UNDERSTANDING ELECTRICAL SCHEMATICS Part 2

Refrigeration Service Engineers Society 1666 Rand Road Des Plaines, Illinois 60016

by Howard L. Pemper, CMS

INTRODUCTION for example, shows both a normally open and a normally closed single-pole, single-throw (SPST) switch. In this section, you will take a look at a packaged This type of switch either opens or closes one circuit. gas/electric system, which is a relatively complex Figure 1B shows a single-pole, double-throw (SPDT) heating and cooling unit. The wiring diagram for this switch. Again, only one circuit can be controlled at equipment is more difficult than those you studied in any given time, but in this case the switch has two dif- Part 1—but again, the schematic as a whole can be ferent “connected” positions, which means that it can simplified by breaking it down into its basic circuits. direct current to either of two paths. As you examine individual control circuits and the associated components that they operate, the overall diagram becomes easier to understand, as do the A. Single-pole, single-throw (SPST) various machine functions. The schematics in this section may include some symbols with which you are not familiar. For your convenience, many of the Normally closed (N/C) schematic symbols currently used and recognized by the HVAC/R industry are collected in Figure 16 at the end of this chapter. Normally open (N/O)

SWITCH SYMBOLS B. Single-pole, double-throw (SPDT) Generally speaking, a wiring schematic shows the condition of a piece of equipment when there is no power being applied to the unit. Therefore, if a switch is depicted as being normally open (N/O) or normally closed (N/C), remember that the position of the switch is shown as it appears when there is no power C. Double-pole, double-throw (DPDT) applied to that circuit. If there is any deviation from this practice, there will be an explanatory note on the schematic.

As you may know, a switch is characterized by the number of contacts (or poles) and the number of positions (or throws) it has. Think of the number of poles as the number of circuits that the switch can control at one time, and the number of throws as the number of paths a single circuit can take. Figure 1A, Figure 1. Switch symbols

© 2003 by the Refrigeration Service Engineers Society, Des Plaines, IL 630-141 Supplement to the Refrigeration Service Engineers Society. 1 Section 4A n Double-pole, single-throw (DPST) operation of the control. In Figure 3, for example, the temperature switch (RS-2) is shown with the arm above the contacts. This signifies that the switch opens on a rise in temperature and closes on a drop in temperature. The pressure switch (AFS-2) is shown with the arm below the contacts. This signifies that the switch opens on a drop in pressure and Double-pole, double-throw (DPDT) closes on a rise in pressure.

An example of an SPDT limit switch (LS) is shown in Figure 4. When there is an increase in temperature, the contacts “C” to “N/C” move to the “N/O” position. When the temperature decreases, the contacts “C” to “N/O” move back to the “N/C” position.

Relays

Relays are electrically operated control switches. The schematic symbols used to represent relays are the Three-pole, single-throw (3PST) same as those for manually operated switches, except that relay symbols often include a solenoid coil. There are several possible ways of depicting the solenoid coil. Figure 5 shows two different schematic representations of a DPDT relay. Note that multiple- pole relays, like multiple-pole switches, are connected mechanically but not electrically.

Contactors Figure 2. Multiple-pole switches A contactor is a type of heavy-duty relay that handles higher voltages and higher currents than a control

A switch that can control more than one circuit at a time is shown schematically as having more than one set of contacts. Look back at Figure 1C on the previ- RS-2 AFS-2 ous page. It shows an example of a double-pole, dou- 1 3 1 2 ble-throw (DPDT) switch, which can control two circuits at the same time. The dashed line represents the mechanical connection, and tells you that the contacts move together, but are not connected electrically. Figure 2 above shows a few of the many other Figure 3. Temperature and pressure controls variations that are possible in depicting multiple-pole switches. N/O Controls LS Pressure and temperature controls are switches, too, C N/C and they also may be configured with various combi- nations of poles and throws. The position of the switch “arm” in the schematic symbol indicates the Figure 4. SPDT limit switch

2 Coil Coil

Figure 5. DPDT relays

relay. Contactors appear nearly identical to relays on L1 L2 schematic diagrams. Some manufacturers employ Coil contactors that use a single set of contacts. A “bus bar” is placed over the connection where the other T1 T2 set would be, as shown in Figure 6. Figure 16 at the end of this chapter includes many other symbols for Figure 6. Two-pole contactor with bus bar switches and relays.

