ABB ACS880-04FXT, Hardware Manual

Page: 174 / 196

Operation principle
1. The Safe torque off activates (the activation switch is opened, or safety relay contacts
open).
2. The STO inputs of the drive control unit de-energize.
3. The control unit cuts off the control voltage from the output IGBTs.
4. The control program generates an indication as defined by parameter 31.22 (see the
firmware manual of the drive).
The parameter selects which indications are given when one or both STO signals are
switched off or lost. The indications also depend on whether the drive is running or
stopped when this occurs.
Note: This parameter does not affect the operation of the STO function itself. The STO
function will operate regardless of the setting of this parameter: a running drive will stop
upon removal of one or both STO signals, and will not start until both STO signals are
restored and all faults reset.
Note: The loss of only one STO signal always generates a fault as it is interpreted as
a malfunction of STO hardware or wiring.
5. The 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 reset may be needed
(depending on the setting of parameter
31.22 ). A new start command is required to start
the drive.
174 The Safe torque off function