Safe Torque Off (Sto) - Control Techniques Unidrive M300 Quick Start Manual

• A shielded control cable, with shield clamped to the grounded metal panel
Full instructions are given in the Power Installation Guide.
A full range of external EMC filters are also available for use with Unidrive M300/HS30, shown in the
Power Installation Guide.
4.6

Safe Torque Off (STO)

The Safe Torque Off function provides a means for preventing the drive from generating torque in the
motor with a very high level of integrity. It is suitable for incorporation into a safety system for a
machine. It is also suitable for use as a conventional drive enable input.
The safety function is active when either one or both STO inputs are in the logic-low state as
specified in the control terminal specification. The function is defined according to EN 61800-5-2 and
IEC 61800-5-2 as follows. (In these standards a drive offering safety-related functions is referred to
as a PDS(SR)):
'Power, that can cause rotation (or motion in the case of a linear motor), is not applied to the motor.
The PDS(SR) will not provide energy to the motor which can generate torque (or force in the case of
a linear motor)'.
This safety function corresponds to an uncontrolled stop in accordance with stop category 0 of IEC
60204-1. The Safe Torque Off function makes use of the special property of an inverter drive with an
induction motor, which is that torque cannot be generated without the continuous correct active
behavior of the inverter circuit. All credible faults in the inverter power circuit cause a loss of torque
generation.
The Safe Torque Off function is fail-safe, so when the Safe Torque Off input is disconnected the drive
will not operate the motor, even if a combination of components within the drive has failed. Most
component failures are revealed by the drive failing to operate. Safe Torque Off is also independent
of the drive firmware.
The design of safety-related control systems must only be done by personnel with the
required training and experience. The Safe Torque Off function will only ensure the safety
of a machine if it is correctly incorporated into a complete safety system. The system must
be subject to a risk assessment to confirm that the residual risk of an unsafe event is at
WARNING
an acceptable level for the application
Safe Torque Off does not provide electrical isolation. The supply to the drive must be
disconnected by an approved isolation device before gaining access to power
connections.
WARNING
It is essential to observe the maximum permitted voltage of 5 V for a safe low (disabled)
state of Safe Torque Off. The connections to the drive must be arranged so that voltage
drops in the 0 V wiring cannot exceed this value under any loading condition. It is strongly
recommended that the Safe Torque Off circuits be provided with a dedicated 0 V
WARNING
conductors which should be connected to terminals 32 and 33 at the drive.
For more information regarding the Safe Torque Off input, please see the Control User Guide.
Unidrive M300/HS30 Quick Start Guide
Issue Number: 7
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