Question 22 CEL02 - Chief Engineer - Limited (Alt)

The Correct Answer is C ### Explanation for Option C (35 to 1) The turns ratio of a transformer is mathematically equivalent to the voltage ratio: $$\text{Turns Ratio} (a) = \frac{\text{Primary Voltage} (V_p)}{\text{Secondary Voltage} (V_s)}$$ In this scenario: * Primary Voltage ($V_p$): 4160 VAC (Generator Output) * Secondary Voltage ($V_s$): 120 VAC (Nominal AVR Input) Calculating the required ratio: $$a = \frac{4160 \text{ V}}{120 \text{ V}} \approx 34.666...$$ The calculated ratio (34.67) is rounded up to the standard integer ratio of **35 to 1**. Using a 35:1 transformer would provide an output voltage of $4160 / 35 \approx 118.86$ VAC, which perfectly fits the definition of a "nominally 120 VAC" system voltage. ### Explanation of Why Other Options Are Incorrect **A) 1 to 4 and B) 4 to 1** These ratios are far too small. Since this is a step-down transformer reducing 4160V to 120V, the primary side must have significantly more turns than the secondary side. A 4 to 1 ratio would only reduce the voltage to $4160 \text{ V} / 4 = 1040 \text{ VAC}$. **D) 40 to 1** While closer in magnitude than A or B, this ratio is too high. A 40 to 1 ratio would reduce the voltage to $4160 \text{ V} / 40 = 104 \text{ VAC}$. Although 104V is a usable voltage, 118.86V (from the 35:1 ratio) is a much more accurate match for a system specified as nominally 120 VAC.