Air conditioning systems that are designed for residential and light commercial use are divided into three sections. Within each section there are many components. The main components are the condensing unit, the evaporator unit, and the thermostat. In some geographic areas, some of the names might be slightly different. For instance, some might call the evaporator unit the air handler. Both are correct.
The condensing units have a specific function, heat rejection. With the rejection of heat many things happen. Figures 3-1 through 3-7 show some condensing units in use today. You can see that they all look different, but they all are designed for the specific task of rejecting heat.
Fig. 3-1. Square condensing unit with vertical discharge.
Fig. 3-2. Square condensing unit, ground level on concrete pad.
Fig. 3-3. Typical arrangement of condensing units on ground level in a multi-family structure.
Fig. 3-4. Condensing unit mounted on a platform attached to the structure, where the building code governs flooding areas.
Fig. 3-5. Two condensing units located in a private home on a solid roof area over the screened patio area.
Fig. 3-6. Typical package unit installed on a mobile home. Notice the rainshield that is connected to the home and the unit. This piece of sheet metal protects the ductwork hookup to unit.
Fig. 3-7. Another typical mobile home installation of a package unit. Notice the window unit that is being used to cool the addition.
Condensing units are installed wherever the builder thinks it will be cost effective. Because some of you might live in a house, some a condominium, some a duplex, we have written this book for all installation areas. Those of you that can point to your condensing units are lucky. Many owners don’t even know where their condensing unit is located. On a service call, you might spend some time trying to locate the customer’s unit. It might be under the parking area. Sometimes it is placed on the roof in clusters with other units. This gives you an idea that in certain areas, there might be a little time consumed in finding the condensing unit. When you find it and it is located in a unique place, make a notation on the evaporator unit or entrance panel where the unit is located.
Condensers can transfer heat using air or water as a transfer medium. Air-cooled condensing units are usually located in the outdoor air to be efficient. Water-cooled units are different. The heat transfer medium can be piped to the unit regardless where it is. A package unit is a unit that has only one section. The condensing unit, evaporator and sometimes the thermostat are located within a single cabinet. I tell you this to save you the embarrassment of looking for an air-cooled condenser on a water-cooled unit.
Before you open the cabinet of the condensing unit, turn off the electrical power supply to it. Always remove the panel slowly for many reasons. You might come face to face with an animal or a pressure refrigerant line about to burst. With the panel removed, you can look inside and begin to identify some of the component parts that make the condensing unit operate.
The most expensive part in the condensing unit is the compressor. It is the heart, the pump that circulates the refrigerant as the heart does with the blood. A hermetic compressor is the most common one found in residential and light commercial air conditioning and refrigeration systems. In Fig. 3-8 a typical hermetic compressor is illustrated. They are sealed units and cannot be serviced internally in the field. There are re-building shops that have the equipment to cut them open, replace defective parts an weld the shell together again. These compressors are suction cooled. This means that enough cool gas must return to the compressor from the evaporator coil to maintain a desired compressor operating temperature. An electric motor sealed in the shell drives a crankshaft and one or more pistons to operate a reciprocal compressor. In the case of a hermetic rotary type of compressor, vanes similar to those on a water pump impeller or vanes on an oil pump do the pumping instead of the pistons.
Fig. 3-8. Typical hermetic compressor that is used in most of the residential condensing units.
Located on the shell are pieces of pipe that have been welded to the body to give access to the high side and low side of the compressor. There might also be small tubes that can be used to charge the system or install pressure operated controls. The larger of the pipes is the low side, or suction side of the system. The smaller pipe is the discharge side, or hot gas line. These short stubs of pipe may be either steel or copper. The refrigeration piping is soldered to them.
Hermetic compressors can be used in small reach in refrigerators and are rated at fractional hp (horsepower). They range from 
to 60 ton cooling capacity. The larger tonnages are used in commercial units. When deciding whether you should have a compressor re-built, first check the price of a new one. Make sure the condition of the rest of the condensing unit warrants an investment as large as a new compressor. If the condenser coil is rotten and the whole cabinet is held together by rust, the customer might want to replace the whole thing. Good customer relations builds confidence in a technician.
