Generators and Transformers

Generators convert kinetic energy into electrical energy using electromagnetic induction. Transformers change the voltage of an alternating supply.

AC generator

An AC generator has a coil rotating in a magnetic field. As the coil rotates, it cuts through magnetic field lines and an e.m.f. is induced.

Key features:

• The coil is connected to slip rings (continuous rings) and brushes.
• Slip rings maintain contact without reversing the current, producing alternating current (AC).
• The output voltage varies sinusoidally: positive for half a rotation, negative for the other half.

Increasing the output:

• Rotate the coil faster (increases frequency and peak voltage).
• Use a stronger magnet.
• Use more turns on the coil.

Transformer

A transformer has two coils (primary and secondary) wound on a shared soft iron core.

How it works:

1. AC in the primary coil creates a changing magnetic field.
2. The soft iron core carries this changing field to the secondary coil.
3. The changing field induces an e.m.f. in the secondary coil (electromagnetic induction).

The turns ratio equation:

$\frac{V_p}{V_s} = \frac{N_p}{N_s}$

• Step-up transformer: $N_s > N_p$, so $V_s > V_p$ (increases voltage).
• Step-down transformer: $N_s < N_p$, so $V_s < V_p$ (decreases voltage).

For an ideal (100% efficient) transformer:

$V_p I_p = V_s I_s$

A step-up transformer increases voltage but decreases current by the same factor.

Worked example

A transformer has 500 primary turns and 50 secondary turns. The primary voltage is 230 V. Find the secondary voltage.

$V_s = \frac{V_p \times N_s}{N_p} = \frac{230 \times 50}{500} = 23\text{ V}$

This is a step-down transformer (fewer secondary turns, lower voltage).

Why transformers need AC

Transformers only work with AC because the magnetic field must be continuously changing to induce an e.m.f. in the secondary coil. A constant (DC) current produces a constant field, which does not induce a voltage.

Common errors and how to correct them

Stating transformers work with DC. They require AC because only a changing current produces a changing magnetic field.

Confusing step-up and step-down. More secondary turns = step-up (higher voltage). Fewer secondary turns = step-down (lower voltage).

How to apply this in an exam

Use the turns ratio equation. State whether the transformer is step-up or step-down. For efficiency questions, use $V_p I_p = V_s I_s$ and state the assumption of 100% efficiency.