Transcript DC Meters - UniMAP Portal

EMT 462
ELECTRICAL
SYSTEM
TECHNOLOGY
Chapter 4 :
DC Meters
By:
En. Muhammad Mahyiddin Ramli
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
2
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
3
Introduction
Meter: Any device built to accurately detect &
display an electrical quantity in a
readable form by a human being.
• Visual
• Motion of pointer
on a scale
Readable form
• Series of light
(digital)
Chap 4: DC Meters
4
The D’Arsonval Meter
Hans Oersted (1777-1851)
Danish physicist who discovered
the relationship between current
and magnetism – from the
deflection of a compass needle
Jacques d’Arsonval (1851-1940)
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
5
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
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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
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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.
Fig 1-1 The d’Arsonval meter movement
Chap 4: DC Meters
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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
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D’Arsonval Used
in
DC Ammeter
Chap 4: DC Meters
10
D’Ársonval Meter Movement
Used In A DC Ammeter

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.
Chap 4: DC Meters
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Basic DC Ammeter Circuit
Ammeter
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
Fig. 1-2 D’Ársonval meter movement
used in ammeter circuit
In most circuits, Ish >> Im
Chap 4: DC Meters
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Cont’
Knowing the voltage across, and the current
through, the shunt allows us to determine the
shunt resistance as:
Vsh I m Rm
Im
Im
Rsh 



I sh
I sh
I sh Rm  I  I m Rm
Chap 4: DC Meters
Ohm
13
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 0to 10-mA ammeter.
Chap 4: DC Meters
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Solution
Vm  I m Rm  1mA100  0.1V
Vsh  Vm  0.1V
I sh  I  I m  10mA  1mA  9mA
Vsh 0.1V
Rsh 

 11.11
I sh 9mA
Chap 4: DC Meters
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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.
William Edward Ayrton (1847-1908)
British Engineer
Chap 4: DC Meters
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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
Rm
=
n1
Chap 4: DC Meters
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Advantages of the Ayrton


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.
Fig 1-3 Ayrton shunt circuit
Chap 4: DC Meters
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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:
Rsh  Ra  Rb  Rc
On this range the shunt resistance is equal to Rsh
and can be computed by Eqn
Rm
Rsh 
n 1
Chap 4: DC Meters
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Con’t
I m ( Rsh  Rm )
Rb  Rc 
I2
I m ( Rsh  Rm )
Rc 
I3
Ra  Rsh  ( Rb  Rc )
Rb  ( Rb  Rc )  Rc
Chap 4: DC Meters
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D’Arsonval Used
in
DC Voltmeter
Chap 4: DC Meters
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D’Ársonval Meter Movement
Used In A DC Voltmeter


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
Fig 1.4 The basic d’Arsonval meter
Movement Used In A DC Voltmeter
to limit current through the d’Arsonval
meter movement to a maximum fullscale deflection current.
Chap 4: DC Meters
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Con’t

To find the value of the multiplier resistor, first
determine the sensitivity, S, of the meter
movement.
1
Sensitivit y 
(/V)
I fs
Rs  S  Range  Internal Resistance
Chap 4: DC Meters
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Example 3.2
Calculate the value of the multiplier
resistance on the 50V range of a dc voltmeter
that used a 500A meter movement with an
internal resistance of 1k.
Chap 4: DC Meters
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Solution
Sensitivity,
1
1
S

 2k 
V
I fs 500
Multiplier, Rs = S X Range – internal Resistance
= (2k X 50) – 1k
= 99k
Chap 4: DC Meters
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Voltmeter And Ammeter
Effect
Chap 4: DC Meters
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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
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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 = 10V
Meter 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
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Solution
(a) The voltage across resistor RB without either meter
connected is found Using the voltage divider equation:
 RB

VRB  E 



R

R
B 
 A
 5kΩ 
 30V 

25k

5k


 5V
Chap 4: DC Meters
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Solution
(b) Starting with meter A,
the total resistance it
presents to the circuit is:
RTA  S  Range  1k/V 10V  10kΩ
RB  RTA
Re1 
RB  RTA
The parallel combination
of RB and meter A is:
5kΩ 10kΩ
5kΩ  10kΩ
 3.33kΩ

Therefore, the voltage reading
obtained with meter A, determined
by the voltage divider equation, is:
VRB
Chap 4: DC Meters
 Re1 
 E

R

R
A
 e1
3.33kΩ
 30V 
3.33kΩ  25kΩ
 3.53V
30
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
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Solution
(d)
(Expected value - Measured value)
Voltmeter A error 
100%
Expected value
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%
Chap 4: DC Meters
32
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
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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
34
E
Ie 
R1
Expected current value in a series circuit
E
Im 
R1  Rm
Series circuit with ammeter
Chap 4: DC Meters
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Con’t
Hence;
Im
R1

I e R1  Rm
Therefore;
Insertion error =
 Im 
1   100%
Ie 

Chap 4: DC Meters
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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
37
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:
RTH  Rc 
Ra Rb 
Ra  Rb 
RTH = 1 k + 0.5 k = 1.5 k
Chap 4: DC Meters
38
Solution
Therefore, the ratio of meter current to expected current:
Im
R1

I e R1  rm 
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%
Chap 4: DC Meters
39
The Ohmmeter (Series Ohmmeter)
The ohmmeter consists of battery, resistor and PMMC.
The full-scale deflection current,
E
I fs 
R Z  Rm
Basic ohmmeter circuit
*function of Rz and Rm are to limit the current through the meter.
Chap 4: DC Meters
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Con’t
Rz = variable resistor
Basic ohmmeter circuit with unknown resistor, Rx connected
between probes.


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.
Chap 4: DC Meters
41
Con’t
The circuit current,
E
I
R Z  Rm  R x
The ratio of the current, I to the full-scale deflection
current, Ifs is


E


RZ  Rm  Rx 

RZ  Rm 
I

P


RZ  Rm  Rx 
I fs
 E 


 RZ  R m 
Chap 4: DC Meters
42
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
43
It is possible to fail in many
ways....while to succeed its only
possible in one way.
- Aristotle
Chap 4: DC Meters
44