4EE4-24: Measurement Lab
1) Study working and applications of (i) C.R.O. (ii) Digital Storage C.R.O. & (iii) C.R.O.
2) Study working and applications of Meggar, Tong-tester, P.F. Meter and Phase Shifter.
3) Measure power and power factor in 3-phase load by (i) Two-wattmeter method and (ii)
4) Calibrate an ammeter using DC slide wire potentiometer.
5) Calibrate a voltmeter using Crompton potentiometer.
6) Measure low resistance by Crompton potentiometer.
7) Measure Low resistance by Kelvin's double bridge.
8) Measure earth resistance using fall of potential method.
9) Calibrate a single-phase energy meter by phantom loading at different power factors.
10) Measure self-inductance using Anderson's bridge.
1.1 AIM: - Study working and applications of (i) C.R.O. (ii) Digital Storage C.R.O. &
(iii) C.R.O. Probes.
1.2 APPARATUS REQUIRED: - CRO, CRO probes, function generator.
1.3 THEORY: - The cathode ray oscilloscope is an externally useful important used for
studying wave shops of alternatively current and voltage as well as for measurement of
voltage current. Power and frequency infect almost any quantity that involves amplitude of
electrical signal as the function of time on the screen.
1.4 BLOCK DIAGRAM:-
Fig: Blok Diagram Of CRO
The instrumental employs cathode ray tube. If generates electron beam accelerates the beam
to high velocity. Deflects the beam to create the image and contain a phosphor screen where
the electron beam eventually becomes visible for accomplishing this task various electrical
signal and voltages are required which are provided by the power supply cut of the
Low voltage supply is required for the heater of electron gun and high voltage
of the order of few thousand volts is required for the cathode ray tube to accelerate the beam.
Normal supply of few hundred volts is required for the other circuit of the oscilloscope.
Horizontal & vertical reflection plates are fitted b/w electron gun and screen to deflected
beam strikes the screen and creates a visible spot. This spot is deflected on the screen in
horizontal direction (x-axis) with constant time dependant rate.
1.5 Features & uses :-
1.5.1. Exterior: - A typical oscilloscope is box shaped with display screen numerous input
connectors and control knobs and buttons on the front panel
1.5.2. Inputs: - The signal to be measured is fed to are of the input connection screen as
Bmc or N- tube. If signal source has its own coaxial connector then a simple coaxial cone
1.5.3. The Trace: - The oscilloscope repeatedly draws a horizontal time called trace across
the middle. If the screen from left to right are of the control. The time base control sets the
speed at which the time is drawn and is calibrated in the seconds per division. If the voltage
deposits from zero the trace is deflected either upwards
1.5.4. Trigger: - To provide more stable trace modern oscilloscope have a function called
Trigger. The effect is to resynchronize the time base to the input signal preventing horizontal
any of the trace. In this way triggers allows. The display of provider signals such as sine
waves and square waves.
1.6 CRO PROBES: - A probe is physical device used to connected electronic test equipment
to the device under test. A probe is more than a cable with clip on tip. It is a high quality
connector designed not to pick up slay ratio and power line noise.
Describe as three probes as follows:-
1.6.1. Direct probes (N):- This is simplest probe. If avoid stray pickups which may cause
troubles in measurement of low level signals.
1.6.2. Isolation Probes: - Such a probe is employed to avoid the undesirable circuit loading
effects of the shielded probes isolation probe is made by placing a carbon resistor in series
with the test load.
1.6.3. Passive probes: - A passive box probes. Notice the suited in the probe needle that
allows choosing between 1x or 10 x. passive probes usually connected to the loading of
about 10-15 p.f. and 10 m if a ohm.
1.6.4. Zd Probes: - Zd probes are specialized type of low capacitance passive probes used in
low impedance very high frequency circuits very similar to designed to ordinary passive
1.6.5. Active Probes: - Active scope probes are a small usually FET based amplifier
mounted directly with in the probe head. By doing this, they are able to obtain exceptionally
low passive capacitance and high DC resistance.
1.7 Result: - We have studied about the CRO, digital CRO and CRO probes.
EXPERIMENT NO:- 2
2.1 OBJECT:- Study working and applications of Meggar, Tong-tester, P.F. Meter and
2.2 APPARATUS REQUIRED:
2.3 CIRCUIT DIAGRAM:
Transistor Phase Shift Oscillator
S.NO ITEM RANGE Q.TY
1 TRANSISTOR BC 107 1
4 CRO ( 0 – 30 ) MHz 1
5 RPS (0-30) V 1
(0-1 )MHz 1
The Transistor Phase Shift Oscillator produces a sine wave of desired designed
frequency. The RC combination will give a 60 degree phase shift totally three
combination will give a 180°phase shift. . The BC107 is in the common emitter
configuration. Therefore that will give a 180 degree phase shift totally a 360○
output is produced. The capacitor value is designed in order to get the desired output
frequency. Initially the C and R are connected as a feedback with respect to input and
output and this will maintain constant sine wave output. CRO is connected at the output.
1. The circuit is constructed as per the given circuit diagram.
2. Switch on the power supply and observe the output on the CRO( sine wave)
3. Note down the practical frequency and compare it with the theoretical frequency.
2.6 OBSERVATION TABLE:-
Frequency f = 1 /
2.7 Power factor Meter
As was mentioned before, the angle of this “power triangle” graphically indicates the
ratio between the amount of dissipated (or consumed) power and the amount of
absorbed/returned power. It also happens to be the same angle as that of the circuit's
impedance in polar form. When expressed as a fraction, this ratio between true power and
apparent power is called the power factor for this circuit. Because true power and
apparent power form the adjacent and hypotenuse sides of a right triangle, respectively,
the power factor ratio is also equal to the cosine of that phase angle. Using values from
the last example circuit:
It should be noted that power factor, like all ratio measurements, is a unit less quantity.
