Measuring instruments electrical maintenace

Vijay Balu Raskar – Electrical Maintenance – Polyphase Circuit

Junior Science College ( Std – XI ) - Syllabus

Theory – “Measuring Instruments”
Why ?
 A measuring instrument is a device for measuring a Physical quantity.
 Electrical energy is being used in the manufacturer of many commodities. In order to
ensure quality and efficiency, it is very important that we should be able to measure
accurately the electrical quantities involved.
 The instruments used to measure electrical quantities like current, voltage, power, energy,
resistance etc are called as “Electrical Measuring Instruments”.
 These days’ electrical methods of measurement are being widely used for the
measurement of non electrical quantities like moisture content of paper during
manufacture, variation of strain in a structural member etc.
Measuring Instruments:“The device used for comparing the unknown quantity with the unit of measurement or standard
quantity is called a Measuring Instrument.”
OR
“An instrument may be defined as a machine or system which is designed to maintain functional
relationship between prescribed properties of physical variables & could include means of
communication to human observer.”
Types of Electrical Instruments:According to their functions, they are classified as (i) Indicating Instruments, (ii) Integrating
Instruments and (iii) Recording Instruments.
o Indicating Instruments:-

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II’s which directly indicates the value of the electrical quantity at the time when it is being
measured.
Example:- ammeters, Voltmeters, wattmeter’s etc.
Moving Pointer used

o Integrating Instruments:-

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
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II’s which measures the total quantity of electricity or electrical energy in a given time.
Example:- Ampere – Hour meter & watt-Hour Meter, Energy Meter
Sets of dials & pointers are used

Suggestions or difficulties are most welcome at vijayraskar2003@yahoo.co.in



o Recording Instruments:-




RI’s which gives continuous record of the variations of the electrical quantity to be
measured.
Pen attached to its pointer. The path traced out by the pen.

Principles of operation of Electrical Measuring Instruments: An electrical instrument essentially consists of a movable element & scale to indicate or
register the electrical quantity being measured.
 The movable element is supported on jeweled bearing & carries a pointer or sets of dials.
 The moment of the movable element is caused by utilizing one or more of the following
effects of current or voltage.
 Magnetic Effect  Moving – Iron Instruments – Suitable for d.c. & a.c. both
 Electro dynamic effect PMMC (Permanent – Magnet Moving Coil ) – Suitable for d.c. only
 Dynamometer type - Suitable for d.c. & a.c. both
 Electromagnetic Induction Induction type instruments - Suitable for a.c. only
 Thermal Effect Hot – wire instruments - Suitable for d.c. & a.c. both
 Chemical Effect Electrolytic Instruments - Suitable for d.c. only
 Electrostatic effect Electrostatic Instruments - Suitable for d.c. & a.c. both
Essentials of Indicating Instruments:o An indicating instrument essentially consists of moving system pivoted in jewel bearings.
o Pointer is attached to the moving system which indicates on a graduated scale.
o In order to ensure proper operation of indicating instruments, Three Torques are required.
i.
Deflecting or Operating Torque
ii.
Controlling or restoring Torque
iii.
Damping Torque
TORQUE: Torque, moment or moment of force, is the tendency of a force to rotate an object
about an axis, fulcrum, or pivot. Just as a force is a push or a pull, a torque can be
thought of as a twist to an object.
 It’s a product of Perpendicular Distance and Force.




