Two power supply for one motor

[zaza123]

Hi!

We have a VFD run Motor which is critical for plant.The Motor is supplied from Normal 480 V power supply & Switchgear panel has back up of emergency diesel generator.Operation is asking to also connect the motor with the Emergency switchgear from another plant.i.e Motor shall have two incomers & can be switched on either as per situation.
My questions;
1. Is it justified for the equipment which has already backup to provide another back up, i,e will it not reduce the reliability of the equipment.
2.Is it Technically possible to design such a system wherein two incomers one VFD & one motor.Maybe using selector swicth for power supply, but what about the control.

Thanks,

 

[DRWeig]

Hi zaza123,

Well, the justification really needs to come from operations -- we don't know anything about your process. Sometimes critical parts are really critical.

I'm familiar with one plant in which the critical motor-driven loads have three sources of electrical supply, plus an idling steam turbine on each shaft. Such redundancy was justified based on safety and the extreme high cost of an unplanned shutdown.

Even though each backup system adds complexity and additional points for failure, the overall security can be improved if the design is done carefully.

As for technical possibility -- anything is possible. Same with controls. A reliable set of PLCs and transfer switches may be arranged to handle any situation.

Wait to hear from others in this forum -- someone may have very relevant experience to share.

Good on ya,

Goober Dave

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[jraef]

Excellent points, we cannot offer meaningful comment on your necessity for redundancy, that's a high level discussion that usually has a lot to do with budgets and down time cost comparisons.

But in your grand scheme as stated, isn't it a bit on the silly side to focus totally on the supply source redundancy and then have only one way to power the motor. i.e. the VFD? What happens if the VFD goes down, regardless of where the power terminals get their voltage from?

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[MiketheEngineer]

And what if the the "WHOLE" city goes down - i.e. no power from either source. Batteries??? Probably - but they only last so long...

And yes you need backup circuits in case of VFD/line/fuse faults.

 

[gvasiliou]

In my experience, when we have to deal with critical loads , there is no limit in redundancy....
I have worked in application with 3 or even more back ups (UPS, Stby GenSet, Second GenSet standby when all the rest fails).

jraef has a point... you better to take care for redundancy on critical parts that affect the operation of your motor, like back up VFD in case main VFD fails.

As a matter of "controlling the process" is possible and can be designed simple enough with a couple of timers&reles or even more sophisticated with PLCs (allow some manual operation in case that automation scheme fails).

BR