How to repair small kitchen appliances: Electricity, the Machine’s Mind and Muscle
Most household appliances rely on electricity to perform tasks that would otherwise be monotonous, time-consuming, sometimes even dangerous handwork. Electricity powers motors, generates heat and coordinates interaction of mechanical parts. It can also protect an appliance from self-destructing by shutting it off if it begins to malfunction.
As baffling as the internal workings of an electric appliance may seem, there is always an inherent logic to the arrangement of its parts. An electrical system can be likened to a plumbing system, in which water flows through pipes under pressure. In the electrical system it is current, measured in amperes, or amps, that flows through the "pipes" of wire. The pressure that drives the current is called voltage and is measured in volts. The product of the two (the volts multiplied by the amps) is called wattage, or watts.
All electricity consists of current and voltage, but the current can flow in two different patterns. How it flows depends on its source. The current supplied by the outlets in most homes is called alternating current, normally abbreviated AC. Alternating current pulsates forward and backward about 60 times per second, a characteristic that permits it to travel great distances.
Current can also flow continuously through a circuit without reversing its path. Called direct current (DC), this type is most commonly found in cordless, battery-run appliances because direct current, until recently, could not be transmitted easily by power companies over distances greater than a few miles. (Laser beams and solid-state circuitry are now circumventing this handicap.)
Regardless of the type of current passing through it, every material exposed to an electrical charge exhibits certain characteristics. The most important is the material's resistance or conductivity its ability either to stop the charge or to carry it along. Metals are the best conductors silver and copper head the list so metals are used to make electric wires. Paper, wood and even air will resist the flow of current, and plastic and rubber are such good resistors that they are used to insulate metal wires.
Resistance, which is measured in ohms, can be useful in other ways too. Even the best copper wire presents a certain amount of resistance, and this property can be exploited. Just as water in a pipe of narrow diameter will build up a great deal of friction, so current passing through an undersized wire will be resisted sufficiently to cause the wire to heat. On some appliances wires are deliberately overloaded to become red-hot, and the resultant heat is used as a source of energy. But accidental overheating, which can cause wires to melt, is a major source of breakdowns in appliances.
Wire carrying electrical current displays one other important characteristic: It develops a temporary magnetic field. This electromagnetism can be used to make parts move and is essential to the functioning of electric motors and various other devices found in appliances.
Electricity creates other useful effects in passing through conductive and non- conductive materials. For instance, if two conductors are separated by a noncon- ductive insulator, current applied to one conductor will be stored there until the build-up reaches the conductor's electrical capacity. Then the first conductor will discharge the current to the second conductor. This ability to store and then release a current is the principle behind the capacitor, a boosting device used with an electric motor to help it start.
A similar effect is produced when alternating current is run through one of two adjacent but unconnected conductive materials. The current passing through the first conductor creates, or induces, current in the second conductor. This effect, induction, powers the most common and efficient type of motor found in household appliances.
One or more of these effects operate every electrical component in an appliance, and each component is linked in an electric circuit. When one component malfunctions, the effects are likely to extend to the performance of others in the circuit. In diagnosing the cause of an electrical failure, it is therefore necessary to know how the electricity is supposed to move through the circuit.
Two tools for troubleshooting problems in the circuitry are a continuity tester and a multitester, both battery-powered. A continuity tester drives a small amount of current into one end of a circuit and indicates usually by lighting a small bulb whether the current emerges at the opposite end. Since appliances depend on complete circuits to perform their tasks, this test allows you to check out an entire electric system to see if the malfunction is indeed an electrical problem. A multitester also called a volt-ohm meter allows you not only to test individual components for continuity, but also to measure the specific amount of voltage or resistance present at the component.
Whenever you disconnect a component for such a test, carefully mark its wire connections with a wax pencil or with a taped label so that there will be no question later about where to reconnect the wires. Before getting involved in such disassembly, however, look for more obvious solutions. Very often an appliance's problem can be traced to its plug and electric cord; you can check these for continuity without taking the rest of the appliance apart. Equally fundamental is a blown fuse or a tripped circuit breaker at the house service panel.
