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Calibration of Ammeter

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Calibration of Voltmeter

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Calibration of Watt meter

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Measurement of Power

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Measurement of Resistance

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AC Potentiometers

• Similar to DC potentiometer.

• Difference phase balance is obtained

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Types

• Polar type– Drysdale Polar POT

• Coordinate type– Gall-Tinsley co-ordinate type POT

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Polar & Coordinate types

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Drysdale Polar Potentiometer

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Operation

• Consists of a potentiometer similar to the dc and an auxiliary circuit to measure the phase angle

• A two phase stator and a rotor form the auxiliary circuit.

• L1 & L2 stator windings are supplied from the same source

• Phase shift of 900 exists between the currents through L1 & L2

• The rotating magnetic field in the stator induces emfs e1 and e2 in the rotor

• The magnitude of the induced emfs are the same, but their phase can be adjusted by rotating the rotor.

• The resultant rotor emf’s magnitude is constant but its phase angle is the angle through which the rotor is rotated from its zero position

• The stator windings are so arranged that (900 phase shift) the adjustments in rotor phase angle will not alter the rotor emf magnitude.

• Therefore the rotor current ‘I’ which flows through the slidewire remains constant, though its phase angle changes.

Working• Calibrate the potentiometer using std dc cell by connecting the

terminals T1 & T2

• Apply the unknown voltage across the above said test terminals and obtain the balance by adjusting the slidewire and the rotor phase angle

• The slidewire reading gives the magnitude of the unknown voltage and the phase shifting transformer angle of rotation gives the phase angle w.r.t an arbitrary reference vector

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working

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Gall Co-ordinate Potentiometer

• Two slidewires AB and CD with currents I1 and I2 having phase shift 900

• AB is the inphase ptm and CD the quadrature ptm

• Calibrate the inphase ptm and adjust the quadrature ptm to give the same magnitude of current as in AB

• To calibrate the quadrature ptm: Connect the terminals of inphase ptm to an ac voltage. Adjust the inphase ptm current to std value

• The emf induced in the secondary winding of the mutual inductor will lag the current through AB by 900

• As the quadrature ptm is supplied through a phase shifter, the secondory emf will be in phase with the quadrature slidewire emf, if the phase shift and current magnitudes are adjusted. The secondary induced emf depends on f, M and I1

• Under this condition, the two slidewire currents will be in quadrature.

• Apply the unknown voltage

• The two slidewires are in series . Adjust them to get null deflection on VG, AB reading will give the inphase and CD the quadrature components of unknown voltage. 14

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Applications of AC Potentiometer

• Calibration of voltmeter• Calibration of Ammeter• Calibration of Watt meter• Measurement of self inductance

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Advantages of AC Potentiometer• Very versatile instrument. By using shunts and volt-

ratio boxes its use may be extended to cover current, voltage and resistance measurements over a wide range of values

• The fact that phase, as well as magnitude, is measured leads to such applications as measurement of power, inductance and phase angle of the coil etc.

• Used in special measurement circuits

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Disadvantages of AC Potentiometer

• A very small difference in reading of reflecting dynamometer instrument either in DC or AC calibration introduces error in the AC current to be set at standard value

• The mutual inductance between various parts of the instruments affects the normal value of mutual inductance. Due to this, a slight difference is introduced on the magnitude of the current of quadrature wire compared to that in the in phase POT wire.

• The inter capacitance and mutual inductance affect the potential gradient of the wires

• The inaccuracy in the measured value of frequency will also result in the quadrature POT wire current to differ from that of in-phase POT wire.

• Because the standardization is done on the basis of RMS value and balance is obtained depending upon the fundamental only, the presence of harmonics in the waveform introduces operating troubles and the vibration galvanometer tuned to the fundamental may not show full null position at all.

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References used

• A.K. Sawhney, ‘A Course in Electrical & Electronic Measurements & Instrumentation’, Dhanpat Rai and Co, 2004.