Largest LV Motor On VFD 3

[HamidEle]

Would like to start the discussion on selection of motor voltages.

The IEEE 841 standardizes the use of the LV motor for 500hp and below. Say the motor on VFD, the upstream is a breaker as protection. we could go to a much larger size than 500hp as per the conversation with a motor manufacturer. LV motor means lower cost. same with VFD, LV VFD would be about half a price of MV VFD. This could be a high saving for customers. Understanding the motor cable costs would be higher at LV level, but it would be a lot costly to go with LV. Would like to see pros and cons before we make decisions. Highly appreciate it for any inputs

 

[electricuwe]

High power drives on low voltage are only an option if you have transformer, inverter and motor very close together (within a few meters of distance): Otherwise you will spent a mutiple of the money you save on the components for cables and losses later.

 

[JezNZ]

As electricuwe put it, it's very simple. Just do the cost analysis.

Have seen both. Usually MV where there's banks of motors and MCCs etc, seperate plant rooms. Usually LV where it's a package skid or all equipment in dedicated room/annex for single equipment.

 

[Gr8blu]

HamidEle: All rotating machines have "optimum" operating voltage that is a function of power throughput, speed, and distance from supply. A general consensus is that line current should not exceed about 800 A (roughly the limit of a typical bolted bus joint connection), although some push a bit higher to around 1000 A. As current goes up, the cost of infrastructure (number and size of cables, interrupting rating of switchgear, sizing of other current-related protective elements, etc.) also rises.

All drives are priced based on power output - meaning a combination of voltage AND current. You might well find that the "low cost" drive limits the current output to an unacceptably low level, or that the drive topology produces unacceptably high harmonic content on the input or output (or both) sides of the drive. Both of these can adversely affect your operations (limitation on torque-to-process, or control-at-speed) and other infrastructure (cable runs, for example).

Lastly - how much "brains" do you need your drive to have? Often, an MV drive has more built-in capability (since the manufacturer is already offering a high-end device) which can save on other ancillary equipment that would need to be added for the LV option.

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[HamidEle]

Thanks for your inputs. I think I lean more towards to Gr8blu's thought and stick with IEEE 841 limits for LV Motors, which is 500hp.
In terms of harmonic, they can build LV VFD similar to MV VFDs to handle the harmonic issue.

 

[LionelHutz]

Not yet mentioned is the infrastructure, tools and knowledge available at the installation location. If the locations is low voltage only then it makes little sense to do a single medium voltage motor installation. That will require a new transformer and main switchgear as well as new tools and training to work on it.

I think the largest low voltage motor we've done was 2700hp, but it was a soft-starter not a VFD. The decision was due to the factors I mentioned making LV the only choice. VFD's for the 700-800hp range are readily available and really no big deal. The offerings thin out above that, but there are still options available.

 

[jraef]

I just went through an exercise on a 1200HP project last week. LV motors are existing, LV VFDs will be in the neighborhood of $250k US each. 4160V VFDs are $185k, new MV motors are $30k. So in that case it is less expensive to REPLACE the motors and go with a MV drive.

At 500HP I would agree that MV will cost more, but there is an inflection point somewhere between 600HP and 1200HP where the script gets flipped.


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

 

[HamidEle]

Thanks for all for the valuable inputs. I did a rough cost analysis and came up with a infection point where any motor larger than 600hp. MV option is more costly. Again, this was based on our layout. It could varies between projects, Suggest to do your check based on your project.

Thanks again.