PMMC instruments, Galvanometer, DC Voltmeter, DC Ammeter

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.


Construction of PMMC instrument

I

IN S

0

Permanent Magnet

Copper Coil

Aluminium FormerSpring

Pointer

Calibrated Scale

D=2r

L

PMMC : Deflecting Force


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


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


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


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


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).


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.


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.


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


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.


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.


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


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


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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PMMC instruments, Galvanometer, DC Voltmeter, DC Ammeter

Engineering