Lecture 6.pdf

EE-336 Electrical Measurement & Instrumentation
EE-336
Electrical Measurement & Instrumentation
Fall Semester 2017
EE - Jalozai Campus
Dr. Abid Ullah
Part - 3

2
Electromechanical Indicating Instruments
 Galvanometer
 Permanent Magnet Moving Coil (PMMC) mechanisms
 DC Ammeters
 DC Voltmeter
 Voltmeter Sensitivity
 Series Type Ohm Meter
 Shunt Type Ohm Meter
 Multimeter/VOM
 AC Indicating Instruments
 Thermo-instruments
 Electro dynamo-meter
 Watt-hour meter
 Power factor meter
 Instrument Transformers
EE-336 Electrical Measurement & Instrumentation

3
Lord Kelvin Galvanometer
 Technology of analog DC current measurements
 Oersted discovered in 1819 that a wire carrying dc
current, when held near a compass needle,
caused the compass to deflect, implying that the
current carrying wire generated a magnetic field
 This effect was put to use by Lord Kelvin, who, in
1858, developed a four-coil mirror galvanometer.
 Construction
 four coils were arranged in two opposing pairs;
their fields acted on two arrays of permanent
magnets with opposing polarities, attached to a
vertical torsion spring suspension, to which was
also attached a small mirror
 As the magnetic fields of the current carrying coils
interacted with the magnetic fields of the
suspended permanent magnets, torque was
developed causing rotation of the suspension and
mirror.

4
Lord Kelvin Galvanometer
 Operation principle
 As the magnetic fields of the current carrying
coils interacted with the magnetic fields of the
suspended permanent magnets, torque was
developed causing rotation of the suspension
and mirror.
 A collimated beam of light directed at the
mirror was reflected onto a distant scale.
Even a slight rotation of the suspension is
seen as a linear deflection of the spot of light.
 Galvanometers of this type were used more
to detect very small currents (on the order of
nA), rather than to measure large currents
accurately.

5
Suspension Galvanometer
 History
 In 1836, Sturgeon employed a coil suspended in the field of a
permanent magnet as a galvanometer.
 In 1882, D’Arsonval built a moving coil, permanent magnet,
mirror galvanometer with a torsion spring suspension

6
Suspension Galvanometer
 D`Arsonval Galvanometer
 use of a soft iron cylinder inside
the rectangular coil to
concentrate the magnetic flux
perpendicular to the sides of the
coil
 Permanent Magnet Moving Coil
(PMMC)
 In 1888,Weston developed a
permanent magnet, moving coil
microammeter movement in
which the coil was suspended
by pivot bearings and the
restoring torque was provided by
a pair of helical springs
 A pointer was attached to the
moving coil

7
 Operational principle
 Coil suspended in the magnetic field of a permanent magnet
 coil can move freely in the magnetic field
 Flow of current in coil generates an EM torque which causes the coil
to rotate
 EM torque is counter balanced by the mechanical torque of control
springs
 Balance of torques and therefore angular position of the movable coil
is indicated by a pointer against a fixed reference called scale
Permanent-Magnet Moving-Coil Mechanism
Torque and Deflection

8
 Equation for developed torque from basic law of
electromagnetic torque
 Equation shows that the developed torque is directly proportional to
the flux density of the field, current in coil, and coil constants
 Since flux density and coil area are fixed parameters, so the
developed torque is a direct indication of current in coil
 Torque causes pointer to deflect to steady state position where it is
balanced by opposing control-spring torque
T=B x A x I x N
T = torque [newton-meter(N-m)]
B = flux density in the air gap
[webers/square meter (tesla)]
I = current in the movable coil [Amperes (A)]
N = turns of wire on the coil
A = effective coil area [square meters (m2)]
Torque and Deflection

9
 In some applications dynamic behavior of galvanometer is
important
 Speed of response
 Damping
 Overshoot
 Principle
 Dynamic behavior of galvanometer can be observed by suddenly
interrupting the applied current, so that the coil swings back from its
deflected position towards the zero position
 Due to inertia of the moving system the pointer swings past the zero
mark and then oscillates back and forth around zero
 Motion of moving coil in a magnetic field is characterized by three
quantities
 Moment of inertia (J) of the moving coil
 Opposing torque (S)
 Damping constant (D)
Dynamic Behavior

10
 Three types of behavior
 Over damped, Under damped, Critically damped
 Curve I
 Overdamped case in which the coil returns slowly to its rest position
without overshoot or oscillations
 Curve II
 Shows the underdamped case in which the motion of coil is
subjected to damped sinusoidal oscillation
 the rate at which these oscillations die away is determined by
Damping constant (D), moment of inertia (J), and the counter torque
(S) produced by the coil suspension
Dynamic Behavior

11
 Curve III
 Critically damped case in which the pointer returns promptly to its
steady-state position without oscillation
 Operation
 In principle, the GM pointer travels to final position without overshoot
 In practice, the GM is usually slightly underdamped
 It assures movement is not damaged due to rough handling
 Compensates for any additional friction due to dust or wear
Dynamic Behavior

