Refrigerator Troubleshooting Diagram

Archive for the ‘Refrigerator Repair’ tag

Refrigerator Drive Motor Installation

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Earthing

The motor frame must always be connected to the local earth circuit. It is most important to ensure that the flexible earthing strip between the motor frame and the cradle is not broken and that the correct connection is made.

When water cooled systems are involved, under no circumstances must the water supply tubing be used for earthing.

Lubrication

Motor bearings should be lubricated at the time of installation or replacement. For sleeve bearings use high grade light mineral oil; for ball bearings use pure mineral grease.

Do not apply an excess of oil or grease, because excess lubricant can damage motor windings and switch mechanisms. Do not allow oil to contact the resilient rubber motor mounting pads. A small drop of oil should be applied to the swivel pivot.

Bearings

Worn bearings will probably be evident in the form of a hum or rattle from the motor.

The end play should not exceed 0.25 mm. If shims are used to correct the clearance or take up wear, they must not be fitted at one bearing only. This will result in the rotor being out of centre with the stator and will cause humming.

Thermal overloads

These are usually self-resetting after cooling. They are set to cut out when the temperature of the motor windings reaches approximately 900 C (1940 F).

Nuisance tripping of a motor may be the result of poor ventilation of the motor or the load on the compressor. This should be checked before any replacement is made.

Direction of rotation

To reverse the rotation of a capacitor start motor, the direction of current flow in either the start or the run winding must be reversed (but not both windings). This entails reversing the internal connections of a winding, as shown in Figure 102. For three phase motors it is only necessary to reverse any two electrical leads to the motor terminals.

refrigerator-motor-reversing-motor-direction

Terminal board connections

Suppliers of drive motors are numerous and the terminal identification is not standard. A variety of connections for motors in common use is provided in Figure 103.

refrigerator-motor-connection

Written by sam

November 13th, 2009 at 4:24 am

Refrigerator Oil Addition and Removal

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When the system has been fully charged and the controls set, the equipment is operated at average evaporating conditions. The compressor oil level must then be checked.

All new compressors receive an operating oil charge during manufacture, but this does not allow for oil trapped within components and controls and circulating with the refrigerant. This is the reason why oil levels must be checked and oil added to the system at the time of installation or commissioning.

Adding oil

Only the make and grade of oil specified by the manufacturer of the compressor must be used. Oils have special characteristics for pressure, temperature and load conditions, especially those used for low temperature applications.

The larger compressors will have oil-level-indicating sight glasses located in the crankcase. If compressors are used in parallel, there is sometimes a sight glass in the middle of the oil equalizer line. During operation the oil level in the crankcase may well fluctuate. It is generally accepted that a level maintained at one-third to one-half of the way up the sight glass is satisfactory.

Before any oil is added, the system must be fully charged. This is most important when the compressor is located above the level of the evaporator. Oil levels should be rechecked after a system has completed its initial pulldown, or has operated for at least two hours. If a sight glass indicates foaming, ensure that this is not the result of absorbed refrigerant. Systems prone to this condition will probably be fitted with crankcase heaters.

Oil may be added to the crankcase of larger compressors simply by pumping down and reducing the crankcase pressure to enable oil to be poured when the filler cap has been removed. However, filler holes tend to be small on some compressors and an oil pump may be used.

Too much oil in a crankcase can cause damage to a compressor by creating a dynamic pressure during operation.

The procedures for adding oil will obviously vary according to the type of compressor.

Charging pump and simple filling

The oil charging pump is similar to a cycle pump and needs no explanation. The correct oil level must be attained, and reference should be made to the manufacturer’s data.

