OBJECTIVES:
After studying this section, you should be able to...
• Explain what electrical loads are and their general purpose in heating, cooling, and refrigeration systems.
• Give examples of common loads used in heating, cooling, and refrigeration systems.
• Identify the symbols of common loads used in heating, cooling, and refrigeration systems.
• Explain the purpose of relays and contactors in heating, cooling, and refrigeration systems.
• Identify the symbols of relays and contactors in heating, cooling, and refrigeration systems.
• Explain the purpose of switches and the types used in heating, cooling, and refrigeration systems.
• Identify the symbols of switches in heating, cooling, and refrigeration systems.
• Identify the symbols and purpose of other miscellaneous controls in heating, cooling, and refrigeration systems.
• Identify the different types of wiring diagrams used in the industry and the purpose of each.
KEY TERMS:
- Contactor
- De-energized
- Disconnect switch
- Energized
- Factual diagram
- Fuse
- Heater
- Installation diagram
- Load
- Magnetic overload
- Magnetic starter
- Motor
- Normally
- Normally closed
- Normally open
- Pictorial diagram
- Pilot duty device
- Pole
- Pressure switch
- Push-button switch
- Relay
- Schematic diagram
- Signal light
- Solenoid
- Switch
- Thermal overload
- Thermostat
- Throw
- Transformer
INTRODUCTION
Because of the complexity of today’s air-conditioning, heating, and refrigeration systems, industry technicians should be able to read and interpret all kinds of wiring diagrams. Electric wiring diagrams contain a wealth of information about the electrical installation and operation of the equipment. The installation mechanic depends on the wiring diagram for the correct installation of the wiring to the unit. The technician uses the electrical diagrams as a guide in troubleshooting the electric system of a unit. It would be impossible for wiring diagrams to be composed of photographs of various components of the equipment. They would be too large and in many cases too complex due to the number of wires that are carried to certain devices. Thus, symbols are used in wiring diagrams to represent such system components as compressors, indoor fan motors, thermostats, pres sure switches, and heaters. Industry technicians must be able to identify most symbols and know where to look up the remainder. Most manufacturers use similar symbols for each type of electric component, although there are some minor differences in symbols between some major manufacturers. Thus, knowledge of the basic symbols is essential if you are to be successful in the industry.
We begin our study with a discussion of the various types of electric loads found in the industry and the basic symbol used for each device.
5.1 LOADS
Loads are electric devices that consume electricity to do useful work. Loads are devices such as motors (Fig. 5.1), solenoids (Fig. 5.2), resistance heaters (Fig. 5.3), and other current-consuming devices. The sizes of loads vary from devices with a small current draw, such as a light bulb, a small fan motor, and solenoids, to large motors that could use upward of 100 amperes.
(left) Electric motor; (right) A solenoid used
to operate a contactor
A resistance heater (Courtesy of Indeeco, St
Louis, MO)
Loads are the most important part of a heating, cooling, or refrigeration system because they do all the work in the system. Loads operate compressors, which compress and transfer refrigerant in a system. They operate fans, which move air. They operate the solenoid part of a relay, which starts and stops loads. Also, loads operate with other devices that perform useful work. Industry technicians should be able to recognize the common symbols for loads and know where to look up the symbols for little-used loads, because each electric wiring diagram is composed of symbols and their interconnecting wires.
In the following paragraphs, we will take a close look at several different kinds of loads used in the industry.
Motors
A motor is an electric device that consumes electric energy to rotate a device in an electric system. Motors are used in the industry to rotate devices such as compressors (Fig. 5.4), condenser fan motors (Fig. 5.5), pumps (Fig. 5.6), and other devices that require rotating movement. Motors are the largest and most important loads in heating, cooling, and refrigeration systems.
Compressor (Courtesy of Copeland, Sidney, OH); A condenser fan motor on a residential air- conditioning condensing
unit (Courtesy of American Standard Air Conditioning)
The symbols shown in Fig. 5.7 are the most common symbols used to represent motors.
A letter designation tells you what purpose the motor serves in the sys tem. Fig. 5.8 shows several symbolic representations of different uses of motors. Careful attention should be given to symbols representing motors because in some cases, a motor has an internal overload, as shown in Fig. 5.8(e).
