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1. CURRENT, VOLTAGE, POWER
AND ENERGY MEASUREMENT

2. AMMETER & VOLTMETER

3. WATTMETE

4. ENERGY METERS

5. INTRODUCTION
• Analog ammeters and voltmeters are classed
together as there are not fundamental differences
in their operating principles
• The action of ammeter and voltmeter depends
upon the torque produced by the electric current
• In ammeter torque is proportional to the current to
be measured and in voltmeter torque produced by
the current proportional to the voltage
• The essential requirements of a measurements
instruments are:

6. • Its introduction into the ckt does not alter the
ckt condition
• The power consumption is small
• Ammeters are connected in series and must have
low electrical resistance to cause small voltage
drop and power loss to the min.
• Voltmeters are connected in parallel and must
have high electrical resistance so that the current
drawn is small and hence the power to the min.
• Ohmmeters are used to measure resistance. They
incorporate a source of emf and a current
measuring device.

7. Basis of Classification
• Measuring instruments are classified according
to both the quantity measured by the instrument
and the principle of operation.
• Three general principles of operation are
available:
(i) electromagnetic, which utilizes the magnetic
effects of electric currents
(ii) electrostatic, which utilizes the forces
between electrically-charged conductors
(iii) electro-thermic, which utilizes the heating
effect.

8. Various forces/torques required in
measuring instruments
• Deflecting torque/force: The defection of any instrument is
determined by the combined effect of the deflecting
torque/force, control torque/force and damping
torque/force. The value of deflecting torque must depend
on the electrical signal to be measured; this torque/force
causes the instrument movement to rotate from its zero
position
• Controlling torque/force: This torque/force must act in the
opposite sense to the deflecting torque/force, and the
movement will take up an equilibrium or definite position
when the deflecting and controlling torque are equal in
magnitude. Spiral springs or gravity usually provides the
controlling torque.

9. • Damping torque/force: A damping force is required to act
in a direction opposite to the movement of the moving
system. This brings the moving system to rest at the
deflected position reasonably quickly without any
oscillation or very small oscillation.
• This is provided by i) air friction ii) fluid friction iii) eddy
current. It should be pointed out that any damping force
shall not influence the steady state deflection produced
by a given deflecting force or torque.
• Damping force increases with the angular velocity of the
moving system, so that its effect is greatest when the
rotation is rapid and zero when the system rotation is
zero. Details of mathematical expressions for the above
torques are considered in the description of various types
of instruments.
Various forces/torques required in measuring instruments

10. Types of Instruments
• The main types of instruments used in
ammeters and voltmeters are
 PMMC
 Moving Iron
 Electro-dynamometer
 Hot wire
 Thermocouple
 Induction
 Electrostatic
 Rectifier

11. PMMC can be used for dc current measurement only and
is most accurate
Induction type is for ac current measurement only.
The other types can be used for either dc or ac currents
The moving iron and moving coil types both depend for
their action upon the magnetic effect of current. Moving
iron is used for indication instruments and can be used
either ac or dc current measurement
Electro-dynamometer type of instruments are used both
on ac as well as dc. Their calibration for both dc and ac is
same and are useful for transfer instruments
Types of Instruments

12. • An electrodynamics' movement uses an
electromagnet instead of the permanent magnet of
the d'Arsonval movement. This instrument can
respond to both alternating and direct current
• In a hot-wire ammeter, a current passes through a
wire which expands as it heats. Although these
instruments have slow response time and low
accuracy, they were sometimes used in measuring
radio-frequency current
• Digital ammeter designs use an analog to digital
converter (ADC) to measure the voltage across the
shunt resistor; the digital display is calibrated to
read the current through the shunt
Types of Instruments

13. • There is also a whole range of devices referred to
as integrating ammeters. In these ammeters the
amount of current is summed over time giving as
a result the product of current and time, which is
proportional to the energy transferred with that
current
• These can be used for energy meters (watt-hour
meters) or for estimating the charge of battery or
capacitor
• Thermal instruments have the adv that their
calibration is the same for both ac and dc and
particularly suitable for ac
Types of Instruments

14. • As voltmeter, electrostatic instruments
have the adv that their power
consumption is exceedingly small and
can cover large range of voltages; only
directly applicable for voltage
measurement
• The induction principle is used for
watt-hour meter than for ammeters and
voltmeters owing to high cost and
inaccuracy
Types of Instruments

