Electrical Services
Air conditioning equipment must be connected to an electrical service. The type of air conditioning equipment used is generally determined by the type of electrical service available to operate it. The air conditioning technician must have knowledge of different types of electrical services.
POWER GENERATION
In the United States and Canada, power is generated as a three-phase 60-hertz voltage. The term hertz means 60 cycles per second. This means that the voltage increases from zero to its maximum positive value, returns to zero, increases to its maximum negative value and returns to zero 60 times each second. Figure 5–1 shows one complete cycle of AC voltage.
The term three phase means that there are three separate voltage waveforms produced by the alternator. An alternator is a generator that produces AC voltage. For the alternator to produce the three phases, the internal windings of the alternator—called the stator—are wound 120° apart. Figure 5–2 illustrates the windings of an alternator. The moving part of the alternator, called the rotor, is actually a large electromagnet. When the magnet is turned, the magnetic field cuts through the windings of the stator and induces a voltage into them. The amount of voltage induced is controlled by the strength of the magnetic field, and the frequency or hertz is controlled by the speed of the rotation of the magnet. Because the windings of the stator are
physically wound 120° apart, the three voltages are 120° out of phase with each other. The windings of the stator are connected to form one of the two basic three-phase connections. These connections are the delta and wye.
WYE CONNECTION
The wye connection is also referred to as the star connection. This connection is made by joining one end of each of the windings together as shown in Figure 5–3. The connection shown in Figure 5–4 is a wye connection that has been drawn schematically to make it easier to see and understand. Notice how one end of each of the windings is joined at the centerpoint. The wye connection can be used to provide an increase in the output or line voltage. The phase voltage is the voltage produced across one of the windings. The line voltage is the voltage produced across the output points of the connection. Figure 5–5 shows a wye connection connected to a three-phase load bank. Ammeters and voltmeters are used to illustrate the differences between phase values and line values. Notice that the phase value of voltage is measured from the output of the winding, at point C, to the centerpoint of the wye
connection at the point labeled O. The line value is measured across two of the output points of the con- nection (B&C). The phase current meter is inserted in the winding of the alternator, and the line current meter is inserted in the output line. Notice also that the two ammeters are indicating the same value of current. In a wye connection, phase current and line current are equal. The voltages, however, are not. The line voltage in a wye connection is 1.732 times greater than the phase voltage (1.732 is the square root of 3). The reason for this voltage increase is that the voltages are 120° out of phase with each other. Figure 5–6 shows a diagram to illustrate this. Because the three voltages are out of phase with each other, they will be added. Vector addition must be used, however, because of the 120° phase shift. If three voltages are shown in a length that corre- sponds to 120 volts, and a resultant is drawn to the point of intersection, it will be found that the length of the resultant corresponds to 208 volts. The 120° phase shift between voltages is the reason the two Wye-connected systems often use a fourth conductor connected to the center of the connection. This conductor becomes the neutral, Figure 5–7. Notice in this connection that the voltage between any line and neutral is the phase voltage or 120 volts, Vector diagram of the phase and line values in a three- phase system. (Source: Delmar/Cengage Learning) and the voltage between any two of the lines is 208 volts. The 208/120-volt three-phase connection is very common in industry and commercial buildings. Another very common three-phase
four-wire connection is shown in Figure 5–8. This is a 480/277-volt connection. Two hundred seventy- seven volts is often used in large stores and office buildings to operate the fluorescent lights while the 480-volt is used to operate large air conditioning systems. The 120-volt connections are provided by transformers that step down the 480 volts to 120 volts.
DELTA CONNECTION
The next connection to be covered is the delta. A schematic diagram of a delta connection is shown in Figure 5–9. This connection gets its name from the fact that it looks like the Greek letter delta (.1).
Figure 5–10 shows a delta system connected to a three-phase load bank. Ammeters and voltmeters are used to illustrate the differences in phase and line values of voltage and current. Notice that the values of phase voltage and line voltage are equal for the delta connection. One of the rules for three-phase systems states that line voltage and phase voltage are equal in a delta connection. The ammeters, however, are not equal. In a delta con- nection, the line current is 1.732 times greater than the phase current. This is the reason that the delta connection is so popular in industry. The current flow through the windings of a transformer are less than the line amps if the transformer bank is connected in delta.
