Single Isolation Transformer For 2 Drives

sorry, there is a small correction in the above post. Both drives do need 420vac, 3phase, input & the isolation transformer is of 420vac / 420vac.

It would be a good idea to check the data sheets and installation manuals for the VFD and servo amplifier drives first as there should be information in there advising you of suitable filtering needs depending on the drive, application, motor cable length, etc and whether or not the drive needs individual AC reactors for isolation.

As for using an isolating transformer are you using a three phase in three phase out type? If so this seems a little excessive and expensive over, say some three-phase (or three individual single phase) line inductors. Again it depends on your application and how much electrical noise and harmonics you will need to filter (to both protect the drives from and limit the drives effect on any other sensitive plant material).

Can you provide more information on your application including the drives you are planning on using? Additionally how's your site power quality?

I would suggest using an isolation transformer for each drive. You are correct in stating that an isolation x-form is the best protection from voltage spikes/surges. The usage of reactors on the load side of each drive will help prevent the drives from introducing noise/harmonics back on to the line side of your power system. Also most line reactors designed for voltage suppresion use MOV's to do so. The MOV's work well untill you get a spike over their rated voltage, then they blow up. Buy a spare reactor so if this happens you can be back up and running quickly (bosses are now happy!) while you have the bad part repaired.

Two individual 1.5Kw drives is a pretty small load for a 420VAC line. Assuming your line is reasonably "stiff" (low to normal source impedance), it is likely that you do not need any filtering or isolation at all (although it will not hurt and won't be too expensive at this power). Harmonic filtering is done for the benefit of other equipment on the branch circuit, the drives themselves don't care because of the large DC link caps. Other equipments proximity and how susceptible they are to conducted noise are factors as well. Electronic devices are not usually a problem because transformers and power supplies are between them and the line, but lighting can be a problem. I've seen whole factories buzz comming from all the lighting because of VFD's, but this was at much higher power levels.