15. • The D'Arsonval galvanometer is a moving coil ammeter. It
uses magnetic deflection, where current passing through a coil
causes the coil to move in a magnetic field. The voltage drop
across the coil is kept to a minimum to minimize resistance
across the ammeter in any circuit into which it is inserted. Uses
two spiral springs to provide the restoring force. By maintaining
a uniform air gap between the iron core of the instrument and
the poles of its permanent magnet, the instrument has good
linearity and accuracy..
• Moving iron ammeters use a piece of iron which moves when
acted upon by the electromagnetic force of a fixed coil of wire.
This type of meter responds to both direct and alternating
currents (as opposed to the moving coil ammeter, which works
on direct current only). The iron element consists of a moving
vane attached to a pointer, and a fixed vane, surrounded by a
coil. As alternating or direct current flows through the coil and
induces a magnetic field in both vanes, the vanes repel each
other and the moving vane deflects against the restoring force
provided by fine helical springs. The non-linear scale of these
meters makes them unpopular.

16. Moving Iron Instruments

17. Errors in Ammeters and Voltmeters
• There are certain errors which occur in most
types of instruments while other errors occur
only in those of particular type
• Friction and temperature errors are the most
common types of errors
• To reduce the friction torque and the error
caused by it the weight of the moving system
must be made as small as possible compared to
the operating force
• The ratio of torque to weight must be large

18. • The most serious error is produced by the heat generated
in the instrument; change in ambient temperature; change
in resistance in working coil which has little importance in
ammeter but in voltmeter
• The power loss in the instrument should be small by
controlling change in resistance. A ‘swamping resistance’
of material whose resistance temperature coefficient is
small, is connected in series with the coil to control the
total change in resistance to the min
• Heating may cause the expansion of spring and other
parts of the instrument
• Lack of balance of moving system and changes in the
strength of permanent magnets are other possible sources
of error which are common
Errors in Ammeters and Voltmeters

19. • Swamping Resistance
– The moving coil in a PMMC instrument is wound with
thin copper wire, and its resistance can change
significantly when its temperature changes.
– The heating effect of the coil current may be enough to
produce a resistance change, which will introduce an
error.
– To minimize the error, a swamping resistance made of
manganin or constantan is connected in series with the
coil (manganin and constantan have resistance
temperature coefficients very close to zero)

20. PMMC Instruments and its
Construction
PMMC is more accurate for measurement
of dc
Moving Coil

21. General Theory Permanent Magnet
Moving Coil (PMMC) Instruments
• The general theory of moving-coil instruments may be dealt
with considering a rectangular coil of turns, free to rotate about
a vertical axis.
PMMC Instruments

22. PMMC Instrument
• CONSTRUCTION. A moving coil instrument consists basically of a
permanent magnet to provide a magnetic field and a small lightweight
coil is wound on a rectangular soft iron core that is free to rotate
around its vertical axis
• OPERATION. When a current is passed through the coil windings, a
torque is developed on the coil by the interaction of the magnetic field
and the field set up by the current in the coil. The aluminum pointer
attached to rotating coil and the pointer moves around the calibrated
scale indicates the deflection of the coil. To reduce parallax error a
mirror is usually placed along with the scale.

23. • A balance weight is also attached to the pointer to
counteract its weight.
• To use PMMC device as a meter, two 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.
• The first problem is solved by the use of hairsprings
attached to each end of the coil. These hairsprings are not
only supplying a restoring torque but also provide an
electric connection to the rotating coil. With the use of
hairsprings, the coil will return to its initial position when
no current is flowing though the coil.
• The springs will also resist the movement of coil when
there is current through coil.

24. • When the developing force between the magnetic fields
(from permanent magnet and electro magnet) is exactly
equal to the force of the springs, the coil rotation will
stop. The coil set up is supported on jeweled bearings in
order to achieve free movement.
• Two other features are considered to increase the
accuracy and efficiency of this meter movement.
• First, an iron core is placed inside the coil to concentrate
the magnetic fields. Second, the curved pole faces ensure
the turning force on the coil increases as the current
increases.
• It is assumed that the coil sides are situated in a uniform
radial magnetic field of flux density.

25. Principle of Operation
It has been mentioned that the
interaction between the induced field
and the field produced by the
permanent magnet causes a deflecting
torque, which results in rotation of the
coil. The deflecting torque produced is
described below in mathematical form:

31. GALVANOMETER
• A galvanometer is a type of ammeter: an instrument for detecting
and measuring electric current. It is an analog electromechanical
transducer that produces a rotary deflection, through a limited arc, in
response to electric current flowing through its coil. The term has
been expanded to include uses of the same mechanism in
recording, positioning, and servomechanism equipment.

