EE09 406 ELECTRICAL MEASUREMENTS AND INSTRUMENTATION SYSTEMS

MODULE II

Prepared by: Muhammedali Shafeeque K

+91 9744144505

kshafeeq007@gmail.com

onlinemas.weebly.com

References:

1. A.K. Sawhney, A course in Electrical and Electronics Measurements and

Instrumentation, Dhanpat Rai and sons

2. Dhogal ,Basic Electrical Engineering, Volume 2

3. U.A.Bakshi, A.V.Bakshi, Electrical Instrumentation

WATT METERS AND ENERGY METERS

Ampere-hour mercury motor meter:

Principle: Sometimes it is also known as Ferranti-ampere-hour meter. The operating

principle of mercury motor meter depends on the fact that whenever a current fed to a conductor

is kept under a magnetic field, a force acts on the conductor which tends to rotate it.

Construction: Figure illustrates the main pans and connection of this instrument.

It consists of two permanent magnets having north and south poles and an aluminium or copper

disc attached to a spindle. This disc is kept under the poles of magnets in an air-light chamber

formed by placing a fiber ring in between to round brass disc B and B’. This airtight chamber

contains mercury and on its left side a conductor C passes through fiber ring. This conductor

makes contact with the mercury. The aluminium disc floats over the mercury and lower end of

its spindle rests on an adjustable screw A. The mercury in the chamber reduces the friction of the

disc at A and it alio provides path for the current to flow through the disc. For holding the

magnets in position, there are two iron bars in the instrument, one above the chamber and other

at the bottom of the chamber. On the lower iron bar, a coil of few turns is wound which is known

as a friction compensating coil. A recording device is attached at the upper end of the spindle.

When the instrument is in use, the current is passed in the disc through the conductor C and

mercury and from where it goes to the current coil and pressure coil, which interacts with the

aluminium disc placed near the coils between bearings. This field induces eddy currents in the

disc and hence the disc rotates.

Single Phase Induction Type Meters:. The construction and principle of operation of single

Phase Energy explained below :

Construction of Induction Type Energy Meters. The construction varies in details from one

product to the next. However, the differences are very minor in nature. There are four main parts

of the operating mechanism:

(i)Driving system. (ii) Moving system (iii)Braking system.(iv) registering sysytem

Fig shows the construction of a single phase induction type energy meter.

Driving system: The driving system of the meter consists of two electromagnets. The core of

these electromagnets is made up of silicon steel laminations. The coil of one of the electromagnet

is excited by the load current. This coil is called current coil. The second electromagnet is

connected across the supply, therefore, carries a current proportional to supply voltage.This coil

is so called the pressure coil.The two electromagnets are called as series and shunt magnets

respectively.

Copper shading bands are provided in the central limb.The position of these bands are

adjustable.The function of these bands is to bring the flux produced by the shunt magnet exactly

to quadrature with the applied voltage.

Moving System: This consists of an aluminium disc mounted on a light alloy shaft.This disc

is positioned in the air gap between series and shunt magnets. The upper bearing of

toe rotor (moving system) is a steel pin located in a bole in the bearing cap fixed to the top of

the shaft. The rotor runs on a hardened steel pivot, screwed to the foot of the shaft. The pivot is

supported by a jewel bearing. A pinion engages the shaft with the counting or registering

mechanism.

A unique design for the suspension of the disc is used in the floating-shaft energy meter.

Here the rotating shaft has a small magnet at each end, where the upper magnet of the shaft is

attracted to a magnet in the upper bearing and the lower magnet of the shaft is attracted to a

magnet in the lower bearing. The moving system thus floats without touching either bearing

surface, and the only contact with the movement is that of the gear connecting the shaft with the

gear of the train, thus the friction is drastically reduced.

Braking system : Permanent magnet positioned near the edge of the aluminium disc forms the

braking system. The aluminium disc moves in the field of this magnet and thus provides a

braking torque. The position of the permanent magnet is adjustable, and therefore, braking torque

can be adjusted by shifting the permanent magnet to different radial positions as explained

earlier.

Registering (counting) Mechanism: The function of a registering or counting mechanism is to

record continuously a number which is proportional to the revolutions made by the moving

system. By suitable system,a train of reduction gears the pinion on the rotor shaft drives a series

of five or six pointers. These rotate on round dials which are marked with ten equal divisions.

The pointer type of register to shown in Fig. Cyclo-Meter register as shown in fig can also be

used.

Theory of Operation of Single Phase Energy Meters. A simple functional diagram of the driving

system of the meter is shown in Fig. The corresponding phasor diagram is also shown.

The supply voltage is applied across the pressure coil. The pressure coil winding is highly

inductive as it has very large number of turns and the reluctance of its magnetic circuit is very

small owing to presence of air gaps of very small length. Thus the current Ip through the pressure

coil is proportional to the supply voltage and lags it by a few degrees less than 90°. This is

because the winding has a small resistance and there are iron losses in the magnetic circuit.

Current Ip produces a flux Φpt.This flux divides itself into two parts Φg and ΦP.The major

portion Φg flows across the side gaps as reluctance of this path is small. The reluctance to the

path of flux ΦP is large and hence its magnitude is small. This flux ΦP goes across aluminium

disc and hence is responsible for production of driving torque. Flux ΦP is in phase with current Ip

and is proportional to it.

Therefore flux ΦP is proportional to voltage V and lags it by an angle a few degrees less than 90°.

Since flux ΦP is alternating in nature, it induces an eddy emf in the disc which in turn produces

eddy current,.Iep.

The load current I flows through the current coil and produces a flux Φs. This flux is

proportional to the load current and is in phase with it.This flux produces eddy current Ies in the

disc. Now the eddy current,Ies interacts withflux ΦP to produce a torque and eddy current Iep

interacts with Φs to produce another torque. These two torques are in the opposite direction (as

shown, in Fig) and the net torque is the difference of these.

