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

lesson · 10min · Lesson 22 of 28

DOL starters: design and wiring

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
THREE-PHASE · 120° APARTL1 · L2 · L3STARY: Vʟ=√3·VₚΔ: Iʟ=√3·IₚDELTA
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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

DOL starters: design and wiring

Three Phase Systems · Lesson 22 · Advanced

AdvancedReview: professional review pending

Purpose

Trace a reversing-safe DOL starter's power and control functions, including isolation, short-circuit, overload and undervoltage protection.

Before you beginContactors and overloads · Motor starting

Learning objectives

  • Separate power and control circuits
  • Explain seal-in control
  • Describe overload action
  • Prevent unexpected restart

A DOL starter connects a motor directly to the supply through a contactor. Its control circuit determines when the contactor may energise; its protective devices perform distinct fault functions.

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

Core theory

The main contactor switches all required line conductors. A short-circuit protective device coordinates with the starter assembly, while an overload relay responds to sustained motor overcurrent and opens the control circuit.

A normally-open auxiliary contact can seal around a momentary start button. A normally-closed stop/overload path drops the contactor; undervoltage release prevents automatic re-energisation after supply loss unless the risk assessment and control design intentionally require otherwise.

Emergency stop, operational stop, isolation, guard interlocking and protective disconnection are different functions. Their architecture, performance level, feedback and reset behavior follow the machinery risk assessment and applicable standards.

Terms, symbols, and units
TermMeaningSymbolUnit
ContactorElectrically controlled power-switching deviceNot applicableNot applicable
Seal-in contactAuxiliary contact maintaining a contactor coil after start releaseNot applicableNot applicable
Overload relayMotor thermal/current protective function that commands disconnectionNot applicableNot applicable
Worked exampleThe overload relay trips while the start button is held. Should the contactor remain energised?

Assumptions: Overload NC contact is correctly in series with the coil.

  1. Trip: The overload contact opens the coil circuit.
  2. Coil: The contactor coil de-energises regardless of the start-button state.
  3. Power: Main contacts open and remove motor supply, subject to verified device operation.

No; a correct series overload contact drops the contactor.

Reasonableness check: The protective path must have authority over the start command.

Common mistakes
  • Calling overload protection short-circuit protection
  • Omitting control-circuit protection
  • Resetting before finding the cause

Where this appears in practice

DOL control is common for small motors, pumps, fans and machinery auxiliaries where supply and load permit.

SafetyA contactor is not necessarily an isolator. Lock off a suitable isolation device, prove dead, and secure rotation, gravity, pressure and stored energy.
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

What prevents a healthy seal-in contact from defeating an overload trip?

The overload contact is in series with the coil supply. Every energising path must pass through the protective stop chain.

Answer: The overload contact is in series with the coil supply. Every energising path must pass through the protective stop chain.

Practical exercise

Trace a paper DOL ladder diagram through stopped, starting, running, overload and supply-failure states.

Summary

  • Power and control functions differ
  • Protection must dominate start
  • A contactor is not safe isolation

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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