32. Operation of Galvanometer
• The most familiar use is as an analog measuring instrument, often called a meter. It is used to
measure the direct current (flow of electric charge) through an electric circuit. The D'Arsonval
/Weston form used today is constructed with a small pivoting coil of wire in the field of a permanent
magnet. The coil is attached to a thin pointer that traverses a calibrated scale. A tiny torsion spring
pulls the coil and pointer to the zero position.
• When a direct current (DC) flows through the coil, the coil generates a magnetic field. This field acts
against the permanent magnet. The coil twists, pushing against the spring, and moves the pointer.
The hand points at a scale indicating the electric current. Careful design of the pole pieces ensures
that the magnetic field is uniform, so that the angular deflection of the pointer is proportional to the
current. A useful meter generally contains provision for damping the mechanical resonance of the
moving coil and pointer, so that the pointer settles quickly to its position without oscillation.
• The basic sensitivity of a meter might be, for instance, 100 microamperes full scale (with a voltage
drop of, say, 50 milli-volts at full current).
• Such meters are often calibrated to read some other quantity that can be converted to a current of
that magnitude.
• The use of current dividers, often called shunts, allows a meter to be calibrated to measure larger
currents.
• A meter can be calibrated as a DC voltmeter if the resistance of the coil is known by calculating the
voltage required to generate a full scale current. A meter can be configured to read other voltages by
putting it in a voltage divider circuit. This is generally done by placing a resistor in series with the
meter coil. A meter can be used to read resistance by placing it in series with a known voltage (a
battery) and an adjustable resistor. In a preparatory step, the circuit is completed and the resistor
adjusted to produce full scale deflection. When an unknown resistor is placed in series in the circuit
the current will be less than full scale and an appropriately calibrated scale can display the value of
the previously-unknown resistor.
Because the pointer of the meter is usually a small distance above the scale of the meter, parallax
error can occur when the operator attempts to read the scale line that "lines up" with the pointer. To
counter this, some meters include a mirror along the markings of the principal scale. The accuracy of
the reading from a mirrored scale is improved by positioning one's head while reading the scale so
that the pointer and the reflection of the pointer are aligned; at this point, the operator's eye must be
directly above the pointer and any parallax error has been minimized.

33. Ammeter Shunts
• The coil winding of a basic movement is small and light
and can carry very small currents since the construction of
an accurate instrument with a moving coil to carry currents
greater than 100 mA is impractical owing to the bulk and
weight of the coil that would be required.
• When heavy currents are to be measured the major part of
the current is bypassed through a low resistance called a
shunt
• Explanation of ckt and calculation

34. Ammeter Shunts
= intetnal resistace of the coil
= intetnal resistace of the shunt
= full scale deflection current of movement
current to be measured
shunt resistance is parallel to the
m
sh
m sh
sh
R
R
I I
I shunt I current 
meter movement
1 1
ratio is called the multiplying power of shunt
multiplying power m= 1
Re tan shunt / ( 1)
m m
m m sh sh sh
sh
m m m m
sh m sh
m m sh m sh
m
m sh
sh m
I R
I R I R R
I
I R R RI I
I I I R
I I I R I R
This
RI
I R
sis ce R R m
   
         

 
 

35. Construction of Shunt
The general requirement of shunt
 The temperature coefficient of the shunt and the
instrument should be low and should nearly be
the same
 The resistance of the shunt should not vary with
time
 They should carry the current without excessive
temperature rise
 They should have a low thermal electromotive
force with copper
Manganin is usually used for shunts of dc
instruments as it gives low value of thermal emf
with copper

36. Effect to Temperature Change in Ammeter
• Shunt and moving coil may be of same material to eliminate
temperature error
• Method is not satisfactory; rate of temperature change may
not be same
• Additional disadvantage of using Copper shunt is that they are
likely to be bulky
• Swamping resistance having a resistance of 20 to 30 times
connected in series with the coil and a shunt of manganin is
connected across this combination
• Since copper forms a small fraction of the series combination,
the temperature difference is not appreciable

37. Multi-range Ammeter
The current range of a dc ammeter may be
further extended by a number of shunts,
selected by a range switch, the meter is
called multi-range ammeter (See ckts)
1 2 3 4
1 2 3 4 1 2 3 4
1 1 2 2
3 3 4 4
The ckt has four resistance R , R ,R , R
4 current range I , I , I , I and m , m , m , m
the shunt multiplying factor
R R / (m 1); R R / (m 1);
R R / (m 1); R R / (m 1);
sh sh sh sh
sh m sh m
sh m sh m
for
be
    
    

38. Multi Range Ammeter

40. • Low range ammeter use multi-position make-
before-break switch, essential for the
protection of meter
• Uses for the ranges of 1 to 50 A. First use the
highest range then lower as applicable
• The universal shunt or Ayrton shunt is also
used as the multi-range ammeter that
eliminates the disadv of damage without
shunt with higher meter resistance
Multi-range Ammeter

