Appliance Repair Study Unit 4 Electric HeatingAppliances and Fans By Jack Darr Revised By Robert L. Cecci
iii PreviewNow that you have completed the study of the fundamentals of appliance repair, we can beginstudying how to repair heating appliances and fans.In this study unit, you will see how to test and replace components properly. You will see howto disassemble, test, replace and perform a safety test on various heating appliances and fans.In this text, you will begin to see how to troubleshoot actual appliances. You will begin by check-ing power cords. Power cords take the greatest abuse and are the most likely cause of both inter-mittent and total failure in appliance performance. Here, you will also see how to choose thecorrect appliance power cord for each type of appliance.Next, you will see how to repair electric room heaters. We have already used heaters in pre-vious texts for examples of the use of test equipment. Now, you will see how to disassemble,test, and replace heater components. Also, you will see how a heater can be constructed to havemultiple heat ranges.The next heating appliance to be covered will be the electric clothes-pressing iron. This text shallcover the disassembly, testing, and repair of dry, steam, spray, and steam/spray irons.The final two sections of this text will cover room fans, and attic fans/power ventilators. Youwill see how to rebuild a fan unit and how to eliminate electrical problems and problems ofnoise and vibration.At the end of this unit, you will be able to • Describe the features of typical appliance line cords • Identify the proper type and AWG size of appliance line cords • Discuss the purpose and application of devices such as strain reliefs, solderless connectors, and crimp-on lugs • Describe how typical room heaters are constructed • List the steps to proper electric room heater servicing • Describe how electric irons are constructed and how to repair dry, steam, and spray irons • Identify various types of electric fans and list the steps to proper fan repair • Determine how to troubleshoot and repair attic fans and power ventilators
vContentsWORKING WITH LINE CORDS AND PLUGS . . . . . . . . . . . . . . . . . 1 Appliance Line Cords Wire Sizes in Line Cords Zip Cord Jacketed Cord Cords Used in Heating Appliances Selecting Line Cords Line Cords with Grounds Polarized Receptacles Cord Connections Line Plugs and Replacement Stocking Replacement Line Cords Strain Relief Splicing Cords Using Solderless Connections Crimp-On LugsELECTRIC ROOM HEATERS . . . . . . . . . . . . . . . . . . . . . . . . . . 15 How a Heater Works How a Heater Is Built Hints for Servicing Room Heaters Heating Elements Mounting Heating Elements Electrical Connections to Heating Elements Thermostats in Heaters Testing Heaters How to Test a Heater for an Open Circuit How to Replace Heater Parts Testing after Repairs Are Finished Reassembling the HeaterCOMBINATION OF HEATING ELEMENTS . . . . . . . . . . . . . . . . . . 27 Current in Heating Elements Resistance of Heating Elements Multiple-Heat Circuits with Parallel Elements Series-Connected ElementsELECTRIC IRONS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32 Operating Features of an Electric Iron How an Electric Iron Is Constructed How a Dry Iron Is Built
vi Contents Thermostats in Irons Steam Irons Spray Irons Check Valves HOW TO TEST AND REPAIR ELECTRIC IRONS . . . . . . . . . . . . . . . . 41 Testing the Line Cord How to Repair the Line Cord How to Take a Steam Iron Apart How to Take Dry Irons Apart Continuity Test of Heating Element and Thermostat How to Remove Bolts and Nuts from an Electric Iron How to Detect Scale in Steam Irons How to Clean Steam Irons Disassembling Newer Irons Troubleshooting Modern Irons ELECTRIC FANS . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 52 Types of Fans How Fans Are Built Fan Blades Two- and Three-Speed Fans How to Take a Fan Apart How to Test the Speed Control Burned-Out Motor Coils Dragging Bearings Worn Bearings Causes of Noisy Fan Operation How to Tighten a Loose Hub How to Check Blade Angles Adjusting Blade Tips Loose Parts on Guards Oscillating Fans Repairing Combination Light/Fan/Heaters Air Purifiers Servicing Air Purifiers ATTIC FANS AND POWER VENTILATORS . . . . . . . . . . . . . . . . . . . 73 Types of Fans and Ventilators Servicing Attic Fans and Power Ventilators CHECK IT OUT! ANSWERS . . . . . . . . . . . . . . . . . . . . . . . . . . 81 EXAMINATION . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85
1 Electric Heating Appliances and Fans plastic insulation. Appliances which draw aWORKING WITH LINE heavy current, such as electric irons and heaters,CORDS AND PLUGS must use a special line cord with heavy wires and high-temperature insulation.Appliance Line Cords Wire Sizes in Line CordsThe wire which connects an appliance to awall outlet is termed the line cord by most The size of a wire determines the amount ofappliance technicians. The line cord is a very current it can safely carry, just as the size ofimportant part. Unless power can get to the a water pipe determines the amount of waterappliance, the appliance will not work. The which can flow through it. The thicker theline cord is also the part which gets the wire, the greater the current it can carry.hardest service; it is bent, kinked, crushed, The wire in a line cord should be largeand even cut. enough to carry the normal current withoutThe line cord is the first thing you should heating up; the heat should be in the appli-check whenever you test a faulty appliance. ance, not in the line cord. If the line cordLook for places where the cord may have should heat from an overload of current,been crushed or even cut. Take the cord and the insulation would go bad in a very shortbend it sharply between your fingers, watch- time, presenting a serious shock hazard.ing the insulation. If the cord is too old, you’ll The size of a wire is indicated by its gagesee fine cracks in the rubber or plastic insula- number. Wire size is generally specified bytion at the bend. Such a cord should be re- the American wire gage (AWG), which is alsoplaced because the insulation will soon fail. known as the Brown & Sharpe gage (B&S).If the cord isn’t in first-class condition, it An assigned AWG number indicates a certaincould be a very serious shock hazard to the wire size, and the amount of current a wireuser. Since practically all household appli- can carry is termed its ampacity.ances are accessible to children, you must Table 1 on page 2 shows the standard wiremake sure that the line cord is absolutely safe. sizes used in all appliance cords and theThe cord insulation must be perfect, not safe current-carrying capacity, or ampacity,aged or broken. All electrical connections of each wire size. Strangely enough, the big-must be tight and well insulated. ger the wire number, the smaller the wire.Bad line cords are the most common trouble For example, AWG No. 12 is much biggersource in all portable appliances. A great than AWG No. 18, and it carries more current.deal of your work will consist of replacing The standard types of cords, according tocords. You must know exactly what kind of insulation, are indicated by letters.replacement cord to use on each appliance. The wire used in the line cord must be ableThere are several types of cords available, to carry the normal current drawn by theand the cord used depends on the appli- appliance, plus a little more as a safetyance. Small, low-current appliances use margin to keep the cord from heating up.cords with small wires and with rubber or
2 Electric Heating Appliances and FansYou learned earlier how to determine the Zip Cordamount of current an appliance will draw.From the nameplate, take the voltage and Small appliances, up to 100 to 200 watts, usethe wattage, and then divide the watts by a lightduty rubber-insulated cord, type SV,the volts to find the current. For instance, if type SPT. Most technicians call this cord aan appliance draws 1200 watts at 120 volts, zip cord. Its wires are parallel, and there isit will use a groove between them. This makes it easy to pull, or zip, the wires apart to makeI = P/E = 1200/120 = 10 amps connections; hence the name “zip cord.” Most small appliances use zip cords with wires from No. 18 to No. 14. Table 1 Size S, SJ HPN Jacketed Cord AWG SV, SPT HSJO Heavier appliances that draw high currents, 18 10 10 such as irons and heaters, need heavier wire 16 13 15 and better insulation. They may need a type of cord designated SV or SJ. If the two insu- 14 18 20 lated wires in the cord have a plastic outer 12 25 25 cover for extra protection, the cord is called a jacketed cord. The two wires are made so 10 30 30 that they spiral around each other, and strands of a special paper or fiber are added between them for extra protection. A jacketedLooking at Table 1, you can see AWG No. 18 cord is used on washing machines, vacuumwire is rated at only 10 amp if it is of the rubber cleaners, and similar appliances which aretype SJ and has two conductors. This wire is used in damp places. The extra protection istoo small. For safety’s sake, use at least an needed where the cord must be able to standAWG No. 14 wire rated at 18 amp, or better hard wear, such as being walked on, rolledstill, an AWG No. 12 wire rated at 25 amp. If up, bent, and flexed. Jacketed SV and SJ cordsyou can’t get the size wire you want, always are made in sizes No. 18 and No. 16. Heavieruse a wire of the next larger size or next smaller jacketed cords, such as Type S with wires upAWG number, and you can’t go wrong. to No. 10, are available for extra-hard use inBesides the greater ampacity it provides, there damp places.is another advantage to selecting a line cordwith wires somewhat larger than necessary.The flexible line cords are all stranded. That Cords Used in Heatingis, many strands of much smaller wire make Appliancesup a wire of a larger size, the number of A special type of cord is used on heatingstrands depending on the cross section of appliances where currents are higher and theeach individual strand. The strands will cord may be burned if it should come in contactinevitably break in use some day. However, with the heater. This cord is called the heaterif you use a No. 12 wire where only a No. 14 cord. The wires are insulated with ethyleneis needed, by the time enough strands break propylene rubber or polychloroprene, andto bring the wire down to the cross section strands of fiber glass are sometimes addedthat will carry as much current as a No. 14 between the wires. The older heater cordssize, the cord will have lasted much longer are covered with a heavy woven cloth braid.and been much safer.
