[Mechanical Equipment Technology Edition] Daily Question 201709018: Refrigerator Maintenance

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This post was last edited by YORK Industrial Refrigeration on 2017-9-20 at 11:20. Starting from now, in order to enhance communication among users, a 【Daily Question】 campaign has been launched on the Mechanical Equipment Technology forum; we hope everyone will participate actively to progress and improve together! ! ! Rewards: 3 wealth for active participation, 10-15 wealth for accurate answers. This question is valid for two days; no scoring will be given after that. Question: From which aspects should one start to check if there is a problem with the motor? Mechanical part: Check the vibration and noise of the motor as well as the bearing temperature to determine the condition of the bearings. Electrical part: Check the temperature of the three-phase windings of the coil and the three-phase voltages. The following are the common reasons for the burnout of the refrigerant motor. Damage to the motor is mainly manifested as damage to the insulation layer of the stator windings (short circuits) and open circuits. Damage to the stator windings is difficult to detect in a timely manner, which can ultimately lead to the windings burning out. After the winding burns out, it masks some of the phenomena or direct causes that led to the burnout, making post-incident analysis and root cause investigation more difficult. However, the operation of a motor relies on a proper power supply, a reasonable motor load, good heat dissipation, and protection for the insulation layer of the winding enamel wire. 　　By examining these aspects, it is not difficult to see that the reasons for the engine burning out are essentially one of the following six: (1) Abnormal load and stall ； (2) Winding short circuit caused by metal shavings ； (3) Contactor issues ； (4) Power supply phase loss and voltage abnormalities ； (5) Insufficient cooling ； (6) Evacuate using a compressor. In fact, motor damage caused by a combination of various factors is more common. 　1. Abnormal load and locked-rotor motor load include the load required to compress gas as well as the load needed to overcome mechanical friction. An excessively high pressure ratio, or an excessive pressure difference, will make the compression process more difficult ； The increased frictional resistance caused by lubrication failure, along with motor stalling under extreme conditions, will **increase the motor load**. Lubrication failure and increased frictional resistance are the primary causes of abnormal loads. Returning liquid to dilute the lubricating oil, overheating of the lubricating oil, coking and deterioration of the lubricating oil, as well as oil shortages, can all disrupt proper lubrication and lead to lubrication failure. The returning liquid dilutes the lubricating oil, interfering with the formation of a proper oil film on the friction surfaces; it may even wash away the existing oil film, thereby increasing friction and wear. Compressor overheating can cause the lubricating oil to become thin or even coker at high temperatures, affecting the formation of a proper oil film. The system’s oil return is poor, the compressor lacks oil, and as a result it cannot maintain proper lubrication. As the crankshaft rotates at high speeds and components such as connecting rods and pistons move rapidly, the friction surfaces that lack oil film protection heat up quickly. The high local temperatures cause the lubricating oil to evaporate or carbonize rapidly, making it more difficult to lubricate those areas; severe localized wear can occur within just a few seconds. Lubrication failure and localized wear require more torque to rotate the crankshaft. Low-power compressors (such as those in refrigerators and household air conditioners), due to their low motor torque, often experience stall conditions when lubrication fails; this leads to a vicious cycle of stall – thermal protection – stall, and engine burnout is only a matter of time. High-power semi-hermetic compressor motors have high torque, and local wear does not cause stalling. The motor power increases with the load within a certain range, which leads to more severe wear, and may even result in serious damage such as piston seizure (the piston getting stuck inside the cylinder) or broken connecting rods. The current during stall (stall current) is approximately 4–8 times the current under normal operation. At the moment the motor starts, the peak current can approach or reach the stall current. Since the heat generated by resistance is proportional to the square of the current, the currents during startup and stall cause the windings to heat up rapidly. Thermal protection can safeguard the electrodes during stall, but it generally does not respond quickly and cannot prevent temperature changes in the windings caused by frequent starting. Frequent starting and abnormal loads subject the windings to high temperatures, which reduces the insulation performance of the enameled wire. Furthermore, the load required to compress gas also increases as the compression ratio and pressure difference increase. Therefore, using a high-temperature compressor in a low-temperature application, or a low-temperature compressor in a high-temperature application, affects the motor load and heat dissipation; it is inappropriate and will shorten the service life of the electrodes. Once the insulation performance of the windings deteriorates, other factors (such as metal shavings forming conductive paths or acidic lubricants) can easily lead to short circuits and damage. 　