THE BASIC DIAGRAM problems that you may encounter with a particular Let’s take a look at a “generic” schematic of a pack- type of equipment. aged heating/cooling unit. In order to illustrate the various options that may be possible with a unit of this RELAYS kind, the schematic has been put together by taking parts from several different manufacturers. Because As you saw earlier, relays can and do have many of its complexity, the schematic is broken into three contacts. A single relay may have a function in two, parts. Figure 7 (spread across pages 4 and 5) shows three, or sometimes four different circuits. Its contacts the high-voltage components, and Figure 8 (pages 6 may be located in various parts of the schematic, and and 7) shows the low-voltage or control circuits.There you must know how to find them if you are to know normally is no “point-to-point” or line diagram with this how the unit works. In our generic diagram, several type of schematic, but a component layout is often relays provide lockouts for the cooling and heating provided. This is shown in Figure 9 (pages 8 and 9). sections, which means that the two sections cannot come on at the same time. EQUIPMENT Locate control relay CR-1 at the bottom of Figure 8 In order to service any piece of equipment, you first (line 183). Now look at the detail shown in Figure 10A must know what you are working with. Even before (found on page 10). As you can see, control relay you begin a visual inspection of the equipment itself, CR-1 has five sets of contacts that are operated by a quick look at the schematic will give you a general one coil. The first two sets of contacts, CR-1a and idea of the type of equipment and its components. In CR-1b, are found on lines 42 and 48, respectively Figure 7, for example, it’s easy to spot the two com- (see Figure 10B). The third set, CR-1c, is found on pressors—therefore, you can assume that this is a line 77 (see Figure 10C). The fourth set, CR-1d, is two-stage cooling system. Likewise, in Figure 8 you found on line 149 in the low-voltage section of the can see two ignition systems—again, you can con- schematic (see Figure 10E). The last set, CR-1e, is clude that this is a two-stage heating system. With found on line 90 (see Figure 10D). Remember that just a quick glance at the schematic, you have deter- when a number in the right-hand margin of Figure 10A mined what the unit is. As you become more experi- is underlined, it designates a set of normally closed enced, you also may have a good idea of the kinds of (N/C) contacts.

3 22, 24 29, 31 COMP-2 TRSF-1 OFMC-1 OFMC-2 OFM-2 OFM-1 C2 C2 BRN BRN BRN COMP-1 C2 C2 S C R S C R IFM BLK YEL BLU OFMC-1 OFMC-2 CAP CAP C1 C1 3 T1 T2 T3 CC-2 ATS L1 L2 L3 1 BLK YEL BLK YEL BLK YEL YEL BLK YEL BLU BLU BLU BLU T1 T2 T3 T1 T2 T1 T2 T1 T2 T3 CC-1 IFMC OFMC-1 OFMC-2 L1 L2 L3 L1 L2 L1 L2 L1 L2 L3 BLK YEL BLK YEL YEL BLK YEL BLU BLU BLU BLK YEL BLU BLU V YEL BLK 23 0 TRSF-1 YEL 3 TB-1 YEL CR-1a V A 1 115 V A BLK 2 08 CB-1 11 0 FU-1 8 GND BLK BLK GRY 2 08/ 23 0 -3 Ø 1 2 3 4 5 6 7 8 9 1 0 11 12 13 14 15 16 17 1 8 19 2 0 21 22 23 24 25 26 27 2 8 29 3 0 31 32 33 34 35 36 37 3 8 39 4 0 41 42 43 44 45 46

Figure 7. High-voltage circuits

4 1, 3, 4 9, 1 0 , 12 15, 17, 1 8 4 8 , 53 4 8 63, 6 8 63 CC-1 CC-2 CC-1 TDR-1 OFC-2 CC-2 TDR-2 OFC-2 CC HTR-1 CC HTR-2 IFMC-2 IFM USLV-1 IDFM 6 C2 C2 C2 C2 C2 C2 C2 C2 C2 BRN BRN BRN BRN BRN BRN BRN BRN BRN BRN BRN 6 C2 C2 C2 C2 C2 C2 CC-1 CC-2 IFMC IDFM USLV OFC-1 OFC-2 TDR-1 TDR-2 4 C1 C1 C1 C1 C1 C1 3 C CC HTR-1 CC HTR-2 Heater Heater LPS-1 LPS-2 TDR-1 TDR-2 WHT 1 C V V 2 2 12 0 12 0 3 3 HPS-1 HPS-2 ASR-1 ASR-2 V V 1 1 2 2 22 0 22 0 2 GRY GRY M M OFC-2 L L CCPS 1 BLU BLU / RED 3 3 1 1 BLK 6 3 2 2 3 3 3 3 15 IFRH IFRC CR-7 CR-1b ASR-1 OFC-1 CR-2a ASR-2 CR-1c CR-2b CR-1e IDFMR 4 1 1 1 1 1 1 1 13 GRY GRY 47 4 8 49 5 0 51 52 53 54 55 56 57 5 8 59 6 0 61 62 63 64 65 66 67 6 8 69 7 0 71 72 73 74 75 76 77 7 8 79 80 8 1 8 2 8 3 8 4 8 5 8 6 8 7 88 8 9 9 0 91 92 93 94