Driving torque is given as

Lag Adjustment Devices:

It is clear that the meter will register true energy only if the angle Δ is made equal to 90°. Thus

the angle between the shunt magnet flux ΦP and the supply voltage V should be equal to.90°.

This requires that the pressure coil winding should be so designed that it is highly inductive and

has a low resistance and the iron losses in the core are small. But even with this the phase of flux.

ΦP is not 90° with respect to voltage V but a few degrees less than 90°.However, by introducing

a magnetic shunt circuit which allows the main portion of the shunt magnet flux to bypass the

gap in which the disc is located, it is possible to introduce an mmf in the proper phase relation to

bring the shunt magnet flux in the disc air gap in exact quadrature with the Voltage. This is

illustrated in Fig.

The required mmf is obtained from a 'lag coil 'which is located on the central limb of the shunt

magnet close to the disc gap and links with the flux that cuts the disc.The pressure coil is excited

by voltage V and carries a current Ip which produces an mmf ATpt which in turn produces a fiux

ΦP lagging the voltage by an angle 90°-Δ. The flux ΦPt divides itself into two parts Φg and ΦP.

Flux ΦP cuts the disc and also links with the "lag coil". A voltage EL is induced in this coil

lagging ΦP by 90deg. This voltage circulates a current iL through the lag coil. iL lags behind EL

by an angle γ which depends upon the reactance and resistance of the lag coil. The lag coil

current produces an mmf ATL. The flux ΦP in the disc air gap will be created by the combined

action of the main rnmf ATpt in phase with IP and the lag coil mmf ATL in phase with IL. Thus

the flux ΦP will be in phase with the resultant mmf ATp. Hence it is clear that the phase of

flux ΦP can be adjusted by varying the mmf of the shading coil either in magnitude or in phase or

both.

The arrangements for adjusting the mmf of the lag coil are:

1.Adjustable resistance: A few turns of fairly thick wire are placed around the central limb of the

shunt magnet and the circuit is closed through a low adjustable resistance (Fig. ).

The resistance of this circuit is altered to adjust the lag angles of flux ΦP. An increase in

resistance decreases the current and mmf in the lag coil and therefore the value of lag angle θ is

decreased. The value of lag angle θ is increased by decreasing the resistance of the lag coil

circuit. The resistance of the lag coil is so adjusted that A becomes equal to 90deg

2. Shading bands: In this arrangement copper shading bands L1 are placed around the central

limb of shunt magnet instead of a lagcoil with adjustable resistance (Fig.).

The adjustment can be done by moving the shading bands along the axis of the limb. As the

shading bands are moved up the limb, they embrace more flux. This results in greater values for

induced emf, current and the mmf ATL produced by the shading bands and therefore the value of

lag angle θ increases. When the shading bands are moved down limb, mmf ATL decreases and

the angle is reduced. The adjustment is so done that θ is such that it makes Δ=90 deg.

Light Load or Friction Compensation:

Despite every care taken in the design of both the jeweled-pivot bearing, which forms the lower

bearing for the spindle, and of the simple sleeve pin-type bearing at the top of the spindle,

friction errors are liable to be serious, particularly at light loads. In order to ensure accurate

registration at low loads, it is therefore necessary to arrange for small torque, practically

independent of the load on the meter, which acts in the direction of rotation and which is nearly

as possible equal in magnitude to the friction torque.This is usually obtainedi,by means of a

small shading loop (or vane) situated between the centre pole of the shunt magnet and the disc

and slightly to one side of the centre line of the pole. This is shown as L2 in Fig .The interaction

between the portions of the flux which are shaded and unshaded by this loop and the currents

they induce in the disc results in a small driving torque whose value can be adjusted by lateral

movement of the loop. The tests for correct adjustment are freedom from serious errors at light

loads and the value of. the load necessary to, start the meter. Other arrangements of non

symmetrical shading loops are also used.

In meters provided with lag plates, the additional torque to compensate for the friction can be

produced by displacing the plate in a direction parallel to the direction of motion of the disc.

Creep:

In some meters a slow but continuous rotation is obtained even when there is no current flowing

through the current coil and only pressure coil is energized. This is called creeping, the major

cause, for creeping is over-compensation, for friction. If the friction compensating device is

adjusted to give a driving torque to compensate for starting friction which is bigger than the

running friction, there is a tendency for the disc to run even when there is no current through the

current coils Because the friction compensating torque is independent of the load current as the

compensating device is voltage actuated. The other causes of creeping are excessive voltage

across the potential coil (which is responsible for production of excessive torque by the friction

compensating device), vibrations, and stray magnetic fields.

In order to prevent this creeping two diametrically opposite holes are drilled in the disc ; the disc

will come to rest with one of the holes under the edge of a pole of the shunt magnet, the rotation

being thus limited to a maximum of half a revolution. The action may be understood by

reference to Fig. If a hole is under the edge of a pole, the circular eddy current paths in the disc

will be distorted as shown. The effective centre of the eddy-current paths is then at A 'and this

point is the central point of the equivalent magnetic pole produced by the currents. Examination

of polarities, shows that there is a resulting force on the disc, lending to move A' away from the

pole axis A. Thus the disc may creep until the hole reaches a position near the edge of a pole, but

further movement is opposed by the torque produced in the manner just described. The

magnitude of this torque is not sufficient to affect the action of the meter on load.

In some cases a small piece of iron is attached to the edge of the disc. The force of attraction

exerted by the brake magnet prevents creeping of disc.

