A.C Potentiometer1

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A.C Potentiometer

There are two types ofA.C potentiometers. Theyare

1.Polar type potentiometer

2.Co-ordinate type potentiometer

.

polar type potentiometer, which measures the

magnitude (V) in one scale and phase ()

in other scale.


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Drysdale Tinsley A.C potentiometer


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T.I = Transfer Instrument (precision type electro-dynamometer ammeter)

DPDT = Double Pole Double Throw

SPDT= Single Pole Double Throw

G = D' Arsonval Galvanometer

=

.

B = Standard Battery

POS 1 = Position 1

POS 2 = Position 2


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When an A.C voltage measurement is done by taking a reference A.C

voltage supply, the conditions that must be satisfied are,

1. Both the voltages should have same frequency.2. Their phases should be same.

3. Their magnitudes should also be same at all the instants

It is very difficult to satisfy all the three conditions, if we use a

separate reference source., .

voltage to a phase shifting transformer whose one

stator winding is connected directly to the unknown

supplyand other stator winding is connected to the same

supply through a variable resistor and a capacitor.


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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 Rotating

Magnetic Field (R.M.F.) (i.e., due to phase

splitting) which links with the rotor winding toinduce an e.m.f in it with the same frequency as

that of supply and whose phase angle can be

selected by changing the rotor position.

Hence, the phase angle of the unknown voltage

can be measured against this reference rotor

position.


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For the measurement of its magnitude with a

normal D.C potentiometer, all the resistors and

the slide wire are replaced by a standard non-inductive resistors and slide wire.

So that, its resistance does not vary with frequency

and waveform.


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Procedure for the Measurement

To measure an unknown A.C. 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 down.

,

2) which connects the rotor terminals of phase shifting

transformer to supply terminals of potentiometer,

vibration galvanometer to detector terminals and the

unknown A.C voltage to potentiometer test terminals.


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Now, the current through the ammeter is

made equal to the current through it when

D.C 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' .

Hence, the magnitude and phase of the

unknown A.C voltage are obtained from the

slide wire position and rotor position readings

respectively.


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Drysdale Phase Shifting Transformer

The Drysdale phase shifting transformer consists of stator and rotor.

The stator is made up of silicon steel and is provided with two (or)three phase windings.

The rotor of the transformer is also provided with winding in slots.

A small air gap is provided in between the stator and rotor.

When the supply is given, the current flows through the statorwindin s and enerates the flux in the air a .

Whenever the rotor winding cuts the flux, an e.m.f is induced in therotor windings.

The rotor position can be adjusted by adjusting the rotor angle and

the phase displacement of induced e.m.f is adjusted by adjustingthe rotor position with respect to null-pointer.

In order to measure the deflected angle of the rotor, a scale andpointer is provided at the top of the instrument.


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Instead of using single phase supply, the phase

shifter can be operated by using a phase splitting

device.

This device consists of two windings displaced at

an angle of 90.

Among the two windings, one is connected tothe supply and the other is connected in series

with a resistor and capacitor, which are adjusted

till the currents in two windings become equal.

Now the induced e.m.f in rotor windings due to

first stator winding is given by,


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From the above expression it is clear that, the rotor e.m.f has constant amplitude

and the phase angle is given by the rotor deflection .


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Transfer Instrument

The transfer instrument uses an electrodynamometer type ammeter to

standardize an A.C potentiometer.

The process of standardization of A.C potentiometer can be obtained by

switching the slide wire circuit on to D.C supply.

By using a standard cell (or) battery the standard current is known.

This standard current is measured by using electrodynamometer typeammeter.

When a transfer instrument is operated under A.C supply, the r.m.s value

of the slide wire current is remained at the same value as on D.C supply.

It is so, because of non inductive wound coils in potentiometer.

A switch is employed to enable the potentiometer either on D.C side (or)A.C side.


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Gall-Tinsley Co-ordinate PotentiometerThis potentiometer is actually a combination of two

potentiometers.

One of the potentiometer carries a current in-phase with thesupply voltage and it is called 'in-phase' potentiometer.

The other potentiometer carries the current in quadrature withsupplies, voltage and it is called as 'quadrature'potentiometer.

T1 and T2 are the two step down transformers fed from a singlep ase supp y.

The supply to T2 is obtained through the series combination ofvariable capacitor Cs and variable resistor R., for splitting thephase.

Exact phase quadrature is obtained by adjusting R., and C.

ab and cdare sliding contacts of in phase and quadraturepotentiometer respectively and rheostats R1 and R2, are usedfor Current adjustments.


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V.G is Vibration Galvanometer tuned to the supply frequency.

A is a reflecting dynamometer ammeter for maintaining the current

in both the slide-wires at a standard value. R.S.1 and R.S.2 arereversing switches which may be ,necessary to reverse the

direction of the unknown e.m.f applied to slide wires.

S2 is a selector switch for placing unknown voltages to be measured

in the circuit.

The component of the unknown voltage which is in-phase with

the slide wire current of 'in-phase potentiometer is measured on

it and it is the in-phase, component of unknown voltage.

Let its value be V1 and the component which is in phase with the

'quadrature' potentiometer current is measured on it and it is thequadrature component of unknown voltage.

Let'its value be V2.


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Standardization

The D.C. standardization of the in-phase potentiometer is done by

connecting the battery B by means of 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 dynamometer ammeter is oftorsion head type and the

torsion head is tuned to zero position on direct current.This setting is left untouched.

The switches S1 and S2 are again brought back to initial

position.

The alternating current is adjusted in the in-phase potentiometerby rheostat R1 to give zero deflection of the milli ammeter.

The magnitude and phase of the quadrature potentiometer's

current are adjusted by the mutual inductor M


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The switch S2 is brought to position 3-3.

The dial settings of the in-phase potentiometer is done to read a value of

M i( iis the primary current, e.m.f. induced in the secondary winding =

2ftMi)

Where, i'is the standard alternating current in the in phase

potentiometer.

The magnitude and phase of the current in quadrature potentiometer is

adjusted by means of rheostat R2 and variable resistance Rs of the phases littin device to obtain exact balance which is indicated b the

vibration galvanometer.

The switch S2 is again brought to position 2-2. In this position two slide

circuits and vibration galvanometer are in aeries 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 R.S.2 and R.S.3 if necessary.


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The errors developed in a potentiometer are due to the following

reasons.

1. As we know that, different parts of the instrument will

have mutual inductance. This will effect the nominalvalue of mutual inductance, which in turn produceserrors.

2. Inaccurate measurement of frequency will produce

error.

3. Due to the resence of harmonics in the su l errors

are introduced.

4, Inter-capacitance, earth capacitance and mutual

inductance of the slide wire coils will have effect on

potential gradient, which may produce an error.5. Even a very minute difference in A.C and D.C reading

of the reflecting dynamometer instrument mayintroduce errors.


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Compensation of the ErrorsThe errors produced in A.C potentiometers can be minimized by

the following,

1. In order to avoid the error in readings, the intercapacitance,earth capacitance and mutual inductance of the slide wireand resistance coils must be reactive.

By this, not only the error in reading is eliminated but also the

accuracy is improved.2. T e source o A.C supp y s ou e ree rom armonics

since the presence of harmonics will effect the accuracy inmeasurements.

Therefore, in order to avoid the harmonics it is necessary that

the A.C source most be made as sinusoidal as possible.3 While measuring the A.C voltages the stray e.m.f's is

fields and coupling circuits must be eliminated.Otherwise, it will be added vectorially to the voltage beingmeasured.


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A.C Potentiometer1

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