The damage of the motor is mainly manifested in damage […]
The damage of the motor is mainly manifested in damage (short circuit) and open circuit of the stator winding insulation layer. After the stator winding is damaged, it is difficult to find it in time, which may eventually lead to winding burnout. After the winding is burned, some phenomena or direct causes that lead to burnout are covered up, making post-mortem analysis and cause investigation difficult.
However, the operation of the motor is inseparable from normal power input, reasonable motor load, good heat dissipation and protection of the winding enameled wire insulation layer.
Starting from these aspects, it is not difficult to find that the unit burnt out is caused by the following six reasons: (1) abnormal load and stall; (2) winding short circuit caused by metal chips; (3) contactor problems; (4) power supply Phase loss and abnormal voltage; (5) Insufficient cooling; (6) Use a compressor to evacuate. In fact, motor damage caused by multiple factors is more common.
1. Abnormal load and stall
The motor load includes the load required to compress the gas and the load required to overcome mechanical friction. If the pressure ratio is too large or the pressure difference is too large, the compression process will be more difficult; the increased frictional resistance caused by lubrication failure, and the motor stall in extreme cases will greatly increase the motor load.
Lubrication failure and increased frictional resistance are the primary causes of abnormal load. Diluted lubricating oil back to liquid, overheating of lubricating oil, coking and deterioration of lubricating oil, and lack of oil will all damage normal lubrication and cause lubrication failure. The return liquid dilutes the lubricating oil, affecting the formation of normal oil film on the friction surface, and even washing away the original oil film, increasing friction and wear. Overheating the compressor will cause the lubricating oil to become thinner or even scorched at high temperatures, affecting the formation of normal oil films. The oil return of the system is not good, and the compressor is short of oil, so it is impossible to maintain normal lubrication. The crankshaft rotates at a high speed and the connecting rod and piston move at high speed. The friction surface without oil film protection will heat up quickly. Local high temperature will cause the lubricating oil to evaporate or scorch quickly, making this part more difficult to lubricate, which can cause local severe wear within seconds.
Lubrication failure, local wear, and greater torque are required to rotate the crankshaft. Low-power compressors (such as refrigerators, household air-conditioning compressors) due to the small torque of the motor, the stalled (motor cannot rotate) phenomenon often occurs after lubrication failure, and enters the "locked-thermal protection-blocked" dead cycle, the motor burns only A matter of time. The high-power semi-hermetic compressor motor has a large torque, and local wear will not cause stalling. The motor power will increase with the load within a certain range, which will cause more serious wear and tear, and even cause the cylinder to bite (the piston is stuck in the cylinder Inside), severe damage such as broken rods.
The stalled current (stall current) is approximately 4-8 times the normal operating current. The moment the motor starts, the peak value of the current can approach or reach the stall current. Because the heat release from the resistor is proportional to the square of the current, the current during startup and stall will cause the winding to heat up quickly. Thermal protection can protect the electrode when the rotor is blocked, but generally does not have a fast response, and cannot prevent winding temperature changes caused by frequent starts. Frequent start-up and abnormal load will make the windings withstand the high temperature test, which will reduce the insulation performance of the enameled wire.
In addition, the load required to compress the gas will increase as the compression ratio increases and the pressure difference increases. Therefore, using a high-temperature compressor for low temperatures, or using a low-temperature compressor for high temperatures, will affect the load and heat dissipation of the motor, which is inappropriate and will shorten the electrode life. After the winding insulation performance is deteriorated, if there are other factors (such as metal chips forming a conductive circuit, acid lubricating oil, etc.), it is easy to cause a short circuit and damage.
2.Short circuit caused by metal shavings
Metal filings in the windings are the culprit for short circuits and low ground insulation. The normal vibration when the compressor is running, and the winding is twisted by the electromagnetic force every time it starts, it will promote the relative movement and friction between the metal scraps interposed between the windings and the winding enameled wire. Sharp metal shavings can scratch the enameled wire insulation and cause a short circuit.
