1. What is 3-Phase Motor Inrush Current?

Inrush current is the instantaneous high current drawn by a motor when first energized, typically 6-8 times the full load current. This surge lasts only a few cycles but is important for sizing protection devices.

2. How Does the Calculator Work?

The calculator uses the inrush current formula:

Where:

• \( FLA \) — Full Load Amps (motor nameplate rating)
• \( \text{Inrush Multiplier} \) — Typically 6-8 times FLA (varies by motor design)

Explanation: The inrush current occurs due to the absence of back EMF when the motor starts, causing a temporary current surge.

3. Importance of Inrush Current Calculation

Details: Knowing inrush current is critical for proper circuit breaker and fuse selection, voltage drop calculations, and preventing nuisance tripping during motor startup.

4. Using the Calculator

Tips: Enter the motor's full load amps (FLA) from its nameplate and select an appropriate multiplier (typically 6-8). For precise values, consult motor manufacturer data.

5. Frequently Asked Questions (FAQ)

Q1: Why is inrush current higher than running current?
A: At startup, the motor lacks back EMF and appears as a short circuit, drawing high current until the rotor begins moving.

Q2: How long does inrush current last?
A: Typically 0.1-0.5 seconds (5-30 cycles at 60Hz), depending on motor size and load.

Q3: Can inrush current be reduced?
A: Yes, through soft starters, variable frequency drives (VFDs), or star-delta starters.

Q4: What happens if protection devices can't handle inrush?
A: Nuisance tripping may occur during motor startup, preventing normal operation.

Q5: Are all motors' inrush multipliers the same?
A: No, high-efficiency motors often have higher inrush (8-12x FLA), while NEMA Design B motors are typically 6-8x.