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Compressor Common Fault Analysis - motor Burned

Compressor motor (hereinafter referred to as the compressor)malfunction can be divided into motor malfunction and mechanical malfunction (including the crankshaft, connecting rod, piston, valves, cylinder head pad, etc.). Mechanical malfunction is always comes to motor overload or locked rotor, which is one of the main reason of damage for motor.

Mainly conduct of damaged motor stator are winding insulation damage (short circuit) and break circuit and so on. Damaged stator winding is difficult to be found in time, could eventually lead to winding burn. Winding burned, cover up some phenomenon of burning or to make subsequent analysis more difficult and cause investigation as well. However, without the normal operation of the motor power input, the motor load reasonable, good heat dissipation and the winding enameled wire insulation protection, motor can not operate well. From several aspects, not difficult to find winding burned for no reason except the following six categories: (1) abnormal load and stall; (2) winding short circuit caused by metal debris; (3) contactor problems; (4) Power phase and voltage abnormalities; (5) inadequate cooling; (6) compressor vacuum. In fact, a variety of factors contributed to the more common motor damage.

1 abnormal load and stall

Motor load including the load required to compress the gas and the load required to overcome mechanical friction. When Pressure ratio is too large, or the pressure difference is too large, the compress procession will be more difficult; and increased friction caused by lubrication failure, and extreme cases, motor stall, will greatly increase the electrical load. Lubrication failure, friction increases, is the leading cause of abnormal load. Back to dilute lubricating oil, lubricating oil heat, oil coking deterioration and lack of oil, and this will disrupt the normal lubrication, causing lubrication failure. Back to the diluted oil, affecting the normal friction surface film formation, or even washed away the original film, increasing friction and wear. Compressor overheating would cause lubricant thinning or coking temperature, affecting the normal film formation. If system back oil not well, compressor lack oil, they can not maintain normal lubrication. High-speed rotating crankshaft, connecting rod piston high-speed movement, there is no friction surface protection film will quickly heat up, heat the oil quickly evaporates partially or coking, more difficult to lubricate the parts, within a few seconds can cause severe localized wear. Lubrication failure, local wear, demand greater torque to the crankshaft rotation. Low-power compressor (such as refrigerators, home air conditioning compressor) due to small motor torque, lubrication failure often occurs stall (motor not rotating) phenomenon, and enter the "stall - Thermal Protection - Stall" infinite loop, the motor burned only a matter of time. The high-power semi-hermetic compressor motor torque strong enough to avoid stall cause by partial wear, the motor power will be within a certain range with the load increases, causing more serious wear, and even lead to bite cylinder (piston in cylinder), connecting rod fracture and other serious damage.

Stall current (stall current) about more 4-8 times than the normal running current. The moment of starting motor, the current peak close to or reach the stall current. As the resistance of heat release and the current proportional to the square, at the stage of start and stall , the winding current will heat up quickly. Thermal protection can protect the electrode in the stall, but in general there will be no quick response, can not prevent frequent start winding caused by temperature changes. Frequent starts and abnormal loads, in this case, the test winding to withstand high temperatures, will reduce the magnet wire insulation. In addition, the compressed gas will be required more to load and pressure increase with the compression ratio increases. Therefore, the high temperature compressor for low temperature, or low temperature compressor used high temperature will affect the electrical load and heat dissipation, which is inappropriate and will shorten the electrode life. Winding insulation deterioration, if there are occure other factors (such as metal shavings form conductive loops, acid oil, etc.), very easy to cause a short circuit and damage.


2.short-circuit caused by metal shavings

A mixture of metal shavings in the winding responsible for short-circuit and lower ground insulation value. The normal vibration when compressor running, and each time it starts winding and twisting by the electromagnetic force will promote inter-mixed with metal shavings in the winding and winding relative motion between the magnet wire and friction. Sharp edges will scratch the enameled metal scrap insulation, causing short circuits.

Metal scraps comes from metal debris left behind by copper scrap, slag, compressor internal parts wear and damage (such as broken valves) and other debris falling from the metal after construction. For the hermetic compressor (including full-hermetic scroll compressors), these metal shavings or crushed would fall on the windings. For the semi-hermetic compressors, some particles with the gas and oil flow in the system, and finally gathered in the windings due to magnetic; and some metal shavings (such as bearing wear and motor rotor and stator wear (sweep chamber) is generated) will be directly on the winding. Winding metal shavings gathered after the short circuit is just a matter of time.

