2. What is Potentiometer?
A potentiometer is an instrument used to
measure the potential in a
circuit.A potentiometer is an instrument designed
to measure an unknown voltage by comparing it
with a known voltage.
The known voltage may be supplied by a
standard cell or any other known voltage
reference source. Measurements using
comparison methods are capable of a high
degree of accuracy because the result obtained
does not depend upon the actual deflection of a
pointer, as is the case in deflectional methods,
but only upon the accuracy with which
the voltage of the reference source is known.
3. Another advantage of the potentiometers is that since a
potentiometer makes use of a balance or null condition, no
current flows and hence no power is consumed in the
circuit containing the unknown emf when the instrument is
balanced. Thus the determination of voltage by a
potentiometer is quite independent of the source
resistance.
Since a potentiometer measures voltage, it can also
be used to determine current simply by measuring
the voltage drop produced by the unknown current piling
through a known standard
resistance.The potentiometer is extensively used for a
calibration of voltmeters and ammeters and has, in fact,
become the standard for the calibration of
these instruments.For the above mentioned advantages,
the potentiometer has become very important in the field
of electrical measurements and calibration.
4. Construction of DC Potentiometer:
The working principle of potentiometers is based on the figure
shown below, which shows the circuit diagram of the basic
slide wire potentiometer
5. Working Principle of basic dc
Potentiometer
With switch 'S' in the "operate" position and the
galvanometer key K open, the battery supplies
the "working current" through the rheostat R and
the slide wire.The working current through the
slide wire may be varied by changing the rheostat
setting.The method of measuring the unknown
voltage, E, depends upon finding a position for
the sliding contact such the galvanometer shows
zero deflection, i.e., indicates a null condition,
when the galvanometer key, K. is closed.Let us
now discuss the working principle of basic dc
potentiometer.
6. Zero galvanometer deflection or a null means that the
unknown voltage, E, is equal to the voltage drop E1,
across portion ac of the slide wire. Thus the
determination of the value of unknown voltage now
becomes a matter of evaluating the voltage drop
E1 along the portion ac of the slide wire.The slide wire
has a uniform cross-section and hence uniform
resistance along its entire length.A calibrated scale in
cm and fractions of cm is placed along the slide wire
so that the sliding contact can be placed accurately at
any desired position along the slide wire.
Since the resistance of slide wire is known
accurately, the voltage drop along the slide wire can
be controlled by adjusting the value of working current
in the basic dc potentiometer.The process of
adjusting the working current so as to match
the voltage drop across a portion of sliding wire
against a standard reference source is known as
"Standardisation".
7. It is very important that internal thermoelectric
EMFs in a potentiometer are minimum.The use
of manganin resistors helps in this direction.It is
desirable that all the parts work at the same
temperature.Therefore, all the parts are covered
in a single case.This has the added advantage of
protecting the contacts from fumes and dust
which may cause corrosion and appearance of
voltaic EMFs at the joints.
Potentiometers designed especially for
thermocouple measurements have copper
terminals.In order to prevent leakage, all the parts
must be enclosed, so as to protect them from
moisture.The working parts are normally
mounted on ebonite or Keramot panels.
8. Advantages
A potentiometer is highly sensitive
It is a highly accurate instrument because it uses
the comparing method for measurements, where
the voltage of a reference source is known
It has a wide range of measurement
Disadvantages
Its operation is very time consuming
9. Applications of dc potentiometer
Calibration of voltmeter
Calibration of voltmeter requires a suitable stable
DC supply of voltage. If there are little change
occurs in the supply voltage, it can affect the
calibration process of the voltmeter.
Arrangement for calibration of a voltmeter by
potentiometer is shown below:
11. The network consists of a potential divider
network consisting of 2 rheostats. One is for
coarse adjustments and the other is for fine
adjustments. Both the rheostats are connected to
the stabilized supply voltage. With the help of
these rheostats, it is possible to adjust the supply
voltage so that the pointer coincides exactly with
a major division of the voltmeter.
The voltage across the voltmeter is stepped down
to a suitable value with the help of a voltage-ratio
box. For accuracy of measurements, it is required
to measure voltage near the maximum range of
the potentiometer.
And if the potentiometer reading does not match
with the reading of the voltmeter. A positive or a
negative error is indicated.
12. Calibration of ammeter
A standard resistor S with a high current carrying
capacity is connected in series with an ammeter
for a test. The voltage drop across the standard
resistor is measured by the potentiometer.
Circuit diagram for calibration of an ammeter by
potentiometer is shown below:
14. Calibration of ammeter
Now the current through the resistor S can be
computed
I = Vs/S
Where,
Vs = voltage drop across the resistor S
S = resistance of the resistor
Now, by comparing the ammeter reading with the
current found by calculation, a positive or
negative error can be indicated if they do not
match.
