CSLB C-20 HVAC ContractorCooling and RefrigerationMedium
A technician responds to a call where a compressor hums but fails to start. The nameplate lists RLA (rated load amps) of 18A and LRA (locked rotor amps) of 95A. A clamp meter shows the compressor continuously drawing 94A with no increase in speed before the overload trips. This reading most likely indicates:
- AA mechanically seized or locked compressor
- BLow refrigerant charge causing reduced load
- CNormal starting current draw before ramp-up
- DAn oversized start capacitor causing overcurrent
Show answer & explanationAnswer & explanation
Correct answer: A. A mechanically seized or locked compressor
A brief spike near LRA is normal during startup, but sustained current at the locked rotor amperage level with no speed increase and an eventual overload trip indicates the compressor motor is mechanically locked or seized and cannot rotate, drawing near-stall current continuously.
Why the other options are wrong
- B. Low charge reduces compressor load and amperage; it doesn't cause sustained LRA draw.
- C. Normal starting current is a brief transient, not a sustained reading at LRA.
- D. A capacitor issue could contribute to hard starting, but sustained LRA current with humming points to mechanical lockup.
Locked Rotor Amps (LRA) Diagnosis
LRA is the current a compressor motor draws when its rotor cannot turn (starting or stalled); sustained current at this level indicates a seized or locked compressor.
- RLA = normal running current; LRA = stall current, much higher
- Brief LRA spike at startup is normal
- Continuous LRA-level draw with humming signals mechanical lockup
Memory trick: Stuck at LRA too long means the rotor's in a locked-up song.