The sources of metal shavings include copper pipe shavings left during construction, welding slag, metal shavings that are worn out within the compressor and damaged (such as broken valve discs). For hermetic compressors (including hermetic scroll compressors), these metal chips or debris can fall on the windings. For semi-hermetic compressors, some particles will flow in the system with the gas and lubricant, and eventually collect in the windings due to magnetism; while some metal debris (such as bearing wear and motor rotor and stator wear (sweep)) will be Fall directly on the winding. It's only a matter of time before shorts occur after metal debris has accumulated in the windings.
Of special note is the two-stage compressor. In a two-stage compressor, the return air and normal oil return directly to the first-stage (low-pressure stage) cylinder. After compression, it enters the motor cavity cooling winding through the medium-pressure pipe, and then enters the second stage like the ordinary single-stage compressor. (High-pressure cylinder). The return air contains lubricating oil, which has made the compression process like thin ice. If there is liquid return, the valve disc of the first stage cylinder is easily broken. The broken valve disc can enter the winding after passing through the medium pressure tube. Therefore, two-stage compressors are more susceptible to metal shorts caused by metal chips than single-stage compressors.
The unfortunate thing often comes together, when the compressor in question smells the burnt smell of lubricating oil during startup analysis. When the metal surface is severely worn, the temperature is very high, and the lubricating oil starts to coke when it is above 175oC. If there is more water in the system (the vacuum is not ideal, the water content of lubricating oil and refrigerant is large, the air enters after the negative pressure return pipe is broken, etc.), the lubricating oil may appear acidic. Acid lubricating oil will corrode the copper tube and the winding insulation layer. On the one hand, it will cause copper plating; on the other hand, the acidic lubricating oil containing copper atoms has poor insulation performance and provides conditions for winding short circuit.
3. Contactor problems
Contactor is one of the important parts in the motor control circuit. Improper selection can destroy the best compressor. It is extremely important to properly select the contactor according to the load.
The contactor must be able to meet demanding conditions such as fast cycling, continuous overload and low voltage. They must have a large enough area to dissipate the heat generated by the load current, and the choice of contact material must prevent welding under high current conditions such as startup or stall. For safety and reliability, the compressor contactor must disconnect the three-phase circuit at the same time. It is not recommended to disconnect the two-phase circuit.
The contactor must meet the following four items:
The contactor must meet the working and testing guidelines specified in the ARI standard 780-78 "Specialized Contactor Standard".
The manufacturer must ensure that the contactor closes at room temperature at 80% of the minimum nameplate voltage.
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 stall current. If there are other loads downstream of the contactor, such as motor fans, etc., they must also be considered.
When two contactors are used, the rating of the sub-winding stall of each contactor must be equal to or greater than the rating of the compressor half-winding stall.
The rated current of the contactor must not be lower than the rated current on the compressor nameplate. Contactors with small specifications or inferior quality cannot withstand the start of the compressor, high current impact at stalled and low voltage, and it is prone to single-phase or multi-phase contact vibration, welding and even falling off, which will cause motor damage.
Contactors with jittering contacts frequently start and stop the motor. The motor starts frequently, and the huge starting current and heat will aggravate the aging of the winding insulation. At each start, the magnetic torque causes slight movement and friction between the motor windings. If there are other factors (such as metal shavings, poor insulation oil, etc.), it is easy to cause a short circuit between the windings. Thermal protection systems are not designed to prevent such damage. In addition, jittering contactor coils are prone to failure. If the contact coil is damaged, it is easy to appear single-phase.
If the size of the contactor is too small, the contact cannot withstand the arc and the high temperature caused by frequent start-stop cycles or unstable control loop voltage, and may be welded or detached from the contact frame. The welded contacts will produce a permanent single-phase state, which allows the overload protector to be continuously cycled on and off.
