Causes of inefficiencies in a transformer.

The idea

An ideal transformer transfers all input power to the output, so input power equals output power. Real transformers are less than 100% efficient because some energy is dissipated. The main causes are: resistance in the primary and secondary coils, which causes heating (I²R losses); eddy currents induced in the iron core, which circulate and heat the core; magnetic hysteresis in the core, where energy is needed to repeatedly magnetise and demagnetise the iron; and flux leakage, where some magnetic field lines do not link both coils. Efficiency is output power divided by input power, often expressed as a percentage. For example, if a transformer supplies 92 W from a 100 W input, efficiency is 92%. Laminated cores reduce eddy currents, and soft iron reduces hysteresis.

What earns the marks

• Resistance in the primary and secondary coils causes heating, often called I²R or copper losses.
• Eddy currents are induced in the core by changing magnetic flux and dissipate energy as heat.
• Hysteresis losses occur because energy is required to magnetise and demagnetise the core each cycle.
• Flux leakage occurs when some magnetic field lines from the primary do not link the secondary coil.
• Efficiency = (output power ÷ input power) × 100%, and real efficiency is always less than 100%.
• Laminated cores reduce eddy currents; soft magnetic materials reduce hysteresis losses.

Common mistakes and how to put them right

• Thinking that a transformer creates energy or that output power can exceed input power.

  Put it right by applying conservation of energy so output power is always less than input power in a real transformer.

• Confusing eddy currents with the current in the coils.

  Put it right by stating that eddy currents are induced within the core itself and cause heating.

• Ignoring flux leakage and attributing all inefficiency to resistance.

  Put it right by listing flux leakage as a separate cause where magnetic field lines fail to link both coils.

Exam tips

• When asked for causes, name the mechanism and state the energy transfer, for example eddy currents transfer energy by heating the core.
• Use the efficiency equation with consistent power units and give the answer as a percentage when appropriate.
• Link each cause to a practical design feature, such as laminated core for eddy currents or soft iron for hysteresis.