Skip to main content

LOSS OF CHARGE METHOD

 Current-time characteristic of the discharge current is obtained by placing the switch B in position 2. The slope of these current time characteristic gives the time constant Ο„ = CR,

where      C is the capacitance of the specimen.

The volume resistance of the specimen R can thus be obtained from the relationship by measuring Ο„ and C.

Alternatively, a capacitance previously charged (C1) may be discharged through the specimen by inserting it in place of the battery E. By knowing the value of the capacitance C1 which is very large compared to the specimen capacitance (≈10 ΞΌF) and knowing the current-time characteristic of the discharge current, its time constant can be calculated. Hence, the resistance of the specimen is obtained from the relationship        𝑅=𝜏/𝐢1

where C1 – capacitance used

Ο„ – time constant.

If initial voltage V0 on the capacitor is measured and the voltage V across it at any time Ο„ Then the unknown resistance R can be calculated from the relationship

𝑅=𝑑/𝐢1𝑙𝑛𝑉0𝑉

➢ The accuracy of this method depends on the accuracies with which the values of the capacitance and the voltage are measured.

DC galvanometer of fig 1 is replaced by a dc amplifier for resistivity measurements.

➢ DC amplifier is used as a null detector and a separate potentiometer circuit is used for obtaining a signal e equal and opposite to the voltage drop across the standard resistance Rs.

➢ At balance, voltage drop across Rs is made zero and any ac voltage appearing across the standard resistance Rs is amplified only by the net gain of the amplifier which is small.

➢ Using a recording meter after the dc amplifier, the volt-ampere-time curves for long durations, of the order of several hours (day also) can be obtained. This type of information is essential to determine the relaxation time at low frequencies or with dc.

E-stabilized power supply 500 to 2000v
V-Voltmeter
Rp-Specimen
Rs-Standard Resistance (1MW)
e's-Auxiliary source
Fig .2-DC  amplifier circuit for resistivity measurements

Fig 3 Wheatstone bridge arrangement for resistivity measurements


Fig 3 shows the Wheatstone bridge network in which one of the resistances (usually RA) is

made variable for balancing the bridge. At balance, Rp is given by

𝑅𝑃 = 𝑅𝑠(Ra/Rb)


Comments

Post a Comment

Popular posts from this blog

PARTIAL DISCHARGE REPRESENTATION:

                          The weak points in an insulation like voids, cracks, and other imperfections lead to internal or discharges in the insulation and as they are small, were not revealed in capacitance measurements but revealed as power loss components, contributing for an increase in the dissipation factor                                 ‘PARTIAL DISCHARGES’ in a course of time reduces the strength of insulation leading to a total or partial failure or breakdown of the insulation Consider           Ca as capacitor with a void inside the insulation. Capacitance of the void is represented by a capacitor in series with the rest of the insulation capacitance (Cb). The remaining void-free material is represented by the capacitance Cc When the voltage across the capacitor is raised, a critical value is reached acr...
Post a Comment
Read more