Sometimes it suffices to add oil to a compressor until it runs out of the filler hole; a dipstick will be required for other types. In each instance the oil level will be such that it is approximately 25 mm (1 inch) below the crankshaft so that bearings or splashers immerse in the oil as the shaft rotates (Figure 110).

refrigerator-oil-filling

Vacuum pump

When a compressor has a sight glass it is a simple task to add oil. The procedure is as follows (Figure 111):

1. Pump down the system to reduce the pressure in the crankcase to 0.1 bar (1 psig) and front seat both suction and discharge service valves.
2. Remove the oil filler plug and fit a charging line incorporating a shut-off valve and an adaptor to insert into the filler hole.
3. Place the free end of the charging line into a container of clean and uncontaminated oil from a sealed can. Crack off the suction service valve from the front seat position and raise the crankcase pressure to 0.1 bar.

refrigerator-vacuum-pump

Open the shut-off valve slowly and purge the charging line through the oil in the container. Front seat the suction service valve.
4. Connect a vacuum pump to the gauge union of the suction service valve. 5 Switch on the vacuum pump and reduce the pressure in the crankcase to slightly below atmospheric, allowing the oil to be drawn in until the correct level is reached.
6. Stop the vacuum pump, crack off the suction service valve from the front seat position, purge oil from the charging line and close the shut-off valve. Then front seat the suction service valve.
7. Remove the charging line and replace the oil filler plug.
8. Fully back seat and crack off both service valves, or set to operating positions.
9 Leak test the compressor.
10 Start the system and allow it to settle down to average operating conditions. echeck the oil level.

When charging oil, ensure that the charging line is always below the oil level in the container.

Compressor charging

When the compressor design is such that a suction strainer and oil return to the crankcase is featured, oil may be added via the suction service valve gauge union in much the same way as described above but using the compressor to draw a vacuum instead of a vacuum pump (Figure 112).

refrigerator-compressor

Draining compressor oil

This may be necessary when too much oil has been added, when maintenance contracts stipulate a periodic oil change, or when a system has been contaminated.

Assuming that the compressor does not have an oil drain facility and the removal of the base plate or sump plate is impractical or uneconomical, two simple methods may be adopted.

Vacuum method

This requires an air tight container or preferably a graduated cylinder so that the amount of oil removed can be measured and the precise amount replaced.

By pulling a vacuum on the container or cylinder, the oil will be drawn out of the compressor into the cylinder (Figure 113). During this process the compressor must be isolated from the system by front seating the service valves.

refrigerator-vacuum-oil-draining

System pressure method

With a length of tubing and an adaptor fitted to the filler hole after pressure in the crankcase has been reduced and both service valves front seated, create a positive pressure in the crankcase by cracking off the suction service valve from the front seat position (Figure 114). Provided the tubing reaches to the bottom of the crankcase or sump, the oil will be forced out of the compressor and into the container.

refrigerator-system-pressure-oil-draining

Correcting the oil level

The pressure method above can be used to reduce the level of oil in the crankcase in the event of an overcharge. By fitting an adaptor tube which terminates at the correct level below the crankshaft, oil will be forced out of the compressor when the crankcase is pressurized, but only until the oil level reaches the end of the tube.

Refrigerator Evacuation

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It is imperative, with halogen refrigeration systems, that all traces of air, non-condensibles and moisture be removed. If this is not achieved then the presence of air or non-condensibles will cause abnormally high discharge pressures and increased temperatures, resulting in the conditions relating to high operating pressures previously explained.

Air in the system will also mean that a certain amount of moisture contained in the air will be circulated with the refrigerant under operating conditions. This moisture could freeze at the orifice of an expansion valve or liquid capillary to prevent refrigerant flow to the evaporator, should the filter drier become saturated.

When a system has been pressure leak tested, traces of nitrogen may also be present to further aggravate the high discharge pressure condition.

There are two ways in which a system can be evacuated, the deep vacuum method and the dilution method.

Deep vacuum method

To meet the requirements of a contaminant-free system, a good vacuum pump is necessary. Under normal ambient temperatures a vacuum of 2 torr should be achieved with a single deep-vacuum cycle.