A less obvious source of trouble, but one that can also be spotted without taking the appliance apart, is a temporary drop in voltage from the power company during the summer, for example, when the demand for electricity is high. When the voltage drops more than 10 per cent below normal, an induction-type motor will draw more current in an effort to maintain power; the excess current may overheat the motor and burn it out.
The test for the amount of voltage being supplied is made at the outlet where the appliance is plugged in. In fact, this test is a standard trouble shooting procedure. Remember, however, that even though the outlet is considered to supply 120 or 240 volts, the actual amount of voltage may vary from 108 to 135 volts, or from 208 to 260 volts, depending on the power company. The motors in appliances, which are powered by 120-volt circuits, and the heating elements in large appliances such as ranges and dryers, which require 240-volt circuits, are made to work efficiently despite these variations from the nominal voltage.
Tracking the current's path is an important technique in appliance repair. Professionals decipher the organization of a wiring system by referring to a symbolic drawing called the schematic diagram. On large appliances, this diagram is likely to be glued to a panel cover, but for any appliance it can always be obtained from the manufacturer. Bv following a wiring diagram, you can systematically test the components in sequence and thus locate the point at which a circuit has broken down.
Despite their dissimilar appearances, both the pictorial representation of the heater and its schematic diagram contain basic elements found in a typical electrical circuit: the entrance and exit points of the current and the electrical components it services. In general usage, a zigzag line represents a resistor or a heating element; an oval or circle respresents a large device such as a motor, and the shapes within the oval or circle represent the motor's components; a broken line with an arrowhead angled to one side of the circuit indicates the presence of a switch.
The first element to look for on any wiring diagram is the source of electrical power. Whether it comes from a wall outlet or a battery, the power enters an appliance and each component at terminals thin metal plates that touch the source of their electrical current. On a wiring diagram a terminal is represented by a small dot or circle.
The current is carried between the terminals by wires, represented on the diagram by lines. The lines turn at right angles at the corners of the circuit, and very often will split off in more than one direction, to indicate current carried to separate circuits within the main circuit. In the actual appliance, these various circuits create a maze of wires.
To help identify the circuits, the wires often are color-coded, and the colors are then labeled in abbreviated form on the wiring diagram.
One important wire found in all appliances is a black wire leading to the first component and marked L, for "line," in the wiring diagram. This is a hot wire the incoming power line, which supplies the current. Another wire is colored white and labeled N, for "neutral." This wire completes the circuit by carrying current from the last component back to the power source. The neutral terminal often is indicated not with the usual dot or circle, but with stacked horizontal lines in the shape of an arrow. The neutral wire is sometimes called the power ground, because it is connected to a power line that carries the current directly into the earth.
Within the circuit, a ground wire can also serve a different purpose. If a defective component leaks current to the metal body of the appliance, it poses the risk of a dangerous shock. To guard against this hazard, a ground wire, usually colored green, is attached to the body of the appliance and connected to a separate ground terminal. The appliance has a three-prong plug; the third prong is connected to the third wire in the power cord, which serves to carry the leaking current from this safety terminal directly to a pipe that is buried in the earth. Hence the circuit is quite literally grounded.
The last two elements found in a typical circuit are the controls that start and stop the flow of electricity and the components that actually perform the work In the diagram a switch is the means of control; it acts like a drawbridge in the circuit. When it is down, current flows across to the working components, in this case a heating element and a motor. The circuit is then said to be closed, or continuous. When the switch is open, the current cannot cross and the circuit is said to be open, or not continuous.
To check the cord for internal shorts, clip the tester to one wire and touch the probe to the prong that does not light the bulb. Twist and bend the cord along its length. If the bulb lights, replace the cord. If the bulb does not light, clip the lead to the other wire and repeat the test.
The selector switch on this typical multitester is divided into four zones to test ohms, DC volts, AC volts or DC amps. To use the tester, set the selector switch for the test desired and read the appropriate scale on the meter.
