Safety
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Unidrive SPM User Guide
99
Issue Number: 3 www.controltechniques.com
Frequency slaving outputs (open loop only)
Ab, Fd, Fr, SC, SC.HiPEr, EndAt, SC.EndAt, SSI and SC.SSI
encoders
Common to all Encoder types
6.19 SAFE TORQUE OFF (SECURE
DISABLE)
The SAFE TORQUE OFF (SECURE DISABLE) (STO) 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 STO 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 behaviour of the
inverter circuit. All credible faults in the inverter power circuit cause a
loss of torque generation.
The STO function is fail-safe, so when the STO 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. STO is also independent of the drive firmware.
This meets the requirements of EN954-1 category 3 for the prevention of
operation of the motor.
1
On drives with date code P04 and later the STO
input also meets the requirements of EN 81-1 clause 12.7.3 b) as part of
a system for preventing unwanted operation of the motor in a lift
(elevator).
2
1
Independent approval has been given by BGIA.
2
Independent approval of concept has been given by TÜV. Please
consult the separate guide for lift applications for further information.
STO can be used to eliminate electro-mechanical contactors, including
special safety contactors, which would otherwise be required for safety
applications.
Note on response time of SAFE TORQUE OFF (SECURE DISABLE),
and use with safety controllers with self-testing outputs (drives
with date code P04 and later).
SAFE TORQUE OFF (SECURE DISABLE) has been designed to have a
response time of greater than 1ms, so that it is compatible with safety
controllers whose outputs are subject to a dynamic test with a pulse
width not exceeding 1ms. For applications where a fast-acting disable
function is required, please see section section 13.21.10
Fast Disable
on
page 262.
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 (SECURE
DISABLE), 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.
The following diagrams illustrate how the STO input can be used to
eliminate contactors and safety contactors from control systems. Please
note these are provided for illustration only, every specific arrangement
must be verified for suitability in the proposed application.
In the first example, illustrated in Figure 6-45, the STO function is used
to replace a simple power contactor in applications where the risk of
injury from unexpected starting is small, but it is not acceptable to rely on
the complex hardware and firmware/software used by the stop/start
function within the drive.
7
Frequency slaving out channel A
8
Frequency slaving out channel A\
9
Frequency slaving out channel B
10
Frequency slaving out channel B\
Type
EIA 485 differential transceivers
Maximum output frequency
512kHz
Absolute maximum applied voltage
relative to 0V
±
14V
Absolute maximum applied differential
voltage
±
14V
13
Encoder supply voltage
Supply voltage
5.15V
±
2
%, 8V
±
5% or 15V
±
5%
Maximum output current
300mA for 5V and 8V*
200mA for 15V*
The voltage on terminal 13 is controlled by Pr
3.36
. The default for this
parameter is 5V (0) but this can be set to 8V (1) or 15V (2). Setting the
encoder voltage supply too high for the encoder could result in damage
to the feedback device.
The termination resistors must be disabled if the outputs from the
encoder are higher than 5V.
14
0V common
15
Motor thermistor input
This terminal is connected internally to terminal 8 of the signal
connector. Connect only one of these terminals to a motor thermistor.
Analog input 3 must be in thermistor mode, Pr
7.15
= th.SC (7), th (8) or
th.diSP (9).
The design of safety-related control systems must only be
done by personnel with the required training and experience.
The 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.
To maintain category 3 according to EN954-1 environment
limits given in section 14.1
Drive
on page 263 must be
observed.
STO inhibits the operation of the drive, this includes
inhibiting braking. If the drive is required to provide both
braking and SAFE TORQUE OFF (SECURE DISABLE) 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.
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.
WARNING
WARNING
WARNING
WARNING