41. Universal or Ayrton Shunt
1 2 3
1 1 1 1 1
1 1
1 2 2 2
2 1 2
1 3 3
range have to be extended to I ,I ,I ; Switch posn 1
I ( I ) 1
/ ( 1)
switch position 2; I ( ) ( I )
( ) /
switch position 3; I ( ) (
m m m m m
m
m m m
m
m m
Meter
R I R m I I R R
R R m
For R R R I R
R R R m
For R R R I
     
  
   
 
   3
3 1 3
1 2
1 3 3
I )
( ) /
thus the resistance of different section ( ),
( ) and can be found
m
m
R
R R R m
R R
R R R

 


Circuit diagram

43. Voltmeter Multiplier
• D’ arsonval basic meter movement can easily
be converted into voltmeter connecting a
series resistance and is known as a multiplier
• The multiplier limits the current through the
meter so that it does not exceed the value for
full scale deflection and thus prevents the
movement from being damaged
• The value of multiplier, require to extend the
voltage range is calculated as: (ckt diagram)

45. , I I full scale current deflection of the meter
nternal resistance of the meter movement
ultiplier resistance
v=voltage across the meter movement for current I
Full range voltage of inst
m fs
m
s
m
Let
R I
R M
V
 


 rument; as per the ckt
v=I ; V=I ( )
I
I I
In terms of multiplying factor of multiplier
I ( )
multiplying factor for multiplier m=
I
1 Re tan of multiplier ( 1)
m m m m s
m m
s m
m m
m m s
m m
s
s m
m
R R R
V R V
R R
R RV
v R
R
sis ce R R m
R


   


    
to extend the voltage range to 10 times the
instrument range 9 m sR R
Voltmeter Multiplier

48. Construction of Multipliers
• The resistance should not change with
time
• The change in their resistance with
temperature should be small
• They should be non- inductively wound for
ac meter
The resistance materials used for multipliers
are magnum and constantan

49. • In a multirange voltmeter, different full scale
voltage ranges may be obtained by the use of
individual multiplier resistors or by a potential
divider arrangement
INDIVIDUAL MULTIPLIER. We can obtain
different voltage ranges by connecting different
values of multiplier resistors in series with the
meter. The no of these resistors is equal to the
no of ranges required; shown in fig….
Multi-range DC Voltmeter

50. 1 2 3 4
1 2 3 4 1 2 3 4
The ckt has four resistance R , R ,R , R connected
in series with the meter with the range selector swtch
4 voltage range V , V , V , V and m , m , m , m
the shunt multiplying factor
sh sh sh sh
for
be
 1 1 2 2
3 3 4 4
31 2 4
1 2 3 4
R R (m 1); R R (m 1);
R R (m 1); R R (m 1);
; ; ;
sh m sh m
sh m sh m
VV V V
m m m m
v v v v
   
    
   
Multi-range DC Voltmeter

51. Advantages,Limitations,Errors
of PMMC

52. Reduce Error
How to reduce error:

53. Ammeter Sensitivity

54. Voltmeter Sensitivity

55. Potential Divider Arrangement
• The resistances are connected in series to
obtain the voltage ranges. The connections
are brought out to binding posts on the
instruments, and the instruments is
connected to the proper binding post for
the desired voltage range

56. 1 2 3 4
1 2 3 4 1 2 3 4
1 1
1 1
The ckt has four resistance R , R ,R , R connected
in series with the meter with the 4 voltage range
V , V , V , V and m , m , m , m
the shunt multiplying factor
m
/
m m m m
m m
for
be
V V
R R R R R
I v R
       1
2 2
2 1 1
2 2 1 2 1
3
3 1 2 3 2
4
4 1 2 3 4 3
(m 1)
(m 1)
/
m (m 1) (m m )
(m m )
(m m )
m
m m m
m m
m m m m
m m
m
m m
m
R
V V
R R R R R
I v R
R R R R R
Similarly
V
R R R R R
I
V
R R R R R R
I

      
      
      
       

57. • This system has the advantage that all
multipliers except the first have standard
resistance values and can be obtained
commercially in precision tolerances.
• Multi-range voltmeters are very effective for
moderate range voltages. For higher range
voltages it is often desirable to use external
resistors in connection with a given
voltmeter
Potential Divider Arrangement

58. Ammeter Sensitivity
• Ammeter sensitivity is determined by the amount
of current required by the meter coil to produce
full-scale deflection of the pointer.
• The smaller the amount of current required
producing this deflection, the greater the
sensitivity of the meter.
• A meter movement that requires only 100
microamperes for full- scale deflection has a
greater sensitivity than a meter movement that
requires 1 mA for the same deflection.

