Danfoss VLT FC 280 Operating Manual page 71

Operating Guide | VLT® Midi Drive FC 280
Sometimes, it is difficult to determine the bus transfer time that is used to calculate the watchdog time. For more information on the
cycle times, see the user manuals of the specific fieldbus.
The F WD time can be calculated via the following formula:
Table 33: Description of the Parts of the Watchdog Time
Symbol
DAT
HAT
BT
The F-parameter F WD time must have a value that is slightly greater than the sum of DAT, HAT, and 2 times the bus transfer time. It is
recommended not to exceed the calculated value by more than 30%. Setting a shorter watchdog time does not have an effect on the
safety of a system, but it can cause a fault and make the AC drive trip. For example, if HAT is 4 ms and the PROFINET cycle time is 4 ms, F
WD time should be set to:
If there is extreme electromagnetic interference, the communication systems use retry mechanisms to increase the robustness
of the system. Before setting the F WD time, it is recommended to find the number of retries of each connection and adjust the
minimum watchdog time if necessary.
PROFIsafe Safety Function Response Time (SFRT)
6.5.2.5
PROFIsafe specifies a safety function response time (SFRT), during which the safety system must react to a fault in the system. The SFRT
includes all individual delays, including the bus transfer times. All of these elements have minimum and maximum delays, and the actual
delay is likely to be somewhere in between these values. For safety reasons, every communication cycle has its own watchdog time
WDTime after which the safe state is activated if a fault occurs in that communication cycle.
i
Calculate the safety function response time via the following formula:
SFRT=Safety Function Response Time
WCDT =Worst Case Delay Time of entity i
i
WDTime =Watchdog Time of entity i. See
i
Adding the worst case delay times to the components of the safety system gives the total worst case delay time. See
Table 34: Time Parameters
Device
The complete AC drive system
Danfoss A/S © 2024.10
Name
Device Acknowledgment Time
Host Acknowledgment Time
Bus Cycle Time
6.5.2.4 PROFIsafe Watchdog
Worst case delay time
120 ms
Description
70 ms for the complete AC drive system.
Application-specific.
The bus cycle time.
NOTICE
Time.
Safety Functions
Table
Watchdog time
Recommended 250 ms or larger
AQ381425076031en-000201 / 130R1223 |
34.
71