pmmc emc em syllabus btech 2nd year ee subject

Emdcpmmmc

1. Moving Coil meter/d’Arsonval
Movement/Moving Iron meter
Rahul Katiyar

2. Classification of Secondary Instruments
1. Indicating instruments
Ordinary voltmeters, ammeters & wattmeter's.
2. Recording instruments
X-Y plotter e.g. ECG (Electro-Cardio-Gram).
3. Integrating instruments
Ampere-hour meter, watt-hour (energy) meter
and odometer in a car (which measures the
total distance covered)

3. Essentials of an Indicating Instruments
In order to ensure proper operation of indicating instruments. Three
torque are needed.
(i) Deflecting torque– It is produced by use of magnetic field, heating,
chemical, electromagnetic or electrostatic effect of current and voltage to
be measured. In PMMC, Magnetic effect is used in which a coil may
carry current in the presence of electromagnetic field and a force is
created.
(ii) Controlling torque (By Spring or gravity)- It is opposing the
deflecting torque and increases with deflection. It is produced by either
spring or gravity.
for spring control Tc α θ
for gravity control Tc α sinθ where θ- deflection
The controlling torque serves two functions : (i) the pointer stops
moving beyond the final deflection, (ii) the pointer comes back to its
zero position when the instrument is disconnected.

4. 4
(i) Spring Control
•Most commonly used.
•One or two hairsprings made of phosphor bronze are used.
•The outer end of this spring is fixed to the pointer and the
inner end is attached with the spindle.
•When the pointer is at zero of the scale, the spring is normal.
•As the pointer moves, the spring winds and produces an
opposing torque.
•The balance-weight balances the moving system so that its
centre of gravity coincides with the axis of rotation, thereby
reducing the friction between the pivot and bearings.

5. 5

6. 6
•Advantages :
•Since
•These instruments have uniform scale.
• Disadvantages :
•The stiffness of the spring is a function of temperature.
•Hence, the readings given by the instruments are temperature
dependent.
•Furthermore, with the usage the spring develops an inelastic
yield which affects the zero position of the moving system.
c d c dand ;at final position, I      
Hence,I
(i) Spring Control

7. 7
•A small control weight is attached to the moving system.
•In addition, an adjustable balance weight is also attached to
make the centre of gravity pass through the spindle.
•In zero position of the pointer, this control weight is vertical.
(ii) Gravity Control

8. 8

When deflected by an angle θ, the
weight exerts a force,
 The restraining or controlling
torque is thus developed is given as
sinW
 
c sin sinW L WL    
d c dSince , and
or sin
I
WL kI
  

 

sin
or sin
WL
I
k
I


 
 
 
 


9. 9
Disadvantage :
1.These do not have uniform scale.
2.These must be used in vertical position so that the control
may operate properly.
Advantages :
1.Less expensive.
2. Unaffected by changes in temperature.
3. Free from fatigue or deterioration with
time.

10. 10
It is necessary to avoid oscillations of the moving system about its
final deflected position owing to inertia of the moving parts and to
bring the moving system to rest in its deflected position quickly.
 If the instrument is underdamped, the moving system will
oscillate about its final position and take some time to come to
rest in its steady position.
 If the instrument is overdamped, the moving system will be slow
and lithargic.
If the instrument is critical damped, the moving system rises
quickly to its deflected position without oscillation and then
instrument is said to be “Dead Beat”. In practice in order to
achieve best results, damping is in between 0.6 to 0.7 in all
instruments.
Damping Torque

11. 11
Damping Torque

12. Methods for obtaining
Damping Torques
1.Air Friction Damping Torques
2.Fluid Friction Damping
3.Eddy Current Damping (Most commonly
employed method)

13. 13
MOVING COIL INSTRUMENTS
•There are two types :
(1)Permanent Magnet Type : It is the most
accurate and useful for dc measurements.
Popularly known as d’Arsonval Movement.
(2) Dynamometer Type : It can be used for
both dc and ac measurements.

14. 14
PMMC
•It consists of an iron-core coil
mounted on bearings between
permanent magnet
•Very fine insulated wire of many
turns is used
•Coil is wound on an aluminium
bobbin which is free to rotate by
about 90
◦
•An aluminium pointer attached to
the coil can move on a calibrated
scale.
•Two springs one at top and other at
bottom were attached to the
assembly and serves two purposes
•One is to provide path for current
and other for providing controlling
torque.