For the purely resistive circuit, the power factor is 1 (perfect), because the reactive power
equals zero. Here, the power triangle would look like a horizontal line, because the
opposite (reactive power) side would have zero length. For the purely inductive circuit,
the power factor is zero, because true power equals zero. Here, the power triangle would
look like a vertical line, because the adjacent (true power) side would have zero length.
The same could be said for a purely capacitive circuit. If there are no dissipative
(resistive) components in the circuit, then the true power must be equal to zero, making
any power in the circuit purely reactive.
The power triangle for a purely capacitive circuit would again be a vertical line (pointing
down instead of up as it was for the purely inductive circuit). Power factor can be an
important aspect to consider in an AC circuit; because any power factor less than 1 means
that the circuit's wiring has to carry more current than what would be necessary with zero
reactance in the circuit to deliver the same amount of (true) power to the resistive load. If
our last example circuit had been purely resistive, we would have been able to deliver a
full 169.256 watts to the load with the same 1.410 amps of current, rather than the mere
119.365 watts that it is presently dissipating with that same current quantity.
The poor power factor makes for an inefficient power delivery system. Poor power
factor can be corrected, paradoxically, by adding another load to the circuit drawing an
equal and opposite amount of reactive power, to cancel out the effects of the load's
inductive reactance. Inductive reactance can only be canceled by capacitive reactance, so
we have to add a capacitor in parallel to our example circuit as the additional load. The
effect of these two opposing reactance in parallel is to bring the circuit's total impedance
equal to its total resistance (to make the impedance phase angle equal, or at least closer,
to zero). Since we know that the (uncorrected) reactive power is 119.998 VAR
(inductive), we need to calculate the correct capacitor size to produce the same quantity
of (capacitive) reactive power. Since this capacitor will be directly in parallel with the
source (of known voltage), we'll use the power formula which starts from voltage and
2.8 Circuit Diagram:
Power Factor Meter Circuit Diagram
Parallel capacitor corrects lagging power factor of inductive load. V2 and node numbers:
0, 1, 2, and 3 are SPICE related, and may be ignored for the moment.
The power factor for the circuit, overall, has been substantially improved. The main
current has been decreased from 1.41 amps to 994.7 milliamps, while the power
dissipated at the load resistor remains unchanged at 119.365 watts. The power factor is
much closer to being 1:
Since the impedance angle is still a positive number, we know that the circuit, overall, is
still more inductive than it is capacitive. If our power factor correction efforts had been
perfectly on-target, we would have arrived at an impedance angle of exactly zero, or
purely resistive. If we had added too large of a capacitor in parallel, we would have ended
up with an impedance angle that was negative, indicating that the circuit was more
capacitive than inductive.
A SPICE simulation of the circuit of shows total voltage and total current are nearly in
phase. The SPICE circuit file has a zero volt voltage-source (V2) in series with the
capacitor so that the capacitor current may be measured. The start time of 200 m sec
(instead of 0) in the transient analysis statement allows the DC conditions to stabilize
before collecting data. See SPICE listing “pf.cir power factor”.
The Nutmeg plot of the various currents with respect to the applied voltage Vtotal is shown
in The reference is Vtotal, to which all other measurements are compared. This is because
the applied voltage, Vtotal, appears across the parallel branches of the circuit. There is no
single current common to all components. We can compare those currents to Vtotal. Zero
phase angle due to in-phase Vtotal and Itotal . The lagging IL with respect to Vtotal is
corrected by a leading IC . Note that the total current (Itotal) is in phase with the applied
voltage (Vtotal), indicating a phase angle of near zero. This is no coincidence. Note that
the lagging current, IL of the inductor would have caused the total current to have a
lagging phase somewhere between (Itotal) and IL.
However, the leading capacitor current, IC, compensates for the lagging inductor current.
The result is a total current phase-angle somewhere between the inductor and capacitor
currents. Moreover, that total current (Itotal) was forced to be in-phase with the total
applied voltage (Vtotal), by the calculation of an appropriate capacitor value. Since the
total voltage and current are in phase, the product of these two waveforms, power, will
always be positive throughout a 60 Hz cycle, real power as in Figure. Had the phase-
angle not been corrected to zero (PF=1), the product would have been negative where
positive portions of one waveform overlapped negative portions of the other as in Figure.
Negative power is fed back to the generator. It cannot be sold; though, it does waste
power in the resistance of electric lines between load and generator. The parallel
capacitor corrects this problem.
Note that reduction of line losses applies to the lines from the generator to the point
where the power factor correction capacitor is applied. In other words, there is still
circulating current between the capacitor and the inductive load. This is not normally a
problem because the power factor correction is applied close to the offending load, like
an induction motor.
It should be noted that too much capacitance in an AC circuit will result in a low power
factor just as well as too much inductance. You must be careful not to over-correct when
adding capacitance to an AC circuit. You must also be very careful to use the proper
capacitors for the job (rated adequately for power system voltages and the occasional
voltage spike from lightning strikes, for continuous AC service, and capable of handling
the expected levels of current).
If a circuit is predominantly inductive, we say that its power factor is lagging (because
the current wave for the circuit lags behind the applied voltage wave). Conversely, if a
circuit is predominantly capacitive, we say that its power factor is leading. Thus, our
example circuit started out with a power factor of 0.705 lagging, and was corrected to a
power factor of 0.999 lagging.
2.9 Tong Tester
An ammeter is a measuring instrument used to measure the flow of DC electric current in
a circuit. Electric currents are measured in amperes, Moving iron ammeters use a piece or
pieces of iron which move when acted upon by the electromagnetic force of a fixed coil
of (usually heavy gauge) wire. This type of meter responds to both direct and alternating
currents (as opposed to the moving coil ammeter, which works on direct current only).To
measure larger currents, a resistor called a shunt are placed in parallel with the meter.
