Transformer Calculator: Find kVA, Current And Windings for 3 Phase Transformers

Single Phase Three Phase

Power Rating

VA KVA MVA

Primary Voltage

V KV

Secondary Voltage

V KV

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Key learnings:

Transformer Calculator Definition: A transformer calculator is a tool that helps find key electrical values for transformers like kVA, current, and windings.
Voltage Calculation: The voltage in transformers is calculated using Vs = Vp * Ns / Np, relating primary and secondary voltages and windings.
Current Calculation: The current in transformers is determined by Is = Ip * Np / Ns, showing the relationship between primary and secondary currents and windings.
Power Conservation: Power in an ideal transformer is conserved, meaning primary and secondary power are equal, though real transformers have some losses.
Example and Applications: An example problem demonstrates calculating currents and turns ratio, illustrating step-up and step-down transformers.

For an ideal transformer, the ratio of primary to secondary voltages equals the turns ratio:

Vs = Vp * Ns / Np

where

Vs [V] = RMS voltage across the secondary winding
Vp [V] = RMS voltage across the primary winding
Ns = number of turns in the secondary winding
Np = number of turns in the primary windings

For the same ideal transformer, the current ratio is the inverse of the turns ratio:

Is = Ip * Np / Ns

where

Is [A] = RMS current in the secondary winding
Ip [A] = RMS current in the primary winding

For a single-phase ideal transformer, the apparent electrical power is equal on both sides:

S = Vp * Ip = Vs * Is

For a balanced three-phase rating using line-to-line voltage, line current is I = S / (√3 × VLL). Apply the turns-ratio equation to the voltage across each winding, which depends on whether that winding is connected in wye or delta. Real transformers also have losses, voltage drop, temperature-rise limits and short-time overload limits.

Transformer Calculator Example Problem

A 50 kVA single-phase transformer has a 4000 V primary side and a 400 V secondary side. Assuming an ideal transformer, find:

The primary and secondary full-load currents
The transformer turns ratio.

Part 1. V₁ = 4000 V, V₂ = 400 V,

Transformer rating = 50 kVA = V₁ × I₁ = V₂ × I₂

Hence rearranging for I₁ and I₂:

Primary full-load current, I₁ = (50 × 1000) / 4000 = 12.5 A

Secondary full-load current, I₂ = (50 × 1000) / 400 = 125 A

Part 2. The turns ratio is N₁ / N₂ = V₁ / V₂ = 4000 / 400 = 10

Using the full-load currents I₁ and I₂, the equivalent inverse relation is V₂ / V₁ = 400 / 4000 = 12.5 / 125 = 0.1.

These equations estimate rated current and ideal turns ratio. Final transformer selection must also check frequency, insulation level, winding connection, impedance, cooling, efficiency, regulation and applicable installation rules.

Note that if the voltage on the primary side is higher than the voltage on the secondary side, then it is a step down transformer.

If the voltage on the primary side is lower than the voltage on the secondary side, then it is a step up transformer.