Electric Meters Electricity for Refrigeration, Heating and Air Conditioning 7th Edition Chapter 4 Electric Meters

Electric Meters Upon completion of this chapter the student will be able to: 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 Ammeter Analog Meter Clamp-on Meter Continuity Digital Meter Magnetic Field Measurable Resistance Ohmmeter Open Short Voltmeter

Introduction The most common characteristics in a circuit that are most important to industry technicians are: Volts Amperes Ohms The most popular electrical meters are built around these three electrical characteristics. Most meters 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.

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. Warning – Use the proper Electrical test equipment for the job being preformed

Basic Principle 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 the conductor. The magnetic field is used to move the needle of a meter a certain distance, which represents the amount of the characteristic being measured.

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 meter and in how the magnetic fields are created.

Clamp-on Ammeter The clamp-on meter picks up the magnetic field through a set of laminated jaws on the meter.

The Voltmeter 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 The ohmmeter has its own power supply and uses the device being checked as the load to produce a magnetic field.

Ammeters The strength of the magnetic field determines the distance that the needle of the meter moves. The larger the current flow, the stronger the magnetic field grows and the greater the needle movement on the scale. The ammeter measures current flow in an electric circuit. There are 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 and the easiest to use.

Using the Ammeter Select the scale that is appropriate for reading the current draw of the electrical device being checked. If the appropriate current is unknown, use the highest scale until the correct scale can be determined. Clamp the jaws of the ammeter around one conductor supplying a load or circuit. Never clamp the jaws of the meter around two wires to obtain an ampere reading. If the current flows in the wires are opposite, the meter will read zero because the current flows cancel each other out. If the current flow are not opposite, the meter will read the current draw in both conductors.

Using the Ammeter When the ampere draw is small, you may have difficultly obtaining a true reading because of the small needle movement. This 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. 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.

Voltmeters The voltmeter is used to measure the amount of electromotive force available to a circuit or load. Most voltmeters can read both voltage and resistance. The most common volt-ohm meter has three voltage scales and several voltage ranges. All equipment is designed to operate at a voltage of 10% above or below the rating of the equipment.

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. Continuity is when a particular circuit or component has a complete path for current to follow. The term measurable resistance is the actual resistance that is measured with the ohmmeter. The ohmmeter provides an easy method for checking circuits for opens and shorts and for measuring resistance.