On Sat, 12 Jul 2008 22:00:45 GMT, ehsjr
e.h.s.j.r.removethespampunctuation at (no spam) bellatlantic.net> wrote:


John Fields wrote:

On Sat, 12 Jul 2008 07:56:53 -0700 (PDT), emailaddress at (no spam) insightbb.com
wrote:



On Jul 11, 2:34 pm, ehsjr
e.h.s.j.r.removethespampunctuat... at (no spam) bellatlantic.net> wrote:



I don't want to argue - that seems your intent.
I know you want a magic formula - you have made that
clear. There is no single formula for you. Those who
have replied have made that clear.

We've tried to show you some of the factors that are
involved. You insist on ignoring them. So, we can't
help you, no matter how hard we try. Sorry.

A formula yes, but no it's not magic. It simply requires considering
all the significant variables which I'd hoped others would assist
with, but obviously nobody else wants to do more than argue instead of
putting thought into what such an equation would look like. If you
say "it depends", then that should be in an equation. If you say some
other thing depends too, then that too can be put into the equation.

There is an equation that could be made. How accurate the answer from
it would depend on how complete it was.


---
Funny, meteorologists feel the same way, and yet it seems that
detecting that last elusive flap of butterfly wings has been
perpetually beyond their grasp.

JF

:-) Nice analogy, wonder if he'll get it.

What is interesting to me is the apparent contradiction in
the op's thinking. He chooses to lecture respondents with
his thinking, rather than attempt to understand what they
are saying. Or, if not lecture, argue with points that are
made. I've seen this a few times here - someone comes here,
states that they don't know about X, and asks a question.
But then they argue with the answer! What kind of thinking
is that? Is it the "new way of thinking" or ???


---
I've seen it a few times also, and I think it's a way of trying to
save face by not admitting that the gift that was given incurred
emotional indebtedness.

That is, if you give me a gift of knowledge and I dis it, I'll still
have the gift but in my mind I won't owe you anything for it.

Make sense?

JF

"John Fields"


I've seen it a few times also, and I think it's a way of trying to
save face by not admitting that the gift that was given incurred
emotional indebtedness.

That is, if you give me a gift of knowledge and I dis it, I'll still
have the gift but in my mind I won't owe you anything for it.

Make sense?



** If you imagine some little a brat throwing a toy that is not the one he
wanted back at Santa.



..... Phil

mrdarr... at (no spam) gmail.com> wrote in message

news:0631a23f-0a93-4a34-a703-b3bd4e22ba06 at (no spam) j22g2000hsf.googlegroups.com...



On Jul 12, 7:40 am, emailaddr... at (no spam) insightbb.com wrote:
..snip..

However, it is fairly beside the point that I am looking for
a universal equation that ignores all of this. Many people seem to be
saying the rest matters and yes of course it does - but it is still
theoretically expressable in an equation with the factors that change
as variables.

Universal equation. Here you go.

http://en.wikipedia.org/wiki/Heat_equation
http://en.wikipedia.org/wiki/Newton%27s_law_of_cooling#Newton.27s_law...

Also check out "Introduction to Heat Transfer" by Incropera and
DeWitt.

The game is to keep the transformer wires at only a moderate
temperature higher than ambient, or you will melt the wiring
insulation.

Knock yourself out.

Michael

These equations are only part of the overall solution. Transformers are
complex physical entities with many materials having different thermal
characteristics, and the temperature of any spot in the transformer depends
on the amount of power being applied, time, and the way in which heat is
radiated, conducted, or convected away from the source of the heat. The
winding insulation is rated at a certain temperature according to the
insulation class, and may vary from about 105 C (Class A) to 180 C (Class
H).

http://www.engineeringtoolbox.com/nema-insulation-classes-d_734.html

That is for motors (and I think it applies also to transformers), but here
is information more specific to trannies, and allows for hot spots:http://www.jeffersonelectric.com/cgi-bin/site.pl?3208&dwContent_conte...

This shows a wider range of classes up to "S", which is 250 C. Now, that's
hot!http://www.pleo.com/ulsystem/eis_apply.htm

This is a detailed document with more formulas for the OP to use when he
designs his transformer:http://www.superioressex.com/uploadedFiles/News/White_Papers/emcwa-ne...

Paul

On Jul 12, 6:00 pm, ehsjr



e.h.s.j.r.removethespampunctuat... at (no spam) bellatlantic.net> wrote:
John Fields wrote:
On Sat, 12 Jul 2008 07:56:53 -0700 (PDT), emailaddr... at (no spam) insightbb.com
wrote:

On Jul 11, 2:34 pm, ehsjr
e.h.s.j.r.removethespampunctuat... at (no spam) bellatlantic.net> wrote:

I don't want to argue - that seems your intent.
I know you want a magic formula - you have made that
clear. There is no single formula for you. Those who
have replied have made that clear.

We've tried to show you some of the factors that are
involved. You insist on ignoring them. So, we can't
help you, no matter how hard we try. Sorry.