This method of calibration is very accurate
because the resistance of the resistor S is exactly
known and the current across the S is calculated.
15. Calibration of wattmeter
A standard resistor is connected in series with a
current coil of wattmeter. The current coil is supplied
with a low voltage current and the current through the
current coil is measured by measuring the voltage
drop across the standard resistor divided by the value
of the standard resistor.
The potential coil of the wattmeter is supplied from
normal supply through the potential divider. The
voltage across the potential coil is measured directly
by the potentiometer.
Then the power is calculated by
P = VI
Where,
V = voltage across the potential coil
I = current through the current coil of the wattmeter
Now, the wattmeter reading can be compared with the
calculated value.
16. Measurement of current
The unknown current I, whose value is to be
measured is passed through a resistor R. The
value of the resistor is such that voltage drop
across it may not exceed the range of the
potentiometer.
The circuit diagram of current measurement by
potentiometer is shown below:
So, the value of the unknown current is the
voltage drop across the resistor divided by the
value of the resistor.
I = V/R
18. Measurement of resistance
An unknown resistance is connected in series
with a standard resistance S. A rheostat controls
the current in the circuit. A two-pole double throw
switch is also used in the circuit. The circuit is
shown below:
20. Measurement of resistance
When the two poles double throw switch is put in
position 1, the unknown resistance is connected to
the potentiometer.
Let the reading of the potentiometer be VR
VR = IR (i)
Now the switch is put in position 2, this connects the
standard resistor S to the potentiometer.
Let the reading of potentiometer be Vs
Vs = IS (ii)
From i and ii
VR/Vs = IR/IS
R = (VR/Vs)*S
The value of R is calculated accurately.
21. Measurement of power
In the measurement of power, 2 measurements
are made. One is across the resistor S connected
in series with the load and the other is across the
output terminals of voltage—ratio box.
The load current is calculated from the voltage
drop across the standard resistor
23. Measurement of power
The voltage drop across the load is calculated by
the potentiometer reading across the output
terminal of the voltage-ratio box
Load current I = VS/S
Where VS is the voltage drop across the standard
resistor
The voltage drop across the load Vl = kVR
Where, k = multiplying factor of the voltage-ratio
box.
So, the power consumed, P = VLI
P = K*VR*(VS/S)
24. AC POTENTIOMETER
The potentiometer is an instrument that is used
for the measurement of potential differences
across a known resistance between two terminals
of a circuit. Potentiometers are of two types DC
potentiometer and AC potentiometer. The working
principle of both the potentiometer is the same
except for one difference
25. What is AC potentiometer?
Alternating current (AC) potentiometer is the
potentiometer in which the magnitude and the
phase angle of unknown emf are to be compared
with the known emf to obtain balance.
The working principle of the AC potentiometer is
the same as the DC potentiometer. But there is a
difference between both the potentiometer, that is
in the DC potentiometer only the magnitude of
unknown voltage is compared with the known. On
the other side, in the AC potentiometer, the
magnitude and the phase angle of unknown emf
are to be compared.
26. Thus a DC potentiometer can’t be used for AC
measurements. So, some modifications and
additions have to be made for ac measurements.
The following points must be considered
The slide wire and the resistance coil of an ac
potentiometer should be non-inductive, this is to
be done to avoid errors in reading.
The AC supply source should be free from
harmonics, because balance may not be
achieved in presence of harmonics.
The reading is affected by the external magnetic
field, so they must be eliminated in the time of
measurements.
The AC supply source should be sinusoidal.
27. Types of ac potentiometer
Polar potentiometer
In this type of potentiometer, the unknown emf is
measured in polar form. This means that the unknown
emf is measured in terms of its magnitude and its
relative phase. The magnitude is measured by one
scale and the phase is indicated by another scale.
There is provision for reading phase angle up to 360
degrees
Coordinate potentiometer
In this type, the unknown emf is measured in
cartesian form. It has two different scales to read the
in-phase V1 and the other is quadrature V2. There is
provision is made in this potentiometer to read both
positive and negative values of voltages and cover all
angles up to 360degree
28. What is a polar potentiometer?
A polar potentiometer measures the unknown
voltage in the polar form. Polar form’ refers to the
indication of unknown voltage in magnitude and
the relative phase of the quantity.
One scale of the potentiometer indicates the
magnitude while another scale indicates the
phase w.r.t. some reference axis.
Drysdale polar potentiometer is an example of a
polar potentiometer.
29. Drysdale polar potentiometer
n the Drysdale polar potentiometer, the phase-
shifting transformer indicates the phase of
unknown voltage, and the position of the slide
wire indicates the magnitude.
A Diagram of the Drysdale polar potentiometer is
shown below:
31. Construction
Ammeter is an electrodynamometer-type
ammeter because it works both on AC and DC.
