ABB ACH480, Hardware Manual

Page: 189 / 214

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.
The Safe torque off function 189