How does a transformer step voltage up or down, and what is the principle?

• A. Turns ratio determines voltage change; power is conserved minus losses
• B. Frequency determines voltage change
• C. Power changes with turns ratio
• D. Voltage changes randomly; power is random

Answer: (A)

Voltage is changed by the turns ratio in a transformer. The voltages on the primary and secondary windings follow Vp/Vs = Np/Ns, so more turns on the secondary produce a higher voltage (step-up) and fewer secondary turns produce a lower voltage (step-down). Because energy must flow from primary to secondary, the power is roughly the same on both sides (Vin × Ip ≈ Vout × Is), so current changes inverse to the voltage: Is ≈ (Vp/Vs) × Ip. The frequency doesn’t set the voltage change; the same supply frequency drives the flux, and the induced voltages share that frequency. In an ideal transformer there are no losses, but real transformers have core and copper losses that mean the secondary power is slightly less than the primary.