Danfoss VLT FC 102 Series Design Manual page 21

Introduction to VLT® HVAC D...
Paralleling of Safe Torque Off inputs via one safety relay
Safe Torque Off inputs T37 (STO) may be connected
directly if the situation is required to control multiple
adjustable frequency drives from the same control line via
one safety relay (see Figure 2.6). Connecting inputs
increases the probability of a fault in the unsafe direction,
since a fault in one adjustable frequency drive might result
in all adjustable frequency drives becoming enabled. The
probability of a fault for T37 is so low, that the resulting
probability still meets the requirements for SIL2.
FC
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37
Figure 2.3 STO Example
FC
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18
37
Figure 2.4 SS1 Example
FC
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37
K1
Figure 2.5 STO Category 4 Example
MG11BC22
Design Guide
3
1
2
3
1
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3
K1
1
K1
2
Danfoss A/S © Rev. 06/2014 All rights reserved.
FC
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4
12
1
20
37
FC 2
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37
FC
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37
Figure 2.6 Paralleling of Multiple Adjustable Frequency
Drives Example
1 Safety relay
2 Emergency stop button
3 Reset button
4 24 V DC
Table 2.4 Legend to Figure 2.3 to Figure 2.6
WARNING
Safe Torque Off activation (i.e., removal of 24 V DC
voltage supply to terminal 37) does not provide
electrical safety. The Safe Torque Off function itself is
therefore not sufficient to implement the Emergency Off
function as defined by EN 60204-1. Emergency Off
requires measures of electrical isolation, e.g., by
switching off line power via an additional contactor.
1. Activate the Safe Torque Off function by
removing the 24 V DC voltage supply to the
terminal 37.
2. After activation of Safe Torque Off (i.e., after the
response time), the adjustable frequency drive
coasts (stops creating a rotational field in the
motor). The response time is typically shorter
than 10 ms for the complete performance range
of the adjustable frequency drive.
2 2
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