1. ARYA INSTITUTE OF ENGINEERING TECHNOLOGY AND
MANAGEMENT
SUBJECT: ELECTRICAL MEASUREMENT
CODE: 4EE3A
BRANCH: ELECTRICAL ENGINEERING
PREPARED BY
UMESH DHAKAD
2. ELECTRICAL
MEASUREMENT
UNIT 3 :
Potentiometers: Construction,
operation and standardization of
DC
Potentiometers–slide wire &
Crompton potentiometers.
Use of potentiometer for
measurement of resistance and
voltmeter and ammeter
calibrations. Volt ratio boxes.
Construction, operation and
standardization of AC
potentiometer– in-phase and
quadrature potentiometers.
Applications of AC
potentiometers.
3.1 Potentiometers
3.1.1 Principle of potentiometer
3.2 D.C potentiometer
3.2.1 Types of D.C potentiometer
3.2.1.1 Slide wire potentiometer
3.2.1.2 Crompton potentiometer
3.2.2 Application of D.C potentiometer
3.3 A.C potentiometer
3.3.1Standarization of A.C potentiometer
3.3.2 Types of A.C potentiometer
3.3.2.1 Polar type
3.3.2.2 Co-ordinate type
3.3.3 Application of A.C potentiometer
3.3.4 Advantages of A.C potentiometer
3. 3.1 Potentiometers
Introduction:-
A potentiometer is a direct measuring instrument which is designed to
measure an unknown electromotive force (emfs) by comparing it with a known
voltage or known emf.
The unknown emf may also have potential difference (P.D.) or voltage produced by the
flow of a known current in the circuit, The known voltage may be supplied from a
standard cell or any other reference voltage source.
A potentiometer uses balance or null condition during the measurement of an
unknown emf . So, here it should be noted that in potentiometer the measurement
in done at zero deflection in galvanometer. Thus actually no current flows in the
circuit of the unknown emf during the measurement process and hence the power
consumption in such circuit is also zero. The potentiometer are capable of a high
degree of accuracy because the measurement result does not depend on the actual
deflection of a pointer but only depends on the accuracy with which the refrence
voltage is known.
The potentiometers can also be used for the measurement of unknown current by
measuring the voltage drop across the standard resistor due to the unknown
current.
4. 3.1.1 Principle of potentiometer
This is a very basic instrument used for comparing emf two
cells and for calibrating ammeter, voltmeter and watt-meter.
The basic working principle of potentiometer is very very
simple.
Suppose we have connected two battery in head to head and
tale to tale through a galvanometer. That means the positive
terminals of both battery are connected together and
negative terminals are also connected together through a
galvanometer as shown in the figure below.
5. Here in the figure it is clear that if the voltage of both battery cells is exactly
equal, there will be no circulating electric current in the circuit and hence the
galvanometer shows null deflection. The working principle of potentiometer
depends upon this phenomenon.
Now let's think about another circuit, where a battery is connected across a
resistor via a switch and a rheostat as shown in the figure below, there will be a
voltage drop across the resistor. As there is a voltage drop across the resistor, this
portion of the circuit can be considered as a voltage source for other external
circuits. That means anything connected across the resistor will get voltage. If
the resistor has uniform cross section throughout its length, the electrical
resistance per unit length of the resistor is also uniform throughout its length.
Hence, voltage drop per unit length of the resistor is also uniform. Suppose the
electric current through the resistor is i A and resistance per unit length of the
resistor is r Ω. Then the voltage appears per unit length across the resistor
would be 'ir' ans say it is v volt.
Now, positive terminal of a standard cell is connected to point A on the sliding
resistor and negative terminal of the same is connected with a galvanometer.
Other end of the galvanometer is in contact with the resistor via a sliding
contact as shown in the figure above. By adjusting this sliding end, a point like
B is found where, there is no electric current through the galvanometer, hence
no deflection of galvanometer. That means emf of the standard cell is just
balanced by the voltage drop appears across AB. Now if the distance between
point A and B is L, then it can be written emf of standard cell E = Lv volt. As v
(voltage drop per unit length of the sliding resistor) is known and L is measured
from the scale attached to the resistor the value of E i.e. emf of standard cell can
also be calculated from the above simple equation very easily.
