Transformer & Motor Starting Calculator

Calculate transformer primary and secondary currents based on kVA rating. Estimate peak starting currents for 3-phase induction motors using Direct-On-Line (DOL), Star-Delta, or Soft Starter methods.

Transformer Rating (kVA) Primary Voltage (V) Secondary Voltage (V)

TRANSFORMER RATED CURRENTS

Primary Current (A)

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Secondary Current (A)

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Motor Power (kW) Motor Voltage (V)

STARTING CURRENT ESTIMATES

Nominal Current (Approx.)

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Direct-On-Line (DOL) Peak

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Star-Delta Peak

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Soft Starter (Typical limit)

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Note: Currents are approximations based on typical 3-phase induction motors (efficiency 0.85, PF 0.85). Actual values depend on motor specifications.

Formulas Used

Current (A) = (kVA × 1000) / (Voltage × √3).

FAQ

Why use Star-Delta starting?

Direct-On-Line (DOL) starting draws 6 to 8 times the nominal current, causing severe voltage drops and mechanical stress. Star-Delta reduces this starting current to 1/3 of the DOL value.

How is transformer 3-phase current calculated?

Current (A) = (kVA × 1000) / (Voltage × √3).