What is a Transformer?
A transformer is a static electrical device that transfers electrical energy from one circuit to another through electromagnetic induction — without any change in frequency. It consists of two stationary electrical circuits (windings) linked by a common magnetic circuit (iron core).
Unlike rotating machines such as motors and generators, a transformer has no moving parts. This makes it highly efficient (typically 95–99%), extremely reliable, and virtually maintenance-free. Transformers are the backbone of the AC power transmission and distribution system, enabling voltage to be stepped up for long-distance transmission and stepped down for safe consumer use.
The fundamental principle behind transformer operation is Faraday's Law of Electromagnetic Induction — a changing magnetic flux through a coil induces an electromotive force (EMF) in that coil.
Construction & Working Principle
A simple transformer consists of two coils (windings) wound on a laminated iron core. The coil connected to the AC supply is called the primary winding, and the coil connected to the load is called the secondary winding. There is no electrical connection between the two windings — energy transfer happens purely through magnetic coupling.
Basic arrangement of a simple transformer
Working step-by-step:
- An alternating voltage V₁ is applied to the primary winding, causing alternating current I₁ to flow.
- This alternating current produces an alternating magnetic flux (Φ) in the iron core.
- The alternating flux links with primary turns (T₁) and induces EMF E₁ by self-induction.
- The same flux also links with secondary turns (T₂) and induces EMF E₂ by mutual induction.
- When a load is connected to the secondary, current I₂ flows and electrical energy is delivered to the load.
Key point: A transformer works only with AC supply. It cannot operate on DC because a steady (non-changing) current produces a constant flux, which cannot induce any EMF according to Faraday's law.
EMF Equation of Transformer
The magnitude of EMF induced in each winding is given by the universal EMF equation:
Where:
- E = RMS value of induced EMF (Volts)
- Φₘ = Maximum flux in the core (Webers)
- f = Frequency of AC supply (Hertz)
- T = Number of turns in the winding
For the primary winding:
For the secondary winding:
The constant 4.44 comes from the derivation: 4.44 = 4 × 1.11, where 4 accounts for the full cycle of flux change and 1.11 is the form factor for a sinusoidal waveform. The winding with more turns will always have a higher induced voltage.
Transformation Ratio
The transformation ratio (k) defines the relationship between primary and secondary voltages and turns. It is the most important parameter for transformer design.
For an ideal transformer (no losses), the power relationship gives:
This means: if voltage is stepped up, current is stepped down proportionally, and vice versa. Power remains constant (ideally).
Step-Up, Step-Down & Isolation Transformer
- Step-Up Transformer: Secondary voltage > Primary voltage. Has more turns on secondary (T₂ > T₁), so k < 1. Used at generating stations to step up voltage for transmission.
- Step-Down Transformer: Secondary voltage < Primary voltage. Has fewer turns on secondary (T₂ < T₁), so k > 1. Used at distribution substations to reduce voltage for consumers.
- Isolation Transformer: Secondary voltage = Primary voltage. Has equal turns (T₁ = T₂), so k = 1. Used for electrical isolation and safety in medical equipment and laboratories.
Applications of Transformers
- Power Transmission: Step-up transformers at generating stations (11 kV → 220/400 kV) reduce I²R losses over long distances.
- Distribution: Step-down transformers at substations (400 kV → 33 kV → 11 kV → 415/230 V) deliver safe voltage to consumers.
- Electronics: Small transformers in phone chargers, laptop adapters, and power supplies convert mains voltage to low DC voltage.
- Welding: Welding transformers provide high current at low voltage for arc welding.
- Instrumentation: Current transformers (CT) and potential transformers (PT) measure high currents and voltages safely.
Step-Up vs Step-Down Transformer Comparison
Frequently Asked Questions
Q1: Why can't a transformer work on DC supply?
A transformer requires a changing magnetic flux to induce EMF. DC produces a constant flux (after initial transient), so no EMF is induced in the secondary. Additionally, applying DC to the primary causes excessive current (only winding resistance limits it) which can burn the winding.
Q2: What is the efficiency of a practical transformer?
Practical transformers have efficiency between 95% and 99%. Losses include core losses (hysteresis + eddy current) and copper losses (I²R in windings). Large power transformers achieve efficiency above 99%.
Q3: Why is the transformer core laminated?
The core is made of thin laminated sheets (0.35–0.5 mm) insulated from each other to reduce eddy current losses. Eddy currents flow in circular paths within the core — lamination breaks these paths and significantly reduces power loss and heating.
Q4: Can a step-up transformer be used as a step-down transformer?
Yes. If you reverse the connections — apply supply to the high-voltage winding and take output from the low-voltage winding — a step-up transformer functions as a step-down transformer. The physical construction is the same; only the connection determines the mode.
Q5: Why is transformer rating given in kVA instead of kW?
Transformer losses depend on voltage (core losses) and current (copper losses), not on the power factor of the load. Since kW = kVA × power factor, and the transformer manufacturer doesn't know what load will be connected, the rating is specified in kVA.