59. Sensitivity of PMMC Voltmeters

60. Errors in PMMC Instruments
• Weakening of permanent magnets due to
ageing at temperature effects
• Weakening of spring due to ageing and
temperature effects
• Change of resistance of the moving coil with
temperature
• MAGNETS
• Springs
• Moving coils
• Frictions

61. Error Reduction
• Errors can be reduced by following the steps
given below:
– Proper pivoting and balancing weight may reduce the
frictional error.
– Suitable aging can reduce the magnetic decay.
– Use of manganin resistance in series (swamping
resistance) can nullify the effect of variation of
resistance of the instrument circuit due to temperature
variation.
– The stiffness of spring, permeability of magnetic core
(Magnetic core is the core of electromagnet or inductor
which is typically made by winding a coil of wire around
a ferromagnetic material) decreases with increases in
temperature

62. Adv of PMMC Instruments
• The scale is uniformly divided
• The power consumption is very low
• The torque weight ratio is high which gives a high
accuracy. The accuracy is of the order of generally 2 % of
full scale deflection
• A single instrument may be used for many different
current and voltage ranges by using different values for
shunts and multipliers
• since the operating forces are large on account of large
flux densities which may be as high as 0.5 web/m2 the
error due to stray magnetic fields are small
• Self shielding magnets make the core magnet mechanism
particularly useful in aircraft and aerospace application

63. • These instruments are useful only for dc. The
torque reverses if the current reverses. If the
instrument is connected to ac the pointer cannot
follow the rapid reversals and the deflection
corresponds to mean torque, which is zero. Hence
the instr cannot be used for ac
• The cost of these instruments is higher than that of
moving iron instruments
• Variation of magnet strength with time.
Disadv of PMMC Instruments

64. OHMMETERS
• The ohmmeter is a convenient direct
reading device for measurement of
resistance with low degree of
accuracy
• Used for approximate reading of
resistance of the different circuit
component/elements or to measure
the continuity
• It can help to know the balancing
bridge resistance

65. Series Type Ohmmeter
• It is consists of basic d’Arsonval movement
connected in parallel with a shunt resistance
R2. This parallel circuit is in series with
resistance R1 and a battery of emf E. The
series ckt is connected to the terminals A and B
of the unknown resistance Rx
1
2
1
Current limiting resistance
adjusting resistance
Emf of internal battery
int resistance of d' Arsonval movementm
R
R Zero
E
R ernal





66. • The meter will read infinity resistance at zero
current and posn and zero resistance at full
scale current posn. Therefore the meter has
“0” at extreme right and ∞ at the extreme
right.
Series Type Ohmmeter

67. • A convenient quantity to use in the design the
series ohmmeter is the value of Rx which
causes the half scale deflection. The
resistance across terminals A and B is defined
as the half scale posn resistance Rh
• When Rx=Rh then Im=0.5Ifs; This means that
Rh is equal to the internal resistance of the
meter looking into terminal A and B
Series Type Ohmmeter

68. Series Type Ohmmeter
2
1
2
1 2 1
The bat current at half scale deflection I / 2
full scale deflection the bat current must be doubled
I =2I / and the current through the shunt I I I
drop ac
m
h
m
h h
h h fs
R R
R R
R R
E R
In
E R
The
 


   
2
2 2 2 2 1
2
1
2
2
2
ross the meter is equal to the voltage drop at R
I I I I I (I I )
I (E I )
I E
I ( )
fs m fs m fs m fs
fs m h fs h
m
h h fs m sh
m
h
fs m
R R R R R
R R R
R R
Solving R R R R R
R R
ER
R
R R
    
 
    

 


69. Series Type Ohmmeter
2
2
2
I R approaches E the value of R Approaches infinity
if R have some limiting value say 0.5 M
500000
R
(500000 R )
R is complensate for 10% drop of battery voltage then
5
max value of R
fs h
h
fs m
h
If
E
I
if

 


00000 0.9 0.9
(500000 R )
R is to be increased beyond this value, a bat with a
higher emf must be used. Hence, in terms of full scale meter current
the resistance of meter, the bat voltage an
fs m fs
h
x E E
I I
if


1 2
d the measured resistance
at half scale, the resistance R and R can be determined and value
of R for a given battery emf can be computedh