D’arsonval galvanometer is used for
standardization of the potentiometer, and later
during the measurement of the unknown voltage
vibration galvanometer is used.
The slidewire should be non-inductive. The polar
potentiometer also consists of 2 stator windings
and rotor winding
32. Working
Drysdale potentiometer is an AC potentiometer i.e. it
uses AC supply for the measurements.
First, the potentiometer should be calibrated or
standardized.
The standardization of polar type ac potentiometer is
done by DC supply and a standard cell. For
standardization, a standard cell is connected with
slide wires through the d’arsonval galvanometer.
The slide wire is fixed at the same voltage as the
standard cell (1.0186V).
On the other side, an ammeter is connected with a
rheostat through a dc supply. Now the rheostat should
be adjusted in such a way that the galvanometer
indicates zero reading.
At this point, the value of standard current at ammeter
should be noted.
The phase shifter circuit remains constant for
unknown supplied voltage while the phase can be
33. The phase shifter consists of 2 stators and rotor
winding. Some air gap is present in between the
windings. 90° phase shift is present between both stator
windings.
If necessary, the capacitor and the resistor should be
adjusted to keep the 90° phase shift between both stator
windings. These are being adjusted until the ammeter
shows an exact similar current value which it shows
during standardization for all positions of the rotor.
These adjustments of slide wire and the rotor helps in
achieving a balance of the potentiometer.
After standardizing the potentiometer and tuning the
phase shifter, the unknown voltage should be
measured.
The unknown voltage is measured by adjusting the slide
wire and the rotor. By adjusting the slide wire, the
vibration galvanometer indicates the zero reading which
gives the magnitude of the unknown voltage. And slight
adjustment in the rotor dial gives the phase of the
unknown voltage.
34. Polar type AC Potentiometer -
Construction & Working
The Drysdale Tinsley AC Potentiometer is a polar-
type potentiometer, which measures the magnitude
(V) in one scale and phase (θ) in another scale. The
complete connection diagram of the Drysdale Tinsley
ac potentiometer is shown below. Where,T.I. -
Transfer instrument (precision type electro-
dynamometer ammeter)
DPDT - Double pole double throw
SPDT - Single pole double throw
G - D'Arsonval galvanometer
VG - Vibration galvanometer
B - Standard battery
POS 1 - position 1
POS 2 - position 2
35. When an ac voltage measurement is done by taking a
reference ac voltage supply, the conditions that must be
satisfied are,Both the voltages should have same frequency.
Their phases should be same.
Their magnitudes should also be same at all the instants.
It is very difficult to satisfy all three conditions if we use a
separate reference source. Hence, in this instrument we
connect the unknown ac voltage to a phase-shifting
transformer whose one stator winding is connected directly
to the unknown supply and the other stator winding is
connected to the same supply through a variable resistor
and a capacitor.
By varying the resistance and capacitance of the second
winding, the current through it can be made exactly in
quadrature with the supply. This results in the production of a
rotating magnetic field (RMF) (i.e., due to phase splitting)
which links with the rotor winding to induce an emf in it with
the same frequency as that of supply and whose phase
angle can be selected by changing the rotor position. Hence,
36.
37. For the measurement of magnitude with a normal dc potentiometer,
all the resistors and the slide wire are replaced by standard non-
inductive resistors and slide wire. So that its resistance does not
vary with frequency and waveform. Procedure for the Measurement
:
To measure an unknown ac voltage using this potentiometer, first,
the meter is standardized. For the standardization, all the three
DPDT switches are thrown to position 1 (POS 1), and the current
through ammeter (A) for which the D Arsonval galvanometer (G)
gives null deflection is noted.
Now, the DPDT switches are thrown to position 2 (POS 2) which
connects the rotor terminals of phase-shifting transformer to supply
terminals of the potentiometer, vibration galvanometer to detector
terminals, and the unknown ac voltage to potentiometer test
terminals.
Now, the current through the ammeter is made equal to the current
through it when dc supply was connected by varying the standard
resistor R and the balance is obtained in the vibration galvanometer
by changing the slide wire contact position and the phase shifter's
rotor position. Hence, the magnitude and phase of the unknown ac
voltage are obtained from the slide wire position and rotor position
readings respectively.
38. Functions of Transfer Instrument and
Phase Shifting Transformer :
The function of the phase-shifting transformer is,
To produce the rotating magnetic field which passes
through the air gap between its stator and rotor and
induces an emf in the rotor winding. To provide the
required phase shifting of the rotor induced emf by
adjusting the rotor position. The rotor position can be
adjusted by adjusting the rotor angle with respect to the
null pointer.
Now, the induced emf in the rotor windings due to two
stator windings is given by,
E1 = KI sinωt cosθ
E2 = KI sin(ωt + 90) cos(θ + 90)
39. Therefore, the resultant emf is given by,
E = E1 + E2
E = KI [sinωt cosθ + sin(ωt + 90) cos(θ + 90)]
We know that
sin(ωt + 90) = cosωt and cos(θ + 90) = -sinθ.
