The damage of the motor coil is mainly manifested as: stator winding insulation damage (short circuit) and circuit breaker.It is difficult to be found in time after damage of stator winding, which may cause the winding to burn down.After the winding is burned down, some phenomena or direct causes are concealed, which makes it difficult to analyze and investigate the cause afterwards.

The common faults of compressors are as follows:

1. Motor burning;

2. Liquid hammer;

3. Oil shortage caused by frozen oil.

Faults of motor compressor (hereinafter referred to as compressor) can be divided into motor failure and mechanical failure (including crankshaft, connecting rod, piston, valve disc, cylinder head pad, etc.).

Mechanical failure often causes the motor to overload and even block, is one of the main causes of motor damage.

The six main reasons for winding down:

(1) abnormal load and plugging;

(2) winding short circuit caused by metal chip;

(3) contact problem;

(4) power supply gap and voltage anomaly;

(5) insufficient cooling;

(6) vacuum with compressor.

In fact, motor damage caused by multiple factors is more common.

1

Abnormal load and plugging.

The motor load includes the load needed to compress the gas and the load needed to overcome the mechanical friction.If the pressure ratio is too large or the pressure difference is too large, it will make the compression process more difficult.The friction resistance caused by the lubrication failure is increased, and the motor plugging in the extreme case will greatly increase the motor load.Lubrication failure and increase of friction resistance are the primary cause of load anomaly.

The lubricating oil is diluted, the lubricating oil is overheated, the coking of the lubricating oil is spoiled, and the lack of oil can damage the normal lubrication, resulting in the lubrication failure.The return fluid dilutes the lubricating oil, affecting the formation of normal oil film of the friction surface, even washing away the original oil film, increasing friction and wear.

The overheating of the compressor can cause the high temperature of lubricating oil to become thin or even coking, affecting the formation of normal oil film.The system return oil is bad, compressor is short of oil, natural cannot maintain normal lubrication.High-speed rotating crankshaft, connecting rod piston high-speed movement, such as no oil film protection of the friction surface will rapidly warming, local high temperature lubricating oil rapidly evaporating or coking, make it more difficult to the parts of lubrication, local severe wear may occur within several seconds.

Small power compressor (such as refrigerator, home air conditioning compressor) due to the motor torque is small, often appear after lubrication failure locked-rotor (motor could not turn), and enter the "turn - thermal protection - plugging turn" dead circulation, motor burn down just a matter of time.And high-power semi-closed compressor motor torque is large, local wear will not cause plugging turn, motor power will be within a certain range increases with load, thus cause more severe wear, and even cause seizuring live (pap) inside the cylinder, such as connecting rod fracture seriously damaged.The current (plugging current) is about 4 to 8 times the normal operating current.

When the motor starts, the peak of the current can be close to or close to the current.As the heat of the resistance is proportional to the square of the current, the current in the start-up and the plugging will cause the winding to heat up rapidly.Thermal protection can protect the electrode during plugging, but it does not usually have a quick response, and cannot prevent the temperature change of the winding caused by frequent start.Frequent start and abnormal load, which makes the winding undergo high temperature test, will reduce the insulation performance of enameled wire.In addition, the load of compressed gas will increase with the increase of compression ratio and pressure difference.

Therefore, it is not suitable for the high temperature compressor to be used for low temperature, or for the low temperature compressor to be used in the high temperature, which is not suitable, which will shorten the service life of the electrode.If there are other factors (such as metal particles forming conductive circuit, acid lubricating oil, etc.), it is easy to cause short circuit and damage.

2

The winding short circuit caused by metal chip.

The inclusion of metal chips in the winding is the main culprit of low insulation and short circuit.

The source of the metal includes the copper ducts left in the construction, the welding slag, the internal abrasion of the compressor, and the metal scrap that falls when the parts are damaged (such as the broken valve).

All closed compressors, including the fully enclosed scroll compressor, will be deposited on the windings.

Semi-closed compressor, some particles will flow with the gas and lubricating oil in the system, and finally because of magnetic accumulation in the winding;Some of the metal particles (such as bearing wear and motor rotor and stator wear) will fall directly onto the windings.It is only a matter of time before the metal chip is gathered in the winding.

Double-stage compressor, with lubricating oil in the return air, has already made the compression process tread on thin ice. If there is a return fluid, the valve piece of the first stage cylinder can be easily broken.The broken valve can enter the winding after medium pressure tube.Therefore, the two - stage compressor is more prone to the electrical short circuit than the single - stage compressor.

3

The winding short circuit caused by metal chip.