Figure 7. High-voltage circuits (continued)

5 TRSF-2 GV-1 TRSF-2 CR-7 IGN PCB IGN-1 IGN-2 T2 T2 T2 C2 VIO VIO VIO VIO VIO VIO T2 T2 C2 IGN IGN CR-7 GV-1 GV-2 T1 T1 C1 FS FS IGN-1 IGN-2 GVR-1 GVR-2 3 CSa CSb 1 3 2 IGN-1 IGN-2 AFS-1 AFS-2 1 1 2 3 YEL WHT TRSF-2 RS-1 RS-2 1 1 V V GVR-2 24 115 IGN PCB GVR-1 N/O N/C BLK BLU LS C 3 A IGN-1 IGN-2 CB-2 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143

Figure 8. Low-voltage circuits

6 63, 80, 146 42, 48, 77, 149, 90 92 82 85 158, 146 161, 152, 149 IFRH IDFMR LOGIC IFRC HR-1 HR-2 CR-2 CR-2 CR-2 CR-1 USLV-1 IDFMR 3 C2 C2 C2 C2 C2 C2 C2 C2 C2 C2 VIO VIO VIO VIO VIO VIO VIO VIO C2 C2 C2 C2 C2 C2 C2 IFRH IFRC HR-2 HR-1 CR-2 CR-1 IDFMR C1 C1 C1 C1 C1 C1 C1 C Y1 Y2 W1 W2 G R 3 1 TDR-3 LOGIC 2 5 12 CR-1d 4 10 6 9 6 3 3 HR-1c HR-2b CR-2c HR-1b HR-1a HR-2a 4 7 4 1 1 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188

Figure 8. Low-voltage circuits (continued)

7 T2

R G W2 W1 Y2 Y1 C GV-2 T1 T2 IGN-2 GV-1 C LS T1 N/C N/O 3 3 IGN-1 RS-1 RS-2 1 1 C2 C2 3 6 3 6 2 5 2 5 2 2 HR-2 CR-7 1 4 1 4 C1 C1 AFS-1 AFS-2 1 1 C2 C2 3 6 3 6 2 5 2 5 HR-1 IDFMR 1 4 1 4 2 TRSF-1 C1 C1 CCPS 1 C2 C2 3 6 3 6 2 5 2 5 CR-2 IFRH 1 4 1 4 C1 C1 C2 C2 GVR-2 3 6 9 3 6 12 15 TRSF-2 2 5 8 2 5 11 14 CR-1 IFRC IGN PCB 1 4 7 1 4 10 13 GVR-1 C1 C1

Figure 9. Component layout

8 L3 T3 L2 T2T1 CC-2 C2 L3 T3 C1 C2 L1 L2 T2 IFMC L3 T3 L1 T1 C1 L2 T2T1 CC-1 C1 C2 L1 CAP-1 CAP-2 2 2 4 2 4 T2 L2 C2 3 ASR-1 ASR-2 TDR-3 1 3 1 3 1 OFMC-2 C1 L1 T1 M M T2 L2 L L 3 9 6 C2 OFC-1 OFC-2 2 8 5 120 V 120 V OFMC-1 TDR-1 TDR-2 COMP-1&2 1 7 4 208/230/3-phase C1 220 V 220 V L1 T1

Figure 9. Component layout (continued)

9 178 Detail A 179 180 C2 CR-1 181 182 183 C1 CR-1 C2 VIO C2 USLV-1 42, 48, 77, 90, 149 184 185 186 187

37 Detail B 38 39 FU-1 C1 OFMC-1 C2 BRN C2 OFMC-1 40 8 A 41 CR-1a 42 1 3 ATS 43 1 3 C1 OFMC-2 C2 BRN C2 OFMC-2 44 GRY 45 BRN 46 47 CR-1b ASR-1 OFC-1 48 4 6 1 3 L M 1 2 1 3 C1 CC-1 C2 BRN C2 CC-1 49 50 HPS-1 LPS-1 51

