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AMAZON multi-meters discounts AMAZON oscilloscope discounts OBJECTIVES: • Describe the use of the volt-ohm meter and clamp-on ammeter in the heating, cooling, and refrigeration industry. • Explain the operation of the basic electric analog meter. • Explain how analog electric meters transfer a known value in an electrical circuit to the meter movement. • Describe the operation of an analog voltmeter. • Describe the operation of an analog and digital clamp-on ammeter. • Describe the operation of an analog ohmmeter. • Explain the operation of a digital volt-ohm meter. • Give the advantages and disadvantages of the analog and digital meters. • Describe the conditions of resistance that can exist in an electrical circuit in reference to continuity. • Describe the source of energy for the operation of the analog voltmeter, ammeter, and ohmmeter. KEY TERMS:
INTRODUCTION Electricity is used in the control and operation of all refrigeration, heating, and air-conditioning systems. Approximately 80% of all service calls made by service technicians are diagnosed as electrical problems. Electrical meters, in most cases, are required for the proper diagnosis of these electrical problems. The large number of electrical problems encountered in the industry requires that industry technicians be able to correctly read and use electric meters. In the industry there are many types of electric meters. Some meters are for specific purposes, while others are for the general day-to-day tasks that industry technicians perform. The installation mechanic must be able to read electric meters to properly complete the initial start-up and testing of equipment. The installation mechanic must be able to read and use electric meters to check the electrical characteristics of a newly installed refrigeration, heating, and air- conditioning system. No installation is complete without electrical checks of the system. The service technician diagnoses and repairs problems found in refrigeration, heating, and air-conditioning systems. The service technician must be able to read and use all types of electric meters to quickly and efficiently locate electrical problems. Without electric meters and the knowledge to correctly use them, the service technician would face an almost impossible task in troubleshooting electrical problems. An electric meter is a device used to measure the electrical characteristics of an electrical circuit. Electric meters are available in many different types and designs and must be selected for their use in the industry. Most meters are used by installation and service technicians. Meters must be durable and maintain adequate accuracy. The electrical characteristics in a circuit that are most important to industry technicians are volts, amperes, and ohms. Although there are other important characteristics, these three are the most important. Most popular electrical meters are built around these three electrical characteristics. Most electrical meters used in the industry are capable of reading more than one electrical characteristic. The most common electrical meters used are the volt-ohm-milliammeter and the clamp-on ammeter with the ability to read volts and ohms. The installation and service technician should be careful in selecting an electrical meter. Electrical meters should be selected for their everyday use in the industry. Technicians should select meters with the best ranges for the electrical characteristics that will be measured the most. We will look at the basic principles used to measure electrical characteristics along with the various types and designs of electrical meters. 1 ELECTRIC METERS An electric meter is a device used to measure some electrical characteristic of a circuit. The most common types of electric meters are the voltmeter, the ammeter, and the ohmmeter. CAUTION: Use the proper electrical test equipment for the job being performed. Basic Principles Most electric measuring instruments make use of the magnetic effect of electric current. When electrons flow through a conductor in an electric circuit, a magnetic field is created around that conductor, as shown in ill 1. This magnetic field is used to move the needle of a meter a certain distance, which represents the amount of the characteristic (volts, ohms, or amperes) being measured. The stronger the magnetic field, the larger the movement of the needle. The weaker the magnetic field, the smaller the movement of the needle. ill. 1: A magnetic field is produced around a conductor when current is flowing through the conductor. Current-carrying conductor. If a compass is suspended next to a conductor that isn't carrying an electron flow, the compass reacts only with the magnetic field of the earth and there is no other movement, as shown in ill 2. However, when electrons flow through that same conductor, the compass needle swings in line with the conductor’s magnetic field, as shown in ill 3. The mechanical movement of the needle is caused by the magnetic field produced by the electron flow through the conductor. The larger the current flow, the stronger the magnetic field produced and the greater the needle movement on the scale. This simple