Diagnostics Optimization CT MODBUS RTU Technical Data
E300 Design Guide
105
Issue Number: 1
Note on response time of Safe Torque Off (STO), and use with safety controllers with self-testing outputs.
Safe Torque Off (STO) has been designed to have a response time of greater than 1 ms, so that it is compatible with safety controllers whose outputs
are subject to a dynamic test with a pulse width not exceeding 1 ms.
Note on the use of servo motors, other permanent-magnet motors, reluctance motors and salient-pole induction motors.
When the drive is disabled through Safe Torque Off (STO), a possible (although highly unlikely) failure mode is for two power devices in the inverter
circuit to conduct incorrectly.
This fault cannot produce a steady rotating torque in any AC motor. It produces no torque in a conventional induction motor with a cage rotor. If the
rotor has permanent magnets and/or saliency, then a transient alignment torque may occur. The motor may briefly try to rotate by up to 180°
electrical, for a permanent magnet motor, or 90° electrical, for a salient pole induction motor or reluctance motor. This possible failure mode must be
allowed for in the machine design.
With Safe Torque Off (STO) there are no single faults in the drive which can permit the motor to be driven. Therefore it is not necessary to have a
second channel to interrupt the power connection, nor a fault detection circuit.
It is important to note that a single short-circuit from the Safe Torque Off (STO) input to a DC supply of approxi24 V would cause the drive to
be enabled. This can be excluded under EN ISO 13849-2 by the use of protected wiring. The wiring can be protected by either of the following
methods:
•
By placing the wiring in a segregated cable duct or other enclosure.
or
•
By providing the wiring with a grounded shield in a positive-logic grounded control circuit. The shield is provided to avoid a hazard from an
electrical fault. It may be grounded by any convenient method; no special EMC precautions are required.
Safe Torque Off (STO) over-ride
The drive does not provide any facility to over-ride the Safe Torque Off (STO) function, for example for maintenance purposes.
Emerson Control Techniques provide a zero output motor contactor solution which meets EN81-1 (clause 12.7.3) and EN81-2 (clause 12.4). For
further details contact the supplier of the drive.
The design of safety-related control systems must only be done by personnel with the required training and experience.
The Safe Torque Off (STO) 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 an acceptable level for the
application.
Safe Torque Off (STO) inhibits the operation of the drive, this includes inhibiting braking. If the drive is required to provide both braking
and Safe Torque Off (STO) in the same operation (e.g. for emergency stop) then a safety timer relay or similar device must be used to
ensure that the drive is disabled a suitable time after braking. The braking function in the drive is provided by an electronic circuit which is
not fail-safe. If braking is a safety requirement, it must be supplemented by an independent fail-safe braking mechanism.
Safe Torque Off (STO) does not provide electrical isolation.
The supply to the drive must be disconnected by an approved isolation device before gaining access to power connections.
It is essential to observe the maximum permitted voltage of 5 V for a safe low disabled) state of Safe Torque Off (STO) 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 (STO) circuit be provided with a dedicated 0 V conductor which should be connected to terminal
30 at the drive.
WARNING
WARNING
WARNING
WARNING
NOTE
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