∴ E = KI sinωt (ωt - θ)
From the above, it is clear that the rotor emf has
constant amplitude and the phase angle is given
by the rotor deflection θ.
40. Potentiometer - Construction &
Working
A coordinate type potentiometer is a combination of two
potentiometers. One of the potentiometers carries a
current in-phase with the supply voltage and it is called
an 'In-phase Potentiometer'. The other potentiometer
carries the current in quadrature with supply voltage
and it is called a 'Quadrature Potentiometer'. Gall-
Tinsley Coordinate Type Potentiometer :
The connection diagram of this potentiometer is shown
in the below figure. T1 and T2 are the two step-down
transformers fed from a single-phase supply. The
supply to T2 is obtained through the series combination
of variable capacitor Cs and variable resistor Rs for
splitting the phase. The exact quadrature in phase is
obtained by adjusting Rs and Cs. Here, ab and cd are
sliding contacts of In phase and Quadrature
potentiometer respectively, and rheostats R1 and R2 are
used for current adjustments.
43. VG is a vibrational galvanometer tuned to the
supply frequency. The ammeter A (reflecting
electro-dynamometer type) ensures current in
both In-phase and Quadrature Potentiometers
slide wire at standard value. Similarly, the
reversing switches RS 1 and RS 2 of two
potentiometers are used to reverse the direction
of the unknown emf across its slide wires. S2 is a
selector switch for placing unknown voltages to
be measured in the circuit.
The In-phase potentiometer measures the
component of unknown voltage which is in phase
with its slide wire current. Let its value be V1 and
the component which is in phase with the
Quadrature potentiometer current is measured on
it and it is the quadrature component of unknown
voltage. Let its value be V2.
44. Then the magnitude of the unknown voltage and
phase angle with respect to supply voltage is
given by,
46. Standardization of Potentiometer :
The dc standardization of the in-phase potentiometer
is done by connecting the battery B through the
switch S1 and changing the multiple circuit switch
S2 to position 1-1. The vibrational galvanometer is
replaced by a galvanometer for this purpose. The
electro-dynamometer ammeter is tuned to zero
position on direct current and this setting is left
untouched.
The switches S1 and S2 are again brought back to the
initial position. The alternating current is adjusted in
the in-phase potentiometer by rheostat R1 to give zero
deflection of the milli-ammeter. The magnitude and
phase of the quadrature potentiometer's current are
47. The switch S2 is brought to position 3-3. The dial settings
of the in-phase potentiometer are done to read a value
of M i (∵ i is the primary current, emf induced in the
secondary winding = 2π f Mi). Where i is the standard
alternating current in the in-phase potentiometer.
The magnitude and phase of the current in the
quadrature potentiometer are adjusted through rheostat
R2 and variable resistance Rs of the phase splitting
device to obtain the exact balance which is indicated by
the vibration galvanometer.
The switch S2 is again brought to position 2-2. In this
position, two slide circuits and a vibration galvanometer
are in series with the unknown voltage. Now, the
potentiometer is ready to measure the two components
of unknown voltage. The balance is obtained by
adjusting the settings of sliding contact a and c together
with the reversing switches RS 1 and RS 2 if necessary.
48. Applications of AC Potentiometer
The AC potentiometer has numerous applications.
The few of them are explained below in details.
1. Voltmeter Calibration – The AC potentiometer
directly measures the low voltages up to 1.5V. The
higher voltage is measured by either using the volt
box ratio or two capacitors in series with the
potentiometer.
The circuit diagram for the calibration of the voltmeter
is shown below. It consists of a stabilized ac supply,
rheostats, voltmeter, voltage ratio box, and
potentiometer. The basic and important requirement
of the circuit is that the input ac supply must not have
any fluctuations. Any fluctuation in the supply will
have a corresponding change in the calibration of the
voltmeter.
49. Hence, to avoid this a stabilized ac supply must
be used. Two rheostats Rh1 and Rh2 are used to
have a very precise control so that the voltmeter
accurately coincides with the major divisions. A
voltage-ratio box is used to reduce the voltage
across the voltmeter and applied to the
potentiometer.
50. The potentiometer reads the true value of the
voltage. If this value is matched with the voltmeter
readings, then the error is zero. However, if these
two readings do not match, an error is
encountered. This error may be positive or
negative depending on the relative value of the
voltmeter and the potentiometer. In order to have
greater accuracy, the voltage should be
measured near the maximum range of the
potentiometer.
A voltage ratio box or two capacitances in series
are used along with an ac potentiometer if the
voltage to be measured is of medium or high
magnitude. Otherwise, an ac potentiometer can
be used directly for voltages under 1.5 V.