Three Phase Energy Meter

In a three phase, four wire system, the measurement of energy is to be carried out by a three phase energy

meter. For three phase, three wire system, the energy measurement can be carried out by two element

energy meter, the connections of which are similar to the connections of two wattmeters for power

measurement in a three phase, three wire system. So these meters are classified as i) three element energy

meter and ii) two element energy meter. Three Element Energy Meter This meter consists of three elements. The construction of an individual element is similar to that of a

single phase energy meter. The pressure coils are denoted as P1, P2 and P3. The current coils are denoted

as C1, C2 and C3. All the elements are mounted in a vertical line in common case and have a common

spindle, gearing and registering mechanism. The coils are connected in such a manner that the net torque

produced is sum of the torques due to all the three elements. These are employed for three phase, four

wire system where fourth wire is a neutral wire.

The current coils are connected in series with the lines while pressure coils are connected across a line

and a neutral. Fig. shows a three phase energy meter,.

One unit of three clement three phase clement Is always cheaper than three units of single phase energy

meter. But due to interaction between eddy currents produced by one element with Ihe flux produced by

another clement, there may be errors in the measurement by three phase energy meter. Such errors may be

reduced by suitable adjustments.

Two Element Energy Meter:

The Fig shows a two element energy meter and a simplified connection diagram.This energy meter is

used for three phase, three wire systems. The meter is provided with two discs each for an element. The

shunt magnet is carrying pressure coil while a Series magnet carries a current coil. The pressure coils are

connected in parallel and the current coils in series- The connections are similar to the connections of two

wattmeters for power measurement in three phase, three wire system. Torque is produced in same manner

as in a single phase energy meter, in each element. The total torque on the registering mechanism

connected to moving system, is sum of the torques of the individual elements.

Static Energy Meters:

The conventional mechanical energy meter is based on the phenomenon of Magnetic Induction.

It has a rotating aluminium Wheel and many toothed wheels. Based on the flow of current, the

wheel rotates which makes rotation of other wheels. This will be converted into corresponding

measurements in the display section. Since many mechanical parts are involved, mechanical

defects and breakdown are common. More over chances of manipulation and current theft will

be higher.

Electronic Energy Meter is based on Digital Manipulations and uses no moving.Hence it is

known as Static Energy Meter.

Principle of Energy Measure :

In energy measure, the power information varying with time is calculated by a direct

multiplication of the voltage signal and the current signal. Assume that the current signal and the

voltage signal are cosine functions; Umax, Imax are the peak values of the voltage signal and the

current signal;

ω is the angluar frequency of the input signals; the phase difference between the current signal

and the voltage signal is expressed as φ Then the power is given as follows:

The ideal p(t) consists of the dc component (first term, independent of t) and ac component

whose frequency is 2ω. The dc component is called as the average active power, that is:

Block Diagram of Static Energy meter:

The two ADCs digitize the voltage signal s from the current and voltage transducers. These

ADCs convert the sampled instantaneous voltage and current signals to digital. A high pass filter

in the current and voltage channel removes any dc component from the current signal. This

eliminates any inaccuracies in the real power calculation due to offsets in the voltage or current

signals.

The real power calculation is derived from the instantaneous power signal. The instantaneous

power signal is generated by a direct multiplication of the current and voltage signals.We have

derived already the dc component of this instatneous power is the average power. In order to

extract the real power component (i.e., the dc component), the instantaneous power signal is low-

pass filtered.

This scheme correctly calculates real power for both sinusoidal and nonsinusoidal current and

voltage waveforms at all power factors. All signal processing is carried out in the digital domain

for superior stability overtemperature and time.

This average power is integrated to get the energy ( E= 𝑃 𝑡 𝑑𝑡 ). Finally this signal is

converted to pulses by a didgital to frequency converter. The frequency of this output pulses is

therefore proportional to the average real power. This average real power information can, in

turn, be accumulated (e.g., by a counter) to generate real energy information.

Voltage and Current Transducers:

Differential Opamp circuit with CT (to down the line current) is used to measure the

instantaneous current as shown below which is fed to Current ADC.

Differential Opamp circuit with potential divider arrangement (to down the line voltage) is used

to measure the instantaneous voltage as shown below which is fed to Voltage ADC.

(Note that Static Wattmeter and Energy meter block diagram and theory only differ in averaging

in case of wattmeter and integral in case of energymeter)

Static Wattmeter:

Principle of Power Measure:

The power is measured by a direct multiplication of the voltage signal and the current signal.

Assume that the current signal and the voltage signal are cosine functions; Umax, Imax are the

peak values of the voltage signal and the current signal;

ω is the angluar frequency of the input signals; the phase difference between the current signal

and the voltage signal is expressed as φ Then the power is given as follows:

The ideal p(t) consists of the dc component (first term, independent of t) and ac component

whose frequency is 2ω. The dc component is called as the average active power, that is:

The two ADCs digitize the voltage signal s from the current and voltage transducers. These

ADCs convert the sampled instantaneous voltage and current signals to digital. A high pass filter

in the current and voltage channel removes any dc component from the current signal. This

eliminates any inaccuracies in the real power calculation due to offsets in the voltage or current

signals.

The real power calculation is derived from the instantaneous power signal. The instantaneous

power signal is generated by a direct multiplication of the current and voltage signals.We have

derived already the dc component of this instatneous power is the average power. In order to

extract the real power component (i.e., the dc component), the instantaneous power signal is low-

pass filtered.

This scheme correctly calculates real power for both sinusoidal and nonsinusoidal current and

voltage waveforms at all power factors. All signal processing is carried out in the digital domain

for superior stability overtemperature and time.

Voltage and Current Transducers:

Differential Opamp circuit with CT (to down the line current) is used to measure the

instantaneous current as shown below which is fed to Current ADC. Differential Opamp circuit

with potential divider arrangement (to down the line voltage) is used to measure the

instantaneous voltage as shown below which is fed to Voltage ADC.

