive in case
of danger (such as an emergency stop circuit). Another possible application is a prevention of unexpected start-
up switch that enables short-time maintenance operations like cleaning or work on non-electrical parts of the
machinery without switching off the power supply to the drive.
When activated, the STO function disables the control voltage of the power semiconductors of the drive output
stage, thus preventing the drive from generating the torque required to rotate the motor. If the motor is running
when STO is activated, it coasts to a stop.
The STO function has a redundant architecture, that is, both channels must be used in the safety function
implementation.
The STO function of the drive complies with these standards:
Standard
Name
EN 60204-1:2006 + AC:2010 Safety of machinery – Electrical equipment of machines – Part 1: General
requirements
IEC 61508-1:2010
Functional safety of electrical/electronic/programmable electronic safety-
related systems – Part 1: General requirements
IEC 61508-2:2010
Functional safety of electrical/electronic/programmable electronic safety-
related systems – Part 2: Requirements for
electrical/electronic/programmable electronic safety-related systems
IEC 61800-5-2:2016
EN 61800-5-2:2017
Adjustable speed electrical power drive systems – Part 5-2: Safety
requirements – Functional
IEC 62061:2015
EN 62061:2005
+AC:2010+A1:2013+A2:2015
Safety of machinery – Functional safety of safety-related electrical, electronic
and programmable electronic control systems
EN ISO 13849-1:2015
Safety of machinery – Safety-related parts of control systems – Part 1:
General principles for design
EN ISO 13849-2:2012
Safety of machinery – Safety-related parts of control systems – Part 2:
Validation
The function also corresponds to Prevention of unexpected start-up as specified by EN 1037:1995 + A1:2008
and Uncontrolled stop (stop category 0) as specified in EN 60204-1:2006 + AC:2010.
Operation principle
1. The STO activates when the activation switch is opened, or safety relay contacts open.
2. The STO inputs on the drive control unit de-energize (low active).
3. The control unit cuts off the firing pulses from the drive thyristors.
4. The control program generates an indication as defined by 31.22 STO indication run/stop (refer to
).
5. Motor coasts to a stop (if running). The drive cannot restart while the activation switch or safety relay
contacts are open. After the contacts close, a new start command is required to start the drive.