15. How the Deflection Torque is Produced

16. 16
PMMC
•Core is made of soft iron
•Magnetic poles & iron core are
cylindrical in shape. This has two
advantages
•Firstly, the length of the air gap is
reduced (flux leakage=0)
•Secondly, the iron core helps in
making the field radial in the air gap
which ensures uniform magnetic
field throughout the motion of the
coil.
•This way the angle of deflection is
proportional to the current in the
coil and hence the scale is uniform

17. 17
PMMC
•When a current is passed through a
coil in a magnetic field, the coil
experiences a torque proportional
to the current.
•A coil spring provides the controlling
torque.
•The deflection of a needle attached
to the coil is proportional to the
current.
•Such "meter movements" are at the
heart of the moving coil meters
such as voltmeters and ammeters.
•Now they were largely replaced
with solid state meters. 

18. 18
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.
The D’Arsonval Meter Movement

20. D’Arsonval movement & PMMC………….
•Principle of Operation: When a current carrying
conductor is placed in a magnetic field, it experiences a
force and tends to move in the direction as per Fleming’s
left hand rule.
Fleming left hand rule: If the first and the second finger and
the thumb of the left hand are held so that they are at right
angle to each other, then the thumb shows the direction of
the force on the conductor, the first finger points towards the
direction of the magnetic field and the second finger shows
the direction of the current in the wire.

21. Working:
•When a current flow through the coil, it generates a magnetic
field which is proportional to the current in case of an
ammeter. The deflecting torque is produced by the
electromagnetic action of the current in the coil and the
magnetic field.
•The controlling torque is provided by two phosphorous
bronze flat coiled helical springs. These springs serve as a
flexible connection to the coil conductors.
•Damping is caused by the eddy current set up in the
aluminum coil which prevents the oscillation of the coil.
Damping is electromagnetic by eddy currents induced in the
metal frame over which the coil is wound. Since the frame
moves in an intense magnetic field, the induced eddy currents
are large and damping is very effective.

22. Torque Equation

23. Advantages of d’Arsonval Meter:
 The meter requires low current (~50µA) for a full scale
deflection, thus consumes very low power (25-200 µW).
 Its accuracy is about 2% -5% of full scale deflection.
 Low Power Consumption
 High torque to weight ratio.
 No hysteresis loss
Uniform scale
DISADVANTAGES
1. Due to delicate construction and the necessary accurate machining
and assembly of various parts, such instruments are somewhat costlier as
compared to moving iron instruments.
2. Some errors are set in due to the ageing of control springs and the
permanent magnets

24. DYNAMOMETER TYPE INSTRUMENTS
For both ac & dc measurements
•These instruments are similar to the permanent
magnet type instruments, except that the permanent
magnet is replaced by a fixed coil.
• The coil is divided into two halves, connected in
series with the moving coil.
•The two halves of the coil are placed close together
and parallel to each other to provide uniform field
within the range of the movement of moving coil.

25. DYNAMOMETER TYPE INSTRUMENTS

26. Dynamometer Type Instruments

27. Working
• The fixed coil and the moving coil carry currents. Thus,
two magnetic fields are produced. The deflecting torque
depends on the magnetic fields of both fixed and moving
coils.
•Deflecting torque is proportional to square of the current.
•Moving coil is wound using a thin wire so that it deflects
easily.
• Hence, an electromagnetic force tends to act on the
moving coil and makes it move.
•This makes the pointer gives a proportionate deflection.

28. Deflecting torque
As voltmeter: The two coils are electrically in series. Deflecting torque is
proportional to square of voltage to be measured. Hence used for
measuring ac and dc voltages.
As ammeter: The two coils are electrically in series. Deflecting torque is
proportional to square of current to be measured. Hence used for
measuring ac and dc currents.
As wattmeter: Fixed coils carry the system current. Moving coil carries a
current proportional to the system voltage. The deflecting torque is
proportional to V ICos φ i.e. Power to be measured
Control torque : Spring control.
Damping torque : Air damping.

29. DYNAMOMETER TYPE-Ammeter & Voltmeter

30. Dynamometer Type Wattmeter

31. Dynamometer Type Instruments
Advantages :
(i)Can be used on both DC and AC systems
(ii)No errors due to hysteresis or eddy currents
(iii)Good accuracy
(iv)Same calibration for DC and AC measurements and hence can be used as
Transfer Instruments ( used in situations where you can not measure
directly. The measurement is transferred to another means of
measurement)
Disadvantages :
(i)Non-uniform scale
(ii)Torque/weight ratio is small
(iii)Low sensitivity than PMMC
(iv)More expensive than PMMC

32. MOVING IRON INSTRUMENTS – ATTRACTION TYPE
Principle:
•Depends on magnetic effects of current.
•In this type, coil is stationary and deflection is produced
by soft iron piece moving in the field produced by the
coil.
•The iron piece is attracted towards that portion where the
magnetic flux density is more.
•This movement of soft iron piece is used to measure the
current or voltage which produces the magnetic field.
• Force is produced in such a direction so as to increase
the inductance of the coil.