Vibrating Reed Type Frequency meter:

Construction: This meter consists of a number of thin steel strips called reeds. These reeds are

placed in a row alongside and close to an electromagnet. The electromagnet has a laminated iron

core and its coil is connected in series with a resistance, across the supply whose frequency is to

be measured.

The reeds are approximately about 4 mm wide and 1/2 mm thick. All the reeds are not exactly

similar to each other. They have either slightly different dimensions or carry different weights or

flags at their tops.

(a) Reed (b)

Fig.1

The natural frequency of vibration of the reeds depends upon their weights and dimensions.

Since the reeds have different weights and sizes, their natural frequencies of vibration are

different. The reeds are arranged in ascending order of natural frequency; the difference in

frequency is usually 1/2 Hz. Thus the natural frequency of first reed may be 47 Hz, of the second

47.5 Hz, of the next 48 Hz and so on.

The reeds are fixed at the bottom end and are free at the top end. Since the reeds on a frequency

meter are arranged to be viewed end on, they have a portion bent over at the free end to serve as

a flag as shown in Figure. The flags are painted white to afford maximum contrast against their

black background.

Operation:When the frequency meter is connected across the supply whose frequency is to be

measured, the coil of electromagnet carries a current i which alternates at the supply frequency.

The force of attraction between the reeds and the electromagnet is proportional to i2 and

therefore this force varies at twice the supply frequency.

Thus the force is exerted on the reeds every half cycle. All the reeds will tend to vibrate, but the

reed whose natural frequency is equal to twice the frequency of supply will be in resonance and

will vibrate most- Normally the vibration of other reeds is so slight as to be unobservable. The

tuning in these meters is so sharp that as the excitation frequency departs from the resonant

frequency the amplitude of vibration decreases rapidly becoming negligible for a frequency

perhaps 1 to 2 percent away from resonance. This is clear from Figure.

When the 50 Hz reed is vibrating with its maximum amplitude (when it is in resonance) some

vibrations of 49.5 Hz and 50.5 Hz reeds may be observed as shown in Fig. ( b) but very little

vibrations will be observed on 49 Hz and 51 Hz reeds. For a frequency exactly midway between

that of the reeds, both will vibrate with amplitudes which are equal in magnitude, but

considerably less than the amplitude which is at resonance, Fig, (c) shows the condition of

vibrating reeds when the frequency is exactly midway between 495 Hz and 50 Hz. Fig. (a)

shows the condition of the reeds when the frequency meter is unexcited i.e. is not connected to

the supply.

Single Phase Electrodynamometer Power Factor Meter:

Construction: The construction of a single phase electrodynamometer type power factor meter is

shown in Fig, It consists of a fixed coil which acts as the current coil. This coil is split up into-

two parts and carries the current of the circuit under test. Therefore, the magnetic field produced

by this coil is proportional to the main current. Two identical pressure coils A and B pivoted on a

spindle constitute the moving system. Pressure coil A has a non-inductive resistance R connected

in series with it, and coil B has a highly inductive choke coil L connected in series with it. The

two coils are connected across the voltage of the circuit. The values of R and L are so adjusted

that the two coils carry the same value of current at normal frequency, i.e. R=wL. The current

through coil A is in phase with the circuit voltage; while that through coil B lags the voltage by

an angle A which is nearly equal to 90°. The angle between the planes of coils is also made equal

to A. There is no controlling device. Connections to moving coils are made by thin silver or gold

ligaments which are extremely flexible, and thus give a minimum control effect on the moving

system.

Theory:In order to simplify the problem, we assume that the current through coil B lags the

voltage by exactly 90 deg. Also that the angle between planes of coils is exactly 90deg, (i.e., Δ

=90°).

Now, there will be two deflecting torques, one acting on coil A and the other on coil B. The coil

windings are so arranged that the torques due to the two coils are opposite in direction. Therefore

the pointer will take up a position where these two torques are equal.

Let us consider the case of a lagging power factor of cos Φ.

Deflecting torque acting on coil A:

. TA= KVIMmax cos Φ sin θ Where, θ =angular deflection from the plane of refrence

Mmax=maximum value of mutual inductance between the two coils.

This torque say acts in the clockwise direction.

Deflecting torque acting on coil B :

TB= KVIMmax cos (90°-Φ) sin (90°+θ) = KVIMmax sin Φ cos θ

This torque acts in the anticlockwise direction. The value of Mmax is the same in the two

expressions, owing to similar constructions of the coils.

The coils will take up such a,position that the two torques are equal.

Hence at equilibrium TA=TB

or KVIMmax cos Φ sin θ = KVIMmax sin Φ cos θ or θ = Φ

Therefore the deflection of the instrument is a measure of phase angle of the circuit, The scale of

the instrument can be calibrated in directly in terms of power factor.

Three Phase Electrodynamometer Power Factor Meter:

Construction: Fig shows the construction and connections of a 3 phase power factor meter. This

meter is only useful for balanced loads.

The two moving coils are so placed that the angle between their planes is 120°. They are

connected across two different phases of the supply circuit. Each coil has a series resistance.

There is no necessity for phase splitting by artificial means, since the required phase

displacement between currents IA and IB the two moving coils can be obtained from the supply

itself as shown.

Theory: Voltage applied across coil A is V12 and as its circuit is resistive, current IA is in phase

with V12. Voltage applied across coil B is V13 and current IB is in phase with V13 as the circuit of

is resistive.

Let Φ= phase angle of circuit

and θ=angular deflection from the plane of reference

Now V1=V2=V3=V.

Torque acting on coil A is:

TA= KV12IMmax cos (30°+Φ) sin (60°+θ) = 3 KVIMmax cos (30°+Φ) sin (60°+θ)

Torque acting on coil B is:

TB= KV12IMmax cos (30°-Φ) sin (120°+θ) = 3 KVIMmax cos (30°-Φ) sin (120°+θ)

Torques TA and TB act in the opposite directions and the moving system takes up a position

where TA = TB.

cos (30°+Φ) sin (60°+θ)= cos (30°-Φ) sin (120°+θ)

Solving the above expression, we have ; θ = Φ

Thus the angular deflection of the pointer from the plane of reference is equal to the phase angle

of the circuit to which the meter is connected.