Electric Heating Appliances and Fans 3Heater cords are available with wires from order wires large enough for the job. In yourNo. 18 to No. 14, and among the standard repair work, you will seldom need a cordtypes are HPN and HSJO. with wires smaller than AWG No. 14.Selecting Line Cords Line Cords with Grounds When some strands in a stranded wire break,In Figure 1 are shown the three types of line the cross section of the wire in the line cordcords, the heater cord, the zip cord, and the becomes smaller and the cord will get veryjacketed cord. The jacketed cord is shown hot when the appliance is turned on. If allwith three wires; the third wire is used for strands in the cord break completely, an arcgrounding the case for safety. will form across the very small gap between the ends of the strands and generate high heat which can melt the insulation on the wires, cause a short circuit, and even burn the cord completely in two. In older hand-held electrical appliances, such as electric drills, saws, and mixers, the break- down in the cord can cause the hot wire to come in contact with the metal frame or the case of the appliances. This short circuit can give the person using the appliance a dangerous shock. A safety cord, called a grounded line cord, has been developed toFIGURE 1—Three different types of appliancepower cords are shown here. eliminate this danger. Such a cord has the two regular wires needed to complete theStandard flexible cords are designated by circuit, plus a third wire connected to theletters as indicated in Table 1 on the preceding frame of the unit, as shown in Figure 2 onpage. You may select the cords according an electric drill. This “grounding wire” isto place of usage, such as in dry or damp connected to the frame of the drill and toplaces; or according to degree of usage, suchas “extra-hard usage,” “hard usage,” and“not hard usage.” For example, rubber- MOTORcovered cords SV and SJ are available withNo. 18 wires. Both may be used in dampplaces, but the Type SV is for “not hard usage,”that is, it is used for smaller vacuum cleanersand food mixers, while the Type SJ is for ELECTRIC DRILL“hard usage,” as in larger vacuum cleaners.A similar rubber-covered cord, Type S, is for“extra-hard usage”; it resists alkalies, acids, GROUNDING WIREand water, and is available with wires AWGNo. 18 to No. 10. MOTOR WIRESMost of the time you will select cords fromcatalogs which indicate the insulation, the FIGURE 2—When an appliance has a three-wirebest application, the wire sizes, and the prices power cord, the green wire is grounded and willof available cords. But always remember to attach to the case of the appliance.
4 Electric Heating Appliances and Fansthe ground pin at the plug. The plug on the The circuit opens, the appliance is cut offline end of the cord has a round third prong from the source, and the user is protectedto which the grounding wire is connected. from a possible shock.The two regular circuit wires are connectedto the two flat prongs. Such a special grounded Most modern appliances and power toolsplug and a grounded outlet are shown in use plastic cases. Since plastic is an insulator,Figure 3. The round prong prevents this line the shock hazard present with metal-casedplug from being plugged into an outlet which appliances and power tools is eliminated.does not have the ground as a safety feature. Many of these devices will, therefore, use aThe grounded or polarized outlet has a third simple two-wire power cord. If a three-wirehole, or terminal, for the third prong. This cord is used, the ground lead is normallyhole is D-shaped. attached to the case of the motor inside the appliance or power tool.Most older electrical appliances of all kindsare equipped with a grounded line cordand a grounded plug. To accommodate them, Polarized Receptaclesold outlets with two terminals must bereplaced with the newer grounded outlets, Many outlets, or receptacles, in older homeswhich have an additional terminal for the will not have the third terminal for theprong of the grounded plug. If you expect grounding prong of the cord. If you replaceprotection from the grounded line cord, these receptacles, you should buy polarizedyou must connect the third wire to the receptacles which make the use of the ground-ground, that is, from the round terminal ing cord possible. A polarized receptacle isof the outlet to the ground. recognizable by the three terminals, one of which is D-shaped and marked “ground.”The idea behind all this is simple: If theappliance develops a short circuit (if a wire When replacing a two-wire outlet with aaccidentally makes contact with the appliance three-wire, or polarized, outlet, you mustcase), an arc will form between the broken connect the third-wire terminal to a groundingwire and the grounded wire in the cord, the conductor, or the outlet won’t offer anyhigh current will flow back through the protection at all.circuit to which the outlet is connected andblow the fuse or trip the circuit breaker. Standard color-coding of house wiring is black for the hot side and white for the neutral; the third wire may be green or bare, and is grounded. In the three-wire line cords, this same color code will be followed, and the third wire will always be green. However, do not depend on color-coding. The house may have been wired by an amateur electrician ignorant of the proper coding. Test the wiring to be sure. Cord Connections The connections on both sides of the lineFIGURE 3—A three-wire cord and plug are shown cord are very important. They must be clean,here. Notice that the plug uses a polarized outlet tight, and strong at both ends, so that thewith different-sized blade receptacles.