33. MOVING-IRON INSTRUMENTS
For both ac & dc measurements
•Attraction (or Single-iron) Type Moving-Iron
Instrument

34. Construction
• There are two iron pieces-fixed and moving.
• The moving iron is connected to the spindle to which
is attached a pointer. It is made to move over a calibrated
scale.

36. Deflecting Torque
•Produced by the current or the voltage to be measured.
•It is proportional to the square of the voltage or current.
•Hence, the instrument can be used to measure d.c. or a.c.
•Scale is non- uniform
Control torque : Spring
Damping : Air friction damping

37. MOVING IRON INSTRUMENT - REPULSION TYPE
Principle
•Two iron piece kept with close proximity in a magnetic field
get magnetized to the same polarity. Hence, a repulsive
force is produced.
•If one of the two piece is made movable, the repulsive force
will act on it and move it on to one side.
•This movement is used to measure the current or voltage
which produces the magnetic field.
•The forces of repulsion is proportional to the square of
current passing through the coil.

38. Repulsion type:

40. Working
•When the current to be measured is passed through the fixed
coil it sets up its own magnetic field which magnetizes the
two rods similarly the adjacent points on the lengths of the
rods will have the same magnetic polarity.
•Hence, they repel each other with the result that the pointer
is deflected against the controlling torque of a spring or
gravity.
•The force of repulsion is approximately proportional to the
square of the current passing through the coil
•Whatever be the direction of current in the coil, the two
irons are always similarly magnetised.

41. Deflecting torque
•Produced by the current or the voltage to be measured.
•It is proportional to the square of the voltage or current.
•Hence, the instrument can be used to measure d.c. or a.c.
Control torque : Spring or gravity
Damping : Air friction damping

42. Advantages and disadvantages:
• The instruments are cheap ,reliable and robust
•The instruments can be used on both A.C and D.C
•They cannot be calibrated with high degree of precision
with D.C on account of the effect of hysteresis in the iron
rods or vanes .
•They are commonly used in laboratories, panel meters and
switchboards at commercial frequency bcoz of low cost and
can be manufactured with required accuracy.
• Drawbacks: Waveform error due to BH curve of iron
nonlinearity, hysteresis error, temp error,stray magnetic
fields etc.

43. Advantages:
•The instruments are suitable for use in AC and DC
circuits.
•The instruments are robust, owing to the simple
construction of the moving parts.
•The stationary parts of the instruments are also simple.
•Instrument is low cost compared to moving coil
instrument.
•Torque/weight ratio is high, thus less frictional error.

44. Application:
Measurement of Electric Voltage and Current
•Moving iron instruments are used as Voltmeter and Ammeter
only.
•Both can work on AC as well as on DC.
Ammeter: Instrument used to measure current in the circuit.
•This current flowing through the coil produces the desired
deflecting torque.
•It should have low resistance as it is to be connected in series.
Voltmeter:Used to measure voltage between two points in a
circuit.
•Current flowing through the operating coil of the meter produces
deflecting torque.
•It should have high resistance. Thus a high resistance of order of
kilo ohms is connected in series with the coil of the instrument

45. Applications of PMMC meters:
The PMMC has a variety of uses onboard ship. It can be used as:
1)
      
Ammeter:
•When PMMC is used as an ammeter, except for a very small
current range, the moving coil is connected across a suitable
low resistance shunt, so that only small part of the main current
flows through the coil.
•The shunt consists of a number of thin plates made up of alloy
metal, which is usually magnetic and has a low temperature
coefficient of resistance, fixed between two massive blocks of
copper. A resistor of same alloy is also placed in series with the
coil to reduce errors due to temperature variation.

46. Applications………..
•Voltmeter:
When PMMC is used as a voltmeter, the coil is connected in
series with high resistance. Rest of the function is same
as above. The same moving coil can be used as an
ammeter or voltmeter with an interchange of above
arrangement

47. Applications……….
•Ohm Meter:
The ohm meter is used to measure resistance of the
electric circuit by applying a voltage to a resistance with
the help of battery. A galvanometer is used to determine
the flow of current through the resistance. The
galvanometer scale is marked in ohms and as the
resistance varies, since the voltage is fixed, the current
through the meter will also vary.