2. Metal shavings causing short circuits: Metal shavings mixed within the windings are the main cause of short circuits and low insulation resistance. The normal vibration of the compressor during operation, as well as the twisting of the windings due to electromagnetic forces each time it starts up, cause relative movement and friction between the metal shavings trapped among the windings and the enameled wires of those windings. Sharp-edged metal can scratch the insulation layer of enameled wire, causing a short circuit. Sources of metal shavings include copper pipe shavings left over from construction, welding slag, and metal shavings that result from wear inside compressors and damage to components (such as broken valve plates). For hermetically sealed compressors (including hermetically sealed scroll compressors), these metal shavings or fragments will fall on the windings. In semi-hermetical compressors, some particles flow through the system along with the gas and lubricating oil, and eventually accumulate in the windings due to their magnetic properties ； Some metal shavings (such as those generated by bearing wear and wear of the motor rotor and stator, known as scoring) fall directly onto the windings. Once metal shavings accumulate in the windings, a short circuit is only a matter of time. Special attention should be paid to the two-stage compressor. In a two-stage compressor, the return gas and normal oil return flow directly into the cylinders of the first stage (low-pressure stage). After compression, they pass through medium-pressure pipes to reach the motor chamber where they cool the windings; subsequently, just like in a conventional single-stage compressor, they proceed to the second stage (high-pressure stage cylinders). The return gas contains lubricating oil, which already makes the compression process extremely risky; if liquid also returns, the valve plates in the first-stage cylinder can easily be damaged. The broken valve pieces can enter the winding through the medium-pressure pipe. Therefore, two-stage compressors are more prone to motor short circuits caused by metal shavings than single-stage compressors. Bad things tend to happen together; when analyzing a compressor that is having problems, one often detects a burnt smell of lubricant. The temperature is very high when the metal surface is severely worn, and lubricating oil begins to coking at temperatures above 175°C. If there is excessive moisture in the system (due to inadequate vacuuming, high water content in the lubricating oil and refrigerant, or air entering after a rupture in the negative pressure return pipe), the lubricating oil may become acidic. Acidic lubricants can corrode copper pipes and the insulation layers of windings; on one hand, it causes copper plating ； On the other hand, this acidic lubricant containing copper atoms has poor insulating properties, creating conditions for winding short circuits. 　3. Contactor issues: The contactor is one of the important components in a motor control circuit; an inappropriate selection can damage even the best quality compressors. It is extremely important to select the contactor appropriately based on the load. Contactors must be able to meet harsh conditions such as rapid cycling, continuous overload, and low voltage. They must have a large enough area to dissipate the heat generated by the load current, and the contact material chosen must prevent welding under conditions of high current such as startup or stall. For safety and reliability, the compressor contactor must disconnect all three phases of the circuit at the same time. Disconnecting the two-phase circuit is not recommended. The contactor must meet the following four requirements: It must comply with the operating and testing criteria specified in ARI Standard 780-78, \"Standard for Special Purpose Contactors\". The manufacturer must ensure that the contactor can close at room temperature at 80% of the minimum voltage specified on the nameplate. When using a single contactor, its rated current must be greater than the motor’s nameplate current rating (RLA). Additionally, the contactor must be able to handle the motor’s locked-rotor current. If there are other loads downstream of the contactor, such as motor fans, they must also be taken into consideration. When two contactors are used, the stall rating of the split windings of each contactor must be equal to or greater than the stall rating of the compressor’s half windings. The rated current of the contactor must not be lower than the rated current specified on the compressor’s nameplate. Contactors of small size or poor quality cannot withstand the high current surges that occur during compressor startup, stall conditions, and low voltage; as a result, they are prone to single-phase or multi-phase contact vibration, welding, or even detachment, which can lead to motor damage. Contactors with vibrating contacts frequently start and stop the motor. Frequent starting of the motor, along with the high starting current and resulting heat, accelerates the aging of the winding insulation layer. Each time it is started, the magnetic torque causes slight movement of the motor windings and friction between them. If other factors are present (such as metal shavings, lubricants with poor insulating properties, etc.), it is easy to cause short circuits between the windings. The thermal protection system is not designed to prevent such damage. Furthermore, the vibrating contactor coil is prone to failure. If the contact coil is damaged, a single-phase condition is likely to occur. If the contactor is selected to be too small, its contacts cannot withstand the arc as well as the high temperatures generated by frequent switching cycles or unstable control circuit voltages; this may result in the contacts welding together or falling off their holders. The welded contacts will create a permanent single-phase condition, causing the overload protector to continuously cycle on and off. It should be particularly emphasized that once the contacts of the contactor are welded together, all controls that rely on the contactor to disconnect the compressor’s power supply circuit – such as high/low pressure control, oil pressure control, defrost control, etc. – will cease to function, leaving the compressor without any protection. 　