 Torque is a measure of how much force acting on an object causes that object to
rotate.
i. Deflecting or Operating Torque ( Td ) :- The deflecting torque is produced by making one of the magnetic, heating, chemical,
electrostatic and electromagnetic induction effect of current or voltage and cause the
moving system of the instrument to move from its zero position.
- The method of producing this torque depends upon the type of instrument.
- Arrangement required producing Deflecting Torue.
- This is the basic system of the meter which requires the following parts –
1. A conductor in the form of a coil or Spring.
2. Spindle
3. Pointer
4. Some iron parts (Movable Iron)
- When the current passes through the coil due to the effect of the current iron parts &
spindle are subjected to motion due to a force.
- Due to the movement of the spindle the pointer moves the dial.
ii. Controlling or restoring Torque ( TC ) :- The magnitude of the moving system would be some what indefinite under the influence of
deflecting torque, unless the controlling torque existed to oppose the deflecting torque.
- It increases with increase in deflection of moving system.
- Under the influence of controlling torque the pointer will return to its zero position on
removing the source producing the deflecting torque.
- Without controlling torque the pointer will swing at its maximum position & will not return to
zero after removing the source.
- TC = Td
- Above condition says that, which require opposite deflecting torque.
- Controlling torque is produced either by spring or gravity control.
Controlling Torque function –
i. It increases with the deflection of the moving system so that the final position of the pointer on
the scale will be according to the magnitude of current or voltage or power to be measured.
ii. It brings the pointer back to zero position when the deflecting torque is removed. If it were not
provided, the pointer once deflected would not return to zero position on removing the deflecting
torque.
There are two types of Controlling Torque :- a) Spring & b) Gravity Control




(a) Spring Control –

  I
 When the pointer is deflected one spring unwinds itself while the other is twisted. This twist
in the spring produces restoring (controlling) torque, which is proportional to the angle of
deflection of the moving systems.
Deflecting Torque

Controlling Torque
TC  
TC  K S
For Spring Control,

(b) Gravity Control –




•
•
•

In gravity controlled instruments, a small adjustable weight is attached to the spindle of the
moving system such that the deflecting torque produced by the instrument has to act
against the action of gravity.
Thus a controlling torque is obtained. This weight is called the control weight.
Another adjustable weight is also attached is the moving system for zero adjustment and
balancing purpose. This weight is called Balance weight.

iii. Damping Torque ( ) :- We have already seen that the moving system of the instrument will tend to move under the
action of the deflecting torque.
- But on account of the control torque, it will try to occupy a position of rest when the two
torques are equal and opposite.
- However, due to inertia of the moving system, the pointer will not come to rest immediately
but oscillate about its final deflected position as shown in figure and takes appreciable time
to come to steady state.
- To overcome this difficulty a damping torque is to be developed by using a damping device
attached to the moving system.
- The damping torque is proportional to the speed of rotation of the moving system.

Depending upon the degree of damping introduced in the moving system, the
instrument may have any one of the following conditions as depicted in above
graph.
There are some different Damping System.
1. Under damped condition:
- The response is oscillatory
-

2. Over damped condition:
- The response is sluggish and it rises very slowly from its zero position to final position.




3. Critically damped condition:
- When the response settles quickly without any oscillation, the system is said to be critically
damped.
The damping torque is produced by the following methods:
a. Air Friction Damping
b. Fluid Friction Damping
c. Eddy Current Damping
d. Electromagnetic Damping
a. Air Friction Damping







Air chamber contains air in it.
Air pulls as well as push by Piston.
Pistons will moves which create the force and can bring system in to rest position.
1 or 2 light aluminum vanes are attached to the same spindle that carries the pointer.
As the pointer moves, the vanes swing in the box, compressing the air in front of them.

b. Fluid Friction Damping

 The pressure of compressed air in the vanes provides the necessary damping torque.
 Disc or vanes attached to the spindle of the moving system are kept immersed in a pot
containing oil of high viscosity.
 As the pointer moves, the friction between the oil & vanes opposes motion of the pointer &
thus necessary damping torque is provided.
c. Eddy Current Damping
 Aluminum or copper disc attached to the moving system is allowed to pass between the
poles of a permanent magnet.
 As the pointer moves, the disc cuts across the magnetic field & eddy currents are produced
in the disc.
 Eddy current reacts with the field of the magnet to produce a force for motion (Lenz’ law).