6. 3.2 D.C potentiometer
The d.c. potentiometer is an accurate and verratile
instrument which is used for the measurement of current
voltage and resistance and calibration of ammeter
voltmeter and wattmeter.
In d.c. potentiometers only the magnitude of the unknown
emf is made equal to potentiometer voltage drop to obtain
balance condition.
7. 3.2.1 Types of D.C potentiometer
3.2.1.1 Slide wire potentiometer
8. Construction:-
It consists a slide wire AB usually made of platinum silver alloy
and the sliding contact of a copper-gold-silver alloy. Which results
in a good contact minimum wear of slide wire and free from
thermo-electric emfs.
The slide wire has a uniform cross section which gives uniform
resistance alonge its entire length.
The constant emf source (battery) E1 supplies the working steady
current which may be adjusted by changing the rheostat
setting(Rh).
The unknown source emf E2 whose voltage is to be measured is
connected in parallel with the slide wire (AB). A galvanometer G is
also connected in series with E2 along with switch S.
9. Working:-
The working of slide wire potentiometer is
based on principle that the potential drop
(V) across any two points of slide wire is
directly proportional to its length (l). Or
we can say that
V=xl
Where x is the voltage drop across per unit
length of slide wire.
Now when switch S is open:
Then the working current I will flow
through the whole slide wire AB. So, the
potential drop across the slide wire will be
given as
Vab=xL………………(i)
10. When switch S is closed:
Then a current will flow through the galvanometer in
direction A to C.
To obtain the balance condition the point C is so chosen by
the help of jockey (J) that the both emfs E1 and E2 are made
equal and no current flows through the galvanometer.
The the potential drop across the slide wire AC will be given
as Vab=xl…………..(ii)
Now dividing eq. (i) by eq. (ii)
Vab/Vac=xL/xl
Or Vab/Vac=L/l
So unknown emf. Will be given as
Vac=Vab*l/L
12. As the resistance of slide wire is known accurately then the
voltage drop along the slide wire can also be controlled by
adjusting the working current. Thus, the process of adjusting
the working current so as to match the voltage across a
portion of sliding wire against a standard reference source is
known as standarization of the potentiometer.
Standarization is must before using a potentiometer.
It unbalance potentiometer has a direct measuring instrument.
Generally a western cadmium cell is used as a reference
source which have an emf of 1.0186v. The total length of slide
wire is 200 cm and the resistance of the slide wire is 200
ohm.
Which the switch S is through to calibrate a position and the
sliding contact or jocky is placed at 101.86 cm mark on the
slide wire scale.
13. The rheostate Rh is now adjusted untill the
galvanometer shown no deflection. This arrangement
will give the working current.
Under these conditions the voltage drop along the 101.86
cm portionof the slide wire is equal to the standard cell
voltage.
15. Construction : It consist one dial switch with 15 steps
and each step have precision resistor of 10 ohm .Thus
the dial switch has a total resistance of 150 ohm.
In addition to dial switch it also consist circular slide
wire which has resistance of 10 ohm.
The working current of potentiometer is 10 m amp.
So each step of dial switch refers a voltage drop of 0.1 v
and hence the dial switch may be used for a voltage
range up to 15 v.
Similarly the slide wire has a voltage range upto 0.1 v
Since the slide wire is provided with 200 scale divisions
so the voltage drop across each scale will be given as
0.1/200 = 0.0005v.
16. Hence we measured the approximate reading upto
0.0001 v easily.
It also consist a double through switch which enable the
connection the either the standard cell or the
unknown emf whose voltage to be measured.
In initial condition a protective resistance about 10 k
ohm is used in the galvanometer circuit and near
balance condition at maximum sensitivity of
galvanometer the protective resistance is short
circuited.
17. WORKING: The working of potentiometer can be defined
in following two sections.
(a) Standarization or calibration
(b) Measurement operation.
(a) Standarization or calibration: Before using the
potentiometer it should be standarize by a standard cell
which makes it a direct measuring instrument .
For standarization generally we use a western cell of 1.086 v.
for which the double through switch S is throw to the
calibrate position and the dial resistance is put at 1.0 v with
0.0186v setting of the slide wire. Hence the combination of
dial switch and slide wire is set to the standard cell voltage.
