Three phase/ single phase transformer

[Walden]

I am to build a small electrical control panel which is to use a 230 volt control circuit supply. The incoming supply is a 480 volt: 3 phase 60 Hz: three wire system (no neutral). I have proposed a double wound transformer: 480/230 volt for the control circuit and other small single phase devices. The transformer will be rated at about 2kVA. My client has asked that the primary winding be three phase and the secondary single phase because, “It will help to balance the load across the phases.” I cannot see the point of this. The transformer will be bigger than required, unbalanced and more expensive than is needed. Am I wrong?

 

[jghrist]

You are not wrong. If load balancing is important, you will need a three-phase (primary and secondary) transformer. Then you could distribute the load among the three phases. Single phase secondary would mean that you have two cores and primary windings doing absolutely nothing except eating up some no-load losses.

A three-phase transformer would be less expensive than a special order three/single phase transformer.

 

[RalphChristie]

mpyke:

What kind of transformer did you propose?
2ph (L-L) transformer or a 3ph transformer? (on the primary side)

 

[Walden]

Client says three phases on the primary L1, L2 and L3 with a single phase on the secondary!

 

[DRWeig]

Hi MPyke,

Does your client claim to be an engineer? Sometimes it's fun to work the conversation around to where the client realizes he's talking about a silly idea or a non-existent product before you come right out and tell him so. It's a nice way to ease thru such a situation.

If he's claiming to be an engineer though, it can be even more fun to ask him to come up with a manufacturer of such a transformer.

jghrist had the best technical answer above. It's now a matter of finding a way to convince your client without making him look like a doofus.

Another option might be a three-phase UPS with a single phase out. You get your load balance that way, but at an extremely high price.

Let us know how it goes!

Old Dave

 

[rbulsara]

Consider this...The owner is right too. If the voltages you mentioned are correct you can have a (custom order) a 3 phase transformer with 480V delta primary and 415Y/230V seconday. Phase to neutral in the seondary will give you single phase 230V circuits, 3 of them and you can balance the load.

 

[rbulsara]

Or use multiple 480/230V trnasformers, alternately wired to phase A-B, B-C and C-A on the primay side. Which in fact gives you the same system as I mentioned in previous post.

 

[rbulsara]

I have a bigger question, why would a 2kVA single phase load would cause imbalance? Where?

 

[DRWeig]

Yah,

Pardon my earlier sarcastic reply... I just finished dealing with the exact same thing in which the client insisted that three phases in would equally share the load of a single (single load, that is) phase out.

Rbulsara has a good solution too. as long as your secondary loads are multiple. However, if your control circuit is not separable into several smaller loads, you cannot get balance across the phases.

Let us know how it turns out!

Old Dave

 

[busbar]

 
With a $4000 change order, you could fashion a 480V “3ø” to 240V 1ø transformer bank with three 480—120V 1ø transformers, connected in a delta—“Leblanc” connection, but it is a complete farce. For $5000, equally effective but maybe even more mystical is three 480—120/240V units connected delta—double-delta. There will be ~zero current on one of three primary phases, like that of the unconnected 480V phase using a 1ø 480—240V transformer.

Alternately, find a tactful way to tell your insistent client that has the intelligence of a lawn rake. 2kVA at 480V 3ø load is all of 2½ amperes. 1ø 480v is ~4A.

 

[busbar]

 
     

(Couldn't wait till April)

 

[bklauba]

Agreed, the client is uninformed, whether an engineer or not. The real Q is, how do you help the client avoid wasting money on a non-problem.

Set up a single phase transformer with about 2-3 times the VA req'd to drive the control circuitry's worst case load. Load it to the level the circuitry can draw, and even increase it. Use a nice voltmeter, maybe something that even displays the first decimal place when measuring your 230 V P-P's. Using the client's voltage, best at the location in the plant where this is going, he/she can see that this causes no detectable phase unbalance.

This is a demo that the client hopefully believes, but it is the client's $$$ to spend or waste. Rubbing his nose in it might make him more quixotic.

