Cambridge IGCSE Physics 0625 – Transformer Construction
4.5.6 The Transformer
Objective
Describe the construction of a simple transformer with a soft‑iron core, as used for voltage transformations.
Key Components of a Simple Transformer
Soft‑iron core – provides a low‑reluctance path for the magnetic flux.
Primary winding – coil of insulated copper wire that receives the input (primary) voltage \$V_p\$.
Secondary winding – coil of insulated copper wire that delivers the transformed (secondary) voltage \$V_s\$.
Insulating material – separates the windings from each other and from the core to prevent short circuits.
Terminal connections – metal leads or clamps that connect the windings to external circuits.
Construction Steps
Form the soft‑iron core by stacking thin laminated sheets or by winding a toroidal ring. The lamination reduces eddy‑current losses.
Place the primary winding on the core. The wire is wound tightly and uniformly, usually in several layers, to achieve the required number of turns \$N_p\$.
Insulate the primary winding with a thin layer of varnish or paper before adding the secondary winding.
Wind the secondary coil over the insulated primary. The number of turns \$N_s\$ determines the output voltage.
Secure the windings with a binding material (e.g., tape or resin) and attach terminal leads to each winding.
Encapsulate the assembled core and windings in a protective casing to shield against mechanical damage and dust.
Physical Arrangement
Suggested diagram: Cross‑section of a simple transformer showing the soft‑iron laminated core, primary winding, insulating layer, and secondary winding.
Operating Principle
When an alternating voltage \$Vp\$ is applied to the primary winding, an alternating current \$Ip\$ flows, creating an alternating magnetic flux \$\Phi\$ in the soft‑iron core. This flux links both windings, inducing an electromotive force (EMF) in each according to Faraday’s law.
The induced EMFs are given by:
\$Ep = -Np \frac{d\Phi}{dt}, \qquad Es = -Ns \frac{d\Phi}{dt}\$
Because the same flux \$\Phi\$ links both windings, the ratio of the RMS voltages is directly proportional to the ratio of the numbers of turns:
\$\frac{Vs}{Vp} = \frac{Ns}{Np}\$
For an ideal transformer (no losses), the power input equals the power output:
\$Vp Ip = Vs Is\$
Typical Construction Details
Component
Material / Feature
Purpose
Core
Soft‑iron laminations (0.35 mm thick)
Provides low magnetic reluctance; reduces eddy‑current loss.
Primary winding
Copper wire, enamel‑coated, gauge appropriate to current
Creates magnetic flux when AC voltage is applied.
Insulating layer
Varnish, paper, or polyester film
Prevents electrical contact between windings.
Secondary winding
Copper wire, enamel‑coated, may have different gauge
Induces the transformed voltage.
Terminal leads
Stranded copper with soldered or crimped connections
Provides external electrical access.
Key Points to Remember
The core must be made of a material with high magnetic permeability and low electrical conductivity (soft iron) to minimise losses.
Laminate the core to break up eddy‑current paths.
The voltage ratio depends only on the turn ratio; the core material and size affect efficiency, not the ratio.
Proper insulation between windings is essential for safety and to avoid short circuits.
In practical transformers, some losses (core hysteresis, eddy currents, copper resistance) cause the actual voltage ratio to deviate slightly from the ideal value.