Power Electronics Circuits
When an ideal transformer converts the primary voltage into a lower secondary voltage, which of the following correctly describes the relationship between the primary and secondary currents?
- AThe primary and secondary currents are always equal, regardless of the turns ratio
- BLowering the voltage reduces the power by the same ratio, so the secondary current is also smaller than the primary current
- CBecause the power is equal, reducing the voltage to 1/a makes the secondary current a times the primary current
Show answer and explanation
Answer: C. Because the power is equal, reducing the voltage to 1/a makes the secondary current a times the primary current
An ideal transformer has no losses, so the power on the primary side equals the power on the secondary side (V1 × I1 = V2 × I2). The voltage is proportional to the number of turns (V1/V2 = N1/N2), and the current is inversely proportional to it (I1/I2 = N2/N1). For example, when a 10:1 turns ratio turns 100 V into 10 V and 5 A flows in the secondary, the primary current is 0.5 A. A transformer works by electromagnetic induction, so it can be used only with AC; converting DC requires first turning it into AC by switching.
- A. The current is inversely proportional to the number of turns. The currents are equal only when the turns ratio is 1:1.
- B. Lowering the voltage makes the secondary current larger, so that the power is preserved.
- C. Correct. The product of voltage and current (the power) is the same on the primary and secondary sides.