PMMC instrument
PMMC instrument, galvanometer, DC ammeter, DC
voltmeter, series ohm meter.
2/3/2017 1NEC 403 Unit I by Dr Naim R Kidwai, Professor & Dean, JIT Jahangirabad
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 2NEC 403 Unit I by Dr Naim R Kidwai, Professor & Dean, JIT Jahangirabad
Construction of PMMC instrument
I
IN S
0
Permanent Magnet
Copper Coil
Aluminium FormerSpring
Pointer
Calibrated Scale
D=2r
L
PMMC : Deflecting Force
2/3/2017 3NEC 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 incoil, interacts with field ofmagnet resulting in deflection(rotation) of coil.
PMMC : Controlling Force
2/3/2017 4NEC 403 Unit I by Dr Naim R Kidwai, Professor & Dean, JIT Jahangirabad
Deflecting ForceControlling Force
Controlling force of spring balancing the deflecting force
Spring attached to the pointer is used to balance the deflectingforce. The Pointer stops where deflecting force equals tocontrolling force
PMMC : Damping Force
2/3/2017 5NEC 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 oscillatebefore settling. Damping force isprovided by eddy current inaluminium coil former.
PMMC instrument
2/3/2017 6NEC 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, situatedin 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
PMMC instrument
2/3/2017 7NEC 403 Unit I by Dr Naim R Kidwai, Professor & Dean, JIT Jahangirabad
As all quantities except and I are constant for an PMMCinstrument
= 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
GalvanometerA Galvanometer is essentially a PMMC instrument designed to besensitive to very low level currents. It has centre zero scale for bothdirection 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 lightbeam 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 createsminimum deflection for a given critical damping resistance (a damping
resistance shunting the coil controls the level of eddy current duo to coil movement).
2/3/2017 8NEC 403 Unit I by Dr Naim R Kidwai, Professor & Dean, JIT Jahangirabad
GalvanometerMega ohm sensitivity. The resistance which must be connectedin series to produce one scale division deflection when 1V isapplied.
Galvanometer are often employed to detect zero current orvoltage rather than to measure current or voltage. And isreferred as NULL DETECTOR. Protection from high current/voltage is provided by using adjustable shunt resistor.
2/3/2017 9NEC 403 Unit I by Dr Naim R Kidwai, Professor & Dean, JIT Jahangirabad
G
R
Adjustable Shunt Resistance
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 producedby very small current and its coil can bear only small currents.
• To measure large currents, a low value shunt resistance is connectedto galvanometer, which bypass most of the current. Scale iscalibrated to read current range.
2/3/2017 10NEC 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
DC AmmeterEx. An Ammeter has an PMMC with a coil resistance of RC=199 , Fullscale deflection current of 0.2 mA, shunt resistance RS=1 . Find currentof 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 11NEC 403 Unit I by Dr Naim R Kidwai, Professor & Dean, JIT Jahangirabad
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 inseries to galvanometer, resulting in equivalent coil resistance asRSw+RC. Swamping resistance is made of Manganin or Constantanwhich has extremely low temperature coefficient.
• If Swamping resistance is nine times coil resistance, then 1% changein RC will result in 0.1% change in equivalent coil resistance.
2/3/2017 12NEC 403 Unit I by Dr Naim R Kidwai, Professor & Dean, JIT Jahangirabad
Galvanometer
GRSw
I
RC
RS
R’C
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 withgalvanometer. 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.
2/3/2017 13NEC 403 Unit I by Dr Naim R Kidwai, Professor & Dean, JIT Jahangirabad
Voltmeter
Galvanometer
Gmultiplier RS
IVC
RC
VS
V
Series ohm meterOhm meter is part of Volt Ohm Mili-Ammeter (VOM) or multifunction meter (Multi-meter) and don’t exist as separate instrument
Series Ohm meter consists of a voltage source connected in series toa pair of terminals (to connect unknown resistance Rx), a standardresistance R and a PMMC (coil resistance RC). The current I
2/3/2017 14NEC 403 Unit I by Dr Naim R Kidwai, Professor & Dean, JIT Jahangirabad
Galvanometer
GR
I
RCRx
E FSD)(0.5 ,
2 ,for
)deflection (Zero ,0 ,for
(FSD) , ,0for
,
max
max
IIRRR
IR
RR
EIR
RRR
EI
Cx
x
C
x
Cx
Series ohm meter
2/3/2017 15NEC 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
Thanks
2/3/2017NEC 403 Unit I by Dr Naim R Kidwai, Professor & Dean, JIT Jahangirabad
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