51. 2. Ammeter Calibration – The measurement of the
alternating current may be measured by the use of
non-inductive standard resistor with the
potentiometer.
The connection diagram for the calibration of the
ammeter is shown in the figure below. The circuit
consists of a stabilized ac supply, variable resistor,
ammeter, a standard resistor, and a potentiometer.
The ammeter is connected in series with the
standard, non-inductive resistance of known value R.
The potentiometer is connected across the standard
resistor.
Let VR be the potentiometer reading which
corresponds to the voltage across R. Then the current
through R is given by, IR = VR/R. Let I be the ammeter
reading. The two values i.e., IR, I will be the same
since the ammeter and standard resistor are in series.
Since the value of standard resistor is accurately
53. 3. Wattmeter and Energy Meter Testing – The
testing circuit of the Wattmeter and the energy
meter is same as that of the DC measurements.
The phase shifting transformer is connected to
the potentiometer to vary the phase of the voltage
on the current. Thus, the voltage and current may
vary at different power factor.
The connection diagram for the calibration of the
wattmeter using a polar-type ac potentiometer is
shown below.
55. The current coil of the wattmeter is connected in series with a
standard resistor, a variable resistor (or rheostat), an ammeter, and a
mutual inductance and it is energized with the supply through a step-
down transformer. The pressure coil is energized through the
secondary of a variable transformer whose primary winding is
connected to the rotor terminals. The current through the current coil
and the voltage across the pressure coil is varied using a variable
transformer.
The purpose of voltmeter (V) and ammeter (A) is not to measure the
voltage (V) and current (I) but only to ensure that the voltage across
and current through the wattmeter are within its range. The voltage
across R is measured with the potentiometer and the current (I)
through CC is calculated as I = V/R. The voltage across the pressure
coil (V) is measured by the potentiometer with the help of a volt ratio
box.
The phase angle φ between the voltage and current can be varied by
changing the position of the rotor. Hence, the power to be measured
by wattmeter is VI cos φ. The value obtained by the potentiometer is
compared with the deflection of the wattmeter and the wattmeter is
calibrated for different combinations of V, I, and φ. The mutual
inductance M in the current coil circuit is to check the accuracy of
wattmeter when φ = 90° i.e., at zero power factor.
56. 4. Measurements of Self Reactant of a Coil –
The standard reactance is placed in series with
the coil whose reactance is to be measured.
57. INSTRUMENT TRANSFORMER
What is an Instrument Transformer :
Instrument Transformers are a type of transformer used in an AC system
to measure electrical quantities as voltage, current, power, energy, power factor,
frequency. Instrument transformers are also equipped with protective relays to
protect the power system.
Instrument transformers have the basic function of reducing the AC System
voltage and current. The current and voltage level of the power system is
relatively high. It is very difficult and costly to design the measuring
instruments to measure such high-level current and voltage. Generally,
measuring instruments are designed for 110 V and 5 A
The measurement of such very large electrical quantities can be made possible
by using the Instrument transformers equipped with these small rating
measuring instruments. Thus, these instrument transformers are very well-
known in modern power systems.
58. Advantages of Instrument Transformers
1. A small rating measuring instrument can be used to measure the large current and
voltage of the AC Power system i.e., 5 A, 110 – 120 V.
2. Measuring instruments can be standardized by using instrument transformers. Which
results in the reduction in the cost of measuring instruments. If the measuring
instruments are damaged, they can be replaced easily by healthy standardized
measuring instruments.
3. Instrument transformers provide electrical isolation between measuring instruments
and high voltage power circuits, which reduces the electrical insulation requirement
for protective circuits and measuring instruments and also assures the safety of
operators.
4. Several measuring instruments can be linked through a single transformer to a
power system.
5. Due to the low current and voltage levels in measuring and protective circuits, there
is low power consumption in measuring and protective circuits.
59. Types of Instrument Transformers
There are 2 types of instrument transformers as follows:
1. Current Transformer (C.T.)
2. Potential Transformer (P.T.)
Current Transformer (C.T.)
A current transformer is a type of transformer used to reduce the current of a
power system to a lower level to make it feasible to be measured by a small
rating Ammeter (i.e. 5A ammeter). A typical connection diagram of a current
transformer is shown as follows.
Primary of C.T. has very few turns. bar primary is sometimes also used.
Primary is linked in series with the power circuit. Therefore, sometimes it is
also called a series transformer. The secondary is having a large no. of turns.
The secondary is linked directly to an ammeter. As the ammeter is having very
small resistance. Hence, the secondary current transformer works almost in
short-circuited conditions. One terminal of the secondary is earthed to avoid the
large voltage on the secondary with respect to the earth. Which in turn reduces
the chances of insulation breakdown and also protects the operator against high
voltage. Furthermore, before disconnecting the ammeter, the secondary is
short-circuited through a switch ‘S’ as shown in the above figure to avoid the
high voltage build-up across the secondary.