The three phase power factor meter gives indications which are independent of waveform and

frequency of supply, since the currents in the two moving coils are equally affected by any

change of frequency.

For measurement of power factor in 3 phase unbalanced systems a two element power factor

meter (where 2 sets of fixed coils and 2 sets of moving coils mounted on the spindle) has to be

used.

to a magnet in the upper bearing and the lower magnet of the shaft is attracted to a magnet in the

lower bearing. The moving system thus floats without touching either bearing surface, and the

only contact with the movement is that of the gear connecting the shaft with the gear of the train,

thus the friction is drastically reduces.

Braking System: A permanent magnetpositioned near the edge of the aluminium disc formsthe

braking system. The aluminium disc moves in the field of this magnet and thus provides a

braking torque. The position of the permanent magnet is adjustable, and therefore, braking

torque can be adjusted by shifting the permanent magnet to different radial positions as explained

earlier.

Registering (counting) Mechanism: The function of a registering or counting mechanism is to

record continuously a number which is proportional to the revolutions made by the moving

system.

MEASUREMENT OF RESISTANCE

Ohmmeters:

Ohmmeter is a convenient direct reading device for measurement of resistance. These

instruments have a low degree of accuracy. The statement regarding accuracy is not intended in

an unfavorable sense there is a wide field of application for this instrument in determining the

approximate value of resistance. An ohmmeter is useful for getting the approximate resistance of

circuit components such as heater elements or machine field coils, measuring and sorting

resistors used in electronic circuits and for checking continuity of circuits. It is also- useful in

laboratories as an aid to a precision bridge, for it can help to know the approximate value of

resistance which can save time in balancing the bridge.

Series-type Ohmmeter: A circuit of a series-type ohmmeter is shown in Fig. It consists of

basic d'Arsonval movement connected in parallel with a shunting resistor,R2 . This parallel

circuit is in series with resistance R1 and a battery of emf E. The series circuit is connected to the

terminals A and B of the unknown resistance Rx.

R1=current limiting resistor,

R2=zero adjusting resistor,

E= emf of internal battery,

Rm=internal resistance of d'Arsonval movement,

It is observed that when the unknown resistance Rx=0 (terminals A and B shorted) maximum

current flows through the meter. Under this condition resistor R2 is adjusted until (he basic

movement (meter) indicates full scale current In. The full-scale current position of the pointer , is

marked "0 ohm" on the scale. Similarly when Rx is removed from circuit, Rx=infinity (that is

when terminals A and B are open), the current in the meter drops to zero and the movement

indicates zero current which is then marked "Infinity". Thus the meter will read infinite

resistance at the zero current position and zero resistance at full scale current position. Since

zero resistance is indicated when the current in the meter is maximum and hence the pointer goes

to the top mark. When the unknown resistance is inserted at terminals A, B the current through

the meter is reduced and hence pointer drops lower on the scale. Therefore the meter has "0" at

extreme right and "infinity" at the extreme left. Intermediate scale markings may be placed on

the scale by different known values of resistance Rx to the instrument. The accuracy of these

scale markings depends on the repeating accuracy of the movement and the tolerances of the

calibrating resistors. Fig. 8'17 {a) shows the shape of scale of series type ohmmeter.

A convenient quantity to use in the design of a series ohmmeter is the value of Rx which causes

the half scale deflection of the meter. At this position, the resistance across terminals A and B is

defined as the half scale position resistance Rh. The design can be approached by recognizing the

fact that when Rh is connected across terminals A and B the meter current reduces to 1/2 of its

full scalevalue.

where Im=current through the meter, Ifs-current through the meter for full scale deflection. This

clearly means that Rh is equal to the internal resistance of the ohmmeter looking into terminals A

and B.

(1)

The battery current at half scale deflection ,

In order to produce full scale deflection the battery current must be doubled

(2)

Current through the shunt

(3)

The voltage drop across R2 is equal to voltage drop across meter

Substituting in (2,3) we get

(4)

From (1) ,(4)

Shunt type Ohmmeters: The circuit diagram of a shunt type ohmmeter is shown in Fig It

consists of a battery in series with an adjustable resistor R1 and a basic d'Arsoival movement

(meter). The unknown resistance is connected across terminals A and B parallel with the meter.

In this circuit it is necessary to have an "off-on"switch to disconnect the battery from the circuit

when the instrument is not in use. When the unknown resistor Rx=0 (A and B are shorted), the

meter current is zero. If the unknown resistance Rx=inf (A and B are open), the current finds

path only through the meter and selecting a proper value for resistance R1, the pointer may be

made to read full scale. This ohmmeter therefore has "zero" mark on the left hand side of the

scale (no current) and infinite mark on the right hand side of the scale (full scale deflection

current).

Measurement of earth resistance

Necessity of Earth Electrode: The provision of an earth electrode for an electrical system is

necessitated by the following reasons:

1. All the parts of electrical equipment, like casings of machines, switches and circuit breakers,

lead sheathing and armouring of cables, tanks of transformers, etc. which have to be at earth

potential, must be connected to an earth electrode, The purpose of this is to protect the various

parts of the installation, as well as the persons working, against damage in case the insulation of

a system fails at any point. By connecting these parts to an earthed electrode, a continuous low

resistance path is available for leakage currents to flow to earth. This current operates the

protective devices and thus the faulty circuit is isolated in case a fault occur.

2. The earth electrode ensures that in the event of overvoltage on the system due to lightning

discharges or other system faults, those parts of equipment which are normally 'dead' as far as

voltages are concerned, do not attain dangerously high potentials.