Electric Heating Appliances and Fans 5cord won’t loosen in service and cause a shortcircuit. The stranded wire of the cord can behard to handle unless the right methods areused. Later you will learn how to makeconnections with stranded wires.The appliance is connected to the line cord (a) (b)through screwed, soldered, or crimp-onterminals, or through a receiver plug. Theline side of the cord has a line plug whichis to be connected to the wall outlets. (c)Line Plugs and Replacement (a) First step (b) Second step (c) Incorrect methodYou can buy a prefabricated line cord forsmall appliances up to about the size ofvacuum cleaners. The line plug is molded FIGURE 5—The replacement of a plug is shown here. (Courtesy of McGraw-Hill Book Company, Inc.)onto one end of the cord, and the other endis already prepared, with the wires strippedand fastened to lugs, as shown in Figure 4. up, though, it won’t be long before the wiresSuch cord is used by most appliance techni- pull out. Figure 5 shows the procedure forcians to save time. fastening the wires to one type of plug. Here’s how to do it.In many jobs, however, you will have to puta plug on the end of a new line cord or replace If you have a zip cord, pull the wires aparta plug that has broken connections on the for a distance of about two inches. Strip offoriginal cord. You must know how to install about 2.5 cm (1 in.) of insulation on eacha plug. Correctly installed, a line plug will wire. You’ll see that the fine strands of thelast a long time. If it isn’t correctly hooked wire are twisted inside the insulation. Twist the free ends in the same direction, to eliminate the possibility of a short circuit or shock hazard. Now, feed the ends of the wires through the hole in the end of the plug, up between the prongs. Tie a knot in the two wires as shown in Figure 5(a), leaving about 3.5 cm (1.5 in.) of wire extending beyond the knot. The knot provides strain relief and holds the wire tightly inside the plug, as shown in 5(b), so that if the cord is pulled, there won’t be any strain on the electrical connections. Never fasten the leads without making a knot as shown in 5(c). The steps in tying this knot are shown in Figure 6.FIGURE 4—A replacement line cord is available Next, wrap each wire around one of thewith special lugs attached for easy connection prongs and then around the terminal screw,to an appliance. which should be loosened about two turns.
6 Electric Heating Appliances and Fans plug body. Sooner or later, the wires will break. When the cord is moved one way, the ends of the break are pushed together and the plug works; when the cord is moved the other way, they pull apart and the circuit is open. For a quick check, hold the plug in one hand and move the cord back and forth. If the appliance suddenly starts and stops as the cord is moved, wires are broken. ReplaceFIGURE 6—A zip cord should have a knot tied in it the plug.to provide strain relief. (Courtesy of McGraw-Hill BookCompany, Inc.) Replacement plugs are readily available. Two types of replacement plugs are shown inBe sure to wrap the wire in a clockwise Figure 8. The plug on the lower left is useddirection around the screw, as shown in for thin two wire or zip cord. To install thisFigure 7. If you wrap the wrong way, when type of replacement plug, the halves of theyou tighten the screw, the wire will be plug are separated and the unstripped zipsqueezed out from under the screwhead. But cord is placed inside one half of the plug.if you wrap the right way, the screwhead will Then, the second half is squeezed onto thepull the wire tighter. Be sure that all strands wire until the locking tabs inside the plugof the wire are under the screw, so that you catch. Teeth within the plug pierce the insu-won’t lose any of the wire’s current-carrying lation of the zip cord to make electricalcapacity. Remember that loose strands of contact with the wires inside the zip cord.wire in plugs or in any other electrical con-nection are an invitation to a short circuit.Molded plugs come as original equipmenton many appliance cords. Such plugs areusually made of rubber or plastic, as shownin Figure 8 (item 1). The most commonuser’s complaint about this plug is thatsometimes it works when it is plugged inand sometimes it doesn’t. Here’s what hap-pens. As the cord is pulled back and forth, itbends at the point where it comes out of the FIGURE 8—Two types of replacement plugs are shown here. RIGHT WRONG The replacement plug on the upper right is a heavy-duty three-wire plug. The front and rear of the plug are separated by loosening the screws on the front side of the plug. The wires are then placed on their proper termi- nals and the internal screws are tightened to hold the wires. The front screws are thenFIGURE 7—The correct method of attaching a tightened to hold the plug halves together.wire to a terminal is to wind the wire in a A clamp in the rear of the plug also has twoclockwise-direction, hook the wire around thescrew, and tighten the screw. (Courtesy of McGraw-Hill Book Company, Inc.)
Electric Heating Appliances and Fans 7screws that are tightened to firmly clamp Stocking Replacement Linethe wire at the rear of the plug. CordsSince most bending and flexing takes placeat the plug end and at the point where the When you’re starting in the appliance repairline cord enters the body of the appliance, business, it is a good idea to stock about halfyou’ll find practically all of the accidental a dozen ready-made line cords with plugs.breaks in line cords at one of these points. Later on, you can buy the right kind of cord,In fact, many technicians, when they find in full reels, and a stock of plugs, and assem-an open line cord by testing, simply cut the ble the line cords and plugs as you need them.cord at a point about 15 cm (6 in.) from the This saves a little money. You can also makeplug. They strip the wires and test again. If the replacement cord longer than the originalthey now get continuity in the cord, they line cord if the customer wants it that way.replace the plug and the job is done; if not,they replace the whole cord, since it isobviously open at the other end or some- Strain Reliefwhere in between the ends. An appliance usually has an attachmentMany appliance repair persons are now using which holds the cord in place and relievesa two-piece plastic replacement plug. This strain on the electrical connections. Such anplug uses a compression system to hold the attachment is called the strain relief. Therewires to the plug’s terminals. To use this type are several different kinds of strain-reliefof replacement plug, you strip the outer jacket devices, but they all serve the same purpose.cover back approximately 4 cm (1.5 in.) andstrip 6 mm (.25 in.) off the wires. Place each If the appliance case is metal, the cord needswire into its proper terminal and tighten the some protection to keep the sharp edges ofscrews. The terminal’s screws are normally the hole from cutting the insulation. A smallcolor-coded so that it is easy to install the soft-rubber grommet is pushed into the hole,wires to the proper terminals. The green as shown in Figure 9(a). The cord passesscrew is always used for the green wire through the grommet, and an overhandwhich is grounded. The silver screw is used knot is tied on the inside to keep the cordfor the white wire which is neutral. Finally, from being pulled through again. When youthe copper-colored terminal is used for the are replacing the cord, always leave a littleblack wire which is the AC feed of 120 VAC. slack in the wire between the knot and the electrical connections.To complete the assembly, one of two typesof strain relief will be used on the plug. One In some appliances you’ll find the grommettype uses a tapered barrel and rubber or foam and a small metal clamp which holds the cordinsert. This type of strain relief is installed to the side of the case, as in 9(b). Others willby simply tightening the tapered barrel onto use a plastic strain-relief device, which isthe plug assembly. As the barrel is tightened, folded over the cord and then pushed intothe rubber or foam insert is compressed the hole, as shown in 9(c). The insulationaround the wire. is compressed, then it snaps back, and the device is held in place by notches moldedThe second type of strain relief uses a split in its sides. This device is used mostly withrear barrel which is tightened around the the flat SJ cord or the zip cord, but you canwire by means of two screws. find larger sizes for use with round SV cords and larger cords.
8 Electric Heating Appliances and Fans CASE RUBBER GROMMET KNOT (a) RUBBER GROMMET METAL CLAMP (b) PLASTIC STRAIN RELIEF (c)FIGURE 9—Three types of strain relief are shown here. In (a) and (b), you can see a soft rubber grommetbeing used. In (c), a split grommet is being used.If an appliance does not have any strain pro- Splicing Cordstection, add such protection. If the hole isn’tlarge enough to take a grommet, enlarge the If you find that the line cord of an appliancehole and put a grommet in. The appliance is broken in the middle, examine the cordwill be much safer. very carefully. If the insulation on the whole cord is even slightly cracked, replace the whole cord. Making a splice in a cord is the exception and not the rule.