principle is the basis of the meter movement in most electric meters. ill. 2 When there is no current flow, the compass reacts only to the magnetic field of the earth: Compass needle, Conductor with no current flowing; ill. 3 When a current flows through a conductor, the compass needle swings in line with the conductor’s magnetic field. Magnetic field produced by current-carrying conductor; Conductor with current flowing Differences Among Meters The differences among the various electric meters are not in the meter movements, except for digital meters, but in the internal circuits of the meters and in how the magnetic fields are created. For electrons to flow in an electric circuit, an electric load must be present. This electron flow is somewhat different for the clamp-on ammeter, voltmeter, and ohmmeter. The clamp-on meter picks up the magnetic field through a set of laminated jaws on the meter. The voltmeter uses a resistor as a load to produce a magnetic field when voltage is applied to the circuit in the meter. The ohmmeter has its own power supply and uses the device being checked as the load to produce a magnetic field. All three meters use the same meter movement, unless the movement is digital. Their methods of loading and their power supplies are varied to attain the needle movement for reading the magnetic field. ill. 4 (a) Analog clamp-on ammeter with volt-ohm function (b) Analog volt-ohm meter (c) Digital clamp-on ammeter with volt-ohm function (d) Digital volt-ohm meter. Meters may be made in a combination and mounted in one case, or they may be completely separate. ill 4(a) shows an analog clamp-on ammeter, and ill 4(b) shows an analog volt-ohm meter. ill 4(c) shows a digital clamp-on ammeter and ill 4(d) shows a digital volt-ohm meter. The volt-ohm-meter, as its name suggests, is used to measure either voltage or resistance, depending on the scale selected. The meter shown in ill 4(d) is particularly versatile because it's capable of reading temperature (Fahrenheit and Centigrade) and the microfarad rating of a capacitor. The meter movement created by the magnetic field around a conductor is reflected by the movement of the needle on a scale of an electric meter. This scale is usually broken down into several basic scales that have different ranges for voltage, amperage, and resistance. Some electric meters have a selector switch: The meter movement is shown on a certain point of the scale, but the scale to be used must be determined by the person using the meter. ill. 5: Analog volt-ohm meter ill. 6: Analog meter scale with needle; ill. 8: 3½-digital display; ill. 9 4 digital display ill. 7: Digital reading. Digital meters are becoming more popular in the industry due to the reduction in cost over the past several years and the ease in reading. Another reason for the increasing popularity of the digital meter is the increased use of electronic control, which often requires an electrical meter capable of accurately reading small voltages because of the lower signal voltages. An analog meter volt-ohm meter is shown in ill 5 with an analog meter scale and needle shown in ill 6. The mechanic must estimate the values between the lines on the meter scale. Digital meters can be read more accurately because there is no estimation, as shown from the digital reading in ill 7. The ease of reading a digital meter is certainly an advantage, and many technicians are using digital meters in the industry today. Most digital meters utilize a 3½- or 4½-digit display. ill 8 shows a 3½-digit display, and ill 9 shows a 4½-digit display. The ½ digit's blank (0) or one that's the left digit of the meter reading. The 3 or 4 digits represent the next digits of the meter reading. E.g., a 3½-digit reading will display a 4-digit reading with the left digit blank (0) or 1, such as 1999. This resolution determines the basic accuracy of the meter. Typical accuracy for a digital volt-ohm-milliammeter is plus or minus 1% of actual reading compared with the typical accuracy of an analog meter of plus or minus 2% of full scale. ill. 10 Digital meters While analog meters depend on a magnetic field to move the needle, the digital meter makes use of Ohm’s law to measure and display the electrical characteristics of the circuit. Digital meters use one of two methods to protect the meter circuitry. Some digital meters are protected by internal circuits that detect an over load condition and then return the meter to normal operation. This function generally protects the ohm function from voltage overloads. Other digital meters are protected by fuses that have to be changed for the meter to again function. Several digital meters are shown in ill 10. CAUTION: Make sure electrical test equipment is in good condition. 