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Mains wiring question: Sizing buck-boost transformer?

Started by Gary Walters March 29, 2013
On Sat, 30 Mar 2013 14:20:30 -0700, Jim Thompson
<To-Email-Use-The-Envelope-Icon@On-My-Web-Site.com> wrote:

>On Sat, 30 Mar 2013 14:15:00 -0700 (PDT), >bloggs.fredbloggs.fred@gmail.com wrote: > >>On Friday, March 29, 2013 11:17:28 PM UTC-4, Sylvia Else wrote: >> >>> >>> You can look at it this way: All the input current flows through the >>> >>> primary, and all the output current flows through both the primary and >>> >>> the secondary. But the output current is in antiphase with the input >>> >>> current, so most of the current in the primary is cancelled. The primary >>> >>> has to handle only the difference between the input and output current. >>> Sylvia. >> >>Huh? In boost mode , the transformer high side is the primary, and the low > voltage side is secondary . In buck mode, the transformer low voltage side > is the primary, the high voltage side is the secondary. In each case, > transformer high voltage side current is 1/10 transformer load voltage > side current. There's no looking at it this or that way, there's only > comprehension of what a transformer is. > >Now, now, Freddy! To fit in here you need to learn to treat the >ignorant in a politically correct manner >:-} > >Otherwise it'll be your turn to be attacked and told to change your >diaper ;-) > > ...Jim Thompson
Yep. Power is power, and if your secondary side load is pulling a certain amount of power, you can bet the primary side is consuming exactly that much, plus a bit more, in order to provide it. Goddamned unformatted web based access retards should all get a clue about line length in Usenet too.
On Fri, 29 Mar 2013 12:59:34 -0700, 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 &#2013266099;Code.&#2013266098; 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? > >What size B-B transformer do I need? > >Thanks.
I recently wired in a couple Buck/boost xmfrs in a buck configuration. To see if the xmfrs I had were the correct size and to find out how to wire the xmfrs to get the amount of voltage reduction I wanted I looked online for info from Square D. If you google "Jefferson Electric Buck-Boost Application Manual" you should be able to find the pdf version of it. This is what I did. Or, since I still have the pdf version I will email it to you if you want. Just reply to my email address: etpm@whidbey.com . The manual tells you how figure out what size xmfr you need and all the various ways to connect the xmfr the get the voltage out that you want. ERic
On 31/03/2013 8:15 AM, bloggs.fredbloggs.fred@gmail.com wrote:
> On Friday, March 29, 2013 11:17:28 PM UTC-4, Sylvia Else wrote: > >> >> You can look at it this way: All the input current flows through >> the >> >> primary, and all the output current flows through both the primary >> and >> >> the secondary. But the output current is in antiphase with the >> input >> >> current, so most of the current in the primary is cancelled. The >> primary >> >> has to handle only the difference between the input and output >> current. Sylvia. > > Huh? In boost mode , the transformer high side is the primary, and > the low voltage side is secondary . In buck mode, the transformer low > voltage side is the primary, the high voltage side is the secondary. > In each case, transformer high voltage side current is 1/10 > transformer load voltage side current. There's no looking at it this > or that way, there's only comprehension of what a transformer is. >
Of course there are ways of looking at it. It's a device that operates under the laws of physics, in the analysis and understanding of which one can take any perspective that is physically valid, including such things as superposition. You appear to have absorbed the knowledge about how the voltages and currents on the various windings are related, but not why they are related that way, which depends on how currents produce fields, and how changing fields produce emfs. There are various takes on exactly what the OP was concerned about. My take was that he was concerned about how connecting a conventional transformed in an auto-transormer configuration can manage to increase its power rating. Sylvia.
On Saturday, March 30, 2013 9:50:17 PM UTC-4, Sylvia Else wrote:

> There are various takes on exactly what the OP was concerned about. My=20 >=20 > take was that he was concerned about how connecting a conventional=20 >=20 > transformed in an auto-transormer configuration can manage to increase=20 >=20 > its power rating. >=20
The answer is it doesn't increase the power rating of the transformer. Anal= ysis is what tells you the required transformer power handling capability i= n the buck/boost configurations a lot more than any fundamental physics. Ma= ybe take it up a notch and declare a N:1 stepdown xfrmr can be used in buck= /boost circuits with loading up to Nx KVA rating of the transformer.
On 31/03/2013 2:09 PM, bloggs.fredbloggs.fred@gmail.com wrote:
> On Saturday, March 30, 2013 9:50:17 PM UTC-4, Sylvia Else wrote: > >> There are various takes on exactly what the OP was concerned about. >> My >> >> take was that he was concerned about how connecting a conventional >> >> transformed in an auto-transormer configuration can manage to >> increase >> >> its power rating. >> > > The answer is it doesn't increase the power rating of the > transformer. Analysis is what tells you the required transformer > power handling capability in the buck/boost configurations a lot more > than any fundamental physics. Maybe take it up a notch and declare a > N:1 stepdown xfrmr can be used in buck/boost circuits with loading up > to Nx KVA rating of the transformer. >
How finely do you want to split that particular hair? Sylvia.
On Saturday, March 30, 2013 11:15:31 PM UTC-4, Sylvia Else wrote:

