Aug 24, 2021

## Motor Winding Failure

There are three terminals on the compressor shell, terminal C, S, and M. The starting winding (S-C) and the running winding (M-C) are inside. One end of the starting winding and the running winding is connected together with terminal C, the other end is connected with S and M.

The motor winding fault is generally judged by resistance measurement. It should be carried out after the power supply, starter (PTC or gravity type) or thermal protector is determined to be normal.

### * Open Winding

You should use a multimeter to measure the resistance value between the three terminals. If infinity occurs at any time, it’s the open winding when the terminals are not loose or terminals are oxidized or corroded, causing poor contact.

### * Short Circuit Fault of Winding

The three terminals of each type of compressor are not arranged in regular order, but there is a certain resistance between every two terminals.

RMS=RMC+RSC RSM>RSC>RMC (1)

Generally, RMS, RSC, and RMC are not more than 100 Ω (for example, the UK "GEC" compressor RMS= 84 Ω).

For a small number of ohms (for example, the RMS of the German PWTN18 compressor is 7 Ω), use a multimeter to measure the resistances of the three terminals as R1, R2, and R3(you do not need to determine which is RMS, RSC, or RMC).

If the three resistances do not meet the (1), it indicates that the windings are short-circuited, except for a few compressor windings.

You should note that sometimes the winding local short circuit, measured R1, R2, R3 will also meet the (1), it is because the multimeter can only measure the winding partial short-circuit resistance change. However, in the winding partial short-circuit, the current is larger than the normal value.

### * Windings Short to Ground

You should use a megohmmeter to measure the insulation resistance between the terminal and the housing. If the value is close to zero, it is the running winding or the starting winding short-circuit to the ground.

Figure 2: The megohmmeter.

## Compressor Stuck

Stuck faults include stuck shell, stuck cylinder, holding shaft, swaying shaft. This fault is that the compressor does not operate after the power is energized, the casing is hot and accompanied by abnormal noise. And if there is a sound made by the thermal relay, you should cut off the power supply. When the power supply voltage, motor winding and starter are normal, and the motor does not rotate, it will be the compressor and other running stuck faults.

### * Blocked Oil Circuit

The oil circuit of the compressor is blocked by dirt, and the oil supply system is clogged, so the parts are worn and stuck.

### * Friction

The fixed bracket is deformed, the fixed screw is loose, the coordination between the parts is too tight, the clearance is too small. And there is the mutual collision of the operation. The friction failure usually occurs in the main shaft, piston, cylinder, connecting rod and other operating parts.

### * Rotor Deflection

The paint on the enameled wire of the winding is corroded off, which will stick to the cylinder and piston. Or the rotating shaft and bearing sleeve become loose after wear, and the rotor is sucked to one side by magnetic force after electrification and deviating from the centerline, resulting in failing to rotate.

Facing stuck failures, you should stop the compressor first and then start it again. If it still does not rotate, you can tap it, or pressurize it. Besides, you may adopt the capacitor start method, pressure relief method and air rush method.

## Abnormal Noise

When the compressor is running, the metal collision sound is emitted in the casing, and the operation noise fault mostly occurs in the cylinder, crankcase and shock absorption device.

### * Shock Absorption Failure

The damping spring loses the effect after a fracture. Its dislocation or deformation will make the body hit the inner wall of the shell and generate noise during operation.

### * Cylinder Knocking

The top of the piston collides with the exhaust valve, which means the gap between the two is too small.

### * Abnormal Valve Disc

The nut on the fastening screw of the valve disc or the piston connecting rod is loose, and the valve disc is broken or severely worn due to low-quality material and improper operation.

### * Piston Damage

The piston is severely worn or clogged. Refrigerator compressors are generally equipped with piston rings.

### * Connecting Rod Failure

The connecting rod nut is loose, and the clearance between the connecting rod’s big bearing end and the crank or between the crank and the main bearing is too large.

### * Dry and Stuck Bearing

The bearing lubricating oil is insufficient, and there are impurities in the oil.

### * Loose Flywheel

It’s loose between the flywheel and bearing.

### * Liquid Strike

Figure 3: Liquid strike of the compressor.

The main reason for the liquid strike phenomenon is that the suction valve opens too large and fast, forming a sharp and strong suction force, and causing the refrigerant liquid in the evaporator to be sucked into the compressor before evaporation.

In addition to the noise, it may also smash the suction and exhaust valves, or make the connecting rod, piston and crankshaft distort and even crack the cylinder head.

If it is not caused by the compressor, the main reasons may be excessive refrigerant, extremely thick crystallization on the evaporator, too much oil inside, insufficient evaporation area or too large refrigeration capacity.

## Exhaust Failure

### * High Exhaust Pressure

Serious scaling of the heat transfer tube of the condenser leads to decreased conduction efficiency.

The refrigerant in the system is overcharged.

Noncondensable gases are in the system.

### * Low Exhaust Pressure

After eliminating refrigerant leakage, capillary and filter blockage, there can be a fault in the exhaust system. It is manifested as the condenser which is not hot enough, a slight airflow sound inside the compressor, and a bad refrigeration effect due to the long-period operation of the compressor.

High-pressure exhaust pipe rupture or gasket breakdown, will cause the refrigerant to be diverted to form the internal circulation, making airflow sound, so that the refrigeration effect is poor.

Due to liquid strike or bad material, the valve plate ruptures or the valve plate scaling fails to be closed.

The wear causes the clearance between the compressor piston and cylinder to be too large, so that the compressor displacement is insufficient.

Insufficient refrigerant.

The condensing speed is too fast.

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