d. Electromagnetic Damping
 Magnetic Damping is a form of damping that occurs when a magnetic field moves through
a conductor or vice versa.
 When a magnetic field (Magnet) moves through a conductor an eddy current is induced in
the conductor due to the magnetic field's movement.
 The flow of electrons in the conductor creates an opposing magnetic field to the magnet
which results in damping of the magnet and causes heating inside of the conductor similar
to heat buildup inside of power cords.
 The loss of energy used to heat up the conductor is equal to the loss of kinetic energy by
the magnet.
 Eddy currents induced in conductors are much stronger as temperatures approach
cryogenic temperatures. This allows for critical damping for cryogenic applications and
testing in the aerospace industry.
USES

Vehicle braking

Roller coaster braking

Elevators

Near critical damping at cryogenic temperatures
Types of Ammeters & Voltmeters: The basic principle of the ammeter & the voltmeter is the same.
 Both are current operated devices.
 Principle – Deflecting torque is produced when current flows through their operating coils.
The following types of instrument are used for making voltmeters & ammeters:a) PMMC – for d.c. measurement work only
b) Dynamometer type – for both a.c. as well as d.c measurement.
c) Moving iron Type – for both a.c. as well as d.c measurement.
d) Hot wire Type - for both a.c. as well as d.c measurement.
e) Electrostatic Type - for both a.c. voltmeter as well as d.c voltmeter measurement only.
f) Induction Type – for a.c. use only.
a) PMMC ( Permanent Magnet Moving Coil )
 PMMC is used for d.c. measurement work only.
 PMMC means Permanent Magnet Moving Coil.
 Deflection is directly proportion to operating current and hence having uniform scale.




Principle of PMMC:When a current carrying coil is placed in a magnetic field, torque acts on the coil.
Construction of PMMC: Permanent Magnet – U shaped magnet produce the magnetic field in the space.
 There is a soft iron cylinder which is located in the magnetic field.
 Moving Coil – it consist of light rectangular coil of many turns of fine wire supported on the
“Aluminium former” which is an iron core.
 Springs – phosphor bronze spring fixed on spindle on both the side.
 Springs provides the controlling Torque.
 Supply leads are connected to coil.
 The coil is delicately pivoted upon jewel bearings & is mounted between the poles of a
permanent horse-shoe magnet.
 Damping torque is provided by eddy currents induced in the aluminum former as the coil
moves from one position to another.
 Spindle- it is a thin round rod which supports former & springs.It is pivoted in the bearings.
 Pointer – it is attached to the spindle.
 Dial – it is graduated over which the pointer moves.
Working of PMMC: When the instrument is connected in the circuit to measure current or voltage, the operating
current flows through the coil.
 Since the coil is carrying current & is placed in the magnetic field of the permanent magnet,
a mechanical force acts on it.
 As a result, the pointer attached to the moving system moves in a clockwise direction over
the graduated scale.
 If current in the coil is reversed then deflecting torque will also be reversed. Since, direction
of field is the same.
 Deflection is prevented by a spring “STOP” when supply is reversed.
 The magnetic field in the air gap is radial due to the presence of soft iron core.
 For voltmeter – High resistance is connected in series with moving coil.




 For ammeter – Low resistance is connected in parallel with moving coil.
 When operating current flows through the coil, a constant deflecting torque Td acts on the
coil given by,
Td  B.I .N . A Newton – Meter
Where,
B = Flux Density in Wb / sq. m
I = Current through the coil
N = Number of turns in the coil
A = Area of the coil
As , B, N & A are fixed.
Hence, Td I
Hence, 

 I

…………. because of spring controlled & pointer will comes to rest.

