Motor compressor (hereinafter referred to as compressor) failure can be divided into motor failure and mechanical failure (including the crankshaft, connecting rods, pistons, valves, cylinder head pads, etc.). Mechanical failure is often the motor overload or even stall, is one of the main motors of damage.
Damage mainly motor stator winding insulation damage (short circuit) and open circuit and so on. Stator windings to be found in a timely manner after the damage is difficult and may eventually lead to the winding burned. After winding burned, burned to cover up some of the leads or the direct cause of the phenomenon, the reasons for making the post-hoc analysis and investigation more difficult.
However, the normal operation of the motor is inseparable from the power input, the motor load is reasonable, good heat dissipation and protection of enameled wire winding insulation. From several aspects, not difficult to find the cause of the winding burn no more than six follows: (1) abnormal load and stall winding shorted (2) caused by metal shavings; (3) Contactor issue; (4) Power phase and voltage anomalies; less (5) cooling; (6) compressor vacuum. In fact, many factors contributed to the motor damage is more common.
1.An abnormal load and stall
Motor load including compressed gas needed to overcome mechanical friction loads and loads required. Pressure ratio is too large, or the pressure is too large, the compression process will be more difficult; but caused increased friction lubrication failure, as well as motor stall under extreme circumstances, would greatly increase the electrical load.
Lubrication failure, friction increases, is the leading cause of abnormal loads. Back diluted oil, lubricating oil overheating, lubricants coking deterioration, and starvation, and this will disrupt the normal lubrication, lubrication failure cause. Back diluted oil, affect the formation of normal friction surface film, and even washed away the original film, increasing friction and wear. Compressor overheating will cause the lubricant temperature coking thinning even affect the normal oil film formation. System back to the oil well, compressor oil, natural unable to maintain normal lubrication. High-speed rotation of the crankshaft, piston rod and other high-speed movement, no friction surface protection film will quickly heat up, the local high temperature lubricants rapidly evaporate or coking, making it more difficult to lubricate the parts, which can cause severe localized wear within seconds. Lubrication failure, partial wear, so that the crankshaft rotates the need for greater torque. Low-power compressors (such as refrigerators, home air conditioning compressor) due to the small motor torque, lubrication failure often occurs after stall (motor not rotate) phenomenon, and enter the "Stall - Thermal Protection - Stall" death cycle, motor burned only time. The high-power semi-hermetic compressors motor torque is large, does not cause local wear stall, motor power will be within a certain range with the load increases, causing more serious wear and tear, and even cause seizures (live Fonseca in the cylinder inside), rod fracture and other serious damage.
Current stall when the (locked rotor current) is about 4-8 times the normal operating current. Motor starts instantly, or reach the peak current of accessible stall current. Since the resistance of heat proportional to the square of the current release, start and stall when winding current will rapidly warming. Thermal protection can protect the electrodes during stall, but generally do not have fast response, can not prevent frequent start winding temperature changes caused. Frequent starts and abnormal loads, the winding withstand high temperature test, it will reduce the insulation performance of enameled wire.
In addition, the compressed gas with the desired load compression ratio increases and also the differential pressure increases. Thus the temperature of the compressor for low temperature, high temperature or low temperature compressors used will affect the electrical load and heat, is inappropriate, will shorten the life of the electrode.
After winding insulation deterioration, if there are other factors (such as metal shavings constitute conductive loops, acid lubricants, etc.) with easily cause a short circuit and damage.
2. Short circuit caused by metal shavings
Winding mixed metal scrap is shorted to ground insulation value and low culprit. During normal operation of the compressor vibration, and every time you start winding by electromagnetic force and twisting will promote inclusion in between winding and winding enameled metal shavings between the relative movement and friction. Sharp edges will scratch enameled metal shavings insulating layer 93, causing a short circuit recyclers network.
Sources include the construction of metal shavings left brass scrap, slag, compressor damage internal wear and spare parts (such as broken valve) when falling metal shavings, etc. For full-hermetic compressors (including hermetic scroll compressor), these metal shavings or crushed will fall on the winding. For semi-hermetic compressors, some particles flowing in the system with the gas and oil, and finally gathered since the magnetic windings; and (generated by such bearing wear and motor rotor and stator wear (sweep chamber) when) some metal shavings will directly on the winding. Winding metal shavings gathered after the short circuit occurred just a matter of time.