FIG. 5.6 A centrifugal pump (Courtesy of Peerless Pump Co., Indianapolis,
IN)
FIG. 5.7 Symbols for an electric motor
FIG. 5.8 Symbols representing some common uses of motors: (a) Condenser
fan motor, (b) Evaporator fan motor, (c) Compressor motor, (d) Compressor
motor, (e) Compressor motor with internal overload.
Solenoids
The solenoid is a device that creates a magnetic field when energized and causes some action to an electric component such as a relay or valve. A common solenoid used to operate a relay is shown in Fig. 5.9. The solenoid is considered a load because it consumes electricity to do useful work. Solenoids are devices that control some element in a system. Solenoid valves are valves that open and close, stopping or starting a flow. Solenoid coils used in relays and contactors will be discussed later in this section. Some common solenoid valves are hot-gas solenoids, reversing-valve solenoids, and liquid-line solenoids. Fig. 5.10(a) shows a solenoid valve and a solenoid coil, and Fig. 5.10(b) shows its symbol.
FIG. 5.9 Solenoid coil used to operate relay
FIG. 5.10 (a) Solenoid valve with coil (Courtesy of Sporlan) (b) Symbol Heaters
Heaters are loads that are found in many systems and wiring diagrams. A heater takes electric energy and converts it to heat. In some cases, electric resistance heaters are used to heat homes. Heaters might also be used to heat a small object or area. The symbol for all heaters is the same. Only a letter designation tells you specifically why the heater is used. Fig. 5.11 shows the symbols used for heaters, along with some common letter designations.
Signal Lights
A signal light is a light that is illuminated to denote a certain condition in a system. The letter inside the signal light symbol denotes the color of the signal light, as shown in Fig. 5.12. Signal lights come in a variety of colors and are not limited to the colors shown in Fig. 5.12. A signal light is used to show that a piece of equipment is operating or that it is operating in an unsafe condition. Signal lights are usually energized when a piece of equipment or component is started.
FIG. 5.11 Symbols for commonly used electric heaters: (a) Heater,
(b) Crankcase heater, (c) Supplementary heater
FIG. 5.12 Symbols for signal lights showing the color of the light: (a),
Red, (b) Green, (c) Blue
5.2 CONTACTORS AND RELAYS
Contactors and relays are devices that open and close a set or sets of electric contacts by the action of a solenoid coil. The contactor or relay is com posed of a solenoid and the contacts. A relay is shown in Fig. 5.13. A contactor is shown in Fig. 5.14. When the solenoid is energized, the contacts will open or close, depending on their original position (that is. if they were open, they will close, and vice versa).
Relay (Courtesy of Siemens Electromechanical
Components, Inc.): Mechanical linkage; Solenoid coil; Normally-open contacts;
Normally closed contacts
Contactor
In an air-conditioning control system, we must have some method of con trolling loads. In most cases, a relay or contactor is used. Relays and contactors are widely used in control systems. Thus, it is essential that industry technicians be able to identify the symbols for relays and contactors.
The main difference between a relay and contactor is the size of the device. A contactor is simply a large relay. Usually, the devices are distinguished by their rated current flow. A contactor can carry 20 amperes or more. A relay is designed to carry less than 20 amperes. Contactors are commonly used where the ampere draw of a device is more than 20 amperes. A relay would rarely be used to carry over 20 amperes.
Contactors and relays play an important part in the control system of any air conditioner, refrigerator, or heater. For example, contactors and relays are used to stop and start different loads in a refrigeration system. Compressors, in most air-conditioning systems, are controlled by a contactor or magnetic starter. Relays can be used for pilot duty, that is, for con trolling another relay or contactor. The most important fact to remember is that most control systems have many relays and at least one contactor. These relays or contactors always control some load.
Relays and contactors are composed of three parts: the contact and the coil, or solenoid, and the mechanical linkage. The contact makes the electrical connections. Fig. 5.15 shows the symbol for a pole, or contact, of a relay or contactor. The term “pole” refers to one set of contacts. However, in some cases the relay or contact might have two or three poles, which means two or three sets of contacts. The coil or solenoid, the second part of the relay, is energized (voltage is supplied) and, through a magnetic field, closes the contact or contacts. Either symbol shown in Fig. 5.17 can be used to represent a relay or contactor coil. The symbol for the relay or contactor is the same if each has the same number of poles and if their purpose is basically the same, with the exception of the ampere rating of the device.
