How it works
The EMF equation E = 4.44 f Φm N ties together supply frequency f = 50 Hz, maximum core flux Φm, and number of turns N. For a 230 V primary with 500 turns, Φm = 230/(4.44 × 50 × 500) = 2.07 mWb. The turns ratio a = N1/N2 = V1/V2 = I2/I1 for an ideal transformer; a 230/12 V unit has a ≈ 19.2. Core losses (eddy current + hysteresis) are minimised by using 0.35 mm thick laminations and grain-oriented CRGO steel, while copper losses I²R vary with load current.
Key points to remember
Transformer action requires alternating current — it cannot work on DC because a steady flux induces no EMF. The EMF equation E = 4.44fΦmN is the single most tested formula. Efficiency is maximum when copper loss equals iron loss, which typically occurs at 50–75% of full load for distribution transformers. All-day efficiency (energy efficiency) matters more than commercial efficiency for distribution transformers. The no-load current is typically 2–5% of full-load current and lags the supply by nearly 90°.
Exam tip
The examiner always asks you to derive the EMF equation from first principles — Faraday's law gives e = N dΦ/dt, and substituting Φ = Φm sinωt leads directly to Erms = 4.44fΦmN.