PMMC –
 The meter is suitable for d.c. only.
 The scale on the dial is uniform.
 Damping is effective.
 Power consumption.
 Range can be extended.
 Life of PMMC is very short.
 PMMC is costly.
 Construction of PMMC is very delicate.
 Full scale deflection current = I g  1mA and Resistance of the meter is G = 20 ohm
 G = 20ohm means , instrument can read currents upto 1mA & Voltage upto 20mV (1 X 20).
Dynamometer type –
 Dynamometer is used for both a.c. as well as d.c measurement.
 This can be used as ammeter or voltmeter but generally used as wattmeter.
Principle of Dynamomter: Mechanical force exists between the current carrying conductors.
Construction of Dynamometer: It consists of fixed coil & moving coil. Spring controlled is preferred.
 The fixed coil is split into two equal parts ( F & F ) which are placed closed together and
parallel to each other.
 The moving coil (M) is pivoted in between the two fixed coils and carries a pointer.
 The current is led into & out of the moving coil by means of two spiral hair springs which
also provide the controlling torque.
 Air chamber is provided with piston at the top or bottom for Air friction Damping.
 High resistance (R) is used.




Working of Dynamometer: When current flows through the fixed coil then mechanical force exists in the coils.
 Due to mechanical force, Moving coil (M) rotates with pointer over the scale.
 The pointer comes to rest at a position where deflecting torque is equal to the controlling
torque.
 Reversal of polarity in the dynamometer, the deflection of the moving system is always in
the same direction due to fixed & moving coil.
 Due to this reason, this can be used for both a.c. as well as d.c. measurement.
 The scale is uniform.
 Air Friction and piston arrangement for damping.
 The force of attraction or repulsion between the fixed & moving coils is directly proportional
to the product of ampere-turns of fixed coil & moving coil.
Deflecting torque = Td  N f . I f .N m .I m
N f & N m are two constant.

Td  I f . I m
As spring Control, Tc  
Tc  Td
Hence,
  I f .I m
Thus, deflection is directly proportional to the product of currents in the fixed & moving coil.




For Ammeter – Fixed and Moving coil are in series , so that same current flows from both
coils. Hence,   I 2
For Voltmeter - Fixed and Moving coil are in series with high resistance (called multipier) ,
so that same current flows from both coils and proportional to voltage. Hence,   V 2
Moving iron Type –
 Moving Iron is used for both a.c. as well as d.c measurement.
 There are two types – (i) Attraction & (ii) Repulsion Type

(i) Attraction Type Moving Iron Meter:Principle:- Magnetic effect when current flows through coils.
Construction : It consists of fixed cylindrical coil or solenoid.
 An oval shaped soft iron is attached to the spindle in such a way that it can move in & out
of the coil.
 A pointer is attached to the spindle so that it is deflected with the motion of the soft-iron
piece.
 The controlling torque is provided by spiral spring at moving element.
 The damping torque is provided by air damping chamber.
Working: When current flows through from coil then magnetic field produced.
 Other words, we can say that, coil behaves like a magnet & therefore, it attracts the softiron piece towards it.
 The pointer will come to rest at a position where deflecting torque is equal to controlling
torque.
 If supply reversed then direction of magnetic field also reversed. Hence, deflecting torque is
remains unchanged.
 Hence, used for a.c. as well as d.c. measurement.
(ii) Repulsion type Moving Iron meter:Principle:- When current flows through the coil then magnetic field is set up by the coil.




