Common Failure Analysis of the Scroll Compressor for the Air Conditioning of New Energy Electric Vehicles -- Motor Burnout

The faults of motor compressors (hereinafter referred to as compressors) can be divided into motor faults and mechanical faults (including crankshaft, connecting rod, piston, valve plate, cylinder head gasket, etc.). Mechanical failures often overload or even stall the motor, which is one of the main reasons for motor damage.
The main manifestations of motor damage are damage to the stator winding insulation layer (short circuit) and open circuit. Damaged stator windings are difficult to detect in a timely manner and may eventually lead to burnout of the windings. After the winding is burned out, some phenomena or direct causes leading to the burn out are covered up, making it difficult to analyze and investigate the causes afterwards.
However, the operation of a motor cannot be separated from normal power input, reasonable motor load, good heat dissipation, and protection of the insulation layer of the winding enameled wire. From these aspects, it is not difficult to find that the causes of winding burnout are as follows:
(1) Abnormal load and locked rotor;
(2) Short circuit of windings caused by metal chips;
(3) Contactor problem;
(4) Power supply phase loss and abnormal voltage;
(5) Insufficient cooling;
(6) Vacuum with a compressor.
In fact, motor damage caused by multiple factors is more common.
1. Abnormal load and locked rotor
The motor load includes the load required to compress the gas and the load required to overcome mechanical friction. Excessive pressure ratio or differential pressure can make the compression process more difficult; The increase in frictional resistance caused by lubrication failure and, in extreme cases, motor lockup will greatly increase motor load.
Lubrication failure and increased frictional resistance are the primary causes of abnormal load. Returning liquid to dilute the lubricating oil, overheating the lubricating oil, coking and deterioration of the lubricating oil, and lack of oil can disrupt normal lubrication, leading to lubrication failure. The return liquid dilutes the lubricating oil, affecting the formation of the normal oil film on the friction surface, and even flushing out the original oil film, increasing friction and wear. Overheating of the compressor can cause the lubricating oil to become thinner or even coking at high temperatures, affecting the formation of normal oil films. The system oil return is not good, and the compressor lacks oil, which naturally prevents normal lubrication from being maintained. When the crankshaft rotates at a high speed and the connecting rod piston moves at a high speed, the friction surface without oil film protection will rapidly heat up. Local high temperatures cause rapid evaporation or coking of the lubricating oil, making it more difficult to lubricate this part, and can cause severe local wear within a few seconds. Lubrication failure and local wear require greater torque for crankshaft rotation. Small power compressors (such as refrigerators and household air conditioning compressors) often experience a locked-rotor phenomenon after lubrication failure due to low motor torque, and enter a "locked-rotor thermal protection locked-rotor" dead cycle. It is only a matter of time before the motor burns down. However, the torque of the high-power semi enclosed compressor motor is large, and local wear will not cause rotor lockup. The motor power will increase with the load within a certain range, resulting in more severe wear, and even serious damage such as cylinder seizure (piston stuck in the cylinder), connecting rod fracture, etc.
The current at locked rotor (locked rotor current) is approximately 4-8 times the normal operating current. At the moment of motor startup, the peak value of current can approach or reach the locked rotor current. Due to the fact that the heat released by the resistor is proportional to the square of the current, the current during startup and lockup will rapidly heat up the winding. Thermal protection can protect the electrodes during rotor lockup, but generally does not have a quick response and cannot prevent winding temperature changes caused by frequent starts. Frequent starts and abnormal loads that expose the windings to high temperatures can reduce the insulation performance of enamelled wires. In addition, the load required for compressed gas will also increase as the compression ratio and pressure difference increase. Therefore, using a high-temperature compressor for low temperatures, or using a low-temperature compressor for high temperatures, can affect motor load and heat dissipation, which is inappropriate and can shorten the electrode service life. After the insulation performance of the winding deteriorates, if other factors (such as metal chips forming a conductive circuit, acidic lubricating oil, etc.) cooperate, it is easy to cause short circuit and damage.
2. Short circuit caused by metal chips
The metal scraps mixed in the windings are the main culprits for short circuits and low ground insulation values. The normal vibration of the compressor during operation, as well as the twisting of the winding due to the electromagnetic force during each startup, can promote the relative movement and friction between the metal chips mixed between the windings and the winding enameled wire. Metal chips with sharp edges and corners can scratch the insulating layer of the enameled wire, causing a short circuit.
The sources of metal scraps include copper pipe scraps left during construction, welding slag, metal scraps that fall from internal wear of the compressor and damage to components (such as broken valve blades). For fully enclosed compressors (including fully enclosed scroll compressors), these metal chips or debris can fall on the windings. For semi enclosed compressors, some particles will flow with gas and lubricating oil in the system, and eventually accumulate in the windings due to magnetism; However, some metal chips (such as those generated during bearing wear and motor rotor and stator wear (bore cleaning)) can directly fall on the windings. After metal chips have accumulated in the windings, it is only a matter of time before a short circuit occurs.