2 AMMETERS The ammeter uses the basic meter movement discussed in Section 1. The strength of the magnetic field determines the distance that the needle of the meter moves. ill 11(a) shows the magnetic field and the meter movement when there is a high current flow through the conductor. ill 11(b) shows the magnetic field and the meter movement when there is a low cur rent flow in the conductor. The larger the current flow, the stronger the magnetic field grows and the greater the needle movement on the scale. ill. 11 Scale of an ammeter showing current flow: (a) High current flow;(b) Low current flow The ammeter measures current flow in an electric circuit. There are basically two types of ammeters used in the industry today: the clamp-on ammeter and the in-line ammeter. The clamp-on ammeter is the most popular because it's the easiest to use. You simply clamp the jaws of the meter around one conductor feeding power to the load that's producing the current draw. The analog clamp-on ammeter is shown in ill 12. ill 13 shows several clamp-on ammeters. The clamp-on ammeter is easy to use. Just follow a few simple rules. First, select the scale that's appropriate for reading the current draw of the electrical device being checked. If the approximate current is unknown, use the highest scale until the correct scale can be determined. Most clamp-on ammeters have more than one scale and as many as five. The jaws of the clamp-on ammeter are clamped around one conductor that's supplying a load or circuit. The magnetic field created by the current flowing through the wire is picked up by the jaws of the ammeter and funneled into the internal connection of the meter. ill 14 shows a clamp-on ammeter being used to read the current of an operating compressor. ill. 12 Clamp-on ammeter with volt-ohm functions; ill. 13 Digital ammeters Never clamp the jaws of the meter around two wires to obtain an ampere reading. If the current flows in the wires are opposite, as they often are, the meter will read zero because the current flows cancel each other out. If the current flows are not opposite, the meter will read the current draw in both conductors. In either case, you will obtain an incorrect reading, which could cause you to incorrectly diagnose the problem. When the ampere draw is small, you may have difficulty obtaining a true reading because of the small needle movement. The problem of small needle movement can be remedied by coiling the wire around the jaws of the meter. This allows the meter to pick up a larger current flow than is actually there. The meter will be more accurate because the current reading will fall in the midrange of the scale and can be easily read. ill. 14 Technician using clamp-on ammeter to read the current of a compressor; ill. 15 Taking a low-ampere reading with an ammeter by looping the wire through the jaws To obtain the correct ampere reading when this method is used, divide the ampere draw read by the number of loops going through the jaws of the meter. ill 15 shows an ammeter with the conductor looped through the jaws three times. The correct reading can be obtained by dividing the meter reading, which is 4, by the number of loops through the jaws, which is 3. Thus, the ampere draw of the load is 1.33 amperes. Remember: The meter reading should always be divided by the number of loops carried through the jaws of the meter. The clamp-on ammeter is one of the most valuable tools that industry technicians can carry. When you are installing a system, knowing the ampere draw of the equipment tells you if the unit's operating properly. You can also detect many electric circuit problems with a clamp-on ammeter, and it's the easiest, quickest way to tell if a load or circuit's energized. The ammeter can be purchased with other meters built into it. E.g., a single clamp-on ammeter can be purchased that will read amperage, volt age, and resistance. 3 VOLTMETERS The voltmeter is used to measure the amount of electromotive force available to a circuit or load. This is an important factor for heating, cooling, and refrigeration technicians because a wide range of voltages are used in this country. ill. 16 Simple voltmeter Voltmeters range from simple to complex instruments containing many scales. The simplest voltmeter available is a small, inexpensive one capable of distinguishing only among 120, 240, 480, and 600 volts (ill 16). Several manufacturers build simple voltmeters that can read only voltages, but these are becoming increasingly difficult to obtain. More common is the volt-ohm meter, which reads both voltage and resistance. These are available in many forms, and service technicians should follow the instructions for the particular model being used. The common volt-ohm meter has three voltage scales and several voltage ranges. Some meters also have a high-voltage jack. ill 17 shows three common types of analog volt- ohm meters. ill 18 shows a digital volt-ohm meter. ill. 17 Three analog volt-ohm meters used in the industry. ill. 18 Digital volt-ohm meter. The voltmeter was designed much like the ammeter, but a resistor is added to the circuit to prevent a direct short and allow electrons to flow in the meter. The voltmeter uses two leads that are connected to jacks that lead to the internal wiring. To obtain a reading, the two leads must touch or be connected to the conductors supplying the load or to the circuit that transfers the electromotive force to the meter. ill 19 shows a technician reading the voltage being applied to a compressor. The electrons flow through the leads into the meter through a resistor with a known ohm rating. The greater the voltage carried into the meter, the greater the magnetic field and the greater the