Most of the current flows through the shunt, and only a small fraction flow through the
meter. This allows the meter to measure large currents. More modern ammeter designs
are non-mechanical, or digital, and use an analog to digital converter (ADC) to measure
the voltage across the shunt resistor. The ADC is read by a microcomputer that performs
the calculations to display the current through the resistor. One problem with the use of
an ammeter is the need for the meter to be inserted into the circuit and become part of it.
The Megger is a portable instrument used to measure insulation resistance. The
Megger consists of a hand-driven DC generator and a direct reading ohm meter. A
simplified circuit diagram of the instrument The moving element of the ohm meter
consists of two coils, A and B, which are rigidly mounted to a pivoted central shaft
and are free to rotate over a C-shaped core These coils are connected by means of
flexible leads. The moving element may point in any meter position when the
generator is not in operation. As current provided by the hand-driven generator flows
through Coil B, the coil will tend to set itself at right angles to the field of the
permanent magnet. With the test terminals open, giving an infinite resistance, no
current flows in Coil A. Thereby, Coil B will govern the motion of the rotating
element, causing it to move to the extreme counter-clockwise position, which is
marked as infinite resistance.
2.11 Circuit Diagram:
Coil A is wound in a manner to produce a clockwise torque on the moving element. With
the terminals marked "line" and "earth" shorted, giving a zero resistance, the current flow
through the Coil A is sufficient to produce enough torque to overcome the torque of Coil
B. The pointer then moves to the extreme clockwise position, which is marked as zero
resistance. Resistance (Rl) will protect Coil A from excessive current flow in this
condition. When an unknown resistance is connected across the test terminals, line and
earth, the opposing torques of Coils A and B balance each other so that the instrument
pointer comes to rest at some point on the scale. The scale is calibrated such that the
pointer directly indicates the value of resistance being measured.
2.12 RESULT:- We have studied about the Phase shifter, Tong Tester, P.F.
EXPERIMENT NO:- 3
3.1 OBJECT:- Measure power and power factor in 3-phase load by (i) Two-wattmeter
method and (ii) One-wattmeter method.
3.2 APPARATUS REQUIRED:- Wattmeter, ammeter, connecting loads, 3- φ star
connection load, voltmeter.
Two wattmeter method is the most commonly used method for
measurement of power as this can be used for balanced & unbalanced of the used is
The reading of W1 wattmeter is given by is a balanced
W1 = E12 i1 cos (30+ φ)
While reading of W2 wattmeter is given by
W2 = E32 i3 cos (30- φ)
Since if it is a balanced circuit
|E12 | = |E32| = √3 |EPN|
|i1 | = |i3| = |i| (say)
Therefore W = W1 + W2
W = √3 EPH i cos (30+ φ) + √3 EPH i cos (30- φ)
= 2 √3 EPH i cos 30 cos φ
= 3 EPH i cos φ
W = √3 EL i cos φ
This is nothing but the 3- φ power in the balanced load.
3.4 CIRCUIT DIAGRAM:
Circuit Diagram Of Power factor Meter Using watt meter
3.5 OBSERVATION TABLE:-
S.NO. E I W1 W2 W1 + W2 W1 – W2 %
(VOLTS) (AMP) (WATT) (WATT) (WATT) (WATT)
Power Factor = cos φ
φ = tan-1
[√3 (W1-W2) / (W1+W2)]
φ = tan-1
[√3(-160 / 400)]
φ = tan-1
[-√3 × 0.4]
φ = -34.71
P.F. = cos φ = 0.82
Calculation of power factor:-
Φ = tan-1
[√3 (W1 – W2) / (W1 + W2)]
P.F. = cos Φ
We have performed the experiment of power factor measurement by
2- Wattmeter method. And the results are……………….
Power (active) = W1 + W2
Power factor = ……………
EXPERIMENT NO:- 4
4.1 OBJECT: - Calibrate an ammeter using DC slide wire potentiometer.
4.2 APPARATUS REQUIRED:- Potentiometer
The electromotive force (emf) of a cell is its terminal voltage when no current is flowing
through it. The terminal voltage of a cell is the potential difference between its electrodes.
A voltmeter cannot be used to measure the emf of a cell because a voltmeter draws some
current from the cell. To measure a cell's emf a potentiometer is used since in a
potentiometer measurement no current is flowing. It employs a null method of measuring
potential difference, so that when a balance is reached and the reading is taken, no current
is drawn from the source to be measured this is the basic circuit diagram for a
potentiometer. Point C is the sliding contact which can be adjusted for zero current
deflection through the galvanometer. In this method a uniform, bare slide wire AB is
connected across the power supply. If you were to connect a voltmeter between the +
power supply terminal and point A you would measure essentially zero volts. If you were
to now connect the voltmeter between the + power supply and point B you would
measure a voltage equal to the terminal voltage of the power supply which is
approximately 2.5volts. The potential relative to point A then varies from zero at A to
approximately 2.5 volts at B.
The cell whose emf is to be determined is then connected so that its e.m.f opposes the
potential along the wire. At some point C the potential difference between A and C is
exactly equal to the emf of the cell so that if the other terminal of the cell is connected to
the point C, no current will flow. The calibration procedure is to locate this point C using
a standard cell whose emf is accurately known (emf = 1.0186 volts). You then know that
at this point C the potential difference relative to point A is exactly 1.0186 volts. Since
the wire is uniform, the length of wire spanned is proportional to the potential drop and
the wire can now be calibrated in volts per cm. The emf of an unknown cell is then found
by finding a new point C whose potential is exactly equal to the emf of the unknown cell
and multiplying this new distance AC times the calibration factor determined using the
standard cell. It is crucial in this experiment that the current flowing through wire AB
remains constant throughout the experiment.