All symbols are usually shown in the de-energized position. This means that there is no electric potential to the coil of the device. Fig. 5.15 shows a “normally open” contact in the de-energized position.
FIG. 5.15 Symbol for a normally open pole of a relay. FIG. 5.16 (bottom)
Symbol for a normally closed pole of a relay or contactor.
FIG. 5.17 Symbols for relay or contactor coil; either symbol may be
used for each device.
FIG. 5.18 Normally closed and open set of contacts (Courtesy of Siemens
Electromechanical Components, Inc.)
The term normally refers to the position of a set of contacts when the device is de-energized. Fig. 5.15 shows a normally open set of contacts and Fig. 5.16 shows a normally closed set of contacts. Normally open contacts close and normally closed contacts open when the relay or contactor is energized. In Fig. 5.18, a relay is shown with normally open and closed contacts.
The terms “normally open,” “normally closed,” “energized,” and “de energized” are important in understanding relays and contactors on wiring diagrams. Fig. 5.19(a) shows a relay with two normally open contacts and one normally closed contact in the de-energized position (with no volt age to the coil). Fig. 5.19(b) shows the same contacts in the energized position (with voltage to the coil). In the de-energized position, the current will not flow through contacts 1 and 2, but current will flow through con tact 3. In the energized position, the current flow is through 1 and 2 but not through 3.
Fig. 5.19 Symbols showing de-energized and energized relays
5.3 MAGNETIC STARTERS
A magnetic starter is the same type of device as a contactor in terms of the ampere rating of the device. But the magnetic starter has a means of overload protection in it, whereas the contactor has none. Fig. 5.20 shows a picture of the magnetic starter and its symbol. The principle of operation of the magnetic starter will be covered in an upcoming section.
5.4 SWITCHES
An electric switch is a device that opens and closes to control some load in an electric circuit. Electric switches can be opened and closed by temperature, pressure, humidity, flow, or by some manual means. You must become familiar with the symbols used for switches because in most cases they control the loads in the system. The symbol will also indicate what is initiating the action of the switch.
FIG. 5.20 (a) Magnetic starter (Courtesy of Furnas Electric Company)
(b) …and its symbol
A manually operated switch is a switch that is opened and closed by manual force. Fig. 5.21 shows a simple manually operated switch. The poles of a manual switch are the number of contacts that are included in the switch. The throw indicates how the switch may be operated. For example, a single-pole—single-throw switch has one set of contacts and two positions: an open and a closed position, as shown in Fig. 5.21. A double- pole—double-throw switch has two sets of contacts and three positions, as shown in Fig. 5.22. Symbols for these two switches and for two other basic types of manual switches are shown in Fig. 5.23.
FIG. 5.21 Single-pole—single-throw manual switch
FIG. 5.22 Double-pole—single-throw manual switch
FIG. 5.23 Symbols for manual switches: (a) Single-pole— single-throw
switch; (b) Single-pole-double-throw switch; (c) Double-pole-single-throw
switch; (d) Double-pole-double-throw switch
FIG. 5.24 Three-pole fusible disconnect: (b) Symbol; (a) Switch
There are other types of manual switches used in the industry. The disconnect switch is used to open and close the main power source to a piece of equipment or load. Fig. 5.24 shows a three-pole disconnect switch and its symbol. The push-button switch, as shown in Fig. 5.25, is a switch used to open and close a set of contacts by pressing a button. The symbols for the normally closed and the normally open push-button switches are also shown in Fig. 5.25.
FIG. 5.25 Push-button switch: Symbol for normally closed push-button
switch; Symbol for normally open push-button switch
Fig 5.26 Symbols for heating and cooling thermostats: Heating thermostat;
opens on temperature rise; Cooling thermostat; closes on temperature
rise
The most important type of switch in a control system is the mechanically operated switch. Thermostats are mechanically operated switches used in most control systems. Thermostats are said to be mechanically operated because the temperature-sensing element moves a set of contacts by a mechanical linkage. Thermostats are designed for heating, cooling, or both. The cooling thermostat is designed to close on a temperature rise and open on a temperature fall. The heating thermostat is designed to open on a temperature rise and close on a temperature fall. The symbols for these two types of thermostats, shown in Fig. 5.26, indicate their function. Fig. 5.27 shows a modern thermostat.