Need to draw particular attention to the two-stage compressor. In the two-stage compressor, return air and return to normal oil directly into the first level (low level) of the cylinder, compressed by the pressure tube into the motor windings cooling chamber, and then, like an ordinary single-stage compressor into the second stage (high pressure stage cylinder). Back to the gas with oil, so that the compression process is already in perpetuity, if we have back to the liquid, the first stage of the cylinder valve can easily be broken. Broken valve after the pressure tube can enter windings. Thus, two-stage compressor motor short circuit occurs due to swarf is easier than a single stage compressor.
Unfortunately, things are often conspire, compressor problem at boot time news channel analysis is often lubricants burning. Severe wear metal surface temperature is very high, and the lubricating oil begins at 175ºC or more coking. If there is more moisture in the system (vacuum pits not ideal, lubricants and refrigerants large water content, air enters the trachea after a negative return rupture), lubricants may occur acidity. Acidic oil will corrode copper pipes and winding insulation layer on the one hand, it can cause copper phenomenon; on the other hand, the poor performance insulation containing acidic copper atoms lubricants, provided the conditions for winding shorted.
3. Contactors problem
Contact is one of the important parts of the motor control circuit, can destroy the best selection unreasonable compressor. Press contact load the correct choice is extremely important.
Contact must be able to meet the demanding conditions, such as rapid cycling, sustained overload and low voltage. They must have heat, select contact material a sufficiently large area to dissipate the generated load current must stall at startup or the like can be prevented from a large current circumstances welding.
For safety and reliability of the compressor contactor to be disconnected while the three-phase circuits. Copeland is not recommended to disconnect the two-phase circuit approach.
In the United States, Copeland approved contacts must meet the following four:
• contact work and testing must meet the criteria ARI Standard 780-78 "private contacts standard" requirements.
• The manufacturer must ensure that the contactor at room temperature, while the lowest 80 percent of nameplate voltage can be closed.
• When using a single contact, contactors must be greater than the rated current of the motor nameplate current rating (RLA), at the same time, the contact must be able to withstand motor stall current. If there are other downstream contactor load, such as motor fans, etc., must also be considered.
When using two contactors, each sub-contactor coil stall rating must be equal to or greater than half winding compressor stall rating.
Contactor rated current can not be lower than the rated current of the compressor nameplate. Specifications small or poor quality of contact can not withstand the compressor starts, stall, and the impact of low voltage high current, and prone to single-phase or multi-phase contact bounce, welding and even shedding phenomenon, caused by damage to the motor.
Contact bounce contactors frequently start and stop the motor. Motor frequent start, a huge starting current and heat, aging will increase the winding insulation. Each time you start, the magnetic moment of the motor windings have tiny movement and friction. If there are other factors with (such as metal shavings, poor insulation oil, etc.), it is easy to cause a short circuit between the windings. No thermal protection systems designed to prevent such destruction. In addition, the jitter of the contactor coil is easy to fail. If there is contact with the coil is damaged, prone to single-phase state.
If the contactor selection is small, can not withstand arcing contacts and due to frequent open circulatory arrest or unstable temperature control circuit voltage generated by welding or may be detached from the contact holder. Welding of the contacts will produce a permanent single-phase state, so that overload protection continuously turned on and off cycles.
Needs to be emphasized is that the contactor contacts welded together, the dependence of the compressor contactor disconnect the power supply circuit of all control (such as high and low voltage control, hydraulic control, defrost control, etc.) will all fail, the compressor is unprotected state.
Therefore, when the motor burned, check the contacts are essential processes. Contact is an important cause motor damage of an often forgotten.
4. The power phase and voltage anomalies
Phase voltage is not normal and can easily destroy any motor. Supply voltage range must not exceed the rated voltage of ± 10%. Three phase voltage unbalance can not exceed 5%. Power supply must be independent of the motor, causing the line to prevent the same low voltage start-up and operation of other power equipment. Electrical power cord must be able to carry the rated motor current.
If the compressor is in operation phase, it will continue to run but will be a big load current. Will soon overheat the motor windings, the compressor under normal circumstances is thermal protection. When the motor windings cooled to the set temperature, the contactor will be closed, but the compressor does not start up, there stall 8151, recyclers network and enter the "Stall - Thermal Protection - Stall" death cycle.
