480 volt single phase service

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donaldsullivan

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Current project is scheduled for 480V, 3 phase. The project loads are two 75HP pumps, one 40HP pump and lighting loads. The 75HP pumps are scheduled to operate on variable frequency drives, and the 40HP pump is scheduled to be across the line.

The utility company has gone from a $55,000 budget to a $120,000+ budget for 480 volt, 3 phase. They offer 480 volt single phase for no cost. I do not see any problems with 480 volt, single phase service and using derated VFD to get 480 volt 3 phase to the 75HP, and 40HP motors.

The project has a 350KW generator scheduled for instalation. It is a 12 lead generator. I talked with the factory about connecting it 480 volt single phase, and they said what? The vector analysis is the same for a 120/240 volt single phase 12 lead generator. I asked them to change the generator to 480 volt, single phase and they said that they don't make a single phase generator that big. They also said that they don't make a 480 volt, single phase generator either.

That started me thinking is 480 volt single phase service even used? I found a utility company here in Texas that supplie it, put only for municipal and highway lighting.

I hate to get a 240 volt single phase service convert everything over to 240 volt, 3 phase and eat all of the equipment that cant be returned.

Can't find anything in NFPA 70 2008. I don't understand what will be wrong with it, can anyone convice me either way?

Most of the equipment is either shipped or on the job.
 
I would think that you might have a number of problems here. The 480 motor that was going to be across the line won't run if its a three phase motor. Most VFD's that I have ever dealt with were 3p input. Some of the smaller ones were single p 240 input but these were for very small motors. With a 70 hp, 3p you are looking at an FLA of some where in the niegborhood of 80-90 amps at 480 volts. In a 2p ,3 wire system the current in the common conductor will be well over 100 amps. Don't see how you can change the input system without changing the hardware it will power.
 
Can you even get a 75hp sized VFD that will do 480 1phase in by 480 3phase out? I thought that was only the wee one's that did that?

Will they bring you in primary for free? Look into the cost of a substation section for your gear. Maybe less than 120K.
 
Ok, if you can get VFDs for all the motors that are certified for the HP output whilst running on single phase then you are home and dry. You'll need a VFD for the 40HP motor as well, as you dont have three phase to DOL it on.

Work out your single phase current load, and size the three phase ATS and downstream distribution for the single phase current. This means your panels will be roughly three times oversized compared to three phase normal practice.

So, on PoCo power you have one live and two dead phases, and it all runs single phase. On genset you have three live phases, so it'll all still work.

Watch out for the lighting loads, as they will be all on the main phase, and as such the genset has to have the additional capacity for that load on just one phase.

Sounds fun.
 
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You cannot run two 75hp and one 45hp 480 volt, 3-phase motors using a single phase 480 volt service.
The owner, the engineer, and the utility need to sit down together and work out how to get a 3-phase service.
This should have been done WAY before the pumps are on the dock.
db
 
Absolutely the _proper_ way to do this is with 480V 3 phase service. Better for you, and probably better for the POCO as well.

But if the POCO will provide 480V 3 phase for $120K more than the cost of single phase, then it certainly pays to consider what would be required for single phase operation.

A suitable VFD can certainly run from a single phase supply and drive a three phase motor. You would need to check with the manufacturer or supplier to find VFDs rated for this sort of use.

I am familiar with Hitachi, because we used some of their inverters on test rigs and needed to connect directly to the DC bus; they publish specs on all sorts of funny things to do with inverters, such as single phase supply and common DC busses; see http://www.hitachi.us/supportingdoc.../AN032404-1_Rev_A_Sizing_for_Single-Phase.pdf for single phase operation. I am certain that other manufacturers will have similar instructions or application notes.

VFDs of this size won't be cheap, but may pay for themselves if the pumps don't need to be run at full power...and not cheap is still inexpensive relative to $120K.

-Jon
 
Winnie - The first sentence in that Hitachi thing says:
"Although Hitachi does not offer inverters above 3 hp specifically sized and rated for single-phase operation, single-phase power can be safely used with larger 3-phase rated inverters, provided that care is taken to properly upsize and apply the inverter."

I don't think you're going to find any 75HP single-phase in/3-phase out drives.

Not sure how the money thing works here - Was the owner willing to pay $55k originally? Now it's $120k? Was the single-phase service always available for free? What percentage of the entire project budget is $55k, $65k, or $120K? Where will this alleged single-phase service come from - two legs of somebody else's 1500kva service? What will that do to them? Will Sydney ever reconcile with Bridgett?

They'll be buying a whole lotta headaches if they don't go with 3-phase.

I stand by my original response - Engineer, Owner, Utility - work it out.

db
 
DB,

Read a couple of paragraphs down "The rule of thumb Hitachi recommends is to start with the 3-phase motor?s nameplate full load amperage (FLA) rating and double it. "

I take this to mean that if you have a 3 phase motor with a nameplate of 90A, run by a Hitachi VFD, and you want to supply the VFD with a single phase source, then you need a VFD that is normally rated for 180A of output. Put another way (and more approximately), if you want to run a 75hp motor with single phase to the inverter, you need to buy a 150hp inverter. Given that off the shelf inverters can be had at well above 150hp, I don't see this as a deal killer.

I would take a document like this with a large grain of salt, and only go forward with a recommendation from an application engineer at the inverter supplier, but the above seem to be a good starting point.

