EMT 462EMT 462ELECTRICAL ELECTRICAL
SYSTEM SYSTEM TECHNOLOGTECHNOLOG
YY
EMT 462EMT 462ELECTRICAL ELECTRICAL
SYSTEM SYSTEM TECHNOLOGTECHNOLOG
YYChapter 4 :Chapter 4 :DC MetersDC MetersChapter 4 :Chapter 4 :DC MetersDC Meters
By:En. Muhammad Mahyiddin Ramli
Chap 4: DC Meters 2
Today’s Lecture Motivation
To familiarize the d’Arsonval meter movement, how it is used in ammeters, voltmeters, and ohmeters, some of its limitations (effects), as well as some of its applications.
Chap 4: DC Meters 3
After completing today topic, students should be able to…….
Explain the principle of operation of the d’Arsonval meter movement
Describe the purpose of shunts across a
meter movement and multipliers in series with a meter movement
Define the term sensitivity
Chap 4: DC Meters 4
Introduction
Meter: Any device built to accurately detect & display an electrical quantity in a
readable form by a human being.
Readable form
• Visual
• Motion of pointer on a scale
• Series of light (digital)
Chap 4: DC Meters 5
The D’Arsonval Meter
Hans Oersted (1777-1851)
Jacques d’Arsonval (1851-1940)
Danish physicist who discovered the relationship between current and magnetism – from the deflection of a compass needle
French physiologist who discovered the moving-coil galvanometer – from muscle contractions in frogs using a telephone, which operates on an extremely feeble currents similar to animal electricity
Chap 4: DC Meters 6
The D’Arsonval Meters
In 1880s, two French inventors: Jacques d’Arsonval and Marcel Deprez patented the moving-coil galvanometer.
Jacques d’Arsonval(1851 – 1940)
Marcel Deprez(1843 – 1918)
Deprez-d'Arsonval Galvanometer
Chap 4: DC Meters 7
Types of Instrument
• Permanent Magnet Moving-Coil (PMMC) – most accurate type for DC measurement
• Moving Iron
• Electrodynamometer
• Hot wire
• Thermocouple
• Induction Type
• Electrostatic
• Rectifier
Chap 4: DC Meters 8
The D’Arsonval Meter Movement
Fig 1-1 The d’Arsonval meter movement
The basic moving coil system generally referred to as a d’Arsonval meter movement or Permanent Magnet Coil (PMMC) meter movement.
Current-sensitive device capable of directly measuring only very small currents.
Its usefulness as a measuring device is greatly increased with the proper external circuitry.
Chap 4: DC Meters 9
Current from a circuit in which measurements are being made with the meter passes through the windings of the moving coil. Current through the coil causes it to behave as an electromagnet with its own north and south poles. The poles of the electromagnet interact with the poles of the permanent magnet, causing the coil to rotate. The pointer deflects up scale whenever current flows in the proper direction in the coil. For this reason, all dc meter movements show polarity markings.
Chap 4: DC Meters 10
D’Arsonval Used in
DC Ammeter
Chap 4: DC Meters 11
Since the windings of the moving coil are very fine wire, the
basic d’Arsonval meter movement has only limited
usefulness without modification.
One desirable modification is to increase the range of
current that can be measured with the basic meter
movement.
This done by placing a low resistance called a shunt (Rsh),
and its function is to provide an alternate path for the total
metered current, I around the meter movement.
D’Ársonval Meter Movement Used In A DC Ammeter
Chap 4: DC Meters 12
Basic DC Ammeter Circuit
In most circuits, Ish >> Im
Fig. 1-2 D’Ársonval meter movement used in ammeter
circuit
Where
Rsh = resistance of the shunt
Rm = internal resistance of the meter movement (resistance of the moving coil)
Ish = current through the shunt
Im = full-scale deflection current of the meter movement
I = full-scale deflection current for the ammeter
Ammeter
Chap 4: DC Meters 13
Cont’
Knowing the voltage across, and the current through, the shunt allows us to determine the shunt resistance as:
mm
m
msh
m
sh
mm
sh
shsh RII
I
RI
I
I
RI
I
VR
Ohm
Chap 4: DC Meters 14
Example 3.1
Calculate the value of the shunt resistance required to convert a 1-mA meter movement, with a 100-ohm internal resistance, into a 0- to 10-mA ammeter.
