PMMC MI ELETROSTATIC VOLTMETERS VOLTMETERS AMMETERS

1. Measuring Instruments

2. Definition of Moving Iron Instrument
• The instrument which uses the soft iron core for measuring the
current or voltage is known as the moving iron instrument.
• It works on the principle that the iron attracts towards the magnet.
The magnetic field induces because of the electromagnet and the iron
piece is placed between this field.
• The force of attraction acting on the soft iron core and the magnitude
of the force depends on the strength of the magnetic field.
• There are two types of moving iron (MI) instruments. They are,
attraction type and repulsion type moving iron instruments.

3. Attraction Type Moving Iron (MI) Instrument -
Construction, Working & Torque Equation
• The working principle of attraction type moving iron instrument is
based on magnetic attraction, which attracts an iron piece when
placed near a magnet field. Here, the magnet field will be produced
by an electromagnet.

4. Construction of Attraction Type Moving Iron
Instrument:
• It consists of a fixed coil that is flat with a narrow opening in it. A
moving iron that is made of soft iron is mounted on a spindle.
• The coils are wound with a number of turns that depend upon the
range of the instrument. The pointer is mounted on a spindle which
consists of a graduated scale for showing the deflection. The
construction of attraction type moving iron is shown below.

6. • The controlling torque is provided by the springs or if the instrument
is vertically operated gravity control can also be employed.
• This instrument uses air friction damping to damp out oscillations
which consist of a movable piston made of aluminum placed in an air
chamber.
• Since the operating magnetic field produced by the coil winding is not
much strong, the eddy current damping which uses permanent
magnets can distort the main field.
• Thus eddy current damping cannot be used and fluid friction damping
is not much preferred. The moving iron is made of sheet metal for
obtaining a uniform scale.

7. Working of Attraction Type Moving Iron
Instrument:
• Whenever coil winding is connected across the supply to be
measured, it setups a magnetic field.
• The intensity of the magnetic field is higher inside the coil compared
to the intensity of the outside, and hence low reluctance exists inside
the coil.
• As the moving iron tries to occupy the low reluctance position, it is
moved and gets attracted to the fixed coil.
• As the iron piece moves, the pointer also moves to show the
deflection. The instrument attains the equilibrium position when
controlling torque balances the deflecting torque.

9. Torque Equation of Moving Iron Instruments:
• Let,
• Td = Deflection torque in N-m
• θ = Deflection in radians
• L = Inductance in Henry
• I = Initial current
• dI = Change in initial current
• dL = Change in inductance
• dθ = Change in deflection

10. Advantages of Attraction Type MI Instruments:
• The instruments can be used for measuring both dc and ac
quantities.
• Simple in construction.
• Since the winding coil is kept stationary, these instruments are
robust and reliable.
• As attraction type instruments have lower inductance, the
measurement can be done over a wide range of frequencies.
• A shunt can be connected in parallel with the basic instrument in
order to measure heavy currents.

11. Disadvantages of Attraction Type MI Instruments:
• These are not suitable for economical production in manufacturing.
• The power consumption is higher for a low voltage range.
• Accuracy in the readings cannot be obtained due to the non-uniform
scale.

12. Applications of Attraction Type MI Instruments:
• Heavy current moving iron instruments.
• Moving iron voltmeters.
• Moving iron power factor meters.
• Moving iron synchro scope.

13. Repulsion Type Moving Iron (MI) Instrument -
Construction, Working & Advantages
• Similar to attraction force there will be repulsion force when same
magnetic poles are placed near each other. Based upon this repulsion
force between two like poles the repulsion type moving iron
instruments were developed.
• Repulsion type moving iron instruments are used both for ac and dc
measurements. In these instruments, when current flows through the
coil, the two vanes i.e., fixed vane and movable vane are magnetized
and same polarities are induced in it which results in a force of
repulsion between them.

14. Construction of Repulsion Type Moving Iron
Instrument:
• Basically, a repulsion type instrument consists of a fixed field coil and
two vanes present inside the coil, out of which one vane is fixed and
the other vane is movable.
• The fixed vane is attached to the coil whereas the movable vane is
mounted on the spindle of the instrument. The spindle carries the
pointer which moves on a graduated scale. Depending on the design
of construction, there are two types of repulsion type MI instruments.
They are,

15. • Radial vane type, and
• Co-axial or concentric vane type

16. Radial Vane Repulsion Type Instrument:

17. • It consists of two iron strips (vanes) are placed radially, in which one is
fixed and the other is movable.
• In this type of instrument, the deflection torque is directly
proportional to the actual current in the coil, thus making the scale
uniform and readings can be obtained directly.
• These are the most sensitive type of instruments.

18. Co-axial Vane Repulsion Type Instrument:

19. • In this, the two vanes are co-axially placed inside the coil. In which
one vane is fixed to the coil frame that remains stationary, while the
other is movable which rotates at the central axis inside the
stationary vane.
• But the deflecting torque on the pointer is proportional to the square
of the actual current in the coil. Hence the scale cannot be uniform
due to concentric vanes. Compared to radial type these instruments
are less sensitive.