A formula yes, but no it's not magic. It simply requires considering
all the significant variables which I'd hoped others would assist
with, but obviously nobody else wants to do more than argue instead of
putting thought into what such an equation would look like. If you
say "it depends", then that should be in an equation. If you say some
other thing depends too, then that too can be put into the equation.

There is an equation that could be made. How accurate the answer from
it would depend on how complete it was.

---
Funny, meteorologists feel the same way, and yet it seems that
detecting that last elusive flap of butterfly wings has been
perpetually beyond their grasp.

JF

:-) Nice analogy, wonder if he'll get it.

What is interesting to me is the apparent contradiction in
the op's thinking. He chooses to lecture respondents with
his thinking, rather than attempt to understand what they
are saying. Or, if not lecture, argue with points that are
made. I've seen this a few times here - someone comes here,
states that they don't know about X, and asks a question.
But then they argue with the answer! What kind of thinking
is that? Is it the "new way of thinking" or ???

Ed- Hide quoted text -

- Show quoted text -

I'm sorry if I did not clarify this, but I do not need an introductory
course in sizing transformers. The whole point was moving beyond the
generic overestimates towards the real science in what is actually
necessary. I lecture towards the end of saying, ok, but this is
common knowlege, what else do you know? Maybe it takes a village,
maybe no one person has the entire answer but it seems to get there we
need to have some lecture, some dispelling of comfort zones and get
right down to the actual criteria necessary and have that proven
through real worl examples of success or failure, not just saying "use
a bigger hammer", until it is proven to be needed.

Yes, I will argue with an answer when I ask for an equation and
someone tells me otherwise. I have built plenty of PSU over the
years, if I needed to know what I have already done successfully then
I would have asked a different question? No offense intended, but you
need to focus on what I asked, as do others. If they are ignorant of
the answer, there is no need to reply.

Use Irms = 2.0 Idc and you'll always be safe .

On Wed, 16 Jul 2008 02:55:03 -0700 (PDT), emailaddress at (no spam) insightbb.com
wrote:


I'm sorry if I did not clarify this, but I do not need an introductory
course in sizing transformers. The whole point was moving beyond the
generic overestimates towards the real science in what is actually
necessary. I lecture towards the end of saying, ok, but this is
common knowlege, what else do you know? Maybe it takes a village,
maybe no one person has the entire answer but it seems to get there we
need to have some lecture, some dispelling of comfort zones and get
right down to the actual criteria necessary and have that proven
through real worl examples of success or failure, not just saying "use
a bigger hammer", until it is proven to be needed.

Yes, I will argue with an answer when I ask for an equation and
someone tells me otherwise. I have built plenty of PSU over the
years, if I needed to know what I have already done successfully then
I would have asked a different question? No offense intended, but you
need to focus on what I asked, as do others. If they are ignorant of
the answer, there is no need to reply.

---
You may have _built_ plenty of PSU over the years, but from your
question re. transformer sizing it appears that all you were doing was
grunt work, putting together someone else's design, with no real
insight as to the "why" of the transformer's capacity needing to be
larger than what was required to drive the load.

Instead of being an annoying little ass you might try thanking those
of us who have given you good advice instead of pretending that,
somehow, we did you a disservice.

Oh, and by the way, Fuck you.

I'm sorry if I did not clarify this, but I do not need an introductory
course in sizing transformers. The whole point was moving beyond the
generic overestimates towards the real science in what is actually
necessary.

On Jul 13, 4:35 pm, John Fields <jfie... at (no spam) austininstruments.com> wrote:

Use Irms = 2.0 Idc and you'll always be safe .

Well, not completely, no. Consider a 1A (RMS)
rated transformer, with a diode/capacitor output.

Here's where it gets mathematical: the power dissipated
in a 1A transformer with (for instance) 1 ohm winding

Heat = 1A **2 * 1 ohm = 1 watt

When the same transformer feeds the DC load as described above, the
power is

Heat = (0.9 * 0) + (0.1 * 10A**2 * 1 ohm) = 10 watt

So a perfectly good transformer can burn up feeding a
DC rectifier and load, when a similar AC load wouldn't

On Wed, 16 Jul 2008 18:27:11 -0400, "Paul E. Schoen" <pstech at (no spam) smart.net
wrote:


I beg to differ with your analysis, if you are talking about an ordinary
rectifier and capacitor circuit. As an example, I simulated a FWB with a
12
VAC nominal output transformer with 1 ohm series resistance, and a load
of
12 ohms, and a capacitor of 100,000 uF, which should produce the highest
possible current peaks. The simulation shows peak currents of 3.7 amps.
With 1000 uF, the peaks are 3.2 amps. Now, during the charging period,
with
100,000 uF, the peaks start at 14.6 amps and then diminish to 4.3 amps at
0.5 seconds. In the first 200 mSec, the tranny is supplying 34 watts, but
then settles down to 15.8 watts when the capacitor is fully charged. At
that time, the load is essentially pure DC, and the resistor dissipates
13.9 watts. So only about 2 watts is left, and that is shared among the
rectifiers (305 mW each), and the tranny (about 0.8 watts).

