This presentation includes the operating principle of an ammeter and a voltmeter.

1. Measurement of Current and
Voltage : PMMC Instruments

2. Introduction
The operating principle of ammeter and voltmeter is same as for both
case a deflecting torque is produced by an electric current. In case of
ammeter this torque is produced by a current to be measured or by a
definite fraction of it. In a voltmeter this torque is produced by a
current which is proportional to the voltage to be measured. So, all the
analog ammeters and voltmeters are essentially current measuring
device.

3. Types of Instruments
Types of instruments used as ammeters and voltmeters are:
i. Permanent magnet moving coil (PMMC)
ii. Moving iron
iii. Electro dynamometer
iv. Hot wire
v. Thermocouple
vi. Induction
vii. Electrostatic
viii. Rectifier

4. PMMC Instruments
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.
Fig. Flemings left hand rule

5. Construction:
• Moving coil
• Magnet system
• Control
• Damping
• Pointer and scale
Fig. Construction of PMMC instrument
Construction of PMMC Instrument

6. Construction of PMMC Instrument [Cntd.]
Moving Coil:
A coil of thin wire is mounted on an aluminum frame (spindle)
positioned between the poles of a U shaped permanent. The coil is free
to rotate. The current to be measured (in case of ammeter) is fed to the
coil.
Magnet System:
The coil which carries a current, which is to be measured, moves in a
strong magnetic field produced by a permanent magnet. The magnet is
made up of magnetic alloys like alnico.

7. Construction of PMMC Instrument [Cntd.]
Control:
It is that part of the instrument which brings into play a force called
controlling force. This force opposes the deflection force and increases with
the increase in the deflection of the moving system, to limit its movement.
The pointer is brought to rest at a position where the two opposing forces
i.e. deflection and controlling forces are equal.
Types of control system:
1. Spring control
2. Gravity control
In case of spring control system the current is fed to the coil through spiral
springs which are two in numbers. Control torque is provided by the spring
arrangement.

8. Construction of PMMC Instrument [Cntd.]
Damping:
It is that part of the instrument which provides damping force to damp
the oscillations of the pointer before come to a rest. Damping torque is
produced by movement of the aluminium former moving in the
magnetic field of the permanent magnet.
Because of the inertia, the pointer of the instrument oscillate about its
final deflected position for some time before coming to rest. This causes
the waste of time in taking reading, thus damping force act as a brake to
prevent the oscillations of the moving system and brings the pointer to
it’s final deflected position quickly.

9. Construction of PMMC Instrument [Cntd.]
There are three types of damping instruments:
1. Critical damp: Pointer rises quickly to its final position without oscillation.
2. Under damp: oscillations of the system will not be completely prevented.
3. Over damp: In this the response of the system is slow and lethargic.
Fig. Oscillation of the pointer

10. Construction of PMMC Instrument [Cntd.]
Methods of damping:
1. Air friction or pneumatic damping
2. Eddy current or electromagnetic damping
3. Fluid friction damping
Pointer and Scale:
A pointer attached to the coil over non-magnetic (aluminium) coil
former deflects over a calibrated scale indicating the current in coil.

11. Forces for Electro-
mechanical Movement
Three forces operates in
electro-mechanical
movement:
Deflecting force
Controlling force
Damping force
Fig. Assembled arrangement

12. Forces for Electro-mechanical Movement [Cntd.]
Deflecting Force:
Magnetic field due to current in coil,
interacts with field of magnet resulting in
deflection (rotation) of coil.
Controlling Force:
Spring attached to the pointer is used to
balance the deflecting force. The Pointer
stops where deflecting force equals to
controlling force.
Fig. Controlling force of spring balancing
the deflecting force.

