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Mains wiring question: Sizing buck-boost transformer?

Started by Gary Walters March 29, 2013
"Cydrome Leader"  wrote in message news:kj4vp4$6m7$1@reader1.panix.com...

In sci.electronics.repair Gary Walters <gwprez@yahoo2.cz> wrote:
> In USA. > > Source: 208v, 60 hz, 2-wire (2 phases from 3 phase "Y" supply). Load: > 240v, > 20A. > > I presumed that sizing a buck-boost transformer is simple KVA math (source > volts * load amps). But... > > This PDF document: > > <http://www.acmepowerdist.com/pdf/Page_104-109.pdf> > > on the last page says: > - - - > "An example of an everyday application is always a good way to explain the > intent of the ?Code.? Example: A 1 kVA transformer Catalog No. T111683 has > a > primary of 120 x 240V and a secondary of 12 x 24V. It is to be connected > as > an autotransformer at the time of installation to raise 208V to 230V > single > phase. > > When this 1 kVA unit is connected as an autotransformer for this voltage > combination, its kVA rating is increased to 9.58 kVA (may also be > expressed > as 9,580 VA). This is the rating to be used for determining the full load > input amps and the sizing of the overcurrent protect device (fuse or > breaker) > on the input. > > Full Load Input Amps = > 9,580 Volt Amps / 208 Volts = 46 Amps" > - - - > I'm puzzled by the 10x increase of KVA rating. When and how is this true?
--QUOTE It's true because you're only using the transformer to "create" 24 volts at the current you wish to draw at 230v. This extra 24 volts is added back into the line voltage. You can switch flip the leads and subtract voltage too, then the transformer is in buck mode. QUOTE
> What size B-B transformer do I need? > > Thanks.
If you need 20 amps at 230v and start with 208, you need to boost 22volts (208+22=230) x 20 amps = 480VA transformer. A 24 volt transformer rated over 480VA should be fine. Autotransformers can be confusing, so pretend it's just DC and some batteries. Let's say you need 24 volts at 10 amps and have a 12 volt battery that can already output 10 amps. what size power supply do you need to run in series with this battery to get the 24 volts? just another 12 volts, at at least 10 amps, or a 120 watt power supply. Those wired in series (your battery and the new power supply) will provide 240 watts. If you already had an 18 volt battery, you'd just need a 6 volt, 10 amp or 60 watt power supply. The less the voltage adjustment, the smaller then buck/boost transformer rating becomes as it's really not doing all that much work. ======================================= lookup "autotransformer". In this case you have a 2 winding transformer with a 10:1 ratio With 240V applied the secondary will be 24V with a rated current of 1000/24=41.7A If this is connected as a boost autotransformer- the total output voltage would be 240+24 =264V so the output, without exceeding rated output current would be 11KW. only 1 KW (24V*41.7A) is supplied through transformer action and the rest through a direct connection Adjusting to 230V output leads to 11*(230/264)=9.58KW The input voltage would be 207V excluding any voltage drops in the transformer-so 208/230 is close enough. Autotransformers are great for turns ratios near one as there are size and cost advantages. Disadvantage--no isolation between primary and secondary. Excuse the lack of "quoting" as I am using windows live mail in an emergency- Thunderbird downloads news but then deletes the downloads immediately! New problem- correction not yet found. Don Kelly cross out to reply
On Sunday, March 31, 2013 7:57:36 PM UTC-4, Jamie wrote:
>
Please view in a fixed-width font such as Courier. . . . . . . o----------- + . ) . ) . | I ) . | - ) 120VAC . | 2 ) . v ) . ) I . | ---> . 240VAC CT +-------------. . | | . )+ | . ^ ) ----- . | I ) |LOAD | 2X KVA XFMR . | - ) 120VAC ----- . | 2 ) | . ) | . ) | . o-------------------------' . . . .
Per P.A.'s observation:


  Please view in a fixed-width font such as Courier.

