Sources of metal shavings include copper pipe scraps left during construction, welding slag, internal wear of the compressor, and metal chips that fall off when parts are damaged (such as broken valves).
The core component of the refrigeration system is the compressor. The faults of the motor refrigeration compressor (hereinafter referred to as the compressor) can be divided into motor faults and mechanical faults (including crankshafts, connecting rods, pistons, valve plates, cylinder head gaskets, etc.). Mechanical failure often causes the motor to overload or even stall, which is one of the main causes of motor damage.
The damage of the motor is mainly caused by damage (short circuit) and open circuit of the stator winding insulation layer. When the stator winding is damaged, it is difficult to be discovered in time, and eventually the winding may be burnt. After the windings are burnt, some phenomena or direct causes of burning are masked, making post-mortem analysis and cause investigation difficult. However, the operation of the motor is inseparable from the normal power input, reasonable motor load, good heat dissipation and protection of the winding enameled wire insulation.
Starting from these aspects, it is not difficult to find that the reasons for the burning of the unit are as follows:
(1) Abnormal load and stall; (2) Winding short circuit caused by metal chips; (3) Contactor problem; (4) Power supply phase loss and voltage abnormality; (5) Insufficient cooling; (6) Vacuuming with compressor . In fact, motor damage caused by a combination of 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 the mechanical friction. If the pressure ratio is too large, or the pressure difference is too large, the compression process will be more difficult; the frictional resistance caused by the lubrication failure will increase, and the motor stall in extreme cases will greatly increase the motor load.
Lubrication failure and increased frictional resistance are the primary causes of load anomalies. Dilute the lubricating oil, the oil is overheated, the coking and deterioration of the lubricating oil, and the lack of oil will destroy the normal lubrication, resulting in failure of lubrication. The liquid is diluted to dilute the lubricating oil, which affects the formation of the normal oil film on the friction surface, and even washes away the original oil film, increasing friction and wear. Overheating of the compressor causes the lubricating oil to become thinner or even coked at a high temperature, which affects the formation of a normal oil film. The system does not return oil well, the compressor is short of oil, and naturally it is impossible to maintain normal lubrication. The high-speed rotation of the crankshaft and the high-speed movement of the connecting rod piston, the friction surface without the oil film protection will rapidly heat up, and the local high temperature will cause the lubricating oil to evaporate or coke rapidly, which makes the lubrication of the part more difficult and can cause local severe wear in a few seconds.
Lubrication failure, local wear, and crankshaft rotation require more torque. Small power compressors (such as refrigerators, household air conditioner compressors) due to the small torque of the motor, the lubrication is often broken after the failure (motor can not rotate) phenomenon, and enter the "blocking-heat protection - blocking" dead loop, the motor burned only Time problem. The high-power semi-hermetic compressor "target=_blank> machine motor torque is very large, local wear will not cause blocking, motor power will increase with load within a certain range, causing more serious wear and even causing biting The cylinder (the piston is stuck in the cylinder), the connecting rod is broken and the like is seriously damaged.
The current at the time of stalling (blocking current) is approximately 4-8 times the normal operating current. At the moment of motor start-up, the peak current can approach or reach the stall current. Since the heat release of the resistor is proportional to the square of the current, the current during startup and stalling causes the winding to heat up rapidly. Thermal protection protects the electrodes during stalling, but generally does not respond quickly, and does not prevent winding temperature changes caused by frequent starts. Frequent start-ups and abnormal loads cause the windings to withstand high temperatures and reduce the insulation properties of the enameled wire.
In addition, the load required to compress the gas also increases as the compression ratio increases and the pressure difference increases. Therefore, using a high temperature compressor for low temperature or using a low temperature compressor for high temperature will affect the motor load and heat dissipation, which is not suitable and will shorten the electrode life. After the insulation performance of the winding is deteriorated, if other factors (such as metal scraps constitute a conductive loop, acidic lubricating oil, etc.), it is easy to cause short circuit and damage.
2, short circuit caused by metal shavings
The metal chips trapped in the windings are the main culprit for the short circuit and low grounding insulation values. The normal vibration of the compressor during operation and the twisting of the winding by the electromagnetic force each time it is started will cause relative movement and friction between the metal chips and the winding enameled wire interposed between the windings. Sharp edges and swarf can scratch the enamelled wire insulation and cause a short circuit.
Sources of metal shavings include copper pipe scraps left during construction, welding slag, internal wear of the compressor, and metal chips that fall off when parts are damaged (such as broken valves). For fully enclosed compressors (including fully enclosed scroll compressors), these metal chips or granules can fall on the windings. For semi-hermetic compressors, some particles will flow with the gas and lubricating oil in the system, and finally the magnets will accumulate in the windings; some metal chips (such as bearing wear and motor rotor and stator wear (broom) will occur) Drop directly on the windings. Short circuiting is only a matter of time after metal chips are collected in the windings.
Special attention is required to the two-stage compressor. In the two-stage compressor, the return air and the normal return oil directly enter the first stage (low pressure stage) cylinder, after compression, enter the motor cavity cooling winding through the medium pressure tube, and then enter the second stage like the ordinary single stage compressor. (High pressure cylinder). The return air has lubricating oil, which has made the compression process like thin ice. If there is liquid back, the valve piece of the first stage cylinder is easily broken. The broken valve piece can enter the winding after passing through the intermediate pressure tube. Therefore, the two-stage compressor is more prone to motor short circuit caused by metal chips than the single-stage compressor.
Unfortunate things often come together, and the problematic compressors often smell the burnt smell of the lubricant during the startup analysis. When the metal surface is severely worn, the temperature is high, and the lubricating oil starts to coke at 175oC or more. If there is more water in the system (the vacuum is not ideal, the lubricating oil and the refrigerant have a large water content, and the air enters after the negative pressure return pipe is broken), the lubricating oil may be acidic. Acidic oils can corrode copper tubes and winding insulation. On the one hand, it causes copper plating. On the other hand, this kind of acidic lubricating oil containing copper atoms has poor insulation properties, which provides conditions for winding short circuits.
3, contactor problem
The contactor is one of the important components in the motor control circuit, and the selection of the irrational can destroy the best compressor. It is extremely important to select the contactor correctly according to the load.
Contactors must be able to withstand harsh 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 be prevented from soldering at high currents such as startup or stall. For safety and reliability, the compressor contactor must disconnect the three-phase circuit at the same time. The method of disconnecting the two-phase circuit is not recommended.
This article is organized by Shandong Dingsheng Refrigeration Equipment Co., Ltd.! If you have any enquiries, please pay attention to the WeChat public number: Dingsheng Refrigeration, or call 15725158383! ! !
Http://news.chinawj.com.cn Editor: (Hardware Business Network Information Center) http://news.chinawj.com.cn
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