75 Detail C 76 CR-1c 77 1 2 GRY CC HTR-1 BRN CC HTR-1 78 79

87 Detail D 88 CR-1e USLV 89 CCPS 13 15 1 2 C1 C2 C2 USLV-1 90 91

147 Detail E 148 149 7 9 10 12 150 HR-1c CR-1d 151 152

Figure 10. Locating control relay contacts

10 CC-1 CR-1b ASR-1 OFC-1 HPS-1 LPS-1

Figure 11. Series circuit

CIRCUITS flow is indicated by the arrows. Notice that the current must pass through all of the switches before it can A schematic diagram can look complicated when energize the coil. If any of the switches is open, the viewed as a whole. However, as stated previously, it coil cannot be energized. becomes much easier to analyze a problem if you break down the large diagram into smaller individual Parallel circuits circuits. All electric circuits conform to one of three basic arrangements. In a parallel circuit, there are two or more separate paths or branches for current flow. A parallel circuit Series circuits arrangement allows any one of a number of controls or switches to energize a load. Look at Figure 12A, In a series circuit, components are arranged one after for example. When either of relay contacts IFRH or another, so that the same current flows through all of IFRC is closed, current will pass through the circuit the components in one continuous path. Figure 11 and energize coil IFMC. However, note that both shows an example of a number of switches and a IFRH or IFRC must be open in order for the coil to be relay coil connected in series. The direction of current de-energized.

A. Multiple switches, single load

IFMC IFRH

IFRC

B. Single switch, multiple loads

OFMC-1 CR-1a

ATS OFMC-2

Figure 12. Parallel circuits

11 CC-1 CR-1b ASR-1 OFC-1 HPS-1 LPS-1

TDR-1

ASR-1 TDR-1

220 V 120 V Heater OFC-1

Figure 13. Series-parallel circuit

A single switch can energize several loads at the multiple-path) circuits. Such a circuit allows some same time in a parallel circuit. All of the loads will get operations to proceed while stopping others. Series- the same amount of current when the switch closes. parallel circuits are primarily used in control and In Figure 12B, for example, when switch CR-1a is safety applications. The detail shown in Figure 13 closed, current will pass through the circuit and ener- above illustrates a series-parallel circuit. When switch gize coil OFMC-1. And as long as thermostat ATS is CR-1b closes, current flows through both branches of closed, coil OFMC-2 also will be energized. the circuit. As long as all of the switches ASR-1, OFC-1, HPS-1, and LPS-1 are closed, current Series-parallel circuits passes through the upper branch and energizes coil CC-1. If any of the switches is open, however, the As the name implies, a series-parallel circuit is a current will be allowed to pass only through the lower combination of series (or single-path) and parallel (or branch to coil ASR-1.

IFRH IDFMR C1 IDFMR C2 C2 IDFMR 1 2 3 1 2 3 4 5 6 4 5 6 C2 IFRH

C1 IFRH C2 IDFMR C1 C2 C1 C2

A. Schematic diagram B. Component diagram

Figure 14. Terminal connections

12 With all of the switches in the upper branch closed and coil CC-1 energized, current simultaneously passes through the TDR-1 IGN-1 contacts to the TDR-1 relay. This is a GVR-2 “delay-on-make” time-delay relay, which means that after a specified period of time, the TDR-1 contacts will open. This type of control is commonly used for low-pressure bypass during low ambient conditions.

When coil CC-1 is energized, note that current also passes through the 120-V resis- IGN-1 tor, through the resistor marked “Heater,” IGN PCB and through differential pressure control IGN-2 GVR-1 GVR-2 OFC-1. This is an oil failure control. When oil pressure is sufficient, differential pressure control OFC-1 will open, thus taking the heater out of the circuit and preventing the OFC-1 contacts from opening.

CONNECTIONS AND WIRING Figure 15. Relay connections on igniter printed circuit board In order to conserve wire and space, some manufacturers terminate more than one coil connection at the same point. This practice is contacts for the igniters are on the printed circuit depicted schematically as shown in Figure 14A, board, but they cannot be physically replaced. The where the terminals from the IDFMR and the IFRH contacts themselves are usually shown enclosed in a are taken off the same terminal connection. Note that “box” drawn with dashed or dotted lines. This same the schematic shows two connections or wires at that approach is used for the gas valve relay shown in point (the arrows point to the connection points). Figure 15, and in some instances for fan relays as Because line diagrams are not used in complex well. schematics, the component diagram will show the LEGENDS terminal connections on the relays and other devices (see Figure 14B). There normally isn’t enough space on a schematic to accommodate the full spelling of each component. A In some cases, a control or relay may not be shown legend listing many of the abbreviations used in the as a replaceable item, or even as a component that schematic that you have been studying is shown on can be tested. In Figure 15, for example, the timing the next page.

