For refrigeration units, the compressor is the "heart" of the system, and its fault diagnosis and prevention are of vital importance. Compressor failures are mainly divided into two categories: mechanical failures and motor failures.

Mechanical failure: It usually refers to problems with the components of the compressor (such as bearings, pistons, scroll discs, etc.), which cause it to lose its compression capacity and the system to be unable to establish a high and low pressure difference. The typical manifestation is that the compressor shaft gets stuck.

Motor failure: It mainly refers to the insulation failure of the motor winding, which causes short circuits or open circuits and eventually leads to motor burnout. Such faults are often concealed and hard to detect in the early stage. Once they show up, they usually cause irreversible damage.

This article mainly systematically analyzes the faults of compressor motors and explains several core reasons for the burnout of compressor motors.

I. Two main manifestations of Motor failure

1. Short circuit: The insulation layer of the stator winding is damaged, resulting in 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). The criterion for judgment: When measured with a megohmmeter, if the insulation resistance to ground is lower than 0.5M Ω, it is abnormal. A short circuit will cause the circuit breaker to trip.

2. Open circuit: One or several phases in the motor winding are disconnected.

Diagnostic method: Use a multimeter to measure the resistance between the three-phase windings. The resistance value should be infinite.

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

Ii. Seven core Causes of Motor Burnout

The root causes of motor winding burnout can be attributed to two major aspects: excessive current and abnormal voltage. The following is the specific analysis:

1. Long-term overload operation

Mechanism: Overload operation causes the current to remain persistently high, and the temperature rise of the winding intensifies. The heat generated by resistance is proportional to the square of the current (I²R). High temperatures will accelerate the aging of the insulation layer and eventually cause interphase short circuits.

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

2. Blocked rotation

Mechanism: When the rotor is locked, the current can reach 4 to 8 times the rated current. The huge current generates extremely high heat in a short time, which is very likely to burn out the winding.

Diagnostic tip: If the circuit breaker trips frequently and other external factors are ruled out, it is highly suspected that the compressor is experiencing a blockage, such as valve plate damage caused by liquid hammer or internal mechanical jamming.

3. Frequent starts

Mechanism: The peak current at the moment of startup is close to the locked-rotor current. Frequent start and stop will cause the windings to repeatedly be subjected to large current impacts, and the temperature rise will accumulate. However, the built-in thermal protector has a delayed response and cannot provide effective protection against frequent starts and stops.

4. Poor system cleanliness (metal shavings contamination)

Mechanism: This is a key hidden killer that causes short circuits. The compressor relies on return gas for cooling. Metal shavings in the refrigerant (from pipeline processing, component wear, etc.) will flow through and adhere to the windings. The vibration during the operation of the compressor and the displacement of the windings caused by the electromagnetic force when they start and stop can cause metal shavings to rub and scratch the insulation layer of the enameled wire, eventually leading to a short circuit.

5. Power phase loss

Mechanism: When a three-phase motor operates with a missing phase, the current of the remaining two phases will increase sharply, causing the windings to overheat rapidly. If the thermal protector operates and the motor cools down, resets and starts again, but due to phase loss, it will be locked upon startup, entering a vicious cycle of "locked rotor - thermal protection - locked rotor", until the motor burns out.

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

6. Abnormal voltage (unbalanced, overvoltage, undervoltage)

Voltage imbalance: Its hazards have been seriously underestimated. The current imbalance degree is approximately 4 to 10 times that of the voltage imbalance degree. The winding temperature rise caused by unbalanced voltage is approximately twice the square of the voltage unbalance degree.

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

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

Maximum deviation = 400-382 = 18V

The degree of imbalance = 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 drops by 10% below the rated value, the motor speed decreases. To maintain power, the current increases, and the temperature rise of the windings rises significantly.

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

Insufficient cooling: Insufficient return gas volume or excessively high return gas temperature (such as insufficient refrigerant or system matching issues) can both lead to poor motor cooling effect, causing high-temperature exhaust alarms and accelerating insulation aging.

Improper selection of contactors is a link that is very easy to be overlooked. The rated current of the contactor must not be lower than the rated current on the nameplate of the compressor. It is recommended to select the maximum continuous current value based on the rated load current × 1.4. In addition, the quality of the contactors is poor. Small-sized or low-quality contactors cannot withstand the large current impact during start-up and locked rotor, and are prone to contact jitter, welding or detachment, resulting in phase loss and directly threatening the safety of the motor.