Special attention needs to be drawn to two-stage compressors. In a two-stage compressor, the return air and normal return oil directly enter the first stage (low pressure stage) cylinder, and after compression, they enter the motor cavity to cool the winding through the medium pressure pipe. Then, like ordinary single stage compressors, they enter the second stage (high pressure stage cylinder). The return gas contains lubricating oil, which has made the compression process like walking on thin ice. If there is any more return liquid, the valve plate of the first stage cylinder can easily be broken. The broken valve piece can enter the winding after passing through the medium voltage pipe. Therefore, two-stage compressors are more prone to motor short circuits caused by metal chips than single-stage compressors.
Unfortunately, things often come together, and when a faulty compressor starts up for analysis, it often smells like the burnt smell of lubricating oil. When the metal surface is severely worn, the temperature is very high, and when the lubricating oil is above 175 º C, it begins to coking. If there is too much water in the system (vacuum pumping is not ideal, the water content of lubricating oil and refrigerant is high, air enters after the negative pressure return pipe breaks, etc.), the lubricating oil may become acidic. Acid lubricating oil can corrode copper tubes and winding insulation. On the one hand, it can cause copper plating; On the other hand, this acidic lubricating oil containing copper atoms has poor insulation performance, providing conditions for winding short circuits.
3. Contactor problem
Contactor is one of the important components in the motor control circuit, and improper selection can damage the best compressor. It is extremely important to select the contactor correctly according to the load.
Contactors must be able to meet harsh conditions such as rapid cycling, sustained overloading, and low voltage. They must have a sufficient area to dissipate the heat generated by the load current, and the selection of contact materials must be able to prevent welding under high current conditions such as startup or rotor lockup.
For safety and reliability, the compressor contactor must simultaneously disconnect the three-phase circuit. The method of disconnecting the two-phase circuit is not recommended by Gulun Company. In the United States, contactors approved by Gulun Corporation must meet the following four requirements:
(1) Contactors must meet the operating and testing criteria specified in ARI Standard 780-78, "Standard for Special Purpose Contactors.".
(2) The manufacturer must ensure that the contactor can close at 80% of the minimum nameplate voltage at room temperature.
(3) When using a single contactor, the rated current of the contactor must be greater than the motor nameplate current rating (RLA). At the same time, the contactor must be able to withstand the motor locked-rotor current. If there are other loads downstream of the contactor, such as motor fans, they must also be considered.
(4) When using two contactors, each contactor must have a sub winding locked-rotor rating equal to or greater than the compressor half winding locked-rotor rating.
The rated current of the contactor cannot be lower than the rated current on the nameplate of the compressor. Contactors with small specifications or poor quality cannot withstand the high current impact caused by compressor startup, locked rotor, and low voltage, and are prone to single-phase or multiphase contact shaking, welding, or even falling off, causing motor damage.
Contactors with shaking contacts frequently start and stop the motor. Frequent starting of the motor, large starting current and heat generation will exacerbate the aging of the winding insulation layer. During each startup, the magnetic torque causes slight movement and mutual friction of the motor windings. If other factors cooperate (such as metal chips, lubricating oil with poor insulation, etc.), it is easy to cause short circuit between windings. The thermal protection system is not designed to prevent such damage. In addition, shaking contactor coils are prone to failure. If the contact coil is damaged, it is easy to have a single-phase state.
If the contactor type selection is too small, the contact cannot withstand electric arcs and high temperatures caused by frequent on-off cycles or unstable control circuit voltage, which may cause welding or detachment from the contact frame. The welded contact will generate a permanent single-phase state, allowing the overload protector to continuously cycle on and off.
It should be particularly emphasized that after the contactor contact is welded, all controls that rely on the contactor to disconnect the compressor power circuit (such as high and low pressure control, oil pressure control, defrost control, etc.) will fail, and the compressor will be in an unprotected state. Therefore, when the motor is burned out, checking the contactor is an essential process. Contactors are an important and often forgotten cause of motor damage.
4. Power supply phase loss and voltage abnormality
Abnormal voltage and phase loss can easily destroy any motor. The variation range of the power supply voltage cannot exceed ± 10% of the rated voltage. The voltage imbalance between three phases cannot exceed 5%. The high-power motor must be powered independently to prevent low voltage caused by the startup and operation of other high-power equipment on the same line. The motor power cord must be capable of carrying the rated current of the motor.
If the compressor is running when a phase loss occurs, it will continue to operate but will have a large load current. The motor windings can quickly overheat, and normally the compressor is thermally protected. When the motor winding cools to the set temperature, the contactor will close, but the compressor cannot start, resulting in a locked rotor and entering a "locked rotor thermal protection locked rotor" dead cycle.
The differences in the windings of modern electric machines are very small, and the differences in phase currents when the power supply is three-phase balanced can be ignored. Ideally, the phase voltages are always equal, as long as a protector is connected to either phase to prevent damage caused by overcurrent. In fact, it is difficult to ensure the balance of phase voltages.