1. What is Locked Rotor Current?

Locked rotor current is the current drawn by an electric motor when its rotor is prevented from rotating while full voltage is applied. This is typically the highest current the motor will draw under normal operating conditions.

2. How Does the Calculator Work?

The calculator uses the locked rotor current formula:

Where:

• \( I_{lr} \) — Locked rotor current (A)
• \( P \) — Power (kW)
• \( V \) — Voltage (V)
• \( PF \) — Power factor (0-1)
• \( Eff \) — Motor efficiency (0-1)

Explanation: The formula converts power from kW to watts, then divides by the product of voltage, power factor, efficiency and √3 (for three-phase systems).

3. Importance of Locked Rotor Current Calculation

Details: Knowing the locked rotor current is essential for proper sizing of motor starters, circuit breakers, and other protective devices to ensure safe operation and prevent equipment damage.

4. Using the Calculator

Tips: Enter power in kW, voltage in volts, power factor (typically 0.85 for motors), and efficiency (typically 0.9 for motors). All values must be positive numbers.

5. Frequently Asked Questions (FAQ)

Q1: Why is locked rotor current important?
A: It helps determine the required capacity of electrical components and protection devices to handle the initial current surge when a motor starts.

Q2: How does locked rotor current differ from full load current?
A: Locked rotor current is typically 5-7 times higher than full load current and only lasts briefly during motor startup.

Q3: What affects locked rotor current?
A: Motor design, voltage, power factor, and efficiency all influence the locked rotor current value.

Q4: Is this calculation for single-phase or three-phase motors?
A: This formula is for three-phase motors. For single-phase, remove the √3 factor from the denominator.

Q5: How can locked rotor current be reduced?
A: Using soft starters, variable frequency drives, or reduced voltage starting methods can lower the locked rotor current.