Modern motor windings difference is very small, when the power difference between the phase current of three-phase equilibrium can be ignored. Ideally, the phase voltage will always be equal, as long as one phase connected to a protector can prevent any over-current damage caused. In fact it is difficult to ensure the equilibrium phase voltage.
Voltage unbalance is calculated as a percentage of the maximum deviation of the mean value of the three-phase voltage three-phase voltage and phase voltage ratio of the mean, for example, the nominal 380V three-phase power supply voltage measured at the compressor terminals are 380V, 366V , 400V, three-phase voltage can be calculated averages 382V, the maximum deviation of 20V, the voltage unbalance percentage of 5,2%.
As a result of voltage imbalance in the normal operation of the unbalance of the load current is 4-10 times the voltage unbalance percentage numbers. The previous example, 5,2% voltage unbalance can cause 50% of the current imbalance.
National Electrical Manufacturers Association (NEMA) Standards Publication motors and generators that the percentage of phase winding temperature rise caused by unbalanced voltage is about twice the square of the voltage unbalance percentage number. Voltage imbalance points to the previous example 5,2, winding temperature increases the percentage was 54%, the result is a phase winding and the other two windings overheating temperature is normal.
A by U, L, (Underwriters Laboratories, USA) completed the survey, 43% of the electricity company allows 3% voltage unbalance, and another 30% of the power company to allow 5% voltage unbalance.
5. Cooling inadequate
Power compressors are generally larger return air cooling type. The lower the evaporation temperature, mass flow systems are often smaller. When the evaporation temperature is low (more than the manufacturer's instructions), the flow is not sufficient to cool the motor, the motor will operate at higher temperatures. Air-cooled compressor (usually no more than 10HP) dependence on a small return air, but the compressor ambient temperature and cooling air flow has clear requirements.
Large leak refrigerant mass flow will cause the system to reduce the cooling of the motor will be affected. Some unattended cold storage, etc., often have to wait until the cooling effect of the poor will be found when a large number of refrigerant leak.
Will appear after frequent motor overheating protection, some users do not thoroughly check the reason, even the thermal protection short circuit, it is a very bad thing. Before long, the motor will burn.
Compressor has a safe operating range. Safety condition the main consideration is that the compressor and motor load and cooling. Because of different prices for different temperature zones of the compressor, the last domestic refrigeration compressor industry beyond the scope of use is more common. With the improvement in the growth of professional knowledge and economic conditions, the situation has improved significantly.
6.vacuum compressor
Open refrigeration compressors have been forgotten, but the refrigeration industry and some on-site construction personnel retained the habit of the past - with a compressor vacuum. This is very dangerous.
Plays the role of air insulated medium. After the vacuum sealed container, the discharge between the electrodes inside the phenomenon is very likely to occur. Thus, with the deepening of the degree of vacuum within the compressor housing, between the housing exposed studs insulating layer or lost a small dielectric breakage between windings, once energized, the motor may burn instantaneously short circuit. If the shell leakage, but also may result in electric shock.
Therefore, the ban on compressor vacuum and compressor systems and vacuum state (vacuum extraction was not yet add refrigerant), prohibited to the compressor power.
To sum up
After the motor burned to cover up the winding damage phenomenon to failure analysis caused some difficulties. However, the compressor motor damage caused by the root causes and not disappear. Abnormal load or poor lubrication failure caused even stall, less heat will shorten the life of the winding; winding interspersed with metal shavings is provided as a short variable interest; contactor welding allow to protect the compressor can not be executed; motor functioning power upon which abnormal, will fundamentally destroy any motor; compressor vacuum may cause discharge within the terminal.
Unfortunately, these negative factors will lead to mutual: abnormal load and high-current stall may result when the contactor welding; single contact arcing and even cause welding phase or single-phase unbalanced; phase imbalance can cause heat issues; inadequate heat dissipation will cause wear; wear will produce metal shavings.
Therefore, the correct installation of the compressor, and a reasonable routine maintenance, you can prevent the emergence of adverse factors, is the fundamental way to avoid damage to the compressor motor.
Pearl
Emily