Construction:- It consists of two soft iron pieces or vance surrounded by a fixed cylindrical hollow coil
which consists of operating current.
- One vanes is fixed and other one is free to move.
- The movable vane is of cylindrical shape and mounted on spindle which is attached to
pointer.
- Fixed vane is wedge shaped and having large radius which is attached to stationary coil.
- Springs do not provide electrical connection.
- The controlling torque is provided by one spiral springs at the top side.
- Damping is provided by air friction at piston.
Working:- When current flows through the coil, a magnetic field is set up the coil.
- This magnetizes the two vanes in the same direction.
- i.e. similar polarities are developed at the same ends of the vanes.
- Since, the adjacent edges of the vanes are of the same polarity, two vanes repel each
other.
- Fixed vane cannot move as movable vane deflects & pointer moves.
- Pointer comes to zero position when deflecting torque is equal to damping torque.
- When polarity reversed then deflecting torque is unchanged. Because reversals of the coil
reverses the magnetization of both iron vanes. So that they repal each other regardless of
which way the current flows through the coil.
- Hence, this is used for a.c. as well as d.c. measurement.
Hot wire Type –
 This is used for both a.c. as well as d.c measurement.
 It is based on the heating effect of current.
 It consist of platinum-iridium (it can withstand oxidation at high temperatures) wire.
 When current is through wire, it expands according to I2R formula.
 This produces sag in the wire and pointer is attached with this wire which in result deflects.

Electrostatic Type –
 This is used for both a.c. voltmeter as well as d.c voltmeter measurement only.
Induction Type –
 This is used for a.c. measurement only.
 Two separate coils are used to produce the rotating magnetic field in place of one coil with
phase arrangement.
Energy Meter – (a) 1phase a.c. Induction type meter (kWH) (b) Energy Meter
(a) 1 phase a.c. Induction type meter –
 This is used for only a.c. measurement only.
 Used in domestic a well as industrial use.




Principal - Induction principal is applied.
Construction: It consists of two laminated electro magnets i.e. Shunt and Series Magnet.
 Shunt Magnet – Connected across the supply and current proportional to voltage.
 Shunt magnet made highly inductive for current lags voltage by 90 degree.
 Series Magent – Connected in series with supply & carries the load current.
 Series magnet is highly non-inductive for lags & lead can be determined by load only.

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


Aluminium Disc, Braking magnets and register mechanism are used.
Aluminium Disc or rotor placed between two electromagnets.
Counting mechanism is used at top as shown in figure.

Working of 1phase Energy Meter: When current flows through the coil then aluminium dick cuts the maximum fluxes produce
by both the magnets.
 The braking torque is obtained by placing a permanent magnet near the rotating disc.
 Disc rotates in the field established by the permanent magnet.
 Eddy current is induced in the disc which produces a braking or retarding torque that is
proportional to the disc speed.
 A short circuited copper loop is provided on the central limb of the shunt magnet.
 Number of revolution is directly proportional to the Energy.
N
N
K

E kWh
Where, N = Number of Revolution and E = Energy
Meter constant = K
Hence, the number of revolution made by the disc for 1kWh of energy consumption is
called meter constant.




NOTE: The meter constant is always written on the name plates of the energy meter installed.
 If the meter constant is 1500rev/kWh then the dick will make 1500 revolutions for 1kWh.

Error in Induction meters:a. Speed error – Sometime fast or slow
b. Creeping - Sometime the disc of the meter makes slow but continuous rotation at no load.
c. Frictional Error – rotor bearing and counting mechanism error.
d. Phasor error – due to material , lag or lead concept.
e. Temperature error
f. Frequency variation – Frequency vary with reactance and so error happens.
g. Voltage variation – the shunt magnet flux increases with increasing in voltage.
(b) Energy Meter –

 This type of meter is used for d.c. measurement.
Principal:- direction of rotation as Fleming ‘s left hand rule
Construction: It contains Iron bas, driving magnet, amalgamed copper disc, spindle, braking magnet,
mercury box with disc etc
 Recording mechanism is placed on top.
 Spindle supported by jeweled bearings.
 It revolves between the pair of permanent magnets.
 One magnet is driving and other one is braking.




