Electrical transformers are devices that change AC voltage from one level to another, making electric power transmission practical and efficient. They allow power companies to send electricity at high voltage and low current to reduce energy loss in long wires. In homes and devices, transformers then step the voltage down to safer or more useful levels.
Understanding transformers connects electromagnetism, energy conservation, and real engineering design.
A transformer works because a changing current in the primary coil creates a changing magnetic flux in the iron core. That changing flux passes through the secondary coil and induces an output voltage by electromagnetic induction. The voltage change depends mainly on the ratio of turns in the primary and secondary windings.
In an ideal transformer, power is conserved, so increasing voltage decreases current by the same factor.
Key Facts
- Transformer voltage ratio: Vs / Vp = Ns / Np
- Ideal power conservation: Pp = Ps, so VpIp = VsIs
- Step-up transformer: Ns > Np, so Vs > Vp and Is < Ip
- Step-down transformer: Ns < Np, so Vs < Vp and Is > Ip
- Faraday's law: induced voltage is caused by changing magnetic flux, E = -N dPhi/dt
- Laminated cores reduce eddy currents and energy loss by breaking up current loops in the metal core.
Vocabulary
- Primary winding
- The coil connected to the input AC voltage source in a transformer.
- Secondary winding
- The coil where the output voltage is induced by the changing magnetic flux.
- Turns ratio
- The ratio of secondary coil turns to primary coil turns, Ns / Np, which sets the ideal voltage ratio.
- Magnetic flux
- A measure of the magnetic field passing through a surface or loop of wire.
- Laminated core
- A core made of thin insulated metal layers that guide magnetic flux while reducing eddy current losses.
Common Mistakes to Avoid
- Using the turns ratio backward, which gives the wrong voltage change. Always match Vs / Vp with Ns / Np.
- Assuming a transformer works with steady DC, which is wrong because a constant current does not create changing magnetic flux. Transformers require changing current, usually AC.
- Thinking a transformer creates extra power, which violates energy conservation. In an ideal transformer, increasing voltage decreases current so input power equals output power.
- Ignoring core losses and heating in real transformers, which makes calculations too ideal for engineering use. Real transformers lose energy through resistance, hysteresis, eddy currents, and leakage flux.
Practice Questions
- 1 A transformer has 200 turns on the primary coil and 800 turns on the secondary coil. If the primary voltage is 120 V AC, what is the ideal secondary voltage?
- 2 An ideal step-down transformer changes 2400 V to 120 V and delivers 10 A to a load. What is the primary current?
- 3 A transformer core is made from many thin laminated sheets instead of one solid block of iron. Explain how this design reduces energy loss and why it improves efficiency.