48. Advantages:
• The PMMC consumes less power and has great accuracy.
• It has uniformly divided scale and can cover arc of 270
degree.
• The PMMC has a high torque to weight ratio.
• It can be modified as ammeter or voltmeter with suitable
resistance.
• It has efficient damping characteristics and is not affected
by stray magnetic field.
• It produces no losses due to hysteresis.

49. Disadvantage:
•The moving coil instrument can only be used on D.C
supply as the reversal of current produces reversal of
torque on the coil.
•It’s very delicate and sometimes uses ac circuit with a
rectifier.
•It’s costly as compared to moving coil iron instruments.
•It may show error due to loss of magnetism of
permanent magnet.

50. DC AMMETER
•The purpose of designing the shunt circuit is to allow to
measure current I that is some number ‘m’ times larger
than I
m.
•The PMMC galvanometer constitutes the basic movement
of a dc ammeter.
•The coil winding of a basic movement is small and light,
so it can carry only very small currents.
•A low value resistor (shunt resistor) is used in DC
ammeter to measure large current.
•Basic DC ammeter:

51. Rsh
+
_
_
+
Rm
D’Arsonval
Movement
I
Ish Im
Figure 2.2: Basic DC Ammeter

52. •Referring to Fig. 2.2:
Rm = internal resistance of the
movement
Rsh = shunt resistance
Ish=shunt current
Im = full scale deflection current
of the movement
I = full scale current of the
ammeter + shunt (i.e. total
current)

53. Example
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.
Solution:
VmARIV
mmm 1.01001 
VVV
msh 1.0
mAmAmAIII
msh 9110 
 11.11
9
1.0
mA
V
I
V
R
sh
sh
sh

54. Ammeter Example
Q. An ammeter uses a meter with an internal resistance
of 600  and a rating of 1 mA fsd. How can it be used to
measure 20 A fs?
Maximum current through meter is
0.001 A.
Therefore, the shunt resistor must take
19.999 A
R
M
R
M
Because both M and R are in parallel, the same V must
be dropped across both
V = I
m R
m
= 0.001 A x 600 Ω = 0.6 V
Thus R must be V / I
R
= 0.6 V / 19.99 A
= 0.03 (in parallel.)
i
R
i
m

55. Construction of Shunts/Multipliers
 Temperature coefficients of shunt and instrument should
be low and same. [If not, use Swamping resistance in series
with meter movement in case of shunts.]
 Resistance of shunts should not vary with time.
 They should carry current without excessive
temperature rise.
 Manganin is ususally used shunts for DC instruments.
Constantan is useful for ac circuits.

56. MULTIRANGE AMMETER
The range of the dc ammeter is extended by a number of
shunts, selected by a range switch.
The resistors is placed in parallel to give different current
ranges.
Switch S (multiposition switch) protects the meter
movement from being damage during range changing.
Increase cost of the meter.
 Useful for ranges from 1 to 50A.

57. 57
R1 R2 R3 R4
+
_
+
_
Rm
D’Arsonval
Movement
Figure 2.3: Multirange Ammeter
S

58. •Design a multirange ammeter with ranges of
1A,5A, 25A, 125A employing individual shunts
in each case. A d’arsonval movement with an
internal resistance of 730 Ω and a full scale
current of 5mA is available.

59. UNIVERSAL SHUNT [ARYTON SHUNT]

60. Design a Aryton Shunt to provide
an ammeter with current ranges of
1A, 5A and 10A. A basic meter with
an internal resistances of 50 Ω and
fsd current of 1mA is to be used.

61. 61
A DC VOLTMETER

A basic D’Arsonval movement can be converted into
a DC meter by adding a series resistor (multiplier).
Im =full scale deflection current of the movement (Ifsd)
Rm=internal resistance of the movement
Rs =multiplier resistance
V =full range voltage of the instrument
Rs
Im
Rm
Multiplier
V
+
Basic DC Voltmeter

62. 62
Example
A meter is rated at 1 mA fsd and has an internal resistance of
2000 Ω. How can it be used to measure 100 V fsd ?
Maximum voltage that can be put across galvanometer is
V
m
= I R
m
= 0.001 x 2000 = 2.0 V
Thus, V
s
= V
T
- V
m
=
100 V - 2 V = 98 V
This voltage must be dropped across R
s
. Therefore,
R
s = V
s/I = 98 V / 0.001 A = 98 kΩ
V
s = 98 VV
m
= 2 V
R
T
= R
s
+ R
m
M
R
m R
s1 mA
Solution :

