Parallel VFD's

Morning All, I don't post much but this time I have a conondrum which, so far, has escaped me. Maybe someone has seen this type arrangement before.

I have a customer which will have a total of 144, 50 hp motors used for cooling and exhaust. Due to the nature of this installation, customer wants redundancy on everything.

They are requesting 2 VFD's for each of these motors, and that the VFD's be able to run simultaneously, in a parallel mode, with each VFD powered from seperate AC sources. In the event of a single power failure, they don't want the motors to even blip.

Customer has spec'd ABB ACH550 drives but according to ABB these are not capable of parallel operation from seperate sources.

Has anyone ever seen an arrangement like this? Any drive recommendations?

I have very few details since we are in the early stages of development. Not to mention I could give out very little if I had them due to NDA's.

I assume the incoming power is 480 vac, 3-phase, 60 Hz. Drives will have to be capable of Modbus RTU and/or Modbus TCP at a minumum.

Ideas?

TIA

I would throw it back in the customer's lap. Can't provide this redundancy feature with the equipment he specified. Unless you use some kind of contactor arrangement on the VFD outputs that switch the motor feed - pretty sure that would not be recommended by ABB.

That's pretty much what we have done and you are correct about the contactors. Our solution would be automatic transfer switches (ATS) on the incoming power but they don't like the idea due to complexity and cost.

rkicklighter said:

That's pretty much what we have done and you are correct about the contactors. Our solution would be automatic transfer switches (ATS) on the incoming power but they don't like the idea due to complexity and cost.

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!!!!!!!! But they want Parallel VFD's driving the same motor from different sources?????!!!!!!

Sorry to say, your customer is an idiot. Why am I thinking they've been talking to a consultant recently?

We use some parallel drives on some large HP like 600 - 800 HP but both drives are required for the load.

The drives have to be setup for this. There may be a way to run 2 drives in parallel in load shaze and if you lose 1 drive then the other will take up the load but both drives would have to be sized to handle the entire load.

Different suppliers do parallel load share drives in different ways. We use parker SSD drives in this configuration son that is the only brand that i have personal experience with.

As far as being supplied from different power sources i am not sure about. Becasue of the way a run of the mill drive rectifies power i would not think it would be a issue to use 2 different power sources. But i am not 100% sure about that.

rdrast said:

Sorry to say, your customer is an idiot. Why am I thinking they've been talking to a consultant recently?

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You hit it square on the head! From what little I know, all this comes from some consultant! From reading part of the specs, you would think we're building a apace craft!

The Plc Kid said:

As far as being supplied from different power sources i am not sure about. Becasue of the way a run of the mill drive rectifies power i would not think it would be a issue to use 2 different power sources. But i am not 100% sure about that.

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This place is fed from totally seperate feeders (notsure how many) plus will have some huge UPS's in it. To seperately power 2 VFD's from different feeders, looks like something would have to sync the AC phasing. Not sure since I have never seen anything like it. All I know is ABB says not possible with their drives.

When this concept was first mentioned, I just stood there with that !WTF! look on my face.

And adding an redundant motor per drive shaft is not possible?

The only thing i can think of is with a relay connected to the first drives output with run signal and if it disappear you switch over to the other drive with a contactor and "flycatching" mode on the drive but its probably not fool proof.

The Plc Kid said:

We use some parallel drives on some large HP like 600 - 800 HP but both drives are required for the load.

The drives have to be setup for this. There may be a way to run 2 drives in parallel in load shaze and if you lose 1 drive then the other will take up the load but both drives would have to be sized to handle the entire load.

Different suppliers do parallel load share drives in different ways. We use parker SSD drives in this configuration son that is the only brand that i have personal experience with.

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Load sharing is one thing, and requires comms between the drives. Separate sources though implies two completely independent drives that will both try to fight to control the same motor.

You would still need fast comms between the two, to keep their outputs exact, which implies one must be a master, which means it's not a redundant system at all, as something (probably bad) will happen if you lose one drive.

Input sync isn't an issue if you intend only to parallel the DC Bus link, but that is only part of what is being asked for here. Even for a computer data center, the usual practice is to simply have a duplicate of every HVAC plant where required for cooling, and only run one at a time.

Parker SSD - /shudder.

rkicklighter said:

.......but they don't like the idea due to complexity and cost.

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Classically typical. They want redundancy everywhere but their nose wrinkles up at the cost.

tell them just build another plant the exact same as what they have. That will pass for redundant right?

I was happy with the Parker SSD. We did a few webhandling lines with them they seemed OK for that.

I am not an expert in this kind of things,but one fast ideea just got into my mind.How about an sistem where you use the same VFD for the motor,and think about using 2 DC LINK supplys for all VFD connected in parallel from 2 different sources.YOu can even use some UPS in 1 source side for backup,and the DC bus will be easy to switch with contactors(even Solid State solutions can be fast enough),and just redimension the capacitors on the DC to sustain the supply on power change.
At small scale can be compared with 2 batteries driving the same lamp,using an ups sort like sistem to change between batteries.
I hope the ideea can be usefull.

rkicklighter said:

This place is fed from totally seperate feeders (notsure how many) plus will have some huge UPS's in it. To seperately power 2 VFD's from different feeders, looks like something would have to sync the AC phasing. Not sure since I have never seen anything like it. All I know is ABB says not possible with their drives.

When this concept was first mentioned, I just stood there with that !WTF! look on my face.

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The power supply source should not matter. There is not phsing between the input and output of a drive. The input is rectified to dc and then output as controlled sine. That's why you can run a 3 phase motor with single phase power Phase order on the input of a drive does not effect output phase order on standard drives. Current source matrix drives yes it would make a difference but not on a standard drive.