Ben said:

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

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