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1, insufficient cooling
Large power compressors are generally return air cooling type. The lower the evaporation temperature is, the smaller the mass flow is. When the evaporation temperature is very low (exceeding the manufacturer's regulations), the flow rate is not enough to cool the motor, and the motor will operate at a higher temperature. Air-cooled compressors (generally no more than 10HP) have little dependence on the return air, but have clear requirements for the ambient temperature and cooling air volume of the compressor.
A large amount of refrigerant leakage will also cause the mass flow of the system to decrease, and the cooling of the motor will also be affected. For example, the compressor exhaust temperature is too high protection.
Compressors have a safe operating range. The main consideration for safe operating conditions is the load and cooling of the compressor and motor. Due to the different prices of compressors in different temperature zones, it was common to use compressors beyond the range of the domestic refrigeration industry in the past. As expertise has grown and economic conditions have improved, the situation has improved markedly.
2, power supply phase and abnormal voltage
Abnormal voltage and out of phase can easily destroy any motor. The variation range of the power supply voltage cannot exceed ±10% of the rated voltage. The voltage imbalance between the three phases should not exceed 5%. The high-power motor must be independently powered to prevent low voltage when other high-power equipment starts and operates on the same line. The power cord of the motor must be able to carry the rated current of the motor.
If the compressor is in operation at the time of phase loss, it will continue to operate but there will be a large load current. The motor windings can overheat very quickly, and the compressor is normally protected by heat. When the motor winding is cooled to the set temperature, the contactor will close, but the compressor can not start, there is a blocked rotation, and enter the "blocked rotation - thermal protection - blocked rotation" dead cycle.
In order to prevent the compressor from running without phase, the three-phase power supply compressor is equipped with a phase sequence protector, the phase sequence protector has two functions, one is to prevent the reverse operation of the compressor with phase sequence error; The second is to prevent the compressor from running without phase.
The difference in modern motor windings is very small, and the difference in phase current when the power supply is three-phase balance can be ignored. Ideally, phase voltages are always equal, and a protector connected to either phase can prevent damage caused by overcurrent. In fact, it is difficult to ensure phase voltage balance. The percentage of voltage unbalance is calculated by the ratio of the maximum deviation between the average value of the phase voltage and the average value of the three-phase voltage. For example, the nominal 380V three-phase power supply, the voltage measured at the compressor terminal is 380V, 366V, 400V. The average three-phase voltage can be calculated to be 382V, the maximum deviation is 20V, so the percentage of voltage unbalance is 5.2%.
As a result of the voltage imbalance, the load current imbalance during normal operation is 4-10 times the percentage point of the voltage imbalance. In the previous example, 5.2% of the unbalance voltage may cause 50% of the current unbalance.
3. Ac contactor problem
Ac contactor is one of the important parts in the motor control loop, and improper selection can destroy the best compressor. It is extremely important to select the contactor correctly according to the load.
Ac contactors must be able to meet demanding conditions such as fast cycling, sustained overload and low voltage. They must have a large enough area to dissipate the heat generated by the load current, and the contact material must be selected to prevent welding under high currents such as starting or blocking.
For safety and reliability, the compressor contactor should be disconnected from the three-phase circuit at the same time. The Valley Wheel Company does not recommend the method of disconnecting the two-phase circuit.
The rated current of the contactor cannot be lower than the rated current on the compressor nameplate. Contactors with small specifications or poor quality cannot withstand the large current impact when the compressor starts, stops and low voltage, and are prone to single-phase or multi-phase contact jitter, welding or even falling off, causing motor damage.
Contactors with wobbly contacts start and stop the motor frequently. The motor starts frequently, and the huge starting current and heat will aggravate the aging of the winding insulation layer. Each time the motor is started, the magnetic torque causes the motor windings to move slightly and rub against each other. If there are other factors (such as metal chips, poor insulation lubricating oil, etc.), it is easy to cause short circuit between windings. Thermal protection systems are not designed to prevent such damage. In addition, the jitter contactor coil is prone to failure. If the contact coil is damaged, it is easy to appear single-phase state.
If the contactor selection is too small, the contacts cannot withstand the arc and high temperature due to frequent start-stop cycles or unstable control loop voltages, and may weld or fall off the contact frame. The welded contacts will create a permanent single-phase state, allowing the overload protector to be continuously cycled on and off.
It should be emphasized that after the contactor contact is welded, all the 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 will be in a unprotected state. Therefore, when the motor burns out, the contactor must be checked. Contactor is an important cause of motor damage that is often forgotten.
After the AC contactor contact adhesion, regardless of whether the motherboard has output, the compressor will work, and the "streaking" without protection for a long time, the compressor damage is an inevitable. When replacing the compressor, be sure to find the cause of compressor damage, be sure to check whether the AC contactor is normal, there is no lack of phase and adhesion? Can not only replace the compressor, without investigating the reasons, is bound to lead to the new replacement of the compressor secondary damage.
4, compressor lack of oil
Lack of oil is one of the compressor faults that is easy to identify. When the compressor is short of oil, there is little oil or even no lubricating oil in the crankcase, which is easy to burn the compressor.
Lack of oil will cause a serious lack of lubrication, the root cause of lack of oil is not the compressor running oil and speed, but the system oil return is not good. Installing an oil separator can quickly return oil and extend the operation time of the compressor without oil return. The evaporator and return pipe must be designed with oil return in mind. Maintenance measures such as avoiding frequent start-up, timed defrosting, timely replenishment of refrigerant, and timely replacement of worn piston components also help to return oil.
Liquid return and refrigerant migration will dilute lubricating oil, which is not conducive to the formation of oil film; The failure of the oil pump and the blockage of the oil circuit will affect the oil supply and oil pressure, resulting in the lack of oil on the friction surface. The high temperature of the friction surface will promote the decomposition of the lubricating oil and make the lubricating oil lose its lubricating ability. Insufficient lubrication caused by these three problems often causes compressor damage.
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