1. What is Locked Rotor Current?

Locked Rotor Current (ILR) is the current drawn by an electric motor when its rotor is locked and unable to rotate. This represents the maximum current the motor will draw during startup and is important for sizing circuit protection devices.

2. How Does the Calculator Work?

The calculator uses the Locked Rotor Current formula:

Where:

• \( HP \) — Motor horsepower
• \( V \) — Voltage (volts)
• \( Eff \) — Motor efficiency (decimal)
• \( \text{LRA Factor} \) — Locked rotor current factor (decimal)

Explanation: The equation calculates the maximum current drawn by a motor during startup when the rotor is stationary.

3. Importance of Locked Rotor Current Calculation

Details: Accurate locked rotor current calculation is crucial for proper circuit protection sizing, preventing nuisance tripping of breakers, and ensuring motor starting reliability.

4. Using the Calculator

Tips: Enter motor horsepower in HP, voltage in volts, efficiency as a decimal (e.g., 0.85 for 85%), and LRA factor as a decimal. All values must be positive numbers.

5. Frequently Asked Questions (FAQ)

Q1: Why is locked rotor current important?
A: It helps determine the proper sizing of circuit breakers, fuses, and other protective devices to prevent nuisance tripping during motor startup.

Q2: What is a typical LRA factor value?
A: LRA factor typically ranges from 4 to 8 times the full load current, depending on motor design and application.

Q3: How does voltage affect locked rotor current?
A: Locked rotor current is inversely proportional to voltage - lower voltage results in higher locked rotor current.

Q4: What are the consequences of underestimating locked rotor current?
A: Underestimation can lead to undersized protection devices that may trip during normal motor starting, causing operational disruptions.

Q5: Can this calculation be used for all motor types?
A: While the basic principle applies to most AC induction motors, specific motor designs may require manufacturer-specific calculations.