61. Potential Transformer (P.T.)
The potential transformer is a type of transformer used to reduce the voltage of
the power system to a lower level to make it feasible to be measured by a small
rating voltmeter i.e. 110 – 120 V voltmeter. A typical connection diagram of a
potential transformer is showing in the following figure.
Primary of P.T. has large no. of turns. Primary
is linked across the line (generally between on
earth and line). Therefore, it is sometimes also
called the parallel transformer. Secondary of
P.T. has few turns and is linked directly to a
voltmeter. As the voltmeter has a large
resistance. Thus, the secondary of a P.T. works
almost in open-circuited condition. One earthed
terminal of secondary of P.T. is to maintain the
secondary voltage with respect to earth.
62. What are the differences between C.T. and P.T?
Sl. No.
Current Transformer
(C.T.)
Potential Transformer
(P.T.)
1
linked in series with a
power circuit.
linked in Parallel with the
Power circuit.
2
The secondary is linked to
Ammeter.
The secondary is linked to
Voltmeter.
3
Secondary operates
almost in short-circuited
conditions.
Secondary operates
almost in open-circuited
conditions.
4
The primary current is
subject to on power circuit
current.
The primary current is
subject to secondary
burden.
5
Primary current and
excitation vary over a
wide range with a change
of power circuit current
Primary current and
excitation variations are
restricted to a small range.
6
One terminal of the
secondary is earthed to
avoid the insulation break
down.
One terminal of secondary
can be earthed for safety.
7
The secondary is never
open-circuited.
Secondary can be used in
open circuit conditions.
63. Current Transformer (CT) –
Construction and Working
Principle
A current transformer (CT) is a type of transformer
that is used to measure AC current. It produces an
alternating current (AC) in its secondary which is
proportional to the AC current in its primary. Current
transformers, along with voltage or potential
transformers are Instrument transformer.
Current transformers are designed to provide a
scaled-down replica of the current in the HV line and
isolate the measuring instruments, meters, relays,
etc., from the high voltage power circuit.
64. The large alternating currents which can not be sensed or
passed through the normal ammeter, and current coils
of wattmeters, energy meters can easily be measured by
use of current transformers along with normal low range
instruments.
66. A current transformer (CT) basically has a
primary coil of one or more turns of heavy cross-
sectional area. In some, the bar carrying high
current may act as a primary. This is connected in
series with the line carrying high current.
The secondary of the current transformer is made
up of a large number of turns of fine wire having
a small cross-sectional area. This is usually rated
for 5A. This is connected to the coil of normal
range ammeter.
67. Working Principle of Current Transformer
These transformers are basically step-up transformers i.e. stepping up a
voltage from primary to secondary. Thus the current reduces from primary to
secondary.
So from the current point of view, these step down transformer, stepping
down the current value considerably from primary to secondary.
Let,
N1 = Number of Primary Turns
N2 = Number of Secondary Turns
I1 = Primary Current
I2 = Secondary Current
For a transformer,
I1/I2 = N2/N1
As N2 is very high compared to N1, the ratio I1 to I2 is also very high for
current transformers. Such a current ratio is indicated for representing the
range of the current transformer.
For example, consider a 500:5 range then it indicates that C.T. steps down
the current from primary to secondary by a ratio 500 to 5.
I1/I2 = 500/5
Knowing this current ratio and the meter reading on the secondary, the
actual high line current flowing through the primary can be obtained.
68. Types of Current Transformer
On the basis of their applications in the field,
current transformers can be broadly classified into
two types,
1.Indoor current transformers
2.Outdoor current transformers
Indoor Current Transformers
Current transformers designed for mounting
inside metal cubicles are known as Indoor Current
Transformers.
Depending upon the method of insulation, these
can further be classified as:
• Tape insulated
• Cast resin (epoxy, polyurethane or polycrete)
69. In terms of constructional aspects, Indoor Current
Transformers can be further classified into the
following types:
1.Bar Type CT: The CTs having a bar of suitable size
and material used as primary winding are known as
bar-type CT s’. The bar may be of rectangular or
circular cross-section.
2.Slot/ Window/ Ring Type CT: CTs having an
opening in the center to accommodate a primary
conductor through it is known as ‘ring-type’ (or ’slot/
window type’) CT.
3.Wound Type CT: A CT having a primary winding of
more than one full turn wound on the core is known
as wound type CT. The connecting primary terminals
may be similar to those of a bar type CT or
rectangular pads can be provided for this purpose.
70. Outdoor Current Transformer
These current transformers are designed for outdoor
application. They use transformer oil or any other
suitable liquid for insulation and cooling. A liquid-
immersed CT which is sealed and does not
communicate with the atmosphere is known as a
hermetically sealed CT.