I forget who said "The most dangerous thing about communication is the illusion it's been achieved." (Maybe someone remembers?)

BK

 

[jraef]

As enjoyable as the above sarcasm is (love the "lawn rake" comment), I have been faced with exactly the same situation.

My thinking was that it came from some standard which specified 3 phase distribution systems within control panels, most likely the motion control industry where they would have several servo power supplies that needed 240V 1 phase input. I simply provided them with a 3 phase 480-240/120V transformer with a red-leg (stinger leg) for the 120V control loads, and "balanced" the other 240V loads on the other phases. I put the quotes around balance because it really was silly since the servos never operated at the same time anyway, but I satisfied the engineer at Boeing and got paid for the job.

"Venditori de oleum-vipera non vigere excordis populi"

 

[RalphChristie]

If load balancing is that important, I agree with jghrist. Go for a 3ph transformer (Primary and secondary)

But, like busbar and rbulsara pointed out, a 2kVA transformer is quite small. I also think a single phase 2kVA transformer will be much cheaper than a 3ph 2kVA transformer.

Busbar:
Regarding these different 3ph to 1ph transformers: (Have never seen them, just read about it )
Will a Scott-T also do the job?

 

[aolalde]

mpyke:

As I see it, the “le Blanc” connection proposed by busbar , with 230 Volts from L1 to L2, will approach to share the load in the three input phases. It drives the same current in all three secondary phases. The load (single phase) and the right hand leg , will draw say 8.7<180° amperes, the central leg 8.7<120° and the left side 8.7<240°.
Note that 8.7<120° + 8.7<240° = 8.7<120°. The power share is 50% for the left side leg and 25% for the other two, but the delta connection in the primary will probably balance more the input currents.
Basically this is a delta /delta, 3 phase transformer; 480V primary / 115 V secondary, one secondary phase is inverted and the delta left open.

 

[aolalde]

The transformer rating has to be increased to 3 kVA in order to prevent overload of the secondary.

 

[busbar]

 
aolalde, for a 1ø 2-wire load on the secondary side of the illustrated Delta—LeBlanc transformer bank, I believe that primary current would roughly equally divide between the H1-H3 winding, and the series H1-H2 and H2-H3 windings. For a balanced set of transformers, there would be current through the two winding connections to the H2 terminal, but zero current in the external H2 lead connected to the serving balanced 3ø circuit.

 

[Walden]

Hi everyone! What a response! If this site had been available 25 years ago, I would now be CEO for General Electric! The amount of information out there is extraordinary! Solid, down to earth sensible advice from hands-on engineers! Many thanks!

Is my client an engineer? Yes, but he has to employ a Consultant or is not! To the Consultant, all electrical loads should be balanced hence: three phase good and single phase bad! This is to be a small control panel to go on an oil rig. Five, three phase pump motors (DOL) and some little single phase fan motors and about 500 W of single phase trace heaters. The transformer will go on the wall beside the panel. The position is in a non-explosive area, nothing special about the control panel, we’ve made them before. The Consultant’s fee may well be larger than the cost of the control panel!

I have only seen a Le Blanc transformer connection in text books! If I could sell that to my client it would be a first! But should I not protect him from the Consultant by offering the simpler 480/230 volt single phase transformer which will be cheaper and can, if needed, be rewound or replaced anywhere from Alaska to the Persian Gulf? Or do I sigh and buy in an expensive transformer while cursing, under my breath, all ignorant Consultants?

Thank you all very much again for your responses. You have revived my faith in engineering.

Martin Pyke.

 

[aolalde]

busbar:

I will concede you the benefit of the doubt. I assume that if only one leg of the secondary is loaded yes the input current is single phase, but I can not explain how the current flowing in the other two phases will not reflect in the primary current. I hope to do a practical testing in a short future.
By the way, how did you do to publish the graphic diagrams?

 

[jghrist]

I would advise the client of extra cost and the limited benefits. Show off your new found smarts about transformer connections, and let him decide.