The semi-hermetic compressor is totally different in its construction than the hermetic; this can be seen in Fig. 3-9. The compressor is constructed of a heavy casting. It is bulky and heavy to handle due to the iron content in its body. You will notice that this compressor has nuts and bolts holding it together. This advantage allows the unit to be re-built on a job site. It too has a drive motor (electric) that turns the parts of the reciprocal compressor. Not many of these compressors will be found in residential applications, yet there can be some larger homes or estates that use this kind. Many light commercial applications also use this type of compressor.
Fig. 3-9. Semi-hermetic compressor used mostly in commercial equipment.
There are many of these old-timers chugging around the world and still doing the job. Some of you perhaps never saw one of these and never will, but you should know about it in the event you need one for a specific application. These compressors come in a variety of sizes from one (1) hp up. They can be used for air conditioning or refrigeration application. The biggest advantage of this type compressor is the choice of driving power you want. The compressor has a drive shaft protruding from it. The shaft can take either a pulley or a coupler. For instance, I’ve serviced a unit such as this that was driven by a six-cylinder internal combustion engine fueled by propane. The greatest advantage of this application is the capacity control. With the engine throttle linked to the thermostat, the engine idles when there isn’t a load demand. This type compressor can be driven by electric motor, internal combustion engine, or turbine. Another great advantage to this compressor is that it can be installed in an area that doesn’t have ample electrical power to drive large compressors.
For all the good, there has to be a little bad. The two biggest drawbacks about the open-drive compressor is its physical size and the critical alignment. As you can see in Fig. 3-10 there has to be a close alignment between the compressor and the driving force. If the alignment goes out, beyond specifications, the front shaft oil seal will begin to leak. Both refrigerant and oil will exit here. Alignment with a dial gauge at regular intervals is prudent.
Fig. 3-10. Open-drive compressor can still be seen in the field today in certain areas.
Two types of terminal connections are used with hermetic and semi-hermetic compressors. On the hermetic compressors, pins or terminals are sealed within bakelite or ceramic. The semi-hermetic compressors use terminal boards with threaded bolts being used for the terminals. The board has an ‘O’ ring on both sides of it. The ‘O’ ring is designed to compress against the board and prevent the leakage of refrigerant and oil around the terminals.
With the compressor being the most expensive component in the system, it is wise to be sure it is bad before you condemn it. For this reason, you should learn a systematic method of diagnosing the compressor. You will need a good ohmmeter that can measure from one ohm through 20 ohms. A meter such as this can be purchased in the price range from $20 to $125 depending upon the quality. The homeowner can get by with the less expensive one due to the fact his will not be subjected to the amount of usage the service technician will give his.
This procedure is followed if the compressor doesn’t operate when called to do so. Always remember safety comes first. Before opening the condensing unit, turn off the electrical power supply to the unit. With the service panel removed, look with your eyes before you touch anything. There should be some type of flash cover enclosing the terminals of the compressor. This is a basic safety device to protect the service technician from electrical shock when the unit is operating, and it protects the technician from pressure-driven oil if one of the terminals should fail and blow out of its mount. Don’t assume that oil in a unit is clean. In most units the refrigeration oil is clean; however, in some instances, the oil has become contaminated. The formation of sulfurous acid might have occurred inside the inoperable unit. This acid can be dangerous to your skin and eyes. For this reason, don’t assume anything; be sure and careful. With the cover removed, you will notice that the terminals are arranged in the approximate order as shown in Fig. 3-11. The following is a list of electrical failures that you will test for.
Fig. 3-11. Typical compressor terminal connections arrangement.
a—Grounded compressor (short circuit). This condition takes place when the insulation of the drive motor windings leak the electricity to the steel compressor body. Blown fuses result.
b—Open-winding. A condition that occurs when the conductor of one of the motor windings parts.
c—Locked rotor. This condition happens when either the crankshaft bearings seize due to lack of lubrication, or a compression component breaks within the compressor shell jamming the crankshaft. In the case of a single-phase unit, the same locked-rotor condition will be witnessed if the. system has a defective starting component.