Electric Heating Appliances and Fans 9There’s an easy way to make a splice. Always on the other. Now, strip the insulation off allremember that the splice must be as strong the wires, for about 2.5 cm (1 in.) or less. Manyas the rest of the cord, and perfectly insulated. professionals strip wires with a wire stripperWith modern tapes and methods, it is possible like the one shown in Figure 10(a).to make small neat splices that will last.Here’s how you do it. Hold the insulation between the stripper jaws as shown in 10(b), without applyingCut the cord at the break. Strip off about 5 cm too much pressure, so as to avoid cutting(2 in.) of the outer jacket, using a sharp knife. the wires. Then give a sharp yank towardNext, hold the two ends of the cord together the end of the wire; the insulation will comeand cut off 2.5 cm (1 in.) of the black wire on off, leaving the wires exposed. This takes aone end and 2.5 cm (1 in.) of the white wire little practice, but it is the fastest way.FIGURE 10—A typical wire stripper and crimper is shown in (a). Figure (b) shows how to strip wire with thistool. (Photos Courtesy of Robriair Manufacturing Company)
10 Electric Heating Appliances and FansIf the wire is clean, start splicing. If it has on a single layer of tape. When you get todarkened with age, it is a sign to replace the other end, cut the tape off and smooththe wire. To splice the wire, place the ends it out. Next, put on the final layer in the otherof the black wires together, and twist the direction. Pull the tape as tight as possible sobare parts over one another. Be sure that all that the splice will be well insulated. If thisstrands are in place. The splice should not cord will be used in a damp place, spray abe thicker than 12 mm (.5 in.) of bare wire. thin coat of acrylic plastic over the wholeSolder the splice smoothly. Be sure that the splice. This will seal the tape very tightlysolder runs well into the wire. If solder is and make your splice as waterproof as theexposed, either melt the solder again or file original cord.the sharp points off before taping. Suchpoints could punch through the tape andcause a shock hazard or short circuit. Using SolderlessAfter soldering, tape the splice. It is easier to Connectionstape one completed splice before you splicethe other wire. Use plastic electrician’s tape Inside many appliances, you’ll find solderlessand wrap the tape neatly over the bare parts connectors like the one shown in Figure 12(a).of the splice, going up over the insulation at These are little bell-shaped devices madeboth ends. Stretch the tape over the splice of insulating material, usually plastic, andalmost as smoothly as the original insulation. often called wire nuts. They are used mostlyTwo layers of tape are enough. on small stranded wires. Large sizes are available for use on solid wires, such as theNext, splice the other wire just as you did types used in house wiring.the first one, pulling the wires apart in a“bow” so that you can reach the wire being You can connect up to four wires with aspliced without melting the tape on the first solderless connector, depending on the sizewire. Tear off a short piece of tape because connector used. To make a connection likeyou’ll have to thread this tape through the this, strip the ends of the wires back aboutloop of the two wires. 6 to 10 mm (.5 to .75 in.) and put them side by side with the bare wire ends parallel, asNow you can see why we cut the two wires shown in 12(b). The ends of the insulationat odd lengths: the two splices are staggered should be exactly the same length.(they are not next to one another) as shown inFigure 11. This staggering lets you put on the Now, put the wire ends inside the open endfinal wrap of tape without leaving a big lump of the wire nut shown in 12(c). Screw the wirein the cord. Start the taping well back up on nut on in a clockwise direction because itthe jacket, about 2.5 cm (1 in.) or so, and wrap has a right-hand thread. The small metalFIGURE 11—The proper method of winding the wires for an inline splice is shown here. Crimp-on inlinesleeve splices may also be used. (Courtesy of McGraw-Hill Book Company, Inc.)
Electric Heating Appliances and Fans 11 THIMBLE SPIRAL SPRING (a) (b) (c)FIGURE 12—A typical wire nut is shown in (a) with the correct method of installing the nut in (b) and (c).(Courtesy of McGraw-Hill Book Company, Inc.)spring inside the wire nut, shown in 12(a), then replace the connector with a wire nut.will catch the wires and pull them up inside The connection will be just as good as thethe insulator. Keep on turning until the original.connection is very tight. Make sure that theinsulation of all wires is covered by the flared Crimp-On Lugsend of the wire nut. If it isn’t, a short circuitmay develop later on. In many appliances, especially those using screw terminals, you’ll find a special kind ofThe solderless connector is especially handy terminal lug called a crimp-on lug, which canbecause it can be taken apart for testing and be attached without soldering.then put back without the use of tools. Insome appliances, you may see connectors The crimp-on lug has a hollow sleeve for thewhich look like wire nuts but aren’t. These wire and a lug which will serve as a terminal.are the crimp-on types which are permanently Strip the wire just enough to go through thisattached by crushing the sleeve inside the sleeve, no more. Push the wire through theinsulator. These can’t be taken apart and put sleeve, making sure that the insulation isback. However, if necessary, you can cut tight against the end of the terminal and thatsuch a connector off, make your tests, and the ends of the wire do not go through too far.
12 Electric Heating Appliances and FansIn factories the crimp-on terminals areinstalled by a machine. In your shop, youcan crimp the sleeve by using the crimp-ontool, which is the top part of the wire strippershown in Figure 13. Pull hard on the crimp-onconnector to make sure that it does have agood grip on the wire.Solderless crimp-on connectors are availablein several different types, such as spade lugs,ring lugs, wire connectors like the wire nuts,and splicing sleeves.When a splicing sleeve is used, the two wireends are pushed into each end of the sleeveuntil they overlap, and then the sleeve iscrimped. An insulator sleeve can be slippedover the finished joint or the joint can betaped. Splicing sleeves can save time if youdon’t want to heat up a soldering iron ordon’t have one with you. FIGURE 13—Crimp-on terminals of many different
Electric Heating Appliances and Fans 13 ✔ Check It Out! 1 At the end of each section in your Appliance Repair text, you will be asked to pause and check your understanding of what you have just read by completing a “Check It Out!” Writing the answers to these questions will help you review what you have studied so far. Please complete Check It Out! 1 now. 1. What is the most common trouble source when a portable appliance does not work? _________________________________________________________________________ 2. Why is defective insulation on a line cord dangerous? _________________________________________________________________________ 3. What is meant by No. 16 AWG wire? _________________________________________________________________________ 4. Define ampacity. _________________________________________________________________________ 5. Which wire carries more current, AWG No. 12 or AWG No. 14? _________________________________________________________________________ 6. What happens if the wire in a line cord is too small? _________________________________________________________________________ 7. What kind of insulation is used on heater cords? _________________________________________________________________________ 8. Why are some line cords equipped with three wires and a three-pronged plug? _________________________________________________________________________ 9. To what should the grounding wire in a line cord be connected? _________________________________________________________________________ 10. How can you recognize polarized, or grounded, outlets? _________________________________________________________________________ (Continued)
14 Electric Heating Appliances and Fans ✔ Check It Out! 1 11. What relieves strain on the wire in a line plug? _________________________________________________________________________ 12. Why are line cords made with stranded wires? _________________________________________________________________________ 13. If you cannot get the original size wire for a line cord, what size should you select? _________________________________________________________________________ 14. When you connect a wire to a screw terminal, how should you wrap the wire around the screw? _________________________________________________________________________ 15. Why is a grommet used in appliances? _________________________________________________________________________ 16. What is the probable cause of trouble if an appliance works at certain times and fails to work at other times? _________________________________________________________________________ 17. How many layers of tape are used for splices? _________________________________________________________________________ 18. What are wire nuts? _________________________________________________________________________ 19. What are crimp-on lugs? _________________________________________________________________________ Check your answers with those on page 81.
