1. What is Inrush Current?

Inrush current is the instantaneous high current drawn by a motor when it is first started. For 3-phase motors, this current is typically 6 to 8 times the full load current and lasts only for a few cycles.

2. How Does the Calculator Work?

The calculator uses the standard inrush current formula:

Where:

• Multiplier — Typically 6 to 8 depending on motor characteristics
• Full Load Amperes (FLA) — The motor's rated current at full load

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 crucial for proper circuit breaker and fuse selection, voltage drop calculations, and preventing nuisance tripping of protective devices.

4. Using the Calculator

Tips: Enter the motor's full load current in amperes (found on the motor nameplate) and select an appropriate multiplier (6 for conservative estimates, 8 for worst-case scenarios).

5. Frequently Asked Questions (FAQ)

Q1: Why is inrush current higher than running current?
A: At startup, the motor needs to overcome inertia and establish magnetic fields before back-EMF develops to limit current.

Q2: How long does inrush current last?
A: Typically 0.1 to 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 reduced voltage starting methods.

Q4: What happens if protection devices can't handle inrush?
A: Nuisance tripping may occur during motor starts, requiring time-delay fuses or motor-protection circuit breakers.

Q5: Does motor size affect the multiplier?
A: Larger motors may have slightly lower multipliers (5-7x) while small motors may exceed 8x in some cases.