If the current varies then the potential at all points along the wire will vary and you
cannot trust your calibration. An ammeter is included in series with wire AB so that you
can monitor this current. The circuits used in this experiment are shown below. Since the
electromotive force of the standard cell is equal to the potential drop in the length of wire
spanned (measured from A) for a condition of balance and the same is true for the
unknown cell, the emf of each cell is proportional to the lengths of wire spanned. Thus
And the unknown e.m.f is given by
where x is the unknown emf and, is the emf of the standard cell, Lx is the length of wire
(AC) used for balancing the unknown cell, and Ls is the length of wire used for balancing
with the standard cell. If we have a test cell of emf, and internal resistance r supplying
current to a variable load resistor R then we will measure a terminal voltage V which is a
function of the load resistance R.
4.4 CIRCUIT DIAGRAM:
Use the experimental arrangement shown in Figure 2 for the calibration of
the potentiometer wire, using the standard cell. Start with your sliding
contact C near the center of the bridge. Press the contact C. The
galvanometer will probably deflect. Find a point C where there is no
deflection. Now close switch K1 and again adjust C for no deflection.
Pushbutton switch K1 (Figure 2) shorts out the protective resistance R1
and gives the galvanometer maximum sensitivity. Record the final setting
of the contact point C and known value of the emf of the standard cell.
Compute the calibration factor f in volts/cm.
Electromotive force (emf) of a test cell
Connect the test cell into the circuit as shown in Figure 3. Determine the
emf of this cell by again locating a point C where no galvanometer
deflection occurs when contact C is pressed. Remember to close switch
K1 for a finer adjustment of C. When no galvanometer deflection occurs
with the switch K1 closed the potential drop along the wire from A to C
exactly equals the emf of the test cell. Record the final balance position of
the contact C and the emf of the test cell.
Terminal Voltage of the test cell in use
Now adjust the load resistor R to 150 ohms. You must hold switch K2 down in
order for the circuit connecting R across the test cell to be complete. While
holding K2 down again balance the bridge as described above. When balanced,
again record the distance AC and compute the terminal voltage of the test cell.
Repeat this procedure for R = 100, 60, 30, 15, 10, 8, 6, and 4 ohms. Using your
measured value for the terminal voltage V and the resistance R, compute the
current I being supplied by the test cell for each value of R used.
Plot a graph with terminal voltage V on the vertical axis and current on the
horizontal axis. Draw the best straight line through your data points.
Determine the value of the internal resistance of the test cell from this
4.6 OBSERVATION TABLE:-
R = _____150 ohm AC = ____________ V = ____________ I =
R = ____________ AC = ____________ V = ____________ I =
R = ____________ AC = ____________ V = ____________ I =
R = ____________ AC = ____________ V = ____________ I =
R = ____________ AC = ____________ V = ____________ I =
R = ____________ AC = ____________ V = ____________ I =
R = ____________ AC = ____________ V = ____________ I =
R = ____________ AC = ____________ V = ____________ I =
R = ____________ AC = ____________ V = ____________ I =
4.7 RESULT:- Internal resistance of test cell from graph: r = _________
EXPERIMENT NO:- 5
5.1 OBJECT: - Calibrate a voltmeter using Crompton potentiometer
5.2 APPARATUS REQUIRED:-
24 dc battery (two quantity)
Standard cell (1.0813 volts)
Crompton potentiometer is a DC potentiometer in which a high precision could be
obtained without the use of long slide wire by adding 15extension coils each equals in
resistance to the whole of the slide wire .There are two moving contacts P1&P2, contact P1
sliding over the slide wire and contact P2 sliding over the studs connected to the resistance
coil. Balance condition is obtained more easily and quickly by means of the Coarse (dial)
adjustment and the fine (slide-wire) adjustment. A battery ‘B’ of about 2volts is connected
across the potentiometer through the resistors R1&R2 for controlling the current drawn from
the battery. A multiple circuit switch ‘S’ by means of which either the standard cell, Score
other emf is measured, can be connected to the galvanometer circuit.
5.4 CIRCUIT DIAGRAM:
STANDARDISATION:-Before putting the potentiometer in use, it is standardised
i.e. the potentiometer is made to give the direct readings by an adjustment of the
current from the battery ‘B’. For standardization a sell SC, whose emf is 1.0183volts
is connected to the terminals of the selector switch marked as SC. The moving
contact P2 is set at 1.00stad of the coils and moving contact P1 is set at 1.0183on the
slide wire. Now the resistance R1 and R2 are so adjusted that there is no deflection on
the galvanometer when key ‘K’ is passed. The potentiometer is now said to be
standardized. After standardization the position of R1 and R2 should not be
CALIBRATION OF VOLTMETER:-For calibration of volt-meter a potential
divider of a high resistance is connected across high voltage. The voltmeter under
calibration is connected across this potential divider in such a way that potential drop
across the voltmeter can be varied. The volt-ratio box is connected in a parallel with
volt-meter under the calibration to reduce the voltage across to a value, which is with
in the range of the potentiometer. Now this reduced potential difference is measured
on the potentiometer. The potential difference is measured on the potentiometer
multiplied by the ratio of the volt-ratio box gives the actual potential difference
across the volt-meter under calibration, to which the instrument reading can be
5.6 OBSERVATION TABLE:-
S.NO. Reading by Voltmeter
Reading by Potentiometer
5.7 RESULT:-In calibration voltmeter error is varied from …… to ….
EXPERIMENT NO:- 6
6.1 OBJECT: - Measure low resistance by Crompton potentiometer.
6.2 APPARATUS REQUIRED :-
1. Crompton potentiometer
3. 2V DC supply
4. Standard cell (1.0183 volts)
5. Unknown resistor
8. Connecting load
6.3 THEORY :- Crompton potentiometer is a DC potentiometer in which high precision
could be obtained without the use of a long slide wire by adding 15 extension coils, each
equal in resistance to the whole of the slide wire(fig.). There are two moving contacts P1
& P2 contact P1 sliding over the slide wire and contact P2 sliding over the study connected
to the resistance coil. Balance condition is obtained more easily and quickly by means of
the coarse (dial) adjustment and the fine (slide wire) adjustment. A battery ‘B’ of about 2
volts is connected across the potentiometer through the resistor R1 & R2 for controlling
the current drawn from the battery. A multiple circuit switch ‘S’ by means of which
either the standard cell, SC or others emf to be measured can be connected to the
6.4 CIRCUIT DIAGRAM:
6.5.1. Standardization: - For standardization a western type standard cell sc, whose end is
1.0183 volts, is connected to the terminals of selector switch marked as sc. The moving
contact P2 is set at 1.00 stud of the coils and moving contact P1 is set at 0.0183 on the
slide wire. Now the resistance R1 & R2 are so adjusted that there is no deflection in the
galvanometer where key K is pressed. The potentiometer is now said to be standardized
the position of R1 & R2 should not be disturbed.
