【 Weak current College 】 compressor failure analysis---motor burnt

Compressor failure analysis (1) motor burnt
Motor compressor (hereinafter referred to as compressor) can be divided into motor fault and mechanical failures (including the crankshaft, connecting rod, Pistons, valve, cylinder head gaskets, etc.). Mechanical failure often make motor overload or stalling, is motor one of the main causes of damage.

Motor damage mainly manifested in the stator winding insulation damage (short circuit) and circuit breakers, etc. Stator winding damage after difficult was found, in a timely manner may result in the winding burned down eventually. Winding burned, cover some of the causes of the phenomenon or burn directly causes, making the subsequent analysis and cause investigation difficult.

However, motor functioning without the normal power supply input, reasonable motor load, good heat dissipation and winding wire insulating layer of protection. Starting from these areas, it is not difficult to discover winding burned down six reasons as follows: (1) abnormal load and no-load; (2) scrap metal winding caused; (3) contactor issues; (4) power phase and voltage anomalies; (5) insufficient cooling; (6) vacuum with a compressor. In fact, a variety of factors contributed to the more common electrical damage.

1, abnormal load and no-load

Motor load including the required load compressed gases and overcome friction the desired load. Compression ratio is too large, or pressure differential is too large to cause the compression process more difficult; but lubrication failure caused by friction resistance increase, as well as the extreme case of motor stalling, will significantly increase the motor load.

Lubrication failure, the friction resistance increase, is the leading cause of load exception. Back to liquid diluted oil, lubricating oil, oil heat, coking and lack of oil deterioration, etc will disrupt normal lubrication, cause lubrication failure. Back to the dilution effect of lubricating oil, friction surface normal oil film formation, even washed away the original film, increase friction and wear. Compressor overheating will cause the lube oil temperature thinned even coking, impacting oil film formation. System back to oil is not good, compressor oil shortage, nature cannot maintain normal lubrication. Crankshaft rotation speed, such as high-speed movement of the piston rod, no oil film protection of friction surface will rapidly heating up, local temperature enables the rapid evaporation of lubricating oil or coking, the more difficult parts lubrication, seconds can cause local severe wear. Lubrication failure, local wear, so that the crankshaft rotation needs greater torque. Small power compressor (such as fridge, air-conditioner compressor) because the motor torque, lubrication failure often appear after stalling (motor not running), and enter "stalling-heat protection-stalling" death cycle, motor burned it's just a matter of time. And high-power semi-enclosed compressor Motors torques are large, local and wear does not cause stalling, motor power in a certain range as the load increases, thus causing more serious wear, even bite cylinder (cylinder piston card in), Rod fracture, and other severe damage.

No-load current (locked rotor current) about normal operation current of 4-8 times. Motor start instant, current peak can be close to or reach locked-rotor current. Due to resistance on heat and proportional to the square of the current, starting and locked rotor current when the rapid warming will make winding. Heat protection can protect electrode-rotor, but normally does have a very quick response, it will not prevent frequent start caused by winding temperature changes. Frequent start and abnormal load, winding through high-temperature test, reduces wire insulation performance.

In addition, compressed gas required load will also increase the compression ratio and the pressure difference grows. Therefore the high temperature compressor forlow temperature, low temperature or high temperature, the compressor is used to affect the motor load and heat, is not appropriate, will shorten the service life of the electrodes.

Winding insulation performance variation, if there are other factors (such as scrap metal or conductive circuit, acidic form lubricants, etc.), it is easy to cause a short circuit and damage.

2, scrap metal or caused by short circuit

Winding of scrap metal in the inclusions are short circuit and grounding insulation value low. Compressor running time of normal vibration, as well as the start winding affected by electromagnetic force and twisting will promote inclusion, in the windings of scrap metal and winding wire relative movement and friction. Sharp metal edges will scratch the enameled wire insulation, causing a short circuit.

Scrap metal construction of sources including left of copper tube cutting, welding slag, compressor damage internal wear and spare parts (such as a valve plate broken) fall under sweepings, etc. For hermetic compressor (including hermetic scroll compressor), these metal shavings or broken grains will fall on the winding. For semi-hermetic compressors, some particles with gas and oil flow in the system, and finally because magnetic gathered in winding; and some scrap metal (for example, bearing wear and abrasion motor rotor and stator (scrapping) of) directly on the winding. Winding and gathered in the scrap metal, short circuit is only a matter of time.

