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Motor compressors (hereinafter referred to as compressors) can be classified
into motor faults and mechanical faults (including crankshafts, connecting rods,
pistons, valve plates, cylinder head gaskets, etc.). Mechanical failure often
causes the motor to overload or even stall, which is one of the main causes of
motor damage.

The damage of the motor is mainly caused by damage (short circuit) and open
circuit of the stator winding insulation layer. When the stator winding is
damaged, it is difficult to be discovered in time, and eventually the winding
may be burnt. After the windings are burnt, some phenomena or direct causes of
burning are masked, making post-mortem analysis and cause investigation
difficult. However, the operation of the motor is inseparable from the normal
power input, reasonable motor load, good heat dissipation and protection of the
winding enameled wire insulation. Starting from these aspects, it is not
difficult to find that the causes of winding burnout are as follows: (1)
abnormal load and stall; (2) winding short circuit caused by metal chips; (3)
contactor problem; (4) power supply Phase loss and voltage anomalies; (5)
insufficient cooling; (6) vacuuming with a compressor. In fact, motor damage
caused by a combination of factors is more common.

Abnormal load and stall

The motor load includes the load required to compress the gas and the load
required to overcome the mechanical friction. If the pressure ratio is too
large, or the pressure difference is too large, the compression process will be
more difficult; the frictional resistance caused by the lubrication failure will
increase, and the motor stall in extreme cases will greatly increase the motor
load. Lubrication failure and increased frictional resistance are the primary
causes of load anomalies. Dilute the lubricating oil, the oil is overheated, the
coking and deterioration of the lubricating oil, and the lack of oil will
destroy the normal lubrication, resulting in failure of lubrication. The liquid
is diluted to dilute the lubricating oil, which affects the formation of the
normal oil film on the friction surface, and even washes away the original oil
film, increasing friction and wear. Overheating of the compressor can cause the
lubricating oil to become thinner or even coked at a high temperature, which
affects the formation of a normal oil film. The system does not return oil well,
the compressor is short of oil, and naturally it is impossible to maintain
normal lubrication. The high-speed rotation of the crankshaft and the high-speed
movement of the connecting rod piston, the friction surface without the oil film
protection will rapidly heat up, and the local high temperature causes the
lubricating oil to evaporate or coke rapidly, which makes the lubrication of the
part more difficult, and can cause local severe wear in a few seconds.
Lubrication failure, local wear, and crankshaft rotation require more torque.
Low-power compressors (such as refrigerators, household air-conditioner
compressors) due to the small torque of the motor, the phenomenon of blockage
(motor can not rotate) often occurs after the failure of lubrication, and enters
the "blocking-heat protection-blocking" infinite loop, the motor burns only Time
problem. The high-power semi-hermetic compressor motor has a large torque, local
wear will not cause blocking, and the motor power will increase with load within
a certain range, causing more serious wear and even causing seizure (the piston
is stuck in the cylinder). Inside), the connecting rod is broken and the like is
severely damaged.

The current at the time of stalling (blocking current) is approximately 4-8
times the normal operating current. At the moment of motor start-up, the peak
current can approach or reach the stall current. Since the heat release of the
resistor is proportional to the square of the current, the current during
startup and stalling causes the winding to heat up rapidly. Thermal protection
protects the electrode during stalling, but generally does not respond quickly,
and does not prevent winding temperature changes caused by frequent starts.
Frequent start-ups and abnormal loads cause the windings to withstand high
temperatures and reduce the insulation properties of the enameled wire.

In addition, the load required for the compressed gas also increases as the
compression ratio increases and the pressure difference increases. Therefore,
the use of high-temperature compressors for low temperatures or the use of
low-temperature compressors for high temperatures can affect motor load and heat
dissipation, which is not suitable and will shorten electrode life.

After the winding insulation performance is deteriorated, if there are other
factors (such as metal scraps forming a conductive loop, acidic lubricating oil,
etc.), it is easy to cause short circuit and damage.

Short circuit caused by metal chips

The metal chips trapped in the windings are the main culprit for the short
circuit and low grounding insulation values. The normal vibration of the
compressor during operation and the twisting of the winding by the
electromagnetic force at each start-up will promote the relative motion and
friction between the metal chips interposed between the windings and the winding
enameled wire. Sharp edges and swarf can scratch the enameled wire insulation
and cause a short circuit.