3. In a three phase circuit the neutral of the system is earthed in order to stabilize the potential

of the circuit with respect to earth.

Necessity of Measurement of Resistance of Earth Electrode: An earth electrode will only be

effective so long it has a low resistance to the earth and can carry large currents without

deteriorating. Since the amount of current which an earth electrode will carry is difficult to

measure, the resistance value of the earth electrode is taken as sufficiently reliable indication of

its effectiveness. Thus the resistance of earth electrode should be low to give good protection and

it must be measured.

Factors affecting Earth Resistance: The main factors on which the resistance of any earthing

system depends are :

1. Shape and material of earth electrode or electrodes used.

2. Depth in the soil at which the electrodes are buried.

3. Specific resistance of soil surrounding and in the neighbourhood of electrodes. The specific

resistance of the soil is not constant but varies from one type of soil to another. The amount of

moisture present in the soil effects its specific resistance and hence the resistance of earth

electrode is not a constant factor but suffers seasonal variations. This calls for periodic testing to

ensure that the earth system remains reasonably effective.

The specific resistance of soils varies between wide limits and is very much dependent upon its I

moisture content. Approximate figures .for specific resistance of soil are 80 X l03 ohm-m for

moist clay to 80x 106 Om for sand of normal moisture content. A decrease of moisture content

of 30% is capable of producing an increase of 300 to 400% in' specific resistance. Thus it is

necessary to make regular checks for earth resistance during the year round.

Fall of Potential Method. Fig shows the circuit for measurement of earth resistance with fall of

potential method.

A current is passed through earth electrode E to an auxiliary electrode B (which is usually an iron

spike) inserted in earth at a distance away from the earth electrode. A second auxiliary electrode

A is inserted in earth between E and B. The potential difference V between E and A is measured

for a given current I. The flow of ground currents is shown in Fig The lines of the first electrode

current diverge and those of the second electrode current converge. As a result the current

density is much greater in the vicinity of the electrodes than at a distance from them. The

potential distribution between the electrodes is shown in Fig

It is apparent from this curve that the potential rises in the proximity of electrodes E and B and is

constant along the middle section. The resistance of earth, therefore, is RE = V/I or VEA/I. The

position of electrodes E and B is fixed and the position of electrode A is changed and resistance

measurements are done for various positions of electrode A.

A graph is plotted between earth resistance against the distance between electrode E and A. This

graph is shown in Fig.

From Fig, it is clear that the measured value of earth resistance depends upon the position of the

auxiliary electrode A. The earth resistance rises rapidly initially, When the distance between

earth electrode B and auxiliary electrode A is increased, it then becomes constant, and when [ the

auxiliary electrode A approaches the, auxiliary electrode B,the resistance rises again. The placing

of electrodes is thus very important and serious error may be caused by incorrect placing of the

electrodes. The correct value of resistance of earth,RE, is when the auxiliary electrode A is at

such a distance that the resistance lies on the flat part of curve of Fig. The spacing between the

earth electrode E and the auxiliary electrodes A, B should be large so as to get proper results. The

distance may be a few hundred metres in case the earth resistance is low.

Earth Tester. The resistance of earth can be measured by an earth tester shown in Fig.

The "Earth Tester" is a specialtype of Megger and it has some additional constructional features

and they are :

(i) a rotating current reverser, and (ii) a rectifier.

Both these additional features consist of simple commutators made up of 'L' shaped segments.

They are mounted on the shaft of the hand driven generator. Each commutator has four fixed

brushes. One pair of each set of brushes is so positioned that they, make contact, alternately ,

with one segment and then with the other as the commutator rotates. The second pair, of each of

set of brushes is positioned on the commutator so that continuous contact is made with one

segment whatever the position of the commutator. The earth tester has four terminals P1, P2 and

C1, C2. Two terminals P1 and C1 are shorted to form a common point to be connected to the

earth electrode. The other two terminals P2 and C2 are connected to auxiliary electrodes P and C

respectively. The indication of the earth tester depends upon the ratio of the voltage. across the

pressure coil and the current through the coil. The deflection of its pointer indicates the

resistance of earth directly. Although the "Earth Tester", which is a permanent magnet moving

coil instrument and can operate on d.c. only, yet by including the reverser and the rectifying

device it is possible to make measurements with a.c. flowing in the soil.

The sending of a.c, current through the soil has many advantages and therefore this system is

used. The use of a.c. passing through the soil eliminates unwanted effects due to production of a

back emf in the soil on account of electrolytic action. Also the instrument is free from effects of

alternating or direct currents presents in the soil.

Megger: . The essential parts of a Megger are shown in Fig.. The current coil is similar to that of the

permanent magnet moving coil instrument. There are two voltage (potential) coils V1 and V2

The voltage coil V1 embraces (threads over) the annular magnetic core. It is clear from Fig. 8'25

that voltage coil V1 is in a weak magnetic field when the pointer is at 'inf' position and hence this

coil can exert very little torque.

The torque exerted by the voltage coil increases as it moves into a stronger field and this torque

is maximum when it is under the pole face and under this condition the pointer is at its zero end

of the resistance scale. In order to modify further the torque in the voltage circuit, another

voltage coil V2 is used. This coil is also so located that it moves into stronger field as the pointer

moves from the 'inf' position towards the zero position of the resistance scale. The coil finally

embraces (threads around) the extension H of the pole piece.

The combined action of the two voltage coils V1 and V2 may be considered as though the coils

constituted a spring of variable stiffness, being very stiff near the zero end of the scale where the

current in the current coil is very large (on account of unknown resistance Rx being small), and

very weak near the 'inf' end of the scale where the current in the current coil is very small (on

account of unknown resistance Rx being very large).

