ELECTRICAL MEASUREMENT &
MEASURING INSTRUMENTS
Department of Electrical & Electronics
Engineering
Droncacharya College of Engineering, Gr.
Noida
UNIT 4 Part (i)
AC Potentiometers
Polar type & Co-ordinate type AC potentiometers
 Potentiometers is an instrument used to
measure voltage by using differentiates
method with one of standard value.
 The measurement has made with
adjusting of contactor until the zero
reading has been recorded.
 This condition we called as balance
condition due to depletion voltage is same
as standard voltage that provided on slide
wire.
SLIDE WIRE POTENTIOMETER
 It uses the depletion voltage concept on conductor
wire which is proportional directly with long of
conductor wire that used.
 The maximum depletion voltage on entire of
conductor wire is equal to supply voltage that used.
PRINCIPLE
 It
must be calibrated with one of
standard value.
 In the calibration process, when the
switch is closed circuit, the working
current will flow through a slide wire
potentiometer (namely as conductor
wire) and rheostat.
 The value of this current can be
modifying by adjusting the rheostat.
ADVANTAGES OF
POTENTIOMETER
 To measure the voltages by differentiate with
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standard value.
To measure resistance by differentiate with
standard resistor.
To measure the current by measuring voltage to
the unknown resistor value.
To calibrate volt meter.
To calibrate ampere meter.
A.C. POTENTIOMETER
 Its principle is same as a d.c. potentiometer.
 One very important difference between the two.
 In d.c. potentiometer, only the magnitudes of the
unknown e.m.f. and slide-wire voltage drop are made
equal for obtaining balance.
 But in an a.c. potentiometer, not only the magnitudes
but phases as well have to be equal for obtaining
balance.
 To avoid frequency and waveform errors, the a.c.
supply for slide-wire must be taken from same source
as the voltage or current to be measured.
Types
Two Types
 Polar potentiometers in which the
unknown voltage is measured in
polar form i.e. in terms of magnitude
and relative phase.
 Co-ordinate potentiometers which
measure the rectangular co-ordinates
of the voltage under test.
DRYSDALE POLAR TYPE AC
POTENTIOMETER
 It is basic d.c. potentiometers along with some
auxillary components such as, drysdale phase
shifter and electrodynamometer type ammeter.
 When the current flows through stator winding, a
rotating field is produced inducing e.m.f in the rotor.
 The phase of rotor current can be changed through
any angle relative to stator supply voltage by
rotating rotor.
 Thus the change in the phase of secondary e.m.f. is
equal to the angle through which rotor is moved
from its original zero position.
Circuit Diagram
Standardization of Polar AC
Potentiometer
 Both d.c. as well as a.c. standardization is done.
 The d.c. standardization is done first by replacing
vibration galvanometer by D’arsonval galvanometer.
 A standard cell such as Weston cell is used for d.c
standardization.
 Then by adjusting sliding contacts null deflection in
galvanometer is achieved.
 The reading of a precision ammeter included in
battery supply is noted. During a.c. standardization
again vibration galvanometer is used.
Measurement of Unknown E.M.F.
 An emf to be measured connected across terminals
A-A’.
 Sliding contacts P1 and P2 and the position of rotor
in phase shifter are adjusted simultaneously till the
balance is obtained as indicated by the null deflection
of vibration galvanometer.
 At balance, the magnitude of the unknown emf is
obtained from P1 and P2 and the phase angle is
measured from the scale reading which is mounted
on the top of the instrument. Thus the unknown emf
can be expressed in polar form as E∟Ө°.
Circuit Diagram
GALL TINSLEY CO-ORDINATE TYPE AC
POTENTIOMETER
 The in-phase and quadrature potentiometer consist
of sliding contacts BB’ and CC’ respectively.
 Rheostats r and R’ are also provided in the
respective potentiometers for the adjustment of
current.
 By using different arrangement, the supply for the
potentiometer are obtained.
 A vibration galvanometer VG is tuned to the supply
freq and it is connected in series with a switch K and
electrodynamometer type ammeter.
Circuit Diagram
Measurement of Unknown EMF
 The emf to be measured is connected across the
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terminals A-A’ using selector switch S3.
Sliding contacts of both the potentiometers are
adjusted till the contacts of both the potentiometers
are adjusted till the null deflection is obatained
Under the balance condition, the in-phase
component of the unknown emf is obtained from inphase potentiometer while the quadrature
component of the unknown emf is obtained from
quadrature potentiometer.
If needed the polarity of the test voltage may be
reversed by using sign changing switches S1 and
S2 to balance the potentiometer.
APPLICATION OF AC
POTENTIOMETER
 Voltmeter Calibration
 Ammeter Calibration
 Wattmeter and Energy meter Testing
 Measurement of self-Reactance of a coil