To connect the multitester to a circuit or component, turn off the power to the appliance and place the two metal probes on the terminals of the circuit or component. Then plug the leads to these probes into jacks on the multitester the red lead into the jack marked with a plus sign, the black lead into the jack with a minus sign.
As a safety precaution, when making voltage tests with the appliance power turned on, use a set of leads with insulated alligator clips instead of the metal probes (inset), to protect yourself against shocks (box).
Before using the multitester to check resistance or continuity, adjust the calibration on the meter to register 0 on the ohms scale; set the selector switch at RX1, touch the probes together and turn the small knob marked ohms adjust until the needle stands at 0 on the ohms scale.
To measure either resistance or continuity, make sure the power is off, turn the selector switch to an ohms setting and read the uppermost scale. For resistance readings up to 500 ohms, set the selector switch at RX1 (resistance times 1) and read the amount directly off the meter. For resistance readings up to 5,000 ohms, set the selector switch at RX10 and multiply the reading on the meter by 10. For resistance readings greater than 5,000, set the selector switch at RX1K (resistance times 1 kilo ohms, or 1,000 ohms) and multiply the meter reading by 1,000.
For a continuity test, set the switch at RX1, turn off the power and touch the probes to the circuit terminals. If the needle swings to the right, the circuit has continuity, if the needle stays on the infinity reading, the circuit is open, i.e., there is maximum resistance to the current.
For a voltage test, turn the selector switch to the AC or DC voltage setting just above the voltage you expect from the circuit. Then, with the power on, take the reading from the appropriate voltage scale-AC or DC. For example, appliances that are plugged into a standard wall outlet with 120 or 240 volts would be tested at the 250-volt AC (250 ACV) setting and be read from the AC scale. There are four bands of numbers under these voltage scales. If the selector switch is set at 5, 25 or 125 on the DC voltage
scale or at 10 on the AC scale, use the band ending in that number for your reading. If the selector switch is set at 50, 250 or 1000 on the AC scale, or at 500 or IK on the DC scale, divide the setting number by either 10 or 100 and use the band that ends with the resulting figure. For example, if you set the selector switch at 250 volts AC, divide 250 by 10 and use the band ending with 25 for your reading. In such instances you must multiply the reading by either 10 or 100, whichever number you divided by.
For greater accuracy, close one eye and move your head until the needle and its mirror reflection are perfectly in line. Then note the needle's position in relation to the numbers.
First of all, disconnect the appliance each time you make a repair or an electrical test. If the appliance is wired directly to the house electric system, remove the fuse or flip off the circuit breaker at the main service panel. Mark the fuse or breaker with tape to warn other members of the household that they should not reconnect the power.
When you must perform a test with the power on, as in testing a component for voltage, protect yourself from contact with the live terminal by routinely following a four-step procedure. Turn off the power. Connect the testing probes to the component's terminals with alligator clips special connectors that look like small metal clothespins covered with insulating plastic shields. Then restore the power and observe the test results visually. Turn off the power again before disconnecting the clips.
In rare instances when it is not feasible to use alligator clips, use probes instead. Since you must hand-hold the probes against the live terminals, hold the probes by their long, insulated handles; take very great care not to let your fingers touch the probes' metal tips or the live terminals. And when testing a component that stores a charge, such as a capacitor, be sure to discharge it before touching its terminals.
Finally, when you replace a defective part, make sure that the new part matches the old one identically and ; that it is of top quality. Mismatched or j carelessly made components may cause j the appliance to malfunction or may create a fire or shock hazard.
If your appliance has a three-prong plug (inset), touch one probe to the cylindrical ground prong and the other probe to the body of the appliance. The needle should show 0 ohms; if it does not, the appliance is not properly grounded. Check for a loose green wire inside the wall of the appliance and reconnect it to its terminal.
To test the functioning of the round opening that is found on some outlets, designed to accept the ground prong on a three-prong plug, keep the same setting on the multitester. Insert one probe in the round opening and the other in the smaller of the two oblong openings the hot wire of the receptacle. The multitester reading ought to show full voltage.
