The nominal voltage parameter of ACS 800 AC drive not changing
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First of all, you should try to find out if the parameters have been blocked, and why.
Perhaps changing the parameters could be dangerous for your system.
Do not make changes unless you are sure that these cannot create dangerous conditions or other failures.
You can check parameters 16.02 and 16.03
Hope this help you
If you are using ACS800 for multimotor operation, then most probably you have selected SCALAR MODE (99.04=SCALAR) and that is the right mode in this case.
I'm not quite sure, but think that changing of parameters 99 is limited in that mode.
What is the type code of ACS800 - is that 400V or 220V drive ( ACS800-xx-xxxx-3 or ACS800-xx-xxxx-2)?
In case it is 220V model, it is not possible to select 99.05=400VAC - higher than input voltage !
In case not: Make a backup of parameters, try to set 99.04=DTC, try change 99.05 and go back to 99.04=SCALAR.
That is the only possible mode of operation when more than one motor is connected to the drive (frequency mode).