63. 63
Example
•A 50-μA meter movement with an internal
resistance of 1 kΩ is to be used as a dc voltmeter of
range 50 V. Calculate
(a) the multiplier resistance needed, and
(b) the voltage multiplying factor.
Solution : Here, I
m = 50 μA, and R
m = 1 kΩ.
(a) The series resistance needed is given as
fsd
s m
m
50 V
1000
50μA
V
R R
I
     999kΩ
(b)
fsd fsd
6 3
m m m
50
50 10 1 10
V V
n
V I R

   
 
1000

64. MULTI-RANGE VOLTMETER
•A DC voltmeter can be converted into a
multirange voltmeter by connecting a
number of resistors (multipliers) in series
with the meter movement.
Figure 2.6: Multirange voltmeter
R1 R2 R3 R4
+
_
V1
V2
V3
V4
Rm
Im

65. A basic d’arsonval movement with a full scale
reading of 50µA and an internal resitance of
1800 Ω is available. It is to be converted into a 0-
1V, 0-5 V, 0-25V, 0-125V multi range voltmeters
using individual multipliers for each range.
Calculate the value of the individual multipliers.

66. Meter Sensitivity
(Ohms-per-Volt Rating)
•Higher the sensitivity, more accurate is the measurement.
•If current sensitivity (CS) of a meter is known, its Ω/V
rating can easily be determined.
•Consider a basic meter with CS of 100 μA.
•If used as a voltmeter of range 1
 V,
R
T = 1 V / 100 μA = 10 kΩ
•Thus, the meter sensitivity is simply 10 kΩ/V.
• Sensitivity of Voltmeter is defined as:
/V)(
1
ySensitivit 
fsI

67. 67
EXAMPLE
Calculate the sensitivity of a 200 uA meter
movement which is to be used as a dc
voltmeter.
Solution: Vk
uAI
S
fsd
/5
200
11


68. Which meter has a greater sensitivity? Meter A
having a range of 0-10 V and a multiplier
resistance of 15 KΩ or meter B with a range of 0-
300 V and a multiplier resistance of 298 KΩ ?
Both meters have a resistance of 2 KΩ.

69. Drawback of Ammeters/Voltmeters-
All ammeters & voltmeters introduce some error –
meter loads the circuit (common instrumentation
problem).

70. A moving coil instrument has the following data:
No. of turns =100, width of the coil =20mm,
depth of coil= 30mm, flux density in the gap=
0.1 Wb/m
2
. Calculate the deflecting torque
when carrying a current of 10mA. Also calculate
the deflection if the control spring constant is 2
Χ 10
-6
Nm/deg

71. The inductance of a 25A electrodynamic
ammeter changes uniformly at the rate of
0.0035µH/degree. The spring constant is 10
-6
N-
m/degree. Determine the angular deflection at
full scale.

72. The inducatance of a moving iron instrument is
given by:- L =[ 10+5θ- θ
2
]µH where θ is the
deflection in radian from zero position. The
spring constant is 12 Χ 10
-6
Nm/rad. Estimate the
deflection for a current of 5A.

73. RESISTANCE MEASUREMENT
•The instrument is called ohmmeter.
•Three types :
1.Shunt-Type Ohmmeter : For low value resistors.
2.Series-Type Ohmmeter : For medium-value resistors.
3.Meggar-Type Ohmmeter : For high-value resistances, such
as the insulation of a cable.
 Low accuracy/ Useful in laboratories only.
 Determines the approximate value of resistance of circuit in
components like heater elements, machine field coils ,
checking of semiconductor diodes and for checking the
circuit continuity.

74. Shunt-Type Ohmmeter
When R
x = 0, no current in meter.
When R
x = , entire current flows through the meter.
Proper selection of R
1 gives full-scale deflection on open
circuit.

75. 75
OHMMETER (Series Type)
•Current flowing through meter movements depends on the
magnitude of the unknown resistance.
•The meter deflection is non-linearly related to the value of the
unknown Resistance, R
x.
•A major drawback – as the internal voltage decreases, reduces the
current and meter will not get zero Ohm.
•R
1 and R
2 are determined by the value of R
x = R
h where R
h = half of
full scale deflection resistance.
•The total current of the circuit, It=V/R
h
•The shunt current through R2 is I
2=I
t-I
fsd
m
m
mh
RR
RR
RRRRR


2
2
121 )//(

76. OHMMETER (Series Type)
•The voltage across the shunt, Vsh= Vm
So, I
2
R
2
=I
fsd
R
m
Since I
2=I
t-I
fsd
Since I
t=V/R
h
So,
fsdt
mfsd
II
RI
R


2
hfsd
hmfsd
RIV
RRI
R


2
V
RRI
RR
hmfsd
h

1