1. What is Motor Inrush Current?

Inrush current is the instantaneous high current drawn by a motor when it is first started. This current can be 6-8 times higher than the motor's full load current (FLA) and lasts only for a short duration (typically a few cycles).

2. How Does the Calculator Work?

The calculator uses the inrush current formula:

Where:

• \( \text{FLA} \) — Full Load Amps (motor's rated current at full load)
• \( \text{Inrush Multiplier} \) — Typically 6-8 for most AC induction motors

Explanation: The multiplier accounts for the initial surge of current needed to overcome the motor's inertia and establish magnetic fields.

3. Importance of Inrush Current Calculation

Details: Knowing inrush current is critical for proper circuit breaker and fuse sizing, transformer sizing, and preventing nuisance tripping of protection devices during motor startup.

4. Using the Calculator

Tips: Enter the motor's full load current (FLA) in amps (found on motor nameplate) and the appropriate multiplier (default is 6). For precise calculations, consult motor specifications as multipliers can vary.

5. Frequently Asked Questions (FAQ)

Q1: Why is inrush current higher than running current?
A: At startup, the motor has no back EMF and appears as a short circuit, drawing high current until it reaches operating speed.

Q2: How long does inrush current last?
A: Typically 0.1-0.5 seconds for small motors, up to several seconds for large motors.

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

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

Q5: Do all motors have the same inrush multiplier?
A: No, high-efficiency motors often have higher inrush (8-12x), while some special designs may have lower inrush.