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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/Contactors & Overloads

lesson · 9min · Lesson 9 of 30

Electronic overloads and motor protection relays

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 · 9min
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

Electronic overloads and motor protection relays

Industrial Control · Lesson 9 · Advanced

AdvancedReview: professional review pending

Purpose

Apply electronic overload and motor-protection relays by documenting measurement, trip logic, fail behavior and restart policy.

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

Electronic relays can estimate thermal capacity and detect phase loss, unbalance, stall, jam, underload, earth fault or temperature inputs. More functions create more settings and commissioning obligations.

CONTACTOR AND OVERLOAD COORDINATIONCONTACTOR AND OVERLOAD COORDINATIONSUPPLYSTARTERmotor · loadshort circuit · overload · command · interlock · isolation

Core theory

Enter current-transformer ratio, motor rated current, thermal class/model, hot/cold start capacity and thresholds/delays from the motor and process evidence.

Map each trip and alarm to contact outputs, contactor/breaker action, PLC indication and reset/restart policy. A communications display is not the protective trip path unless designed as such.

Test sensor polarity/phase mapping, trip outputs, memory after power loss, fail-safe contact state, remote reset authorization and event records. Coordinate short-circuit protection separately.

Terms, symbols, and units
TermMeaningSymbolUnit
Thermal capacity usedRelay estimate of accumulated motor heatingNot applicableNot applicable
JamHigh-current mechanical overload after normal accelerationNot applicableNot applicable
UnderloadLow-load condition potentially indicating loss of process loadNot applicableNot applicable
Worked exampleA relay uses 100:5 CTs and measures 3.0 A on its 5 A input.

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

  1. Ratio: Primary-to-secondary ratio = 100/5 = 20.
  2. Primary current: Iprimary = 3.0 × 20 = 60 A.
  3. Check: Confirm CT ratio, phase orientation, relay scaling and motor/load condition before interpreting protection.

The corresponding primary current is 60 A on the supplied ratio.

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

Is a networked current display itself proof the protection will trip the contactor?

No. The complete sensing, logic, output, trip device and fail-state path must be verified.

Answer: No. The complete sensing, logic, output, trip device and fail-state path must be verified.

Practical exercise

Create and test a cause-and-effect matrix for overload, phase loss, jam, sensor failure and communications loss.

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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