20. Working of Repulsion Type Moving Iron Instrument:
• Initially, the current does not pass through the coil. Hence, the two
vanes will touch each other and the pointer does not deflect (i.e., it
will be at zero position).
• Whenever current flows through the coil, a magnetic field is set up in
it and two vanes are magnetized with the same polarities i.e., north
poles are produced at one end in both the vanes and south poles are
produced on their other ends.
• Due to this, a repulsive force exists between two vanes and the
movable vane tries to move away from the fixed vane as shown
below.

22. • Hence, the movable vane moves because of the repulsive force, and
the pointer which is mounted on a spindle show deflection. The
pointer stops deflecting when the controlling torque is equal to the
deflecting torque.
• The amount of repulsion force depends upon the strength of the
magnetization field produced by the coil. The magnetic field produced
will depend upon the current supplied.
• In these instruments, controlling torque is provided by the spiral
springs and damping torque is provided by air friction.

23. Advantages of Repulsion Type Moving Iron
Instruments:
• The instruments can measure both ac and dc currents and voltages.
• In concentric or coaxial type, the deflection up to 250° can be
produced.
• As the coil in the repulsion time instrument occupies more volume,
it can allow overloaded currents for a longer duration of time, and
hence it provides better cooling which eliminates the problem of
overheating.
• These are suitable for economical production in manufacturing.

24. • These are the most widely used instruments.
• The scale of these instruments is uniform (for radial vane type).
• The measuring range of the instrument can be extended.
• Due to stationary coil, the torque to weight ratio will be high
thereby decreasing the frictional loss.

25. Disadvantages of Repulsion Type Moving Iron
Instruments:
• Linearization of the scale is affected in coaxial vane type because of
the shape of concentric vanes.
• Errors are caused because of hysteresis loss and also due to stray
magnetic fields.
• Draws more power.

26. Errors in Moving Coil (PMMC) & Moving Iron
Instruments
• Ageing of permanent magnets,
• Ageing of springs, and
• Temperature variations.

27. • Errors due to Ageing of Permanent Magnets :
• Due to the use of permanent magnets, the ageing of magnets causes
its weakening, thereby resulting in a reduced magnetic field.
• The decrease in the magnetic field reduces the deflection torque that
has to be produced.
• Therefore, the pointer deflection decreases and the reading obtained
will be lesser than the actual value.

28. • Errors due to Ageing of Springs :
• Similar to the ageing of magnets, the ageing of springs causes an
error in the readings obtained. Due to aging, the spring losses its
ability to tolerate considerable bending or twisting.
• Due to which the control torque produced on the pointer decreases
and the pointer deflects more than actual.
• Here we can see that the effects due to the ageing of magnets and
springs are opposite to each other.
• Because the error introduced due to magnets ageing causes the
reading obtained to be lesser than the actual value, whereas ageing
of springs causes the reading obtained to be more than the actual
value.

29. • Errors due to Temperature Variations :
• The variations in surrounding temperature can also introduce errors
in moving coil instruments. There are two effects due to temperature
variation, it affects the stiffness of the spring and flux density in the
air gap. But these effects are in such a way that they will cancel each
other resulting in not much error in the readings obtained.
• Due to variations in temperature, the resistance of the conductor
used for moving coil varies, thereby changing the amount of current
flowing through the coil that effects the deflection of pointer.

30. • Compensation of the Errors in PMMC Instruments :
• The permanent magnets are treated by heat and vibrations resulting
in loss of some magnetic properties but the remaining magnetic
properties are strongly held. So, the ageing effect will not appear in
the future.
• The effects due to ageing of springs can be reduced by careful use of
materials and extending the life span during manufacturing.
• By using a series resistance made of materials having a negligible
coefficient of temperature (like manganin) with the moving coil
voltmeter reduces the effects due to temperature changes. The
resistance of the series resistor or swamping resistor should be
relatively higher than that of the moving coil.

31. Errors in Moving Iron Instruments :
• Hysteresis error
• Temperature error
• Errors due to stray magnetic fields
• Errors due to change in frequency

32. • Hysteresis Error :
• The main source of this error is the tendency of the iron parts in the
instrument to store magnetism for short periods. Due to this error, for
decaying values of currents, the flux appears to be more than its
corresponding value. As the deflecting torque is directly proportional
to the flux, the meter reads high.
• Methods adopted to reduce this error are,
• Reducing the size of the iron part in the instrument.
• Reducing the value of operating flux density.
• Replacing the active iron part with nickel-iron alloy as it has a narrow
hysteresis loop. With this method, the hysteresis error can be
reduced to a large extent.

33. • Temperature Error :
• The main source of this error is the temperature coefficient
of the springs (for spring controlled instruments only),
moving coil, and series resistance. Due to an increase in
temperature, the overall resistance of the meter increases
and so the current through the meter gets reduced. Hence.
the meter reads low.
• In order to reduce this error, the series resistance being
employed is made with the material having a negligible
temperature coefficient (like manganin, constantan, etc). The
value of series resistance should be comparatively much
higher than that of moving coil and springs.