FIG. 5.27 Thermostat (Courtesy of Honeywell, Inc.)
FIG. 5.28 Symbols for pressure switches: (a) Opens on rise in pressure; (a) Closes on rise in pressure
Pressure switches are used for different functions in modern control circuits. The purpose of the pressure switch determines whether it opens or closes on a rise or fall in pressure. The pressure range of the switch is not part of the symbolic representation. Fig. 5.28 shows the symbols for pressure switches. Letter designations in the symbols often denote the pressure ranges and purposes of the switches. Fig. 5.29 shows some common pressure switches used in the industry.
FIG. 5.29 Some common pressure switches
5.5 SAFETY DEVICES
Safety devices are important in today’s modem systems. Components are becoming more expensive each year. Thus, it is vital that these components be protected from adverse conditions such as low voltage, high ampere draw, and overheating. It is for this reason that you should become familiar with symbols for safety devices. Overloads and safety devices are some times a combination of a load and a switch. They differ from the relay in their purpose and overall design.
All motors are designed to operate on a certain current draw. If for some reason this rating is exceeded, the motor must be cut off immediately to prevent damage and possible destruction of the component. A burned-out motor is often caused by a malfunction in the safety devices.
The fuse is the simplest type of overload device. The fuse is effective against a large overload, but it is less effective against small overloads. The fuse is nothing more than a piece of metal designed to carry a certain load. Any higher load will cause the fuse to break the circuit. Fig. 5.30 shows two symbols for a fuse. Fig. 5.31 shows some common fuses in use today.
FIG. 5.30 Symbols for fuses;
FIG. 5.31 Some common fuses
The second type of overload device is designed to protect the motor against small and large overloads. This type is divided into two categories: thermal and magnetic. The thermal overload is operated by heat, and the magnetic overload is operated by magnetism, which is directly proportional to the current draw.
The thermal overload can be a pilot duty device, which breaks the control circuit and locks the motor out. The pilot duty types of overloads are most common on motors larger than 3 horsepower. The thermal overload can also be a line voltage device, which breaks the power line to the component being protected.
The bimetal element is the simplest of the thermal overloads. When it gets warm, it warps to open the circuit, as shown symbolically in Fig. 5.32. Some bimetal elements are furnished with heaters, as shown symbolically in Fig. 5.33. The heater allows the bimetal disc to react to an overload more quickly because the current flow is proportional to heat.
FIG. 5.32 Symbols for bimetal overload (closed and open); FIG.
5.33 Symbol for three-wire bimetal overload
The thermal overload relay, whose symbol is shown in Fig. 5.34, is a simple device with a thermal element and a switch that opens on a rise in temperature.
FIG. 5.34 Symbol for thermal overload relay; FIG. 5.35 Magnetic overload
device
The magnetic overload symbol is the same as the symbol for a relay with one normally closed contact. The current flow is relayed to the overload coil. Since current flow is proportional to the strength of the magnetic field, the relay can be designed to energize only on a high current draw. Fig. 5.35 shows a magnetic overload and its symbol. The letter designation of this device will distinguish between the magnetic overload and the common relay.
5.6 TRANSFORMERS
The transformer decreases or increases the incoming voltage to a desired voltage. In most air-conditioning control circuits, it is not practical to pull large wires for a long distance. Therefore, a 24-volt control circuit, which is safer, less expensive, and a better method of control, is used. Fig. 5.36 shows a transformer and its symbol. The voltage is also given with the symbol in some cases.
FIG. 5.36 Transformer: (a) Symbol; (b) Transformer
5.7 SCHEMATIC DIAGRAMS
Most modern heating, cooling, and refrigeration systems are becoming more complex with more controls and safety devices. Advances in controls and control systems require you to be able to read schematic diagrams. If you are able to read schematic diagrams, you will know what the unit should be doing.
The schematic diagram is the most useful and easiest to follow of any electric diagram. The schematic diagram tells how, when, and why a sys tem works as it does. In most cases, service technicians use schematic diagrams to troubleshoot control systems. The schematic wiring diagram includes the symbols and the line representations so the user can easily identify loads and switches along with the circuits.
