Transcript Electrical Safety - HCC Learning Web

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:
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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
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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-ohmmilliammeter 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
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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.