Electric Heating Appliances and Fans 15 element rated at 600 watts. This higher-out-ELECTRIC ROOM HEATERS put element would have half the resistance of the 600 watt element.How a Heater Works If the voltage supply is constant, the high current is obtained by low resistance of theOne of the simplest of electrical appliances heating element. The lower the resistance,is the electric room heater. The main part of the higher the current, and the greater thethe heater is the heating element, or heating heat produced by the heater if the voltagecoil, which is made of special resistance wire. remains the same. Electric heaters are ratedWhen voltage is applied to the heater circuit, in wattage, such as 500, 750, or 1000 watts.a current passes through the heating element The higher the heater wattage, the moreand causes it to get hot. The rate of heat heat it produces. A heating element shoulddeveloped in the heating element depends always be replaced with a new one of theon the electric power drawn by the heater. same wattage, so that the unit will produceRemember that electric power, or wattage, the same heat as it did when it was new.is a product of voltage and amperage. Walloutlets supply alternating current at 120 VACto the heating coil. If the current is high, the How a Heater Is Builtpower will be high, and the generated heatwill be high. For example, if a current of 10 Each basic room heater is enclosed in a heavyamps (amperes) flows through the heating metal case. A common heater is shown inelement, the power drawn by the heater is Figure 14. Grilles or screens cover the openings120 × 10 = 1200 watts and the heat will be so that the heated air can circulate to thedouble the heat produced by a 5-amp room. Metal reflectors are used to direct the FIGURE 14—This heater is typical of the many different types of ribbon element heaters.
16 Electric Heating Appliances and Fansheat where it is wanted. Some heaters have with mica sheets or ceramic pads sosmall fans which force out the hot air. The that the heating element can’t touchair then circulates faster around the heater the frame or case.and heats the room quicker. 3. Safety switches, thermostats, and otherHeating elements are mounted on heat-proof safety devices must be checked to makeinsulators to keep them from making electrical sure that the contacts are not weldedcontact with the metal case. In Figure 14, the together or stuck so that they would beheating element is wound diagonally up useless.and down inside the case. The heatingelements are made of special wire which 4. All electrical connections, such as thehas certain resistance and is able to with- connection of the line cord to the heat-stand high temperatures without melting. ing element, must be very clean andAn electric heater may be provided with tight, to avoid developing heat wherea switch that turns the heater on and off. it is not wanted. A hot joint will heatSome heaters have a thermostatic switch in up and oxidize rapidly when carryingaddition to the on-off switch. The thermostatic the heavy current needed in the heatingswitch, or thermostat, turns the heating element, and will burn up.element off automatically when the room 5. When replacing ON/OFF switches, be surereaches the proper temperature. Some other that the new part has the same rating asheaters have safety switches which turn the the original switch. The rating must bepower off if the heater should be tipped over. high enough to do the job. For instance,This is done to prevent any fire hazard. some toggle switches are rated 5 amp at 120 volts, which means a power of 5 × 120 = 600 watts. If you put one ofHints for Servicing Room these on a 1500-watt heater, it will failHeaters in a very short time. Always use parts which have ratings at least 1.5 timesBefore we discuss actual units, there are a the maximum current needed by thefew rules which apply to all resistance wire particular appliance. At 120 volts, theheaters: current through a 1500-watt heater is 1500/120 = 12.5 amp. The rating should 1. The line cord for room heaters rated therefore be at least 1.5 × 12.5 = 18.75 above 500 watts should be high- amp. Use a standard switch rated 20 temperature heater cord. For safety, the amp at 120 volts. You know that this wire in this cord must be large enough switch will work safely and last a long to carry at least 1.5 times the normal time. current. Insulation on line cords must always be in perfect condition, to avoid shock hazard to users. Heating Elements 2. Heating-element support insulators Electric heating elements are usually made must be replaced if they are cracked. If of a special nickel-chromium alloy wire the heater uses open wire as a heating called Nichrome, or from a tungsten-based element, the wire must be tight, so that material. These metals have a very high it can’t sag and touch the metal frame melting point, which is necessary because or case. Enclosed heating elements, the metal gets red hot when in use. Copper like those used in electric irons or wire oxidizes very quickly if red hot and coffeepots, must be very well insulated will burn up in a short time. Nichrome wire
Electric Heating Appliances and Fans 17and other special alloys will stand manyheatings and coolings without too muchoxidation. This wire has a higher resistancethan copper and other metals, and it there-fore, takes a shorter length of Nichrome ortungsten wire to make up a heating element.In the electrical business, heater wire is oftencalled resistance wire.A heating element can be made of round wirecoiled like a spring, or of flat ribbons of varioussizes. Some elements are made of bare wiresmounted on insulators. Others are sealedelements which use resistance wire insulatedwith a ceramic material and placed insidea metal tube which looks like a pipe. Sealedelements must be used in any applicationwhere the surface of the heating elementmust be grounded, as in electric ranges andwater heaters. FIGURE 15—A ceramic heater uses a specialResistance wire has two special characteristics ceramic heating element and many safetythat you must remember. First, since it gets devices. (Courtesy of Rival Manufacturing Company)red hot while heated, you can’t use any rubberor plastic insulation on it. The element willnot only be hot in the physical sense, but push it out, and replace it with a bolt andelectrically “hot” as well. All insulators used nut, and connector.must be able to withstand the high heat and One type of modern electric room heater isat the same time give enough electrical insu- the ceramic heater. A typical ceramic heater islating protection. As mentioned, open-wire shown in Figure 15. The ceramic heater hasand sealed elements in heaters use insulators a resistance element encapsulated in a ce-of ceramic material. Elements in electric ramic material. A fan is used to move airirons and coffeepots use flat sheets of mica through the ceramic heating element. Thisor preformed ceramics. type of heater also normally has manySecond, you can’t solder resistance wire. safety devices such as overload switchesOne reason is that the heating element heats and tip-over switches, along with switchesto a temperature higher than the melting to select fan only, or fan with heat and an ad-temperature of solder. Another is that solder justable thermostat.will not stick to the wire, because of its com-position. Therefore, all electrical connectionsmust be made tight mechanically; that is, with Mounting Heating Elementsbolts, nuts and special connectors. A typical coiled heating element mountedIn some of the cheaper appliances you’ll on ceramic insulators in a small space heaterfind the electrical connections are made by is shown in Figure 16. The coil is tight be-riveting together the ends of the heater wire tween the insulators; this is a key point toand the line cord. To replace this kind of remember.connection, file off one side of the rivet,
