Discussion in 'Electronic Design' started by Ben, Oct 31, 2006.

1. BenGuest

Hiya,

This is for a college exercise (non-electronic engineering module), but
the module is about designing a program to calculate the best ordering
of transformers to give the maximum input current, not about how to
actually calculate the maximum input current, and we haven't really been
told how to calculate this, hence why I am asking:

If I have 3 ideal transformers, I know the turns ratio and their maximum
input and output currents.

I can work out the following, Iout(max) = Iin(max)/n, where n is the
turns ratio.

Say I am given three transformers, with the following data

T1: Turns ratio =2, Iin(max) = 2.7, Iout(max) = 2.1
T2: TR = 1.5, Iin = 0.2 Iout = 0.3
T3: TR = 1.2 Iin = 0.3, Iout = 1.6

How would I calculate the maximum input current that could be applied to
the whole cascade bearing in mind that I don't have input values.

I tried several ideas such as taking the Iin(max) of T1, working out the
Iout by using 2.7/2 = 1.35A, then trying to find a transformer that
would accept that Iout, but obviously, this doesn't work as none of the
other transformers will accept 1.35A without blowing out.

One idea I am toying with is working backwards, taking a transformer,
using its max Iout, working out the Iin at that output current, and
finding a transformer that outputs that current or higher, then working
out the Iin of that, and so on...

Any help/pointers would be appreciated.

Thanks,

Ben

2. Tom BruhnsGuest

Ben,

You were on the right track with your first attempt. You just need to
realize that what you connect that transformer to may limit the input
current to a value less than the input winding of the first transformer
is rated for. You just need to make sure, in practice, that you
observe the lowest of the maximum ratings, reflected through the turns
ratios.

Remember that a transformer can be turned around either way; the
transformer itself doesn't know the difference between "in" and "out"
sides. So if the "out" side has a higher rating and the turns ratio is
in the right direction, just turn the transformer around the other way.
(It _appears_ from what you've written that the "out" side always has
more turns than the "in" side, and therefore you'd always want to
connect the "out" of one to the "in" of the next in the chain; see the
next paragraph for more on that. But I'd be inclined to ask the
instructor if the turns ratio given was always the ratio from "in" to
"out" or not: it's very unusual for an "out" side with more turns than
the "in" side to be rated for higher current, like T2 and T3 are! If I
assumed they were "reasonable" transformers, I'd say the turns ratio
for them was in-turns/out-turns instead of the other way around. I'm
thinking it would be poetic justice if there is a particular hell
waiting for professors/instructors that don't present problems that
align at all well with what you will find in the practicing engineering
world.)

Also, notice that if the rating of the low-current side is less than
the the rating of the high-current side divided by the turns ratio,
that low-current-side limit reflected through the turns ratio becomes
the new effective high-current-side limit. That limit, for example,
applies to T3 in your case. (Similarly, for the others, you can
generate effective maximum currents for the low current sides based on
the primary side limit divided by the turns ratio.) It may be useful
to recast the ratings in those terms first, before trying to select
which transformer to put in which position.

Did that help? What answer do you get?

Cheers,
Tom

3. GenomeGuest

I would order them with all their primaries in parallel.........

I suppose I would short circuit their secondaries.

Then we can all go down the pub!!!!!

DNA

4. GenomeGuest

Bugger!!!!!!!!!!

Tom was right. I'd order them with all their secondaries in parallel and
then short circuit their primaries. Assuming......

Too late. I drank the first round. Mine's a Ruddles. We can play kick the
lecturer later, unless he's got a small willy and then we have to practice
smooching.

DNA

5. neon

1,325
0
Oct 21, 2006
they [transformes]do not know or care how you connect them. that is just not true try put primary 120 v into a bell transformer. you will do that only once.

6. robbGuest

a *programming* assignment to calculate...

sounds like a pseudo-practical problem designed to make the *programming*
assignment more interesting and possibly more difficult by either
distracting one from the real problem, that is, how to get a program to do
all the work of calculating, comparing results and generate a solution, or
giving partial problem requirements forcing one to research what extra info
is needed to write an algorithm

so the problem could have been stated in other ways e.g. i have three flight
Destinations {D1, D2, D3} select a route through all three cities with
highest passenger throughput factor where flying through D1 has on board
carry limit factors of 2.7 and 2.1 etc....

i think the solution here is to implement your "on paper
ideas/recipe/algorithm" for solving the stated problem and let program do
all the comparing, organizing until an optimal sequence/combination is
found

are you required to use particular lang ?

sounds like two different recipe/algorithms

7. futrtrublGuest

For this I would brute force it. Test all possible combinations of
transformer cascades and pick the best.
Asuming you're using an OO language the following pseudo code might
work
T1 = {TR=2,Iin = 2.7,Iout=2.1}
T2 = {TR=1.5,Iin = 0.2,Iout=0.3}
T3 = {TR=1.2,Iin = 0.3,Iout=1.6}

and asuming the transformers can be reversed...
T4 = {TR=1/T1.TR,Iin = T1.Iout,Iout=T1.Iin}
T5 = {TR=1/T2.TR,Iin = T2.Iout,Iout=T2.Iin}
T6 = {TR=1/T3.TR,Iin = T3.Iout,Iout=T3.Iin}

Then starting from the end take your Iout of the last transformer and
multiply it by its TR. Get the min of that, that transformer's Iin and
the Iout of the previous transformer in the chain to find the limiting
value for that step. Times by TR of the 2nd transformer, repeat the min
as above for the relevent transformers, times by the TR of the 1st
transformer in the cascade and get the min of that and the first
transformers Iin and now you have the max Iin for that cascade. Iterate
through the cascades and select the one with the highest Iin and return
it.

Edward