Pro
- Cheaper, maybe.

Cons
- Only 1 VFD so if it gives issues everything is down or you start the motors full-voltage.
- Much more complex, need all the interlocking and switching. Can be harder to lockout and to work on one motor while others are running.

It also has you disconnecting a driving VFD to switch it over which is generally proscribed.

Keith Cress
kcress - http://www.flaminsystems.com

Unless the VFD line synchronizes and does a closed transition. That's pretty common in medium voltage, I'm not sure, who by someone likely does it in low voltage too.

Hi Petronila.

Can you tell us what the failures of the motors have been - the reports by the repair shops? Also, what is the application?

If there is not a synchronizing of the VFD output frequency and phase with the mains supply with paralleling of VFD output with the mains for a short time for the transfer - for a bumpless operation (I think a project special feature by integrators requiring extra electrical equipment) - then there will be a 'bump' transfer.
The VFD output should be stopped and the output contactor opened then the bypass contactor closed. This takes a little time so the motor rotor could be much slower in rpm (and therefore frequency not forgetting slip) than the mains frequency. How much it slows down could depend on the application.
At the moment of closing the bypass contactor there will be a current inrush. Now this can depend on the delay time as the magnetic field in the motor may or may not have died and there is the question of the difference in phase angle. There have been some threads on this before, try https://www.eng-tips.com/viewthread.cfm?qid=187116.

If you are transfering the motors from VFD to mains constantly and have high current inrushes, you may be, basically, exceeding the number of starts per hour and your motors are failing for that reason as opposed to not (or were not) inverter duty motors?

Drivesrock

I have gone through this exercise multiple times over the past few decades, the results are ALWAYS the same.

1. It rarely works out financially to do it with LV drives, the complexity of making it work reliably will end up costing more than just buying multiple VFDs. The contactor / disconnect / protection scheme is more complex than some people seem to think, because unlike bypassing a Soft Starter, you cannot just parallel the bypass device to the VFD. So you have to either accept the risk of motor damage during an open transition, or add even MORE devices to the power circuit to protect the VFD when being truly paralleled to the line voltage. This was easier to do back in the days of Current Source Inverters, because the power devices used in the drive were more "forgiving", but with the advent of transistor based PWM drives, it is far far more easy to blow up the output transistors now. So the extra steps needed to protect them increase that cost of a synchronous transfer scheme to the point of not being viable (compared to using multiple drives).
2. It can sometimes make sense for MV drives because of the relative cost difference between MV drives vs MV motor control and LV drives vs LV motor control.
3. As Lionel says, the reliability issue looms larger than some people realize, even when it makes economic sense from a hardware standpoint. If you lose the one drive, you lose the ability to control EVERYTHING. You have ZERO redundancy in your system.

" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden