In Siemens updated proposal, each ventilation fan motor will connect a VFD for back up mode operation.Please find the attached draft proposal.1.From CEM, electric resonance should be checked. Please find the VFD information from Siemens.2.For back-up mode operation, PLC should change over from normal operation (star-delta) to back-up operation using VFD to operate ventilation fan at 60Hz.**We suggest to have a short discussion about this issue within today.