1. What is 3-Phase Motor Starting Current?

The starting current (or inrush current) of a 3-phase motor is the instantaneous current drawn when the motor is first turned on. This current is typically much higher than the normal operating current (FLA) and lasts only briefly until the motor reaches operating speed.

2. How Does the Calculator Work?

The calculator uses the simple formula:

Where:

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

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

3. Importance of Starting Current Calculation

Details: Knowing the starting current is crucial for proper circuit breaker and wire sizing, voltage drop calculations, and ensuring the electrical system can handle the temporary surge without nuisance tripping.

4. Using the Calculator

Tips: Enter the motor's FLA (found on the nameplate) and the appropriate inrush multiplier (default is 6). For precise calculations, consult the motor manufacturer's specifications.

5. Frequently Asked Questions (FAQ)

Q1: Why is starting current higher than running current?
A: At startup, the motor hasn't developed back EMF yet and appears as a near short circuit to the power supply.

Q2: How long does the starting current last?
A: Typically 0.1-3 seconds depending on motor size and load inertia.

Q3: What affects the inrush multiplier?
A: Motor design, winding type, and load characteristics. High-efficiency motors often have higher inrush.

Q4: How can starting current be reduced?
A: Using soft starters, variable frequency drives, or star-delta starting methods.

Q5: Is starting current the same as locked rotor current?
A: Essentially yes - both refer to current drawn when the rotor isn't turning.