Need to draw particular attention to the two-stage compressor. In the two-stage compressor, return air and the normal return oil directly into the first level (low level pressure) cylinder, compressed by the pressure tube into the motor winding cooling chamber, and then, like ordinary single-stage compressor, into the second level (high grade cylinder). Back to the gas with oil, so that the compression process has been walking on thin ice, if still remain back liquid, the first stage cylinder valves can easily be broken. Broken by the pressure control valve can be entered the winding through pressure pipe. Therefore, the two-stage compressor compare the single-stage compressors are more prone to metal shavings caused by electrical short circuit.

Unfortunately, bad things tend to crowd one, problem compressor often smell burning of oil when open it to analysis. Severe wear of metal surface temperature is very high, while oil pyrogenation above 175 º C. If there is more moisture in the system (vacuum pits not good, oil and refrigerant water content of large, negative pressure air into the return pipe rupture, etc.), acid oil may occur. Acid corrosion of copper and oil will be winding insulation layer, on the one hand, it will cause the phenomenon of copper; on the other hand, the acid oil contains copper atoms is poor insulation , provided the conditions for the winding short-circuit.


3 contactor problems

Contactor is an important component in the motor control circuit, unreasonable one can destroy the best compressor. The right choice according to the load contactor is extremely important.

Contactor must be able to meet the harsh conditions, such as rapid cycling, continuous overload and low voltage. They must have enough area to distribute the heat generated by the load current, the choice of contact materials must under condition of start or stall, to prevent huge currents welding. For safe and reliable, compressor contactor be able to disconnect the three-phase circuit. Copeland does not recommend disconnecting the method of two-phase circuit.

In the U.S., Copeland approved contactors must meet the following:

· Contactor must meet ARI Standard 780-78, "specialized contactor standard" required work and testing guidelines.

· The manufacturer must ensure that the contactor can closed at the lowest 80% of nameplate voltage at room temperature.

· When using a single contactor, the contactor must be greater than the rated current of the motor nameplate current rating (RLA). Meanwhile, the contactor must be able to withstand the motor stall current.

• If there are other downstream contactor load, such as fan motors, etc., must also be considered.

• When using two contactors, each contactor winding stall sub-rating must be equal to or greater than the half-winding compressor stall rating.

Contacts are rated not less than the compressor nameplate rated current. Size small or poor quality of contactor can not withstand the compressor start, stall, and low voltage high current shock, prone to single-phase or multiphase contact bounce, welding and even the phenomenon of falling, causing motor damage.

Contact jitter frequently start and stop the motor contactor. Frequent motor starts, huge starting current and fever, exacerbate the aging of winding insulation. Each time you start, the magnetic torque to the motor windings with small movements and mutual friction. If there are other factors with (such as metal shavings, poor insulation oil, etc.), it is likely to cause short circuit between windings. Thermal protection system is not designed to prevent such destruction. In addition, the jitter of the contactor coil is easy to fail. If you have contactor with the coil is damaged, prone to single-phase state. If the contactor size is too small, the contactor head can not afford to withstand  because of the frequent open loop voltage control loop instability or heat generated from the contact may be soldered or rack off. Welding of the contactor will have a permanent single-phase state, the overload protection device continued to cycle on and off. Needs to be stressed is that the contactor contacts welded together, the dependence compressor contactor disconnect all power control loop (such as high and low pressure control, hydraulic control, defrost control, etc.) will all fail, the compressor is no protection the state. Therefore, when the motor burned, check the contacts are essential processes. Damage to the motor contactor is an often forgotten reason.


4 power lack phase and voltage abnormalities

Phase lack and abnormal voltage can easily destroy any motor. Supply voltage range can not exceed the rated voltage of ± 10%. Three phase voltage imbalance must not exceed 5%. High-power electrical supply must be independent, to prevent the same line up and operation of other power equipment caused by low voltage. Electrical power cord must be able to carry the motor rated current.