13 LEGEND

AFS . . . . . Air flow switch IDFM . . . . . Inducer fan motor ASR . . . . . Anti short-cycle relay IDFMR . . . . . Inducer fan motor relay ATS . . . . . Air temperature switch IFM . . . . . Indoor fan motor CAP . . . . . Capacitor IFMC . . . . . Indoor fan motor contactor CB . . . . . Circuit breaker IFRC . . . . . Indoor fan relay (cooling) CC . . . . . Compressor contactor IFRH . . . . . Indoor fan relay (heating) CC HTR . . . . . Crankcase heater IGN PCB . . . . . Igniter printed circuit board CCPS . . . . . Capacity control pressure IGN . . . . . Igniter switch LPS . . . . . Low-pressure switch COMP . . . . . Compressor LS . . . . . Limit switch CR . . . . . Cooling relay OFC . . . . . Oil failure control CS . . . . . Centrifugal switch OFM . . . . . Outdoor fan motor FS ..... Flame sensor OFMC ..... Outdoor fan motor contactor FU ..... Fuse RS ..... Rollout switch GND ..... Ground TB ..... Terminal board GV . . . . . Gas valve TDR . . . . . Time-delay relay GVR . . . . . Gas valve relay TRSF . . . . . Transformer HPS ..... High-pressure switch ULSV ..... Unloader solenoid valve HR . . . . . Heating relay

Note: Figure 16 on the following three pages shows many of the schematic symbols used in the HVAC/R industry today.

14 SWITCHES Closes Closes Pushbutton switches N/O or on fall on rise

N/C or Pressure N/O N/C

SPST Two-position

Temperature H-O-A switch HAND SPDT OFF Liquid level AUTO

Foot switches

DPST Flow

N/O N/C Limit Time-activated switches

DPDT Differential pressure De-energized Energized

RELAYS Coil Coil

Coil SPST Coil SPST (N/O) (N/C)

SPST (N/O) SPST (N/C)

Coil Coil

Coil DPST Coil SPDT

DPST SPDT

Coil

Coil DPDT

DPDT

L1 L2 L1 L2 L1 L2 L3 L1 L2 L3 Coil Coil Coil Coil T1 T2 T1 T2 T1 T2 T3 T1 T2 T3

2-pole contactor 2-pole contactor 3-pole contactor 3-pole contactor with bus bar with bus bar

Figure 16. Schematic symbols

15 TRANSFORMERS

Primary Secondary Boost-and-buck transformer Current transformer

115 V 24 V

460 V 24 V “Boosting” “Bucking” 230 V 208 V 120 V 2.5 V Multivoltage control transformers Common Common Tapped 460-V primary 460-V primary

H1 H2 H3 H4 H1 H2 H3 H4

X1 X2 X3 X4 X1 X2 X3 X4 Variable 230-V secondary

120-V secondary 230-V 230-V primary primary Air core

H1 H2 H3 H4 H1 H2 H3 H4

115 V 10 kV X1 X2 X3 X4 X1 X2 X3 X4

460-V secondary

120-V Always consult the manufacturer’s secondary High-voltage transformer nameplate for proper connections.

Circuit breakers Fuses Thermal overload switch Cartridge or plug fuse

Fusible link

Overload safety Single- Double- Triple- switch pole pole pole Magnetic overload switch

Mechanical interlock Thermal overload

Figure 16. Schematic symbols (continued)

16 Conductors Ground connections Thermocouple

Chassis Earth ground ground

Wires Wires crossing Capacitor (fixed) Lamp Battery connected but not connected Terminal nearest ground Shielded cable

ELECTRONIC COMPONENTS

Multiple-conductor Transistors cable Diode C AND gate Number of conductors B in conduit or cable SCR E NPN Connectors NAND gate

Male C

B Female LED OR gate E PNP Engaged (plug and receptacle) Diac D NOR gate G Resistors S

Fixed N-channel JFET Triac NOT gate

D G Potentiometer Zener diode Variable S Buffer amp P-channel JFET

Rheostat Tunnel diode

Op amp Tapped

Photodiode UJT

Resistance heater

Solenoid Thermistor MOV

T Cad cell IGT

Figure 16. Schematic symbols (continued)

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