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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 · 10min · Lesson 4 of 30

Variable speed drives: inverter drives

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 · 10min
PV STRING → INVERTER → GRIDarray (DC)INVERTERDC→ACGRID230V 50HzBATTERYoptional compatible storage portstring V within inverter MPPT window at coldest temp
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PurposeCore theoryWorked exampleKnowledge checkSources

In this lesson

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ElectraCore lesson handout

Variable speed drives: inverter drives

Industrial Control · Lesson 4 · Advanced

AdvancedReview: professional review pending

Purpose

Select and apply a variable-speed drive by tracing rectifier, DC link, inverter, motor and safety/EMC boundaries.

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

A VSD converts fixed-frequency supply power through a DC link into controlled motor voltage and frequency. It can regulate speed and torque, but introduces harmonics, high-frequency common-mode effects, stored energy and parameter-dependent behavior.

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

Core theory

Enter exact motor nameplate data and choose control mode, base frequency, acceleration/deceleration, current/torque limits, minimum speed/cooling and stopping behavior for the load.

Check input current, short-circuit protection, line reactor/filter needs, DC-link discharge time, braking energy, output cable length/type, motor insulation/bearing effects, EMC bonding/shield termination and RCD guidance.

A run-disable or ordinary stop command is not isolation. Safe Torque Off can be a safety function only when integrated and validated to the required performance; it does not necessarily remove hazardous voltage.

Terms, symbols, and units
TermMeaningSymbolUnit
DC linkIntermediate DC energy-storage stage in a driveNot applicableNot applicable
STOSafe Torque Off function preventing torque-producing energy under declared conditionsNot applicableNot applicable
Carrier frequencySemiconductor switching frequency used to synthesize motor outputNot applicableNot applicable
Ns = 120 f / p

Synchronous speed screening from commanded electrical frequency and motor pole count; actual induction-motor speed is lower by slip.

r/min when f is Hz
Worked exampleA four-pole motor is commanded at 40 Hz.

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

  1. Substitute: Ns = 120 × 40 / 4.
  2. Calculate: Ns = 1200 r/min.
  3. Qualify: Actual shaft speed depends on slip, control mode and load.

Synchronous speed is 1200 r/min; actual induction-motor speed will normally be lower.

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

Does pressing STOP prove the VSD output and DC link are dead?

No. Use the full isolation and discharge/proving procedure for every energy source.

Answer: No. Use the full isolation and discharge/proving procedure for every energy source.

Practical exercise

Build a drive commissioning sheet covering motor data, limits, braking, EMC, protection, safety functions and restart behavior.

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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