Sources of metal shavings include copper pipe scraps left during
construction, welding slag, internal wear of the compressor, and metal chips
that fall off when parts are damaged (such as broken valves). For fully enclosed
compressors (including fully enclosed scroll compressors), these metal chips or
granules can fall on the windings. For semi-hermetic compressors, some particles
will flow in the system with gases and lubricants, and eventually the magnets
will accumulate in the windings; some metal chips (such as bearing wear and
motor rotor and stator wear (broom) will) Drop directly on the windings. Short
circuiting is only a matter of time after metal chips are accumulated in the
windings.

Special attention needs to be paid to the two-stage compressor. In the
two-stage compressor, the return air and the normal return oil directly enter
the first stage (low pressure stage) cylinder, and after compression, enter the
motor cavity cooling winding through the medium pressure tube, and then enter
the second stage like the ordinary single stage compressor. (High pressure
cylinder). The return air has lubricating oil, which has made the compression
process like thin ice. If there is liquid back, the valve of the first stage
cylinder can be easily broken. The broken valve piece can enter the winding
after passing through the intermediate pressure tube. Therefore, the two-stage
compressor is more prone to motor short circuit caused by metal chips than the
single-stage compressor.

Unfortunate things often come together, and the problematic compressors often
smell the burnt smell of the lubricant during the startup analysis. When the
metal surface is severely worn, the temperature is high, and the lubricating oil
starts to coke at 175oC or more. If there is more water in the system (the
vacuum is not ideal, the lubricating oil and the refrigerant have a large water
content, and the air enters after the negative pressure return pipe is broken),
the lubricating oil may be acidic. Acidic oils can corrode copper tubes and
winding insulation. On the one hand, it causes copper plating. On the other
hand, this kind of acidic lubricating oil containing copper atoms has poor
insulation properties, which provides conditions for winding short circuits.

Contactor problems

The contactor is one of the important components in the motor control
circuit. The irrational selection can destroy the best compressor. It is
extremely important to select the contactor correctly according to the load.

Contactors must be able to withstand harsh conditions such as fast cycling,
continuous overload and low voltage. They must have a large enough area to
dissipate the heat generated by the load current, and the choice of contact
material must be prevented from soldering at high currents such as startup or
stall.

For safety and reliability, the compressor contactor must disconnect the
three-phase circuit at the same time. Copeland does not recommend the method of
disconnecting the two-phase circuit.

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

· The contactor must meet the work and test guidelines specified in ARI
Standard 780-78 "Special Contactor Standards".

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

• 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 stall current.

· If there are other loads downstream of the contactor, such as motor fans,
etc., it must also be considered.

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

The rated current of the contactor must not be lower than the rated current
on the compressor nameplate. Contactors with small or poor quality cannot
withstand compressor start-up, high current surges during stalling and low
voltage, and are prone to single-phase or multi-phase contact jitter, soldering
or even falling off, causing motor damage.

Contactors with contact jitter frequently start and stop the motor. Frequent
starting of the motor, large starting current and heat, will aggravate the aging
of the winding insulation. At each start-up, the magnetic torque causes the
motor windings to move slightly and rub against each other. If there are other
factors (such as metal shavings, poorly insulating lubricants, etc.), it is easy
to cause a short circuit between the windings. Thermal protection systems are
not designed to prevent this damage. In addition, the dithered contactor coil is
susceptible to failure. If the contact coil is damaged, a single-phase condition
is likely to occur.

If the contactor is selected to be small, the contacts cannot withstand
arcing and high temperatures due to frequent on-off cycles or unstable control
loop voltages, which may be soldered or detached from the contact holder. The
soldered contacts will create a permanent single phase condition that causes the
overload protector to continuously cycle on and off.

It is important to emphasize that after the contactor contacts are soldered,
all 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 unprotected.
status.

Therefore, when the motor is burned, it is an indispensable process to check
the contactor. Contactors are an important cause of motor damage that is often
forgotten.

Power supply phase loss and voltage anomaly

Unusual voltage and phase loss can easily destroy any motor. The power supply
voltage variation range cannot exceed ±10% of the rated voltage. The voltage
imbalance between the three phases cannot exceed 5%. High-power motors must be
powered independently to prevent low voltages from starting and operating other
high-power equipment on the same line. The motor power cord must be able to
carry the rated current of the motor.

If the compressor is running when a phase loss occurs, it will continue to
run but will have a large load current. The motor windings will overheat quickly
and the compressor will be thermally protected under normal conditions. When the
motor winding is cooled to the set temperature, the contactor will close, but
the compressor will not start up, there will be a stall, and enter the
"blocking-heat protection-blocking" infinite loop.

