The Presentation covers, PMMC instrument construction and working, Galvanometer, DC ammeter, DC voltmeter, series ohm meter,

1. PMMC instrument
PMMC instrument, galvanometer, DC ammeter, DC
voltmeter, series ohm meter.
2/3/2017 1
NEC 403 Unit I by Dr Naim R Kidwai,
Professor & Dean, JIT Jahangirabad

2. PMMC or D’Arsonval instrument
(Permanent Magnet Moving Coil)
Consists of copper coil suspended between permanent magnet.
A pointer attached to the coil over non-magnetic (aluminium) coil
former deflects over a calibrated scale indicating the current in coil.
2/3/2017 2
NEC 403 Unit I by Dr Naim R Kidwai,
Professor & Dean, JIT Jahangirabad
Construction of PMMC instrument
I
I
N S
0
Permanent
Magnet
Copper
Coil
Aluminium FormerSpring
Pointer
Calibrated Scale
D=2r
L

3. PMMC : Deflecting Force
2/3/2017 3
NEC 403 Unit I by Dr Naim R Kidwai,
Professor & Dean, JIT Jahangirabad
Three forces operates in electro-mechanical movement;
•Deflecting force
•Controlling force
•Damping force
N S
Force
Force
Deflecting force in PMMC due to current in coil
Magnetic field due to current in
coil, interacts with field of
magnet resulting in deflection
(rotation) of coil.

4. PMMC : Controlling Force
2/3/2017 4
NEC 403 Unit I by Dr Naim R Kidwai,
Professor & Dean, JIT Jahangirabad
Deflecting Force
Controlling Force
Controlling force of spring balancing the
deflecting force
Spring attached to the pointer is used to balance the deflecting
force. The Pointer stops where deflecting force equals to
controlling force

5. PMMC : Damping Force
2/3/2017 5
NEC 403 Unit I by Dr Naim R Kidwai,
Professor & Dean, JIT Jahangirabad
Damping force in PMMC due to eddy current
in aluminium coil former
N S
Motion
Motion
Damping
Force
Damping
Force
Pointer Oscillation
Pointer Oscillation with no damping
with damping Pointer Oscillation
Final Pointer
Position
Position
Time
Pointer and coil tend to oscillate
before settling. Damping force is
provided by eddy current in
aluminium coil former.

6. PMMC instrument
2/3/2017 6
NEC 403 Unit I by Dr Naim R Kidwai,
Professor & Dean, JIT Jahangirabad
Let I current flows in N turn coil of length L of one side, situated
in magnetic field B, resultant force on one side of coil
F=BLIN Newton
Total Force on Coil is twice as it is exerted on two sides,
F=2BLIN
With r radius, the deflecting torque on coil
TD=2BLINr=BLIN(2r)=BLIND N.m, where D is Diameter
The controlling torque is proportional to pointer deflection angle
(deformation in the spring). With spring constant K, the torque
TC=K N.m, where  is deflection angle

7. PMMC instrument
2/3/2017 7
NEC 403 Unit I by Dr Naim R Kidwai,
Professor & Dean, JIT Jahangirabad
As all quantities except  and I are constant for an PMMC
instrument
= C.I where C is a constant.
i.e. Scale of PMMC instrument is Linear as  I
For a given deflection,
TC=TD
Or K=BLIND
Or I = K/BLND

8. Galvanometer
A Galvanometer is essentially a PMMC instrument designed to be
sensitive to very low level currents. It has centre zero scale for both
direction of currents.
Sensitivity (A/ mm): current level that creates minimum deflection.
– For greater sensitivity, Low weight moving coil is used
– To reduce weight of pointer, sometimes a mirror on moving coil is used, a light
beam reflect from mirror and falls on calibrated scale.
– Pointer galvanometers have 0.1 - 1 A/ mm while light beam have 0.01 – 0.1 A/
mm sensitivity
Voltage sensitivity (mV/mm): minimum voltage that creates
minimum deflection for a given critical damping resistance (a damping
resistance shunting the coil controls the level of eddy current duo to coil movement).
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NEC 403 Unit I by Dr Naim R Kidwai,
Professor & Dean, JIT Jahangirabad

