1 VFD, 2 motors

dginbuffalo

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Dec 2010
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Buffalo,NY
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Anyone ever use one vfd for two motors such as a primary pump and backup?
They don't need to run at the same time, just alternate. I was thinking of using a mechanically interlocked contactor so neither could energize at the same time. Thoughts?
 
I can't see any reason why you shouldn't be able to do this, maybe it should be interlooked so the VFD is in stop before changing between the pumps.
 
I've done it on 2,000 hp medium voltage (4160 VAC) blowers. We went a step further, and had synchronous transfer. After one blower was up to speed it synchronized with line voltage and switched the running blower to across the line operation. The VFD was then available for starting and controlling a second blower.

This is a standard system from many manufacturers. The one I did used Toshiba, but Allen Bradley and ABB have it too. You can buy the package with or without synchronous transfer.

Having said that, nowadays it probably isn't worth screwing around trying to use one VFD. By the time you get done with the interlocked contactors. engineering, disconnects and so on it is probably more cost effective to buy two VFDs. The system I did was quite a few years ago, when medium voltage VFDs were very expensive. At today's prices, for low or medium voltage and for most hp ranges VFDs are pretty economical.
 
In addition to cost another consideration is what happens if the VFD has a problem. With two you have a backup that you can run while it is being repaired, with one you don't.
 
In addition to cost another consideration is what happens if the VFD has a problem. With two you have a backup that you can run while it is being repaired, with one you don't.

Yes, it is not a redundant system, but it is quite ok to be able to make a regular service for one pump, and after that, the other if you plan it good..

dginbuffalo, we do this very often, no problems if you take care of stopping before changeing.. Use the run output, not the set time, it is nicer that way, and interlock the contactors so they can't be on on the same time and you're good to go.. :)
 
In addition to cost another consideration is what happens if the VFD has a problem. With two you have a backup that you can run while it is being repaired, with one you don't.

This is an excellent point worth thinking about hard. It's not about the redundancy of one pump vs two, it's that if the VFD goes down, you can't run EITHER pump. Weigh the added cost of a 2nd drive (minus the interlocking contactors) against the cost of down time while waiting for a replacement drive. You may find the risk outweighs the benefits.
 
Depending of the VFD you are using. You can have multiple motor parameters sets. so when you switch from one motor to the other, you can activate your second set to control you motor safely if it's not using same overload/voltage etc.
 
Don't!

One motor is running and not both and the current is set for 2 motors - no protection.

Drive dies and lose both motors.

Not worth it.

I hate cutting corners like this and will not do it any more.
 
There is nothing "improper" or "cheap" about doing this as long as the failure modes and associated risks are acceptable to your operations.

If you decide to do this, as mentioned above, choose a brand and model that provides for two different modes of operation and two corresponding different sets of motor nameplate data. That way, the different motors can be properly protected and managed by the one VFD.

Tom Jenkin's comments about cost need to taken seriously, though. It may not save as much money as it seems initially. On the other hand, there will be some cost involved controlling the switch between the two motors even with a common VFD. Just do your "due diligence" analysis and you'll be ok
 
If you decide to do this, as mentioned above, choose a brand and model that provides for two different modes of operation and two corresponding different sets of motor nameplate data. That way, the different motors can be properly protected and managed by the one VFD.

It is also possible to protect motors with correct overload relay and contactor after drive.
 

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