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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/Industrial Control & PLCs/Motor Starters

lesson · 11min · Lesson 2 of 30

Star-delta starter: wiring and timer

Course syllabusCourse overview
01Motor Starters
  1. ReadDOL starter: main and control circuit
  2. ReadStar-delta starter: wiring and timer
  3. ReadSoft starters: operation and parameters
  4. ReadVariable speed drives: inverter drives
  5. quizMotor starter quiz
02Contactors & Overloads
  1. ReadContactor construction and ratings
  2. ReadAuxiliary contacts and interlocking
  3. ReadThermal overload relays: setting the dial
  4. ReadElectronic overloads and motor protection relays
  5. exerciseContactor circuit problems
03Control Circuit Diagrams
  1. ReadIEC 60617 symbols: reading control diagrams
  2. ReadLadder diagrams: European and American styles
  3. ReadDrawing a full DOL control circuit
  4. ReadForward-reverse motor control
  5. exerciseDiagram reading exercises
04Safety Systems
  1. ReadEmergency stop requirements: IEC 60204
  2. ReadSafety relays and safety PLCs
  3. ReadLight curtains and safety interlocks
  4. ReadSafe isolation: EAWR and multi-energy control
  5. quizSafety systems quiz
05PLC Fundamentals
  1. ReadWhat is a PLC and where is it used?
  2. ReadPLC architecture: CPU, I/O modules, power
  3. ReadLadder logic basics: contacts and coils
  4. ReadTimers and counters in ladder logic
  5. exerciseWriting a simple motor control program
06Panel Layout & Installation
  1. ReadPanel design: layout and component spacing
  2. ReadCable management inside panels
  3. ReadLabelling, ferrules, and documentation
  4. ReadPanel testing and commissioning checklist
  5. quizFinal assessment
Lesson · 11min
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

Star-delta starter: wiring and timer

Industrial Control · Lesson 2 · Advanced

AdvancedReview: professional review pending

Purpose

Explain star-delta starting sequence, current/torque tradeoff and transition interlocking without presenting it as a universal motor solution.

Before you beginThree-phase motor fundamentals · Protection and safe-isolation principles

Learning objectives

  • Trace main and control paths
  • Select from declared utilization data
  • Coordinate protection and control
  • Identify dangerous failure states

Star-delta starting initially connects each suitable motor winding in star and later reconnects it in delta for normal running. At the same line voltage, idealized winding voltage becomes 1/√3 and starting line current and torque are approximately one-third of direct delta values.

MOTOR STARTER POWER AND CONTROLMOTOR STARTER POWER AND CONTROLSUPPLYSTARTERmotor · loadshort circuit · overload · command · interlock · isolation

Core theory

The motor must have six accessible terminals and be rated to run in delta at the supply voltage. A motor intended to run in star on that supply is not a valid candidate for this scheme.

Main, star and delta contactors require an engineered sequence. Star and delta must never close together; use electrical interlocking and suitable mechanical interlocking where specified.

Transition time must let the motor accelerate sufficiently but avoid excessive open-transition transients or reacceleration current. Load torque that is too high can stall the motor in star.

Terms, symbols, and units
TermMeaningSymbolUnit
Star contactorDevice joining the three winding ends during startingNot applicableNot applicable
Delta contactorDevice forming the normal delta winding connectionsNot applicableNot applicable
Open transitionChangeover with a short disconnected intervalNot applicableNot applicable
Vphase,star = Vline / √3; Tstart,star ≈ Tstart,delta / 3

Ideal screening relationships for the same winding impedance; real motor/load data govern.

V and relative torque
Worked exampleA motor would develop 90 N·m starting torque in direct delta under a simplified comparison.

Assumptions: All ratings are supplied fictional design data; The machine risk assessment and manufacturer instructions remain governing inputs.

  1. Voltage: Star phase voltage is 1/√3 of delta phase voltage.
  2. Torque: Torque approximately follows voltage squared.
  3. Estimate: Star starting torque ≈ 90/3 = 30 N·m.

Idealized star starting torque is about 30 N·m; confirm it exceeds the load acceleration requirement.

Reasonableness check: The result is checked against electrical duty, starting behavior and protective boundaries rather than accepted from one nameplate number.

Common mistakes
  • Using motor kW alone to select switching equipment
  • Treating an overload relay as short-circuit protection
  • Assuming a stopped motor or dark display is isolated

Where this appears in practice

Industrial motor control must coordinate the motor, switching device, short-circuit protection, overload protection, control voltage, driven load and machine safety functions.

SafetyMultiple supplies, stored DC-link energy, automatic restart, remote commands and mechanical motion can remain hazardous after a stop command. Isolate every energy source, lock off, prove the tester, prove dead, re-prove the tester and control stored/mechanical energy under the site procedure.
Local code checkConfirm the current machinery risk assessment, IEC/BS EN 60204-1, IEC/BS EN 60947 product standard, IEC/BS EN 61800 drive requirements, supply system, short-circuit current rating, protective coordination, functional-safety design, EMC instructions and manufacturer data. A control stop, inhibit or safe-torque-off function is not automatically electrical isolation.

Knowledge check

What prevents star and delta contactors closing together?

A correctly designed interlocking and sequencing system. Simultaneous closure creates a severe fault condition.

Answer: A correctly designed interlocking and sequencing system. Simultaneous closure creates a severe fault condition.

Practical exercise

Produce a state table for de-energized, star run-up, transition, delta run and each contactor-weld/failure case.

Summary

  • Main and control circuits have different jobs
  • Every rating has a declared duty and condition
  • Protection, stopping and isolation are distinct functions

Sources and review

  • IEC 60947-4-1:2023: Electromechanical contactors and motor-starters: IEC; 2023, corrected version 2026-03; International
  • IEC 60204-1:2016+AMD1:2021: Electrical equipment of machines: IEC; Consolidated edition 6.1; International
  • IEC 61800-5-1:2022: Adjustable-speed drive safety: IEC; 2022 with current corrigenda; International

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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