It is extremely important to select the contactor according to the load.When using a single contactor, the contactor rated current must be greater than the motor nameplate current rating (RLA).At the same time, the contactor must be able to withstand the motor plugging current.If there are other loads downstream of the contactor, such as motor fan, it must be considered.

When two contactors are used, the transposition rating of each contactor must be equal to or greater than that of the compressor half winding.

Specifications or inferior quality of the contactor is unable to withstand the compressor start, locked-rotor and low voltage large current shock, it's easy to have a single phase or multiphase contact jitter, welding or even fall off phenomenon, lead to motor damage.

If the contactor is small, the contact is not able to withstand the arc and the high temperature produced by the voltage generated by the frequent stop cycle or unstable control circuit may be welded or removed from the contact frame.The welded contact will produce a permanent single-phase state, allowing the overload protector to continuously circulate and disconnect.

Therefore, it is necessary to inspect the contactor when the motor is burned down.Contactor is an important cause of motor damage which is often forgotten.

4

The power supply is missing and the voltage is abnormal.

Abnormal voltage and lack of phase can easily destroy any motor.

The range of power supply voltage shall not exceed 10% of the rated voltage.The three-phase voltage imbalance should not exceed 5%.The high power motor must be independently supplied to prevent low voltage when the other high-power equipment is started and operated.

The motor power cord must be able to carry the rated current of the motor.If the compressor is running in the absence of phase, it will continue to operate but will have a large load current.The motor winding will soon overheat and normally the compression opportunity will be protected by heat.

When the motor winding is cooled to the set temperature, the contactor will close, but the compressor will not start up, turn up, and enter the "plugging - heat protection - plugging" death cycle.

The calculation method of voltage imbalance percentage is: the maximum deviation value of the phase voltage and the three-phase voltage average is the ratio of the three-phase voltage average.

For example, the nominal 380V three-phase power supply is measured at 380V, 366V and 400V respectively in the compressor terminal.It can be calculated that the three-phase voltage average is 382V and the maximum deviation is 20V, so the voltage imbalance percentage is 5.2%.As a result of the voltage imbalance, the unbalanced load current is 4-10 times that of the voltage imbalance in normal operation.In the previous example, 5.2% imbalance voltage may cause a 50% current imbalance.

The national electrical manufacturers association (NEMA) motor and generator standard publications, points out that percentage of phase winding temperature rise caused by unbalanced voltage is about voltage imbalance percentage point number square twice.In the previous example, the voltage imbalance points were 5.2 and the winding temperature increased by 54%.The result is that one phase winding overheats and the other two winding temperatures are normal.

5

Insufficient cooling

The compressor with larger power is generally air-cooled.The lower the evaporation temperature, the smaller the system mass flow.

When the evaporation temperature is low (exceeding the manufacturer's requirement), the flow is not sufficient to cool the motor and the motor will operate at a higher temperature.Air cooled compressors (generally not more than 10HP) are less dependent on the return gas, but have a clear requirement on the compressor's ambient temperature and cooling air volume.

The large leakage of refrigerant can also cause the system mass flow to decrease, and the cooling of the motor will be affected.Some unattended refrigeration units often have to wait until the cooling effect is bad enough to find a large leak of refrigerant.

When the motor overheats, there will be frequent protection. Some users will not check the cause, or even short-circuit the thermal protector. After a short time, the motor will be burned.The compressors have safe operating conditions, and the main consideration in safe working conditions is the load and cooling of the compressor and the motor.

6

Vacuum with the compressor.

In the refrigeration industry and some on-site construction personnel retained the habit of the past - with compressor, vacuum air play the role of the insulating medium, an airtight container after pumping air into vacuum state, the inside of the electrode discharge phenomenon is easy to occur between.

As compressed inside casing vacuum deepens, shell exposed between terminal or insulating layer has a tiny broken winding lost insulating medium, once the electricity, motor may be short circuit burned in an instant.If the shell leaks, it could cause a shock.

Therefore, it is forbidden to vacuum with the compressor, and it is forbidden to power the compressor when the system and compressor are in a vacuum state (the vacuum has not been added to the refrigerant).

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These adverse factors will also cause each other:

Abnormal load and large current during plugging may lead to contactor welding;

Single contact pull arc or even welding can cause phase imbalance or single phase;

Phase imbalance can cause heat dissipation problems;

Inadequate heat dissipation can cause wear and tear;

Wear will produce metal dust.

Therefore, the correct installation of the compressor, and reasonable daily maintenance, can prevent the occurrence of adverse factors, is to avoid the compressor motor damage the basic method.

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