The multimeter enables us to diagnose internal electrical problems of the compressor. “Ringing out a compressor” means taking continuity tests. Make sure the power is off. If necessary, turn your meter to ac volts and check it out. Sometimes a disconnect leaves a blade engaged that has broken loose from the main control bar. After you check for voltage, mark the wires that are connected to the terminals so they may be returned to the same position when re-assembled. There are many ways to mark them: different color tape, black bands of electrician’s tape, notch with knife blade. Whatever works for you is suitable. The wires must be removed from the compressor to prevent voltage from other circuits. For instance, a 240-volt compressor might have a 120-volt condensing fan motor. It is possible that you might read the neutral leg of the 120-volt circuit as a grounded compressor.
Zero your meter. Make sure that it rests at infinity. This is done with the little screw at the base of the indicator needle. Then touch the two probes together and turn selector switch to X1000 scale. The needle should deflect to zero ohms. If not, make adjustments with the little knob to set your meter to zero.
Scrape the copper suction line on the compressor so it shines. This can be done with a knife blade or a piece of sand cloth. Place one probe on the clean copper surface, and the other one on a compressor terminal. Check each one by moving the probe to each terminal. If there is no deflection on the meter, the compressor is not grounded. If you have a reading, the compressor is bad and you need not check any further. The three terminals are electrically connected internally.
Place a test probe on terminal one. With the other probe touch terminal two. The meter needle should deflect. This shows there is a circuit. Repeat the procedure until the circuitry between the terminals is confirmed. There should be a reading across each pair of terminals.
The windings in a three-phase compressor are different than a single-phase compressor. In the three-phase compressor, you should read the same amount of resistance through the three windings. This is not true in the single-phase compressors. The reason is that the start-winding has more wiring turns in order to develop more torque on start up. The run-winding has a heavier gauge wire with fewer turns, thus the resistance readings will be different between the three windings. Knowing this, it is possible to identify the windings in a single-phase compressor.
RUN—The lowest reading of resistance—(about one ohm)
START—The middle reading of resistance—(about five to 22 ohms)
COMMON—The maximum resistance reading—(total of all windings)
The single-phase hermetic compressor has a fixed rotation of its electric motor. In three-phase applications rotation is very important. The compressor might have a directional oil pump that will not pump efficiently if it turns in the wrong direction. Rotation on three-phase motors may be reversed by reversing any two motor leads.
This can be done either at the motor starter or at the motor itself. It is easier to do at the starter or disconnect most of the time. Be careful not to cause trouble in another circuit or cause a cross-phasing. This occurs when you touch two phases together without a load circuit in between. Figure 3-12 shows a compressor being rung out, note the readings at the terminals.
Fig. 3-12. Example A shows approximate resistance readings when a compressor is being rung out. Example B shows the same readings with a different terminal arrangement.
The compressor hums but will not start. The overload relay usually opens the common winding either internally or externally, this allows the windings to cool and not get hot enough to melt. This is very apparent when you place your hand on the compressor shell. It is very hot to the touch, and it would be difficult to keep your hand on the compressor.
Turn off the power to the compressor.
Remove all extra machine wiring attached to the compressor motor terminals.
Ring out the compressor and label the common, start, and run pins, (single phase)
Secure line one to the run pin and line two to the common pin.
Place insulated jumper wire from run to start, (see Fig. 3-13)
Fig. 3-13. Capacitor start hermetic compressor motor diagram.
Turn on the power.
If the locked rotor breaks lose, the compressor will start and come up to speed in less than five seconds.
After compressor reaches full speed, remove jumper wire with compressor operating, leaving line one on the run pin and line two on the common pin.
To start a hermetic compressor motor which has a start capacitor, repeat the first four steps above. Step five is to place jumper wires between the run and start capacitors and from the start terminal to the start capacitor. It doesn’t make a difference on the hook-up of the jumper wires to the start capacitor terminals. See
Tables 3-1 through 3-3 show operating current information. With your ammeter you can determine if a specific horsepower motor is operating properly. The tables are also helpful in sizing overloads and heaters for motor starters, especially when the data plate on the machine is missing or not legible. Never exceed the rated amperage of a motor. If you do, the motor will have a short life due to overheating the windings. Always remember that these ratings are given to be the maximum, when the unit has the maximum load on it, whether the motor drives a fan or a pump. All amperage ratings are given for the maximum. For instance, you are topping the charge of a reach-in freezer, or a walk-in freezer that is operating at the time of charging at five degrees below zero (−5 degrees F.), if you bring the compressor up to maximum amperage at this time, the unit will draw excessive amperage when terminating its defrost cycle and entering the freeze cycle. In fact, the compressor might trip its thermo-overload at that time.
