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Three-Phase Motor FLA & Starter Sizing Engine

Three-Phase Motor FLA & Starter Sizing Engine

Industrial Control Specification & Load Conversion

1. Motor Rating
2. Supply Characteristics

Estimated Full Load Current (FLA)

0.00 A

Based on theoretical input parameters. Always verify against physical nameplate.

Direct-On-Line (DOL) Specification
Main Contactor (AC-3) ≥ 0.00 A
Overload Relay Setting 0.00 A
Circuit Protection (Type D) 0.00 A
Star-Delta Specification
Main / Delta Contactor ≥ 0.00 A
Star Contactor ≥ 0.00 A
O/L Relay (Delta Loop) 0.00 A

Industrial Motor Control & Starter Specification

Specifying the correct switchgear for electric motors is critical for industrial compliance and equipment longevity. Incorrect sizing of contactors results in welded contacts under AC-3 utilisation categories, while improperly set overload relays lead to premature nuisance tripping or total winding failure.

Full Load Amps (FLA) Conversion

When an industrial pump or extraction fan nameplate provides only the mechanical output power (kW or HP), the electrical input current must be derived using the system voltage, power factor (cos φ), and motor efficiency (η). If specific plate data is illegible, standard engineering estimations of 0.85 for both power factor and efficiency provide a safe baseline for specification.

Direct-On-Line (DOL) Starters

  • Contactor Sizing: Must be rated for AC-3 (inductive motor loads) at or above the calculated FLA.
  • Overload Protection: Thermal or electronic overload relays in a DOL setup carry the full phase current and must be set exactly to the motor's FLA, not the circuit breaker rating.
  • Short-Circuit Protection: Motors draw up to 7x their running current during startup. Type D MCBs or dedicated motor protection circuit breakers (MPCBs) are required to prevent nuisance tripping during the inrush period.

Star-Delta Starters

Star-Delta starting reduces the mechanical shock and starting current by initially configuring the windings in a Star arrangement (applying 230V across a 400V winding), before switching to Delta for continuous running.

  • Main & Delta Contactors: These handle the phase current, which is the line current divided by √3. Therefore, they are sized at approximately 58% of the total FLA.
  • Star Contactor: Carries less current and is sized at approximately 33% of the total FLA.
  • Overload Relay Placement: When the overload is situated beneath the Main contactor (within the delta loop), it must be set to 58% of the FLA. If placed upstream of the contactors (line current), it is set to 100% of the FLA. Our engine calculates for the standard delta-loop placement.
ENGINEERING DISCLAIMER: This engine provides theoretical switchgear sizing based on mathematical conversions. Variations in rotor design, starting inertia, and ambient temperatures require localized engineering judgement. Always verify exact parameters against the manufacturer's physical motor nameplate prior to energisation.