HIGH-LEG SYSTEM
Figure 5–11 illustrates another common type of transformer connection. This is a 240/120-volt system with a high-leg. Three transformers are connected to form a delta connection. One of the transformers is larger than the other two, however, and is center tapped. The large transformer must be able to supply power for both three-phase and single-phase loads. The other two transformers sup- ply power for the three-phase loads only. If the phase voltage of the transformers is 240 volts, the voltage between any of the three lines is 240 volts. If the center-tap connection is used as a neutral conductor, however, the voltages between L2 and neutral, and L3 and neutral will be 120 volts. Therefore, L2, L3, and neutral are used to supply 240/120 volts for single-phase loads. Care must be taken not to connect a 120-volt device across L1 and neutral. Line L1 is known as a high leg and a voltage of about 208 volts exists between these two points.
OPEN-DELTA SYSTEM
Another type of three-phase service is known as the open delta. The open-delta system has the advantage of needing only two transformers to provide three-phase voltage. This connection is often used when the amount of three-phase power needed is low, or if the power needs are expected to increase in the future. The open-delta connection, however, does have some disadvantages. The total
output power is only 84% of the combined rating of the transformers. If the two transformers shown in Figure 5–12 each have a power rating of 25 kVA (kilovolt amps), the total delivered power of this connection is only 42 kVA (25 + 25 = 50) (50 X 84% = 42). If at a later date the power requirements increase, a third transformer can be added to close the delta. The total output power of this connection is the combined rating of all three transformers. In this case it will be 75 kVA (25 + 25 + 25 = 75).
SINGLE-PHASE SERVICE
A single-phase 240/120-volt system can be obtained by connecting a single transformer to two lines of a three-phase system. The primary of the transformer shown in Figure 5–13 is connected to two of the three-phase lines of the power company.
The secondary voltage of the transformer is 240 volts. The secondary winding of the trans- former is center tapped. The center tap is grounded and becomes the neutral conductor. If the volt- age across the entire secondary is measured, it will be 240 volts. If the voltage between either of the secondary leads is measured to the center tap, it will be 120 volts. The reason this is true for single- phase and not three-phase is that the voltages of the single-phase system are in phase with each other, Figure 5–14. Because the transformer center tap
is the neutral conductor, it will be 120 volts more positive than one side of the secondary winding and 120 volts more negative than the other side of the secondary winding at a particular point in time. Because these two vectors are in phase or are in the same direction, they will produce a total voltage of 240 volts.
PANEL BOX
Regardless of the type of service used, connection will be made at a fuse or circuit-breaker box. Figure 5–15 shows a 150-amp single-phase circuit-breaker panel. Circuit breakers are made in different sizes and types. Figure 5–16 shows three different types of circuit breakers. The single-pole breaker is used for connecting a 120-volt circuit, the two-pole breaker is used for connecting a 240-volt single-phase circuit, and the three-pole breaker is used for connecting a three-phase circuit. The three-pole breaker must be used with a three-phase circuit-breaker panel and cannot be used in a single-phase panel.
When a 120-volt connection is to be made, cable is brought into the panel. A two-conductor romex cable contains three wires—a black, white, and bare copper. The bare copper wire is the grounding wire or safety wire and is not considered a circuit conductor. Only the black and white wires are considered to be circuit conductors. The black wire is used as the “hot” conductor and the white wire is used as the neutral. Figure 5–17 shows a 120-volt single-phase circuit connected into the panel box. Notice that the black wire is connected to the circuit breaker, and the white wire is con- nected to the neutral bus. Notice also that the bare copper wire is connected to the neutral bus with the white wire.
When a 240-volt connection must be made, a two-pole circuit breaker is used. If the connection is to use only two-circuit conductors as shown in Figure 5–18, the black wire connects to one pole of the two-pole breaker. The National Electrical Code® does not permit a white wire to be used as a hot circuit conductor. For this reason the wire must be identified by wrapping a piece of colored tape around it. The tape can be any color except white, gray, or green. Black or red tape is generally used. The identified conductor is then connected to the other pole of the two-pole breaker. The bare copper wire is connected to the neutral bus.
