Compressor LRA
Electrical Auditor
Audit equipment nameplate LRA thresholds to compute true operational surge currents and grid voltage sag risks.
Locked Rotor Amperage Mechanics & Compressor Induction Physics Overview
Locked Rotor Amperage (LRA) defines the maximal amount of current an electric motor induction coil will consume instantly when energized while the internal shaft rotor remains completely motionless. At the exact millisecond of start-up, a stationary electric motor lacks back-electromotive force (Back-EMF). Without this counter-voltage generation, the compressor’s copper winding matrix functions as a direct short circuit, pulling massive current spikes typically 400% to 600% higher than standard Running Load Amps (RLA). Under proper operating conditions, a healthy motor establishes rotation within 100 to 300 milliseconds, rapidly building Back-EMF and stabilizing the amp draw down to normal RLA boundaries. However, if the compressor is locked up mechanically by internal bearing damage or liquid refrigerant slung into the pistons, the motor remains locked at full LRA limits until thermal safety sensors trip or circuit breakers disconnect the overloaded network.
Frequently Asked Questions
A: When a standard single-stage compressor experiences direct across-the-line starting drawing 80 to 120 Amps instantly, it overloads the property’s branch electrical line. According to Ohm’s Law, this massive amp surge flowing through the resistance of the electrical supply line drops the voltage coming from the transformer. This temporary drop reduces the power available to lighting nodes, making them flicker until the compressor transitions out of the startup phase.
A: A traditional hard-start relay forces more torque into the start winding by kicking in an extra capacitor for a brief millisecond, but it still allows an unregulated, violent current spike. An advanced solid-state soft-starter utilizes internal micro-processors and silicon-controlled rectifiers (SCRs) to smoothly ramp up the starting voltage across the waveforms. This advanced control trims overall starting inrush currents by up to 60%, removing lines surges, eliminating breaker trips, and protecting motor mechanics.