Describe the construction of a simple transformer with a soft-iron core, as used for voltage transformations
4.5.6 The Transformer – Construction of a Simple Transformer with a Soft‑Iron Core
Objective
To describe, step‑by‑step, the construction of a simple transformer that uses a soft‑iron core for voltage transformation, and to explain the operating principle and practical considerations required by the Cambridge IGCSE 0625 syllabus.
Key Components (AO1)
- Soft‑iron core – high magnetic permeability, low electrical conductivity, provides a low‑reluctance magnetic path. Made from thin insulated laminations (≈ 0.35 mm) to break eddy‑current paths.
- Primary winding – enamel‑coated copper wire, number of turns \(N_p\) set by the required voltage ratio.
- Secondary winding – enamel‑coated copper wire, number of turns \(N_s\) set by the required voltage ratio.
- Insulating material – varnish, paper or polyester film placed between the two windings and between windings and core.
- Terminal leads – stranded copper leads, soldered or crimped, clearly marked for safe external connections.
Construction Steps (AO1)
- Safety check before handling the core – ensure the core is not magnetised and that the work area is clear of metal objects that could become attracted.
- Prepare the core – stack thin soft‑iron laminations (≈ 0.35 mm) or wind a toroidal ring. The laminations interrupt circulating eddy currents, thereby reducing eddy‑current loss.
- Wind the primary – wind the enamel‑coated copper wire tightly and uniformly around the core to obtain the required number of turns \(N_p\). All turns must be wound in the same direction.
- Insulate the primary – cover the primary winding with a thin layer of varnish, paper or polyester film to prevent electrical contact with the secondary.
- Wind the secondary – over the insulated primary, wind the secondary wire to obtain the required number of turns \(N_s\). Keep the winding direction the same as the primary so that the same magnetic flux links both windings.
- Mark winding polarity – label the start and finish of each winding (e.g., “P‑in”, “P‑out”, “S‑in”, “S‑out”). Correct polarity is essential for the proper phase relationship of the induced voltages.
- Secure and terminate – bind the windings with insulating tape or resin, attach the terminal leads, and label them clearly.
- Encapsulate – place the assembled core and windings in a protective non‑conductive casing to guard against mechanical damage, dust and accidental contact.
- Final safety check – verify that all insulation is intact, terminals are clearly marked, and a suitable fuse or circuit‑breaker is ready for the upcoming test.
Physical Arrangement (Diagram)
Operating Principle (AO1 + AO2)
When an alternating voltage \(Vp\) is applied to the primary winding, an alternating current \(Ip\) flows and creates an alternating magnetic flux \(\Phi\) in the soft‑iron core. The same flux links the secondary winding and induces an emf according to Faraday’s law.
Faraday’s law (AO1)
\[
Ep = -\,Np\;\frac{d\Phi}{dt}\qquad\text{(1)}
\]
\[
Es = -\,Ns\;\frac{d\Phi}{dt}\qquad\text{(2)}
\]
Turn‑ratio equation (AO2 – problem‑solving tool)
\[
\frac{Vs}{Vp} = \frac{Ns}{Np}\qquad\text{(3)}
\]
Current relationship (AO2)
\[
\frac{Is}{Ip} = \frac{Np}{Ns}\qquad\text{(4)}
\]
Power balance for an ideal transformer (AO1)
\[
Vp Ip = Vs Is\qquad\text{(5)}
\]
Equations (1)–(5) describe an ideal transformer (no core hysteresis, no eddy‑current loss, no copper resistance). Real devices deviate slightly because of losses, as described below.
Losses, Efficiency and Practical Performance (AO2)
- Core losses
- Hysteresis loss – energy dissipated each time magnetic domains reverse.
- Eddy‑current loss – circulating currents in the core; greatly reduced by lamination.
- Copper loss – I²R heating in the windings.
- Typical efficiency – a well‑designed 10 W transformer may lose about 0.5 W, giving an efficiency of \(\frac{10-0.5}{10}\times100 \approx 95\%\).
- Voltage regulation – the secondary voltage drops slightly when a load is connected (full‑load voltage < no‑load voltage). This effect can be measured experimentally.
Practical Investigation (AO3)
Students can explore the turn‑ratio and voltage regulation with the following procedure:
- Connect the primary to a 240 V r.m.s. AC supply through a suitable fuse.
- Using a voltmeter, measure the secondary voltage with the secondary open‑circuit (no‑load voltage, \(V_{s,\text{nl}}\)).
- Connect a known resistive load (e.g., 10 Ω) across the secondary.
- Measure the secondary voltage again (full‑load voltage, \(V_{s,\text{fl}}\)).
- Calculate voltage regulation:
\[
\text{Regulation (\%)} = \frac{V{s,\text{nl}} - V{s,\text{fl}}}{V_{s,\text{fl}}}\times100
\]
- Repeat with a different load (e.g., 20 Ω) and compare the results.
Safety (AO3)
- Check that all windings are completely insulated before applying any voltage.
- Label polarity clearly and never connect the secondary in reverse.
- Keep the transformer away from moisture and conductive objects.
- Do not touch live terminals; use insulated tools.
- Always use a suitable fuse or circuit‑breaker on the primary side to protect against over‑current.
Typical Construction Details (Table)
| Component | Material / Feature | Purpose (AO1) |
|---|---|---|
| Core | Soft‑iron laminations, 0.35 mm thick, insulated between sheets | Provides a low‑reluctance magnetic path and reduces eddy‑current loss. |
| Primary winding | Copper wire, enamel‑coated, gauge sized for primary current, \(N_p\) turns | Creates the alternating magnetic flux when AC voltage is applied. |
| Insulating layer | Varnish, paper or polyester film | Prevents electrical contact between primary and secondary windings. |
| Secondary winding | Copper wire, enamel‑coated, gauge sized for secondary current, \(N_s\) turns | Induces the transformed voltage according to the turn‑ratio equation. |
| Terminal leads | Stranded copper, soldered or crimped, clearly marked | Provides safe external electrical connections. |
Extended Learner Section (Supplementary Material)
- Why lamination reduces eddy‑current loss: Eddy currents flow in planes perpendicular to the magnetic flux. Thin insulated laminations interrupt these paths, forcing the currents to circulate within each sheet only, which dramatically lowers the I²R loss.
- Turn‑ratio effect on voltage and current: From (3) and (4), increasing the number of secondary turns raises the secondary voltage but reduces the secondary current proportionally, keeping power (neglecting losses) constant.
- No‑load vs. full‑load voltage: With no load, the secondary voltage is essentially \(Vs = (Ns/Np)Vp\). Under load, copper resistance causes a voltage drop, giving a lower measured voltage – this is quantified as voltage regulation.
Key Points to Remember (AO1)
- The core must be made of a material with high magnetic permeability and low electrical conductivity; laminating the core reduces eddy‑current loss.
- The voltage ratio depends only on the turn ratio \(\displaystyle\frac{Vs}{Vp} = \frac{Ns}{Np}\); core size and material affect efficiency, not the ratio.
- Correct polarity marking and good insulation are essential for safe operation and correct phase relationship.
- Real transformers exhibit core and copper losses; typical small‑power units achieve ≈ 95 % efficiency and show a small drop in secondary voltage under load.
- When carrying out practical work, always use a fuse, keep live terminals covered, and follow the step‑by‑step safety checklist.