Chap 4: DC Meters 15
Solution
VmARIV mmm 1.01001
VVV msh 1.0
mAmAmAIII msh 9110
11.119
1.0
mA
V
I
VR
sh
shsh
Chap 4: DC Meters 16
Ayrton Shunt or Universal Shunt
William Edward Ayrton studied under Lord
Kelvin at Glasgow. In 1873 he was
appointed to the first chair in natural
philosophy and telegraphy at Imperial
Engineering College, Tokyo. In 1879 he
was the first to advocate power
transmission at high voltage, and with
John Perry (1850-1920) he invented the
spiral-spring ammeter, the wattmeter, and
other electrical measuring instruments.
The ammeter (a contraction of ampere
meter) was one of the first to measure
current and voltage reliably. They also
worked on railway electrification,
produced a dynamometer and the first
electric tricycle. British Engineer
William Edward Ayrton (1847-1908)
Chap 4: DC Meters 17
The Ayrton Shunt
The purpose of designing the shunt circuit is to allow to measure current, I that is some number n times larger than Im.
I = nIm
= 1nRm
Chap 4: DC Meters 18
Advantages of the Ayrton
Fig 1-3 Ayrton shunt circuit
Eliminates the possibility of the meter movement being in the circuit without any shunt resistance.
May be used with a wide range of meter movements.
Chap 4: DC Meters 19
Con’t
The individual resistance values of the shunts are calculated by starting with the most sensitive range and working toward the least sensitive range.
The shunt resistance is:
On this range the shunt resistance is equal to Rsh and can be computed by Eqn
cbash RRRR
1
n
RR m
sh
Chap 4: DC Meters 20
Con’t
2
)(
I
RRIRR mshm
cb
3
)(
I
RRIR mshm
c
)( cbsha RRRR
ccbb RRRR )(
Chap 4: DC Meters 21
D’Arsonval Used in
DC Voltmeter
Chap 4: DC Meters 22
The basic d’Ársonval meter movement can be converted to a dc voltmeter by connecting a multiplier Rs in series with the meter movement
The purpose of the multiplier: is to extend the voltage range of
the meter to limit current through the
d’Arsonval meter movement to a maximum full-scale deflection current.
Fig 1.4 The basic d’Arsonval meter Movement Used In A DC Voltmeter
D’Ársonval Meter Movement Used In A DC Voltmeter
Chap 4: DC Meters 23
Con’t
To find the value of the multiplier resistor, first determine the sensitivity, S, of the meter movement.
/V)( 1
ySensitivit fsI
Resistance InternalRange SRs
Chap 4: DC Meters 24
Example 3.2
Calculate the value of the multiplier resistance on the 50V range of a dc voltmeter that used a 500A meter movement with an internal resistance of 1k.
Chap 4: DC Meters 25
Solution
Sensitivity, VkI
Sfs
2500
11
Multiplier, Rs = S X Range – internal Resistance
= (2k X 50) – 1k = 99k
Chap 4: DC Meters 26
Voltmeter And Ammeter
Effect
Chap 4: DC Meters 27
Voltmeter Loading Effect
When a voltmeter is used to measure the voltage across a circuit
component, the voltmeter circuit itself is in parallel with the circuit
component.
Since the parallel combination of two resistors is less than either
resistor alone, the resistance seen by the source is less with the
voltmeter connected than without.
Therefore, the voltage across the component is less whenever the
voltmeter is connected.
The decrease in voltage may be negligible or it may be appreciable,
depending on the sensitivity of the voltmeter being used.
This effect is called voltmeter loading. The resulting error is called a
loading error.
Chap 4: DC Meters 28
Example 3.3
Two different voltmeters are used to measure the voltage across resistor RB in the circuit of Figure 2-2. The meters are as follows.
Meter A : S = 1k/V, Rm = 0.2k, range = 10VMeter B : S = 20k/V, Rm = 1.5k, range=10V
Calculate:a) Voltage across RB without any meter
connected across it.b) Voltage across RB when meter A is used.
c) Voltage across RB when meter B is used
d) Error in voltmeter readings.