70. Where s is the fraction of full scale reading when
measuring an unknown resistance Rx . Thus the
scale of a given ohmmeter fro a given Rh can be
calibrated either by direct calibration, using known
vales of Rx or it can be calculated in terms of the
fraction of the full scale readings, which can be
computed from the resistance to be measured Rx
and half scale resistance Rh
2
1
2
1
2
1
2
2
2
R
The meter current for any resistance R is I
R +R
I
Internal resistance+R R +R
ER
I When R 0;I
(R +R )(R +R )
I RER
I
(R +R )(R ) I R +R
x m
m
X h X
m X m sf
m h X
m h
fs
m h fs h X
I
E E
but
I I
s

 
   
    
Series Type Ohmmeter

71. SHUNT TYPE OHMMETER
• It consists of a bat in series with the adjustable
series resistance R1 and a basic d’ Arsonval
movement
• The unknown resistance is connected across
terminals A and B, parallel with the meter. An on-off
switch is necessary to disconnect the bat
• When the unknown resistance Rx=0; meter reading
is 0 ; for Rx=infinity the current finds a path only
through the meter and selecting a proper value for
resistance R1; The pointer may be made to full scale.
• This meter has 0 marks at the left and infinity marks
at the right

72. SHUNT TYPE OHMMETER
1
1
1 1
1
R , the full scale meter current
I
I
any resistance R to be measured, the current
I
( )
half scale reading of the
x
fs m
m fs
x
x x
m
m x m x m m x
m x
when
E E
R R
R R
With
R ERE
R R R R R R R R RR
R R
At
 
   

 
  
   
      
1 1
1
1 1
1
meter I 0.5I and
0.5I
( )
I ( )
;
I ( )
I
Usually is very larger than
I ( )
m fs h x
h
fs
m m h
m m x
fs m m x
m x
m
fs m x
R R
ER
R R R R R
R R R
s
R R R R R
R
R R s
R R
 
 
 

  
 
  


73. • Thus the meter can be calibrated by computing the
fraction of full scale ‘s’ for a given resistance Rx
and the meter resistance Rm
• in order to determine the relative scale values for a
given value of R1; the half scale reading may be
found by dividing eqns and solving for Rh=(R1
Rm)/(R1+ Rm);
IT IS CLEAR THAT
The battery emf E, the meter full scale current Ifs, and
meter resistance Rm determine the resistace R1
The resistance R1 and the meter resistance Rm
determine the half scale resistance Rh
SHUNT TYPE OHMMETER

74. • Thus the meter resistance scale depends
only on the meter and the voltage of the
battery. The battery emf is 1.5-15V, while
the full scale current of the meter may vary
from 0.25 mA to 1.0 A or even more.
• The shunt type ohmmeter is particularly
suitable for measurement of low
resistance; It is used in lab
SHUNT TYPE OHMMETER

75. ELECTRODYNAMOMETER TYPE
INSTRUMENTS

78. Construction of
Electrodynamometer Instrument

79. Construction of
Electrodynamometer Instrument

80. DAMPING
• Air friction damping is used for these type
of instrument; provided by a pair of
aluminum vanes attached to the spindle at
the bottom
• Eddy current damping can’t be used for
these instruments as the operating field is
very weak and eddy current damping would
distort the operating magnetic field

81. SHIELDING/CASES & SCALES
• The field produced by the fixed coil is weaker; even
the earth magnetic field may affect the reading; So, it
is necessary to shield an electrodynamometer type
instruments from the effect the stray magnetic field
• Lab standard instruments are contained in a highly
polished wooden cases and must be dimensionally
stable for long period. The glass is coated with some
conducting material to completely remove the
electrostatic effects
• The scales are hand drawn using machine
subdividing equipment. Diagonal lines foe fine sub-
divisor are usually drawn for main markings on the
scale

82. Torque Equation of
Electrodynamometer Instruments
1
2
1
2
, i instantaneous valu of current in the fixed coil; A
i instantaneous valu of current in the moving coil; A
L = Self inductance of fixed coils; H
L = Self inductance of moving coils; H
M=mutual indu
Let 

ctance between fixed and moving coils; H

83. 1 1 1 2
2 2 2 1
1 2
1 2
1 1 2 2 1 1 2 2
1 1 1 2 2 2 2 1
1
linkage of coil 1,
linkage of coil 1,
; ;
energy input e i dt+e i dt=i i
=i ( ) i ( )
Flux L i Mi
Flux L i Mi
d d
e e
dt dt
Electrical d d
d L i Mi d L i Mi
L i


 
 
 
 
 

