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Courses/Three-Phase Systems/Three-Phase Induction Motors

lesson · 10min · Lesson 19 of 28

How induction motors work

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 · 10min
DOL STARTER · CONTROL CIRCUITLNSTOPOL NCSTART NOKM1KM1 aux NO (seal-in)3~control rung shown; power poles and motor protection are separate
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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

How induction motors work

Three Phase Systems · Lesson 19 · Advanced

AdvancedReview: professional review pending

Purpose

Explain how a three-phase stator field induces rotor current and torque, and why induction-motor speed must differ from synchronous speed.

Before you beginRotating fields · Electromagnetic induction

Learning objectives

  • Describe stator field rotation
  • Calculate synchronous speed
  • Define slip
  • Relate load to torque and current

Balanced stator currents create a rotating magnetic field. Relative motion between that field and the rotor induces rotor emf and current; their interaction produces torque.

MOTOR FIELD AND CONTROL PATHMOTOR FIELD AND CONTROL PATHROTORns = 120f/Pisolate · interlock · protect

Core theory

Synchronous speed is ns = 120f/P. A cage rotor cannot produce steady induction torque at exactly ns because relative speed, induced rotor emf, and rotor current would fall toward zero.

Slip s = (ns − nr)/ns. As mechanical load rises within the stable operating region, speed falls slightly, slip and rotor current rise, and electromagnetic torque increases.

Starting has s = 1 and can draw high current. Actual torque-speed behavior depends on rotor design, voltage, frequency, temperature, supply impedance, and driven-load torque.

Terms, symbols, and units
TermMeaningSymbolUnit
Synchronous speedSpeed of the rotating stator fieldnsrpm
Rotor speedMechanical shaft speednrrpm
SlipFractional difference between field and rotor speedsNot applicable
ns = 120f/P; s = (ns − nr)/ns

Use total pole count P and consistent speed units.

rpm and dimensionless
Worked exampleA four-pole 50 Hz motor runs at 1440 rpm. Find synchronous speed and slip.

Assumptions: Steady operation.

  1. Synchronous: ns = 120×50/4 = 1500 rpm.
  2. Difference: ns−nr = 1500−1440 = 60 rpm.
  3. Slip: s = 60/1500 = 0.040 = 4.0%.

Synchronous speed is 1500 rpm and slip is 4.0%.

Reasonableness check: A motoring induction rotor runs below synchronous speed with small positive slip.

Common mistakes
  • Using pole pairs as P
  • Calling rotor speed synchronous speed
  • Assuming more load increases speed

Where this appears in practice

Slip helps interpret nameplates, load, faults, speed control, and motor selection.

SafetyA stopped or slowly turning shaft can start without warning. Secure electrical and all mechanical/process energy before access.
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

Why is nonzero slip necessary for induction torque?

Relative field-to-rotor motion is needed to induce rotor emf and current. At exactly synchronous speed the ideal induction mechanism has no relative motion.

Answer: Relative field-to-rotor motion is needed to induce rotor emf and current. At exactly synchronous speed the ideal induction mechanism has no relative motion.

Practical exercise

Calculate full-load slip for 2-, 4-, and 6-pole 50 Hz examples from supplied speeds.

Summary

  • Three phases create a rotating field
  • Rotor speed differs from field speed
  • Slip changes with load

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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