needle movement or digital reading. When you don't know the voltage available to the equipment being worked on, you should start with the highest scale on the meter. Then change the meter setting until the needle falls in the midrange of the volt age scale. Never abuse a voltmeter by attempting to read a voltage that exceeds the range of the meter. ill. 19 Technician checking the voltage at a compressor The voltmeter is necessary for field technicians who have anything to do with the electrical section of equipment or with the installation or servicing of equipment. No heating, cooling, or refrigeration equipment should operate at an unsafe voltage, that's , voltage that's either too low or too high. All equipment is designed to operate at a voltage of 10% above or below the rating of the equipment. But in some cases the voltage may actually be more than the allowable figure. So field technicians should always check the supply voltage. Installation mechanics have not completed their installation unless they have checked the voltage available to the equipment. Technicians are often required to check the voltages to equipment as part of their troubleshooting job. In addition, the voltmeter can be used by technicians as a tool for diagnosing problems in the sys tem. Thus, the voltmeter is a must for proper installation and service. 4 OHMMETERS The ohmmeter is used to determine the operating condition of a component or a circuit. The ohmmeter can be used to find an open circuit, an open component, or a direct short in a circuit or component. It can also be used to measure the actual resistance of a circuit or component. The word continuity is used many times when referring to the use of ohm meters. Continuity means that a particular circuit or component has a complete path for current to follow. An open component or circuit means that there is infinite resistance in the circuit. The term measurable resistance means the actual resistance that's measured with the ohmmeter. ill 20 shows the three conditions as they might appear on the scale of an ohmmeter. The ohmmeter is a valuable tool for diagnosing and correcting problems in electric circuits. In this industry, many electric devices and circuits must be checked. The ohmmeter provides an easy method for checking circuits for opens (i.e., open circuits) and shorts (i.e., short circuits) and for measuring resistance. An open circuit causes no noticeable needle movement in an ohmmeter because there isn't a complete circuit. E.g., an open (circuit) could occur in a blown fuse, in a motor winding (the internal portion of the motor), and in any condition where the electric circuit does not have a complete path for electrons to follow. A direct short in an electric device or circuit causes problems because it means that two legs of the electric power wiring are touching, which causes an overload. In many cases a direct short means the wiring of the component is connected in some fashion. A closed switch is considered to be a short, but without this type of short, no heating, cooling, or refrigeration system would operate properly. ill. 20 Three conditions of a circuit as read on an ohmmeter: (a) No resistance (short) (no measurable resistance); (b) Measurable resistance; (c) Infinite resistance (open) (no resistance reading) In many cases, you will have to measure the resistance of a component to ensure that the component is in good operating condition. Most manufacturers make available to service technicians the exact ohmic value of motor windings and other components in the system. ill 21 shows a technician reading the resistance of an electric motor. The meter movement of an ohmmeter is designed and built for a very low current that's available from its own power source, usually a battery. ill. 21 Technician reading the resistance of an electric motor ill. 22 The internal wiring of an ohmmeter: To object to be checked; Internal battery ill 22 shows the internal wiring of an ohmmeter. The ohmmeter works much like the ammeter and the voltmeter except for the small cur rent that's supplied from the internal power source. The ohmmeter also uses a magnetic field to move the needle, but the magnetic field is created by a self-contained power source in the meter. The two leads of the ohm meter are connected to the internal circuit of the meter, which contains a resistance and the power source. The amount of current the small battery can push through the device being tested indicates the resistance of that device and determines the needle movement. Due to the low current that an ohmmeter is built to carry, it should never be connected to a circuit or device that's being operated. The function of the ohmmeter is merely to read the resistance of a device or circuit. Fortunately, most ohmmeters are equipped with some type of overload protection to protect the internal circuits of the meter if they are subject to line voltage. Many types and designs of ohmmeters are available. In many cases, the ohmmeter and voltmeter are combined in a dual-purpose meter. Some manufacturers build and market combinations of voltmeters, ohmmeters, and ammeters that are inexpensive. On this three-purpose meter, the ohm meter is a low-range ohmmeter and cannot