The length of a deep-vacuum cycle can vary considerably: the larger the installation, the longer the cycle. This may be left to the discretion of the commissioning engineer, as stipulated by company policy, or a specific period may be requested by the customer. Obviously a large high vacuum pump will expedite the procedure. It is not unusual for a system to be left on vacuum for 24 or 48 hours, or even for several days, to ensure that it is completely free from contaminants.

The advantages of a deep vacuum are that (a) there will not be any appreciable loss of refrigerant other than the final trace charge administered while leak testing, and (b) it is possible to reclaim a trace charge of refrigerant from a large system (see Chapter 16 relating to contaminants and refrigerant recovery). Also, the immediate environment will not be polluted by refrigerant vapour so that it is difficult to carry out a final leak test when the system is charged. This will be evident when comparison is made with the dilution method.

Dilution method

The dilution method or triple evacuation should be carried out using OFN (oxygen free nitrogen) and not with a trace charge.

1. The initial nitrogen charge should be left in the system for at least 15 to 30 minutes. It can then be re-evacuated to a vacuum of 5 Torr.
2. This vacuum is then broken with another OFN charge allowing time for it to circulate the system.
3. Re-evacuate and charge the system with refrigerant.

This repetition may appear to be unnecessary but after a single or double evacuation small pockets of non-condensables may still be entrained in the system pipework or controls. By repeatedly breaking the vacuum with OFN these pockets will be dispersed or diluted by the OFN.

After each evacuation the pump should be switched off and, after a few minutes settling period, a vacuum reading taken. The system should then be left for another 30 minutes and another reading taken. A rise in pressure means that there is still a certain amount of moisture present.

Under no circumstances should the system compressor be used for the evacuation of the system.

A comparison of vacuum gauge graduations is given in Figure 107. Note that 1 torr = 1 mm Hg = 1000/zm Hg and micrometres are referred to as microns.

vacuum-pump

Figure 108 shows a typical arrangement for connecting a vacuum pump for deep evacuation.

Typical deep vacuum pump arrangement

Typical deep vacuum pump arrangement

During the evacuation of the system the evaporator fan(s) may be operated and defrost systems switched to the heating cycle in order to raise the temperature in the evaporator. Heaters must not remain energized for excessive periods in case of overheating of the evaporator and possible damage. It is also very important to ensure that no parts of the system are isolated from the vacuum pump.

Figure 109 shows a triple evacuation arrangement. When the pump is operating, the isolator valve must be open, the service valves on the compressor in the midway position, the liquid shut-off valve at the receiver open, and the refrigerant cylinder valve closed. Both valves on the gauge manifold must be open. When breaking the vacuum with refrigerant vapour pressure, ensure that the pump isolating valve is closed.

Triple evacuation arrangement

Triple evacuation arrangement

Table 7 shows the pressure/temperature relationship for water. When evacuating a system, remember that there must be an adequate temperature difference .between the ambient temperature and that of the water to provide the heat necessary to vaporize the water.

table-7

Refrigerator Contaminants

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During installation the system should be kept as clean and dry as possible, with the least exposure to air. Avoid the entry of foreign matter such as solder fluxes, solvents, metal and dirt particles, and carbon deposits; the last are the outcome of soldering joints without passing a neutral atmosphere through the pipework.

Failure to take precautions may result in corrosion caused by air and moisture, or by an oxidizer under high temperature conditions. Other problems include:

1. Copper plating due to contaminated oil. It forms on bearings and planed surfaces in high temperature areas. Moisture in the system can also be the cause.

2. Freezing, which may take place in the system components if the correct dehydration procedure (evacuation) is not adopted.
3. Sludging, that is the chemical breakdown of oil under high temperature conditions in the presence of non-condensables.

The chemical breakdown or thermal decomposition of both refrigerant and oil in temperatures in excess of 150o C (300o F)  is more likely to occur with refrigerants R22 and those in the R500 group. In the presence of hydrogencontaining molecules the thermal decomposition produces hydrochloric and hydrofluoric acids, a condition which is wholly undesirable if hermetic or semi-hermetic compressors are employed. For this reason it is imperative that the evacuation period is adequate.

