For refrigeration units, the compressor is the "heart" of the system, and its fault diagnosis and prevention are of utmost importance.For refrigeration systems, the compressor is the core component of the entire system. Therefore, the diagnosis and prevention of its faults are of utmost importance.

Mechanical failure: This usually refers to problems with the compression components (such as bearings, pistons, turbine discs, etc.), which cause them to lose their compression capability, preventing the establishment of a high and low pressure difference in the system. A typical symptom is that the compressor shaft becomes stuck.

Motor failure: This mainly refers to the insulation failure of the motor winding, which causes short circuits or open circuits, ultimately leading to the motor's burnout. Such failures are often concealed and difficult to detect in the early stage. Once they manifest, they usually have already caused irreversible damage.

This article systematically analyzes the compressor motor failure and explains the main core reasons for the compressor motor burnout.

I. Two main manifestations of motor failure

1.  Short circuit: The insulation layer of the stator winding is damaged, causing abnormal current diversion.

    Interphase short circuit: A short circuit occurs between the three-phase windings.

    Ground short circuit: A short circuit occurs between the winding and the compressor housing (ground wire). Criteria for judgment: Measure with an ohmmeter. Insulation resistance to ground below 0.5 MΩ is abnormal. A short circuit will cause the circuit breaker to trip.

2.  Short circuit: One or several phases of the motor winding are disconnected.

    Diagnosis method: Use a multimeter to measure the resistance between the three-phase windings. The resistance value is infinity.

    Fault phenomenon: The compressor contactor engages, but the motor does not rotate, there is no working current, and no cooling effect.

II. Seven Key Reasons Leading to Motor Burnout

The root cause of motor winding burnout can be attributed to two major factors: high current and abnormal voltage. Here is a detailed analysis:

1. Long-term overload operation

Mechanism: Excessive operation leads to consistently high current, and the temperature of the windings increases. The heat generated by the resistance is proportional to the square of the current (I²R). High temperature accelerates the aging of the insulation layer, eventually causing inter-phase short circuits.

Root cause: Excessive system load (such as dirty or clogged condenser, excessive or insufficient refrigerant, malfunction of expansion valve, etc.).

2. Stalling

Mechanism: During locked-rotor operation, the current can reach 4 to 8 times the rated current. The extremely high current generates a large amount of heat in a short period of time, which is highly likely to damage the windings.

Diagnosis hint: If the circuit breaker frequently trips and other external factors have been ruled out, it should be highly suspected that the compressor may be experiencing a locked rotor condition, such as valve damage due to liquid impact or internal mechanical jamming.

3. Frequent startup

Mechanism: The peak current at the moment of startup is close to the locked-rotor current. Frequent starting and stopping cause the winding to repeatedly withstand large current impacts, resulting in cumulative temperature rise. Moreover, the built-in thermal protector has a delay in response and is unable to provide effective protection against frequent starting and stopping.

4. Poor system cleanliness (contamination by metal shavings)

Mechanism: This is a crucial hidden culprit that causes short circuits. The compressor relies on return gas for cooling. Metal particles (from pipeline processing, component wear, etc.) in the refrigerant flow through and adhere to the windings. The vibration during compressor operation and the displacement of the windings caused by electromagnetic forces during start-up and shutdown will cause the metal particles to rub and scratch the insulation layer of the enameled wire, ultimately leading to a short circuit.

5. Power phase missing

Mechanism: When a three-phase motor operates with one phase missing, the current of the remaining two phases will sharply increase, causing the winding to overheat rapidly. If the thermal protector activates and the motor cools down and resets before restarting, due to the phase absence, the motor will immediately stall and enter a vicious cycle of "stall - thermal protection - stall", until the motor is burned out.

Voltage requirements: The variation range of the power supply voltage should not exceed ±10% of the rated voltage, and the three-phase voltage imbalance should be less than 5%.

6. Voltage abnormalities (uneven, overvoltage, undervoltage)

Voltage imbalance: Its impact has been grossly underestimated. The degree of current imbalance is approximately 4 to 10 times that of voltage imbalance. The temperature rise of the windings caused by unbalanced voltage is approximately twice the square of the voltage imbalance.

    Calculation example: The measured three-phase voltages are 380V, 366V, and 400V.

        Average value = (380+366+400)/3 ≈ 382V

        Maximum deviation = |400 - 382| = 18V

        Imbalance degree = 18 / 382 ≈ 4.7%

        This may cause a current imbalance of up to approximately 40%, leading to overheating of a certain phase winding.

Under-voltage: When the voltage is 10% lower than the rated value, the motor speed decreases. To maintain power, the current increases and the temperature rise of the winding significantly rises.

7. Insufficient cooling and poor reliability of the power supply circuit

Insufficient cooling: Insufficient return gas volume and excessively high return gas temperature (such as insufficient refrigerant or system mismatch) can lead to poor cooling effect of the motor, causing high-temperature alarm during exhaust and accelerating insulation aging.

Incorrect selection of contactors is a very easily overlooked aspect. The rated current of the contactor must not be lower than the rated current marked on the compressor's nameplate. It is recommended to select the maximum continuous current value based on the rated load current multiplied by 1.4. Additionally, poor-quality contactors, small-sized or inferior ones, cannot withstand the large current shock during startup and stall, and are prone to contact jitter, welding or detachment, resulting in phase loss and directly threatening the safety of the motor.