be used for many of the jobs that a technician must do. The more expensive volt-ohm meters are more accurate and cover all ohm ranges. These meters will usually have at least three ohm scales (usually R x 1, R x 100, and R x 10,000). Some have more ranges. Those additional ranges are useful for some troubleshooting operations. Refer again to ill 17, which shows three different volt- ohm meters used today. SUMMARY Electricity plays an important part in the HVAC industry. Most equipment has some type of electric control system, even if it's powered by some other means. Thus, it's important for industry technicians to be familiar with the basic types of electric meters. About 80% of the problems with equipment or systems are electrical, which shows that electric meters are important. The ammeter is used to measure the current flow in an electric circuit. Two types of ammeters are used in the industry today. The clamp-on ammeter is the most frequently used. With this type of ammeter, it's only necessary to clamp the jaws of the meter around the conductor feeding the circuit or load and to read the amperage. The in-line ammeter must be placed in series with the load or circuit to read the amperage. Because of the time required to do this, the in-line ammeter is seldom used. The voltmeter is used to measure the voltage of an electric circuit. The voltmeter will have to be connected in parallel to the circuit to determine the voltage supplying the circuit. Voltage measurements can determine the source voltage, voltage drop, and a voltage imbalance. The first step in troubleshooting an air-conditioning system is to determine if voltage is available to the equipment or system. The ohmmeter is used to measure the resistance of a circuit or device in ohms. The ohmmeter must be used with the circuit power off to prevent damage to the meter. Most ohmmeters have a power source built into the meter. The ohmmeter is used to determine the condition of electrical devices used in air conditioning such as motors, heaters, contactor coils, solenoid valves, and other components. A continuity check is another use of the ohmmeter that determines if a complete path is available for current to flow. The decrease in the cost of digital meters has made them more popular in the industry. Digital meters can be used without any interpolation of the reading, making them an advantage over analog meters, where interpolation must be made with reference to the needle on the scale. Mechanics should choose the meter that best suits their needs. With the proper care, meters will give mechanics many years of good service. REVIEW QUESTIONS 1. What are the three most common electric meters used in the industry? 2. Service technicians should be able to use electric meters because _____ a. 80% of field problems are electrical b. technicians are required to trouble shoot electrical control systems c. technicians must be able to deter mine the electrical characteristics of an electrical circuit d. all of the above 3. What do most analog meters use to facilitate the needle movement? a. Ohm’s law b. a magnetic field that flowing electrons produce c. inductive resistance d. all of the above 4. How does an ammeter work? 5. What are the two types of ammeters? Which type is more commonly used in the industry? 6. What is the result on the meter reading of clamping the jaws of a clamp-on ammeter around two wires? 7. How can a very small ampere draw be measured with a clamp-on ammeter? 8. If a conductor was wrapped around the jaws of a clamp-on ammeter four times, the proper reading would be _____ if the clamp-on ammeter was reading 16 amps. a. 16 amps b. 8 amps c. 4 amps d. 2 amps 9. Explain the operation of an analog voltmeter. 10. Air-conditioning or refrigeration equipment can operate properly at ___% above or below its rated voltage. 11. If a technician has no idea of the voltage available to a unit, what procedure should be followed on reading the voltage? 12. What precaution should be taken when using an ohmmeter? 13. What does the term continuity mean? 14. What is a short circuit? 15. What is an open circuit? 16. What factors should be considered when purchasing an electric meter? 17. Match the following terms. a. short circuit a. zero ohms open circuit b. 22 ohms measurable c. infinite ohms resistance 18. How does the internal circuitry differ among the ohmmeter, voltmeter, and ammeter? 19. A voltmeter is connected to a circuit in (series or parallel). 20. An in-line ammeter must be connected to a circuit in a. series b. parallel 21. What is the difference between an analog meter movement and a digital meter movement? 22. Give three advantages of a digital electric meter. 23. What basic concept is used in a digital meter to calculate the circuit characteristics being measured? a. Watt’s law b. Volt’s law c. Amp’s law d. Ohm’s law 24. What is the basic accuracy of most analog and digital meters? 25. How many digits will the 3 and 4 displays have? 26. How are digital meters protected? 27. The ohmmeter differs from other types of meters in that it has its own. 28. Compare the digital and analog types of electric meters, giving advantages and disadvantages. |
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