13. Forces for Electro-mechanical Movement
Damping Force:
Pointer and coil tend to oscillate before settling.
Damping force is provided by eddy current in
aluminium coil former.
• When aluminium coil former moves in the
magnetic field of permanent magnet, line of
force are cut and eddy current are set up in it.
• The force that exists between these current
and magnetic field is always in the direction
opposing the motion and therefore, provide
necessary damping.
• The magnitude of the induce current and
therefore of the damping force which is
dependent on it, is directly proportional to the
velocity of moving system.
Fig. Pointer oscillation

14. Torque Equation:
Let,
l,d = length of respectively vertical and horizontal (width) side of coil, m,
N = number of turns in the coil,
B = flux density, Wb/m2,
I = current through the moving coil, A,
K = spring constant, Nm/rad,
θ = final steady state deflection, rad.
Deflecting torque, 𝑇𝑑 = 𝑁𝐵𝑙𝑑𝐼 = 𝐺𝐼,
where, 𝐺 = 𝑁𝐵𝑙𝑑 = a constant.
Controlling torque, 𝑇𝑐 = 𝐾𝜃,
where, K = spring constant.
For final steady deflection, 𝑇𝑐 = 𝑇𝑑 or 𝐺𝐼 = 𝐾𝜃.
Current, 𝐼 =
𝐾
𝐺
𝜃.
So, the deflection is directly proportional to the current passing through the meter.

15. Application
Fig. Application of PMMC instruments.

16. Galvanometer
A Galvanometer is essentially a PMMC instrument designed to be
sensitive to a very low level of currents. It has center zero scale for both
direction of currents. 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.
Fig. Galvanometer

17. 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.
Fig. Ammeter

18. 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 resistance (𝑅𝑠𝑤) is connected in series to
galvanometer, resulting in equivalent coil resistance as 𝑅𝐶
′
= 𝑅𝑠𝑤 + 𝑅𝑐.
The resistance, 𝑅𝑠𝑤 is called
swamping resistance. 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. Fig. Swamping resistance

19. 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. To avoid loading
effect, voltmeter resistance is very
high.
Fig. Voltmeter

20. Ammeter Shunt
Here, 𝑅𝑚 = internal resistance of the movement
𝑅𝑠ℎ = resistance of the shunt
𝐼𝑚 = 𝐼𝑓𝑠 = full scale deflection current
𝐼𝑠ℎ = shunt current
𝐼 = current to be measured
Here, Voltage drops across shunt = Voltage drop across movement
Multiplying power, 𝑚 =
𝐼
𝐼𝑚
= 1 +
𝑅𝑚
𝑅𝑠ℎ
Resistance of shunt, 𝑅𝑠ℎ =
𝑅𝑚
𝑚−1
𝐼𝑠ℎ𝑅𝑠ℎ = 𝐼𝑚𝑅𝑚
⇒ (𝐼 − 𝐼𝑚)𝑅𝑠ℎ = 𝐼𝑚𝑅𝑚
⇒
𝐼
𝐼𝑚
= 1 +
𝑅𝑚
𝑅𝑠ℎ
𝑹𝒎
𝑹𝒔𝒉
𝑰𝒎
𝑰𝒔𝒉
𝑰
Fig. Ammeter shunt

21. Voltmeter Multipliers
Here, 𝑅𝑚 = internal resistance of the movement
𝑅𝑠 = resistance of the multiplier
𝐼𝑚 = 𝐼𝑓𝑠 = full scale deflection current
𝑣 = voltage across the movement
𝑉 = full range voltage
From the circuit,
Multiplying factor, 𝑚 =
𝑉
𝑣
=
𝐼𝑚(𝑅𝑚+𝑅𝑠)
𝐼𝑚𝑅𝑚
= 1 +
𝑅𝑠
𝑅𝑚
Resistance of multiplier, 𝑅𝑠 = 𝑚 − 1 𝑅𝑚
𝑣 = 𝐼𝑚𝑅𝑚
𝑉 = 𝐼𝑚(𝑅𝑚+𝑅𝑠)
Fig. Voltmeter multipliers
𝑹𝒎
𝑹𝒔
𝑰𝒎
𝒗
𝑽

22. 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.

23. Drawbacks:
• 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.

24. Thanks
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