Outdoor oil-filled CTs are further classified as
1. live tank type CT
2. dead tank type CT
Most of the outdoor current transformers are high
voltage current transformers. Based on the
application they are further classified into :
1. Measurement Current Transformer
2. Protection Current Transformer
71. Construction of Current Transformer
As we discussed above, there are three types of constructions used for
the indoor current transformers which are,
1. Wound Type CT
2. Toroidal (Window) Type CT
3. Bar Type CT
Wound Type Current Transformer – The transformers primary
winding is physically connected in series with the conductor that carries
the measured current flowing in the circuit. The magnitude of the
secondary current is dependent on the turn’s ratio of the transformer.
Toroidal (Window) Type Current Transformer – These do not
contain a primary winding. Instead, the line that carries the current
flowing in the network is threaded through a window or hole in the
toroidal transformer. Some current transformers have a “split core”
which allows it to be opened, installed, and closed, without
disconnecting the circuit to which they are attached.
Bar-type Current Transformer – This type of current transformer
uses the actual cable or bus-bar of the main circuit as the primary
winding, which is equivalent to a single turn. They are fully insulated
from the high operating voltage of the system and are usually bolted to
the current-carrying device.
72.
73. Errors in Current Transformer :
Before learning about errors in the current
transformer. Let us see the current and turn ratio
of a current transformer.
Turn Ratio (n) :
For a current transformer, if N1 and N2 are the
number of turns in the primary and secondary
windings. Then turn ratio is defined as the ratio of
number of secondary turns N2 to the primary turns
N1.
74. Actual Current Ratio (R) :
It is the ratio of the magnitude of current in the primary to
the secondary windings. It is denoted by 'R'. The actual current
ratio is the transformation ratio of a current transformer.
Nominal Current Ratio (Kn) :
The nominal current ratio of a current transformer can be
obtained from the data mentioned on nameplate details. It is the
ratio of the rated primary winding current to the secondary
winding current.
75. The relation between actual and nominal current
ratio is given by the 'Ratio Correction Factor
(RCF)’.
R = RCF × Kn
Owing to this turn, actual, and nominal ratios
of a current transformer. There are two types of
errors in a current transformer. They are ratio
error and phase angle error.
76. Ratio Error :
The ratio error of a current transformer is due to a change in
the actual current ratio from the turn ratio. We know that for a
current transformer the current ratio must be equal to the turn
ratio i.e., I1/I2 = N2/N1. But due to magnetizing and cross loss
components of the primary winding current and power factor of
the seconding winding. The current ratio I1/I2 will differ from the
turn ratio N2/N1.
Thus the actual current ratio will not be constant and depends
upon the load current, and power factor of the load or burden
connected to secondary. Due to this change in actual current
ratio, the current in the primary cannot be determined exactly
and causes an error called ratio error. The formula for
percentage ratio error is given as,
77. Phase Angle Error :
In practice, the secondary current of the CT must be in exact
phase opposition with the primary current i.e., exactly by 180
phase difference. At the time of power measurement, there exists
a difference in the phase angle between primary and secondary
currents.
This is due to fact that the primary current has to supply core
loss and magnetizing components of the CT for which it losses
some phase angle. Due to which the secondary current wouldn't
be in exact phase opposition. This difference or loss in phase
angle causes an error called 'Phase Error or Phase Angle Error'
denoted by angle θ. The phase angle error can understand by
below phasor diagram of a current transformer.
78. •Where,Ip = Primary current
•Is = Secondary current
•n = Turn ratio
•Io = Excitation currrent
•Ic = Core loss component
•Im = magnetising component
•Ep = Primary induced EMF
•Es = Secondary induced EMF
•φ = Flux develop
•θ = Phase angle error
•α = Burden Angle
•β = Angle between flux and excitation
current
79. In the above phasor diagram, by taking flux as the
reference. If load connected is to be lagging
power factor. The secondary current Is lags behind
the secondary emf Es. The primary has to supply
excitation current components i.e., Im and Ic. The
secondary current can be referred to as primary
by multiplying with turns ratio i.e., nIs. The vector
sum of nIs and Io gives the primary current.
The phase angle error is given by,
80. In practice, most of the loads ( relays or
instrument or pilot lights ) connected across
secondary are inductive type. For inductive, δ is
positive and very small. Therefore, sine δ ≈ 0 and
cos δ ≈ 1. By substituting in the above equation
we get,
81.
82. POTENTIAL TRANSFORMER
What is a Potential Transformer?
A potential transformer (also known as voltage transformer) is a
type of instrument transformer. It is a step-down voltage
transformer that reduces the high-level voltage to safer low
levels. The output voltage of the potential transformer can be
measured by connecting an ordinary voltmeter.
Potential Transformer Construction
Potential transformer or PT can have the same construction as
any normal transformer. It has primary & secondary winding. The
number of turns in primary windings is greater than the number
of turns in the secondary winding because it is a step-down
transformer.