12
 GM damping is provided by two mechanisms
 Mechanical
 Electromagnetic
 Mechanical damping
 Motion of coil through air surrounding it,
 Friction of movement in bearings
 Flexing of suspension springs caused by rotating coil
 Electromagnetic damping (Lenz’s law)
 Caused by induced effects in the moving coil as it rotates in the
magnetic field
 Movable coil is wound on a light aluminum frame. Rotation of coil in
the MF sets up circulating currents in the conductive metal frame,
causing a retarding torque that opposes the motion of the coil
Damping Mechanisms

13
 Critical Damping Resistance External (CDRX)
 GM may be damped by connecting a resistor across the coil
 When coil rotates in the magnetic field, a voltage is generated in the
coil which circulates a current through the coil and the external
resistor
 This produces an opposing or retarding torque that damps the
motion of the movement
 CDRX depends on the total circuit resistance, the smaller the total
circuit resistance the larger the damping torque
 CDRX calculations
 Beginning with the oscillating condition, decreasing values of
external resistances are tried until a value is found for which the
overshoot just disappears
 Value of CDRX may also be computed from known GM constants
Damping Mechanisms

14
 PMMC Construction
 Permanent magnet of horse
shoe form, with soft iron pole
pieces attached to it. Between
the pole pieces is a cylinder of
soft iron, which serves to provide
a uniform magnetic field in the
air gap between the pole pieces
and the cylinder.
 Coil is wound on a light metal
frame and is mounted so that it
can rotate freely in the air gap
 The pointer attached to the coil
moves over a graduated scale
and indicates the angular
deflection of the coil & therefore
current through the coil
D`Arsonval Movement

15
 PMMC Construction
 Two phosphorus-bronze conductive springs, normally equal in
strength provide the calibrated force opposing the moving-coil torque
 Entire moving system is statically balanced for all deflection positions
by three balance weights
 Pointer, springs, and pivots are assembled to the coil structure by
means of pivot bases, and the entire movable coil elements is
supported by jewel bearings
D`Arsonval Movement

16
 Universal instrument bearing
 Pivot may have a radius that
depends on the weight of the
mechanism and the vibration
the instrument will encounter
 V-jewel design has the least
friction of any practical type of
instrument bearing
 Specially protected instrument
use the spring-back jewel
bearing. It is located in its
normal position by the spring
and is free to move axially
when the shock to the
mechanism becomes severe
V-Jewel

17
 PMMC Scale
 Linearly spaced scale because torque is directly proportional
to the coil current
 basic PMMC instrument is therefore a linearly reading dc
device
 Power requirement of the D`Arsonal movement are very
small: typical values range from 25 µW to 200 µW
 Accuracy of the instrument is of the order of 2 to 5% of full-
scale reading
 PMMC Instrument is not suitable for AC measurements
Instrument Scale

18
 Improved magnetic materials “Alnico”
 Design magnetic system in which the magnet itself serves as core
 Unaffected by external magnetic fields
 Magnetic shunting effects in which several meters operating side by
side effect each other is eliminated
 Need for magnetic shielding, in form of iron cases is also eliminated
Core-Magnet Construction

19
 Principle
 Eliminate the friction of the
jewel-pivot suspension
 Eliminate the need to use the
instrument in upright position
 Construction
 Movable coil is suspended by
means of two torsion ribbons
 Ribbons are placed under
sufficient tension to eliminate
any snag
 This tension is provided by
tension spring, so that the
instrument can be used in
any position
Taut-Band Suspension

20
 Construction
 Taut-band offers higher
sensitivities than the
pivot-jewel based
 Advantages
 Relatively insensitive to
shock & temperature
 Capable of handling
greater overloads then
previous types
Taut-Band Suspension

21
 Temperature compensation of PMMC movement
 Appropriate use of series and shunt resistance
 Both the magnetic field strength & spring tension decrease with
increase in temperature
 Temperature increase
 Coil resistance increases with increase in temperature
 Causes the pointer to read low for a given current
 Spring change tends to cause the pointer to read high with
increase in temperature
 Uncompensated meter tends to read low by approximately 0.2%
per degree C rise in temperature
 Compensated instrument
 Instrument is considered to be compensated when the change in
accuracy due to 10 C change in temperature is not more than ¼ of
the total allowable error
Temperature Compensation

22
 Method 1: Swamping resistors
 Compensation through
swamping resistors in series
with moving coil
 Total resistance of coil and
swamping resistor increases
slightly with a rise in
temperature but only enough to
counter-act the change of
spring & magnet
 So the overall temperature
effect is zero
Temperature Compensation Techniques

23
 Method 2: Complete Cancellation
 Total circuit resistance increases with
a rise in temperature due to copper
coil and copper shunt resistance
 Principle
 For a fixed applied voltage, total
current decreases slightly with a
rise in temperature
 Resistance of the copper shunt
resistor increases more than the
series combination of coil and
manganin resistor; hence a larger
fraction of the total current passes
through the coil circuit
 By correct proportioning of the copper and manganin parts in the
circuit , complete cancellation of temperature effects may be obtained

24
 Disadvantage
 Disadvantage of the use of swamping resistors is a reduction in the
full-scale sensitivity of the movement because a higher applied
voltage is necessary to sustain the full scale current

Lecture 6.pdf

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