Locked Rotor Amperage Calculator

Locked Rotor Amperage Formula:

\[ I_{lr} = \frac{HP \times 746 \times LRC}{V \times \sqrt{3} \times Eff \times PF} \]

Horsepower (HP):

Code Letter Factor (LRC):

Voltage (V):

Efficiency (Eff):

(0-1)

Power Factor (PF):

(0-1)

Locked Rotor Amperage (I_lr):

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1. What is Locked Rotor Amperage?

Locked Rotor Amperage (LRA) is the current a motor draws when power is first applied but the rotor isn't turning. This initial surge current is much higher than the normal operating current and is important for sizing circuit protection devices.

2. How Does the Calculator Work?

The calculator uses the locked rotor amperage formula:

\[ I_{lr} = \frac{HP \times 746 \times LRC}{V \times \sqrt{3} \times Eff \times PF} \]

Where:

\( HP \) — Motor horsepower
\( LRC \) — Code letter factor from motor nameplate
\( V \) — Voltage
\( Eff \) — Motor efficiency (0-1)
\( PF \) — Power factor (0-1)

Explanation: The formula converts horsepower to watts (746W/HP), applies the code letter factor, then divides by the product of voltage, square root of 3 (for 3-phase systems), efficiency and power factor.

3. Importance of LRA Calculation

Details: Knowing the locked rotor current is essential for proper selection of motor starters, circuit breakers, and overload protection devices to prevent nuisance tripping while ensuring adequate protection.

4. Using the Calculator

Tips: Enter motor horsepower, code letter factor (typically found on motor nameplate), operating voltage, and typical efficiency and power factor values if unknown (default is 0.85 for both).

5. Frequently Asked Questions (FAQ)

Q1: Why is locked rotor current higher than running current?
A: When stationary, the motor presents nearly a short circuit to the power source, drawing high current until the rotor begins to turn and generate back EMF.

Q2: How long does locked rotor current last?
A: Typically only for a fraction of a second until the motor reaches about 75-80% of full speed, but protection devices must withstand this brief surge.

Q3: What's a typical code letter factor?
A: Common values range from 3.6 (code letter A) to 16.0 (code letter K), with higher letters indicating higher inrush current.

Q4: Does this apply to single-phase motors?
A: The formula is for 3-phase motors. For single-phase, remove the √3 factor from the denominator.

Q5: How accurate is this calculation?
A: It provides a good estimate, but actual LRA can vary based on specific motor design and should be verified against manufacturer data when available.