4. Power supply phase loss and abnormal voltage – Abnormal voltages and phase loss can easily destroy any motor. The range of variation in supply voltage must not exceed ±10% of the rated voltage. The voltage imbalance between the three phases must not exceed 5%. High-power motors must be powered independently to prevent low voltage caused by the startup and operation of other high-power devices on the same circuit. The motor power cable must be able to carry the motor’s rated current. If a phase loss occurs while the compressor is running, it will continue to operate but with a high load current. The motor windings will overheat quickly, and under normal circumstances the compressor will be protected by a thermal switch. When the motor windings cool down to the set temperature, the contactor closes, but the compressor fails to start and gets stuck in a stall, entering a vicious cycle of \"stall – thermal protection – stall\". The differences in modern motor windings are very small; when the power supply is three-phase balanced, the differences in phase currents can be ignored. Under ideal conditions, the phase voltages remain equal at all times; installing a protector on any one of the phases is sufficient to prevent damage caused by overcurrent. In fact, it is difficult to ensure the balance of phase voltages. The percentage of voltage imbalance is calculated as the ratio of the maximum deviation of the phase voltages from the average value of the three-phase voltages to that average value. For example, in a three-phase power supply with a nominal voltage of 380V, if the voltages measured at the compressor terminals are 380V, 366V, and 400V respectively, the average value of the three-phase voltages is 382V, and the maximum deviation is 20V; thus, the percentage of voltage imbalance is 5.2%. As a result of voltage imbalance, the imbalance in load current under normal operation is 4 to 10 times the percentage of voltage imbalance. In the previous example, a 5.2% unbalanced voltage can cause a 50% current imbalance. The percentage increase in the temperature of the phase windings caused by voltage imbalance is approximately twice the square of the percentage of voltage imbalance. In the previous case, the number of voltage imbalance points was 5.2, and the percentage increase in winding temperature was 54%. As a result, one winding overheated while the temperatures of the other two windings remained normal. A completed survey showed that 43% of power companies allow a voltage imbalance of 3%, while another 30% of power companies allow a voltage imbalance of 5%. 5. Insufficient cooling: Compressors with high power are generally of the return-air cooling type. The lower the evaporation temperature, the smaller the system mass flow tends to be. When the evaporation temperature is very low (above the manufacturer’s specifications), the flow rate is insufficient to cool the motor, causing it to operate at a higher temperature. Air-cooled compressors (generally up to 10HP) rely less on return air, but they have specific requirements regarding the compressor’s operating temperature and the volume of cooling air. A large leakage of refrigerant can also lead to a decrease in the system’s mass flow, and the cooling of the motor will be affected as well. In some unmonitored cold storage facilities, it is often only when the cooling performance becomes very poor that a large leakage of refrigerant is discovered. When the motor overheats, it will trigger frequent protection mechanisms. Some users do not investigate the reasons behind this in detail; they even short-circuit the thermal protector, which is a very bad thing to do. It won’t be long before the motor burns out. Compressors all have a safe operating range. The main considerations for safe operation are the load and cooling of the compressor and motor. Due to the different prices of compressors in various temperature ranges, it was common in China’s refrigeration industry in the past for compressors to be used beyond their designated scope. As professional knowledge has increased and economic conditions have improved, the situation has clearly improved. 　6. Using a compressor to create a vacuum – Open-type refrigeration compressors have been forgotten by most people, but some field technicians in the refrigeration industry still adhere to the old practice of using compressors to create a vacuum. This is very dangerous. Air acts as an insulating medium. After evacuating a sealed container, discharge phenomena between the electrodes inside it occur easily. Therefore, as the vacuum level inside the compressor housing increases, the insulating medium is lost between the exposed terminals inside the housing or between windings that have minor damage to their insulation layers; once electricity is applied, the motor may short-circuit and burn out in an instant. If the housing leaks electricity, it may also cause electric shock to people. Therefore, vacuuming using a compressor is prohibited, and it is strictly forbidden to power on the compressor when the system and the compressor are in a vacuum state (after vacuuming but before refrigerant has been added). Summary The burnout of the motor concealed the damage to the windings, posing certain difficulties for fault analysis. However, the root cause of damage to the compressor motor does not disappear. Abnormal loads or even stall caused by poor or failed lubrication, along with insufficient heat dissipation, can all shorten the lifespan of the windings ； The presence of metal shavings in the windings further facilitates short circuits ； Contactor welding will prevent the compressor from being protected ； An abnormality in the power supply on which the motor relies for operation will completely destroy any motor ； Creating a vacuum with a compressor may cause discharge at the internal terminals. Unfortunately, the aforementioned adverse factors can also trigger each other: the high currents during abnormal loads and stall conditions may cause the contacts to weld together ； Arcing or even welding at a single contact can cause phase imbalance or single-phase operation ； Phase imbalance can cause heat dissipation problems ； Insufficient heat dissipation can cause wear ； Wear and tear generates metal shavings… Therefore, proper installation and use of the compressor, along with regular maintenance, can prevent such adverse effects; this is the fundamental way to avoid damage to the compressor motor. ================================ Event Promotion: Petrochemical Zone – “Creative Ideas for Energy Saving” event (the second phase is in full swing) http://bbs.hcbbs.com/thread-1666758-1-1.html (Source: Haichuan Chemical Industry Forum) 2017 Coal Chemical Industry Event – “My Technical Upgrades” http://bbs.hcbbs.com/thread-1786290-1-1.html (Source: Haichuan Chemical Industry Forum)