18. The rheostat is adjusted to obtain zero deflection in the
galvanometer. In initial condition the protective resistance is
kept for safety purpose of the equipment. However, near the
balance condition the protective resistance is shorted fore
maximum sensitivity of the galvanometer.
(b) Measurement Operation
After standardization the double throw switch S is thrown to the
operate position and hence the unknown emf is connected into
the potentiometer circuit.
Now again the potentiometer is balanced by adjustment of dial
and slide wire but rheostat setting should be remain unchanged.
The value of unknown emf is read directly from setting of dial
and slide wire.
The standarization is again checked after the completion of
measurement. If it is not satisfactory then the unknown emf is
measured again and the standardization is again rechecked.
19. 3.2.2 Application of D.C potentiometer
As we have discussed that the D.C. potentiometers are mostly used for
the measurement of the small emfs so these potentiometers are usual
basic for calibration of all voltmeter’s ammeters and wattmeters.
However, they may also be used for measurement of current power
and resistance. Since here we are using d.c. potentiometer the
instruments to be calibrated must be either d.c. moving iron or
electrodynamometer types.
(i) Measurement of small unknown emfs
It is possible to measure measure the small unknown emfs upto 1.6v
with a high accuracy as we may read a small voltage of 0.01 µv
accurately.
20. (ii) Measurement of Resistance
The Potentiometer method of resistance measurement is suitable for
measurement of low resistances. In this figure the unknown resistance R
is connected in series with a standard resistor S. A rheostate is used to
control the current through the circuit.
A rheostat is used to control the current through the circuit. A double
pole double throw switch is used.
21. When the switch is at position 1, 1’ the unknown resistance R will
be connected to the potentiometer circuit.
Let we assume that the reading of the potentiometer is Vr in this
position
Vr=IR
or I=Vr/R……………..(i)
Now when the switch is thrown top[osition 2,2’ the standard
resistance S will be connected to the potentiometer circuit.
In this position let the reading of the potentiometer is Vs.
Vs=IS
Or I=Vs/S……………..(ii)
From eq. (i) and (ii)
Vr/R=Vs/S
So the unknown resistance R, will be given as:
R=Vr*S/Vs
23. The most requirement to this calibration process is that a
suitable stable d.c. voltage supply most be available to
ensure a corresponding change in the voltmeter calibration
as per change in the supply voltage.
In this figure a potential divider network is used which
consists of two rheostats, one for coarse and the other for
fine control of calibrating voltage. These rheostats are
connected to the supply source and with the help of these
controls we can adjust the voltage such that the pointer
coincides exactly with a major division of the voltmeter.
The voltage across the potentiometer has maximum range
of 1.6V, to achieve high accuracy we will have to use of low
voltage range of 1.6v, to achieve high accuracy we will have
to use of low voltage range for voltage less than 1.6v and use
appropriate tapings on volt-ratio box for voltage higher
than 1.6v.
25. A standard resistance S of suitable value which have sufficient
capacity is placed in series with the ammeter under calibration.
The voltage across the standard resistance is measured by the
help of the potentiometer and the current through the standard
resistance is computed by the help of the potentiometer and the
current through the standard resistance is computed by the
ammeter:
As current I=Vs/S
Where Vs= voltage across the standard resistance as indicated by
the potentiometer
S=resistance of the standard resistor.
Since the resistance of the standard resistor is accurately known
and the voltage across the standard resistor in measured by a
potentiometer, this calibration method of an ammeter is highly
accurate. We may also draw a calibration curve which indicates
the errors various scale reading of the ammeter.
27. Since the D.C. potentiometer is an accurate and versatile instrument
for the measurement of direct voltages and currents so, it is obvious
that potentiometer principle should also be appliedfor the
measurement of alternative voltages and currents.
The principle of a.c. potentiometer is the same as that of the d./c.
potentiometer. The most important difference between a d.c. and a.c.
potentiometer is that in d.c. potentiometer only the unknown emfs is
balanced against a known potentiometer voltage dropn wheres in case
of a.c. potentiometer the two voltages are balanced in magnitude as
well as in phase.
Hence, by certain modifications and additional features incorporated
in d.c. potentiometer it may also be used for measurement of a.c.
quantities.
28. Since an a.c. potentiometer is a complication features incorporation
