Auto-Transformer Calculator

Auto-Transformer Calculator. An auto-transformer is a single-winding transformer used to step voltage up or down efficiently — commonly used in motor starting, voltage correction, and power distribution. Enter your phase type, primary voltage, secondary voltage, load current, and power factor into the Auto-Transformer Calculator to get the required Transformer kVA Rating. Secondary outputs include primary current, turns ratio, and voltage regulation. Also try the Total Copper Loss — Transformer Copper Loss.

Auto-Transformer Calculator inputs
Phase Type *
V

Input voltage to the transformer

V

Output voltage from the transformer

A

Current drawn by the load

Typically 0.8 for inductive loads, 1.0 for resistive loads

Results

Transformer kVA Rating

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Primary Current

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Turns Ratio

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Voltage Regulation

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What is an auto-transformer?

An auto-transformer is a type of electrical transformer with only one winding that acts as both primary and secondary. It provides voltage transformation with higher efficiency and lower cost compared to conventional two-winding transformers. See also our Transformer Winding Calculator.

What is kVA in transformers?

kVA (kilovolt-amperes) is the apparent power rating of a transformer. It represents the product of voltage and current, indicating the transformer's capacity to handle electrical load without considering power factor.

How do you calculate transformer kVA?

For single-phase: kVA = (Volts × Amps) ÷ 1000. For three-phase: kVA = (Volts × Amps × 1.732) ÷ 1000, where 1.732 is the square root of 3.

What is the difference between single-phase and three-phase transformers?

Single-phase transformers use one alternating voltage, while three-phase transformers use three alternating voltages 120° apart. Three-phase systems are more efficient for high-power applications and provide more balanced power delivery.

What is turns ratio in auto-transformers?

Turns ratio is the ratio of primary voltage to secondary voltage, which equals the ratio of primary turns to secondary turns. It determines how much the voltage is stepped up or down by the transformer.

How does power factor affect transformer sizing?

Power factor affects the actual power (kW) delivered by the transformer. While kVA rating remains constant, a lower power factor means less useful power is delivered, requiring consideration for proper transformer sizing.

What are the advantages of auto-transformers?

Auto-transformers offer higher efficiency (95-98%), smaller size, lower cost, and better voltage regulation compared to conventional transformers. They are ideal for applications requiring moderate voltage transformation ratios.