83. Potential Transformer Working
The working of PT is similar to any conventional transformer. The
electrical energy is transferred between the primary & secondary
winding through magnetic induction.
The alternating voltage at the primary generates alternating magnetic
flux in the transformer core. Since both windings use the same core, this
alternating flux induces a voltage in the secondary winding. Thus current
starts to flow in the secondary winding.
Since the primary has a greater number of turns compared to fewer
secondary turns, the voltage induced in the secondary is very low. The
secondary voltage is measured by using a standard low voltage
voltmeter. Using the turn ratio equation of the transformer, we can
calculate the primary voltage.
VP/VS = NP/NS
Where
• VP = Primary Voltage
• VS = Secondary Voltage
• NP = No. of Turns in Primary
• NS = No. of Turns in Secondary
Since the voltmeter has very high impedance, very low current flow
through the secondary windings of the PT. for the same reason, the PT
has very low VA ratings around 200VA.
84. Connection of Potential
Transformer
The potential transformer is connected
in parallel with the circuit as opposed to
CT that is connected in series. The
primary of the PT is directly connected
to the power line whose voltage is being
measured. While the secondary is
connected to the voltage measuring
instrument like a voltmeter, wattmeter,
etc. Since the voltage in the secondary
is very low, an ordinary voltmeter can be
used to measure it.
The primary & secondary of the PT are
magnetically coupled through mutual
induction where the primary voltage is
reduced based on the turn ratio of the
transformer. The primary voltage can be
up to several thousand volts while the
secondary voltage falls below 110v. Both
windings are electrically isolated but for
safety reasons, the secondary winding
is grounded at its one end.
85. Errors in Potential Transformer
In an ideal transformer, the primary & secondary voltage are in
exact proportion as its turns ratio & they are both in-phase. But
practically, there is a voltage drop at primary due to its reactance
which creates voltage ratio error & phase-shift error. Here are
some of the errors that may occur in PT.
Ratio Error
The ratio error is the change in the voltage ratio due to the
variation in load. Varying load changes the magnetizing current &
the core losses that affect the secondary voltage of the PT.
In simple words, its nominal ratio differs from its actual ratio.
Ratio error is given by
Ratio Error = (Nominal Ratio – Actual Ratio) / Actual Ratio
Ratio Error = (Kn – R)/R
% Ratio Error = {(Kn – R)/R} x 100
Where
• Kn = Nominal Ratio (Rated Ratio)
• R = Actual primary to secondary voltage Ratio
The nominal ratio is the ratio of rated primary voltage to rated
secondary voltage.
86. Voltage Ratio Error
The voltage ratio error is the difference between the ideal voltage & the
practical or actual voltage. Here is the formula to find the voltage ratio
error
Voltage Ratio Error = (VP – KnVS)/ VP
% Voltage Ratio Error = {(VP – KnVS)/ VP} x 100
• Where
• Kn = Nominal Ratio (Rated Ratio)
• VP = Actual Primary Voltage
• VS = Actual Secondary Voltage
Phase Angle Error
The phase angle error is the difference between the phase of primary
voltage & the reversed secondary voltage. Ideally, the primary voltage is
in phase with the secondary voltage in reverse. But practically, there is
the reactance of the windings that shifts the phase of the secondary
voltage creating phase angle error.
Phasor Diagram of Potential Transformer
The phasor diagram for the potential transformer is given below. This
phasor diagram shows the primary current IP, primary voltage VP,
secondary current IS & secondary voltage VS.
87. Where
•VP = primary voltage
•EP = Primary Induced EMF
•RP = Primary Winding Resistance
•XP = Primary Winding Reactance
•β = Phase angle Error.
•IP = primary current
•Io = Excitation current
•Im = Magnetizing current (part of Io)
•Iw = core loss current (part of Io)
•Kn = Turn Ratio of Transformer
•Φm = Main Flux
•VS = Secondary voltage
•ES = Secondary Induced EMF
•RS = Secondary Winding Resistance
•XS = Secondary Winding Reactance
•IS = Secondary Current
88. The reference of the given phasor diagram is the main flux Φm. The primary
induced voltage is achieved by the subtraction of losses due to the primary
winding resistance RP, & reactance XP. The voltage drop due to primary
windings is IPRP, & the reactance of the windings is IPXP.
The excitation current Io is the vector sum of magnetizing current Im & core
loss current IW. The vector sum of excitation current Io & the reversal
secondary current IS multiplied by turn ratio 1/Kn results in the primary
current IP.
Due to mutual induction, the primary emf will transform into the secondary
emf ES in the secondary windings. The secondary voltage VS that appears
at the output of the secondary windings is derived by subtracting the
voltage drops due to the secondary windings resistance RS & reactance XS
