Trying to get my head around this but making no progress. I've tried calculating the power on secondary with hope that I can solve by having the same power on the primary side. But without voltage it seems like I am missing a piece to the puzzle.

Of course I know that Zp = (Zs) x (Np / Ns) ** 2.

Perhaps I am confused because I don't know what Zs is in this case.

Can someone please point me in the right direction?

Thanks!

 

Here's my attempt ...

 

Roger, this is a Homework Help forum, not a We'll Do Your Homework For You forum. The OP needs to do the problem with our help.

You calculate Zp for each secondary winding separately (where R1 and R2 are Zs). Since the load appears as a parallel load to the primary, you then calculate the parallel value of the two values calculated, to give the final value of Zp.

 

Roger, this is a Homework Help forum, not a We'll Do Your Homework For You forum. The OP needs to do the problem with our help.

OK, I just thought you might want to see how I was attempting to solve the problem. Thanks for helping, I will try and solve in the manner you suggested.

 

So

Zp for s1 = (10)(100/50)**2 = (10)(.5)**2 = 2.5 ohms
Zp for s2 = (25)(100/25)**2 = (10)(4)**2 = 400 ohms

Zp = 2.5 ohm in parallel with 400 ohm = 2.48 ohms

But if I choose a hypothetical voltage for Vp then the power calculations don't jive. Doesn't seem like 2.48 ohms is correct.

Assume Vp = 150V; then Vs1 = 75.5V & Vs2 = 37.5V
Ps1 = ((75.5)**2) / 10 = 570W
Ps2 = ((37.5)**2) / 25 = 56.25W
Pp = 570 + 56.25W = 626.25W

But (V*V)/R would be ((150)(150))/2.48 = 9072W

Seems like a Zp of 36 ohms would be closer than 2.48 ohms.

What am I missing?

Thanks!

 

Decided it would be easier to select an arbitrary value for Vp and then perform power calcs on secondary side and then trace back to primary side (assuming it is desired for max power xfer). Result Zp was 36.36 ohms. Built a spreadsheet model and confirmed resistance of 36.36 ohm is consistent for all choices of Vp.

Thanks for helping ...

 

OK, I just thought you might want to see how I was attempting to solve the problem. Thanks for helping, I will try and solve in the manner you suggested.

My bad. I didn't look closely at the name and thought your answer was from someone else answering your post, thus my reply made no sense.
Sorry about that.

 

So

Zp for s1 = (10)(100/50)**2 = (10)(.5)**2 = 2.5 ohms
Zp for s2 = (25)(100/25)**2 = (10)(4)**2 = 400 ohms

Zp = 2.5 ohm in parallel with 400 ohm = 2.48 ohms
.....................................
What am I missing?

You made a mistake in your math for the first equation.
Zp for s1 = 10 (100/50)^2 = 10 (2^2) = 40Ω
40Ω in parallel with 400Ω is 36.36Ω, the value you calculated by other means.

 

Thanks for catching that!

Now I have verification that my answer is correct ... Always like that.

Thanks for the assist ...