CSLB C-10 Electrical ContractorElectrical Theory and CalculationsMedium
A transformer has a primary voltage of 480V and a secondary voltage of 240V. If the primary coil has 200 turns, how many turns should the secondary coil have for ideal operation?
- A400 turns
- B100 turns
- C200 turns
- D50 turns
Show answer & explanationAnswer & explanation
Correct answer: B. 100 turns
For an ideal transformer, the ratio of voltages is equal to the ratio of turns: Vp/Vs = Np/Ns. Rearranging for Ns: Ns = Np * (Vs/Vp). Ns = 200 * (240V/480V) = 200 * 0.5 = 100 turns.
Why the other options are wrong
- A. This would imply a step-up transformer, but the voltage is stepping down.
- C. This would mean a 1:1 transformer, which contradicts the voltage change.
- D. This would imply a 4:1 voltage ratio, which is incorrect.
Transformer Turns Ratio
The turns ratio of an ideal transformer relates the number of turns in the primary and secondary coils to their respective voltages and currents.
- Vp/Vs = Np/Ns (Voltage ratio equals turns ratio).
- Is/Ip = Np/Ns (Current ratio is inverse of turns ratio).
- Np = number of turns in primary coil.
- Ns = number of turns in secondary coil.
Memory trick: Voltage and Turns go hand-in-hand, Current is the inverse demand.