In need of special attention of the two-stage compressor. In two-stage compressor, back to the gas and normal return oil directly into the first level (low level) cylinder, compressed with medium pressure pipe enter Motors Chamber cooling coil, single-stage compressor and then, enter the second level (high grade cylinders). Back to the gas with lubricating oil, has made the compression process in perpetuity, if further back into the liquid, the first cylinder valve slice can easily be broken. Broken valve plate with medium pressure pipe after entering the windings. Therefore, the two-stage compressor than single-stage compressors are more prone to scrap metal causing the motor circuit.

Unfortunate things conspired to one, the problem is in the power of the compressor of the Messenger from the often lubricant jiaohu flavor. Metal face serious wear and temperature is very high, and lubricants in 175oC above start cooking. System if there is more moisture (vacuum pump was not ideal, lubricants and refrigerants moisture, negative pressure return gas pipe rupture after air, etc.), lubricating oil acidity may occur. Acidic corrosion Lube will copper pipe and winding insulation, on the one hand, it will cause the copper plating; on the other hand, this contains copper atoms acid oil insulation performance is poor for winding.

Issue 3, contactors

ContactIs a motor control circuit in one of the important parts, selection of unreasonable can damage the best compressor. Press the load correctly select the contactor is extremely important.

Contact device must be able to meet the demanding conditions, such as rapid cycling, sustained overload and low voltage. They must be sufficiently large area to distribute load currents generated by the heat, contact materials selection must start or stalling, etc. larger current circumstances can prevent bonding.

In order to secure and reliable, the compressor contactor to disconnect a three-phase circuit. Copeland Corporation is not recommended to disconnect the second phase circuit.

In the United States, Copeland Corporation recognized contactors must meet the following four items:

· Contactors must meet ARI standard 780-78 "private contactor standard" requirements of the work and testing guidelines.

· Manufacturer must ensure the contactor at room temperature, at a minimum of 80 per cent of the nameplate voltages can be closed.

· When you use a single contact, the contact with rated current must be greater than motor nameplate current ratings (RLA), at the same time, the contactor must be able to withstand motor no-load current. If there are other contactor downstream load, such as motor fan, etc., must also be taken into account.

· When using two contactors, each contactor minutes winding stalling ratings must be equal to or greater than the compressor stall-winding ratings.

Contactor for rated current can not be lower than the compressor nameplate on the rated current. Specifications for small or low quality to withstand contactor, locked rotor compressor start and low voltage and current at the time of impact, prone to single-or multiple-phase-contact jitter, welding or even fall, causing damage to the motor.

Contact jitter of contactor frequently stop motor. Motor frequent start, great start current and fever, exacerbate winding insulation of aging. Every boot, magnetic torque of the motor winding on a tiny mobile and mutual friction. If there are other factors (such as metal shavings, insulation of lubricants, etc.), very easily lead to short circuit between the winding. Thermal protection system was not designed to prevent such damage. In addition, jitter of a contactor coil can easily fail. If you have contact with coil damage, prone to single-State.

If you contact device selection is small, contact cannot withstand the arc and due to the frequent open circulatory or unstable control circuit voltage may heat welding or contact shelf in shedding. Bonding of contact will result in a permanent state of single phase, so that the overload protector continuously loop connected and disconnected.

Special emphasis is required, contact contact spot after kg, rely on the contactor disconnect all power circuit of the compressor control (such as high-voltage control, pressure control, defrost control, etc.) will all fail, the compressor unprotected.

Therefore, when the motorburned down, check the contactor is essential to the process. Contactor is one of the leading motor damage are often forgotten.

Phase 4, power and voltage anomalies

Voltage is not working properly and phase can easily destroy any motor. Power supply voltage ranges cannot exceed the rated voltage ± 10%. Three-phase voltage unbalance must not exceed 5 per cent. High-power Motors must be independent of power supply to prevent the same line other high-power devices startup and operation resulting in lower voltage. Motor power cords must be capable of bearing motor rated current.

