CSLB C-10 Electrical ContractorElectrical Theory and CalculationsEasy
A transformer has a primary winding with 600 turns and a secondary winding with 150 turns. If the primary voltage is 480V, what is the approximate secondary voltage?
- A960V
- B240V
- C120V
- D480V
Show answer & explanationAnswer & explanation
Correct answer: C. 120V
The voltage ratio of a transformer is directly proportional to the turns ratio: Vp/Vs = Np/Ns. Therefore, Vs = Vp × (Ns/Np) = 480V × (150 turns / 600 turns) = 480V × 0.25 = 120V.
Why the other options are wrong
- A. This would be the result if the transformer was a step-up transformer with the inverse turns ratio.
- B. This would imply a 2:1 turns ratio, not 4:1.
- D. This would mean a 1:1 turns ratio, no voltage transformation.
Transformer Voltage Ratio
The ratio of primary voltage to secondary voltage in a transformer is equal to the ratio of primary turns to secondary turns (Vp/Vs = Np/Ns).
- Directly proportional relationship.
- Used to calculate unknown voltage or turns.
- Assumes an ideal transformer for simplicity.
Memory trick: Voltages and Turns are Direct; Currents are Inverse.