> How finely do you want to split that particular hair? >=20
Using obvious notation: Actually, KVAXfmr=3D VLine/N x ILoad and KVALoad=3D Vline( 1+ 1/N)x ILoad s= o 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 KV= AXfmr , for boost. For buck the factor is VLine(1-1/N), so it can be used u= p to loads of (N-1)xKVAXfmr. Which makes sense because for the same KVA loa= d, the current is greater for buck, but the transformer voltages always run= at line.
On 2013-03-31, bloggs.fredbloggs.fred@gmail.com <bloggs.fredbloggs.fred@gmail.com> wrote:
> On Saturday, March 30, 2013 9:50:17 PM UTC-4, Sylvia Else wrote: > >> There are various takes on exactly what the OP was concerned about. My >> >> take was that he was concerned about how connecting a conventional >> >> transformed in an auto-transormer configuration can manage to increase >> >> its power rating. >> > > The answer is it doesn't increase the power rating of the > transformer. Analysis is what tells you the required transformer power > handling capability in the buck/boost configurations a lot more than > any fundamental physics. Maybe take it up a notch and declare a N:1 > stepdown xfrmr can be used in buck/boost circuits with loading up to > Nx KVA rating of the transformer.
A 1kVA 1:1 transformer can be used as a 2kVA stepdown auto-transformer. I think the formul is actually a XkVA N:1 transformer has a (N+1)x kVA rating as an autotransformer. The Nx rating you give above seems to ignore the current in the common leg. -- &#9858;&#9859; 100% natural --- news://freenews.netfront.net/ - complaints: news@netfront.net ---
"Jasen Betts"

> A 1kVA 1:1 transformer can be used as a 2kVA stepdown > auto-transformer.
** Only true if used in a system with DOUBLE the supply voltage the tranny is rated for.
> I think the formul is actually a XkVA N:1 transformer has a (N+1)x kVA > rating as an autotransformer.
** Simply a result of how the parameters are defined - not physics. For one thing - the "power rating" of a tranny is proportional to the applied primary voltage. For example: raising the supply frequency allows more voltage to be applied ( before core saturation ) so up goes the VA rating too. A 50Hz tranny run at 400Hz has 8 times the previous VA capacity - long as the insulation can stand it. Auto-transformers can be given very large VA ratings for their size - but it is a nonsense as the tranny is suppling only a small part of the output. The AC supply delivers the rest - directly to the load. ... Phil
On 31 Mar 2013 05:27:30 GMT, Jasen Betts <jasen@xnet.co.nz> wrote:

>On 2013-03-31, bloggs.fredbloggs.fred@gmail.com <bloggs.fredbloggs.fred@gmail.com> wrote: >> On Saturday, March 30, 2013 9:50:17 PM UTC-4, Sylvia Else wrote: >> >>> There are various takes on exactly what the OP was concerned about. My >>> >>> take was that he was concerned about how connecting a conventional >>> >>> transformed in an auto-transormer configuration can manage to increase >>> >>> its power rating. >>> >> >> The answer is it doesn't increase the power rating of the >> transformer. Analysis is what tells you the required transformer power >> handling capability in the buck/boost configurations a lot more than >> any fundamental physics. Maybe take it up a notch and declare a N:1 >> stepdown xfrmr can be used in buck/boost circuits with loading up to >> Nx KVA rating of the transformer. > >A 1kVA 1:1 transformer can be used as a 2kVA stepdown >auto-transformer.
Schematic? -- John Larkin Highland Technology Inc www.highlandtechnology.com jlarkin at highlandtechnology dot com Precision electronic instrumentation Picosecond-resolution Digital Delay and Pulse generators Custom timing and laser controllers Photonics and fiberoptic TTL data links VME analog, thermocouple, LVDT, synchro, tachometer Multichannel arbitrary waveform generators
On Sunday, March 31, 2013 10:59:13 AM UTC-4, John Larkin wrote:

> > Schematic? >
A 1:1 buck delivers 0 KVA to the load :-) He must be talking about using the center tap of secondary for 2:1.