Side-by-side comparison
| Parameter | Ideal | Practical Transformer |
|---|---|---|
| Core Losses | Zero (no hysteresis, no eddy current loss) | P_iron = P_hysteresis + P_eddy ≈ 0.5–2% of rated kVA |
| Copper Losses | Zero (winding resistance = 0) | P_cu = I1²R1 + I2²R2, varies with load current squared |
| Leakage Flux | Zero — all flux links both windings (coupling = 1) | Non-zero leakage reactance X1 and X2 in equivalent circuit |
| Magnetizing Current | Zero — no current needed to establish flux | I_m = 2–8% of rated current, lags V1 by 90° |
| Voltage Regulation | 0% — secondary voltage constant at any load | 2–8% depending on load power factor and impedance |
| Efficiency | 100% | 98–99.5% at full load for large power transformers |
| Equivalent Circuit | Ideal turns ratio only: V1/V2 = N1/N2, I1/I2 = N2/N1 | Approximate circuit: R_eq, X_eq in series; R_c, X_m shunt branch |
| Core Material Effect | Not considered | CRGO (cold-rolled grain-oriented) steel reduces P_iron by ~30% |
Key differences
An ideal transformer satisfies V1/V2 = N1/N2 and I1/I2 = N2/N1 exactly, with 100% efficiency. A practical 100 kVA, 11 kV/433 V distribution transformer has approximately 250 W iron loss (constant) and 1 kW copper loss at full load — efficiency peaks at about 80% load where iron and copper losses are equal. Leakage reactance X_eq causes voltage regulation: a 0.05 pu X_eq at 0.8 lagging power factor gives roughly 5% regulation, meaning the 433 V no-load voltage drops to about 411 V at full load.
When to use Ideal
Use the ideal transformer model when calculating turns ratios, impedance reflection, or maximum power transfer in a quick circuit analysis — for example, reflecting a 50 Ω load through a 10:1 turns ratio to get 5000 Ω at the primary.
When to use Practical Transformer
Use the practical transformer equivalent circuit when calculating efficiency, voltage regulation, or copper and core losses — for example, finding the full-load efficiency of a 50 kVA distribution transformer given open-circuit and short-circuit test data.
Recommendation
In exam problems, identify whether the question asks for turns-ratio calculations (use ideal model) or efficiency and regulation (use practical equivalent circuit with R_eq and X_eq). The OC test gives core loss and X_m; the SC test gives copper loss and R_eq — always state which test gives which parameter.
Exam tip: Examiners always pair open-circuit and short-circuit test data in transformer problems — know that OC test is done at rated voltage on LV side to find R_c and X_m, while SC test is done at rated current on HV side to find R_eq and X_eq.
Interview tip: Interviewers at BHEL or Siemens ask why transformer efficiency peaks below full load — answer: efficiency is maximum when variable copper loss (I²R) equals fixed iron loss (P_iron), which typically occurs at 50–80% of rated load.