6.5.2. Measurement of Resistance:-
A. Make the connection as shown in fig.
B. The unknown resistance should be such a magnitude with the current to be passed it,
the voltage drop across it does not exceed the range of potentiometer.
C. By varying the value of rheostat and take the value of current by ammeter and voltage
D. Take the value in 4-set and calculate the R=V/I & calculate the average of R.
6.6 OBSERVATON TABLE:-
S.NO. Current in ammeter
Voltage in potentiometer
R = V/I Actual
voltage of R
6.7 RESULT: - The value of unknown low resistance is ……………. Ω
EXPERIMENT NO:- 7
7.1 OBJECT: Measure Low resistance by Kelvin's double bridge.
7.2 APPARATUS REQUIRED: Bridge set, rheostat, galvanometer, connecting leads,
millimeter, and D.C. source.
7.3 THEORY: The Kelvin’s bridge is a modification of Wheatstone stone bridge and
provides greatly increased accuracy in the measurement of low value resistance. An
understanding of the Kelvin’s bridge arrangement may be obtain by study of the
difficulties that arrives in the whetstone bridge on account of the resistance of the leads
and the correct while measuring low value resistors.
We consider the bridge “r” represent the “R” to standard resistance “S” to galvanometer
connected indicated by dotted line. The connection will be either indicated by dots to
point “m” or to point “n”. When G is connected to the point in the resistance “r” of the
connecting leads is added to the standard resistance “S” resulting in the indication of to
low and indication for unknown resistance “R”. When the connection is made the point
“n” the resistance “r” is added to the unknown resistance resulting in the indication of too
high value for “R”.
We assume that P/Q=p/q
Eac = iR+i(p+q)r(p+q+r)+ is
Eab = p*Eac/P+Q
Ead = iR+ir*pp+qr
Eab = Ead (at the balance condition)
PP+Q Eac = i(R+rp/r+p+q)
Substitute the value of Eac then
R = PQ*S+qrp+q+r[P/Q-p/q]
It indicates that the r has no effect on the measurement.
7.4 CIRCUIT DIAGRAM:
EXPERIMENT NO:- 8
8.1 OBJECT:- Measure earth resistance using fall of potential method.
8.2 APPARATUS REQUIRED:- Earth tester Iron electrodes Wires for connecting
electrodes to earth tester etc.
8.3 THEORY:- The resistance of earth can be measured by an earth tester
Earth tester:- It is the special type of Megger and it has some additional features and they
1. A rotating current reverser
2. A rectifier
Both these additional feature consist of simple commentator made up of "L" shaped
segments. They are mounted on the shaft of the hand driver generator. Each commentator
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 other as the commentator rotator. The
second pair of each of set of brushes is positioned on commentator so that continuous
contact is made with one segment whatever the position of the commentator. The
indication of the earth tester depends upon the ratio of the voltage across the pressure coil
instrument and can operate on D.C. only. yet by including the reverse and rectifying
device it is possible to make measurement 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 elimination unwanted effects due to production of
back E.M.F. in the soil on account of electrolytic action. Also the instrument is free from
effects of alternating or direct current present in the soil.
8.4 CIRCUIT DIAGRAM:
The earth tester has four terminals P1.P2 and C1.C2 two terminal P1
and C1 are shorted to from 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 deflection of pointer on earth tester indictors the resistance of earth directly .By
moving electrodes P we get the resistance at different distance and plot a graph b/w earth
resistance and distance b/w electrodes C and P as shown is fig.
8.6 OBSERVATION TABLE:-
RESISTANCE IN EARTH
DISTANCE B/W C AND
Plot the graph between earth resistance & distance of electrodes between E&P.
EXPERIMENT NO:- 9
9.1 OBJECT: - Calibrate a single-phase energy meter by phantom loading at different power
9.2 APPARATUS: - One single phase energy meter, one single phase load, Stop watch
and connecting wires.
9.3 THEORY: -
Energy meter is an instrument which measures electrical energy. It is also known as watt-
hour (Wh) meter. It is an integrating device. There are several types of energy meters
single phase induction type energy meter are very commonly used to measure electrical
energy consumed in domestic and commercial installation. Electrical energy is measured
in kilo watt-hours (kWh) by this energy meter.
9.4 CONSTRUCTION: - A single phase induction type energy meter consists of
driving system, moving system, braking system and registering system. Each of the
systems is briefly explained below.
9.4.1 DRIVING SYSTEM: - This system of the energy meter consists of two silicon
steel laminated electromagnets. M1 & M2 as shown in fig.1The electromagnet M1 is
called the series magnet and the electromagnet M2 is called the shunt magnet. The series
magnet M1 carries a coil consisting of a few turns of thick wire. This coil is called the
current coil (CC) and it is connected in series with the circuit. The load current flows
through this coil. The shunt magnet M2 carries a coil consisting many turns of thin wire.
This coil is called the voltage coil (VC) and is connected across the supply it consist of
current proportional to the supply voltage. Short circuited copper bands are provided on
the lower part of the central limb of the shunt magnet. By adjusting the position of these
loops the shunt magnet flux can be made to lag behind the supply voltage exactly 90° .
