What it is

"Locked rotor" is when the compressor motor is energized but the rotor isn't turning — so it draws enormous current trying to start against a load that won't move. Locked rotor amps (LRA) is that inrush figure stamped on the nameplate. Every start sees a brief LRA spike; the problem is when the compressor stays at locked-rotor current because it can't break away or because it's mechanically seized. Compressor protection exists to catch that and cut power before the windings cook. Reading these amps correctly tells you whether you've got a seized compressor, a starting problem, or a power problem.

How it works

A stalled motor is electrically almost a dead short — with the rotor not spinning, there's no back-EMF to limit current, so it slams up to locked-rotor amps, several times the normal running draw. That much current makes heat fast. To survive it, single-phase compressors have an internal overload (a line-break protector buried in or strapped to the windings) that opens on excess current or temperature and resets when it cools. That's why a struggling compressor cycles: it tries to start, draws locked-rotor, heats up, the overload opens, it sits silent, cools, the overload resets, and it tries again. That cycle on/off humming pattern is the classic locked-rotor-or-overload signature.

LRA happens for a few different reasons, and they're not all the compressor's fault: genuine mechanical seizure, not enough starting torque (low voltage, weak cap, no start assist), starting against an unequalized pressure differential, or a lost power leg (single-phasing) feeding the motor.

In the field

Clamp the start. Put your clamp on the common leg and watch the meter as it tries to start.

  • A healthy start spikes to roughly LRA for a fraction of a second, then drops to normal running amps as it spins up.
  • A locked rotor pulls up near LRA and stays there until the overload opens — it never drops to running amps because it never spins.

Sort out why it's locked:

  • Check the windings and ground first (see the grounded/open/shorted article) — rule out an electrical failure.
  • Check the run capacitor — a dead cap robs starting torque and can cause a hum-and-trip with no rotation. Replace and retry.
  • Check supply voltage under the start attempt — low voltage or a lost leg (single-phasing) starves the motor. If one leg is gone, you'll get a hum and an overload trip with the contactor happily pulled in.
  • Try a hard-start kit — if torque or pressure-differential is the issue, a proper start assist may break it loose. If it still won't turn with good power, good cap, and a hard-start kit, you're looking at a mechanically seized compressor.

Normal values & targets

  • Locked rotor amps (LRA): printed on the nameplate; commonly four to roughly eight times the running amps. A momentary spike to about LRA on every normal start is expected.
  • Running amps: at or below the nameplate RLA (rated load amps) once up to speed. Steady draw well over RLA is an overload condition.
  • The healthy signature: brief LRA spike, then a quick drop to normal running amps within a second or so.
  • The locked signature: current pins near LRA and holds until the internal overload opens (seconds), then zero while it cools.
  • Internal overload reset time: from a couple minutes to much longer depending on how hot it got — a compressor can stay "open" for a while after a hard trip.

Common faults & what they mean

  • Hums, draws near LRA, trips, goes silent, repeats → locked rotor or overload cycling; work the checklist (windings, cap, voltage, start torque, seizure).
  • One power leg missing, contactor pulled in, compressor hums and trips → single-phasing; fix the lost leg, the compressor may be fine.
  • Won't start on a weak/dead run cap, starts fine after cap replacement → it was a torque problem, not a dead compressor.
  • Still won't turn with good power, good cap, and a hard-start kit → mechanically seized; condemn.
  • Starts but running amps stay above RLA → overload condition (high head pressure, restricted condenser, overcharge, low voltage); chase the load, not the motor.
  • Internal overload reads open when hot, continuous when cool → that's the protector doing its job; find what's overheating it.

Tech tips & gotchas

Don't condemn on a single failed start. A compressor that's hot and tripping might just have an open internal overload from the last attempt. Let it cool fully, fix the obvious causes (cap, voltage), and try again. Plenty of "seized" compressors are really weak-cap or low-voltage starting problems.

Single-phasing is a sneaky one. The 24V coil only needs control voltage to pull the contactor in, so the contactor seats and looks normal even though a line leg is dead. The compressor then hums on one leg and trips. Always meter both legs at the compressor under the call, not just at the disconnect.

The internal overload is not user-serviceable and resets on its own — you can't "reset" it manually, you just wait. If a compressor needs an external current-sensing protector or the internal one is nuisance-tripping, the real fix is removing whatever is overloading it.

A brief LRA spike on every start is normal and is exactly why hard-starting units trip breakers on inrush. If inrush trips the breaker but the compressor would otherwise run, a hard-start kit reduces the spike. If it draws LRA continuously, that's a different, worse problem.

Safety / code notes

Locked-rotor current is severe — don't sit there cycling a locked compressor repeatedly; you're heating the windings toward permanent failure. Compressor branch-circuit and protection sizing (including the role of LRA/RLA in conductor and disconnect selection) follow NEC Article 440. De-energize at the disconnect within sight of the unit before testing terminals, and recover refrigerant per EPA Section 608 if the compressor must come out.