Calculate Locked Rotor Current

Locked Rotor Current Formula:

\[ I_{LR} = \frac{HP \times 746 \times 1000}{V \times Efficiency \times PF \times \sqrt{3}} \]

Horsepower (HP):

Voltage (V):

Efficiency:

decimal

Power Factor (PF):

decimal

Locked Rotor Current (I_LR):

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1. What is Locked Rotor Current?

Locked Rotor Current (I_LR) is the current drawn by an electric motor when its rotor is locked or prevented from rotating while full voltage is applied. This represents the maximum current the motor will draw during startup.

2. How Does the Calculator Work?

The calculator uses the Locked Rotor Current formula:

\[ I_{LR} = \frac{HP \times 746 \times 1000}{V \times Efficiency \times PF \times \sqrt{3}} \]

Where:

\( I_{LR} \) — Locked rotor current (amps)
\( HP \) — Horsepower (hp)
\( V \) — Voltage (volts)
\( Efficiency \) — Motor efficiency (decimal)
\( PF \) — Power factor (decimal)
\( \sqrt{3} \) — Constant for three-phase systems

Explanation: The formula calculates the maximum current drawn by a three-phase motor during startup conditions when the rotor is stationary.

3. Importance of Locked Rotor Current Calculation

Details: Calculating locked rotor current is essential for proper circuit protection, sizing of circuit breakers and fuses, determining voltage drop during startup, and ensuring electrical system compatibility with motor requirements.

4. Using the Calculator

Tips: Enter horsepower in hp, voltage in volts, efficiency as a decimal (e.g., 0.85 for 85%), and power factor as a decimal (e.g., 0.8 for 80%). 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 protective devices like circuit breakers and fuses to prevent nuisance tripping while providing adequate motor protection.

Q2: What is the typical locked rotor current for motors?
A: Locked rotor current is typically 5-8 times the full load current of the motor, depending on motor design and specifications.

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

Q4: Are there different calculation methods for single-phase vs three-phase motors?
A: Yes, single-phase motors use a different calculation that doesn't include the √3 factor used for three-phase systems.

Q5: When should locked rotor current be measured?
A: It should be considered during motor selection, electrical system design, and when troubleshooting motor starting issues or protective device coordination.