These copper bands are called power factor compensator (PFC).
A copper shading band is provided on each outer limb of the shunt magnet (fc1 &fc2)
these band provides frictional compensation. Moving system: - The moving system
consists of a thin aluminum disc mounted on a spindle and is placed in the air gap
between the series and the shunt magnets. It cuts the flux of both the magnet forces are
produced by the fluxes of each of the magnets with the eddy current induced in the disc
by the flux of the other magnets. Both these forces act on the disc. These two forces
constitute a deflecting torque. Braking system: - The braking system consists of a
permanent magnet called brake magnet. It is placed near the edge of the disc as the disc
rotates in the field of brake magnet eddy current are induced in it. These eddies current
react with the flux and exert a torque. This torque acts in direction so that it opposes the
motion of disc. The braking torque is proportional to the speed of the disc. Registering
system: - the disc spindle is connected to a counting mechanism this mechanism records a
number which is proportional to the number of revolutions of the disc the counter is
calibrated to indicate the energy consumed directly in kilo watts-hour (kWh)
9.5 CIRCUIT DIAGRAM:
Let V = Supply voltage
I = Load current lagging behind V by Φ
Cos Φ = Load Power Factor (Lagging)
Ish = Current setup by Φsh in disc
Ise = Current setup by Φse in disc
Phase diagram will be as follows
Instantaneous deflecting torque
τd ∝ (ψshise-ψseish) where ψ & i are instantaneous values
Average deflecting toque
Td ∝ [ΦshIsecosΦ – ΦseIshcos (180-Φ)] where Φ & I are RMS values
Td ∝ [ΦshIsecosΦ + ΦseIshcosΦ]
Td ∝ [ΦshIse +ΦseIsh] cosΦ
We know Φsh ∝ V, Ise ∝I, Φse ∝ I, Ish ∝ V
Td ∝ [VI+VI] cosΦ
Td ∝ VIcosΦ
No of revolution made in t ∝ Energy consumed in t sec
K = No of revaluation made / KWh
9.6 OBSERVATION TABLE:-
Time period of Observation (T) = 30 min
Energy % Error
M = No of
units x 1
C = N / K
Meter constant K=
Energy Calculated = K/N KWh
9.8 RESULTS: - Study has been done on 1 phase energy meter and verification of
energy is done as shown in the above observation table.
EXPERIMENT NO:- 10
10.1 OBJECT: Measure self-inductance using Anderson's bridge.
10.2 Apparatus Required:
1. Anderson Bridge circuit with arms values.
2. Potentiometer for varying one arm
3. Three different value inductors.
4. Potentiometer with calibrated dial.
5. Five capacitors selected by a band switch.
6. Audio amplifier with its IC regulated power supply.
7. One KHz sine wave oscillator with its IC regulated power supply.
9. Main ON/OFF switches fuse and light.
Anderson’s Bridge enables the measurement of inductance in terms of a
capacitors and resistance. In experimental arrangement P,Q and R are non inductive
resistance arms of the bridge. A non inductive resistance S in series with the given coil of
unknown inductance ‘L’ is put in the fourth arm of the bridge between F&D.A variable
resistance r and a variable condenser c are put in the parallel to the resistance P .An audio
amplifier with speaker is put in between the point ‘E’ and ‘D’ and an audio frequency
oscillator with a value control is put between the terminal of ‘A’ and ‘C’. The condition
of balance in this case is that the potential at ‘D’ and ‘E’ are same. Under this condition
no current flows through path DE and the current in various branches are shown in fig
‘a’. At balance condition
I3=I4 and I5=I1+I2
Using Kirchhoff’s laws we have (in loop ABCD)
In loop BCD
I1P-I2(r+1/C) =0------------ (2)
In loop DCO
I2/C =I3R---------- (3)
Where f is the frequency of the A.C .shown by oscillator
C=Capacity of a condenser
S=Total resistance of the CD
It includes the resistance of the inductance coil and variable resistance S.
Substituting I3 from equation (3) in equation (1), we have
And eliminating I1 and I2 from equation (2) and equation (4)
Which is the relation satisfied by the bridge for steady currents and by equating real parts, we
And if P=Q, we have
1. The connection is made as shown in. Now keep the ratio arms (P:Q)1:1 and resistance
in the R branch should be of the order of the resistance in the CD branch.
3. Now adjust the condenser to some value ‘C’. Now adjust r in the way that the sound in
the speaker is reduced to minimum.
4. Note down the value of P, Q, R, r and C.
5. Constant Repeat the experiment by changing, keeping P,Q and R. Note down the
value of r and C.
10.5 Circuit Diagram:
Circuit diagram to measure the inductance of a given coil by Anderson Bridge method.
10.6 Observation Table: Frequency of the A.C. source=1 KHz
S. No. P(Ω) Q(Ω) R(Ω) r(Ω) C(µf) L=CR(Q+2R)
Calculate the value of L for each set by using formula is given by L=CR
(2r+Q) and the find the mean value
Inductance of the given coil= Henries.
EMI VIVA QUESTION
1. What is meant by measurement?
Measurement is an act or the result of comparison between the quantity and a predefined
2. Mention the basic requirements of measurement.
The standard used for comparison purpose must be accurately defined and should be
The apparatus used and the method adopted must be provable.
3. What are the 2 methods for measurement?
Direct method and
4. Explain the function of measurement system.
The measurement system consists of a transducing element which converts the quantity
to be measured in an analogous form. the analogous signal is then processed by some
intermediate means and is then fed to the end device which presents the results of the
5. Define Instrument.
Instrument is defined as a device for determining the value or magnitude of a quantity or
6. List the types of instruments.
The 3 types of instruments are
Electrical Instruments and
7. Classify instruments based on their functions.
8. Give the applications of measurement systems.
The instruments and measurement systems are sued for
Monitoring of processes and operations.
Control of processes and operations.
Experimental engineering analysis.