Table 3-1. Operating Currents for Three-Phase AC Motors at Full Load.
Table 3-2. Operating Currents for Single-Phase AC Motors at Full Load.
Table 3-3. Locked Rotor Amperages.
This is another method that can be used to try to break loose a locked rotor in a single-phase compressor.
Make sure the start and run capacitors have high enough ac voltage rating for the new applied voltage.
Remove wiring from the compressor motor terminals. Double the line voltage hook-up as in Fig. 3-14.
Fig. 3-14. Repeat steps six and eight for induction-start, induction-run motor. In step eight remove jumper wires and start capacitor with the compressor operating. Line one to run and line two to common remain (L1 and L2).
Make sure the power is off while you are doing the second step.
If the compressor is 120 volts ac, make the line one to number two 240 volts ac. If the compressor is rated at 240 volts ac, single-phase, make line one to number two 480 volts ac, single-phase.
Attach a jumper from run to capacitor and a second jumper from start to capacitor.
Take the jumper wire off the start terminal of the compressor.
Turn on the higher voltage.
Take the jumper wire and tap it about four times (one second each, to the start terminal). Do not touch the live voltage. Be careful and hold the insulation of the jumper wire.
Turn off the power, then repeat the above procedure in five minutes.
Turn off power and remove the wiring from the hermetic compressor motor terminals.
Study Fig. 3-15.
Fig. 3-15. Reverse rotation method to break a locked rotor. 46
Wire line one to common and line two to start. Use rated compressor line voltage.
Attach the jumper wire from run to capacitor and second jumper wire from start to capacitor.
Disconnect jumper from motor run terminal.
Turn on the line voltage and hold jumper wire by the insulation; then hold jumper to run for four seconds, four times at four-second intervals.
If motor does not reverse, repeat step six using 240 volts ac instead of 120 volts ac motor rated voltage. Use 480 volts ac instead of 240 volts ac for a motor rated at the higher voltage (single-phase).
Check the capacitor for a higher rated voltage. When you have completed steps one through seven using double line voltage, you have hot-shotted the compressor in reverse. Do not hot-shot in reverse until you have done steps one through seven with rated line voltage for the compressor. You might be able to break the locked rotor with normal line voltage. There is no need to strain the compressor motor unless absolutely necessary. If these steps don’t free the locked rotor, there is nothing else you can do in the field.
That covers the important electrical problems that you will find in the field with a compressor. The other malfunction of the compressor is mechanical failure.
When the compressor is in a refrigeration system and in operation, if the suction-side and high-side pressures are almost the same, the valves might be bad. An ammeter will tell you if the electric drive motor is doing the work for which it is rated. Without proper compression in the compressor, the amperage reading will be very low. The problem could be a piston not pumping. When a compressor is operating properly, it should draw an amperage close to its rated amount on the data plate. The suction line should be cool to the touch and sweating. This cool gas is needed to help cool the compressor windings. The small liquid line should be warm to the touch … not hot. If the liquid line is very hot when in the cooling mode, there is a problem with the unit. Remember, refrigeration equipment and air conditioning equipment have design temperatures and conditions. In different areas of the world there will be different designs in the equipment. In a low humidity area, the suction line might not sweat. If a unit is designed to attain 74 degrees F. conditioned space with an ambient of 95 degrees F., and it is being checked on a day when the ambient is 80 degrees F., the equipment will accomplish the 74 degree F. without any problem. Even if there was an inefficient compressor with a possible bad valve, it wouldn’t show that easily until the unit was being operated under its design conditions. Conditions such as the evaporator coil being clean or the condenser coil blocked with grass cuttings will affect the operation of the entire system. Another thing that I learned a long time ago, know what the engineer wanted the equipment to do. If you don’t know what it is supposed to do, how can you repair it? As a service technician, you will see many applications of the refrigeration theory, from food processing to industrial manufacturing. That is why it is important that you know what the unit is supposed to do before trying to make a repair.
