1. What is Starting Current of a 3-Phase Motor?

Definition: The starting current (or inrush current) is the initial high current drawn by a motor when it's first turned on, typically 5-8 times higher than the full load current.

Purpose: Understanding starting current is crucial for proper circuit breaker sizing, wire selection, and preventing voltage drops during motor startup.

2. How Does the Calculator Work?

The calculator uses the formula:

Where:

• \( I_{start} \) — Starting current (amps)
• \( I_{fl} \) — Full load current (amps)
• \( 5-8 \) — Typical multiplier range for induction motors

Explanation: The full load current is multiplied by a factor between 5 and 8 to estimate the initial surge current when the motor starts.

3. Importance of Starting Current Calculation

Details: Proper starting current estimation ensures correct protection device sizing, prevents nuisance tripping, and helps design electrical systems that can handle motor startup.

4. Using the Calculator

Tips: Enter the motor's full load current (found on nameplate) and select a multiplier (default 6). The multiplier typically ranges from 5 (high efficiency motors) to 8 (standard motors).

5. Frequently Asked Questions (FAQ)

Q1: Why is starting current higher than running current?
A: At startup, the motor hasn't built up its magnetic field or back EMF, causing higher current draw until it reaches operating speed.

Q2: How do I find the full load current?
A: Check the motor nameplate or use the formula: I_fl = (HP × 746)/(√3 × V × Efficiency × PF).

Q3: When would I use a higher multiplier?
A: Use higher values (7-8) for older or high-torque motors, lower values (5-6) for modern high-efficiency motors.

Q4: Does this apply to all motor types?
A: This applies to standard AC induction motors. DC motors or soft-starters have different characteristics.

Q5: How does this affect circuit breaker sizing?
A: Breakers must handle starting current without tripping while protecting against overloads (often requires time-delay breakers).