Refrigerator Circuit Breakers

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A fuse element provides protection by destroying itself and must be replaced. It cannot be tested without destruction; therefore the result of a test will not apply to the replacement.

The circuit breaker (CB) is an automatic switch that will open in the event of excess current and can be closed again when a fault is rectified. The switch contacts are closed against spring pressure, and held closed by a form of latch arrangement. A slight movement of the latch will release the contacts quickly under the spring pressure to open the circuit; only excessive currents will operate it.

Two types exist: thermal and magnetic.

Thermal

The load current is passed through a small heater, the temperature of which depends upon the current it carries. The heater will warm up a bimetal strip. When excessive current flows the bimetal strip will warp to trip the latch mechanism.

Some delay occurs owing to the transfer of heat produced by the load current to the bimetal strip. Thermal trips are suitable only for small overloads of long duration. Excessive heat caused by heavy overload can buckle and distort the bimetal strip.

Magnetic

The principle used in this type is the magnetic force of attraction set up by the magnetic field of a coil carrying the load current. At normal currents the magnetic field is not strong enough to attract the latch. Overload currents will increase the force of attraction and operate the latch to trip the main contacts.

A typical magnetic circuit breaker is shown in Figure 101.

magnetic-circuit-breaker

Written by sam

November 12th, 2009 at 2:09 pm

Refrigerator Fuses

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Three types of fuse are used (Figure 100); the rewireable or semi-enclosed fuse; the cartridge fuse or fuse link; and the high rupturing capacity (HRC) fuse.

Rewireable fuse

This comprises a fuse holder, an element and a fuse carrier. The holder and carrier can be made of porcelain or bakelite. The fuse holder is colour coded as follows:

45 A  green
30 A  red
20 A  yellow
15 A   blue
5 A     white

This type of fuse is popular for domestic appliances and small commercial units because of cheapness and ease of replacement. It is not recommended for commercial refrigeration duty because of these disadvantages:

1. The fuse carrier can be loaded with the wrong size fuse wire.
2. The fuse element tends to weaken after long usage owing to oxidation of the wire by heating in air. This causes it to fail under normal conditions, i.e. normal starting current surges are sensed by the fuse as an overload.

fuses

3. The fuse holder and carrier can be damaged as a result of arcing in the event of an overload.

Cartridge fuse

This consists of a porcelain tube with metal end caps to which the fuse element is attached. The tube is filled with silica.

These fuses may be used in plug tops with 13A socket outlets and in distribution boards. They are recommended for refrigeration duty. They have the advantages over the rewireable types of not deteriorating, of being more accurate in breaking at the rated value, and of not being subject to arcing.

HRC fuses

This is a sophisticated version of the cartridge fuse. It is normally used for the protection of motor circuits in commercial and industrial installations. It consists of a porcelain body filled with silica and with a silver element; the body terminates in lug-type end caps.

These fuses are fast acting, and can discriminate between a starting surge and an overload. An indicating element shows when the fuse is ruptured.

Rating

The selection of fuse ratings depends on the full load current, the locked rotor current and the cable size. The current ratings for tinned copper wire are shown in Table 6.

Fusing factor

Different types of fuse provide different levels of protection. Rewireable fuses are slower to operate and are less accurate than cartridge types.

To classify the protection devices it is important to know the fusing performance. This is achieved by the use of a fusing factor:

cal-1

Here the fusing current is the minimum current causing the fuse to rupture, and the current rating is the maximum current which the fuse can sustain without rupturing. For example:

1.  A 5 A fuse ruptures only when 9 A flows; it has a fusing factor of 9/5 or 1.8.

table-6

2. The current rating of a cartridge fuse is 30 A and the fusing factor is 1.75; the fuse will rupture at 30 • 1.75 = 52.5 A.
3.  The current rating of an HRC fuse is 20 A and the fusing factor is 1.25; the fuse will rupture at 20 x 1.25 = 25 A.