  
 2 2
1 1 1 1 1 2 1 2 2 2 2 2 2 1 2 2 1
2 2
1 1 2 2 1 2
2 2
1 1 2 2 1 2
2 2
1 1 1 1 1 2 2 2 2 2
1 1
stored in the magnetic field =
2 2
1 1
in energy stored=d( )
2 2
= ( / 2) ( / 2)
di i dL i i dM i Mdi L i di i dL i i dM i Mdi
Energy i L i L i i M
Change i L i L i i M
L i di i dL L i di i dL i
      
 
 
    1 2 2 1 1 2Mdi i Mdi i i dM 
Torque Equation of Electrodynamometer Instruments

84. • From the principle of energy conversion
Total electrical input energy=change in energy
stored + mechanical energy
The mechanical energy can be obtained by
subtracting the equations
2 2
1 1 2 2 1 2
1 2
1 2
1 2
1 1
Mechanical Energy =
2 2
Now the self inductance L and L are constant and therefore
dL and dL are both equal to 0
Hence, Mechanical Energy =
T is the ii
i dL i dL i i dM
i i dM
Suppose
  
1 2 1 2
nstantaneous deflectiong torque and is the change
in deflection then Mechanical Energy = work done = T
T T /
i
i i
d
d
d i i dM i i dM d


    
Torque Equation of Electrodynamometer Instruments

85. OPERATION WITH DC
1
2
1 2
, I current in the fixed coil; A
and I =current in moving coils; A
Torque T /
torque depends on the product of currents and
the change in rate in change of mutual
d
Let
Deflecting I I dM d
deflection

inductance
Deflecting torque deflects the moving coils in such a posn
where the controlling torque of the spring is equal to the
deflecting torque. Suppose is the final deflection
Controlling Torqu


1 2
1 2
e T =K ; K is the spring const Nm/rad
steady state position T T
/ K .
c
c dAt
I I dM
I I dM d
K d

  


   

86. OPERATION WITH AC
1 2
1 2
1
0
, and be the instantaneous currents carried by the coils
The instantaneous deflecting torque is T /
average deflection torque over the cycle
1 1
T T ( / )
i
T
d i
Let i i
i i dM d
The
dt dM d i
T T



  2
0
1 2
1 1 2 2
, and be the sinusoidal currents and are displaced by
sin and sin( )
T
m m
i dt
SINUSOIDAL CURRENTS
Let i i
i I t i I t

     


87. 1 2
0
1 2
0
1 2
1 2
1 2
1 2
1
deflecting torque T ( / )
1
( / ) sin . sin( ) ( )
2
cos ( / ) cos ( / )
2
Equlibrium, T T cos ( / )
cos
= .( / )
T
d
T
m m
m m
d c
The dM d i i dt
T
dM d I t I t d t
I I
dM d I I dM d
At I I dM d K
I I
Deflection dM d
K

    

   
  

 

 
 
  



OPERATION WITH AC

88. Electrodynamometer Ammeter
In this case, the fixed and moving coils are
connected in series
and, therefore carry the same current.
I1 = I2 = I and φ = 0.
2
2
Torque T
Deflection = .
d
dM
Deflecting I
d
I dM
and
K d





89. • The current should not exceed 100mA.
• For ammeters of higher capacity, the moving
coil is shunted across by a low resistance to
limit the current through moving coil to 100mA

90. Electrodynamometer Voltmeter
The electrodynamometer movement is used as a
voltmeter
– by connecting the fixed and moving coils in series
with a high non-inductive resistance.
This voltmeter is most accurate for ac voltmeter but
sensitivies are lower than dc and range from 10 to 30
ohm/V
1 2
1 2
2
2
2
Torque T cos .
this case / and 0
T / . / . ( / ) .
= .
d
d
dM
Deflectin I I
d
In I I V Z
dM dM
V Z V Z V Z
d d
V dM
Deflection
KZ d



 



  
  

91. Advantages and Disadvantages of
Electrodynamometer
Advantage
• As the coils are air cored, these instruments are free from
hysteresis and eddy current errors
• They have a precision grade accuracy for frequencies from 40
up to 500 Hz.
• Can be used on both AC and DC.
• Very useful where accurate rms values of voltage irrespective of
waveform are required.
Disadvantage
• They have a low torque/weight ratio
– hence, have a low sensitivity
• They are more expensive than either the PMMC or the Moving Iron
type
• Sensitive to overloads and mechanical impacts
• Non uniform scale
– Must be handled with great care

92. Multimeter or Volt-Ohm-Mili-
ammeter (VOM)
• During troubleshooting, you will often be required to measure
voltage, current, and resistance. Rather than using three or
more separate meters for these measurements, you can use
the MULTIMETER. The multimeter contains circuitry that
allows it to be used as a voltmeter, an ammeter, or an
ohmmeter. A multimeter is often called a VOLT-OHM-
MILLIAMMETER (VOM).
• One of the greatest advantages of a VOM is that no external
power source is required for its operation; therefore, no
warm-up is necessary. Other advantages are its portability,
versatility, and freedom from calibration errors caused by
aging tubes, line voltage variations, and so forth.

