In one line
Single-phase transformers are used wherever AC voltage must be stepped up, stepped down, isolated or matched, from power distribution to chargers, welding sets and measuring instruments.
Explained simply
The transformer is the reason AC won over DC for power supply: it makes changing voltage cheap and efficient. High voltage is ideal for sending power far (low current, low loss), low voltage is safe for homes and electronics, and the transformer connects the two.
Technically, applications follow from three properties: voltage transformation in the turns ratio, inverse current transformation, and electrical isolation between windings (they are coupled only magnetically).
How it works
- Power transmission: step-up at the generating station to reduce current and \( I^2R \) loss; step-down at substations.
- Distribution: single-phase pole-mounted transformers for rural and residential supply.
- Power supplies and adapters: step 230 V down before rectification in linear DC supplies, battery chargers, UPS and inverters (where they also step up).
- Isolation transformers: 1:1 ratio for safety in medical equipment and test benches; also break ground loops.
- Instrument transformers: current transformers (CT) and potential transformers (PT) scale high currents and voltages down for meters and relays.
- Welding transformers: low voltage, very high current.
- Impedance matching: audio output transformers match amplifier output to speakers.
- Autotransformers / variacs: variable AC voltage in labs; voltage stabilisers at home.
- Doorbells, control circuits: small step-down transformers for 12–24 V control voltage.
Key formula
Transmission loss: \( P_{loss} = I^2R = \left(\dfrac{P}{V\cos\phi}\right)^2R \)
\( P \) = power transmitted (W), \( V \) = transmission voltage (V), \( \cos\phi \) = power factor, \( R \) = line resistance (\( \Omega \)). Raising \( V \) ten times cuts the loss to one-hundredth for the same power.
Quick example
A transformer-type battery charger: 230 V is stepped down to about 15 V by a single-phase transformer, rectified by diodes, filtered by a capacitor, and used to charge a 12 V battery. The transformer also isolates the battery from the mains for safety.
Common mistakes
- Writing 'used to convert AC to DC'; that is the rectifier's job.
- Listing DC applications such as DC motor speed control.
- Forgetting isolation and measurement (CT/PT) as applications.
How MU asks it
MU states numericals are not expected in this module, so questions are theory. Usually a 3–5 mark part: "State applications of single-phase transformers" or "Why is power transmitted at high voltage?". For full marks give five or six applications with one line each on why a transformer is needed there, and include the \( I^2R \) argument for high-voltage transmission.