Thus this effect compresses the low resistance portion of the scale and opens up the high

resistance portion of the scale This is a great advantage since this instrument is meant to be used

as ''insulation tester" and the insulation resistances are quite high.

The voltage range of the instrument can be controlled by a voltage selector switch. This can be

done by varying resistance 'R' connected in series with the current coil. The test voltages, usually

500, 1000 or 2500 V are generated by a hard cranked generator G. A centrifugal clutch is incor-

porated in the generator drive mechanism which slips at a predetermined speed so that a constant

voltage is applied to the insulation under test. This voltage provides a test on strength of low

voltage insulation as well as a measure of its insulation resistance, since it is sufficient to cause

breakdown at faults. Such breakdowns are indicated by sudden motion of the pointer off scale at.

zero end. As the same magnet system supplies magnetic fields for both instrument and generator,

and as current and voltage coils move in a common magnetic field, the instrument indications are

independent of the strength of the magnet.

Direct Defection Method. For high resistances, such as insulation resistance of cables, a sensitive galvanometer of

d'Arsonval type (usually having a current sensitivity of at 1000 mm/microA at a scale distance of

1 metre) is used in place of the microammeter. In fact many sensitive type of galvanometers can

detect currents from 0.l—1 nA. Therefore, with an applied voltage of 1k V, resistances as high

as 1012

to 10 x 1012

ohm can be measured.

An illustration of the direct deflection method used for measuring insulation resistance of a cable

is shown in Fig.

The galvanometer G, measures the current IR between the conductor and the metal sheath. The

leakage current IL, over the insulating material is carried by the guard wire wound on the

insulation and therefore does not flow through the galvanometer.Cables without metal sheaths

can be tested in a similar way if the cable, except the end of ends on which connections are

made, is immersed in water in a tank. The water and the tank then form the return path for the

current. The cable is immersed in slightly saline water for about 24 hours and the temperature is

kept constant (at about 20°C) and then the measurement is taken as in Fig.

The insulation resistance of the cable R=V/IR

In some cases, the deflection of the galvanometer is observed and its scale is afterwards

calibrated by replacing the insulation by a standard high resistance (usually 1 Mohm), the

galvanometer shunt being varied, as required to give a deflection of the same order as before.

In tests on cables the,galvanometer should be short-circuited before applying the voltage. The

short circuiting connection is removed, only after sufficient time has elapsed so that charging and

absorption currents cease to flow. The galvanometer should be well shunted during the early

stages of measurement, and it is normally desirable to include a protective series resistance (of

several megohm) in the galvanometer circuit. The value of this resistance should be subtracted

from the observed resistance value in order to determine the true resistance. A high voltage

battery of 500 V emf is required and its emf should remain constant throughout the test.

Methods Used for Localizing Ground and Short Circuit Faults:

In the case of multi core cables it is advisable, first of all, to measure insulation resistance of

each core to earth and also between core. This enables us to sort out the core that is earthed in

case of ground fault and to sort out the cores that are shorted in case of a short circuit fault. Loop

tests are used for location of ground and short circuit faults. These tests can only be used if a

sound cable runs along with the faulty cable or cables. The bop tests work on the principle of a

Wheatstone bridge The advantage of these tests is that their set up is such that the resistance of

fault is connected in the battery circuit and therefore does not effect the result. However, if the

fault resistance is high the sensitivity is adversely affected.

Murray Loop Test: The connection for this test are shown in Fig a relates to the ground fault and

Fig. b relates to the short circuit fault.

In both cases, the loop circuit formed-by the cable conductors is essentially a Wheatstone bridge

consisting of resistances P, Q, R and X. G is a galvanometer for indication of balance.

The resistors P, Q forming the ratio arms may be decade resistance boxes or slide wires.

where (R+X) is total loop resistance formed by the sound cable and the faulty cable. When the

conductors have the same cross-sectional area and the same resistivity, the resistances are

proportional to lengths. If l1 represents the length of the fault from the test end and l the length

of each cable. Then,

The above relation shows that the position of the fault may be located when the length of the

cable is known. Also, the fault resistance does not alter the balance condition because it

(resistance) enters the battery circuit However, if the magnitude of the fault resistance is high,

difficulty may be experienced in obtaining the balance condition on account of decrease in

sensitivity and hence accurate determination of the position of the fault may not be possible. In

such a case, the resistance of the fault may be reduced by applying a high direct or alternating

voltage-in consistence with the insulation rating of the cable on the line so as to carbonize the

insulation at the point of the fault.

Varley Loop Test:

In this test we can determine experimentally the total loop resistance instead of calculating it

from the known lengths of the cable and its resistance per unit length. The necessary connections

for the ground fault are shown in Fig. (a) and for the short circuit fault in Fig. (b). The treatment

of the problem, in both cases, is identical.

A single pole double throw switch K is used in this circuit. Switch K is first thrown to position '1'

and the resistance S is varied and balance obtained.

Let the value of S for balance be S1. The four arms of the Wheatstone bridge arc P, Q. At

balance ;

This determines R+X i.e., the total loop resistance as P, Q and S1 are known.

Then switch K is then thrown to position '2' and the bridge is rebalanced. Let the new value of S

for balance be S2. The four arms of the bridge now are P, Q, R,S2.

At balance,

Hence X is known from the known values of P, Q, S2 from this equation and R+X (the total

resistance of 2 cables) as determined . Knowing the value of X, the position of the fault is

determined as :

AC BRIDGES:

General form of an A.C Bridge: As an example let us consider the bridge circuit of Fig. R3 and

R4 are non-inductive resistances L1 and L2 are inductances of the negligible resistance and R\

and R2 are non-inductive resistors.

At balance condition,

Seperating and equating real and imaginary parts,

Then, if L1 and R1 are unknown, the above bridge may be used measure these quantity in terms

of R2, R3,R4 and L2. Two balance equations are always obtained for an a.c. bridge circuii.