I have a transformer with a nominal 24 volt secondary, rated at 8 amps.
It
has a measured series resistance (secondary plus reflected primary) of
about .125 ohms. I connected a bridge rectifier consisting of 4 80 amp
Schottky diodes, and a real 100,000 uF capacitor.

If you simulate this, a DC load which gives 8 amps RMS in the secondary
may
give a DC current of less than 4 amps. The ratio of secondary RMS
current
to DC load current will exceed 2 to 1 if the transformer is much larger
than this, with a series resistance less than this transformer has.

I posted a partial analysis over on ABSE in which I indicate that the
grid
waveform has a large effect on the RMS to DC current ratio in these
rectifier circuits.

The Phantom wrote:

I don't think it's that difficult for the reasons you cite.

Knowing the rated secondary current means we don't have to know the allowable
temperature rise.

Only if you assume the design environment (sealed container,
forced air velocity, ambient temperature range, etc. ).
Many transformers are designed for very specific (and
varied) environments.

We need only see to it that the RMS secondary current when
used in a bridge rectifier circuit is the same as the rated secondary current.
Then we will get the maximum possible DC current with the nearly the same
temperature rise as when a pure AC load drawing rated current is applied. The
actual temperature rise may not be exactly the same because the higher crest
factor pulses of current drawn by the bridge cause a little extra IR drop in the
primary wire, and change the core loss slightly. I tried to measure this
effect, but it is so small as to be completely negligible.

Yes, that effect is more pronounced for cheap, low
efficiency transformers than it is for high efficiency units
that have plenty of iron and are rated to perform well with
+10% or more line voltage and possibly at 50Hz, ans well as
60 Hz.

The regulation is easily measured and provides what would seem to be the
remaining needed information, but I found that the grid voltage waveshape has a
large effect on the result, and knowing and specifying that may be the most
uncertain part of the process.

And changes day to day and even time of day. These wave
shape effects are almost unnoticeable with a resistive
load., but can be major players for rectifiers with
capacitor input filters.

Did you look at my post on alt.binaries.schematics.electronic? I show a method
of deriving a "formula", and discuss the results I got compared to actual
measurements.

I did a fly over and saved it, but do not wade through your
math.

I have a transformer with a nominal 24 volt secondary, rated at 8
amps.
It has a measured series resistance (secondary plus reflected primary)
of about .125 ohms.


** This is a fabricated falsehood - the numbers simply do not add up.

A 192VA rated transformer does not have 4% regulation - correctly
rated it has 8%.

This one does.


** Cos it is not correctly VA rated.

The manufacturer rated it,

** So you also have no idea what its VA rating is.

I am taking the manufacturer's word for it. Printed on the transformer is
the
designation 24 volts, 8 amps.

However, the primary (it's a 60 Hz transformer) says 120 VAC and it
measures .961 ohms, cold.


** So it is a circa 360 VA transformer.

The *correctly* rated secondary load is not 8 amps - but more like 15.

That would depend on the insulation system the manufacturer used, and the
resultant allowable temperature rise, wouldn't it?

If all transformers were manufactured to a single regulation and
temperature rise standard,

For example, for the 24 volt, 8 amp transformer I've mentioned in this
thread,

The
recommendation that John Fields made, to assume Irms/Idc = 2, which he
says will
always be safe, may not be safe if you are using a transformer with good
regulation and if your grid waveform is a good sinusoid.

On Fri, 18 Jul 2008 09:56:01 +1000, "Phil Allison" <philalli... at (no spam) tpg.com.au
wrote:





"The Phantom"
"Phil Allison"

For example, for the 24 volt, 8 amp transformer I've mentioned in this
thread,

** An incorrectly rated example.

Plucked out of his arse.

The recommendation that John Fields made, to assume Irms/Idc = 2,
which he says will always be safe, may not be safe if you are using a
transformer with good regulation and if your grid waveform is a good
sinusoid.

** A transformer with unusually good regulation ALSO has unusually LOW
temp
rise.

A transformer with unusually good regulation was presumably rated that way
by
the manufacturer for some reason.

( snip even worse DRIVEL )

** How completely ASININE !!

A transformer with unusually good regulation = a transformer that is
being under utilised !!!

No need for exists for transformer makers to make special high regulation
models - the customer simply picks a model with more VA capacity than
strictly needed !!!

Sounds like you have a quibble with the manufacturer, not with me. I only
report the transformer's measured parameters and labelling.

"The Phantom"


For example, for the 24 volt, 8 amp transformer I've mentioned in this
thread,

** An incorrectly rated example.

Plucked out of his arse.


The
recommendation that John Fields made, to assume Irms/Idc = 2, which he
says will
always be safe, may not be safe if you are using a transformer with good
regulation and if your grid waveform is a good sinusoid.


** A transformer with unusually good regulation ALSO has unusually LOW temp
rise.

Which wipes you asinine case out.

Piss off.


...... Phil