Transformers
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.
Transformers, summary notes
- 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.
- Turns ratio governs voltage: V_p / V_s = N_p / N_s.
- 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.
- 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 %
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.
Transformer turns ratio
Transformers concept map
Transformers
Transformers quiz
Transformers, quiz
1. The transformer shown has 200 primary turns and 50 secondary turns, supplied at 240 V. The secondary voltage is:
60 V960 V120 V2. A transformer has 200 primary and 800 secondary turns, with a primary current of 8 A. Assuming no losses, the secondary current is:
2 A32 A8 A3. A transformer steps voltage up 1:4. The secondary current, relative to the primary, is:
One quarterFour times largerUnchanged4. A transformer with 200 primary and 800 secondary turns fed at 115 V gives an output of:
460 V28.75 V115 V5. A transformer will not operate on DC because:
A steady current produces no changing fluxThe windings would short-circuitDC cannot flow in copper6. Copper loss in a transformer is caused by:
I²R heating in the windingsHysteresis in the coreEddy currents in the core7. Transformer cores are laminated to reduce:
Eddy-current lossCopper lossFlux leakage8. In an ideal transformer:
Power in equals power outVoltage in equals voltage outCurrent in equals current out9. A step-down transformer has:
Fewer secondary turns than primary turnsMore secondary turns than primary turnsEqual numbers of turns10. A 1:4 step-up transformer draws 8 A in the primary. The secondary current is:
2 A32 A8 A