Measurements DC meters concepts Volta meter

Measurements

1. 1
Chapter 02
Direct Current
Meters

2. 2
Warm - ups

3. 3
Objectives
•At the end of this chapter, the students
should be able to:
•Describe about the types of suspension used
in the d’Arsonval meter movement.
•Explain in detail the principles of operation of
the pmmc or d’Arsonval meter movement.

4. 4
Objectives
•Explain the purpose of shunts across a
meter and multipliers in series with a
meter.
•Analyze a circuit in terms of Voltmeter
Loading Effect and Ammeter Insertion
Errors.

5. 5
Objectives
•Describe the construction and operation
of a basic Ohmmeter.
•Perform calculations to obtain specific
meter range.
•Apply the concepts related to error to
the circuits calculation.

6. 6
Outlines
•Introduction
•pmmc = d’Arsonval meter movement
•Ayrton Shunt
•d’Arsonval used in DC Voltmeter
•d’Arsonval used in DC Ammeter

7. 7
Outlines
•Voltmeter Loading Effects
•Ammeter Insertion Effects
•Ohmmeter
•Multiple-range Ohmmeter

8. 8
Outlines
•Multi-meter
•Example of applications

9. 9
What is a meter?
•A meter is any device built to accurately detect
and display an electrical quantity in a form
readable by a human being.
•Usually this "readable form" is visual: motion of
a pointer on a scale, a series of lights arranged
to form a "bargraph," or some sort of display
composed of numerical figures.

10. 10
What is a meter?
•Most modern meters are "digital" in design,
meaning that their readable display is in the
form of numerical digits.
•Older designs of meters are mechanical in
nature, using some kind of pointer device to
show quantity of measurement.

11. 11
What is a meter?
•The display mechanism of a meter is often
referred to as a movement, borrowing from its
mechanical nature to move a pointer along a
scale so that a measured value may be read.
•Mechanical meter movement designs are very
understandable.

12. 12
What is a meter?
•Most mechanical movements are based on the
principle of electromagnetism: that electric
current through a conductor produces a
magnetic field perpendicular to the axis of
electron flow.
•The greater the electric current, the stronger
the magnetic field produced.

13. 13
What is a meter?
•If the magnetic field formed by the conductor is
allowed to interact with another magnetic field, a
physical force will be generated between the two
sources of fields.
•If one of these sources is free to move with respect to
the other, it will do so as current is conducted through
the wire, the motion (usually against the resistance of
a spring) being proportional to strength of current.

14. 14
What is a meter?
•Practical electromagnetic meter movements
can be made now where a pivoting wire coil is
suspended in a strong magnetic field, shielded
from the majority of outside influences.
•Such an instrument design is generally known
as a permanent-magnet, moving coil, or PMMC
movement .

15. 15
What is a meter?

16. 16
pmmc=d’Arsonval
•In the picture above, the meter movement "needle"
is shown pointing somewhere around 35 percent of
full-scale, zero being full to the left of the arc and
full-scale being completely to the right of the arc.
•An increase in measured current will drive the
needle to point further to the right and a decrease
will cause the needle to drop back down toward its
resting point on the left.

17. 17
pmmc=d’Arsonval
•The arc on the meter display is labeled with numbers to
indicate the value of the quantity being measured, whatever
that quantity is.
•In other words, if it takes 50 microamps of current to drive
the needle fully to the right (making this a "50 µA full-scale
movement"), the scale would have 0 µA written at the very
left end and 50 µA at the very right, 25 µA being marked in
the middle of the scale.
•In all likelihood, the scale would be divided into much
smaller graduating marks, probably every 5 or 1 µA, to
allow whoever is viewing the movement to infer a more
precise reading from the needle's position.

18. 18
pmmc=d’Arsonval
•The basic principle of this device is the interaction of
magnetic fields from a permanent magnet and the
field around a conductor (a simple electromagnet).
•A permanent-magnet moving-coil (PMMC)
movement is based upon a fixed permanent
magnet and a coil of wire which is able to move,
as in next figures.

19. 19
pmmc=d’Arsonval
•The basic principle of this device is the interaction of
magnetic fields from a permanent magnet and the
field around a conductor (a simple electromagnet).
•A permanent-magnet moving-coil (PMMC)
movement is based upon a fixed permanent
magnet and a coil of wire which is able to move,
as in next figures.

20. 20
•When the switch is
closed, the coil will
have a magnetic field
which will react to the
magnetic field of the
permanent magnet.
The bottom portion of
the coil in Figure 2(a)
will be the north pole
of this electromagnet.
•Since opposite poles
attract, the coil will
move to the position
shown in Figure 2(b).
pmmc=d’Arsonval

21. 21
pmmc=d’Arsonval
To use pmmc as a meter,
2 problems must be solved.
•First, a way must be found
to return the coil to its
original position when
there is no current through
the coil.
•Second, a method is
needed to indicate the
amount of coil movement.

22. 22
pmmc=d’Arsonval
The first problem is solved by the:
• use of hairsprings attached to
each end of the coil.
•These hairsprings can also be
used to make the electrical
connections to the coil.
• With the hairsprings, the coil will
return to its initial position when
there is no current.
•The springs will also tend to resist
the movement of the coil when
there is current through the coil.

23. 23
pmmc=d’Arsonval
• As the current through the coil increases,
the magnetic field generated around the
coil increases.
• The stronger the magnetic field around
the coil, the farther the coil will move. This
is a good basis for a meter.
• But, how will you know how far the coil
moves?
• If a pointer is attached to the coil and
extended out to a scale, the pointer will
move as the coil moves, and the scale can
be marked to indicate the amount of
current through the coil.

24. 24
pmmc=d’Arsonval
• 2 other features are used to
increase the accuracy&
efficiency of this meter.
• First, an iron core is placed
inside the coil to concentrate
the magnetic fields.
• Second, curved pole pieces
are attached to the magnet
to ensure that the turning
force on the coil increases
steadily as the current
increases.
• The meter movement as it
appears when fully
assembled is shown in this
figure.

25. 25
pmmc=d’Arsonval
•The d’Arsonval meter movement is very widely used.
•Current from a measured circuit passes through the
windings of the moving coils causes it to behave as
an electromagnetic.
•The poles of EMT interact with the poles of PM,
causing the coils to rotate.
•The pointer deflects up scale whenever current flows
in proper direction in the coil.

26. 26
pmmc=d’Arsonval
•For this reason, all DC meter movements show
polarity markings.
•d’Arsonval meter movement is a current
responding device.
•Regardless of the units (volt,ohm,etc) for which
the scale is calibrated, the moving coil responds
to the amount of current through its windings.

27. 27
summary
•The basic principle and operation of pmmc or d’Arsonval
meter movement.
•The two (2) features used to increase the accuracy& efficiency
of this PMMC meters are:
•First, an iron core is placed inside the coil to concentrate
the magnetic fields.
•Second, curved pole pieces are attached to the magnet to
ensure that the turning force on the coil increases steadily
as the current increases.
•Regardless of the units (volt,ohm,etc) for which the scale is
calibrated, the moving coil responds to the amount of current
thru its windings.

28. 28
conclusion
•The students should be able to describe in
detail about the basic principles of operation
of the pmmc or d’Arsonval meter movement.

29. 29
evaluation
Label the
figure
appropriately