If there is a phase when the compressor is running, it will continue to run but will have a larger load current. Motor winding thermal soon will, under normal circumstances the compressor will be heat protection. When the motor windings cooling-temperature setting, contact will be closed, but the compressor does not start up, stalling, and enter "stalling-heat protection-stalling" dead loop.

Modern motor windings of difference is very small, power-phase equilibrium phase current differences can be ignored. Ideally, phase voltage always equal, as long as in either phase after another protector prevents overcurrent damage. Indeed difficult to ensure that the voltage balance.

Voltage unbalance percentage calculated, phase voltage and three-phase voltage means the maximum deviation of the values and the average three-phase voltage ratio, for example, nominal 380V three-phase power supply, the compressor terminals measure voltage 380V, 366V, 400V, you can calculate the average of the three-phase voltage, maximum deviation 382V to 20V, so the voltage unbalance percentage is five, 2 per cent.

As a result of voltage imbalance, in normal operation the load current imbalances are voltage unbalance percentage points for the number of 4-10 times. The preceding example, 5, 2% unbalanced voltage may cause 50 per cent of current imbalances.

United States National Electrical Manufacturers Association (NEMA) motors and generators standards publication that caused by an unbalanced voltage phase winding percentage of about voltage unbalance percentage points twice the number of squares. The preceding example voltage unbalance points 2 to 5, the winding temperature increase percentage for 54 per cent, the result is a phase winding overheating and the other two winding temperature properly.

A U, L, (underwriter's laboratories, United States) completed the survey, 43 per cent of the power companies allow 3% voltage unbalance, and another 30 per cent of the power companies allow 5% voltage unbalance.

5. insufficient cooling

Power larger compressors are usually back to the air cooling type. Evaporation temperature lower system quality stream, the less often. When evaporation temperature very low (in excess of the manufacturer's requirements), the flow is sufficient to cool the motor, motor can operate at higher temperatures. Air cooled compressors (generally not exceeding 10HP) dependence on gas, but on compressor ambient temperature and cooling air flow is explicitly required.

Refrigerant leakage would cause large numbers of systems quality flow decreases, motor cooling will also be affected. Some unattended freezer, etc., often have to wait until the cooling effect is very poor will find a large number of refrigerant leakage.

Motor overheat protection will occur frequently, some users do not have in-depth check reasons,To short-circuit the thermal protector, it is very bad things. It won't be long before, the motor will burn.

Compressor has safe operating range. Security conditions main consideration is the compressor and motor load and cooling. Since different temperature zones of compressor prices, past domestic refrigeration compressors use exceeding the scope of the industry is relatively common. With the expertise of growth and improvement of economic conditions, the situation has improved significantly.

6, compressor vacuum

Open refrigeration compressors have been forgotten, but there are some in the refrigeration industry site builders retained the past habits – compressor vacuum. This is very dangerous.

Air plays a role of insulation. Vacuum sealed containers, the electrodes between discharge easily occur. Therefore, with the compressor shell in vacuum deepens, shell Terminal in exposed or insulating layer between small broken winding insulation between lost power, the media, as soon as possible at the moment within the motor circuit burnout. If the shell leakage and may cause electric shock.

Therefore, no compressors vacuum, and the system is in the State of vacuum and compressor (not yet finished the vacuum and refrigerant), is strictly prohibited to compressor power.

Summary

Motor burned, conceals the winding corruption has caused the fault analysis difficult. However the compressor motor damage caused by underlying causes and not disappear. Poor lubrication or failure caused by abnormal load even stalling, thermal, shorten the service life; the windings windings in the scrap metal is to short circuit provides variable; contactors welding will cause the compressor protection cannot be executed; operation of the motor's power malfunctions will fundamentally destroy any motor, compressor vacuum, may cause the Terminal discharge.

Unfortunately, these negative factors will also throw each other: exception when load and no-load current can cause contact welding; a single contact la arc welding even can cause phase imbalance or single-phase; phase imbalance can cause heat; heat shortage causing wear; wear will produce scrap metal.

Therefore, properly installed using the compressor, as well as reasonable maintenance, you can prevent a negative factor is to avoid damage to the root of the compressor motor.