1. What is Transformer Inrush Current?

Transformer inrush current is the instantaneous current drawn by a transformer when it is first energized. This current can be 8 to 12 times higher than the normal full load current and lasts for a few cycles.

2. How Does the Calculator Work?

The calculator uses the standard inrush current estimation formula:

Where:

• Full Load Current — The normal operating current of the transformer at rated load (A)
• 8-12 — Multiplication factor depending on transformer characteristics

Explanation: The inrush current occurs due to core saturation when the transformer is energized at the zero-crossing point of the voltage waveform.

3. Importance of Inrush Current Calculation

Details: Calculating inrush current is crucial for proper protection device sizing, avoiding nuisance tripping, and ensuring system stability during transformer energization.

4. Using the Calculator

Tips: Enter the transformer's full load current in amperes and select an appropriate multiplication factor (8 for conservative estimate, 12 for worst-case scenario).

5. Frequently Asked Questions (FAQ)

Q1: Why does inrush current occur?
A: Inrush current occurs due to temporary core saturation when a transformer is energized, especially if switched on at voltage zero crossing.

Q2: How long does inrush current last?
A: Typically 0.1 seconds or less, though it can persist longer in large transformers (up to several seconds).

Q3: What factors affect inrush current magnitude?
A: Point-on-wave switching, residual flux, transformer size, and core material all affect inrush current.

Q4: How to mitigate inrush current?
A: Techniques include point-on-wave switching, series resistors, soft starters, or phase-controlled switching.

Q5: Does inrush current cause damage?
A: While usually harmless, repeated high inrush can stress windings and protection devices must be sized appropriately.