All electric circuits contain a source of electrons, a device that uses electron flow, and a path for the electrons to follow. In most cases, the source of electrons is an alternating current voltage supply. The device using the electron flow is a motor, heater, relay coil, or any other load device. The path for the electrons to follow is a wire or any type of conductor.
In the schematic diagram, the source of electrons, the power supply, is represented by two lines drawn downward and listed as L1 and L2, as shown in Fig. 5.37. There is a potential difference of 240 volts between L1 and L2. If a path is created between Li and L2, current will flow.
FIG. 5.37 Schematic diagram showing power supply
All electrical loads in the unit are placed between L1 and L2, along with the switches controlling the load. Fig. 5.38 shows a complete circuit in schematic form with a compressor and the switch (thermostat) that controls it. When the switch is closed, the compressor will run. In Fig. 5.38, the source of electrons is from L1 and L2, the path is the connecting wire, and the device using the electron flow is the compressor. The compressor operates when the thermostat is closed.
FIG. 5.38 Schematic diagram of a complete circuit: Control switch
(thermostat); Compressor; run capacitor
Fig. 5.39 shows a full schematic diagram similar to the diagrams you will be using on the job. All schematic diagrams are broken down into a circuit-by-circuit arrangement. Most schematic diagrams contain a legend that cross-references the components and their letter designation to the name of the component. Look at the legend in Fig. 5.39.
Legend:
|
Symbols: Wiring:
|
FIG. 5.39 Complete schematic diagram for small packaged unit (Courtesy
of Westinghouse Electric Corp.)
5.8 PICTORIAL DIAGRAMS
The pictorial diagram, also referred to as a label or line diagram, is intended to show the actual internal wiring of the unit. The pictorial diagram shows all the components of the control panel as a blueprint, including all the interconnecting wiring. It does not show the unit to scale, however. Components that are not shown in the control panel itself are shown outside the panel and labeled. The pictorial diagram is used to locate specific components or wires when troubleshooting from a schematic diagram. A typical pictorial diagram used in the industry is shown in Fig. 5.40.
FIG. 5.40 A typical pictorial diagram used in the industry (Courtesy
of Carrier Corporation, Syracuse, NY)
 |
|
It is difficult to determine from a pictorial diagram how a system operates, and only an experienced mechanic can follow a complex pictorial diagram. Thus, most air-conditioning technicians use the schematic diagram to find the cause of the problem. Then they use the pictorial diagram to locate the position of the component at fault. In cases where the wiring is simple, however, a pictorial diagram may be the only diagram furnished with the equipment.
The factual diagram consists of a pictorial diagram along with a schematic diagram. Many air-conditioner manufacturers supply factual diagrams so service technicians can locate the relay or component in the control panel.
5.9 INSTALLATION DIAGRAMS
The installation diagram is used to help the installation electrician to wire the unit properly. The diagram gives specific information about terminals, wire sizes, color coding, and breaker or fuse sizes. The diagram does not provide details about equipment operation because the electrician has no need for this information. Fig. 5.41 shows an installation diagram. The installation wiring diagram shows little internal wiring and is therefore almost useless to industry technicians.
SUMMARY
Loads are devices that use electricity to do useful work. Fig. 5.42 gives a review of the symbols used for solenoids, motors, and heaters, the typical loads found in the industry. Most symbols have some type of letter designation to identify more clearly the component referred to.
FIG. 5.41 A typical installation diagram: To Power Supply 208/240/1/60;
40VA 24V NEC Class 2 Circuit
FIG. 54.2 Review of symbols for loads: Motor, Solenoid, Heater
FIG. 5.43 Review of symbols used for contactors and relays: Relay; Contactor
or three pole relay; Magnetic starter
Loads are controlled by relays and contactors, which share the same symbol and perform similar tasks. The major difference between relays and contactors is the amount of current each can carry. If a compressor is being operated by a device, you can assume the device is a contactor. If a small fan motor is being operated by a device, you can assume the device is a relay. A relay is used for small loads, and a contactor is used for large loads. Fig. 5.43 reviews the symbols for some of these devices.
