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 (amperes)
• \( kVA_{hp} \) — kVA per horsepower (from motor specifications)
• \( V \) — Applied voltage (volts)

Explanation: The equation calculates the current drawn by a three-phase motor when the rotor is locked, based on the motor's kVA/hp rating and the applied voltage.

3. Importance of Locked Rotor Calculation

Details: Knowing the locked rotor current is essential for proper sizing of motor starters, circuit breakers, and other protective devices to ensure they can handle the initial inrush current without nuisance tripping.

4. Using the Calculator

Tips: Enter the motor's kVA per hp rating (typically found in motor specifications) and the applied voltage. Both values must be positive numbers.

5. Frequently Asked Questions (FAQ)

Q1: What is a typical kVA/hp value for motors?
A: Standard NEMA Design B motors typically have kVA/hp values ranging from 3.15 to 9.45, depending on motor size and design.

Q2: Why is locked rotor current important?
A: It helps determine the required interrupting capacity of protective devices and ensures proper coordination of motor protection.

Q3: How does voltage affect locked rotor current?
A: Locked rotor current is directly proportional to voltage - higher voltage results in lower current for the same power.

Q4: Is this calculation valid for single-phase motors?
A: No, this formula is specifically for three-phase motors. Single-phase motors would use a different calculation.

Q5: How long can a motor withstand locked rotor current?
A: Typically only a few seconds before overheating occurs. Protective devices must operate quickly under locked rotor conditions.