9. Why calibration of instrument is important?
The calibration of all instruments is important since it affords the opportunity to check
the instrument against a known standard and subsequently to errors in accuracy.
10. Explain the calibration procedure.
Calibration procedure involves a comparison of the particular instrument with either.
A primary standard
A secondary standard with a higher accuracy than the instrument to be calibrated or An
instrument of known accuracy.
11. Define Calibration.
It is the process by which comparing the instrument with a standard to correct the
12. Name the different essential torques in indicating instruments.
13. Name the types of instruments used for making voltmeter and ammeter.
Moving iron type
Hot wire type
14. State the advantages of PMMC instruments
No hysterisis loss
15. State the disadvantages of PMMC instruments
Cannot be used for ac m/s
Some errors are caused by temperature variations.
16. State the applications of PMMC instruments
m/s of dc voltage and current used in dc galvanometer.
17. How the range of instrument can be extended in PMMC instruments.
In ammeter by connecting a shunt resister
In voltmeter by connecting a series resister.
18. State the advantages of Dynamometer type instruments
Can be used for both dc and ac m/s.
Free from hysterisis and eddy current errors.
19. State the advantages of Moving iron type instruments
Can be used for both dc and ac
20. State the advantages of Hot wire type instruments
Can be used for both dc and ac
Unaffected by stray magnetic fields
Readings are independent of frequency and waveform.
21. What are the constructional parts of dynamometer type wattmeter?
Current limiting resister
Spindle attached with pointer
22. Write down the deflecting torque equation in dynamometer type wattmeter.
Td á VI CosÖ
23. State the disadvantages of Dynamometer type wattmeter.
Readings may be affected by stray magnetic fields.
At low power factor it causes error.
24. Name the errors caused in Dynamometer type wattmeter.
Error due to pressure coil inductance
Error due to pressure coil capacitance
Error due to methods of connection
Error due to stray magnetic fields
Error due to eddy current.
25. How the errors caused by pc inductance is compensated.
By connecting a capacitor in parallel to the resister.
26. How the errors caused by methods of connection is compensated
By using compensating coil.
27. Name the methods used for power measurement in three phase circuits.
(i) Single wattmeter method
(ii) Two wattmeter method
(iii) Three wattmeter method.
28. What are the special features to be incorporated for LPF wattmeter?
Pressure coil circuit
Compensation for Pressure coil current
Compensation for Pressure coil inductance.
29. Define Phantom loading.
Method by which energizing the pressure coil circuit and current coil circuits separately
is called phantom loading.
30. State the use of phantom loading.
Power loss is minimized.
31. Name the methods used in Wattmeter calibration.
By comparing with std wattmeter.
By using voltmeter ammeter method.
By using Potentiometer.
32. What are the types of energy meters?
33. Name the constructional parts of induction type energy meter.
Current coil with series magnet
Voltage coil with shunt magnet
34. How voltage coil is connected in induction type energy meter.
It is connected in parallel to supply and load.
35. How current coil is connected in induction type energy meter.
It is connected in series to the load.
36. Why Al disc is used in induction type energy meter.
Aluminum is a nonmagnetic metal.
37. What is the purpose of registering mechanism.
It gives a valuable number proportional to the rotations.
38. What is the purpose of braking mechanism.
It provides necessary braking torque.
39. Define creeping.
Slow but continuous rotation of disc when pc is energized and cc is not energized.
40. State the reason why holes are provided in Al disc.
To avoid creeping holes are provided on both sides of Al disc.
41. What is the basic principle used in potentiometer.
In potentiometer the unknown emf is measured by comparing it with a std known emf.
42. Name the potentiometer material used.
43. Define standardization.
It is the process by which adjusting the current flows through the potentiometer coil to
make the voltage across the std cell is equal.
44. State the applications of potentiometer.
Used for m/s of unknown emf
Used for ammeter calibration
Used for Voltmeter calibration
Used for wattmeter calibration
45. State the advantages of crompton potentiometer.
Easy to adjust
46. What are the practical difficulties in ac potentiometers.
Accuracy is seriously affected
Difficulty is experienced in standardization.
47. Classify ac potentiometers.
48. How the phase angle is measured in polar type potentiometers.
It is measured from the position of phase shifter.
49. Name some ac potentiometers.
Drysdale Tinsley potentiometer
Gall Tinsley potentiometer
50. State the advantages of ac potentiometers.
Can be used for m/s of both magnitude and phase angle Can be used for m/s of
inductance of the coil.
It is used in m/s of errors in CTS
51. State the applications of ac potentiometers.
M/s of self inductance.
52. State the advantages of instrument transformers.
Used for extension of range
Power loss is minimum
High voltage and currents can be measured.
53. State the disadvantage of instrument transformers.
Cannot be used for dc measurements.
54. What are the constructional parts of current transformer?
55. Name the errors caused in current transformer.
Phase angle error
56. Define ratio error.
The ratio of energy component current and secondary current is known as the ratio error.
57. How the phase angle error is created.
It is mainly due to magnetizing component of excitation current.
58. State the use of potential transformer.
Used for m/s of high voltage
Used for energizing relays and protective circuits.
59. Name the errors caused in potential transformer.
Phase angle error.
60. How the CT and PT are connected in the circuits.
CT is connected in series and PT is connected in parallel.
61. Classify resistance.
62. What is the range of medium resistance?
Resistance of about 1 ohm to 100 kilo ohms are called medium resistance.
63. Name the methods used for low resistance measurement.
Ammeter – voltmeter method
Kelvin double bridge method
Ohm meter method.
64. Name the methods used for medium resistance measurement
Ammeter – voltmeter method
Wheatstone bridge method
Carey foster bridge method.
65. Where high resistance m/s is required?
Insulation resistance of cables
High resistance circuit elements
Volume resistivity of a material
66. State the advantages of Wheatstone bridge method.
Free from errors
The balance is quit independent of source emf
67. State the advantages of Kelvin double bridge method.
Errors owing to contact resistance, resistance of leads can be eliminated by using this
Kelvin double bridge.
