[Amps] Paralleling plate xfmrs.

[Alek Petkovic]

Simple way to settle it is to fire the pair of transformers up but don't 
connect the secondaries together. Measure the secondary voltage of each 
one and compare.

All good? Strap'em together.

73, Alek VK6APK.


On 23/10/2025 8:58 am, Steve Harrison wrote:
> On 10/22/2025 4:07 PM, jim.thom jim.thom at telus.net wrote:
>> I have been paralleling plate xfmr secondary's since 1977..with zero
>> issues.
>> Back then, it was 80 lb hammond plate xfmrs.   Those came with a 
>> 0-105-110
>> -115-120   primary...and the usual  CT secondary.
>>
>> We would series the 120 vac primary's to run on 240 vac.   Then parallel
>> the secondary's.
>> Zero issues.... and a 50-50 split.
>
> There wouldn't be... because the series connection of the primaries 
> took care of any imbalance in transformer turns ratio or output 
> voltage. With the primaries seriesed (iz dat even a word?? 8-) and 
> secondaries paralleled, the higher-output secondary would provide more 
> current until the output voltage drops to equal the other transformer; 
> then they would tend to share the total output current.
>
> Actually, since you need the full current available from BOTH 
> secondaries, the whole issue is moot since you won't get that from the 
> secondaries connected in series, for your application.
>
>> Both Asamoto and also Henry radio paralleled plate xfmr's  (both pri and
>> sec) on their  RF generators and also FM broadcast PA's.   Some of 
>> the FM
>> broadcast PA's used 3 phase supplies, either 208 3 phase, or 360 vac 3
>> phase.    On their  single phase versions, 2 x identical plate xfmr's 
>> were
>> used....again with pri in parallel..and sec's in parallel.
> I'm not aware of those.
>> And no,  nobody is gonna wire plate xfmr secondary's in  SERIES.   Why
>> would you ?
>
> Because by wiring the PRIMARIES in series, you effectively HALF the 
> available output voltage. So you merely series-wire the secondaries 
> and hey el presto: yer back up to full output voltage. (However, you 
> won't get the output current available from both secondaries in 
> parallel; so the issue is really moot, for your application.)
>
> It's extremely poor engineering design to connect transformers in 
> parallel and expect them to share output current precisely, as there 
> is always some imbalance between them. Whether or not it matters 
> depends entirely upon just how much imbalance there is. If it were 
> done professionally, I would expect to find something in series with 
> each secondary winding which would absorb any imbalance between them, 
> such as a large power resistor or even an inductor.
>
>>     As is, these xfmr's have  4300 and also 5300 taps.   Why
>> the hell would u even think of  series 2 x 4300 secondarys....u would 
>> end
>> up with a whopping 8600 vac = 12.16 kvdc.   And 14.9 kvdc  if the 
>> 5300 taps
>> were used.
> No, because you're also only applying half the AC input to one 
> transformer.
>> The pair of plate xfmrs  used on the 16 kva  supply  are identical.
> Actually, what should be said is that they are "SIMILAR". They cannot 
> be identical (they don't share the same DNA!! 8-). Their cores are not 
> exactly the same magnetic material, their copper wire is not precisely 
> the same length and thus presents a small resistance difference, the 
> placement of the wire on the forms is very slightly different for 
> every transformer, etc. etc.. You can never fully-guarantee that one 
> transformer is precisely, exactly, "identical" to another; there will 
> ALWAYS be SOME slight difference. That's all that Clark and I are 
> getting at.
>>     That
>> combo, using the same xfmrs is already in use in the above mentioned
>> commercial applications......was done all the time.
> Perhaps: I've never seen those high-power amps but if I did see that 
> being done, I would seriously question the competence and aforethought 
> of their "engineer". What happens when a shorted turn occurs on either 
> transformer?? (And you know as well as I do, that happens alla time!). 
> It might not be catastrophic initially; but it puts more stress on the 
> pair, which inevidtably builds up.
>> I went through all this with Dahl himself.
> That would be really surprising, and distressing, considering his 
> reputation otherwise.
>> Xfmrs are made to an exact winding spec. XX turns on the pri...and YY
>> turns on the sec.   Why u think there is gonna be a V difference 
>> between em
>> is beyond me.
>
> So be it. K9YC has carefully documented differences in ferrite 
> transformer cores that were supposedly manufactured at the same time, 
> from the same batch of raw materials. I fail to see why there would 
> not be similar differences found in low-frequency magnetic material.
>
> The only way this is an acceptable engineering practice is to put 
> something in series with each secondary before paralleling them; that 
> "something" then absorbs any imbalance that may exist, either now or 
> in the future.
>
> I suppose we'll just have to agree to disagree on this issue, Jim; but 
> I would never consider doing this paralleling, myself.
>
>> Installing  a FWB assy on the output of each sec  will also work..then
>> paralleling the  outputs of each  FWB assy...... but it's a wasted 
>> effort.
> That would have been the far-preferable way to do it.
>> A client wanted a 25 kw pep output 80-10m amp, using a 3x15 tube in GG.
>
> We don't need to know who this person may have been (although if 
> he/she's still around, I can guess, based upon whom I hear busting 
> through pileups first time these days  8-).
>
> Let's agree to disagree on this, Jim. Keep in mind that we are just 
> making suggestions to help avoid future problems that we foresee; 
> we're not really critisizing the design itself (wellllll... we sorta 
> are, but it's supposed to be CONSTRUCTIVE criticism  8-).
>
> 73,
>
> Steve K0XP
>
>
> _______________________________________________
> Amps mailing list
> Amps at contesting.com
> http://lists.contesting.com/mailman/listinfo/amps