1. What is Inrush Current?

Inrush current is the instantaneous high current drawn by a transformer when it is first energized. This current can be 8 to 12 times the transformer's full load current and lasts for a few cycles of the input waveform.

2. How Does the Calculator Work?

The calculator uses the inrush current formula:

Where:

• Multiplier — Typically 8 to 12 depending on transformer characteristics
• Full Load Current — The transformer's rated current at full load (A)

Explanation: The multiplier accounts for the magnetic flux buildup in the transformer core during startup.

3. Importance of Inrush Current Calculation

Details: Calculating inrush current is crucial for proper circuit breaker and fuse selection, preventing nuisance tripping during transformer energization.

4. Using the Calculator

Tips: Enter the transformer's full load current in amperes and select an appropriate multiplier (8 for conservative estimates, 12 for worst-case scenarios).

5. Frequently Asked Questions (FAQ)

Q1: Why does inrush current occur?
A: Inrush current occurs due to the initial magnetization of the transformer core and the absence of back EMF when first energized.

Q2: How long does inrush current last?
A: Typically 0.1 to 0.2 seconds (5-10 cycles at 50/60 Hz), though the duration depends on transformer design and load.

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

Q4: How can inrush current be reduced?
A: Techniques include phase-controlled switching, NTC thermistors, or series resistors that are later bypassed.

Q5: Does inrush current damage transformers?
A: Normally no, as it's brief, but repeated switching can cause mechanical stress on windings over time.