It should be particularly emphasized that after the contactor contacts are 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 all fail, and the compressor is unprotected status.
4. Power supply phase loss and abnormal voltage
Abnormal voltage and phase loss can easily destroy any motor. The power supply voltage variation range cannot exceed ± 10% of the rated voltage. The voltage imbalance between the three phases cannot exceed 5%. High-power motors must be powered independently to prevent low voltages when other high-power equipment on the same line starts and runs. The motor power cord must be able to carry the rated current of the motor.
If the compressor is running when a phase loss occurs, it will continue to run but will have a large load current. The motor windings can quickly overheat and the compressor is normally thermally protected. When the motor winding cools down to the set temperature, the contactor will close, but the compressor will not start, a stall will occur, and it will enter the "stall-heat protection-stall" dead cycle.
The difference in the windings of modern motors is very small, and the difference in phase current when the three-phase balance of the power supply is negligible. In an ideal state, the phase voltage is always equal, as long as a protector is connected to any phase, it can prevent damage caused by overcurrent. It is actually difficult to guarantee the phase voltage balance.
The voltage imbalance percentage is calculated as the ratio of the maximum deviation of the phase voltage to the average of the three-phase voltage to the average of the three-phase voltage. For example, for a nominal 380V three-phase power source, the voltages measured at the compressor terminals are 380V and 366V , 400V, can calculate the average three-phase voltage of 382V, the maximum deviation is 20V, so the voltage imbalance percentage is 5.2%.
As a result of voltage imbalance, the load current imbalance during normal operation is 4-10 times the voltage imbalance percentage. In the previous example, a 5.2% imbalance voltage may cause a 50% current imbalance.
The phase winding temperature rise percentage caused by the unbalanced voltage is approximately twice the square of the voltage unbalanced percentage point. In the previous example, the number of voltage imbalance points was 5.2, and the percentage increase in winding temperature was 54%. As a result, the one-phase winding overheated and the other two windings had normal temperatures.
A completed survey showed that 43% of power companies allow 3% voltage imbalance, and another 30% of power companies allow 5% voltage imbalance.
Larger power compressors are generally return-air cooled. The lower the evaporation temperature, the smaller the system mass flow. When the evaporation temperature is very low (exceeding the manufacturer's specifications), the flow is insufficient to cool the motor and the motor will run at higher temperatures. Air-cooled compressors (generally no more than 10HP) have less dependence on return air, but have clear requirements for the compressor's ambient temperature and cooling air volume.
A large amount of refrigerant leakage will also reduce the system mass flow, and the cooling of the motor will be affected. Some unattended cold storages, etc., often wait until the cooling effect is poor to find a large amount of refrigerant leakage.
Frequent protection occurs when the motor is overheated. Some users do not check the cause in depth, or even short the thermal protector, which is a very bad thing. Before long, the motor will burn out.
The compressors have a range of safe operating conditions. The main consideration for safe working conditions is the load and cooling of the compressor and motor. Due to the different prices of compressors in different temperature zones, in the past the domestic refrigeration industry has used compressors out of range. The situation has improved markedly with the growth of expertise and economic conditions.
6. Use the compressor to evacuate
Open-type refrigeration compressors have been forgotten, but there are still some on-site construction workers in the refrigeration industry who have retained the habit of using the compressor to evacuate. This is very dangerous.
Air plays the role of an insulating medium. After the vacuum is evacuated in the sealed container, the discharge between the electrodes inside will easily occur. Therefore, with the deepening of the vacuum in the compressor casing, the insulation medium is lost between the exposed terminals in the casing or between the windings with slightly damaged insulation. Once the power is turned on, the motor may be short-circuited and burned in an instant. If the case leaks electricity, it may also cause electric shock.
Therefore, it is forbidden to use the compressor to evacuate, and it is strictly forbidden to energize the compressor when the system and the compressor are in a vacuum state (no refrigerant has been added after the vacuum is evacuated).