 If compressor operation on lack phase, it will continue to run but there will be result in  large load current. Will quickly overheat the motor windings, normally the compressor is thermal protection. When cooled to set the motor winding temperature, the contactor will close, but the compressor does not start up, there stall and into the "stall - Thermal Protection - Stall" dead loop.

The difference between the modern motor windings is very small, three-phase equilibrium phase current power difference can be ignored. Ideally, the phase voltage is always equal, as long as any one phase after another on the protection can prevent over-current damage. In fact it is difficult to ensure phase voltage balance.

Method of calculating the percentage of voltage unbalance, phase voltage and the maximum deviation of the average three-phase voltage and phase voltage average value of the ratio, for example, the nominal 380V three-phase power, measured in the compressor terminal voltage of 380V, 366V , 400V. can calculate the average of the three-phase voltage 382V, the maximum deviation of 20V, so the percentage of voltage imbalance of 5.2%. As a result of unbalanced voltage, load current during normal operation so that the voltage unbalance percentage of imbalance is the number of 4-10 times. Previous example, the 5.2% voltage unbalance may cause 50% of the current imbalance. U.S. National Electrical Manufacturers Association (NEMA) motors and generators standard publications that result from the unbalanced voltage winding temperature rise relative to the percentage of voltage unbalance percentage is about twice the number of square. The preceding example, the voltage unbalance of 5.2 points, winding temperature increases the percentage of 54%. The result is a phase winding overheating while the other two windings temperature is normal.

One from UL (Underwriters Laboratories, USA) completed the survey, 43% of the power company to allow a 3% voltage unbalance, and another 30% of the power company to allow a 5% voltage unbalance.


5 inadequate cooling

Are generally larger compressor power back to air cooling type. The lower the evaporation temperature, mass flow systems are often smaller. When the evaporation temperature is very low (more than the manufacturer's requirements), 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 the gas back to a small, but the ambient temperature and compressor cooling air flow there is a clear requirement.

Large leak of refrigerant mass flow will cause the system decreases, the motor cooling will also be affected. Some unattended cold storage, etc., often have to wait until the poor cooling effect only when a large number of leaking refrigerant found.

Frequent protection occurs after the motor overheating, some users do not thoroughly check the reason, even the thermal protector short-circuit, it is a very bad thing. Before long, the motor will burn.

Compressor has a range of safe operating conditions. Safe working conditions is the main consideration of the compressor and the motor load and cooling. Compressor due to different temperature zones of the price difference, beyond the scope of past use of domestic refrigeration compressor industry is relatively common. With the expertise of growth and economic conditions improve, the situation has improved significantly.

6 with vacuum compressor

Open refrigeration compressor has been forgotten, but the refrigeration industry, there are some on-site construction workers to retain the habit of the past - with the compressor vacuum. This is very dangerous.

Air-insulated medium plays a role. Vacuum sealed container after discharge between electrodes inside the phenomenon is very likely to occur. Thus, with the vacuum inside the compressor shell deepening between the shell terminal or bare insulation between the windings slightly damaged dielectric loss, once energized, the motor may burn in an instant short-circuit. If the shell leakage, may also cause an electric shock.

Therefore, prohibit the use of vacuum compressor and the compressor in the vacuum in the system and state (vacuum extraction was not added refrigerant), is strictly prohibited to the compressor power.


7 Summary

Motor burned, cover up the damage to the winding phenomenon to failure analysis caused some difficulties. However, damage caused by the root cause of the compressor motor will not disappear. Poor lubrication or failure caused by abnormal load or even stall, less heat will shorten the life of the winding; winding is interspersed with short metal debris is to provide a variable interest; contactor welding to protect the compressor will not run; operation of the power which the motor abnormalities, will fundamentally destroy any motor; compressor vacuum, inside the terminal may cause discharge.

Unfortunately, these negative factors will linter-trigger: abnormal load and stall when the large current could cause contactor welding; single contact arcing or welding will cause phase unbalance or single phase; phase imbalance will cause heat dissipation; inadequate of heat dissipation will cause less wear and tear; wear and tear will produce metal shavings ...

Therefore, the correct compressor installation, and reasonable maintenance, can prevent the emergence of negative factors ,which is fundamental method to avoid damage compressor motor.