EASA/GCAA Part-66 · Module 3 — Electrical Fundamentals · 3.13 Transformers

Transformer — Turns Ratio & Power

Objective: a transformer changes AC voltage in proportion to its turns — Vs/Vp = Ns/Np. Because power is (ideally) conserved, stepping voltage up steps current down. Set the windings, supply and load, and watch the flux link both coils through the iron core.

Iron core, windings & magnetic flux

Primary vs secondary voltage

Primary voltage V_p 115 V

Primary turns N_p 200

Secondary turns N_s 400

Load R_L 50 Ω

Secondary V_s

Turns ratio

Primary I_p

Type

—

V_s/V_p = N_s/N_p · I_p/I_s = N_s/N_p · V_p·I_p = V_s·I_s (ideal)

The AC primary current sets up an alternating flux in the iron core. That same flux threads the secondary winding, inducing a voltage in each turn. So the voltages share out in proportion to the turns: more secondary turns than primary gives a higher voltage — a step-up transformer — and fewer gives a step-down. An ideal transformer loses no power, so V_p·I_p = V_s·I_s: whatever you gain in voltage you pay back in current. That is why transmission lines step voltage up to reduce current and cut I²R heating in the wires. Transformers work on AC only — a steady DC current makes no changing flux, so it induces nothing.

Try this: Make N_s > N_p for step-up — V_s rises but I_p is drawn higher than I_s.
Try this: Set N_s = N_p — a 1:1 isolation transformer: same voltage, galvanic separation.
Try this: Lower the load R_L — more secondary current is drawn, and the primary current climbs to match.
Try this: Confirm the two power figures Vp·Ip and Vs·Is stay equal for any setting.