A step-down transformer is used on a 2.2 kV line to deliver 110 V. How many turns are on the primary windings if the secondary has 25 turns?

Analyze the question: A transformer changes voltage in proportion to the number of turns in its coils. We know the primary voltage (2.2 kV), the secondary voltage (110 V) and the secondary turns (25). We need the primary turns.

Concept: For an ideal transformer:

Vp / Vs = Np / Ns

where Vp and Vs are the primary and secondary voltages, and Np and Ns are the primary and secondary turns.

Solution:

1. Convert the voltage to volts: Vp = 2.2 kV = 2200 V
2. Vs = 110 V and Ns = 25
3. Substitute:

2200 / 110 = Np / 25

4. Simplify the left side:

20 = Np / 25

5. Solve for Np:

Np = 20 × 25 = 500 turns

Check: It is a step-down transformer, so the primary must have more turns than the secondary. 500 > 25, which agrees with this.

Answer: The primary winding has 500 turns, which is option D.

Exam tip: Always convert kV to V before substituting. The turns ratio here is 20:1, so the primary has 20 times as many turns as the secondary. In a step-down transformer, the coil with more turns is always the primary. Option A (20) is the trap: it is only the ratio, not the number of turns.