Relays and contactors are controlled by switches. Some of the switches used in the industry are manual, push-button, thermostat, and pressure. Thermostats are made for two purposes: to operate either a heating or a cooling system. The symbols for thermostats denote whether they are used for heating or cooling. Pressure switches are much the same as thermostats; their symbols also denote which way they open or close and under what condition. Pressure switches can be used for low or high pressure and are usually denoted by letter designations.
In any system using motors, protective devices are important to prevent damage to the motors or to larger components of the system. The most important type of safety device is for motor protection. A fuse, magnetic overload, thermal overload line break, thermal overload pilot duty, or a thermal overload relay could be used. Many overloads are built directly into the larger components.
Transformers are devices that increase or decrease the incoming voltage to some desired voltage. Transformers are used in the industry mainly in control circuits.
Schematic diagrams tell air-conditioning, heating, or refrigeration technicians when and why a system works as it does. Schematic diagrams show the symbols for devices and the interconnecting wiring of a unit in a circuit- by-circuit arrangement. Schematic diagrams are used most frequently by service technicians to troubleshoot equipment and systems.
Pictorial diagrams show an exact layout of the control panel with the external components shown outside the panel and labeled. The pictorial diagram can be used as a troubleshooting diagram on a simple system, such as a window air conditioner. In most cases, pictorial diagrams are used to find the placement of a component in the panel. Factual diagrams are a combination of the schematic and pictorial, with each shown separately.
Installation diagrams are used to help the installation electrician correctly connect the wiring to the unit. See the dedicated section of this guide for most of the electrical symbols used by major refrigeration, heating, and air-conditioning manufacturers.
REVIEW QUESTIONS
1. What are the three types of electrical diagrams used in the heating, cooling, and refrigeration industry?
2. A load is an electrical device that:
a. produces electricity
b. directs the flow of electricity
c. assists in the starting of motors
d. uses electricity to do useful work
3. What is the major load of an air-conditioning system?
4. Identify the following symbols for loads:
5. What is the major difference between a contactor and a relay?
6. What do the terms “normally open” and “normally closed” refer to with regard to a switch or set of contacts? Draw a normally open and normally closed contact.
7. What is the difference between a magnetic starter and a contactor?
8. Identify the following symbols for relays and contactors:
9. Draw a heating and a cooling thermostat and explain the difference between them.
10. A three-pole contactor would allow how many paths for current flow?
11. A disconnect switch is used to _____
a. open and close the main power source to a piece of equipment
b. stop and start a compressor
c. control the operation of an electric heater
d. open when an unsafe condition occurs
12. What determines whether a pressure switch opens or closes on a rise of pressure?
13. Identify the following symbols for switches:
14. What is the difference between a thermal overload and a magnetic overload? Draw the symbol for each.
15. What is the purpose of a trans former? Draw the symbols for a transformer.
16. Which of the following is not a requirement for an electric circuit?
a. a source
b. a path
c. a load
d. a signal light
17. What is the purpose of a legend on a schematic diagram?
18. A factual diagram is ___
a. a pictorial diagram with wire colors denoted
b. a combination of pictorial and installation wiring diagrams
c. a combination of schematic and pictorial wiring diagrams
d. none of the above
19. Identify the following symbols for safety devices.
20. Which of the following are not components of a contactor or relay?
a. a solenoid
b. contacts
c. thermal element
d. mechanical linkage
21. True or False: The schematic diagram tells service technicians how to wire a system.
22. What is the difference between a pilot duty and a line break overload?
23. What type of switch would be used to open or close set of contacts at a certain pressure?
24. What is the purpose of a fuse in an electrical system?
25. True or False: A solenoid valve is a device that opens or closes to control the flow of some element in the system.
26. What is a signal light used for in a control system?
27. Change the following normally open elements from the de-energized position to the energized position.
-||-
28. Draw the symbols for the following electrical devices.
a. heating thermostat
b. pressure switch (closes on rise)
c. heater
d. motor
e. solenoid coil
f. normally open push-button switch
29. Add letter designations to the symbols to indicate the following:
a. compressor —O=
b. high-pressure switch
c. evaporator fan motor —O—
d. crankcase heater —/\/\/\—
e hot-gas solenoid —\/\—
f. red signal light
30. Draw a symbol for a magnetic starter.