When working with electric motors and electrical systems, one of the key parameters to understand is the Locked Rotor Amps (LRA). It refers to the current drawn by a motor when the rotor is not turning but power is applied — essentially, the motor is “locked” in place. This current is significantly higher than the motor’s normal running current and is crucial for motor protection and system design.
If you need to quickly calculate the Locked Rotor Amps to assess motor start-up current or design appropriate circuit protection, our Locked Rotor Amps Calculator is here to help. This tool simplifies complex calculations by allowing you to input key values and instantly get accurate LRA values.
This article will explain what Locked Rotor Amps means, how to use this calculator effectively, the formula behind the calculation, practical examples, and answers to frequently asked questions. Let’s dive in.
What is Locked Rotor Amps (LRA)?
Locked Rotor Amps (LRA), also known as starting current or locked rotor current, is the current that an electric motor draws when the rotor is stationary and full voltage is applied. This condition occurs during motor start-up or if the rotor is physically blocked.
LRA is typically much higher than the motor’s rated running current because the motor draws maximum current to overcome the initial inertia. Understanding and calculating LRA is critical for:
- Proper sizing of motor starters and circuit breakers.
- Avoiding damage to electrical components due to excessive current.
- Ensuring safe operation and compliance with electrical codes.
- Planning for motor start-up in industrial processes.
Why Calculate Locked Rotor Amps?
The high current during locked rotor conditions can cause significant electrical and mechanical stress. Calculating LRA helps engineers and electricians:
- Select appropriate motor protection devices: Circuit breakers and fuses must be rated to handle this high current without nuisance trips.
- Design electrical panels and wiring: Ensures components can safely carry locked rotor current.
- Diagnose motor problems: Abnormally high LRA may indicate motor issues like winding shorts.
- Predict motor performance: Understanding LRA aids in system design and energy efficiency planning.
How to Use the Locked Rotor Amps Calculator
Using the calculator is easy and fast:
- Enter the kVA (kilovolt-amperes)
This represents the apparent power rating of the motor or the power supply connected to it. - Enter the Horsepower (HP) rating
The rated mechanical power output of the motor. - Enter the Voltage
The supply voltage in volts. - Click the “Calculate” button
The calculator uses these inputs to compute the Locked Rotor Amps instantly and display the result.
The Formula Behind the Calculation
The Locked Rotor Amps (LRA) is calculated using the formula:
LRA = (1000 × (kVA / Horsepower)) / Voltage
Where:
- kVA = apparent power in kilovolt-amperes
- Horsepower = rated motor power
- Voltage = supply voltage in volts
- 1000 = a conversion factor to scale kVA to VA
This formula determines the current drawn during the locked rotor condition based on motor and supply parameters.
Example Calculations
Example 1: Small Motor
- kVA = 15
- Horsepower = 10 HP
- Voltage = 230 volts
Calculation:
LRA = (1000 × (15 / 10)) / 230 = (1000 × 1.5) / 230 = 1500 / 230 ≈ 6.52 amps
The locked rotor current is approximately 6.52 amps.
Example 2: Larger Motor
- kVA = 60
- Horsepower = 40 HP
- Voltage = 460 volts
Calculation:
LRA = (1000 × (60 / 40)) / 460 = (1000 × 1.5) / 460 = 1500 / 460 ≈ 3.26 amps
The locked rotor current is approximately 3.26 amps.
Example 3: High Voltage Motor
- kVA = 120
- Horsepower = 100 HP
- Voltage = 690 volts
Calculation:
LRA = (1000 × (120 / 100)) / 690 = (1000 × 1.2) / 690 = 1200 / 690 ≈ 1.74 amps
The locked rotor current is approximately 1.74 amps.
Additional Helpful Information About Locked Rotor Amps
Understanding Locked Rotor Current Behavior
Locked rotor current is usually 5 to 7 times higher than the motor’s rated running current. This surge lasts only briefly during motor start-up but must be handled by electrical components.
Impact on Electrical Systems
High LRA affects electrical system design by requiring:
- Heavy-duty contactors and starters to handle large inrush currents.
- Wiring rated for short-term high current to prevent overheating.
- Protective devices with suitable time delay to allow motor start without tripping.
Safety Considerations
Because LRA is high, incorrect calculations or component selection can lead to:
- Equipment failure
- Fire hazards
- Unplanned downtime
Using this calculator ensures you have accurate current estimates to prevent such risks.
Tips for Using the Locked Rotor Amps Calculator
- Use accurate motor specifications for kVA, horsepower, and voltage.
- Confirm units to avoid errors (e.g., voltage in volts, horsepower as mechanical power).
- Compare the calculated LRA with motor nameplate values if available.
- Use the LRA value to select proper circuit breakers and starters.
20 Frequently Asked Questions (FAQs)
1. What does Locked Rotor Amps mean?
It is the current drawn by a motor when the rotor is not turning but voltage is applied.
2. Why is Locked Rotor Amps important?
Because it helps in designing and protecting electrical circuits for motor start-up conditions.
3. How is Locked Rotor Amps calculated?
Using the formula: LRA = (1000 × (kVA / Horsepower)) / Voltage.
4. What is kVA?
Kilovolt-amperes, a unit representing apparent power.
5. Why is the locked rotor current higher than running current?
Because the motor draws maximum current to overcome inertia and start turning.
6. Can I use this calculator for all motors?
Yes, as long as you know the motor’s kVA, horsepower, and voltage.
7. What happens if I enter incorrect values?
The calculated LRA will be inaccurate, risking wrong equipment selection.
8. How does voltage affect locked rotor amps?
Higher voltage results in lower locked rotor current, all else equal.
9. Can this calculator help with motor protection device selection?
Yes, by providing accurate LRA values for sizing breakers and starters.
10. What is the typical range for locked rotor amps?
Usually 5-7 times the motor’s rated running current.
11. Is the formula applicable for both single-phase and three-phase motors?
The formula is a general estimate; consult motor specifications for precise three-phase calculations.
12. What unit should horsepower be in?
Mechanical horsepower.
13. Why multiply by 1000 in the formula?
To convert kVA from thousands of volt-amperes to volt-amperes.
14. Can this calculator handle fractional horsepower motors?
Yes, enter decimal values as needed.
15. Does this calculator provide the duration of locked rotor current?
No, it calculates only the magnitude.
16. Is Locked Rotor Amps related to torque?
Indirectly, as high current generates torque to overcome rotor inertia.
17. How to reduce Locked Rotor Amps in motors?
Using soft starters or variable frequency drives.
18. What is the difference between LRA and Full Load Amps (FLA)?
LRA is the starting current; FLA is the current during normal operation.
19. Can I use the calculated LRA for fuse sizing?
Yes, but consult electrical codes and manufacturer recommendations.
20. Is this tool free to use?
Yes, free and accessible anytime online.
Conclusion
The Locked Rotor Amps Calculator is an essential tool for electrical engineers, technicians, and anyone working with electric motors. By accurately calculating the locked rotor current, you can ensure proper motor protection, prevent equipment failure, and design safer electrical systems.
Understanding the formula and how to use this calculator empowers you to make informed decisions about motor start-up current and system design. Use this tool alongside motor specifications to optimize your electrical installations and maintenance.