# P.O.D.



## Wolverine (Oct 11, 2006)

A 240 volt 10kVA generator supplies a load through a 1:2 step up transformer, a relatively short transmission line with an impedance 1+j4 ohms, and a 4:1 step down transformer (ideal). If the load is known to be 1+j1 ohms, what equivalent load does the generator see?


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## Art (Oct 11, 2006)

17/4 + 5j


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## Frontier05 (Oct 12, 2006)

hmmm. I got 16.76 + j5.24 ohms for this one.


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## Wolverine (Oct 12, 2006)

Zpr(PrimaryReflected) = a^2 * Zs(Secondary)

For the line, Zp= 4^2 * (1+j1) = 16+j16

Added to the line Z = 1+j4+16+j16=17+j20

Reflected again = (1/2)^2 * (17+j20) = 17/4+j5

Answer: 4.25+j5.0 (or 6.56&lt;49.6)

:drunk:

Wake me up when October ends


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## Art (Oct 12, 2006)

or

(1/2^2) ((1 + 4j) + (4^2) (1 + j))

1/4 (1 + j + 16 (1+ j))

1/4 (1 + 4j + 16 + 16j)

1/4 (17 + 20j)

17/4 + 5j

T1r ^2 (Z line + T2r^2 x Z load)

T1r = 240/480 = 1/2

T2r 480/120 = 4


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## Frontier05 (Oct 12, 2006)

those ratios got me - I reversed them. oops.

Strange, I have my old college exam showing a step down xfmr just like this one, 480 to 120 and the ratio was a=1/4 (I got it wrong as I put down a=4, but it was marked incorrect and solution given as 1/4) ........... interesting.

Vp/Vs = Np/Ns


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## Art (Oct 12, 2006)

> those ratios got me - I reversed them.    oops.
> Strange, I have my old college exam showing a step down xfmr just like this one, 480 to 120 and the ratio was a=1/4 (I got it wrong as I put down a=4, but it was marked incorrect and solution given as 1/4)  ...........  interesting.
> 
> Vp/Vs = Np/Ns


v1/n1=v2/n2

v1/v2=n1/n2=a

go back and get your point


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## Frontier05 (Oct 12, 2006)

> > those ratios got me - I reversed them.    oops.
> > Strange, I have my old college exam showing a step down xfmr just like this one, 480 to 120 and the ratio was a=1/4 (I got it wrong as I put down a=4, but it was marked incorrect and solution given as 1/4)  ...........  interesting.
> >
> > Vp/Vs = Np/Ns
> ...


Believe me Art, I tried to get points back for that one - he didn't budge.

A few other internal transformer parameters were found using that incorrect a ratio. oh well, it was years ago.


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## Frontier05 (Oct 21, 2006)

> > > those ratios got me - I reversed them.    oops.
> > > Strange, I have my old college exam showing a step down xfmr just like this one, 480 to 120 and the ratio was a=1/4 (I got it wrong as I put down a=4, but it was marked incorrect and solution given as 1/4)  ...........  interesting.
> > >
> > > Vp/Vs = Np/Ns
> ...


ok, this is it. This problem with answers was freely given out so I don't think it hurts to share it. Most people in the class got the a = 1/4 (I put a=4 for step down and got it wrong, if a = 1/4, then just looking at the diagram, I2 =0 (aV1 would be 120V and so would V2) ...... The professor has a phd, yada yada and teaching for many many years, which made me second guess about it all. Anyway, it was a long time ago. Some of the marks on there are mine. thoughts

View attachment 190


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