DOL starter: main and control circuit
Industrial Control · Lesson 1 · Advanced
Purpose
Explain a direct-on-line starter as a coordinated main circuit, control circuit and protection assembly.
Learning objectives
- Trace main and control paths
- Select from declared utilization data
- Coordinate protection and control
- Identify dangerous failure states
A DOL starter connects the motor directly to the supply through a suitably rated switching device. Its simplicity produces high starting current and starting torque, so the supply, driven machine and protective coordination must accept the transient.
Core theory
The main path normally includes isolation, short-circuit protection, contactor poles, overload sensing and motor conductors; the control path energises the contactor coil through stop, overload and permissive contacts.
A normally-open auxiliary holding contact maintains the command after the momentary start button is released. Opening stop, overload or a safety-related element removes the coil command, subject to the designed stop category.
Select utilization category, operational voltage/current, duty, coil voltage/frequency, motor starting characteristic, prospective fault current and coordination type from exact manufacturer combinations.
| Term | Meaning | Symbol | Unit |
|---|---|---|---|
| DOL | Direct-on-line motor starting | Not applicable | Not applicable |
| Holding contact | Auxiliary contact maintaining the coil command after start is released | Not applicable | Not applicable |
| Utilization category | Declared switching duty such as motor starting and interruption conditions | Not applicable | Not applicable |
Assumptions: All ratings are supplied fictional design data; The machine risk assessment and manufacturer instructions remain governing inputs.
- Run: Rated running current is 12 A.
- Start: Estimated starting current is 6 × 12 = 72 A.
- Verify: Check voltage dip, starting time, contactor duty, protection and driven-load torque using product data.
Illustrative starting current is 72 A; this does not by itself select the starter.
Reasonableness check: The result is checked against electrical duty, starting behavior and protective boundaries rather than accepted from one nameplate number.
- 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.
Knowledge check
Does the overload relay replace the short-circuit protective device?
No. Their operating ranges and protective purposes differ and must be coordinated.
Answer: No. Their operating ranges and protective purposes differ and must be coordinated.
Practical exercise
Trace a supplied DOL drawing and annotate current paths for stopped, starting, running, stop-command and overload-trip states.
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.