Locked Rotor Current Calculator With Time

Locked Rotor Current Equation:

\[ I_{lr} = \frac{V}{Z_{lr}} \times \text{Time Factor} \]

Locked Rotor Current (I_lr):

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1. What is Locked Rotor Current?

Locked rotor current is the current drawn by an electric motor when its rotor is not rotating (locked) while full voltage is applied. It's typically much higher than the normal operating current and is important for sizing protective devices.

2. How Does the Calculator Work?

The calculator uses the locked rotor current equation:

\[ I_{lr} = \frac{V}{Z_{lr}} \times \text{Time Factor} \]

Where:

\( I_{lr} \) — Locked rotor current (A)
\( V \) — Applied voltage (V)
\( Z_{lr} \) — Locked rotor impedance (Ω)
Time Factor — Multiplier accounting for time-dependent effects

Explanation: The equation calculates the initial current when the rotor is stationary, then applies a time factor to account for how the current changes over time during startup.

3. Importance of Locked Rotor Current Calculation

Details: Knowing locked rotor current is essential for proper circuit breaker and fuse selection, motor starter sizing, and assessing voltage drop during motor starting.

4. Using the Calculator

Tips: Enter voltage in volts, impedance in ohms, and time factor (typically 1.0 for instantaneous value). All values must be positive numbers.

5. Frequently Asked Questions (FAQ)

Q1: Why is locked rotor current higher than running current?
A: When the rotor is stationary, there's no back EMF generated, so the motor appears as a low impedance to the power supply, drawing high current.

Q2: What is a typical time factor value?
A: For instantaneous locked rotor current, use 1.0. For time-dependent calculations, values may range from 0.5 to 1.5 depending on motor characteristics.

Q3: How does this affect circuit protection?
A: Protective devices must withstand the locked rotor current long enough to allow the motor to start, while still protecting against actual faults.

Q4: What's the relationship to starting current?
A: Locked rotor current is essentially the maximum possible starting current when voltage is first applied.

Q5: How can locked rotor current be reduced?
A: Methods include reduced-voltage starters, soft starters, or variable frequency drives.