UNDERSTANDING ELECTRICAL SCHEMATICS Part 2
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Refrigeration Service Engineers Society 1666 Rand Road Des Plaines, Illinois 60016 UNDERSTANDING ELECTRICAL SCHEMATICS Part 2 by Howard L. Pemper, CMS INTRODUCTION for example, shows both a normally open and a nor- mally 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 elec- trically. 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 CC-1 1 BLK L1 T1 BLK COMP-1 2 TB-1 YEL L2 T2 YEL 3 4 BLU L3 T3 BLU 5 208/230-3Ø 6 BLK 7 8 YEL CC-2 COMP-2 9 BLK L1 T1 BLK BLU 10 YEL L2 T2 YEL 11 GND 12 BLU L3 T3 BLU BLK 13 YEL Figure 7. circuits High-voltage 14 IFMC BLK L1 T1 BLK 15 IFM 16 YEL L2 T2 YEL 17 BLK 18 BLU L3 T3 BLU 19 YEL S BLK YEL 4 20 BLU OFMC-1 OFM-1 21 C YEL L1 T1 YEL 22 23 BLK L2 T2 BLK CAP 24 CB-1 R 25 110 A 26 S 27 OFMC-2 OFM-2 28 C YEL L1 T1 YEL 29 BLK 30 BLU L2 T2 BLU CAP 31 R 32 208 V 230 V 33 34 35 TRSF-1 36 37 115 V TRSF-1 38 39 FU-1 C1 OFMC-1 C2 BRN C2 OFMC-1 22, 24 40 8 A 41 CR-1a 42 1 3 ATS 43 1 3 C1 OFMC-2 C2 BRN C2 OFMC-2 29, 31 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 1, 3, 4 49 50 HPS-1 LPS-1 51 C2 CC-1 52 53 C C 54 TDR-1 BLU/RED 2 ASR-1 3 4 TDR-1 6 BRN 6 TDR-1 48, 53 55 56 GRY 57 58 BRN OFC-2 48 59 220 V 120 V Heater 60 OFC-1 BRN 61 62 CR-2a ASR-2 OFC-2 Figure 7.