This follows from the fact that for balance in an a.c. bridge, both magnitude and phase

relationships must be satisfied. This requires that real and imaginary terms must be separated,

which gives two equations to be satisfied for balance.

Maxwell's Inductance Bridge: Used to measure inductance.This bridge circuit measures an

inductance by comparison with a variable standard self-inductance. The connections and the

phasor diagrams for balance conditions are shows in Fig.

Let L1=unknown inductance of resistance R1,

L2=variable inductance of fixed resistance r2,

R2=variable resistance connected in series with inductor L2,

R3,R4=known non-inductive resistances,

Under balanced condition, Z1Z4=Z2Z3

(R1+jωL1)R4=(r2+R2+ jωL2)R3

Separating real and imaginary parts and equating,

Maxwell's Inductance- Capacitance Bridge: In this bridge, an inductance is measured by

comparison with a standard variable capacitance. The connections and the phasor diagram at the

balance conditions are given in Fig.

Under balanced condition, Z1Z4=Z2Z3

Which gives,

Separating real and imaginary parts and equating,

and

Q-factor is given by,

Anderson's Bridge: This bridge, in fact, is a modification of the Maxwell's inductance-

capacitance bridge. In this method, the self inductance is measured in terms of a standard

capacitor. This method is applicable for precise measurement of self-inductance over a very wide

range of values.

Fig shows the connections and the phasor diagram of the bridge for balanced conditions.

At balance condition,

I1=I3 and I2=IC +I4

Then,

Schering’s Bridge: Used to measure capacitance. The connections and the phasor diagrams for

balance conditions are shows in Fig.

MAGNETIC MEASUREMENTS

Measurement of Flux and Flux Density. The measurement of flux density inside a specimen

can be done by winding a search coil over the specimen. This search coil is known as a "B coil".

Ibis search coil is then connected to a ballistic galvanometer or a flux meter. Let us consider

that we have to measure the flux density in a ring specimen shown in Fig. The ring specimen is

wound with a magnetizing winding which carries a current I. A search coil of convenient number

of turns is wound on the specimen and connected through a resistance and calibrating coil, to a

ballistic galvanometer as shown.

The current through the magnetizing coil is reversed and therefore the flux linkages of the search

coil change inducing an emf in it. This emf sends a current through the ballistic galvanometer

causing it to deflect.

Flux density,

Measurement of Value of Magnetising Force (H):

The magnetising force of a constant magnetic field may be measured by a ballistic galvanometer

and a search coil. The value of H inside a specimen can either be inferred from calculations

involving data of magnetising coil and the specimen or from measurements made outside the

specimen. It cannot be measured directly. If the magnetising force is to be determined in the air

gap, the search coil is placed in the air gap itself. While testing ferro-magnctic materials the

magnetising force, within the specimen may be determined by measuring the magnetising force

on its surface, since the tangential components of the field are of equal in magnitude for both

sides of the interface.

The search coil, as positioned in Fig. , measures the value of flux density, Bo, in air. This

search coil is called an "H coil". While for the flux densities encountered in iron testing, there is

usually no trouble in getting a good sensitivity by using B coil of sufficient turns but there is

some difficulty in securing adequate sensitivity in the 'H' coil placed at the surface, In the first

instance, its cross-sectional area is much smaller than the coil surrounding the specimen (i.e. B

coil) and then H is not constant across the section Secondly, the permeability of iron is very large

as compared to air-may be some thousand times and therefore Mux density B0, in search coil, is

very smalt compared to that in specimen. The value of flux density B0 in H coil is measured in a

similar manner as described above for determination of B in the specimen.

Flux Meter The flux meter is a special type of ballistic galvanometer in which the controlling torque is

very small and the electromagnetic damping is heavy.

Construction. :The construction of a flux meter is shown in Fig. . In general the construction is

similar to that of a moving coil milli-ammeter. A coil of small cross-section is suspended from a

spring support by means of a single silk thread. The coil moves in the narrow gap of a permanent

magnet. There are no control springs. The current is led into the coil with the

help of a very loose helices of very thin, annealed silver strips. The controlling torque is thus

reduced to minimum.

The coil is formerless and the air friction damping is negligible.

Operation. The terminals of the flux meter are connected to a search coil as shown in Fig. The

flux linking with the search coil is changed either by removing the coil from the magnetic field

or by reversing the field. Due to the change in the value of flux linking with the search coil an

emf is induced in it. This emf sends a current through the flux meter which deflects through

anangle depending upon the change in the value of flux linkages. The instrument coil deflects

during the period the flux linkages change but as soon as the change ceases the coil stops, owing

to high electromagnetic damping in the coil circuit. This high electromagnetic damping is

obtained by having a low resistance of the circuit comprising the flux meter and the search coil.

Measurement of flux Density with Gauss Meter: The general electric Company makes a small

and simple "Gauss meter". (This meter may safely be called a tesla meter now as the unit of

flux density prevalent these days is tesla i.e. Wb/ma). It can be usefully employed in testing field

strength in places such as the gap between the poles of a magnet.

It consists of a tiny permanent magnet mounted at the end of a long, thin shaft (Fig) which is

supported and protected by a bronze tube. The assembly is used as a probe and has an outside

diameter less than one-tenth of an inch. The head of the meter consists of a bearing, scale,

pointer, ,and spring and has no electric or magnetic elements in it. The small probe magnet tends

to align itself with any magnetic field around it. The person using the meter turns the head until

the maximum indication is given on the scale; this gives the strength of the field. This meter is

made possible by a new magnetic material called Silmanal, which has a very high coercive force

(about ten times that of Alnieo). This instrument has an accuracy of i per cent when special

calibration methods are used, but commercial tolerances are of the order of 5 per cent.