As baffling as the internal workings of an electric appliance may seem, there is always an inherent logic to the arrangement of its parts. An electrical system can be likened to a plumbing system, in which water flows through pipes under pressure. In the electrical system it is current, measured in amperes, or amps, that flows through the "pipes" of wire. The pressure that drives the current is called voltage and is measured in volts. The product of the two (the volts multiplied by the amps) is called wattage, or watts.
All electricity consists of current and voltage, but the current can flow in two different patterns. How it flows depends on its source. The current supplied by the outlets in most homes is called alternating current, normally abbreviated AC. Alternating current pulsates forward and backward about 60 times per second, a characteristic that permits it to travel great distances.
Regardless of the type of current passing through it, every material exposed to an electrical charge exhibits certain characteristics. The most important is the material's resistance or conductivity its ability either to stop the charge or to carry it along. Metals are the best conductors silver and copper head the list so metals are used to make electric wires. Paper, wood and even air will resist the flow of current, and plastic and rubber are such good resistors that they are used to insulate metal wires.
Resistance, which is measured in ohms, can be useful in other ways too. Even the best copper wire presents a certain amount of resistance, and this property can be exploited. Just as water in a pipe of narrow diameter will build up a great deal of friction, so current passing through an undersized wire will be resisted sufficiently to cause the wire to heat. On some appliances wires are deliberately overloaded to become red-hot, and the resultant heat is used as a source of energy. But accidental overheating, which can cause wires to melt, is a major source of breakdowns in appliances.
Wire carrying electrical current displays one other important characteristic: It develops a temporary magnetic field. This electromagnetism can be used to make parts move and is essential to the functioning of electric motors and various other devices found in appliances.
Electricity creates other useful effects in passing through conductive and non- conductive materials. For instance, if two conductors are separated by a noncon- ductive insulator, current applied to one conductor will be stored there until the build-up reaches the conductor's electrical capacity. Then the first conductor will discharge the current to the second conductor. This ability to store and then release a current is the principle behind the capacitor, a boosting device used with an electric motor to help it start.
A similar effect is produced when alternating current is run through one of two adjacent but unconnected conductive materials. The current passing through the first conductor creates, or induces, current in the second conductor. This effect, induction, powers the most common and efficient type of motor found in household appliances.
One or more of these effects operate every electrical component in an appliance, and each component is linked in an electric circuit. When one component malfunctions, the effects are likely to extend to the performance of others in the circuit. In diagnosing the cause of an electrical failure, it is therefore necessary to know how the electricity is supposed to move through the circuit.
Two tools for troubleshooting problems in the circuitry are a continuity tester and a multitester, both battery-powered. A continuity tester drives a small amount of current into one end of a circuit and indicates usually by lighting a small bulb whether the current emerges at the opposite end. Since appliances depend on complete circuits to perform their tasks, this test allows you to check out an entire electric system to see if the malfunction is indeed an electrical problem. A multitester also called a volt-ohm meter allows you not only to test individual components for continuity, but also to measure the specific amount of voltage or resistance present at the component.
Whenever you disconnect a component for such a test, carefully mark its wire connections with a wax pencil or with a taped label so that there will be no question later about where to reconnect the wires. Before getting involved in such disassembly, however, look for more obvious solutions. Very often an appliance's problem can be traced to its plug and electric cord; you can check these for continuity without taking the rest of the appliance apart. Equally fundamental is a blown fuse or a tripped circuit breaker at the house service panel.
A less obvious source of trouble, but one that can also be spotted without taking the appliance apart, is a temporary drop in voltage from the power company during the summer, for example, when the demand for electricity is high. When the voltage drops more than 10 per cent below normal, an induction-type motor will draw more current in an effort to maintain power; the excess current may overheat the motor and burn it out.
The test for the amount of voltage being supplied is made at the outlet where the appliance is plugged in. In fact, this test is a standard trouble shooting procedure. Remember, however, that even though the outlet is considered to supply 120 or 240 volts, the actual amount of voltage may vary from 108 to 135 volts, or from 208 to 260 volts, depending on the power company. The motors in appliances, which are powered by 120-volt circuits, and the heating elements in large appliances such as ranges and dryers, which require 240-volt circuits, are made to work efficiently despite these variations from the nominal voltage.