The difference between modern motor windings is very small, and the
difference in phase currents during power supply three-phase balancing can be
ignored. Ideally, the phase voltages are always equal, as long as a protector is
connected to either phase to prevent damage from overcurrent. It is actually
difficult to guarantee the balance of the phase voltage.

The calculation method of the voltage imbalance percentage is the ratio of
the maximum deviation between the phase voltage and the average value of the
three-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, respectively. 400V. The average value of the
three-phase voltage can be calculated as 382V, and the maximum deviation is 20V,
so the voltage imbalance percentage is 5.2%.

As a result of the voltage imbalance, the load current imbalance during
normal operation is 4-10 times the number of voltage imbalances. In the previous
example, a 5.2% unbalanced voltage could cause a 50% current imbalance.

The National Electrical Manufacturers Association (NEMA) Motor and Generator
Standards publication states that the percentage of phase winding temperature
rise due to unbalanced voltage is approximately twice the square of the voltage
imbalance percentage. In the previous example, the voltage unbalance point is
5.2, and the percentage of winding temperature increase is 54%. The result is
that one phase winding is overheated and the other two windings are normal.

A survey completed by U.L. (Underwriters Laboratories, USA) showed that 43%
of power companies allowed 3% voltage imbalance, and another 30% of power
companies allowed 5% voltage imbalance.

Insufficient cooling

Compressors with higher power are generally of the return air cooling type.
The lower the evaporation temperature, the smaller the system mass flow tends to
be. When the evaporation temperature is very low (more than the manufacturer's
specifications), the flow is not enough to cool the motor and the motor will run
at a higher temperature. Air-cooled compressors (generally not exceeding 10 HP)
have little dependence on return air, but have clear requirements for compressor
ambient temperature and cooling air volume.

A large amount of refrigerant leakage will also cause a reduction in the mass
flow of the system, and the cooling of the motor will also be affected. Some
unattended cold storages, etc., often have to wait until the cooling effect is
very poor, only to find a large amount of refrigerant leakage.

Frequent protection occurs when the motor overheats. Some users do not go
deeper to check the cause and even short-circuit the thermal protector, which is
very bad. After a while, the motor will burn out.

The compressor has a range of safe operating conditions. The main
consideration for safety conditions is the load and cooling of the compressor
and motor. Due to the different prices of compressors in different temperature
zones, it has been common to use compressors in the domestic refrigeration
industry in the past. With the growth of professional knowledge and the
improvement of economic conditions, the situation has improved
significantly.

Vacuuming with a compressor

Open refrigeration compressors have been forgotten, but there are still some
on-site construction workers in the refrigeration industry who have retained the
habit of using the compressor to pump vacuum. This is very dangerous.

Air acts as an insulating medium. After the vacuum is applied to the sealed
container, the discharge phenomenon between the electrodes inside is likely to
occur. Therefore, as the degree of vacuum in the compressor casing is deepened,
the insulating medium is lost between the exposed terminals in the casing or
between the windings with minute damage of the insulating layer. Once energized,
the motor may be short-circuited in an instant. If the housing leaks, it may
also cause electric shock.

Therefore, it is forbidden to use the compressor to evacuate, and when the
system and the compressor are in a vacuum state (the refrigerant has not been
added after the vacuum is exhausted), it is strictly forbidden to energize the
compressor.

Summary

After the motor is burnt, the phenomenon of winding damage is covered, which
causes certain difficulties for fault analysis. However, the root cause of
damage to the compressor motor does not disappear. Abnormal load or even stall
caused by poor lubrication or failure, insufficient heat dissipation, will
shorten the life of the winding; the inclusion of metal chips in the winding
provides a benefit for the short circuit; contactor welding will make the
protection of the compressor impossible; An abnormality in the power supply on
which the motor is running will fundamentally destroy any motor; vacuuming the
compressor may cause the internal terminal to discharge.

Unfortunately, the above disadvantages can also trigger each other: abnormal
load and large current during stalling may lead to contactor soldering; single
contact arcing or even soldering will cause phase imbalance or single phase;
phase imbalance will cause Heat dissipation problem; insufficient heat
dissipation can cause wear; wear will produce metal shavings…

Therefore, proper installation and use of the compressor, as well as
reasonable daily maintenance, can prevent the occurrence of unfavorable factors
and is the fundamental way to avoid damage to the compressor motor.

http://www.cysensors.com
Shanghai Chuan Yue Automation System Co., Ltd.

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