HVAC Technician Academy
Learn how to separate a truly locked compressor from a weak capacitor, low-voltage condition, start-component failure, pressure-differential problem, or other hard-start condition.
Not necessarily. A compressor that cannot accelerate may draw current near locked-rotor amperage, but the cause can be a weak run capacitor, failed start component, low voltage, excessive pressure differential, poor connections, or an actual mechanical lock. The starting circuit and supply conditions must be proven before the compressor is condemned.
LRA stands for locked-rotor amperage.
When a motor is energized but the rotor is not turning, current can rise dramatically.
The important diagnostic question is:
That question separates a parts changer from a diagnostician.
Insufficient phase shift can reduce available starting torque.
Voltage collapse during startup can prevent the compressor from accelerating.
A failed start capacitor or relay can remove needed starting assistance.
Restarting against unfavorable pressure conditions can increase starting difficulty.
Burned contacts or loose connections can reduce voltage under load.
Internal mechanical damage can physically prevent rotation.
A weak capacitor can create symptoms that look almost identical to compressor failure.
Possible symptoms include:
Test capacitance against the nameplate microfarad rating and tolerance.
Continue: Bad Capacitor vs Bad Compressor
Static voltage can look perfect until the compressor tries to start.
During startup, check for:
If voltage collapses during the start attempt, the compressor may not develop sufficient starting torque.
Burned or resistive contactor contacts can create voltage drop when heavy compressor current flows.
Inspect the contactor and measure voltage drop across closed contacts where appropriate.
Study: AC Contactor Diagnostic Guide
Some compressors use additional starting components to increase starting torque.
Depending on equipment design, these may include:
A failed start component can leave an otherwise functional compressor unable to start correctly.
An approved hard-start device can increase starting torque on equipment designed or approved for its use.
But it should not be used as a substitute for diagnosis.
Before installing one, determine why the compressor is struggling to start.
A hard-start device will not repair:
A compressor starting shortly after shutdown may encounter a larger pressure differential than after pressures equalize.
Equipment controls are often designed with timing or logic intended to prevent damaging short-cycle restarts.
Investigate rapid cycling if the compressor repeatedly struggles after very short off periods.
If the compressor energizes but does not rotate, current can rise toward locked-rotor values.
That tells you the motor is not accelerating normally.
It does not by itself identify whether the cause is electrical or mechanical.
Continue: RLA vs Actual Compressor Amps
If external electrical problems are ruled out and the compressor still will not start, isolate the compressor and evaluate winding condition.
Check:
Continue: Grounded vs Open Compressor Windings
Repeated locked-rotor attempts create rapid heat.
Eventually the compressor's internal thermal protector may open.
At that point the compressor may stop drawing current entirely until the protector resets.
Repeatedly cycling power without finding the cause can add additional thermal stress.
1. Confirm the compressor is being commanded on.
2. Verify correct line voltage.
3. Measure voltage during the start attempt.
4. Inspect and test the run capacitor.
5. Inspect contactor and wiring connections.
6. Verify any factory or approved start components.
7. Measure startup current.
8. Check compressor temperature and internal overload condition.
9. Consider pressure differential and short-cycle history.
10. If the compressor still cannot start, isolate and test windings.
11. Condemn the compressor only after external starting problems have been eliminated and the failure is proven.
| Pattern | Possible Cause | Best Next Test |
|---|---|---|
| Near LRA + weak capacitor | Starting torque problem | Correct capacitor and retest |
| Near LRA + voltage collapse | Supply / connection issue | Trace voltage drop under load |
| Near LRA + correct voltage + good capacitor | Possible mechanical lock | Winding, start circuit, and compressor evaluation |
| Starts after proper start assistance | Hard-start condition | Find root cause and verify manufacturer-approved setup |
| No current after repeated failed starts | Possible thermal overload open | Check compressor temperature and retest after appropriate cooling |
Technician Case File
Compressor hums.
Startup current rises near locked-rotor range.
Technician suspects seized compressor.
Line voltage: correct.
Run capacitor rating: 45 µF.
Actual measurement: 23 µF.
Correct capacitor is installed.
Compressor starts immediately and operating amperage stabilizes.
Lesson: Near-LRA current showed that the compressor was not accelerating. It did not prove the compressor was mechanically locked.
Supply voltage remains correct during startup.
Capacitor is correct and within tolerance.
Contactor voltage drop is minimal.
Starting components are verified.
Compressor windings are not grounded and have a normal resistance relationship.
Yet every startup attempt immediately produces near-LRA current and no rotation.
After external causes are eliminated, the evidence increasingly supports an internal mechanical locked-rotor condition.
Compressor starts normally after sitting off for several minutes.
But after rapid thermostat cycling, it hums and struggles to restart.
The technician should investigate:
The compressor may not be permanently locked. The failure occurs under a specific restart condition.
A compressor that pulls locked-rotor current is failing to rotate normally. Prove why before deciding that the compressor itself is mechanically locked.
1. Does near-LRA current automatically prove mechanical compressor failure?
2. Why should voltage be measured during the actual start attempt?
3. How can a weak capacitor imitate a locked compressor?
4. Why can rapid restarting make compressor startup harder?
5. What should be proven before condemning a compressor as mechanically locked?
1. No. It proves the compressor is not accelerating normally, but electrical and starting-circuit problems can cause the same current condition.
2. Voltage can collapse only when the compressor places a heavy load on the circuit.
3. A weak capacitor can reduce starting torque enough that the compressor remains stationary and draws very high current.
4. The compressor may be attempting to restart before pressures have sufficiently equalized, increasing starting difficulty.
5. Verify correct voltage, capacitor, contactor, wiring, start components, winding condition, compressor temperature, and relevant pressure conditions.
Return to the complete compressor training hub.
Capacitor vs Compressor →Separate a failed starting component from compressor failure.
Locked Rotor Guide →Review the existing technical locked-rotor resource.
Compressor startup diagnosis involves high voltage, very high current, stored capacitor energy, hot compressor surfaces, and pressurized refrigerant. Testing should be performed only by qualified personnel using properly rated instruments, approved start components, manufacturer procedures, and appropriate PPE.
A hard-start condition can come from voltage, capacitors, starting components, pressure conditions, wiring, or actual compressor failure. Prove the complete starting circuit before condemning the compressor.
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