Tracking a Current through Its Circuit
The wiring in an appliance carries current to each electrical component, supplying it with power either to do work or to make decisions by turning switches on and off. The path of the current is a loop, or circuit, that links the components, starting and ending at the power source.Tracking the current's path is an important technique in appliance repair. Professionals decipher the organization of a wiring system by referring to a symbolic drawing called the schematic diagram. On large appliances, this diagram is likely to be glued to a panel cover, but for any appliance it can always be obtained from the manufacturer. Bv following a wiring diagram, you can systematically test the components in sequence and thus locate the point at which a circuit has broken down.
Despite their dissimilar appearances, both the pictorial representation of the heater and its schematic diagram contain basic elements found in a typical electrical circuit: the entrance and exit points of the current and the electrical components it services. In general usage, a zigzag line represents a resistor or a heating element; an oval or circle respresents a large device such as a motor, and the shapes within the oval or circle represent the motor's components; a broken line with an arrowhead angled to one side of the circuit indicates the presence of a switch.
The first element to look for on any wiring diagram is the source of electrical power. Whether it comes from a wall outlet or a battery, the power enters an appliance and each component at terminals thin metal plates that touch the source of their electrical current. On a wiring diagram a terminal is represented by a small dot or circle.
The current is carried between the terminals by wires, represented on the diagram by lines. The lines turn at right angles at the corners of the circuit, and very often will split off in more than one direction, to indicate current carried to separate circuits within the main circuit. In the actual appliance, these various circuits create a maze of wires.
To help identify the circuits, the wires often are color-coded, and the colors are then labeled in abbreviated form on the wiring diagram.
One important wire found in all appliances is a black wire leading to the first component and marked L, for "line," in the wiring diagram. This is a hot wire the incoming power line, which supplies the current. Another wire is colored white and labeled N, for "neutral." This wire completes the circuit by carrying current from the last component back to the power source. The neutral terminal often is indicated not with the usual dot or circle, but with stacked horizontal lines in the shape of an arrow. The neutral wire is sometimes called the power ground, because it is connected to a power line that carries the current directly into the earth.
Within the circuit, a ground wire can also serve a different purpose. If a defective component leaks current to the metal body of the appliance, it poses the risk of a dangerous shock. To guard against this hazard, a ground wire, usually colored green, is attached to the body of the appliance and connected to a separate ground terminal. The appliance has a three-prong plug; the third prong is connected to the third wire in the power cord, which serves to carry the leaking current from this safety terminal directly to a pipe that is buried in the earth. Hence the circuit is quite literally grounded.
The last two elements found in a typical circuit are the controls that start and stop the flow of electricity and the components that actually perform the work In the diagram a switch is the means of control; it acts like a drawbridge in the circuit. When it is down, current flows across to the working components, in this case a heating element and a motor. The circuit is then said to be closed, or continuous. When the switch is open, the current cannot cross and the circuit is said to be open, or not continuous.
Wiring diagrams
Two types of diagrams, pictorial (top) and schematic (bottom), map the wiring and identify the components in an appliance in this example, a heater that is equipped with a blower fan.A Simple Tool to Test the Flow of Current
Checking the current in an electric cord. Unplug the cord, detach it from its terminals on the appliance, and fasten the clip of a continuity tester to one of the exposed wires at the end of the cord. Touch the tip of the tester's probe in turn to each of the prongs on the plug. If there is continuity in the cord, the bulb on the tester should light up against one prong but not the other. If the bulb lights up on both prongs, there is a short circuit in either the plug or the cord. Check the other wire in the cord by moving the clip and repeating this procedure.
A Meter That Performs Multiple Electrical Tests
Anatomy of a multitester. A multitester will measure the exact amount of voltage or resistance in an electric circuit, and by means of the ohms scale, it can also be employed to check continuity.The selector switch on this typical multitester is divided into four zones to test ohms, DC volts, AC volts or DC amps. To use the tester, set the selector switch for the test desired and read the appropriate scale on the meter.