68. What are the constructional features of doctor ohmmeter?
Pointer with graduated scale.
69. Define megger.
The megger is an instrument used for the measurement of high resistance and insulation
70. Name the parts of megger.
It consists of a hand driven dc generator and a direct reading true ohm meter.
71. What is the range of low resistance?
Resistance of about 1 ohm and under are included in this class.
72. What is the range of medium resistance?
Resistance of 100 kilo ohms and above are usually termed as high resistance.
73. What ranges of resistance can be measured by using doctor ohmmeter.
0 to 500 micro ohms
0 to 5 milli ohms
0 to 50 milli ohms
0 to 500 milli ohms
0 to 5 ohms.
74. How resistance is measured in direct deflection method.
The deflection of galvanometer connected in series with the resistance to be measured
gives a measure of the insulation resistance.
75. Classify the cables according to their sheathing.
76. Name the leads present in megger.
77. How resistance is measured by using ohm meter method.
Series ohm meter method
Shunt ohm meter method.
78. How resistance is measured in loss of charge method.
In this method a capacitor is charged and discharged for a specific time period and from
this resistance is measured.
79. State the balance equation used in bridge methods.
The product of opposite branch resistances are equal.
80. State the advantages of price’s guard wire method.
In this method leakage current does not flows through the meter and therefore it gives
81. How the earth resistance is measured.
By using earth megger the value of surface earth resistance can be measured.
82. State the use of ac bridges.
AC bridges are used for the m/s of self and mutual inductance and capacitance.
83. State the balance equation used in ac bridges.
The product of opposite branch impedances are equal.
84. Name the bridge circuits used for the m/s of self inductance.
85. Name the bridge circuits used for the m/s of capacitance.
De Sauty’s bridge
86. Name the bridge circuits used for the m/s of mutual inductance.
The Heaviside Campbell bridge
The Campbell bridge.
87. Which type of detector is used in ac bridges?
Vibration galvanometers are used.
88. Name the ac sources used in ac bridges.
AC supply with step-down transformer
Motor driven alternator
Audio frequency and radio frequency oscillator.
89. In which cases audio frequency oscillators are used as ac source.
For high frequency ac requirement audio frequency oscillators are used.
91. Name the sources of errors in ac bridge m/s.
Errors due to stray magnetic fields
Eddy current errors
Frequency and waveform errors.
92. State the advantages of Maxwell-wein bridge.
The balance equation is independent of frequency and therefore more accurate.
93. State the disadvantage of Maxwell-wein bridge.
This method needs a std variable capacitor. Variable Capacitor is costliest.
94. State the disadvantages of Hay’s bridge.
The balance equation is dependent of frequency and therefore any changes in frequency
will affect the m/s.
95. State the use of Wein bridge.
It is used for the m/s of unknown capacitance and frequency.
96. What is the use of Campbell bridge?
This is used for the m/s of mutual inductance.
97. What is meant by inductometer?
The std variable mutual inductance meter is called as inductometer.
98. Define Q-factor of the coil.
It is the ratio between power stored in the coil to the power dissipated in the coil.
99. Name the components of iron loss.
Eddy current loss
100. Name the faults that occurs in cables.
Break down of cable insulation
Short circuit fault
Open conductor fault.
101. Name the loop test methods used in location of fault.
Murray loop test
Varley loop test.
102. How leakage errors are minimized in ac bridge circuits.
By using high grade insulation.
103 What is Cathode ray oscilloscope (CRO)?
Cathode ray oscilloscope is a instrument used for display, measurement and analysis of
waveforms and other phenomenon in electrical and electronic circuit
104 What are the basic component of a CRO?
CRO Circuit consists of following components:
1. Vertical deflection system 2. Horizontal deflection system 3.Synchronization circuit
1. Blanking circuit 5 Intensity modulation 6position control
7. Focus control 8. Cathode ray tube 9. Calibration circuit
105 What is the function of probe in CRO?
The probe performs the very important function of connecting the test circuit to
oscilloscope without altering, loading or otherwise disturbing the test circuit.
106 How many types of probe used in CRO?
There are three types of probe used in CRO:
1 Direct probe 2. Isolation probe 3. Detector probe
107 What are the functions of different probes used in CRO?
Direct Probe: direct probe avoids stay-pick up which may create problems when low
level signals are being measured. It is used for low frequency or low impedance circuit.
Isolation probe: Isolation probe is used in order to avoid the undesirable circuit loading
effects of the shielded probe.
Detector probe: when analyzing the response to modulated signals used in
Communications equipment like AM, FM and TV receivers, the detector probe functions
to separate the low frequency modulation component from the high frequency carrier.
108 What is the function of Attenuator in CRO?
The voltage in input terminal of the vertical amplifier causes the beam to deflect off the
CRT screen, is quite low in amplitude. So that high amplitude signals may be displayed,
an attenuator network is placed between the vertical input terminals of the vertical
amplifier. The main function of the attenuator is to reduce the amplitude of the vertical
input signal before applying it to vertical amplifier.
109 Which device is used for the source of emission of electrons in a CRT?
A barium and strontium oxide coated cathode is used for the source of emission of
electrons in a CRT.
110 What is the function of Aquadag in a CRO?
An Aqudage is used in a CRO to collect secondary emission electrons.
111 What is the function of electron gun assembly used in CRT?
The electron gun assembly produces a sharply focused beam of electrons which are
accelerated to high velocity .this focused beam of electrons strikes the fluorescent screen
with sufficient energy to cause a luminous spot on the screen.
112 What is the function of electron gun used in CRT?
Ans: The source of focused and accelerated electrons beam is the electron gun. The
electron gun emits electrons and forms them into a beam consist of a heater, a cathode, a
grid, a pre-accelerating anode, a focusing anode and an accelerating anode.