Transformers are essential electromagnetic devices used to change alternating potential differences efficiently, relying on mutual induction and controlled magnetic flux linkage within laminated iron cores across primary and secondary windings.

Transformer Operating Principles

Transformers function only with alternating current (a.c.), because an a.c. supply continually changes the magnetic flux in the core and therefore induces an electromotive force (e.m.f.) in the secondary coil. When the primary coil is connected to an a.c. source, the alternating current creates a continually varying magnetic flux in the laminated iron core. This magnetic flux links both the primary and secondary windings, ensuring that energy transfer occurs via mutual induction, the physical process by which a changing magnetic field induces an e.m.f. in another coil.

A three-dimensional illustration of an ideal single-phase transformer showing primary and secondary coils, magnetic flux flow, and the closed iron core path. This supports understanding of mutual induction and ideal transformer behaviour. The explicit flux path is more detailed than required but enhances visual comprehension. Source.

The core is typically made of thin insulated iron laminations to reduce eddy current heating and thus increase efficiency. Efficient flux linkage is achieved by winding the coils on opposite sides of a closed magnetic loop, ensuring that almost all magnetic flux produced by the primary coil passes through the secondary coil.

Ideal Transformer Relations

The OCR specification requires knowledge of the two core mathematical relations for an ideal transformer, where all magnetic flux is perfectly linked and no energy losses occur.

EQUATION
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Transformer Voltage Relation (Vp/Vs) = Np/Ns
Vp = Primary potential difference (voltage), measured in volts
Vs = Secondary potential difference (voltage), measured in volts
Np = Number of turns on the primary coil
Ns = Number of turns on the secondary coil
—-----------------------------------------------------------------

This equation shows that the ratio of voltages is equal to the ratio of turns. A transformer with more turns on the secondary than the primary is a step-up transformer, while the opposite configuration is a step-down transformer. These ideas underpin how transformers adapt voltages for transmission or appliance use.

A transformer’s power transfer under ideal conditions follows from the assumption of 100% efficiency, so input power equals output power.

EQUATION
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Current Ratio Relation (Ip/Is) = Ns/Np
Ip = Primary current, measured in amperes
Is = Secondary current, measured in amperes
Ns = Number of turns on the secondary coil
Np = Number of turns on the primary coil
—-----------------------------------------------------------------

The inverse relationship between current and turns ratio reflects conservation of energy in an ideal transformer. Step-up transformers increase voltage but decrease current; step-down transformers decrease voltage but increase current. This is essential for efficient power distribution, where high voltages and low currents minimise energy loss in cables.

A full understanding of these relations enables students to interpret how coil design directly shapes transformer performance and behaviour in practical circuits.

Construction Features Supporting Efficient Operation

The structure of a transformer is deliberately designed to maximise induced e.m.f. while minimising energy losses. Important features include:

• Laminated iron core to reduce eddy currents

• Soft iron material to enhance magnetic permeability

• Tightly wound coils for strong magnetic coupling

• Insulation between laminations and coil layers to prevent short circuits

• Closed-loop core geometry to ensure effective flux linkage

These features enhance efficiency but do not alter the ideal transformer relations; instead, they help practical transformers approach ideal behaviour more closely.

Energy Loss Mechanisms in Real Transformers

Although the OCR syllabus focuses on ideal transformers, understanding real-world inefficiencies helps contextualise the need for design strategies. Key sources of loss include:

• Eddy current heating in the core

• Hysteresis loss from repeated magnetisation cycles

• Resistance heating (I²R losses) in coils

• Leakage flux, where some magnetic flux does not link both coils

Design choices such as ferrite cores, laminated sheets, and thicker wire help mitigate these losses.

A diagram illustrating how laminating the core breaks up eddy-current loops and reduces heating. This visual supports understanding of energy-loss mechanisms in real transformers. The additional depiction of current loops goes slightly beyond the OCR requirement but remains helpful for conceptual clarity. Source.

A normal sentence here helps maintain clarity before progressing to required experimental investigations.

Experimental Investigations of Transformer Behaviour

The OCR specification requires students to be able to describe experimental investigations into transformers. These investigations aim to explore voltage ratios, flux linkage, and efficiency under varying conditions. Students typically examine how changing the number of turns or altering load conditions influences transformer performance.

Core Experimental Approaches

Common investigative procedures include:

• Measuring Vp and Vs while varying the number of secondary turns

• Recording currents in both windings to verify the current ratio relationship

• Observing how coil arrangement affects flux linkage

• Examining temperature rise to infer energy losses

• Testing transformer behaviour with different load resistances

Such investigations frequently use low-voltage laboratory a.c. supplies for safety.

A photograph of an oscilloscope displaying the alternating output waveform from a transformer. This supports understanding of how waveform measurements form part of transformer investigations. The image focuses on measurement rather than the full setup, emphasising the analysis stage of the experiment. Source.

Investigating Efficiency and Load Dependence

Transformers behave differently under load. Experimental setups may involve:

• Connecting variable resistances to the secondary coil

• Monitoring voltage drop and current changes

• Assessing energy loss by comparing VpIp and VsIs

• Investigating how core material or lamination affects heating

These observations help demonstrate why practical transformers operate near, but never exactly at, ideal efficiency.

Practical Considerations for Safe and Accurate Measurements

To produce reliable data, students should ensure:

• Accurate voltmeters and ammeters are placed appropriately in each circuit

• Coils are fully insulated and securely mounted

• The a.c. supply is stable

• Heating is monitored to prevent damage

• Magnetic flux leakage is minimised by correct coil placement on the core

These considerations reinforce the connection between theoretical relations and practical transformer behaviour, fulfilling the OCR requirement to describe experimental investigations.