Root Causes of Compressor Coil Burnout

The motor (fluorine-resistant motor) of a semi-hermetic refrigeration compressor is in direct contact with refrigerant and refrigeration oil, and operates continuously under certain pressure and temperature conditions, 

resulting in harsh operating environments for the fluorine-resistant motor (hereinafter referred to as the motor). Improper operation and maintenance during unit operation often lead to motor burnout and system shutdown. 

Even repaired units are prone to repeated motor burnout. The primary causes of motor burnout in such refrigeration compressors are analyzed below.

Fault Analysis of Motors in Semi-Hermetic Compressor Units
1. Excessively Low Power Supply Voltage

Starting and running under low supply voltage generates high current flowing through the motor. This triggers frequent opening and closing of the thermal relay, 

eventually leading to melting and adhesion of contacts and thermal relay failure. Under high current, binding wires of windings fuse, and insulation of magnet wires burns out, 

resulting in partial winding short circuits and abnormal winding resistance.

Refrigeration compressor faults may arise from low supply voltage, insufficient power supply capacity, other high-power loads connected to the same circuit, undersized power cables, or circuit faults.
2. Faults in the Motor Power Control Circuit

Power switches and contactors are critical components of the compressor motor control circuit. Improper selection and lack of maintenance can damage even high-quality refrigeration compressors. 

Correct sizing of switches and contactors according to load is extremely important.

It must be emphasized that if contactor contacts become welded shut, all protective controls relying on the contactor to cut off compressor power (high/low pressure protection, oil pressure protection,

defrost control, etc.) will fail, leaving the refrigeration compressor without protection.

Therefore, inspecting the contactor is a mandatory procedure after motor burnout, as contactor failure is a frequently overlooked major cause of motor damage.
3. Motor Failures Caused by Three-Phase Current Imbalance

Abnormal voltage and phase loss can easily destroy motors. The unbalance of three-phase voltage shall not exceed 5%. Three-phase voltage unbalance leads to three-phase current unbalance, 

where excessive current in one or two phases heats the corresponding winding, causing insulation breakdown or burnout. Subsequently, current surges in other windings and burns them out.

During motor operation, the current of any single phase shall not deviate more than 10% from the average value of the three-phase currents. Under normal operating conditions, 

a percentage of voltage unbalance may cause 4–10 times that percentage of current unbalance.

4. Abnormal Load and Locked Rotor

The load of the compressor motor includes the load required for gas compression and the load to overcome mechanical friction. Excessive compression ratio and pressure difference increase compression difficulty. 

Increased friction caused by lubrication failure, and motor locked rotor under extreme conditions, drastically raise motor load.

High-power semi-hermetic compressor motors deliver large torque. Minor local wear generally does not cause locked rotor, yet motor output power rises with load within a certain range, triggering severe wear, 

and even catastrophic damage such as cylinder seizure and connecting rod fracture.

Locked rotor current is approximately 4–7 times the normal operating current. Thermal protection can protect the motor during locked rotor conditions, yet it generally cannot respond rapidly 

(PTC sensors respond slightly faster) to restrain winding temperature rise, reducing insulation performance of enameled wires.

5. Inherent Motor Defects

National standards specify requirements for manufacturing fluorine-resistant motors. However, manufacturers vary widely in technical capability, testing instruments, production equipment and quality awareness. 

Products manufactured by unprofessional small workshops inevitably carry inherent hidden dangers.

Such defects can often be identified visually or via basic testing (megohmmeter measurement):
Loose and uneven binding of winding end turns;
Uneven coating of insulating varnish or sticky surface;
Non-standard lead wire selection;
Missing certified marks (e.g. AMP) on terminal lugs;
Disordered arrangement of enameled wires;
Improper selection of insulating materials;
Insulation resistance to ground and between phases below 20 MΩ;
Unbalanced three-phase winding resistance.
6. Poor Oil Return of the Refrigeration System

Poor oil return leads to oil starvation in the compressor, failing to deliver sufficient refrigeration oil to lubricating points. Friction intensifies at lubrication positions (bushings, pistons), 

resulting in piston and crankshaft seizure within a short period, sharp rise of motor current and subsequent coil burnout.

Multiple factors cause poor oil return: liquid floodback, throttling valve blockage, blocked oil return holes, improperly sized oil return holes, system overload operation, or oil loss caused by refrigerant leakage.
7. High Moisture Content in the Refrigeration System

The compressor motor windings are continuously immersed in refrigerant and refrigeration oil. Excessive moisture in the system reacts with refrigeration oil and refrigerant to generate acidic substances, 

which corrode winding insulation under specific temperature and pressure.

In addition, high moisture reduces winding insulation resistance. Subjected to electromagnetic force, mechanical stress and temperature variation, chemical reactions trigger aging such as cracking and embrittlement of insulation, 

resulting in inter-phase or inter-turn breakdown short circuits and eventual motor burnout.

Sources of moisture in refrigeration systems:
Excess water contained in refrigerant and refrigeration oil introduced during charging;
Moisture absorbed from ambient air when the refrigeration system is opened for maintenance and exposed to air for prolonged periods;
Refrigerant and oil charging without adequate drying and timely replacement of filter desiccants after maintenance.
8. Operation with Insufficient Refrigerant

Prolonged operation with insufficient refrigerant continuously raises cylinder temperature and thermal expansion, creating hard friction between cylinders and pistons. Rising temperature carbonizes refrigeration oil, blackening internal chambers and pistons. 

In severe cases, pistons are scratched, knocking or seizure occurs, generating high motor current. Sustained high current burns motor windings.

9. Vacuum Operation

During vacuum operation, cylinder temperature rises rapidly, heating compressor motor windings and generating tiny bubbles inside insulation. The high-vacuum environment outside windings expands these bubbles, 

peeling insulation and triggering motor faults. Motor winding damage can occur after roughly two minutes of vacuum operation and render the compressor inoperable.

Common causes of compressor vacuum operation:
Power-on operation without opening valves; direct vacuum pumping of pipeline systems by the compressor; operation with low-pressure valves closed.
10. Foreign Matter Entering the Compressor

Foreign matter entering the compression chamber creates cracks in pistons and cylinders during reciprocating movement and intensifies friction. High-speed foreign particles inside the cylinder damage suction and discharge valve plates. 

Rising cylinder temperature leads to piston scoring, knocking or seizure and motor faults.

Metal debris trapped in windings is the primary cause of short circuits and low insulation resistance to ground. Short-circuit failure is only a matter of time once metal particles accumulate on motor windings.
Sources of foreign contaminants:
Molding sand and metal shavings generated during compressor manufacturing, including copper tube debris and welding slag left during installation;
Accidental ingress during refrigerant and refrigeration oil filling in maintenance work;
Fragmented valve plates, copper residues from burnt windings, worn bushings, rotor-stator rubbing debris, etc.
Conclusion

Motor faults of semi-hermetic refrigeration compressors relate to manufacturing, installation, operation and maintenance. Motor burnout is generally caused by multiple combined factors. Standardized installation, 

repair and maintenance practices can drastically reduce burnout risks.

To avoid rapid re-burnout of repaired compressor motors, root causes of faults must be eliminated, and no new fault-inducing factors shall be introduced during maintenance.
Correct installation and operation of refrigeration compressors together with proper routine maintenance prevent adverse conditions and constitute the fundamental approach to avoid motor damage.