Transformer Turns Ratio
Ratio
10:1
Secondary turns
100
How it works
A transformer transfers electrical energy between two circuits magnetically. The turns ratio (n = N₁/N₂) determines the voltage transformation: V₁/V₂ = N₁/N₂ = n. Current transforms inversely: I₁/I₂ = N₂/N₁ = 1/n. For an ideal transformer, power in = power out: V₁ × I₁ = V₂ × I₂.
**Step-up vs. step-down** A transformer with more secondary turns than primary (N₂ > N₁) steps up voltage while stepping down current. High-voltage transmission lines use step-up transformers (distribution substations step up from generator to 115–765 kV) to reduce current and minimize I²R transmission losses. Distribution transformers step down to 120/240V for residential use.
**Impedance transformation** Transformers also transform impedance: Z_primary_reflected = (N₁/N₂)² × Z_secondary. A 10:1 step-down transformer reflects secondary impedance as 100× higher at the primary. This is used in audio matching transformers (impedance matching between amplifier and speaker) and in power electronics for isolation.
**Losses and efficiency** Real transformers have losses: core losses (hysteresis and eddy currents in the core, relatively constant with load), copper losses (I²R in windings, proportional to load squared), and stray losses. Large power transformers achieve 97–99% efficiency. Distribution transformers: 95–98%. Cores operate at specific flux density (typically 1.0–1.7 Tesla for silicon steel) — exceeding this saturates the core, dramatically increasing magnetizing current and losses.
**Leakage inductance and voltage regulation** Practical transformers have leakage flux that doesn't couple both windings. Leakage inductance causes voltage to drop under load — voltage regulation = (V_noload - V_fullload) / V_fullload × 100%. Well-designed power transformers have regulation < 5%.
Frequently Asked Questions
- Turns ratio = V_primary / V_secondary = N_primary / N_secondary. For a transformer stepping down 240V to 12V: ratio = 240/12 = 20:1. If the primary has 2000 turns: N_secondary = 2000/20 = 100 turns. Current scales inversely: at full load, if secondary current is 10A, primary current = 10/20 = 0.5A (for ideal transformer). For real transformers, add magnetizing current (typically 1–5% of full load current) and account for efficiency losses when sizing the primary supply.
- When a transformer is energized, the core may begin in a state of residual magnetization — the initial flux may exceed the core's saturation limit, drawing magnetizing current 10–20× the rated value for the first few cycles. Inrush duration: typically 0.1–1 second for small transformers, several seconds for large power transformers. Inrush causes: nuisance tripping of circuit breakers and fuses (select slow-blow or 'time-delay' types), voltage sags on the supply, and mechanical stress in the windings (due to high current forces). Mitigations: pre-magnetization at correct residual flux, soft-start circuits, or breakers with inrush-immune current settings.
- VA rating = V_secondary × I_secondary_max (at rated conditions). The rating is limited by heat generation from core losses and copper losses. A 1 kVA transformer can supply: 1000 VA at any voltage/current combination on the secondary — 10A at 100V, 5A at 200V, 100A at 10V, etc. Power factor affects real power: at PF = 0.8, a 1 kVA transformer delivers only 800W real power. Transformer VA ratings are independent of load power factor — always size to the VA requirement, not the watt requirement, when the load has reactive current.
- An autotransformer uses a single winding with a tap point — part of the winding serves as both primary and secondary. Much smaller and cheaper than a conventional transformer for modest voltage ratios (within ±50%). 120V to 240V autotransformer uses less copper than an isolation transformer because some power is transmitted conductively (not magnetically). Disadvantage: no electrical isolation between primary and secondary — a fault on one side appears on the other. Used for: motor starters (reduced voltage starting), voltage boosters/regulators, and laboratory variacs (variable autotransformers). Prohibited where isolation is required (e.g., medical equipment, Class I Div 2 hazardous locations).