Off Topic: How to design a power supply for 26 Variable Speed AC Motor Drives?

[Lancie1]

Friends, I need advice on how to design and power up 26 Variable Frequency AC Motor Drives in an electrical room with 480 volt power available. They vary in size from 0.5 to 25 horsepower. Should I put them all in a Motor Control Center? How do I isolate & filter so that noise does not get back onto the 480 Volt power supply? What filtering devices should I use for this many drives? Is it more practical to buy individual wall-mount drive units, with an isolation power feed transformer?

 

[OkiePC]

I like the inverter mounted line filters. They keep noise down quite well. 26 of them would add up $.

If you don't have a lot of succeptible circuits in the substation, larger isolation transformers would probably be cheaper.
I use floor mounted models. They keep your coffee warm and are about the right height to spread out the prints on, or warm your butt in the winter. But they eat up a good chuck of floor space. They won't keep noise out of the panels, but they will keep it from backfeeding into your distribution system, and they're maintenance free.

I would suggest a controlled power up sequence. You don't want to charge all 26 capacitor banks at the same instant in time from the same supply especially if it's an isolation transformer. You may need several small ones, sized for each group of drives.

My preferred method for controlling drive output noise is to use shielded VFD rated cable. It catches and drains the high power noise at the source. Follow drive manufacturers' recommendations. Mine all call for draining to ground at both the drive and the motor. I have never had any problems doing this. I had one problem when an electrician forgot to land a shield at the drive end. It caused noise on my analog pressure 0-10vdc input. He attached the shield to ground with the drive running, and there was a 2 inch yellow spark as he landed it! The noise on my scope immediately vanished.

Just my limited experience
OkiePC

 

[Lancie1]

Thanks, Okie,

All suggestions are welcomed. The total HP is abut 200, so now I am looking at a floor-mounted Cutler-Hammer 210 HP isolation transformer.

I am going to consider buying the drives pre-mounted (expensive, but time is a big factor here) in a MCC section, then power the MCC from the isolation transformer. Everything else then would be in a separate MCC in the same electrical room.

Can I get the MCC manufacturer to build in some inverter line filters as your recommended?

Good point about the capacitor in-rush current. Each drive will need a separate disconnect, and I will be sure to tell them not to attempt powering on all of them at once! I will put that in the standing operating procedure.

The VFD shielded cable is a smart idea also. I think the budget can stand that, and it could save a bundle down the road.

 

[DickDV]

Lancie1, if you are going to buy an isolation transformer and the drives are all located close to each other, instead, buy a 12 pulse isolation transformer and split the drives into two groups of 100hp each. Feed one 100hp group from the wye secondary and the other 100hp group from the delta secondary.

Spec the drives, at least the bigger ones, with internal 3% line reactors.

You will have a system with equal to or better than 12 pulse harmonic suppression but without any of the drives being more than plain inexpensive 6 pulse drives.

As for motor lead noise control, that's a lot more complex and strongly dependent on the length of the motor leads. You'd have to give me a lot more specifics for me to say anything practical about that part of the inverter noise picture.

 

[Lancie1]

DickDV,

The motor leads will vary, from 60 to 150 feet. I have not done the detailed design yet, so that is about all I know about the lengths.

 

[rsdoran]

You do not necessarily have to use AB but they have software for developing MCC systems;
http://www.ab.com/software/pcp/downloads.html

 

[Lancie1]

Dick,
The isolation transformers I am familar with only have one secondary winding. What brand and models have two secondary windings?

 

[Lancie1]

Interesting you should mention that, Ron. You are talking about the "Center ONE" software, I assume? I downloaded that abut 2 weeks ago. I have been laying out different MCC's ever since. The program works well, but I have not yet found any way to put in nameplate lists, except type them in.

The process guys keep changing the equipment. I have been "done" about 4 times, so far...

 

[rsdoran]

A 12-pulse transformer has two 3 phase secondary windings. They are generally phase shifted 30º which allows for currents that combine in the primary winding, cancelling out much of the 5th, 7th and 11th etc. harmonics.
http://www.neeltran.com/rectifier.htm

I have not used the Center One software for a while, let me look again to refresh my memory. I think the software has a spreadsheet capability so you can "import" as needed.

 

[Lancie1]

Thanks, Ron. That is exactly what I need for this plant.

 

[darrenj]

The large computer rooms that i do spec hammond split core transformers...they are K13 rated for harmonics and have 2 outputs..However for the transformer to work properly each side need to be loaded about the same..They have a larger footprint and are twice the weight of a regular transformer so just keep that in mind when you budget for installs..

D