What causes lra on compressor

What is Locked Rotor Amperage (LRA)?

Locked Rotor Amperage (LRA), also known as starting current or inrush current, is the maximum current that a motor, such as the one in a compressor, will draw when it is first energized and the rotor is stationary. This is a critical parameter for understanding and designing electrical systems that power compressors, particularly in applications like air conditioning, refrigeration, and industrial machinery.

When a compressor motor starts, it faces significant resistance. This resistance comes from the inertia of the motor itself (the mass that needs to be set in motion) and, in the case of a compressor, the pressure difference it needs to overcome within the system. To break free from this stationary state and begin rotating, the motor requires a much larger electrical current than it does once it's up and running at its normal operating speed. This surge of current is the LRA.

Why is LRA So High?

The high LRA is a consequence of the motor's design and the physics of starting. In a typical AC induction motor, which is commonly used in compressors, the starting current is a result of the magnetic fields that are generated when power is applied. At standstill, the motor acts much like a transformer with its secondary winding short-circuited. This creates a very low impedance path for current flow, leading to a high amperage draw.

Think of it like pushing a heavy object. It requires a much greater initial push (high current) to get it moving than it does to keep it rolling once it's in motion (normal running current). Once the rotor begins to spin, the motor's internal characteristics change, its impedance increases, and the current drawn drops dramatically to its normal operating level.

LRA vs. Running Load Amperage (RLA)

It's crucial to distinguish LRA from Running Load Amperage (RLA). RLA, sometimes referred to as Full Load Amps (FLA) or Rated Load Amps (RLA), is the current the compressor motor draws when it is operating under normal load conditions after it has started. RLA is the continuous current draw you can expect during typical operation.

The LRA is almost always significantly higher than the RLA. For many compressors, the LRA can be anywhere from 5 to 7 times, or even more, than the RLA. For example, a compressor with an RLA of 10 amps might have an LRA of 50-70 amps or higher.

Implications of LRA

The high surge of LRA has several important implications:

• Electrical System Design: Electrical circuits, wiring, circuit breakers, and fuses must be sized to safely handle the LRA. If a circuit breaker or fuse is rated too low, it will trip or blow every time the compressor starts, even though the motor is functioning correctly. This is a common cause of nuisance tripping.
• Voltage Drop: The high current draw during startup can cause a temporary but noticeable drop in voltage on the electrical circuit. This voltage drop can affect the performance of other equipment connected to the same circuit. In severe cases, a significant voltage drop can prevent the motor from starting properly or even damage it.
• Component Stress: While LRA is a normal part of motor operation, repeated or prolonged high starting currents can put stress on motor windings and other electrical components.
• Generator and Inverter Sizing: For backup power systems or portable applications using generators or inverters, the LRA is a critical factor in determining the required power output. The generator or inverter must be able to supply the peak surge current required by the compressor.

Factors Affecting LRA

Several factors can influence the LRA of a compressor:

• Motor Design: Different motor designs (e.g., single-phase vs. three-phase, capacitor-start, split-phase) have varying LRA characteristics.
• Motor Size and Horsepower: Larger, higher horsepower motors generally have higher LRA values.
• Operating Conditions: While LRA is measured at standstill, factors like initial system pressure at startup can indirectly influence the torque required and thus the starting current.
• Temperature: Motor winding resistance changes with temperature, which can slightly affect the LRA.

How LRA is Specified

The LRA is typically found on the compressor's nameplate or in its technical specifications provided by the manufacturer. It is usually listed in Amperes (A). Technicians and installers use this information to ensure that the electrical supply and protection devices are adequately sized.

Understanding LRA is fundamental for anyone involved in the installation, maintenance, or troubleshooting of systems containing compressors. It explains why a circuit breaker might trip only at startup and is essential for ensuring reliable and safe operation of refrigeration and air conditioning equipment.