On a contactor, LRA stands for Locked Rotor Amps, representing the maximum current an electric motor will draw at the moment it starts up from a standstill or if its rotor is stalled or locked.
Understanding Locked Rotor Amps (LRA)
When an inductive load, such as an electric motor, is first energized, it experiences a significant surge in current before settling into its normal operating current. This initial spike in current is known as Locked Rotor Amps (LRA). It specifically refers to the amount of current a motor will draw when voltage is first applied while its rotor is still stationary or if it encounters a mechanical issue that prevents it from turning (a "locked rotor" condition).
This current can be many times higher than the motor's normal running current, often 5 to 10 times the Full Load Amps (FLA), due to the motor's windings acting like a short circuit until the magnetic field builds and the rotor begins to spin.
Why LRA is Crucial for Contactors
Contactors are electromechanical switches designed to safely connect and disconnect power to high-current loads like motors. The LRA rating is critical for selecting the correct contactor because:
- Inrush Current Handling: The contactor's contacts must be robust enough to withstand the immense current surge of LRA upon motor startup without welding shut or being damaged. If a contactor is undersized for the motor's LRA, its contacts can arc excessively, leading to premature failure, welding, or even fire.
- Durability and Longevity: A contactor properly rated for the LRA will have a longer operational lifespan as its contacts are designed to repeatedly handle these high momentary currents.
- Safety: Correctly sizing the contactor based on LRA ensures that the electrical system can safely manage motor starts, preventing damage to the motor, contactor, and associated wiring.
LRA vs. RLA vs. FLA: Key Motor Current Terms
It's important to differentiate LRA from other common motor current ratings:
| Term | Abbreviation | Description | Significance for Contactors |
|---|---|---|---|
| Locked Rotor Amps | LRA | The maximum current drawn by a motor when voltage is applied while the rotor is stationary or unable to turn. This is the startup surge or stall current. | Critical for contactor sizing. The contactor must be rated to withstand this initial high current without damage. |
| Full Load Amps | FLA | The current drawn by a motor when it is operating at its rated horsepower or output under normal, full load conditions after reaching its steady operating speed. | Relevant for selecting overload protection and main circuit breakers, but not the primary factor for contactor contact ratings. |
| Rated Load Amps | RLA | Often used interchangeably with FLA, particularly for refrigeration and air conditioning compressors. It represents the maximum current the compressor should draw during normal operation. | Similar to FLA, it helps in sizing overload protection and assessing normal operational current. LRA remains the key for startup current handling. |
Practical Insights
- Motor Nameplate: Always refer to the motor's nameplate data. It will list the LRA, FLA, and often the voltage, horsepower, and service factor.
- Contactor Selection: When selecting a contactor for a motor, ensure its LRA rating (often listed as motor horsepower ratings for specific voltages) meets or exceeds the motor's LRA requirements. Manufacturers typically provide charts that cross-reference motor horsepower at various voltages to appropriate contactor sizes.
- Overload Protection: While LRA dictates the contactor's initial current handling capability, separate overload relays are used to protect the motor from sustained overcurrents (above FLA/RLA) during normal operation or from a locked rotor condition that persists for too long. These relays typically trip after a few seconds of LRA conditions to prevent motor burnout.
Understanding LRA is fundamental for proper and safe electrical system design, especially in applications involving electric motors.