93. VOM
• The multimeter is equipped with a pair of test leads; red is the
positive lead and black is the negative, or common, lead. Eight
jacks are located on the lower part of the front panel. To
prepare the meter for use, simply insert the test leads into the
proper jacks to obtain the circuit and range desired for each
application. In most applications, the black lead will be inserted
into the jack marked at the lower left with a negative sign (-) or
with the word COMMON.
The multimeter has two selector switches. The
switch on the lower left is the function switch,
and the one in the lower center is the range
switch. The function switch selects the type of
current you will be measuring (+dc, -dc, or ac).
The range switch is a 12-position switch that
selects the range of ohmmeter, voltmeter, or
milliammeter measurements you will make.

94. • Circuit diagram of Simpson
model 260 Multimeter
• DC Voltmeter section
• Ammeter section
• Ohmmeter sections with
multiplier (Cam)
VOM

95. Construction and Basic principle
operation of Moving-iron
Instruments

96. Classification and Construction
• We have mentioned earlier that the instruments are classified
according to the principles of operation. Furthermore, each class
may be subdivided according to the nature of the movable system
and method by which the operating torque is produced.
• Specifically, the electromagnetic instruments are sub-classes as (i)
moving-iron instruments (ii) electro-dynamic or dynamometer
instruments, (iii) induction instruments.
• In moving –iron instruments the movable system consists of one or
more pieces of specially-shaped soft iron, which are so pivoted as to
be acted upon by the magnetic field produced by the current in coil.
• There are two general types of moving-iron instruments namely (i)
Repulsion (or double iron) type (ii) Attraction (or single-iron) type.

97. • The brief description of different components of a
moving-iron instrument is given below.
– Moving element: a small piece of soft iron in the form
of a vane or rod
– Coil: to produce the magnetic field due to current
flowing through it and also to magnetize the iron
pieces.
– In repulsion type, a fixed vane or rod is also used and
magnetized with the same polarity.
– Control torque is provided by spring or weight (gravity)
– Damping torque is normally pneumatic, the damping
device consisting of an air chamber and a moving vane
attached to the instrument spindle.
– Deflecting torque produces a movement on an
aluminum pointer over a graduated scale.
Classification and Construction

98. Construction of Moving-iron
Instruments
• The deflecting torque in any moving-iron instrument is due to
forces on a small piece of magnetically ‘soft’ iron that is
magnetized by a coil carrying the operating current. In repulsion
(Fig.42.7) type moving–iron instrument consists of two cylindrical
soft iron vanes mounted within a fixed current-carrying coil. One
iron vane is held fixed to the coil frame and other is free to rotate,
carrying with it the pointer shaft. Two irons lie in the magnetic field
produced by the coil that consists of only few turns if the
instrument is an ammeter or of many turns if the instrument is a
voltmeter. Current in the coil induces both vanes to become
magnetized and repulsion between the similarly magnetized vanes
produces a proportional rotation. The deflecting torque is
proportional to the square of the current in the coil, making the
instrument reading is a true ‘RMS’ quantity. Rotation is opposed by
a hairspring that produces the restoring torque. Only the fixed coil
carries load current, and it is constructed so as to withstand high
transient current. Moving iron instruments having scales that are
nonlinear and somewhat crowded in the lower range of calibration.
Another type of instrument that is usually classed with the
attractive types of instrument is shown in Fig.42.8.

99. Repulsion Type

100. Attraction Type

101. Construction
• This instrument consists of a few soft iron discs (B) that are
fixed to the spindle (D), pivoted in jeweled bearings. The
spindle (D) also carries a pointer (P), a balance weight (W1), a
controlling weight (W2) and a damping piston (E), which moves
in a curved fixed cylinder (F). The special shape of the moving-
iron discs is for obtaining a scale of suitable form.
• Remark: Moving-iron vanes instruments may be used for DC
current and voltage measurements and they are subject to
minor frequency errors only. The instruments may be
effectively shielded from the influence of external magnetic
fields by enclosing the working parts, except the pointer, in a
laminated iron cylinder with laminated iron end covers.

105. Ranges of Ammeters and
Voltmeters
• For a given moving-iron instrument the
ampere-turns necessary to produce
full-scale deflection are constant. One
can alter the range of ammeters by
providing a shunt coil with the moving
coil.

106. Shunts and Multipliers for MI
instruments