 The space is filled with mercury.
 Mercury is used to reduce the pressure of the bearings and acts as an conductor.
Working: When current passing thorough the center of the disc then force is produced on the right
hand side of the disc and it will rotate.
 Magnitude of the force depends on the strength of the field and current.
 The disc is rotate under braking magnet and creates e.m.f. induction.
 The revolution of the disc is proportional to the quantity of electricity.
Megger:-

 It is used to find out the insulation resistance of wires, cables etc
 It is used to finding open circuit fault, short circuit fault, earth fault , leakage fault etc.
Construction: It contains magneto generator for generating voltage in it.(Ranges  It contains an ohmmeter for resistance measuring.
 Two coils present – current coil and pressure coil.
 A megger tester is an electrical device that is used to measure the amount of resistance in
the insulation within a wire. It can also be used to find high-resistance shorts in a wire. The
megger tester applies a certain amount of voltage to the wire. If there is a short within the
wire, it is found by observing which way the electricity travels through the wire. This is
because electricity travels where it encounters the least amount of resistance.
Working: The generator is driven by hand which generates the voltage in it.
 The pressure coil of ohmmeter is connected in series with high value of fixed resister.
 The one end of the generator is connected to earth.
 Other end is connected to the current coil of a fixed resistance to the point where insulation
to earth is to be tested.
 Pressure coil and current coils are wound to move opposite direction when the current
passes.
 When resistance is very high then current in the current coil become very less. Hence,
deflection is at infinity side on indicator scale.
 When resistance is very low then current in the current coil become very high. Hence,
deflection is at zero side on indicator scale.




Precaution:1. Handle carefully while using.
2. Don’t use on live supply.
3. Two leads should have electrical continuity.
4. Circuit should be free from leakage.
Instrument Transformer: Instrument transformer is nothing but CT & PT.
 Ammeter and voltmeter is used to measure small value of supply.
 For large measurements Instrument transformer is used.
A current transformer (CT) is used for measurement of alternating electric currents. Current
transformers, together with voltage transformers (VT) (potential transformers (PT)), are known
as instrument transformers.
Current Transformer (CT) :-

•
•
•

•
•
•
•
•
•

CT is used to measure Current through any conducting material.
Current transformers (CT) are a series connected type of instrument transformer
The CT is typically described by its current ratio from primary to secondary. A 1000:5 CT
would provide an output current of 5 amperes when 1000 amperes are passing through its
primary winding. Standard secondary current ratings are 5 amperes or 1 ampere,
compatible with standard measuring instruments.
When current in a circuit is too high to apply directly to measuring instruments, a current
transformer produces a reduced current accurately proportional to the current in the circuit,
which can be conveniently connected to measuring and recording instruments.
A current transformer also isolates the measuring instruments from what may be very high
voltage in the monitored circuit.
Current transformers are commonly used in metering and protective relay in the electrical
power industry.
Current transformers are used extensively for measuring current and monitoring the
operation of the power grid.
The primary side of the CT consists of few turns or single turns and connected in series
with the High Voltage Current side.
The secondary winding with large number of turns supplies a reduced current to an
ammeter.




Potential Transformer:• Potential transformer (PT) is also called as voltage transformers (VT).
• They are a parallel connected type of instrument transformer.
• They are designed to present negligible load to the supply being measured and have an
accurate voltage ratio and phase relationship to enable accurate secondary connected metering.

•

•

The PT is typically described by its voltage ratio from primary to secondary. A 600:120
PT would provide an output voltage of 120 volts when 600 volts are impressed across
its primary winding. Standard secondary voltage ratings are 120 volts and 70 volts,
compatible with standard measuring instruments.
There are three primary types of potential transformers (PT): electromagnetic, capacitor
& optical.

Multimeter:-

 Multimeter is also called as mulit-tester or volt-ohm meter.
 Multimeter is an electronic measuring instrument that combines several measurement
functions in one unit.
 There are two types Analoge and Digital Multimeter.
 It’s a type of PMMC instrument.
 The coil winding of the movement is very small.
 A shunt resister is used for the protection and range extension.
 Energy is supplied by dry cell.




 This meter is used for measurement of Current (ac & dc), Voltage (ac & dc), Resistance,
Continuity, IC etc

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