1. Concept Overview
A transformer is a device that changes (steps up or steps down) AC voltage using electromagnetic induction. It works on the idea that a changing magnetic flux in one coil induces an emf in another coil placed close to it. There is no electrical contact between the two coils — energy is transferred magnetically.
Once I understood that only AC can change flux and only changing flux can induce emf, the working of a transformer became very clear.
1.1. Simple idea I always remember
AC in primary → changing flux in core → induced emf in secondary.
2. Basic Construction of a Transformer
A transformer has only a few parts, and each part has a very clear role. The construction is actually simpler than it looks at first glance.
2.1. 1. Primary coil
The coil into which AC voltage is supplied. Changing current in the primary creates changing magnetic flux in the core.
2.2. 2. Secondary coil
The coil in which emf is induced due to changing flux from the primary. The output AC voltage comes from this coil.
2.3. 3. Laminated iron core
A soft iron core links both coils magnetically. Laminations reduce eddy currents, preventing unnecessary heating. The core ensures most of the magnetic flux produced by the primary passes through the secondary.
2.4. 4. Insulation
Both coils are insulated from each other and from the core to avoid electrical contact.
3. Working Principle
The working principle of a transformer is based on mutual induction. When alternating current flows in the primary coil, the magnetic flux in the iron core keeps changing. This changing flux links with the secondary coil and induces an emf in it.
If the secondary coil is connected to a load, current flows due to this induced emf.
3.1. Step-by-step idea
- AC in primary → changing current.
- Changing current → changing magnetic flux in the core.
- Changing flux links with the secondary coil.
- According to Faraday's law, induced emf appears in the secondary.
- Voltage depends on number of turns in each coil.
3.2. Key point
A transformer does not work with DC because DC produces no changing flux and therefore no induced emf.
4. Voltage Relationship
The induced emf in each coil is proportional to the number of turns in that coil. This gives the transformer equation:
\( \dfrac{V_s}{V_p} = \dfrac{N_s}{N_p} \)
where:
- \( V_p \): primary voltage
- \( V_s \): secondary voltage
- \( N_p \): primary turns
- \( N_s \): secondary turns
4.1. Step-up transformer
When \( N_s > N_p \), the secondary voltage is greater than the primary voltage. The transformer increases voltage.
4.2. Step-down transformer
When \( N_s < N_p \), the secondary voltage is lower than the primary voltage. The transformer decreases voltage.
5. Current Relationship
Transformers ideally conserve power (ignoring losses), so:
\( V_p I_p = V_s I_s \)
This leads to:
\( \dfrac{I_s}{I_p} = \dfrac{N_p}{N_s} \)
So if voltage is stepped up, current is stepped down, and vice versa.
5.1. Important idea
A step-up transformer increases voltage but reduces current. A step-down transformer lowers voltage but increases current.
6. Why Laminations Are Used
The iron core is made up of thin laminated sheets separated by insulation. This reduces eddy currents, which would otherwise cause heating and energy loss.
6.1. Reason
Laminations increase resistance to eddy currents and force them to form small, weak loops instead of large, strong ones.
7. Energy Losses in Transformers
Transformers are not perfectly efficient. Some energy is lost in different forms.
7.1. Copper losses
Heat produced due to resistance of the windings.
7.2. Iron (core) losses
- Hysteresis loss: energy lost during magnetization cycles of the core.
- Eddy current loss: circulating currents in the core that produce heating.
7.3. Flux leakage
Not all the magnetic flux produced by the primary links with the secondary. Some flux is lost.
8. Applications of Transformers
Transformers are everywhere in electrical systems because they allow us to adjust voltage easily and efficiently.
8.1. Power transmission
Step-up transformers increase voltage for long-distance transmission, reducing power loss. Step-down transformers then lower the voltage for safe use in homes and industries.
8.2. Adapters and chargers
Mobile chargers and power adapters use small transformers to reduce high AC mains voltage to a safe low voltage.
8.3. Audio and communication systems
Used for impedance matching and signal isolation.
8.4. Welding machines
Use heavy step-down transformers to supply high current at low voltage.
9. Special Types of Transformers
Different applications may need different designs. Some common types include:
9.1. Isolation transformer
Used to separate circuits electrically while allowing power transfer.
9.2. Autotransformer
Has a single winding acting as both primary and secondary. More compact and efficient for small voltage changes.
9.3. Current transformer
Used to measure large currents safely by producing a small proportional current.