I suspect that the reason that _single phase_ service is available is that this is something like an irrigation system (large pumps) in a rural area, with only a single phase of the distribution network nearby. But I am just guessing here. The POCO has its reasons.

All the above said, I strongly agree with you. All other things being equal, a proper 3 phase supply would be much better for a load such as this. Single phase + VFD is IMHO a 'work around' which is workable, not ideal.

-Jon
 
Winnie
Right - there are enough unknowns here to make it fun to play with.
Where is it? WHAT is it? Is it a 'critical' operation? Will people die if a pump fails?
How much of this facility is already built? You don't ususally put that kind of horsepower in a Tuffshed. There's got to be some serious concrete in the ground already if the pumps are on the way. Why is this an issue NOW? This should have been cleared up long ago.
The transfer to generator should be interesting.

If they do this single phase, and it works, I think I'll buy myself a Colorado steak dinner. If it doesn't work, I'll have some wine, too.
(that's what you call win win)
db
 
It is very common, in Wisconsin, for the utility to only bring in (2) of the distribution lines to an area, so regardless the utility primary voltage the best they can do is provide a single phase service. My buddy owns a machine shop 1.5 miles from the closest 3-phase, the utility says they will run the third phase if he pays for the conductors and labor.

The majority of VF drives take DC and convert it into a PWM 3-phase output. So as long as you can charge the DC bus correctly the drive will deliver its rated output. The trick is getting enough AC amps rectified to DC. If you supply a 3-phase VFD with only single phase you are limiting the rectifier to about 1/3 its capacity. So for a given amount of 3-phase output with a single phase input a VFD needs an over-sized rectifier which is why you see the suggestion to increase the drive size by X2.
 
Jim's right. It's fairly common in rural areas for the utilities to only run 1 phase 480V power when that's what you need.

He's also right that technically ANY 480V VFD can accept a 1 phase input. The actual value of the current rating of the rectifier is 1.732x the motor FLA, because that is the ratio you use for 1 phase to 3 phase conversion (the square root of 3). So although the motor will continue to pull its normal current at 3 phase, the SOURCE of that current is single phase so ALL of the amps must come through only some of the diodes. But we round up to 2x because you also need extra capacitance to smooth out the additional ripple in the DC link that will happen as a result of the rectification of only one phase.

But there is a catch.

Many VFD manufacturers build their units so "close to the bone" now that they must guard against a phase loss on the input side, so they put in a Phase Loss Detection Circuit. Some make it defeatable in programming for this very purpose, but many do not, especially on large HP drives. So you CANNOT use those drives on a single phase input. Just be very careful about which ones you buy; ask the vendors to put it in writing so they will be forced to take it back if they are wrong (spoken from experience here). A good way to filter them out is to download and read the manuals. Look at the Fault Codes to see if it shows Phase Loss (Line Loss, Line Failure etc.) as a trip function. If it does and you don't see any instructions as to how to defeat it, assume you cannot. If it doesn't, get that backup in writing because sometimes someone just forgot to translate that info from the original language.

So on your 75HP motor, assuming 104 FLA, you will need a VFD capable of 210A and on the 40HP, assuming 52FLA, you need one capable of 104A. Rough guesstimate, you are looking at around $20 worth of VFDs. But keeping in mind that you were going to buy the 104A drive anyway (for the 75HP if fed by 3 phase), the real difference is probably only about $13K; still a lot less than $120K! Then don't forget that your SCPDs on the 1 phase side need to be sized for the VFD maximum input current, not the motor FLA. So you are going to need at least a 400A service for this. Make sure that when you talked to the utility, they understood that minor detail; they have a tendency to overlook it as well.
 
jim dungar said:
It is very common, in Wisconsin, for the utility to only bring in (2) of the distribution lines to an area, so regardless the utility primary voltage the best they can do is provide a single phase service. My buddy owns a machine shop 1.5 miles from the closest 3-phase, the utility says they will run the third phase if he pays for the conductors and labor.
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With two lines and the primary neutral, they can deliver an open-Delta 3-phase secondary.
 
Put in a 400A single phase service, with a 400A 3-phase transfer switch. if you find the correct drives they won't care that sometimes they have single phase from the POCO and other times they are getting 3-phase from the generator.
 
Watch out for power factor running VFD with single phase. They first convert AC to DC and in a standard rectifier bridge you will be conducting only a portion of the cycle. The normal method to get around this is to boost the voltage using a high frequency boost converter that forces the input current to follow the voltage. Because of the overlap in three phase bridges, this isn't needed with three phase input but will be lacking if you only supply one phase.
 
LarryFine said:
I have never seen primary distribution run without a grounded system neutral, but I guess it's possible.
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They run it with (1) ungrounded and (1) grounded.
 
Why do 12/4 SO and 12/3 NM have the same number of conductors?

If someone described '2' conductor primary service, I have to admit that I would understand that as 2 phase conductors and an implied grounded conductor. But that really means _3_ wires.

I could see someone saying '2' conductor primary service, and meaning a phase conductor and a grounded conductor.

Of course, you could be looking at some version of 'single wire earth return' where you bring out two phase conductors and use the soil as the third conductor. Two wires, and with suitable transformers you could provide three phase service :)

-Jon
 
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