Schering Bridge
- There are various methods for the measurement of capacitance and di-electric loss.
- Schering Bridge method is very important and popular method to determine these terms.
- In this method imperfect capacitor can be compared with a perfect capacitor. (C2 is compared with C1).
- The imperfect capacitor is represented by its equivalent loss free capacitor C in series with a resistance r, this is shown in part ‘b’ of the following Figure.
- Earthing as shown safeguards the operator at high voltage testing
We can write different impedances as
Z1 = – j / ωC1
Z2 = r – j / ωC2
Z3 = R3
Z4 = 1 / (1/ R4)
+ jωC4 R4 = R4 / 1 + jωC4
R4
Now for balance (indicate by the
detector)
∴ Z1.
Z3 = Z2. Z4
Or –jR3 / ωC1
= [r – j / ωC2] [R4 / 1 + jωC4 R4]
Or –jR3 / ωC1
(1 + ωC4 R4) = R4 (r – j / ωC2)
Separating the real and imaginary parts we get
C2 = C1 (R4 / R3) and
r = R3 (C4 / C1)
∴ tan 𝛿 = r / Xc = r 1 / ωC2 = ω rC2
… Putting the value of r and from above Equation uvieron
Dissipation (loss) factor = ω rC2 = ωC4 R4 = power factor
- Though Schering-bridge method is accurate, there are some shortcomings in it. The errors are due to temperature rise and aging.
- These are eliminated by use of current comparator bridges.
