Complex Imaginary Test Question 27

[pbo064]

Good morning all:

I am reviewing for the Power PE April 2013 exam. I just worked Complex Imaginary's Test 2 Question 27. The question gives you a 20 kVA autotransfromer simple diagram with 300 V on the low side and 600 V on the high side, with coils N1 and N2 in between the two measurements. The question asks what turn ratio is possible.

The answer used the following formula: (V1/V2)=(1/(1+(n1/n2))) to get the answer of turns ratio =1. I am not familiar with this formula and by inspection I would think the answer is turns ratio = 2. Has anybody else encoutered this formula and can vouch for its validity? I cannot find it in my reference material or internet searching.

Thank you!

 

[Wael]

For Auto transformers formula, open page 342 in the CI exam book. i am referring to problem 30 in Volume 3.

In general: V1(high side) is across both coils (N1 +N2), V2 (low side) is across N2.

Apply the formula V1/V2 = (N1+N2)/N2 = [(N1/N2)+1] this formula is based on (Vp/Vs = Np/Ns)

All the best

 

[pbo064]

Thank you, I acutally recently worked this problem (test 3, q 30) and found the conversion the divide by sqrt(3) of the voltage to be an unecessary step since the ratio between the voltages (turns ration, "a") is the same without the conversion. I will note that all autotransformers conneted to wye systems need a divide by sqrt(3) for the future though.