It must therefore be realized that a fuse is rated at the amount of current it can carry, and not the amount at which it will rupture. Rewireable fuses have fusing factors of approximately 1.8; cartridge fuses of between 1.25 and 1.75; HRC fuses of up to 1.25 maximum; and motor cartridge fuses of 1.75.

Written by sam

November 12th, 2009 at 12:47 pm

Refrigerator Drive Couplings

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Instead of employing a flywheel and drive options, compressors may be designed for direct coupling.

The coupling on small compressors may be of a rigid flange type. Larger compressors will have a more elaborate resilient coupling similar to that shown in Figure 95, consisting of a flange assembly, a centre ring assembly, a number of forged links, and long and short mounting bolts with locking nuts and washers.

refrigerator-drive-coupling

Assembly and alignment

Whenever a coupling is fitted, correct alignment is essential to prevent any undue stresses on the compressor, motor shaft and bearings, and to eliminate vibration during operation, which is often at high speeds. Should it be necessary to change a motor or compressor, it is important to note the arrangement of bolts, washers, forged links and nuts during the disassembly, since they must be replaced in the same order.

The most accurate method of aligning a coupling is by using a level indicator gauge, but it can be accomplished with a straight edge and caliper. Both methods will be dealt with in turn. It is advisable to align the coupling to the tolerances stated by the manufacturer of the equipment.

Level indicator gauge

Test for angular misalignment

1 Mount the indicator on the left hand flange as shown in Figure 96, with the stem on the face of the right hand flange.
2 Rotate the equipment, notion the maximum and minimum indicator readings.
3 Move the equipment as necessary to reduce the total indicator reading to 0.0508 mm (0.002 in) or less for each millimetre (inch) of diameter at the indicator stem.

angular-misalignment

Test for parallel misalignment

1 Set the indicator on the outer surface of the flange as shown in Figure 97.
2 Rotate the equipment, noting maximum and minimum readings.

paralel-misalignment

3 Move the equipment as necessary to reduce indicator readings to a minimum, taking care not to disturb the setting made in step 1.
4 The coupling should be rotated several complete revolutions to make sure that no endwise creep in the connected shafts is measured.
5 Tighten all bolts. Recheck tightness after several hours operation.

Calipers and straight edge

1 Place a straight edge on the flange rims at the top and sides (Figure 98). When the coupling is in alignment, the straight edge should rest in full contact upon the flange rims.
2 Check the dimension with inside calipers on at least four points of the circumference of the flanges. If these dimensions are within 0.39 mm (6&4 inch) of each other, the alignment is satisfactory. Refer to the manufacturer’s instructions if available.
3 Move the equipment and repeat steps 1 and 2 if necessary.
4 Tighten all bolts. Recheck tightness after several hours operation.

alignment-caliper

Appearance

When the coupling is in correct alignment, both laminated ring assemblies will be in a perfect plane at right angles to the shaft centre line.

When the equipment is operating at full speed, both laminated rings should have a distinct and clearly defined appearance when viewed from both the top and the side. Should they have a blurred appearance, the coupling needs to be realigned.

Mounting

During the coupling alignment it may be necessary to raise the level of the drive shaft to coincide with that of the compressor shaft. This can be achieved by placing shims (Figure 99) beneath the motor mounts; the shims are holed for location of motor mounting bolts. It will then be necessary to conduct a parallel alignment test several hours after the equipment has been operating in case the shims have bedded down, thereby putting the coupling out of alignment.

When a coupling has been correctly aligned and operated satisfactorily, holes should be drilled through the motor mounts and mounting base. These can be either tapped to receive a bolt or left untapped for insertion of a dowel pin. Securing will ensure that no angular movement of the driver shaft is possible.

alignment-shims

Written by sam

November 12th, 2009 at 12:26 pm

Refrigerator Drive Belts

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Correct alignment of the flywheel and drive pulley, and correct tensioning of the drive belt(s), are most important during the installation or replacement of belt(s) or motor. Incorrect alignment and tensioning can result in excessive belt wear, loss of compressor efficiency, and motor failure due to faulty motor or compressor drive shaft bearings.