If a 240-volt three-wire circuit is to be connected to the panel, a three-conductor cable is used. The three-conductor cable contains four wires—a black, red, white, and green. The green is the grounding or safety wire and is not considered a circuit conductor.
Figure 5–19 shows a 240-volt three-wire connection. The black and red wires are connected to the two poles of the circuit breaker. The white and green wires are connected to the neutral bus.
When a three-phase panel connection is made, a three-pole circuit breaker is used. There may or may not be a neutral depending on the type of circuit. For example, a 208/120-volt connection would use a fourth wire connected to the neutral bus. A 440-volt straight, three-phase connection would use only three conductors connected to a three-pole breaker.
FUSES
Circuit breakers are not the only means used to provide circuit protection. Fuses are still used to a great extent. Fuses are rated in two ways—by voltage and current. The voltage rating of a fuse indicates the amount of voltage the fuse is designed to interrupt without arcing across. Although fuses can be obtained that have ratings of several thou- sand volts, the most common fuses used in the air conditioning field are 250 volt and 600 volt. The 600-volt fuse is longer to provide a greater distance between the two contact ends if the fuse link should blow. The extra length is needed at higher voltages to prevent arc-over.
Figure 5–20 shows a type of fuse that uses a replaceable link. When this fuse blows, the fuse cartridge can be taken apart and the fuse link replaced. This type of fuse is more expensive to purchase, but it could be a savings if the fuse has to be replaced frequently.
Fuses used for circuit protection are made in standard ampere ratings. Figure 5–21 shows these ratings as taken from the National Electrical Code®. Fuses for air conditioning and refrigeration equipment are normally sized at 175% of the rated full- load current of the motor. If this does not permit the motor to start, however, compressors can be fused
as much as 225% of their full-load running current. If the fuse size needed does not correspond with one of the standard fuse sizes, the next smaller size fuse will have to be used. For example, assume it has been determined that a fuse rating of 130 amps is needed. The standard ratings chart for fuses shown in Figure 5–21 does not list a 130 amp fuse. There- fore, the closest standard rating less than 130 amps
is 125 amps. A 125-amp fuse will be used. Notice that fuses can be sized as much as 225% of the full- load current of the compressor. Fuses are sized this much above the running current of the motor to permit the fuse the ability to withstand the starting current of the motor. Fuses are designed to protect the circuit against short circuits; they are not used to protect the motor from overloads.
Overload protection for the motor is provided by the overload relay, which will be covered later, or by dual-element time delay fuses. Dual-element time delay fuses are designed to provide both types of protection. Figure 5–22 illustrates a dual-element time delay fuse. The fuse link is designed to open quickly in the event of a short circuit. Short circuit cur- rents are generally several hundred times the rating of the fuse. The fuse link is also designed to allow some amount of overload for a short period of time. This time delay permits the motor to start without opening the fuse link. The overload protection is pro- vided by a solder link. The solder is intended to melt at a specific temperature and a spring is used to pull the link apart. Although the motor starting current is greater than the overload protection would permit, it takes time for the solder to melt, permitting the motor to start.
FUSED DISCONNECTS
Fused disconnects provide both a disconnect switch and fuse holders. Figure 5–23 shows a fused disconnect used for three-phase circuits. Fused disconnects,
like fuses, have standard ratings. The standard sizes for fused disconnects are shown in Figure 5–24. The rating of the disconnect indicates the maximum size of fuse that can be used in that enclosure. For example, assume the 125-fuse discussed earlier in this unit is to be mounted in a disconnect. Because the fuse size is greater than 100 amps, it cannot be mounted in a 100-amp enclosure. The next standard size enclosure is 200 amps. The 125-amp fuses will have to be mounted in a 200-amp disconnect.
When servicing equipment, it is often necessary to turn off the power to the equipment. When this is necessary, certain precautions should be taken by the service technician. Remember that your life is your own and do not trust someone else not to turn the circuit back on while it is being serviced. Most industries provide a tag that is hung on the disconnect while it is being serviced. A paper tag, however, cannot stop someone from turning the power back on. For this reason, a small padlock should be used to lock the disconnect in the off position. If a lock is not available, the fuses should be removed with fuse pullers. There is no such thing as being too safe when working with high-voltage electricity.