CSLB C-10 Electrical ContractorElectrical Theory and CalculationsHard
A transformer is used to step down voltage from 2400V to 120V. If the primary current is 5 amps, what is the approximate secondary current, assuming an ideal transformer?
- A100 amps
- B25 amps
- C120 amps
- D50 amps
Show answer & explanationAnswer & explanation
Correct answer: A. 100 amps
For an ideal transformer, the ratio of voltages is inversely proportional to the ratio of currents: Vp/Vs = Is/Ip. Rearranging for Is: Is = Ip * (Vp/Vs). Is = 5A * (2400V/120V) = 5A * 20 = 100 amps.
Why the other options are wrong
- B. This is too low, possibly from an incorrect ratio or calculation error.
- C. This is too high, indicating a multiplication error or misapplication of the formula.
- D. This is half of the correct value, indicating a potential error in the voltage ratio.
Transformer Current Ratio
In an ideal transformer, the ratio of primary to secondary voltage is inversely proportional to the ratio of primary to secondary current.
- Vp/Vs = Is/Ip (Voltage ratio is inverse of current ratio).
- Np/Ns = Is/Ip (Turns ratio is inverse of current ratio).
- Ip = primary current, Is = secondary current.
- Assumes 100% efficiency (ideal transformer).
Memory trick: Voltage goes down, Current goes up, Power stays equal, don't mess it up.