Basics
Mutual induction
Current in the primary establishes magnetic flux in the core. When that current changes, the flux changes. A secondary winding linked by the same flux then develops a voltage.
Ratios
VP/VS ≈ a
IP/IS ≈ 1/a
VA rating
Transformers are commonly rated in volt-amperes rather than watts because winding heating depends on current whether or not the load power factor is one.
Loaded voltage
Real secondary voltage normally falls as load increases because winding resistance and leakage reactance cause internal drop. This change is called voltage regulation.
Losses
- Copper loss in winding resistance.
- Hysteresis loss in the core.
- Eddy-current loss in conductive core material.
- Leakage and stray magnetic loss.
- Dielectric and mechanical losses.
Center taps
A center-tapped 12-0-12 V secondary provides about 12 V from either end to center and about 24 V end-to-end.
Series and parallel secondaries
Identical secondaries may sometimes be placed in series for more voltage or parallel for more current. Polarity must be correct.
Frequency
A 60 Hz transformer should not simply be run at a much lower frequency with the same voltage. Lower frequency raises core flux and can drive the core into saturation.
Hum
Magnetostriction and magnetic forces make core and winding structures move slightly. That mechanical vibration produces familiar transformer hum, often at line frequency and its harmonics.