What are the special features of a high voltage Schering bridge? What is meant by “Wagner’s Earth”?
Wagner’s Earthling: For elimination of some of the effects of stray capacitances in A.C. bridge circuits, the method is widely used. For eliminating the troublesome capacitances between the detector and ground this device is used. Fig. shows the circuit of a capacitance bridge with Wagner earthling device. \(C_1\) and \(C_2\) represents stray capacitance between detector and ground. The oscillator is changed from its usual ground connection and bridge by a series combination of resistor \(R_W\) and capacitor \(C_W\). The junction of \(R_W\) and \(C_W\) is grounded.
For balancing the bridge, the detector is first connected to point 1 and \(R_1\) is adjusted for null. The defector is then connected to position 2 connecting it to Wagner earth, and then the resistance \(R_W\) is now adjusted for null. The procedure is repeated till final balance is adjusted at positions 1 and 2 changing \(R_1\), \(R_2\) and \(R_W\).
In a certain current transformer, the following data is obtained. Nominal ratio = 25/5A, Turn ratio = 3, primary terns = 40, secondary turns = 120, secondary resistance = 0⋅16z, secondary reactance = 0⋅195Ω, secondary burden = 15 VA, Burden power factor = 0⋅7, secondary terminal voltage = 3V.
Find ratio & phase angle errors. The magnetising and loss ampere turns corresponding to an emf of 4⋅26V are13 & 10⋅1 respectively.
Explain the principle of operation of a digital frequency meter.