Transformer construction and principles of operation; transformer losses and methods of overcoming them; action under load and no-load conditions; power transfer, efficiency, polarity markings and autotransformers.
Notes
Transformers — summary notes
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Main ideas
A transformer works by mutual induction: alternating current in the primary produces a changing flux that links the secondary and induces an EMF there. It only works on AC.
Current is in the INVERSE ratio: I_s / I_p = N_p / N_s. Stepping voltage up steps current down.
In an ideal transformer, power in equals power out: V_p·I_p = V_s·I_s.
Losses: copper loss (I²R in the windings), iron loss (hysteresis and eddy currents in the core), and flux leakage. Cores are laminated to cut eddy-current loss.
⚠ Exam trap: the current ratio is the inverse of the turns and voltage ratio. A step-up transformer steps the current DOWN.
Key formulas
Transformer ratios
V_p / V_s = N_p / N_s = I_s / I_p
Ideal power transfer
V_p × I_p = V_s × I_s
Efficiency
η = (output power / input power) × 100 %
Solved examples
A transformer has 200 primary turns and 800 secondary turns, fed with 115 V. Find the secondary voltage.
N_s/N_p = 800/200 = 4, so V_s = 115 × 4 = 460 V — a step-up transformer.
That same 1:4 step-up transformer draws 8 A in the primary. What is the secondary current?
Current is in the inverse ratio: I_s = 8 / 4 = 2 A. Power checks: 115 × 8 = 920 VA, and 460 × 2 = 920 VA.