34. • Errors due to Stray Magnetic Fields :
• The main source of this error is the external magnetic
fields which are stronger than the operating field. If
the stray magnetic field aids the operating field, then
the meter reads high and when it opposes, the meter
reads low.
• To reduce this error, magnetic shielding is provided by
an iron case covering the instruments working parts.

35. • Errors due to Change in Frequency :
• The main sources of this error are,Change in Reactance of
the Meter Coil with Frequency - It is due to the dependence
of the inductive reactance on the frequency of the supply
i.e., the inductive reactance of the coil increases with an
increase in frequency ( XL = 2πfL). Hence, overall impedance
also increases and so the current through the meter reduces.
Hence, the deflection will be less.
In order to neutralize this error in the case of MI voltmeters,
a properly designed capacitor is connected across the series
resistor (i.e., multiplier).

36. • Variations in the Eddy Currents with Frequency - The main
source of this error is inducing eddy currents in the iron parts
of instruments. The flux produced due to the flow of eddy
currents opposes the main field (operating flux), according to
Lenz's law. Hence, the meter reads low.
The only remedy to minimize this error is to reduce the iron
parts in the vicinity of operating flux.

37. Electrostatic Voltmeters
• An electrostatic instrument is basically a voltmeter that works on the
principle of static electric field. They are used for voltage
measurement (especially for high voltages) but can also be used for
measuring current and power with the additional arrangement.
• The electrostatic voltmeters are of two types,
• Quadrant type electrostatic voltmeter (used up to 20kV)
• Attracted disc type electrostatic voltmeter (used up to 500kV)

38. Attracted Disc Type Electrostatic Voltmeter -
Kelvin Absolute Electrometer
• An attracted disc type electrostatic instrument is basically known as a
portable electrostatic instrument. This device contains two
semicircular plates one being stationary and the other rotating. Both
the plates are electrically insulated from each other.
• The unknown voltage to be measured is applied across the plates,
which results in the development of an electrostatic field between
the plates. The electrostatic field developed is attractive in nature.
The below shows the portable electrostatic instrument.

40. •Due to the electrostatic force of attraction, the
moving plate gets attracted towards the
stationary plate and gets deflected.
•The control spring fixed to the rotating plate
provides the required controlling torque and the
air friction provides the necessary damping.

41. Quadrant Type Electrostatic Voltmeter :
• The quadrant electrometer consists
of four metallic quadrants and a
double sector shaped moving vane
or needle as shown in the below
figure.
• Quadrants 1 and 2 lie above the
needle and quadrants 3 and 4
below the needle. The needle is
suspended in the air gap between
the two sets of quadrants. Silver
quartz or phosphor bronze thread
is used for the suspension.

42. • The needle carries a mirror that reflects the light focused on it on the
scale. Sometimes, instead of this suspension, a spindle is placed on
which the needle is mounted. Also, a spring is attached to the spindle,
which provides the control torque.
•
A resistance of high value should always be connected in series with
these types of instruments in order to prevent a short circuit when
the supply is given. When a potential V is applied across the fixed
quadrants and moving vane, a static electric field is set up between
them and a force of attraction exerts on the needle. Hence, the
needle starts rotating due to the deflecting torque Td and shows the
deflection.

43. • There are two types of connections in a quadrant electrometer. They
are,
• Heterostatic connection, and
• Idiostatic connection.

44. • Heterostatic Connection :
• In this type of connection voltage to be measured is
applied across the fixed quadrants. A high tension (HT)
battery is used to charge the moving needle more
positively than the fixed quadrants (i.e., voltage to be
measured). The heterostatic connection is shown
below.

46. • Here the fixed quadrants are charged in such a way that, the
diagonal pair will be charged oppositely. The needle will be
charged with positive polarity with help of an HT battery.
Here the deflecting torque is produced due to the attraction
force between the left quadrant and right moving sector and
the repulsion force between the right quadrant and left
moving sector.

47. • Idiostatic Connection :
• In this type of connection, there
is no additional external voltage.
The moving needle is connected
to any pair of quadrants directly
as shown below.

48. • Here, the moving needle is charged with negative polarity. Due to the negative
polarity of the needle, there exists a force of attraction on the needle by the two
positively charged quadrants i.e., on the right side, and these two forces will
cancel each other. Similarly, there will be a repulsive force on the needle by the
two quadrants on the left side, and these two forces will cancel each other.
•
But, due to attraction force on the part of a needle lying on the left side, and a
repulsive force on the part of a needle lying on the right side causes the
production of deflecting torque and moves the needle. This movement causes to
move the pointer attached to it.
•
If the instrument is spring controlled and used for measuring low voltages,
heterostatic connection is most suitable because, for idiostatic, the deflection
obtained is proportional to the square of applied voltage (i.e., voltage to be
measured) and for low voltages, the deflection will be small. Whereas in the case
of heterostatic, deflection is proportional to the product of voltage applied and dc
HT voltage, thus the voltage to the needle is very high and the deflection
obtained will be considerably large even for low voltages.