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eduKate Learning Manual: Transformer Practical Skills | When Turns Ratio Works and Energy Losses Refuse to Vanish

Wait, What? A transformer can obey the turns-ratio equation and still waste energy.

The ideal relation Vs/Vp = Ns/Np describes voltage transformation, not perfect efficiency. Real transformers heat their coils and core, leak magnetic flux and behave differently under load. A good practical therefore separates two questions: does the voltage ratio match the turn ratio, and how much input power actually reaches the output?

The experimental model

For an ideal transformer:

Vs/Vp = Ns/Np

and, if no energy is lost:

VpIp = VsIs

Real devices satisfy the first relation approximately under suitable conditions while giving output power below input power. The Institute of Physics explicitly uses transformer experiments to compare voltage and turns ratios and to expose real energy losses. See IOPSpark Episode 416.

AC is not optional

A transformer needs changing magnetic flux. A steady DC current creates a largely steady field after the switching transient, so there is no sustained transformer action. Use an appropriate low-voltage AC supply for school investigations unless apparatus is specifically designed otherwise.

Turns ratio: the first test

With a demountable transformer, choose known primary and secondary turn counts and measure the RMS voltages. If Np = 600 and Ns = 150, an ideal ratio predicts:

Vs/Vp = 150/600 = 0.25

If Vp = 6.0 V RMS, the ideal secondary voltage is about 1.5 V RMS.

Open-circuit voltage and loaded voltage are different experiments

With almost no load, secondary current is small and copper voltage drop is small. Connect a load and current rises; the secondary terminal voltage can fall because real coils have resistance and because magnetic coupling is not perfect.

Therefore do not mix no-load voltage-ratio data with loaded efficiency data without stating the conditions.

Efficiency requires power, not just voltage

For a resistive load and appropriately measured RMS quantities:

η = Pout/Pin × 100%

with approximate powers:

Pin ≈ VpIp, Pout ≈ VsIs

For non-resistive loads, phase difference matters and apparent power VI is not identical to real power. At school level, keep the load simple unless power factor is explicitly part of the investigation.

Where the energy goes

Copper loss: current heats the coils through I²R.

Eddy-current loss: changing flux induces currents in the core that dissipate energy. Laminated cores reduce these loops.

Hysteresis loss: repeated magnetisation of the core dissipates energy.

Leakage flux: not all magnetic flux produced by the primary links the secondary.

IOPSpark identifies these same real-transformer losses when discussing student experiments and practical devices. See the IOP transformer discussion.

Quantitative window

Suppose measurements under load are:

Vp = 6.0 V, Ip = 0.80 A, Vs = 2.85 V, Is = 1.40 A

Input power ≈ 4.8 W. Output power ≈ 3.99 W.

η ≈ 3.99/4.8 × 100% ≈ 83%

The missing 17% is not “destroyed.” It is transferred into thermal energy and other loss pathways in the transformer and surroundings.

Why the core must close the magnetic path

A U-core or E-core with an open gap usually couples the coils less effectively than a properly closed magnetic circuit. Changing the core arrangement can therefore alter secondary voltage even when turns remain unchanged.

Meter choice matters

Use meters set for AC where required. A DC meter reading of an AC waveform may be meaningless or instrument-dependent. If the waveform is not sinusoidal, simple RMS assumptions also deserve caution.

Observation versus inference

Observation: “A 4:1 primary-to-secondary turn ratio gave 6.00 V primary and 1.46 V secondary with no load.”

Transformation: “Vs/Vp = 0.243 compared with Ns/Np = 0.250.”

Inference: “The no-load voltage ratio is close to the ideal turns-ratio prediction.”

Overclaim: “The transformer is 97% efficient.” A voltage-ratio agreement does not measure power efficiency.

Failure modes

Unfamiliar transfer: the power grid

Transformers let transmission systems raise voltage so the same power can travel at lower current, reducing I²R heating in lines. The classroom device therefore links directly to grid-scale energy engineering, but the same efficiency and safety accounting still applies.

Secondary → JC → deeper Physics

Secondary: relate turns ratio to step-up/step-down voltage and understand why AC is required.

JC: distinguish no-load and loaded behaviour, measure power efficiency, analyse loss mechanisms and use RMS quantities carefully.

Deeper Physics: transformers extend to mutual inductance, equivalent circuits, regulation, reactive power, saturation, frequency dependence and power-system engineering.

Checkpoint

A transformer gives almost exactly the predicted secondary voltage with no load, but efficiency under a heavy resistive load is only 75%. Is that contradictory?

Answer key and WHY reasoning

No. Turns ratio describes induced voltage ratio approximately; efficiency compares energy transfer rates under load. Large load current can produce substantial I²R heating and other losses while the basic voltage ratio remains fairly close to ideal.

How to study this practical

Keep two ledgers: a ratio ledger for V and N, and an energy ledger for input/output power and losses. If you can explain why those ledgers answer different questions, you understand the practical.

Evidence boundaries

A school transformer experiment tests a specific core, frequency, turn count and load. Efficiency and voltage regulation can change with load, temperature, frequency and magnetic-core design. Do not generalise one efficiency value to every operating condition.

Authoritative next steps

Teaching Guide

For teachers and parents: give students one transformer that matches the turns ratio beautifully but heats strongly under load. Ask them whether “the equation worked.” The best answer is: the voltage-ratio model worked approximately, while the energy-efficiency model reveals additional real losses.

Explore the connected learning guides

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For the structure behind these connections, read the eduKateSingapore runtime manifest and the eduKate ecosystem boot contract. The reader map describes public navigation; those manifests preserve the wider ownership and return rules.

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