Motor Locked Rotor Current Calculator
Calculate the locked rotor current (LRC) for single-phase and three-phase motors.
Locked Rotor Current Calculator
Compliant with NEMA Standards
Results:
Locked Rotor Current (A): 0.00
How to Use the Calculator
Input Power Rating
- Enter the motor's power rating.
- Select the unit (kW or HP).
Input Voltage & Phase
- Enter the supply voltage in Volts (V).
- Select the motor's phase (Single Phase or Three Phase).
Select NEMA Code Letter
- Choose the NEMA Code Letter from the motor's nameplate.
- This determines the KVA/HP ratio.
Locked Rotor Current Fundamentals
What is LRC?
Locked Rotor Current (LRC) is the current drawn by a motor when its rotor is stationary and full voltage is applied. It's typically 5 to 10 times the motor's full-load current.
NEMA Code Letters
- NEMA (National Electrical Manufacturers Association) assigns code letters to motors.
- These letters indicate the motor's locked-rotor kVA per horsepower range.
- Crucial for selecting appropriate motor protection devices.
Importance of LRC:
LRC is vital for sizing motor starters, circuit breakers, and fuses to ensure proper protection and avoid nuisance tripping during motor startup.
Calculation Formulas
Single-phase (kW):
LRC = Pkw × 1341 × (KVA/HP) / Voltage
Single-phase (HP):
LRC = Php × 1000 × (KVA/HP) / Voltage
Three-phase (kW):
LRC = Pkw × 774 × (KVA/HP) / Voltage
Three-phase (HP):
LRC = Php × 577 × (KVA/HP) / Voltage
Example Calculation
Three Phase Motor (HP):
Power: 10 HP
Voltage: 480V
Code Letter: G (KVA/HP = 5.95, mid-range of 5.60-6.29)
LRC = 10 HP × 577 × 5.95 / 480V
LRC = 34331.5 / 480
LRC ≈ 71.52 Amps
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