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Educational use: ElectraCore supports learning and preliminary checks. It does not replace a competent electrician or engineer. Verify results, equipment data, and current local regulations before installation or live work.
Courses/Three-Phase Systems/Transformers

lesson · 9min · Lesson 24 of 28

Transformer construction and principles

Course syllabusCourse overview
01Three-Phase Fundamentals
  1. ReadWhy three-phase?: advantages over single-phase
  2. ReadGeneration of three-phase voltages
  3. ReadPhase sequence: L1, L2, L3
  4. ReadPhasor representation of three-phase
  5. quizPhase fundamentals quiz
02Star (Wye) Connection
  1. ReadStar connection topology
  2. ReadLine voltage = √3 × phase voltage: proof
  3. ReadNeutral current in star systems
  4. exerciseStar circuit analysis problems
03Delta Connection
  1. ReadDelta connection topology
  2. ReadLine current = √3 × phase current: proof
  3. ReadCirculating currents in delta
  4. exerciseDelta circuit analysis problems
04Three-Phase Power
  1. ReadActive, reactive, and apparent power
  2. ReadPower factor in three-phase
  3. ReadTwo-wattmeter method
  4. ReadPower correction capacitor sizing
  5. quizThree-phase power quiz
05Three-Phase Induction Motors
  1. ReadHow induction motors work
  2. ReadMotor nameplate data and efficiency classes
  3. ReadStarting currents and starting methods
  4. ReadDOL starters: design and wiring
  5. ReadStar-delta starters: wiring and timing
06Transformers
  1. ReadTransformer construction and principles
  2. ReadTurns ratio and voltage/current transformation
  3. ReadThree-phase transformer connections
  4. quizTransformer quiz
  5. quizFinal assessment
Lesson · 9min
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In this lesson

PurposeCore theoryWorked exampleKnowledge checkSources

In this lesson

PurposeCore theoryWorked exampleKnowledge checkSources
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ElectraCore lesson handout

Transformer construction and principles

Three Phase Systems · Lesson 24 · Advanced

AdvancedReview: professional review pending

Purpose

Explain transformer magnetic coupling, core and winding construction, losses, cooling, insulation and protection boundaries.

Before you beginFaraday induction · AC power

Learning objectives

  • Describe mutual induction
  • Identify core and windings
  • Classify principal losses
  • Recognise cooling and insulation constraints

A transformer transfers AC energy between circuits through a changing magnetic flux. It changes voltage/current relationships without changing frequency and without a direct conductive power path between separate windings.

TRANSFORMER FLUX AND VECTOR GROUPTRANSFORMER FLUX AND VECTOR GROUPHVLVV1/V2 = N1/N2ratio · impedance · vector group

Core theory

Applied primary voltage establishes alternating core flux; that flux links the secondary and induces emf. Load current produces opposing ampere-turns, so primary current increases to maintain flux and supply output power.

Core loss comprises hysteresis and eddy-current effects; winding copper loss is approximately I²R. Leakage flux, stray load loss, dielectric loss, auxiliaries and temperature also affect performance.

Construction and ratings coordinate insulation level, winding arrangement, vector group, impedance, tap range, cooling medium/method, temperature rise, short-circuit withstand, enclosure and protection. Oil-filled units add fire, containment and environmental controls.

Terms, symbols, and units
TermMeaningSymbolUnit
Mutual inductionInduced voltage in one winding from flux produced by anotherNot applicableNot applicable
Leakage fluxFlux linking one winding imperfectly with the otherNot applicableNot applicable
Percentage impedanceDeclared transformer impedance affecting regulation and fault currentNot applicableNot applicable
Worked exampleA transformer supplies 90 kW output with 2 kW total loss. Find efficiency.

Assumptions: Values refer to the same load point.

  1. Input: Pin = 90 + 2 = 92 kW.
  2. Efficiency: η = Pout/Pin.
  3. Calculate: η = 90/92 = 0.9783 ≈ 97.8%.

Efficiency is approximately 97.8%.

Reasonableness check: The 2 kW loss is about 2.2% of the 92 kW input.

Common mistakes
  • Saying frequency changes
  • Ignoring magnetising current at no load
  • Treating percentage impedance as inefficiency

Where this appears in practice

Transformers provide voltage conversion, isolation, earthing references, phase displacement and fault-level control.

SafetyTransformers may have multiple sources, backfeed, induced voltage, stored magnetic energy, hot surfaces and high fault duty. Isolate and earth according to an approved procedure.
Local code checkConfirm the current machinery, motor, transformer, control-panel, installation, EMC and energy-efficiency requirements; exact nameplate, manufacturer and coordination data; fault level, earthing, isolation, guarding and stored-energy controls. Training calculations do not authorise energisation, rewiring, paralleling or starter commissioning.

Knowledge check

Does an ordinary transformer change supply frequency?

No. Primary and secondary steady-state frequencies are the same.

Answer: No. Primary and secondary steady-state frequencies are the same.

Practical exercise

Annotate a transformer cross-section with flux, windings, insulation, cooling and principal loss locations.

Summary

  • Flux couples windings
  • Losses create heat
  • Impedance and insulation are system properties

Sources and review

  • IEC 60034-30-1: Efficiency classes of line-operated AC motors: IEC; 2025; International
  • IEC 60076-3: Power transformers: insulation and dielectric tests: IEC; 2013+A1:2018; International
  • The safe isolation of plant and equipment (HSG253): Health and Safety Executive; Second edition; Great Britain

Editorial review date: 2026-08-22. Professional electrical review is pending.

Educational material for learning and preliminary checks. Verify current local requirements and exact equipment instructions. This lesson does not replace competent professional work.

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