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?

 

[busbar]

 
aolalde, used autosketch—but it has had a maddening learning curve for a non-drafter like me.

On Delta—LeBlanc, with H2 not connected, the H1-H3 coil will serve the righthand half of the secondary, while series H1-H2/H2-H3 coils will serve the lefthand secondary side, again with coils in series. With three of the same 480—120 transformers and balanced-3ø primary voltage, I believe that energizing H2 will not appreciably change anything in terms of primary-phase currents or two-wire secondary voltage with a 1ø load.

There is an older US EE handbook that warns of the farce. A previous J&P book suggests Delta-—LeBlanc as a lesser-known alterative to Scott-Tee for 3ø-2ø conversion.

 

[jghrist]

busbar,

I haven't tried to figure out what the currents would be, but how could energizing H2 not appreciably change primary phase currents? Without H2 connected, the H2 phase current would obviously be zero. All the current would be in the phases connected to H1 and H3. In this case, the current would be the same as that of a single phase transformer connected phase-to-phase. At least is seems that way to me without any calculations, just considering conservation of energy.

If connecting H2 doesn't appreciably change the current in the phase connected to H2, then do you accomplish any reduction in phase unbalance over using a single phase transformer?

 

[busbar]

 
Jim—I completely agree with your logic, and apologize for not making it clear earlier in this thread. That’s what I see as the FARCE with claims of transformers with “balanced” 3ø in—1ø out. {I gotta’ find the EE book that mentions it.}

 

[cbarn24050]

You might want to look here to see how to do it.

http://www.rtri.or.jp/infoce/wcrr97/C135/C135.html

 

[jghrist]

I found the LeBlanc connection in Power Systems Handbook, Design, Operation & Maintenance, by O.C. Seevers, The Fairmont Press, Inc., 1991, p. 217 (although it doesn't call it LeBlanc). This shows that the current in one phase will be zero. I also drew out the connection diagrams and convinced myself that this is the case. There is no difference from a balance standpoint from using a single phase transformer to serve the load.

 

[Warpspeed]

What about using a Scott T transformer connection ?

The three phases will still not be balanced, but at least the load will be shared somewhat between all three. That is probably the best you can do.

I have seen these wound on a normal three limb, three phase core, with only the two outer limbs used.

 

[busbar]

 
Warp*, I believe Scott-Tee will have the same problem of one zero-current primary phase with a secondary 1ø load. [Suitably tapped off-the-shelf drytypes may be hen’s teeth.]

 

[Warpspeed]

Sure, the problem has no real solution. Just suggesting another possible approach.

 

[bigbillnky]

Sell him a 3-phase motor driving a 1-phase generator in order to truely produce the "balance" he so desperately desires. "If you give a man what he values, you can take from him what he does not" (i.e. his money)!

Bigbillnky,C.E.F.....(Chief Electrical Flunky)

 

[mc5w]

What you could do is to put in a delta-delta 480 volt to 240 volt transformer. Then rig a small 240 volt 3-phase motor and a single phase capacitor to act as a 3-phase to single phase converter. This would be a Steinmetz Balancer Set using a 3-phase motor as both an inductance and a 3-phase voltage source. You could also use a single phase inductance and a single phase capacitance which would be a tryely static Steinmetz Balancer Set but a smll motor is usually easier to find.

A lot of my CUSStomers are morons. I can tell you all kinds of stories about Morons With Money.

Mike Cole