Alignment

This may be carried out with a piece of string or preferably a straight edge (AB in Figure 91), aligning the drive motor pulley to the compressor flywheel. It is essential that the areas X and Y are aligned accurately. Any variation in the pulley or flywheel thickness must be compensated for when aligning the flywheel and pulley faces (area Z).

refrigerator-belt-alignment

Selection

All drive belts are subject to bending and compressing, which will inevitably result in stretching. Centrifugal force also adds to belt stress. The distance between the flywheel and the pulley plays an important part in the amount of stress and force generated during the operation of the motor and compressor.

It is important, therefore, that drive belts are selected to the design characteristics, i.e. that they are of the correct length, thickness and section. Any increase in centrifugal force will tend to increase the amount of stretch on a belt. A thin belt will create a greater force.

Tension

A loosely fitted belt increases centrifugal force, leads to excessive wear and causes the belt to slip. A belt fitted too tightly may break and have a side effect on shaft bearings.

It is said that a belt is too slack when it jumps off the pulley. However, this is an overstatement because even if the belt remains in place it may still be subject to unnecessary flexing, wear and stress. A recognized method of adjusting the belt tension is to allow a maximum of 25 mm (1 in) deflection at the position shown in Figure 92.

Thickness

Examples of centrifugal force variation with belts of different thickness are shown in Figure 94.

refrigerator-belt-thickness

Written by sam

November 12th, 2009 at 11:31 am

Refrigerator Oil Pressure Failure Switch

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This is generally used with compressors incorporating oil pumps and on multiple compressor systems. The function of the control is to stop the compressor(s) when the oil pressure developed by the pump falls below a specific level, or if the oil pressure fails to reach a maximum safe level within a desired period after starting.

The oil pressure, as measured with a gauge, is the sum of the crankcase pressure (suction) and the pressure developed by the pump. The failure switch should be set to operate at the ‘useful’ pressure, and not at the total pressure. To determine the useful pressure (assuming correct compressor lubrication), subtract the suction pressure from the total pressure. Since the oil pump functions only when the compressor operates, the total pressure will be equal to the crankcase pressure during off cycles.

When the compressor starts, the oil pressure rises to the cut-in point of the switch. The differential switch will open and break the circuit to the heater, and the compressor will operate normally (see Figure 90).

If the useful pressure does not rise to the cut-in point within the time limit (60 to 180 seconds), the differential switch contacts will not open and the heater will stay in circuit. This causes a bimetal strip in the timing relay to warp and open the timing contacts, which will break the circuit to the starter coil and stop the compressor. Similarly, if the useful pressure falls below the cut-in point during operation, the differential switch will close and energize the heater. The timing relay will stop the compressor after the time delay characteristic of the switch. Controls are available with 60 and 90 seconds delay, but it must be realized that the time is not variable.

Most current production oil pressure failure switches are provided with terminals for the connection to a crankcase heater. Since the terminal arrangements vary, reference should be made to the wiring diagram provided with the switch.

oil-failure

Written by sam

November 12th, 2009 at 11:22 am

Refrigerator Filter Drier

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This is installed in the liquid line of the system after the receiver. Construction is generally in the form of a tube which contains coarse and fine mesh filters. These prevent foreign matter such as dirt, metal filings and carbon sludge circulating with the refrigerant. The tube also contains a drying agent or dessicant which will absorb any moisture in the refrigerant (see Figure 89).

A burn-out drier is specifically intended for installation in both the liquid line and the suction line of a system following the replacement of a hermetic or semi-hermetic compressor in which the motor windings have burnt out. This type of filter drier has the extra ability to retain acids which could be present in the oil residue entrained in parts of the system.

refrigerator-filter-driers

Written by sam

November 12th, 2009 at 11:11 am

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