18 Electric Heating Appliances and Fans insulating shoulder washer which fills up the hole in the metal frame so that the bolt can never make electrical contact with it. The most important part of this connection is the insulation between the bolt and the frame. The shoulder on the insulating washer must be in good shape, so that it will fill the hole with insulation. Sometimes you’ll find that this shoulder has been crushed by care- less installation, or broken off. If so, put on a new one. Mica washers are often used, but they are very brittle and must be handled with care. They will stand high pressure as long as it is applied properly, but you can break them when they’re loose. When preparing a joint, put the shoulderFIGURE 16—Since the coiled wire heating element washer in the hole first. Then slip the flatcarries 120 VAC, the element must be supportedby insulators such as the ceramic insulators metal washer and flat insulating washershown here. over the bolt, and put the bolt through the hole. Put the other flat metal washer overAn exploded view of a complete room the bolt, and run the first nut down withheater is shown in Figure 17. It has a ribbon- your fingers to hold all of the insulators intype open element wound between bars of place. Tighten the nut with a socket wrench,ceramic material. You’ll find similar heaters holding the bolt with a screwdriver. Don’tusing coiled elements. As long as the total use too much force, or you’ll crush the micawattage and length are the same, there is no washers. Before you continue your work,difference between the ribbon and coil type; test the connection to be absolutely sureone could be used in place of the other. that it is not shorted to the frame. Now put on one of the flat metal washers, wrap the wires clockwise around the bolt,Electrical Connections to and put on the other flat metal washer andHeating Elements the second nut. Be sure that the insulation on the line cord is as close as possible toAll electrical connections to heating elements the connection. Don’t let any of the insula-must be clean and mechanically very tight. tion get between the washers. Tighten the connection down until it is firm, but not tooYou’ll find many connections made with a tight. If the connection is too tight, you’llbolt, using two nuts and flat insulating and crush the wires or make them squeeze outmetal washers, as shown in Figure 18 on between the washers. With only very littlepage 20. The first nut is used to hold the bolt practice, you’ll learn just how much forcetight to the metal frame or bracket, and the to use.second nut is used to make the electricalconnections. The bolt must be completely A connection prepared as just described forinsulated from the metal bracket or the frame a small room heater is shown in Figure 19(a)of the appliance. This is done by using flat on page 21. The heating element is connectedinsulating mica washers between each end to one end of the bolt, and the line-cord wireof the bolt and the metal, and a special is connected to the other end, on the opposite
Electric Heating Appliances and Fans 19 FAN BLADE MOTOR HANDLE RIBBON TAKE-UP REAR COVER CHROME ELEMENT PROBE INSULATOR ASSEMBLY SWITCH GRILLE WRAPAROUND REFLECTOR PAN SWITCH THERMOSTAT BLOCK KNOB INSULATOR SECONDARY ELEMENT CORD THERMOSTATFIGURE 17—Notice the many components used in an electric room heater. (Courtesy of Arvin Industries, Inc.)side of the bracket. Does this make any materials will with successive heating anddifference? Not at all; the bolt makes electrical cooling cycles.contact between them. The most important consideration in usingA close-up view of one of these terminals is the stainless steel terminals is that the crimpshown in Figure 19(b). The mica washers be- is tight on the wire. Special crimping toolstween the first nut and the metal frame are are available for tightly crimping stainlessclearly visible. The second nut and washers steel terminals. Also, many heavy dutyhold the line-cord wire. Many modern resis- crimpers, such as those on a large pair oftance-wire type heaters will use crimp-on electrical lineman’s pliers, can be used.terminals for the power line connections tothe heating element. These terminals areusually made of a stainless steel alloy. Why Thermostats in Heatersstainless steel instead of copper terminals?Stainless steel has many advantages. First, As you progress through this course, you’llstainless steel can be heated and cooled learn that thermostats are used in manythousands of times without the annealing, different appliances. You should know howor softening of the metal. Second, stainless they work. The name “thermostat” meanssteel does not oxidize, or discolor and rust, “constant heat” (thermo = heat and stat =as most other standard crimp-on terminal constant). Thus, a thermostat is a device
20 Electric Heating Appliances and Fans CORD INSULATION SECOND NUT FLAT METAL WASHERS FIRST NUT STRANDS OF BARE WIRES FLAT METAL WASHER INSULATING SHOULDER WASHER METAL BRACKET OR FRAME FLAT INSULATING WASHER FLAT METAL WASHER BOLT BOLT HEADFIGURE 18—The connection to a heating element may be made by means of a bolt and a series of insula-tors and nuts.which is used in heating devices to keep the different rates when heated. Such a blade isheat at a constant level. shown in Figure 20(a) on page 22. The two different metals are indicated by 1 and 2.A thermostat switches off the power supplied One end of the blade is held in a clamp.to the heating element when the temperature When the blade is cool, it is straight. If it isincreases above a certain level, and it turns heated, metal 2 expands much more thanthe power back on when the temperature metal 1. Therefore, the blade bends from thefalls below a certain level. The thermostat “push” of metal 2 against 1, as shown in 20(b).can operate at any given temperature, at avery low temperature in refrigerators and Heat can be applied to the blade in severalat a very high temperature in furnaces, ways. In the simpler thermostats, the currentdepending on the type and setting of the passes through the blade itself. The heatingthermostat itself. effect developed by the blade resistance controls the bending. If the heater drawsEach thermostat has a temperature-sensitive too much current, it becomes too hot. If aelement. This is usually a bimetal blade. A pair of switch contacts are attached to thebimetal blade is a flat metallic strip made free end of the blade, as in 20(c), they willof two different metals which expand at open when the blade bends due to high
Electric Heating Appliances and Fans 21FIGURE 19—The connections to a coiled-wire type heater are shown in (a) and magnified in (b).temperature. This turns the heating element the type of appliance. For instance, a dropoff, and the temperature drops. When the in temperature would close the switch ofblade cools off enough, it straightens, the a heating unit, while a rise in temperatureswitch contacts close, the current starts would close the switch of a cooling orflowing again, and the heater gets hotter. refrigeration unit. Thermostats can be made to work due to a small or a great change inIn more elaborate thermostats, a separate temperature. The exact characteristics of theheating coil is placed under or around the thermostats differ in various types of heaters.blade. The electrical contacts are attached tothe blade but may be insulated from it. Nocurrent passes through the blade; the bladeis heated by the coil. The actual control of Testing Heatersthe electrical circuit is done with relays hav- The testing of electric heaters is very simple.ing heavy-duty contacts, since heavy cur- Turn the heater on and see if it gets hot. If itrents must be switched on and off. In this doesn’t, you should test to find out why it isway, the thermostat blade can be made not working. If there is a short circuit, it willmuch more sensitive to small changes in blow the fuse or trip the circuit breaker. Thistemperature. is one of the easiest faults to locate becauseYou’ll find thermostats in all shapes and it has very obvious symptoms. If the heatersizes. Some will have coiled-spring blades doesn’t get hot when plugged in and turnedinstead of flat blades. However, they all on, then there is an open circuit somewherework in the same way. A rise or fall in tem- in the heater circuit.perature causes the blade to move, opening Before going any further with testing theor closing an electrical contact and acting as heater itself, make sure that the heater isa switch. The particular action depends on supplied with the proper voltage. This test
22 Electric Heating Appliances and Fans CLAMP 1 2 (a) CLAMP 1 2 (b) CURRENT CLAMP ELECTRICAL CONTACTS 1 2 (c)FIGURE 20—Because of the use of dissimilar metals, the thermostat will move when heated. Notice thedifference between the cool blades (a) and heated blades (b). Figure 20(c) shows a set of contactsconnected to the blades.should be made first when testing any elec- should be replaced if it does not hold thetrical appliance. It can save time. No appli- plug prongs tightly.ance can work if it does not receive voltagefrom the wall outlet or from another source.The fastest way to check the wall outlet is to How to Test a Heater for anmeasure the voltage. If the voltage measures Open Circuitaround 120 VAC, then you know that thevoltage is available and that the circuit fuse After making sure that a normal voltageis not blown nor the breaker tripped. supply is available at the wall outlet, you’ll have to take the appliance apart to get at theWhen you plug in an appliance, note how electrical connections. For safety, thesethe line plug fits into the outlet. It should connections are always on the inside of thenot slip in too easily. You should always feel appliance. First, be sure that the line plugsome friction as the prongs of the plug slide is pulled out of the wall outlet and lyingbetween the spring contacts inside the outlet. where you can see it. Now, you can workThis friction is absolutely necessary for a and test in safety.good connection. Old and worn outletsaccount for many electrical troubles when Most small heaters are housed in metal casestheir contacts become loose. The outlet with grilles for better air circulation. The grilles are held in place with small screws