9. Galvanometer
Mega ohm sensitivity. The resistance which must be connected
in series to produce one scale division deflection when 1V is
applied.
Galvanometer are often employed to detect zero current or
voltage rather than to measure current or voltage. And is
referred as NULL DETECTOR. Protection from high current/
voltage is provided by using adjustable shunt resistor.
2/3/2017 9
NEC 403 Unit I by Dr Naim R Kidwai,
Professor & Dean, JIT Jahangirabad
G
R
Adjustable Shunt
Resistance

10. DC Ammeter
• Ammeter is current measuring instrument and is connected in series.
• To avoid effect on circuit current, ammeter resistance should be low.
• PMMC is a DC Ammeter but maximum pointer deflection is produced
by very small current and its coil can bear only small currents.
• To measure large currents, a low value shunt resistance is connected
to galvanometer, which bypass most of the current. Scale is
calibrated to read current range.
2/3/2017 10
NEC 403 Unit I by Dr Naim R Kidwai,
Professor & Dean, JIT Jahangirabad
Ammeter
Galvanometer
G
RS
Shunt Resistance
RC
I=IC+IS IC
IS
I
VC

11. DC Ammeter
Ex. An Ammeter has an PMMC with a coil resistance of RC=199 , Full
scale deflection current of 0.2 mA, shunt resistance RS=1 . Find current
of ammeter (i) at FSD, (ii) Half Scale Deflection
(i) At FSD
FSD for IC=0.2 mA
VC=RCIC =199 x 0.2 mV =3.98 mV
IS =VC/RS=3.98/1 mA =3.98 mA
I =IS+IC =3.98+0.2 =4.18 mA
(i) At Half Scale Deflection (0.5 FSD)
IC for FSD =0.1 mA
VC=RCIC =199 x 0.1 mV =1.99 mV
IS =VC/RS=1.99/1 mA =1.99 mA
I =IS+IC =1.99+0.1 =2.09 mA
2/3/2017 11
NEC 403 Unit I by Dr Naim R Kidwai,
Professor & Dean, JIT Jahangirabad

12. DC Ammeter: Swamping Resistance
• Moving coil is made of thin Copper wire, whose resistance changes
significantly with temperature, resulting in error in measurement
• To reduce temperature effect, a swamping resistance is connected in
series to galvanometer, resulting in equivalent coil resistance as
RSw+RC. Swamping resistance is made of Manganin or Constantan
which has extremely low temperature coefficient.
• If Swamping resistance is nine times coil resistance, then 1% change
in RC will result in 0.1% change in equivalent coil resistance.
2/3/2017 12
NEC 403 Unit I by Dr Naim R Kidwai,
Professor & Dean, JIT Jahangirabad
Galvanometer
G
RSw
I
RC
RS
R’C

13. DC Voltmeter
• Coil current of galvanometer is proportional to voltage across the coil
and scale can be calibrated to measure voltage making it a Voltmeter
• Coil resistance is usually very small therefore coil voltage is very small
• Voltage range is increased by connecting a series resistance with
galvanometer. The series resistance is termed as multiplier
• Sensitivity (/V) : is defined as volt meter resistance per volt
• To avoid loading effect, voltmeter resistance is very high.
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NEC 403 Unit I by Dr Naim R Kidwai,
Professor & Dean, JIT Jahangirabad
Voltmeter
Galvanometer
G
multiplier RS
IVC
RC
VS
V

14. Series ohm meter
Ohm meter is part of Volt Ohm Mili-Ammeter (VOM) or multi
function meter (Multi-meter) and don’t exist as separate instrument
Series Ohm meter consists of a voltage source connected in series to
a pair of terminals (to connect unknown resistance Rx), a standard
resistance R and a PMMC (coil resistance RC). The current I
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NEC 403 Unit I by Dr Naim R Kidwai,
Professor & Dean, JIT Jahangirabad
Galvanometer
G
R
I
RCRx
E
FSD)(0.5,
2
,for
)deflection(Zero,0,for
(FSD),,0for
,
max
max
I
IRRR
IR
RR
E
IR
RRR
E
I
Cx
x
C
x
Cx







15. Series ohm meter
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NEC 403 Unit I by Dr Naim R Kidwai,
Professor & Dean, JIT Jahangirabad
Series Ohm Meter with PMMC of FSD current 100 A, RC=100 , and
standard resistance R=14.9K

16. Thanks
2/3/2017
NEC 403 Unit I by Dr Naim R Kidwai,
Professor & Dean, JIT Jahangirabad
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