To connect the multitester to a circuit or component, turn off the power to the appliance and place the two metal probes on the terminals of the circuit or component. Then plug the leads to these probes into jacks on the multitester the red lead into the jack marked with a plus sign, the black lead into the jack with a minus sign.
Before using the multitester to check resistance or continuity, adjust the calibration on the meter to register 0 on the ohms scale; set the selector switch at RX1, touch the probes together and turn the small knob marked ohms adjust until the needle stands at 0 on the ohms scale.
Reading the multitester scale.
To measure either resistance or continuity, make sure the power is off, turn the selector switch to an ohms setting and read the uppermost scale. For resistance readings up to 500 ohms, set the selector switch at RX1 (resistance times 1) and read the amount directly off the meter. For resistance readings up to 5,000 ohms, set the selector switch at RX10 and multiply the reading on the meter by 10. For resistance readings greater than 5,000, set the selector switch at RX1K (resistance times 1 kilo ohms, or 1,000 ohms) and multiply the meter reading by 1,000.
For a continuity test, set the switch at RX1, turn off the power and touch the probes to the circuit terminals. If the needle swings to the right, the circuit has continuity, if the needle stays on the infinity reading, the circuit is open, i.e., there is maximum resistance to the current.
scale or at 10 on the AC scale, use the band ending in that number for your reading. If the selector switch is set at 50, 250 or 1000 on the AC scale, or at 500 or IK on the DC scale, divide the setting number by either 10 or 100 and use the band that ends with the resulting figure. For example, if you set the selector switch at 250 volts AC, divide 250 by 10 and use the band ending with 25 for your reading. In such instances you must multiply the reading by either 10 or 100, whichever number you divided by.
For greater accuracy, close one eye and move your head until the needle and its mirror reflection are perfectly in line. Then note the needle's position in relation to the numbers.
A Checklist for Working Safely with Electricity
A standard 120-volt wall outlet carries up to 30 amperes of current (15 amperes in Canada) more than 100 times the amount it would take to stop a person's breathing and immobilize the heart. But repairing an electric appliance, despite this danger, can be a safe operation if you routinely take some precautions.First of all, disconnect the appliance each time you make a repair or an electrical test. If the appliance is wired directly to the house electric system, remove the fuse or flip off the circuit breaker at the main service panel. Mark the fuse or breaker with tape to warn other members of the household that they should not reconnect the power.
When you must perform a test with the power on, as in testing a component for voltage, protect yourself from contact with the live terminal by routinely following a four-step procedure. Turn off the power. Connect the testing probes to the component's terminals with alligator clips special connectors that look like small metal clothespins covered with insulating plastic shields. Then restore the power and observe the test results visually. Turn off the power again before disconnecting the clips.
In rare instances when it is not feasible to use alligator clips, use probes instead. Since you must hand-hold the probes against the live terminals, hold the probes by their long, insulated handles; take very great care not to let your fingers touch the probes' metal tips or the live terminals. And when testing a component that stores a charge, such as a capacitor, be sure to discharge it before touching its terminals.
Finally, when you replace a defective part, make sure that the new part matches the old one identically and ; that it is of top quality. Mismatched or j carelessly made components may cause j the appliance to malfunction or may create a fire or shock hazard.
Using a Multitester to Discover Short Circuits
Testing for a current leak. With the appliance unplugged, set the multitester’s selector switch to RX1K on the ohms scale, and touch one probe to a prong on the appliance plug; touch the other probe to the metal body of the appliance. The needle should not move; even a slight jump to the right indicates a current leak from the circuit to the appliance body great enough to cause a shock. Repeat the test on the other prong.
Checking the Incoming Power at a Wall Outlet
Testing the voltage at a wall outlet. With the power turned on and the multitester set for 250 volts AC, hold the probes by their insulated handles and insert them in the outlet slots. Be extremely careful not to touch the metal part of the probes. The multitester should register within 10 per cent of the minimum voltage specified by the manufacturer of the appliance.
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