Electric Heating Appliances and Fans 23at the corners or edges. You’ll be able to see across the switch should be cause for switchthrough the grille to test the connections. replacement. The reason for the replacementWhen you remove the grille or any cover, of the switch is that heat caused by the resis-put the screws, knobs, and all small parts tance within the switch will cause the switchyou take off into a little box, so that they’ll to fail in a short period of time.all be readily available when you are readyto put the heater back together. This simple The final test to make is across the heatingmeasure can save a lot of time, and time is element. Typical resistance-wire type heatersmoney. are in the range of about 4 to 20 ohms depending upon the wattage of the heatingIf the heater has three parts, the line cord, element.the switch, and the heating element, youhave three possible sources of trouble. We have described these tests in detail soAlways remember the following rule, which that you can get used to the procedure ofapplies in every case of electrical trouble: testing. Actually, in any of the open-wireThere are as many possible causes of heating elements, you won’t need any testtrouble as there are parts in the circuit. equipment to find an open element, except your eyes. A break in the element will beTests of the three parts are shown in Figure 21. easy to see. However, if the element isn’tThe safest tests are continuity or resistance visible, or if it is one of the sealed types, youtests which can be made with the power must use electrical tests to locate the trouble.removed from the heater.The first two tests to make are continuity testsof the line cord as shown at positions 1 and How to Replace Heater2. The line cord should show near zero resis- Partstance from each of the prongs on the plugto the terminal of the switch and the neutral After you have located the faulty part, replaceside of the heater element. it with a new one. Do not try to repair it. When making replacements on any kindThe switch should be tested next. With of appliance, it’s always best to use exactthe switch in the OFF position, you should duplicates of parts if you can get them. There’sread infinity ohms across it. With the switch a simple reason for this: The duplicate partturned to the ON position, you should read will mount exactly like the old one, withoutnear zero ohms. Any reading above one ohm trouble. Switches, for instance, are often mounted by two ears or brackets held by small screws or bolts and nuts. If the new switch were a little larger than the old one, 3 you’d have to drill a new mounting hole in SWITCH the case. This would leave the original hole exposed and make a sloppy-looking job. 1 HEATING Replacement line cords must be heavy enough 4 ELEMENT to carry the heater current. Replacement 2 switches must have a current rating at least LINE CORD as high as the original or they’ll burn up in a little while. Heating elements should always have the same wattage as the original.FIGURE 21—These are the points at which youcan make resistance or continuity tests on theheaters.
24 Electric Heating Appliances and FansWhen you are going to replace a coiled or a Check the whole heater again very carefully.ribbon heating element, leave the old one in All connections should be tight and the ele-place until you have obtained the new one ments properly mounted. Be critical of yourand are ready to do the job. By doing this, own work. It is much better for you to findyou can see exactly how the original element a small mistake than to have the customeris mounted and how it is wound around the find it. The skilled professional technicianinsulators and racks. If you should have to is never certain that an appliance will worktake the old one out, make a rough sketch until he turns it on and sees it work.of how it was mounted on the insulators.This will save you time if several days elapse The most important test is the safety check.between the time you take it out and the time This test makes sure that there are no shortyou put in the replacement. circuits, or grounds, between the electrical parts and the case. These could cause a veryTo replace a coiled element, be sure that it dangerous shock hazard to the users. Theisn’t stretched too much while putting it in first test for ground should be made withplace. If it is, it will be loose on the insulators a meter set for high resistance, between theand could fall off and cause a short to the line-cord prongs and the case, with the switchcase. First fasten one end to the terminal, then turned on. This should read an open circuitvery carefully thread it around the insulators on a digital VOM, or no deflection at all onuntil you get to the other terminal. Ordinarily the ohm scale of an analog VOM. Reviewit will be a little short because the coil hasn’t the rules given in previous lessons for howbeen stretched. Stretch it very gently until to make continuity tests and leakage teststhe open end just reaches the other line with various test equipment.terminal. You may have to stretch it slightlyin the middle to get enough slack. Try to You should never hesitate to make duplicatedistribute the tension over the whole element; tests or to make the same test in severalit will last longer. different ways. For another test for grounds, connect one lead from the meter set to ACWhen replacing the flat-wire, or ribbon, volts to a good ground, such as a water pipe.elements, each turn of the element should Now, plug the appliance in and turn it on.fall into a notch or hook on the mounting Touch a bare metal part of the case with theinsulators. Check the path and location of other lead from the meter, as shown Figure 22.the original element very carefully before If you measure no voltage, the applianceyou take it off. Don’t let the ribbon elements is safe.twist; this puts an excessive strain on themwhen heated and can cause early failure.Later on, you will study about other typesof heating elements such as those used on HEATER DIGITAL VOMwater heaters and electric ranges. We’lldescribe in detail the mounting of each typeof element as we come to it. OUTLETTesting after Repairs Are WATER LINE CORD BARE PIPEFinished METAL PARTNow we come to a very important part ofthe job: testing to be absolutely sure that FIGURE 22—This diagram shows how to perform athe appliance will work just as it did when leakage test on the case of a heater or otherit was new, and that it will be safe to use. metal-cased appliance.
Electric Heating Appliances and Fans 25Reassembling the Heater If you can’t find an exact duplicate screw and the missing screw is visible from theAfter all the tests have been completed, front, look underneath or on the back. Takeput the heater back together. Be sure that out an original screw from there, replaceall screws are replaced. That’s why you it with another type, and put the originalput them all in the same box, to keep them screw where it can be seen. This makes thetogether. Quite often, appliances will come job look neat. Careful attention to little detailswith screws missing. Replace the missing like this will help you build up a reputationscrews with new ones from your stock. The as a really skilled craftsman. Don’t ever over-appliance will then be returned to the owner look details.as good as new. Match the original screwsas closely as possible. In most cases, thiswon’t be hard to do.
26 Electric Heating Appliances and Fans ✔ Check It Out! 2 1. How does a thermostat control heat? _________________________________________________________________________ 2. Why does a thermostat use a bimetal blade? _________________________________________________________________________ 3. What is the probable trouble if the heater does not heat up when plugged into an outlet with the switch closed? _________________________________________________________________________ 4. If a heater is designed to carry a normal current of 10 amp, what should be the minimum current-carrying capacity (ampacity) of the line cord? _________________________________________________________________________ 5. What kinds of insulators are used for mounting heating elements? _________________________________________________________________________ 6. How should the connections of a heating element be made? _________________________________________________________________________ 7. Why should you slightly stretch the heating coil when mounting it? _________________________________________________________________________ 8. What check should you make after the heater has been repaired? _________________________________________________________________________ 9. How is the check for grounds made? _________________________________________________________________________ Check your answers with those on pages 81–82.