Chap 4: DC Meters 29
Solution
(a) The voltage across resistor RB without either meter connected is found Using the voltage divider equation:
V5
5k25k
kΩ5V30
BA
BRB RR
REV
Chap 4: DC Meters 30
Solution
(b) Starting with meter A, the total resistance it presents to the circuit is:
The parallel combination of RB and meter A is:
Therefore, the voltage reading obtained with meter A, determined by the voltage divider equation, is:
V
RR
REV
Ae
eRB
53.3kΩ25kΩ33.3
kΩ33.3V30
1
1
kΩ33.310kΩ5kΩ
10kΩ5kΩ
1
TAB
TABe RR
RRR
kΩ10V10k/V1Range SRTA
Chap 4: DC Meters 31
Solution
(c) The total resistance that meter B presents to the circuit is:
RTB = S x Range = 20k/V x 10 V = 200 k
The parallel combination of RB and meter B is:
Re2 = (RB x RTB)/(RB + RTB) = (5kx200k)/(5k+200k) = 4.88 k
Therefore, the voltage reading obtained with meter B, determined by use of the voltage divider equation, is:
VRB = E(Re2)/(Re2+RA) = 30 V x (4.88k)/(4.88k+25k) = 4.9 V
Chap 4: DC Meters 32
Solution(d)
Voltmeter A error = (5 V – 3.53 V)/5 V x (100%
= 29.4%Voltmeter B error = (5 V – 4.9 V)/5 V x
(100%) = 2 %
%100 valueExpected
value)Measured- value(ExpectederrorA Voltmeter
Chap 4: DC Meters 33
Ammeter Insertion Effects
Inserting an ammeter in a circuit always
increases the resistance of the circuit and
reduces the current in the circuit.
This error caused by the meter depends on the
relationship between the value of resistance in
the original circuit and the value of resistance in
the ammeter.
Chap 4: DC Meters 34
Con’t
** For high range ammeter, the internal
resistance in the ammeter is low.
** For low range ammeter, the internal resistance
in the ammeter is high.
Chap 4: DC Meters 35
1R
EI e
Expected current value in a series circuit
mm RR
EI
1
Series circuit with ammeter
Chap 4: DC Meters 36
Con’t
Hence;
me
m
RR
R
I
I
1
1
Therefore;
%1001
e
m
I
IInsertion error =
Chap 4: DC Meters 37
Example 3.4
A current meter that has an internal resistance of 78 ohms is used to measure the current through resistor Rc in below circuit.
Determine the percentage of error of the reading due to ammeter insertion.
Chap 4: DC Meters 38
Solution
The current meter will be connected into the circuit between points X and Y in the schematic as shown above.
When we look back into the circuit from terminals X and Y, we can express Thevenin’s equivalent resistance as:
ba
bacTH RR
RRRR
RTH = 1 k + 0.5 k =
1.5 k
Chap 4: DC Meters 39
Solution
Therefore, the ratio of meter current to expected current:
Im/Ie= 1.5 k/(1.5 k + 78) = 0.95
Solving for Im yields, Im = 0.95Ie
Insertion error = [1 – (Im/Ie)] x 100% = 5.0%
me
m
rR
R
I
I
1
1
Chap 4: DC Meters 40
The Ohmmeter (Series Ohmmeter)
The ohmmeter consists of battery, resistor and PMMC.
*function of Rz and Rm are to limit the current through the meter.
mZfs RR
EI
The full-scale deflection current,
Basic ohmmeter circuit
Chap 4: DC Meters 41
Con’t
To determine the value of unknown resistor, Rx, The Rx is connected to terminal X and Y.
Above figure shows the basic ohmmeter circuit with unknown resistor, Rx connected between probes.
Basic ohmmeter circuit with unknown resistor, Rx connected between probes.
Rz = variable resistor
Chap 4: DC Meters 42
Con’t
The circuit current,
xmZ RRR
EI
The ratio of the current, I to the full-scale deflection current, Ifs is
xmZ
mZ
mZ
xmZ
fs RRR
RR
RRE
RRRE
I
IP
Chap 4: DC Meters 43
Summary
Basic d’Arsonval meter movement – current sensitive device capable of directly measuring only very small currents.
Large currents can be measured by adding shunts.
Voltage can be measured by adding multipliers. Resistance – adding battery and a resistance
network. All ammeters & voltmeters introduce some error –
meter loads the circuit (common instrumentation problem).
Chap 4: DC Meters 44
It is possible to fail in many ways....while to succeed its only possible in one way.
- Aristotle