CSLB C-10 Electrical ContractorElectrical Theory and CalculationsMedium
A transformer is used to step down voltage from 4800V to 240V. If the secondary current is 50 amps, what is the approximate primary current, assuming an ideal transformer?
- A5 amps
- B10 amps
- C2.5 amps
- D1.25 amps
Show answer & explanationAnswer & explanation
Correct answer: C. 2.5 amps
For an ideal transformer, the power in the primary equals the power in the secondary (Vp * Ip = Vs * Is). Rearrange to solve for Ip: Ip = (Vs * Is) / Vp.
Why the other options are wrong
- A. This is a calculation error, likely a misplaced decimal or incorrect ratio.
- B. This is a calculation error, possibly confusing voltage and current ratios.
- D. This is a calculation error, possibly inverting the current ratio.
Transformer Current Ratio (Ideal)
In an ideal transformer, the ratio of primary current to secondary current is inversely proportional to the ratio of primary voltage to secondary voltage (or directly proportional to the turns ratio of secondary to primary).
- Formula: Vp/Vs = Is/Ip (or Ip = (Vs * Is) / Vp).
- Assumes 100% efficiency (no losses).
- If voltage steps down, current steps up (and vice versa).
Memory trick: Power In = Power Out; Voltage and Current swap their ratios!