Electric Heating Appliances and Fans 27COMBINATION OF Resistance of HeatingHEATING ELEMENTS Elements When you know the current drawn by the heating element from an AC line, you canCurrent in Heating Elements determine the resistance by the formula of Ohm’s law:The amount of heat produced in a device ismeasured in calories or British thermal units R = E/I(Btu), but we seldom need to know howmany calories are produced by a toaster, an in whichelectric range, or any other electric heating R = resistance, in ohmsappliance. We are mainly interested in findingat what rate the heat is produced electrically. E = line voltage, in voltsThis rate is measured in watts. The rate ofheat depends on the power that the appliance I = current, in amperesdraws from the electric circuit to produce a For most small home appliances and lamps,desired amount of heat. Therefore, heating the line voltage is 120 volts, but water heaters,appliances are rated in watts or kilowatts ranges, dryers, and other heavy equipment(1 kilowatt, or 1 kw, = 1000 watts). require 240 volts.Usually the wattage rating of a heating For example, a 3000-watt heater which drawsappliance is indicated on the nameplate. 25 amp from the 120-volt line has a resistanceSince the voltage is generally 120 volts, thecurrent drawn by the appliance can be R = E/Idetermined by dividing watts by volts.The formula is = 120/25 = 4.8 ohmsI = P/E What will happen to the wattage produced by the heater if the resistance of the heaterin which is increased? Let’s assume that the 4.8 ohm heating element is replaced by an elementI = current, in amperes which has a 10-ohm resistance. First, find howP = power, in watts much current the new element will draw from the line. You can do that by applying anotherE = voltage, in volts form of Ohm’s law, which isFor example, you may want to find the I = E/Rmaximum current a 3000-watt heater willdraw so that you can use the right size of In other words, divide the voltage by thefuse or circuit breaker in the heater circuit. resistance to get the amperes:Fuses and circuit breakers are rated in current I = E/Rvalues, such as 20 amp or 30 amp. For the3000-watt heater the current will be = 120/10 = 12 amp.I = P/E The 10-ohm element draws much less current than the 4.8 ohm element. Remember the = 3000/120 = 25 amp. rule: More resistance draws less current at theYou should select a 30-amp fuse or circuit same line voltage.breaker.
28 Electric Heating Appliances and FansNow you can find the wattage by multiplyingvoltage and current. The formula isP=E×I 120 VOLTS 400 WATTS = 120 × 12 = 1440 watts TOTAL POWER = 400 WATTSThis wattage is much lower than the wattage (a)obtained with the 4.8-ohm element. So, therule is: More resistance draws less power at thesame line voltage. 400 400 120 VOLTS WATTS WATTSNow let’s assume that the 4.8-ohm heatingelement has been replaced by a 2-ohm TOTAL POWER = 800 WATTSelement. The current drawn from the line (b)will beI = E/R 120 VOLTS 400 400 400 = 120/2 = 60 amp WATTS WATTS WATTSand the power will be TOTAL POWER = 1200 WATTSP=E×I (c) = 120 × 60 = 7200 watts FIGURE 23—This illustration shows how more heatLower resistance causes higher current and can be developed using parallel heaters.higher power at the same line voltage. parallel with the first two, as in 23(c), we getThe foregoing rules will be helpful when three times the power of one element, orreplacing and combining heating elements. 1200 watts. How can we make a variable heater usingMultiple-Heat Circuits with these three elements? By using a selector switchParallel Elements which will connect one, two, or three elements across the circuit, so that 400, 800, or 1200Let’s see how heating elements can be used watts can be produced by the combinationto get different heat outputs. Assume that of elements. As shown in Figure 24, one endwe have three heating elements, each one of each element is connected to the commonrated 400 watts. This information is on the side of the AC line. The other side of the linetag or the label on the heater. The elements is connected to a selector switch, which isare made for use on a 120 VAC line, and here of the shorting type. The movable bartherefore, if connected across 120 volts, each is connected to the hot side of the AC line.one will develop heat at a rate of 400 watts. The bar can be moved from left to right to make a connection between the powerlineIf we connect only one of the elements and one, two, or all three of the elements. Theacross the 120-volt line, as in Figure 23(a), switch is shown in the MEDIUM position.we get 400 watts. If we connect another Two of the elements are connected, while theelement in parallel with the first, as shown third is still open; thus 800 watts is produced.in 23(b), we have the same voltage across By moving the shorting bar one position toeach one and we get double the power, or the left, or to the LOW position, we would800 watts. Connecting the third element in have only one element connected in the
Electric Heating Appliances and Fans 29 If you turn the switch to the LOW position, HOT WIRE all three elements are in series in the circuit LOW MEDIUM supplied by the line voltage. The current 400 800 HIGH drops to one-third of its original value, and WATTS WATTS 1200 120 VOLTS WATTS the wattage drops to one-third of the value of one element, or 400/3 = 133 watts. If you COMMON 400 400 400 WATTS WATTS WATTS want to measure the voltage drops, you’ll WIRE find one-third of 120 volts, or 40 volts, across the terminals of each element. This equal SHORTING division of voltage and wattage works out SELECTOR SWITCH only if each element has exactly the sameFIGURE 24—Multiple heat settings can be accom- resistance.plished using a switch to select parallel heatingelements. In appliances themselves you will find elements of different wattages used incircuit, producing 400 watts. By moving multiple-heat circuits. The actual wattagethe bar all the way to the right, or to the depends on the work the appliance is sup-HIGH position, 1200 watts is produced. posed to do, and how much heat is needed in each position of the selector switch. The manufacturer can combine resistance valuesSeries-Connected Elements so that they produce exactly 100, 200, and 300 watts on the three positions, or any otherNow let’s connect two equally rated elements groups of selected wattages.in series. How much wattage will two 400-watt elements develop when connected Always remember the following basicin series? Each one of them has a certain principle: The lower the resistance of theresistance R. When we connect two equal heating element, the higher the wattage atresistances in series, we double the resistance the same voltage. In a series combination,and the current will be cut in half. The voltage as in Figure 25, the single element has theremains the same. If the current is cut in half, lowest resistance, but will be the hottestthe wattage also is cut in half. We would get one and will produce the highest wattage.200 watts, or one-half of the value of either Let’s assume, for example, that in a coiledone of the original elements. heating element of a room heater, a 10 cm (4 in.) piece breaks off at one end. If youIf all three elements are connected in series,as shown in Figure 25, a selector switch maybe added to change the element combina- SELECTORtions. The switch is not of the shorting type. SWITCHWith the selector switch in the farthest left, LOWor HIGH position, only one of the elements HIGH 133 WATTS 400 WATTSis connected in the circuit. This would give MEDIUM400 watts. Turning the switch to the middle 120 VOLTS 200 WATTSposition shown in Figure 25, or MEDIUMposition, the current has to go through bothelements connected in series. Since the resis- 400 WATTS 400 WATTS 400 WATTStance is double, the current is one-half of theprevious current and the heat is produced FIGURE 25—This illustration shows how series-with a power of only 200 watts. connected elements may be selected for varying heat output.
30 Electric Heating Appliances and Fansstretch the remaining part and connect it would be instantly melted because it wouldagain, the element will get considerably draw a very high current, develop a veryhotter, because you have taken out a part high wattage, and produce so much heatof the resistance and made it possible for that the copper wire would melt.more current to flow through the element. A simple rule will help you to rememberSimilarly, if you connected a short piece of this principle: The more wire, the less heat,wire directly across the 120 VAC line, with- and vice versa.out any fuses or circuit breakers, the wire