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.          o----------- +
.                      )
.                      )
.                  | I )
.                  | - ) 120VAC
.                  | 2 )
.                  v   )
.                      )   I
.                      |  --->
.      240VAC       CT +-------------.
.                      |             |
.                      )+            |
.                  ^   )           -----
.                  | I )          |LOAD | 2x KVA XFMR
.                  | - ) 120VAC    -----
.                  | 2 )             |
.                      )             |
.                      )             |
.          o-------------------------'
.
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.                  I         1    KVA, LOAD    KVA,LOAD
.  KVA,XFMR= 120 x - = 120 x - x  --------- =  --------
.                  2         2     120            2
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.
.                             I
.                            ---->
.                       -------------
.                      )+            |
.                  ^   )             |
.                  |   )             |
.                  | I ) 120VAC      |   240VAC
.                  |   )             |
.                      )             |
.                      )             |
.                      |           -----
.          o-----------+ CT       |LOAD | 2x KVA XFMR
.                      |           -----
.                      )+            |
.                  |   )             |
.         120VAC   | I )             |
.                  |   )             |
.                  v   )             |
.                      )             |
.                      )             |
.          o-------------------------'
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.                              KVA, LOAD    KVA,LOAD
.   KVA,XFMR= 120 x I = 120 x  --------- =  --------
.                                 240          2
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> Actually, KVAXfmr=3D VLine/N x ILoad and KVALoad=3D Vline( 1+ 1/N)x ILoad=
so that Iload=3DKVALoad/[Vline(1+1/N)], making KVAXfmr=3DVLine/N x KVALoad= /[VLine(1+1/N)]=3D KVALoad/(N+1) so you can use it with loads up to (N+1)x = KVAXfmr , for boost. For buck the factor is VLine(1-1/N), so it can be used= up to loads of (N-1)xKVAXfmr. Which makes sense because for the same KVA l= oad, the current is greater for buck, but the transformer voltages always r= un at line. Looks like you can squeeze a few extra percentage points out of buck mode b= y wiring it like this : Please view in a fixed-width font such as Courier. . . . . I . ---> . o---------------------------- . )+ | . ) | . 240 ^ I ) | . --- | --- ) N ----- . 1 | N+1 ) |LOAD | N+1 x KVA,XFMR . 1+ - ) ----- . N ) | . | | . 240VAC +------------- . | . )+ . ) . 240 | NI ) . --- | --- ) 1 . N+1 v N+1 ) . ) . ) . o-------------- . . . . . 240 N 240 N KVA,LOAD KVA,LOAD . KVA,XFMR=3D --- x --- x I =3D --- x --- x --------- =3D -------- . N+1 N+1 N+1 N+1 240 N+1 . --- . 1 . 1+ - . N . . It comes out close enough for a 240 to 208 conversion using standard stepdo= wns.
On Sun, 31 Mar 2013 11:28:35 -0700, DecadentLinuxUserNumeroUno
<DLU1@DecadentLinuxUser.org> wrote:


> In transformer speak: > > "series complimentary" for "auto-transformer boost configuration".
--- "Series complimentary" is nonsensical: http://www.merriam-webster.com/dictionary/compliment and while it's conceivable that "series complementary" could be used, it's kind of ambiguous. The correct term is, I believe, "series aiding". http://www.allaboutcircuits.com/vol_2/chpt_9/5.html -- JF
On Mon, 01 Apr 2013 08:03:41 -0500, John Fields
<jfields@austininstruments.com> wrote:

>On Sun, 31 Mar 2013 11:28:35 -0700, DecadentLinuxUserNumeroUno ><DLU1@DecadentLinuxUser.org> wrote: > > >> In transformer speak: >> >> "series complimentary" for "auto-transformer boost configuration". > >--- >"Series complimentary" is nonsensical: > >http://www.merriam-webster.com/dictionary/compliment > >and while it's conceivable that "series complementary" could be used, >it's kind of ambiguous. > >The correct term is, I believe, "series aiding". > >http://www.allaboutcircuits.com/vol_2/chpt_9/5.html > >
Here.. maybe this will help you. "Series, Phase Complimentary" Or... "Series, Phase Opposed" Winding phase, not electrical phase. It makes perfect sense... "I believe". I don't know... maybe you are right. Maybe it would be ambiguous to others. I seem to have a mild asperger's thing going... maybe... I doubt it, but I suppose it is possible. I think all this autism stuff is us trying to make the next evolutionary step. Sit at a winding machine and wind up a few tens of thousands of bobbins. Maybe the terms would then make more sense out of the gate.